Development of SMAW Electrode for Welding

Published on 21 December 2021
  • book7 min


DEVELOPMENT OF SMAW ELECTRODE FOR WELDING OF DMR 249A & DMR 249B STEELS


1 INTRODUCTION


Indian Navy and NMRL have taken up a project to develop suitable consumables through Indian consumable manufacturers Apart from developing consumables, understanding details pertaining to the welding procedural aspect like joint design, selection of suitable size of consumable, Interpass temperature, number of layers, heat input is essential as they can influence the properties of the weld metal. This paper details the developmental work under taken to develop electrodes for welding low alloy steel plates DMR 249A and very high tensile high strength DMR 249B grade steels and also establish the parameters to get desired properties.


The present steels are nickel bearing micro alloyed steel characterized by higher strength and superior toughness even at sub zero temperatures. These plates were hitherto being imported in quenched and tempered condition. Presence of alloying elements it yields higher strength with goodtoughness. The chemical and mechanical property of these steels is shown in Table-1 & 2. 


Basically, the alloy additions after the transformation characteristics of the steel enable to achieving a higher strength and toughness after heat treatment. Fig.1 shows an isothermal diagram of typical Q&T steel. It can be observed from this diagram. The starting of the austenite to pearlite (or) ferrite transformation requires long durations and therefore even moderate / slow cooling produced martensite / lower bainite and avoid ferrite. (Ref-2)


Indian Navy in association with NMRL has specified the weld metal property requirements.The details of the various properties to be met by the weld metal are specified in the Table-3 & 4.With this understanding, the electrodes development work was under taken.


2 DEVELOPMENTAL WORKS


During development, several batches were produced, tested for all properties with boiler quality mild steel plates before standardizing. During formulating the suitable chemistry, following aspects were also kept in mind.

(a) Lower levels of impurity elements in the weld metal.

(b) Selection of suitable binder to get extra low hydrogen in the weld metal.

(c) Effect of dilution with base material.

(d) Sufficient percentage of Mn & Ni to get desired structure.

Thus electrodes meeting NMRL specification has been developed with the targeted chemistry.


3 EXPERIMENTAL STUDIES


For the purpose of this study, sufficient quantity of electrodes of size 3.15 and 4.0 mm was produced and used for internal laboratory tests and established the parameters with DMR plates. After qualifying the procedure at our R&D, it was sent for testing at NMRL, Ambernath. The procedure established with the consumable is detailed in Table-5 & Table-8. After satisfactory results were received from NMRL, few more batches were produced and established the repeatability at NMRL & user end CSL, Cochin also. The results obtained at user end are shown in Table-6, Table-7, Table-9 and Table-10.

The effects of various parameters are discussed in the following paragraphs.


4 RESULTS AND DISCUSSIONS


The results obtained through the above study were summarized as under:

 

Screenshot table 4 .png


5 CONCLUSIONS:


From the results obtained we can conclude as follows:

(a) SMAW electrodes meeting NMRL specification requirements for welding these steelshave been developed.

(b) Lower IPT seems to have a beneficial effect especially when low temperature toughness properties are desired.

(c) Lower heat input produced better toughness.

(d) A judicious choice of various welding parameters produces desired results.

(e) Proper root face advised to qualify the welders.

(f) Diffusible hydrogen levels control is very important to get good ductility.



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Development of Ferritic High Strength SMAW\r\nElectrodes For Steam Generator Application



1.0 INTRODUCTION


It is well understood that the involvement of welding technology in commercial as well as in industrial fields is inevitable. In contrast to commercial needs, the industry demands more stringent requirements on welding technologies. In view of this, new welding technologies are developed world wide to improve the manufacturing and fabrication of industrial components. Besides this, the main focus is also on


It seems that the novel way of developing welding consumable for specific applications in power generation and petrochemical industry is rather a challenging one. As well, the developed welding consumable must possess an international material standard for its applicability in various related applications. For example, the main industrial products such as boilers, heaters, pressure vessels etc. have to be developed accordingly to enhance the process by various means of increasing steam parameters. In the direction of satisfying the enhanced base metal properties, a high performance welding consumables have to be developed and tested for its reliability. In view of this, a low hydrogen SMAW welding consumable with improved mechanical properties have been developed in-house for fabricating pressure vessel component in nuclear applications. This type of electrodes is regarded as advantageous one, because it eliminates the pre-heat process before carrying out to welding stage. Based on the specification prescribed by client on covered ferritic welding

electrodes for shielded metal arc welding process, extensive research and development has been taken place in-house to achieve the required properties. The details of the specification of weld metal required are presented in Table1.


2.0 DEVELOPMENTAL WORK


2.1.1. All Weld-Joint Preparation

During development, several batches of weld joint have been prepared with

slightly modified electrode compositions to optimize the desired composition of the weld metal. For standardization purpose, various aspects that are taken care as follows,

(a) Reduction of impurity elements in the weld metal

(b) Reduction of hydrogen content by choosing a suitable binder

(c) Welding procedural aspects like influence of Inter Pass Temperature, heat input etc.

(d) Adjustment of chemical composition to get desired properties


The above said methods are optimized and the all weld joint assembly is prepared successfully for metallurgical and mechanical evaluation for its suitable applications. The schematic of the all weld preparation procedure is shown in Figure-1. 20MnMoNi55 forge plate of dimensions 450\u00d7125\u00d720 mm is prepared with the bevel angle of 10 degrees and a root gap distance of 16 mm supported with backing strip. This specimen is welded with our newly developed electrode by using SMAW process. The optimized welding procedure utilized during the welding process is systematically presented in Table-2 (average of seven layers). The test specimens are machined from the weld joint and are subjected to various analyses such as chemical, metallography, mechanical and radiographic examination.

2.1.2. Chemical Composition

The chemical composition (wt. %) of the weld metal determined using wet chemical analysis is given in Table-3. In addition to this, the resulting composition obtained from the root of the weld after the welding process is also given in Table-3. This has been performed to know the extent of dilution. In Table-4, the composition of the forge plate used as a base material is also tabulated.


2.1.3. Metallography Studies

The optical and hardness studies have been carried out using AXIOVERT 100A Optical microscope and Rockwell Hardness tester (0 \u2013 100 RC). Metallographic specimens have been prepared by adopting standard method of polishing procedures using various grades of emery sheets and cloth impregnated with fine alumina particles. This is followed by cleaning with distilled water and methanol. The etchant used for observing the microstructure is made of aqueous solution containing 4% Picric acid and 1% Nitric acid. The etched specimens have been used further for hardness analysis.


2.1.4. Tensile Studies


The tensile property of the pure weld deposit is analyzed using AMSLER Universal Tensile Testing Machine with a load capacity of 200 kN. The tensile measurements have been conducted at room temperature (RT), 200o C and at 350o C respectively.  Figure-2 shows the round specimens of diameter 12.5 mmand guage length 60 mm used for tensile testing prepared as per the ASTMstandard E-21. The tensile data are analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (et) and reduction in area.


2.1.5. Charpy Impact Testing


For Charpy impact testing, the specimens used are cut across the welded joints having dimensions of 10\u00d710\u00d755 mm and type V-notched, with 2 mm of depth. The Charpy transition curves are obtained from room temperature to sub-zero temperatures. The Charpy impact test is accomplished in compliance to ASTM

E23 standard to determine the ductile to brittle transition temperature.


2.1.6. Bend Tester


The welded specimens have been Bend Tested using AMSLER Bend Tester for the evaluation of the ductility and soundness of the weld.


2.1.7. Die Penetration and Radiography


The weld deposits are analyzed with Die penetration and X-ray radiography for

the evaluation of any presence of crack and inclusions.


2.1.8. Drop weight test results


Drop weight test of eight weld samples were tested and the results are satisfactory. Welding procedure for preparation of weld coupon is given in Table-5. From the above weld coupon final size of the test specimen prepared as per ASTM E208. The details are given in Figure-3.


3.0 RESULTS AND DISCUSSIONS


The optical micrographs of the base metal away from the weld region and the as deposited last bead of the weld are shown in Figure-4. In both the cases, the microstructure consists of ferrite and bainite. It is clear that the strength and toughness of this material emanates from the presence of bainite and ferrite fractions. The average Rockwell (RC) hardness values of the forge plate and the as deposited weld metal are 14 and 16 RC respectively. Figure-4a also shows the microstructure of the HAZ of the specimen. It is clear from the figure that the microstructural features are finer than the base metal and the weld metal. This may be due to the effect of cooling rate and dilution.

In Figure-5, the response of weld tensile specimen with the application of load is portrayed. The collage in Figure-5, tested at different temperatures clearly reveals that the tensile strength (YS, UTS) of all the specimens are almost identical, except during the occurrence of fracture. Moreover the tensile strength at high temperature is also not drastically different for 200o C and 350o C tested specimens. It suggests that the required hot tensile property of the weld is attained in accordance to ASME specifications. For comparison purpose, the ASTM value and literature data on tensile properties of base material are gathered. It is observed that the tensile property of the developed weld metal is comparable to the literature data, signifying the strength and quality of the weld. Table-6 also lists the average of three tensile tested specimen properties that are obtained at room temperature and elevated temperatures respectively besides some literature data. In Figure-6, the impact toughness of the weld joints tested at room temperature and at subzero temperatures are displayed. In this figure, the available toughness data on base material as well as similar weld metal is co-plotted. It is seen fromFigure-6 that the toughness values of weld metal is inferior to base metal. In general, the weld samples may have micro-defects, impurities, non-uniform properties due to multi-pass weld, cooling rate dependent transformations etc. However in the present study these parameters are taken care to obtain a high quality weld and hence the improved toughness properties are obtained. This is also made clear by the highlighted data of about 54 J specified by AWS. In Table- 7, the toughness values of the weld metal tested at various temperatures are listed. In addition to this, data on lateral expansion and the percentage of shear fracture area determined from the fracture surface is also listed. In Figure-7, the fracture dependent lateral expansion and the percentage of shear fracture area calculated from the fracture surface is plotted. Both of the parameters decrease gradually with the temperature signifying the change in the fracture mode i.e. ductile to brittle.


In general, a high performance weld material is chosen for stringent applications in nuclear industries. For example, the reactor pressure vessels are made with nuclear grade high strength steel which requires equivalent high strength weld materials for fabrication purposes. In view of this, the results obtained in the present study with regard to mechanical properties evaluation of the high performance SMAW ferritic electrode are discussed below.


The optical microstructures determined at locations of weld centre line, HAZ and base metal suggested the formation of bainite and ferrite phases. The variation of the phase fractions of bainite and ferrite between the base metal, weld and HAZ specimens are determined using the Image Analysis\u00ae software. Because this provides the information about the effect of cooling rate, compositional re- adjustments on the phase transformation of \u03b3-austenite \u2192 \u03b1-ferrite transformation.


It is observed from the image analysis results that the phase fraction of ferrite and bainite is high in base metal as compared to weld and HAZ specimens. This may be due to the effect of cooling rate / composition on the kinetics of \u03b3 \u2192 \u03b1 phase transformation. The phase fraction values have been determined at different locations (more than 10 locations at each region) and the average value is tabulated in Table-8. A typical threshold image of the base metal obtained using Image Analysis is shown in Figure-8 for a comparison. Since the value of the phase fraction depends on the threshold fixing, care is taken in determining the values of respective phase fractions. In Figure-8(b) the white ferrite regions are undecorated with red colour while the dark regions of bainite are decorated with red color using Image anlaysis procedure is shown.

The evaluation of the tensile results of the weld specimen tested at RT, 200 o C and 350 o C suggests that the tensile strength possessed by the weld specimen is adequate for high temperature pressure vessel applications. Besides, the decrease in the yield strength of the weld specimen at 350oC is about 25% and the ultimate tensile strength is about 13% as compared to RT data. 


The decrease in the tensile strengths is attributed to the enhanced thermal activation of dislocations at elevated temperatures. In most cases, the material under fracture investigation must possess homogeneous structure to yield the scatter-less tensile properties. The presence of HAZ and brittle zones in the welded joint has inhomogeneous tensile properties leading to dispersion of the data. But this property is also very important in design considerations [8-11]. In the present study, only the pure weld deposit is tensile tested and the average values of the tensile strengths are presented. It is also clear from the Table-6, the tensile strength of the weld deposit is markedly higher than the base material. The results of the Charpy transition curves of the weld joints having a notch at the weld deposit region clearly suggest that the weld deposit have sufficient toughness to resist the fracture at sub-zero temperatures i.e > 54 J. Even at minus 76o C, the toughness value is found to be 60 J. It is known that the primary design criteria of a component under stringent conditions depend upon the strength and the stability of the microstructure. The fracture toughness is also one of the design criterions, which has to be determined for structural integrity assessments.


4.0 SUMMARY AND CONCLUSION


It is clear from our evaluated results of the weld metal that the SMAW electrode developed in-house has met the specified requirements of BHEL. Careful optimization of the composition of the weld metal yielded good hot tensile as well as toughness properties. The liquid penetrant and radiography test have confirmed that no cracks and inclusions are present on the surface of weld metal.  In addition to this, the diffusible hydrogen mercury test also shows the average level of hydrogen is about 3.9 ml in 100 gms of weld. Hence the optimization of composition of welding consumable, maintenance of weld metal quality, achievement of superior high temperature and low temperature properties makes this product applicable for steam generator applications as well as various demanding structural applications mentioned earlier in this paper.


The major conclusion drawn from this work is as follows:

I.  A high performance low hydrogen E9018-G SMAW electrode meeting specifications is successfully developed.

II. Effect of heat input, Inter Pass Temperature (IPT) on weld metal properties have been analysed and found that lower heat input, interpass temperature of about 100\u00b0C is beneficial for desired mechanical properties. Hence a careful selection of welding parameters is always recommended.

III. The control of hydrogen content in the weld is advantageous to obtain good  ductility of the weld.



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DEVELOPMENT OF SMAW CONSUMABLES FOR OVERLAY & JOINING OF LOW ALLOY FORGE MATERIAL USED IN NUCLEAR SG APPLICATIONS



1 INTRODUCTION


Several structural candidate materials are used for fabricating nuclear power plant materials. For most of the materials, welding consumables are available indigenously. But in case of welding consumables for nuclear applications, we are still importing consumables from overseas to meet certain stringent requirements on specific forged materials for overlay and joining applications. In view of this, SMAW electrodes of \u2248 ENiCrFe-3 and low hydrogen high strength low alloy steel welding consumable with improved mechanical properties have been developed indigenously for fabricating pressure vessel component in nuclear applications.

In this paper, overlaying on 20MnMoNi55 steel with \u2248 ENiCrFe-3 and for joining low hydrogen high strength low alloy steel electrode by SMAW process is reported. The overlaid structural components find application in petrochemical, nuclear, oil & gas industries etc. For such applications, the components should possess good mechanical properties and corrosion resistance.

Hence the objective of this present study is to evaluate the weldability and the essential properties of this weld metal for its suitability in overlay and joining applications related to heat exchangers. The design requirement of this weld metal is tabulated in Table 1 & 6. It is clear from the table that strict control over the weld metal composition besides established welding parameters is necessary to achieve the specified properties.


2 EXPERMENTAL DETAILS FOR INCONEL CONSUMABLE

2.1 Overlay on forge plates

20MnMoNi55 plate of dimension 700\u00d7400\u00d720 mm has been used as a base material for cladding purpose. Cladding has been done with \u2248 ENiCrFe-3 electrode of three different sizes. The typical weld assemblies made using MMAW process is shown in Figure 1. The optimized welding procedure adopted during welding of these cladding assemblies is listed in Table 2.


2.2 Chemical Composition of welding consumable


The chemical composition of the Inconel welding consumable has been optimized on the basis of experience without compromising on weldability characteristics. Following points are kept in mind while designing the product. (i) Effect of impurity elements such as P, B and S contents on solidification cracking of the weld 

(ii) Effect of alloying elements on wetting characteristics to avoid micro-cracking 

(iii) Effect of Si and Fe on formation of low melting laves phase 

(iv) Optimization of Mn and Si to counteract the detrimental effects of S and P (v) Addition of strengthening constituents such as Al and Ti contents.

Few trials have been taken after studying the core wire chemistry with different formulations.

Then it was tested after depositing on a forge plate. Specimen taken from indicated location (CH1) in Figure 1. 

The chemical composition of the weld metal is analyzed by optical emission spectroscopy at five different locations of each weld pad. The locations are 5 mm and 6 mm height from the fusion line. The required weld metal composition is identified from each set of weld pads that are prepared using different sizes of electrodes and the results of the optimized chemical compositions of the weld metal are listed in Table 3.


2.3 Non Destructive Evaluation of Weld


The surface of the prepared weld assemblies has been subjected to liquid penetrant test and ultrasonic test for surface and internal weld defect inspection. Ultrasonic examination has been performed to investigate the presence of any weld bead crack or bonding defects in the weld overlay. As per the requirement, focused 70o angle beam is used for inspection of weld coupons.


2.4 Heat Treatment of Weld Assemblies


As per the requirement, the claded plates have been subjected to a simulated heat treatment cycle, before carrying out any mechanical tests. The overview of the heat treatment procedure is mentioned in Table 1. It consists of heating the weld assemblies that is isothermally held at 300\u00b0C to 550\u00b0C at a rate of 30\u00b0C h-1. At this temperature the weld assemblies is being soaked for over 40h. This is followed by cooling the weld assemblies to 450\u00b0C at 30\u00b0C h-1. And then, the assemblies are taken to 600\u00b0C at a rate of 30\u00b0C h-1 and held at this temperature for 8h and cooled to 450\u00b0C. This particular heating, holding and cooling cycle (600\u00b0C/30\u00b0C h-1, 8h, and 450\u00b0C/ 30\u00b0C h-1) is repeated for three times before cooling to room temperature. This simulated heat treatment process is followed by mechanical evaluation of the weld metal. In general, the heat treatments that are subjected to weld assemblies are aimed at optimizing the properties as well as tempering and promoting stress relief.


2.5 Mechanical Test


2.5.1 Tensile Test


The tensile tests have been carried out at room temperature using an Amsler Universal Testing Machine having a load capacity of 20kN. The two numbers tensile specimens taken from indicated locations (T1 & T2) in Figure 1 of the prepared clad assemblies. Tensile properties of the test specimens are presented in Table 4.


2.5.2 Charpy Impact Test


Charpy impact test is carried out to evaluate the toughness of the welding joints at 20\u00b0C. Charpy tests are conducted on the machined specimens having a 2 mm notch positioned at the centre of the weld. Impact specimens are machined from indicated locations (IP 1-3) in Figure 1 of the prepared clad assemblies. Values for the test specimens are presented in Table 5. An average value of 98-110 J is obtained in this present investigation which is found to be well above the requirement specifications.


2.5.3 Bend Test (Side and Face)


Face and side bend test of the weld specimen has been carried out with Amsler Bend Tester. Side bend test specimens are machined from indicated locations (SB 1-4) in Figure 1 and face bend test specimens are machined from indicated locations (FB 1-2) in Figure 1 of the prepared clad assemblies. The thickness of the weld specimen is machined to about 1/4th of mandrel diameter. The test specimens are bent through an angle of 180\u00b0 slowly to check for it soundness and nature of the defects introduced at the bent side.


2.6 Hot Crack Test


Hot crack test specimens are machined from indicated locations (HC 1-3) in Figure 1 of the prepared clad assemblies. To check resistance to hot cracking, depositing a sequence of cross welded stringer beads on the HC 1-3, after simulated heat treatment. No preheating applied. The beads sequences are displayed in Figure 2. The welding parameters used same as used in the cladding. Liquid penetrant test conducted after grinding. After further grind in steps each of 0.5 mm such that the underlying layer is reached. Conducted liquid penetrant test for every steps and found satisfactory.


2.7 Hot Cracking Sensitivity Test (Thomas Schaeffler Test)


Four numbers of test specimens with a dimension of 45 mm \u00d7 45 mm \u00d7 25 mm have been machined from the SS 347 base material. The schematic of the hot cracking test specimen is shown in Figure 3. This figure demonstrates that how the four pieces of test specimens (A, B, C and D) are arranged for the preparation of hot cracking test. The squarely arranged test specimens having 90 mm length and breadth are welded up to a length of only 50 mm in both directions. After joining, a single V groove is made on this test specimen, whose side view is shown in Figure 3. The side view of the grooved joint has a depth of 12.5 mm and angle 60\u00b0. After making this groove, the weld metal is deposited onto the groove in a clockwise direction by a continuous single pass. The specified discontinuous deposition procedure consists of depositing the weld from a particular point (X) marked on the test specimen to a certain distance (Y) and followed by cleaning and subsequent deposition of remaining portion of the groove from Y to X. The test assemblies are prepared as per the procedure is subjected to liquid penetrant test for crack inspection. The photograph of the grooved and weld deposited test specimen are displayed in Figure 3.


2.8 Metallographic Study


The different microstructures that form during welding govern the toughness and other mechanical properties of a material under investigation. Therefore, the knowledge of compositional effects and welding parameters on micro-structural evolution is important for achieving good weld properties. In Figure 4, the optical micrographs of the weld metal and the HAZ portion of the base metal are shown. The etchants used for revealing the respective microstructure are 10% Oxalic acid for Inconel and 4% Picric acid with 1% Nitric acid for base metal respectively. The dendrite morphology of the weld is found to be composed of fine features of columnar and equiaxed grains. In general, the bright and dark dendrite regions are recognized in the solidification microstructure of the Inconel alloys is due to the segregation of low melting phases such as Nb-rich Laves phases and topologically close packed phases such as sigma, P and \u03bc phase. The investigations of the secondary phases in the present material are currently underway and hence a correct description of secondary phases is not dealt with this present paper. The HAZ regions of the base metal show finer as well as coarser features of ferrite + bainite. This may be due to the effect of maximum temperature reached and the cooling rate influenced by the HAZ region during multi pass welding. This observation suggests that the micro-structural features are not much influenced by the heat input utilized during welding. The typical optical micrograph of the base metal is also shown in Figure 4. The ferrite + bainite structure is clearly evident from this figure.


3 EXPERMENTAL DETAILS FOR LOW ALLOY STEEL CONSUMABLE


3.1 All Weld-Joint Preparation


During development, several batches of weld joint have been prepared with slightly modified electrode compositions to optimize the desired composition of the weld metal. For standardization purpose, various aspects that are taken care as follows:


(a) Reduction of impurity elements in the weld metal

(b) Reduction of hydrogen content by choosing a suitable binder

(c) Welding procedural aspects like influence of Inter Pass Temperature, heat input etc.

(d) Adjustment of chemical composition to get desired properties

The above said methods are optimized and the all weld joint assembly is prepared successfully for metallurgical and mechanical evaluation for its suitable applications. The schematic of the all weld preparation procedure is shown in Figure-5. 20MnMoNi55 forge plate of dimensions 450\u00d7125\u00d720 mm is prepared with the bevel angle of 10 degrees and a root gap distance of 16 mm supported with backing strip. This specimen is welded with our newly developed electrode by using SMAW process. The optimized welding procedure utilized during the welding process is systematically presented in Table-7. The test specimens are machined from the weld joint and are subjected to various analyses such as chemical, metallography, mechanical and radiographic examination.


3.2 Chemical Composition


The chemical composition (wt. %) of the weld metal determined using wet chemical analysis is given in Table-9. In addition to this, the resulting composition obtained from the root of the weld after the welding process is also given in Table-9. This has been performed to know the extent of dilution. In Table-8, the composition of the forge plate used as a base material is also tabulated.


3.3 Metallography Studies


The optical and hardness studies have been carried out using AXIOVERT 100A Optical microscope and Rockwell Hardness tester (0 \u2013 100 RC). Metallographic specimens have been prepared by adopting standard method of polishing procedures using various grades of emery sheets and cloth impregnated with fine alumina particles. This is followed by cleaning with distilled water and methanol. The etchant used for observing the microstructure is made of aqueous solution containing 4% Picric acid and 1% Nitric acid. The etched specimens have been used further for hardness analysis.


3.4 Tensile Studies


The tensile property of the pure weld deposit is analyzed using AMSLER Universal Tensile Testing Machine with a load capacity of 200 kN. The tensile measurements have been conducted at room temperature (RT), 200oC and at 350o C respectively. Figure-6 shows the round specimens of diameter 12.5 mm and guage length 60 mm used for tensile testing prepared as per the ASTM standard E-21. The tensile data are analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (El) and reduction in area. Results are given in Table-11.


3.5 Charpy Impact Testing


For Charpy impact testing, the specimens used are cut across the welded joints having dimensions of 10\u00d710\u00d755 mm and type V-notched, with 2 mm of depth. The Charpy transition curves are obtained from room temperature to sub-zero temperatures. The Charpy impact test is accomplished in compliance to ASTM E23 standard to determine the ductile to brittle transition temperature. Results are given in Table-12.


3.6 Bend Tester


The welded specimens have been Bend Tested using AMSLER Bend Tester for the evaluation of the ductility and soundness of the weld.


3.7 Die Penetration and Radiography


The weld deposits are analyzed with Die penetration and X-ray radiography for the evaluation of any presence of crack and inclusions.


3.8 Drop weight test results


Drop weight test of eight weld samples were tested and the results are satisfactory. Welding procedure for preparation of weld coupon is given in Table-10. From the above weld coupon final size of the test specimen prepared as per ASTM E208. The details are given in Figure-7.


3.9 Metallographic Study


The optical micrographs of the base metal away from the weld region and the as deposited last bead of the weld are shown in Figure-8. In both the cases, the microstructure consists of ferrite and bainite (Table-13). It is clear that the strength and toughness of this material emanates from the presence of bainite and ferrite fractions.


4.0 SUMMARY AND CONCLUSION


It is clear from our evaluated results of the weld metal that the SMAW electrode developed in-house has met the specified requirements. Careful optimization of the composition of the weld metal yielded good hot tensile as well as toughness properties. The liquid penetrant and radiography test have confirmed that no cracks and inclusions are present on the surface of weld metal. In addition to this, the diffusible hydrogen mercury test also shows the average level of hydrogen is about 3.9 ml in 100 gms of weld. Hence the optimization of composition of welding consumable, maintenance of weld metal quality, achievement of superior high temperature and low temperature properties makes this product applicable for steam generator applications as well as various demanding structural applications mentioned earlier in this paper.


The major conclusion drawn from this work is as follows:





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DEVELOPMENT OF CONSUMABLES FOR SUPERCRITICAL GRADE STEEL CASTINGS



1.0 Introduction

To bridge the gap between demand and supply of power, Govt. of India has taken lot of initiatives in their respective five year plans. This resulted several new power plants have started in various strategic locations. Correspondingly all the related industries including fabrication, casting & consumable industries also geared up to meet the demand. To meet power plant component requirements, such as steam turbine, headers, inner & outer casing, valve covers etc are established by Indian casting manufacturers to meet relevant

stringent laid down specifications. In view of this, new welding technologies are developed world wide to improve the manufacturing and fabrication of industrial components. Presently most of the castings are made with 1Cr-Mo-V type steels to resist temperature up to 540\u00b0C. The specification of G17CrMoV5-10 casting is shown in Table-1.

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Indian manufacturers successfully developed suitable consumables for meeting the requirements & matching the specification. Tons of consumables are being used all over India for the above casting applications. The super critical power plants of higher sizes such as 660,800 and 1000 MW units are going to be the future trend in power industry to accelerate the task of bridging the gap between the demand and supply. It seems that developing welding consumable for specific applications in supercritical and ultra supercritical power plant for welded valve casing & Power plant equipments is rather a challenging one. In the direction of satisfying the enhanced base metal properties, a high performance welding consumables have to be developed and tested for its reliability. In view of this, a low hydrogen SMAW welding consumable with improved mechanical properties have been developed in-house for fabricating super critical grade steel casting applications. For meeting super critical & ultra supercritical applications following castings are specified and for welding of these castings certain specifications are laid down by the customer for ASTM A217M-C12A & GX12CrMoWVNbN1011. The details of the same are shown in the Table-2 & 3.

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2.0 Design of product

Developmental work has been designed with performance of the weld to meet the requirement of strength and toughness together with creep. Several trials have been designed by keeping the following parameters in mind.


(i) Variation of chemistry within the specification limits.

(ii) Selection of high purity raw materials in flux formulation.

(iii) Performance characteristics of the product including slag detachability.

(iv) Control on S, P, Sn, Sb, & As to resist cracks.

(v) Selection of proper binder to control pick up of moisture & diffusible hydrogen.

(vi) Selection of suitable core wire & its purity level.


3.0 Experimental study

Several trials have been taken and established the weld metal chemistry to meet the customer requirements. Tramp elements are restricted to a very low level and further reduced the Mn & Si content of the weld metal. The weld metal composition meeting the requirement is confirmed by optical emission spectroscopy. The all weld test coupons were prepared by these electrodes. These test coupons are taken up for characterization and mechanical property evaluations. The optimized chemical composition of the weld metal is listed in Table-4, Table-5& Table-6, with reference to specification Table-1, Table-2 &Table-3.



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3.1 Preparation of Test Coupons

A small section of dimension 300\u00d7170\u00d7 20mm of IS 2062 material is used as a base material for joining purpose. A single V groove having 10 degree bevel angle is made on the base plate. The all weld joint is supported with a backing strip made of mild steel having dimension 325x40\u00d78 mm. In order to avoid dilution, buttering is also made with same type of consumable before actual test coupons are made. In this study we have followed AWS: SFA 5.5 classification guidelines for preparation of weld coupons. The welding parameters such as heat input, preheat & interpass temperatures are controlled to get good quality welds. The WPS followed are shown in the Table-7 & 8. The all weld test specimens are machined out after x-ray from the each weld joint assembly and are subjected to various tests including metallographic & mechanical testing.


3.2 Dye Penetrant and Radiography Test

The weld deposits are analyzed with Dye penetration between the passes and finally sent for X-ray for the evaluation of any presence of crack and inclusions.


3.3 Tensile Studies

The tensile property of the pure weld deposit is analyzed using Universal Tensile Testing Machine. The tensile measurements have been conducted at room temperature (RT). Figure 1 shows the round specimens of diameter 12.5 mm and gauge length 50 mm used for tensile testing prepared as per the ASTM standard E-21.



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3.4 Charpy Impact Testing

For charpy impact testing, the specimens used are cut across the welded joints having dimensions of 10\u00d710\u00d755 mm and type V-notched, with 2mm of depth. The charpy impact test is accomplished in compliance to ASTM E23 standard to determine the toughness of the material.

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3.5 Creep Test Study

For Creep test, the specimens used are cut from the welded portion. The Creep test is accomplished in compliance to ASTM 139-06 standard to determine the strain of the material list in Table-11. We have completed creep test for these material 1000 hrs.

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4.0 Discussion

In general, a high performance weld material is chosen for stringent applications in supercritical steels. For example, supercritical steels are made with grade high strength steel which requires equally capable high strength weld materials for fabrication purposes. In view of this, the results obtained in the present study with regard to mechanical properties evaluation of the high performance SMAW electrode are discussed below. Mn+Ni content also play a critical role to control the AC1 temperatute. The evaluation of the tensile results of the weld specimen tested at RT suggests that the tensile strength possessed by the weld specimen is adequate for supercritical steel applications. In the present study, only the pure weld deposit is tensile tested. It is also clear from the Table-9; the tensile strength of the weld deposit is markedly higher than the requirement. The results of the Charpy shows the toughness value is found to be greater than minimum values.


5.0 Conclusion

\u2022 Suitable SMAW process consumables were developed for welding ASTM

A217M-C12A & GX 12CrMoWVNbN 10-11grade castings.

\u2022 Welding parameters are going to influence the properties. Therefore suitable

parameters are proposed especially Preheat, IPT and PWHT temperature. SR at

760 degree centigrade gives better toughness properties.

\u2022 Consumables with less amount of P, S, As, Sn, and Sb give the good toughness & crack resistance.

\u2022 Alloyed core wire yielded better toughness properties.

\u2022 Basic coated of EXX15 is suitable for meeting toughness & hydrogen limits.


6.0 Future Study

It is known that the primary design criteria of a component under stringent conditions depend upon the strength and the stability of the weldability of the consumables. The fracture toughness is also one of the design criterions, which has to be determined for structural integrity assessments. Hence further work on charpy experiments are planned for fracture toughness investigations of our weld. Creep study at 140 Mpa at 600 0 C also to be established for at least 3000 Hrs.




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Development of SMAW Electrode for Welding SS 316LN Material for Use in Fast Reactor Components


1. Introduction


Austenitic stainless steel of 316 LN SS is the leading potential candidate structural material for most of the high temperature applications (>427\u00b0C) in Prototype Fast Breeder Reactor (PFBR) at IGCAR. Fabrication of these components requires extensive use of welding. Worldwide and IGCAR\u2019s experience on austenitic weldments is mainly concerned about the development of weld metal and heat affected zone (HAZ) against hot and liquation cracking.


It is understood that the resulting solidification transformation mode experienced during welding, segregation of the impure elements to grain boundary forming low melting eutectics play a foremost role in cracking tendency. In order to improve these aspects, the critical factors that have to be considered are:

(i) Optimization of welding consumable composition besides weldability considerations 

(ii) Controlling of impurities which causes property degradation

(iii) Knowledge about the effect of heat input

(iv) Kinetic factors associated with solidification of weld

(v) Control of delta ferrite content in weld metal 

(vi) sensitization effects.


IGCAR\u2019s extensive long-term research on austenitic welds related to solidification cracking, effect of nitrogen on fusion zone and HAZ cracking, effect of ferrite content on embrittlement of weldments due to sigma-

phase formation, improvement of creep properties by adding N content, weldability studies by controlling Ti, Ta and Nb contents provided enormous inputs to improve the properties of austenitic weldments. Based on

this, they have planned to develop special purpose welding electrodes namely, modified E316-15 SS to achieve the desired properties of the weldments.


To meet these challenging requirements of PFBR components, E316-15 (Modified) welding consumable of various diameters have been developed successfully in-house and the properties of weld metal are evaluated jointly in collaboration with IGCAR for its applications. The details of the specification [PFBR/30000/SP/1032/R-1] of weld metal required by IGCAR are presented in Table 1.


2. Developmental Work


In order to develop the electrode material E316\u2013 15 (Modified), the pre-planned activities aimed at developing austenitic stainless weld metal are given as follows;

(i) Optimization of chemical composition of the core wire as per AWS Section II, SFA5.9 to maintain the ferrite level to its 3-7 range for improving hot cracking resistance and avoiding the sigma embrittlement due to service exposures.

(ii) Formulation of ingredients present in the flux to improve the slag detachability and make the electrode user friendly. The formulation is

based on adjusting the ratio of the contents of all basic oxides to all acidic oxides known as basicity index (BI) to achieve the above mentioned properties. The adopted formula for BI in this present work is as follows,

BI = CaO+MgO+CaF2+Na2O+K2O+0.5(FeO+MaO) / SiO2 + 0.5 (TiO2+Al2O3+ZrO2) (1)The basicity index has been fixed as 1.35, which has demonstrated good slag detachability.

(iii) Effect of basicity index on bead appearance, mechanical properties, slagdetachability.

(iv) Optimization of welding parameters in accordance to electrode wire size and effect of voltage, current and speed of deposition on stability of arc, porosity, shape of bead, bead size, depth of penetration and weld distortion.


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(v) Careful optimization of each variable to achieve the specified properties such as solidification mode, tensile properties, toughness requirements on as-welded and heat treated specimens, resistance to cracking test.

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2.1.1 Chemical Analysis and All Weld-Joint Preparation

In order to develop the electrode of required composition, various weld pads have been made with different flux combinations. The electrode composition has been fixed on the basis of 

  1. control of carbon in the core wire 

  2. selecting a core wire having very minimal phosphorus and sulfur content 

  3. strict control of elemental content in both flux and core wire.


The composition test assemblies made as per AWS section II, SFA-5.4/SFA-5.4M is subjected to accurate evaluation of chemical composition by optical emission spectroscopy. The weld composition having the required/specified composition mentioned by IGCAR has been identified from the various weld pads. A careful observation of the composition reveals that the weld specimen prepared from different batches satisfied the required criteria of composition.


Although the composition of all the weld pads prepared seems to have identical values, the presence of Ti+Nb+Ta content in one batch is found to be 0.030 % and it is within the specified limit. The oxygen content determined is also found to be 525 ppm. The typical compositional (wt. %) details of the weld metal prepared with optimized electrode batch is listed in Table 2. 


In addition to this, the composition details of core wire with the IGCAR specification for weld metal composition are also listed in Table 2. Followed by this identification, the all-weld single V joint is prepared with the electrode of the interest. The weld assembly consists of a 316 L plate of dimensions 300\u00d7150\u00d716 mm with the groove angle of 45 degrees and a root gap distance of 6.5 mm. This joint is supported with a backing strip made of SS 316 having dimensions of 325\u00d732\u00d76 mm. The welding conditions used during making of the weld assemblies with the E316-15 (modified) electrode using MMAW

process are tabulated in Table 3.


The test specimens that are machined from the weld joint assembly made of optimized batch have been subjected to metallography, mechanical, corrosion and cracking test, fillet weld test, radiographic examinations. The procedure given in AWS SFA-5.4 section is followed for the analyses wherever it is necessary. In addition to this, some of properties of the weld are also tested in parallel at IGCAR and a comparison is made for its quality and reproducibility.

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2.1.2 Determination of Ferrite Content


The amount of ferrite present in the weld samples have been determined using Ferrite scope and as per AWS SFA 5.4.


2.1.3 Metallographic Studies


The optical studies have been carried out using AXIOVERT 100A Optical microscope. Metallographic specimens have been prepared by adopting standard method of polishing procedures using various grades of emery sheets and cloth impregnated with fine alumina particles. This is followed by cleaning with distilled water and methanol. The electrolytic etchant used for observing the microstructure is 10 % Oxalic acid aqueous solution.

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2.1.4 Tensile Studies


The tensile property of the weld specimen is analyzed using AMSLER Universal Tensile Testing Machine with a load capacity of 100kN. The tensile measurements have been conducted at room temperature (RT) and at

550\u00b0C respectively as per the ASTM standard A370 and E21. Fig 1 show the round specimens of diameter 4.0 mm and guage length 20.0 mm used for hot tensile testing. The tensile data are analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (et) and RA (%).


2.1.5 Charpy U-notch Impact Test


For charpy impact testing, the specimens used are cut across the welded joints having dimensions of 10\u00d710\u00d755 mm and type U- notched, with 2mm of depth. The charpy energy of the as-welded specimen as well as the thermally aged sample (750\u00b0C / 100 h) is obtained at room temperature. The charpy impact test is accomplished in compliance to ASTM E23 standard to determine the

toughness values.


2.1.6 Inter-granular Corrosion Test in As- welded state


Micro fissure and crack evaluation has been performed on the weld specimen having dimensions of 70\u00d710\u00d74 mm using Amsler bend tester. Before carrying out the bend test (90\u00b0angle around a mandrel 6 mm), the weld

specimens with copper turnings are immersed in boiling solution of 10 wt. % CuSO4.5H2O and 10 wt. % H2So4 for 24 h.


2.1.7 Cracking Susceptibility Test

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The procedure recommended by IGCAR for evaluating the non-cracking tendency of weld specimen deposited in 1G position is performed to evaluate the cracking susceptibility. The schematic of such assembly is shown in Fig 2. ASTM SA516 Grade 70 equivalent steel plate of thickness 50 mm having a V groove of 80\u00b0 bevel angle and root gap distance of 2 mm buttered with the electrode to protect from dilution is prepared. The exact procedure mentioned by IGCAR is adopted to achieve the length of bead, distance between the adjacent weld beads, better fusion by using 1/3rd end of an electrode etc. This is followed by the visual, liquid penetrant, microscopic examination of the weld assembly for crack evaluation.

 

2.1.8 Fillet Weld Test


As per the AWS: SFA-5.4 Clause-13, fillet weld test has been performed on the fillet joints prepared in vertical, horizontal and overhead positions. The visual and the convexity of the fillet weld are measured as per the AWS specifications.


2.1.9 Slag Detachability


Slag detachability test has been performed to know the ease of slag removal.


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3. Results


3.1 Metallography examination of weld


Fig 3 displays the optical micrograph of the solidification microstructure of the weld metal and the aged weld specimen (750\u00b0C for 100 h). Fig 3a reveals the dendritic solidified grains believed to be enriched with austenite stabilized elements. The presence of very minimal ferritic inter-dendritic liquids seen as

black in color may have ferrite stabilizers. Fig 3b reveals the microstructure of 7500 C /100 h Aged weld specimen.


It is established that the metastable ferrite phase forms as a result of rapid solidification during welding process along dendritic boundaries having vermicular shape (black colour). In addition to this, the chromium carbides of type M23C6 also form during ageing above 450\u00b0C at grain boundaries and grain interior. The ferrite phase and the M23C6 carbides are clearly evident in Fig 3b. The ferrite content in weld metal determined by Ferritoscope was found about 6.1 and by WRC diagram it is about 4.0 respectively.


3.1.1 Tensile Studies on All Weld Metal


In Fig 4, the extension of the weld specimen recorded at 550\u00b0C with increasing load is shown. The yield strength, ultimate tensile strength, percentage elongation and reduction in area are determined from the tensile graph. The room temperature and elevated temperature tensile properties of the all-weld are presented in Table 4. It is from the Table 4 that the values of the tensile strengths, elongation and reduction in area are well above the limits specified by IGCAR for PFBR applications.


3.1.2 Charpy \u2018U\u2019 Notch Impact Toughness


Charpy U-notch impact test has been conducted both in the as-welded and aged conditions at 7500 C / 100 h. The charpy energy found both in the as-welded and aged conditions are 8.9 daJ cm-2 and 4.5 daj cm-2 respectively.

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3.1.3 Inter-granular Corrosion Test in As-Welded state


The fissure and cracking analysis test conducted on the weld metal as per IGCAR specifications for evaluating the susceptibility against intergranular attack ensures that no crack has been developed in the 90\u00b0 bentregion of the weld.


3.1.4 Cracking Susceptibility Test


The cracking susceptibility test carried out as per IGCAR specifications on the crack test assembly made in 1G position confirmed that no crack is observed and hence it meets the IGCAR requirement.


3.1.5 Fillet Test


The result of the fillet test conducted on the fillet weld joints is listed in Table 5. It is clear from the table that the fillet weld test has been passed and it meets the requirement of IGCAR.


4. Discussion


During welding of austenitic stainless steels, one may encounter four possible solidification modes in the weld fusion zone depending on the composition. With regard to 316 LN austenitic welds, the welding metallurgy essentially concerns about how to control the solidification structure to avoid cracking in the weld metal. The knowledge on the phase diagram specifies that, if the Creq/Nieq ratio of electrode is ~1.5, then the solidification mode is situated along the boundary between the ferrite and austenite region. As a result, the solidification mode may be either austenitic or ferrite-austenite type.


Under non-equilibrium conditions, rapid cooling causes the boundary to shift which in turn affects the solidification mode. Hence the prevailing weld zone composition as well as the welding method strongly decides the solidification structure. Based on the cracking susceptibility studies, it is observed that the weld solidified primarily by austenitic mode is susceptible to cracking due to segregation of impure elements (S+P), formation of low melting eutectics by reaction of Ti and Nb with C, N and S, sensitization by chromium depletion at high temperatures.


In other sense, the weld having austenitic-ferritic structure is better than fully austenitic structure because of higher solubility of impurities in ferrite phase, protection to chromium depletion, effect on ferrite on sensitization kinetics. However long term research on austenitic weldments signifies that, this type of weld is also failed in service due to

  1. sigma phase formation during prolonged high temperature exposure, transformation of ferrite to carbides at high temperatures 

  2.  Formation of nitrides leading to pitting corrosion. It is found that the sensitization of the heat affected zone (HAZ) is not a problem, if the carbon content of the base metal is \u2264 0.03 wt.-%


 Based on the extensive long term research carried out by IGCAR on developing the performance of the austenitic weld and weldments against hot cracking and their prevention, enhancement of creep strength, fatigue and creep-fatigue life have drawn major conclusions. As a result, E316-15 (Modified) electrode for welding PFBR components has been specified by IGCAR. This includes the control over the impurity levels of the weld, control of carbon and nitrogen level in the weld for ensuring improved creep strength and freedom from sensitization.


The permissible amount of ferrite in the weld is specified between 3 to 7 FN. This is followed by specification of hot tensile, toughness properties, bend test etc. The E316-15 (Modified) electrode specified by IGCAR has been developed successfully in- house as a result of sheer hard work with proper planning and execution.

The optimization of flux combination for the welding consumables, welding parameters to achieve high quality weld and the slag detachability characteristics to make welder friendly yielded fruitful results.


The weld assembly made with the optimized electrode composition has been subjected to radiographic examination. It ensures that the weld assembly is free from crack and porosity. The metallurgical testing suggests that it is highly recommended for PFBR applications in IGCAR. The evaluation test performed by IGCAR on the determination of chemical composition of the weld, delta ferrite content in weld metal, mechanical properties, IGC test, fillet test and cracking test provides satisfactory results and as a result, our developed product has been approved by IGCAR for welding PFBR components. However, the creep rupture test of the weld specimen at 650\u00b0C has been planned in near future to evaluate the creep properties. As per IGCAR requirement, the weld pad made in 1G position has to be evaluated for creep rupture analysis under a stress of 130 MPa.


5. Conclusion


(i) Balancing of the composition of the weld to achieve ~5% of delta ferrite makes our product suitable for welding 316 LN austenitic steels.

(ii) By using this welding consumable, it is possible to obtain sound weld joints with high temperature strengths comparable to base metal.

(iii) It is essential to control the heat input to obtain a suitable austenite-ferrite balance in the weld which avoids detrimental phase formation.

(iv) From corrosion point of view, it is beneficial to do post weld cleaning of the weld mechanically.



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DEVELOPMENT OF SMAW ELECTRODE FOR WELDING OF HIGH STRENGTH DMR GRADE STEEL


Introduction


Previously, the Navy used to source special steel plates from overseas suppliers. Now Indian steel manufacturers have developed and produced low alloy steel plates having very high tensile properties with the help of DRDO for use in ship building applications. In association with the Indian Navy and DRDO, SAIL has developed DMR high strength grade steel.


The welding of this material with suitable consumable has become necessary. Therefore Indian Navy and NMRL have taken up a project to develop suitable consumables through Indian consumable manufacturers. Apart from developing consumables, understanding details pertaining to the welding procedural aspect like joint design, selection of suitable size of consumable, Inter-pass temperature, number of layers, heat input is essential as they can influence the properties of the weld metal.


This paper details the developmental work undertaken to develop electrodes for welding high strength DMR grade steel and also establish the parameters to get desired properties. The present steel is a nickel bearing micro alloyed steel characterized by higher strength and superior toughness even at sub zero temperatures as low as minus 50oC. These plates were hitherto being imported in quenched and tempered condition. This steel has got Cr, higher Ni, Cu and Mo also. Because of the presence of these elements it yields higher strength with good toughness at minus 50oC. The chemical and mechanical property of this steel is shown in Table 1. 


Basically, the alloy additions after the transformation characteristics of the steel enable to

achieving a higher strength and toughness after heat treatment. Fig.1 shows an isothermal diagram of typical Q&T steel. It can be observed from this diagram. The starting of the austenite to pearlite (or) ferrite transformation requires long durations and therefore even moderate / slow cooling produced martensite / lower bainite and avoid ferrite. Indian Navy in association with NMRL has specified the weld metal property requirements. The details of the various properties to be met by the weld metal are specified in the Table 2. 

With this understanding, the electrode development work was under taken.

Developmental Works

During development, several batches were produced, tested for all properties with boiler

quality mild steel plates before standardizing. During formulating the suitable chemistry, following aspects were also kept in mind.

(a) Lower levels of impurity elements in the weld metal.

(b) Selection of suitable binder to get extra low hydrogen in the weld metal.

(c) Effect of dilution with base material.

(d) Sufficient percentage of Mn & Ni to get desired structure.


Structural variation with thesealloy percentages are shown in Fig. 2. Thus an electrode meeting NMRL specification has been developed with the targeted chemistry as shown in Table 3.


Experimental Studies

For the purpose of this study, sufficient quantity of electrodes of size 3.15 and 4.0 mm was

produced and used for all internal laboratory tests and established the parameters with DMR plates. After qualifying the procedure at our R&D, it was sent for testing at NMRL, Ambernath. The design of groove is as shown in Fig. 3. The procedure established with the consumable is detailed in Table 4.

After satisfactory results were received from NMRL, few more batches were produced and

established the repeatability at NMRL & user end CSL, Cochin also. The results obtained at user end are shown in Table 5.

The effects of various parameters are discussed in the following paragraphs.


Results and Discussions


The results obtained through the above study were summarized as under:

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Conclusions


From the results obtained we can conclude as follows.

(a) SMAW electrode meeting NMRL specification requirements for welding this steel has been developed.

(b) Lower IPT seems to have a beneficial effect especially when low temperature toughness

properties are desired.

(c) Lower heat input produced better toughness.

(d) A judicious choice of various welding parameters produces desired results.

(e) Proper root face advised to qualify the welders.

(f) Diffusible hydrogen levels control is very important to get good ductility.




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Recent Developments in SMAW Consumables and Future Challenges


1.0 INTRODUCTION

The Manual Metal Arc Welding process continues to dominate the industry and is still being widely used by the fabricators, even though the percentage of weld metal deposited by this process is declining and is slowly being replaced by the automatic and semi automatic welding processes.

The welding electrode industry in India which produced its first electrode in the year 1943 has grown and advanced both in terms of size as well as technologically. The growth and technological achievements were too rapid, that within a short span several indigenous electrodes replaced the imported ones thus conserving a sizeable amount of foreign exchange on one hand and making it easier for the fabricators to fabricate new and sophisticated jobs on the other hand without having to depend on the imported electrodes.

It can be said with certainty that the electrode manufacturers in India have always been enthusiastic and had the spirit to face the challenge of the fabricators in developing and standardizing new consumables.

Thanks to the constant support of the fabricators, the challenges continue to flow to the consumable manufacturers and the developments of new consumables and achievements are seen frequently in this field.

The SMAW process as said earlier is widely used for the fabrication of a variety of materials right from carbon steels to Ni & Ni base alloys. The following pages detail briefly the recent developments in the SMAW consumables and the challenges of the future.

2.0 LOW HYDROGEN ELECTRODES

It is worth observing that the common mild steel low hydrogen electrodes like E-7016, E-7018 & E7018-1 were developed and introduced as early 1962 and since then these electrodes have been used by the fabricators in fabricating several critical components with extremely satisfactory results.

Therefore, recent development in these types is introduction of moisture resistant electrodes with vacuum packing. All these types are well received by the customers and understood its advantages.

Some of the typical requirements to be met in this class are shown in table-1.

2.1 CONSUMABLES FOR OFF-SHORE APPLICATIONS

Indian manufacturers responded quickly to the off-shore requirements with suitable consumables. Meeting AWS Specification alone is not sufficient, since the customers/consultants specifications vary widely and are more stringent. It has to meet stringent requirements of HIC & SSCC tests as per NACE standards.

All these consumables developments are a classic example of the close co-ordination between the user and inspection and manufacturing agencies and the higher technological standards attained by the Indian electrode manufacturers. 

Some of the latest customer requirements are shown in the Table-2.

3.0 LOW ALLOY STEEL ELECTRODES

The increasing use of low alloy steel material for high temperature, low temperature and high tensile service and the continuous development of new steel, with specifically enhanced properties have led to the development of several new types of electrodes.

A reference to the AWS SFA 5.5-1996 and 2006 will reveal that 27 new consumables were introduced and it clearly speak of the rapid pace of standardization of low alloy steel electrodes. Apart from the standard type electrodes there are now a host of low alloy steel electrodes which are tailor-made to suit the specific composition and mechanical property requirements. 

Some of the significant developments are shown in Tables-3 to 7

4.0 STAINLESS STEEL

As we know that the hydro-turbine runners are made up of SS-410 Ni Mo type martensitic stainless steel [13% Cr, 4% Ni and 0.5 % Mo] castings. Welding is used for the assembly and repair of casting defects. Suitable consumables are developed by Indian manufacturers to meet stringent requirements of hardness and toughness. These consumables are specified even for repair of worn out runners because of 

(a) The erosive action of water flow

(b) The abrasion of solid particles in the moving water with themetallic surface.

(c) The corrosive action of water.

The details of the party\u2019s requirements are shown in Table-8

4.1 DUPLEX STAINLESS STEEL

The demand of Duplex stainless steels is increasing day by day for their applications in Oil and Gas fields; Chemical and Processing; and Paper and Pulp industries because of their increased strength and improved corrosion resistance which are not readily attainable by conventional single phase ferritic or austenitic stainless steels. Indigenous products are not only standardized to meet AWS requirements but also it was fine tuned to meet specific requirements.

Some of the customer requirements are shown below:

Special Requirements apart from AWS class are shown below

          : at 90oC with 325 N/mm2 stress & 16 bars partial pressure

5.0 Ni AND Ni BASE ALLOYS

In the past because of non-availability of suitable wires Indian manufacturers used to produce Inconel types with Ni wires and it has got its own disadvantages. But all these consumables are fully tested and standardized to meet stringent requirements even at -196oC impact and lateral expansion. One of the typical customer specifications is shown in Table-9

6.0 PACKING OF ELECTRODES

Electrode packing has always been one of the popularly discussed subjects whenever an electrode manufacturer meets a fabricator. Damaged cartons, torn off polyethylene bags create problems in the use of the electrodes and their quality deteriorates, especially when they are to be stored for quite some time in that condition before they are used.

To a great extent these problems have been overcame by the leading electrode manufacturers who had to carry out a lot of modifications and experiments before arriving at a solution.

The electrodes are now supplied with LDPE, HDPE & vacuum packing to ensure that the electrodes reach the user in good condition. Suitable polyethylene covers inside and outside the carton and outside the shrink wrapping of box ensures the quality of electrodes.

i. Sophisticated vacuum pack machines are being used to meet stringent vacuum pack requirements and increase productivity.

ii. The pouches, which are used, have got three layers. (PP layers minimum 12 micron, Aluminium foil layers minimum 12 micron, polythene layer 98-110 micron.)

iii. Once the vacuum pack sealing is over, the cartons are checked for any leakage and clear for further packing.

For proper identification and traceability even they are supplied in different colours.

7.0 FUTUTRE CHALLENGES

The Indian electrode manufacturers having faced several challenges in the past with the development and marketing of several electrodes look at the future which has several challenges of a mixed nature. The higher productivity at economical cost does not permit the use of SMAW in many cases.

Facing these retarding forces and increasing stringency of quality requirements from customers, the Indian electrode manufacturers\u2019 step into future. There is a lot to be done both technologically in terms of development of new types as well as commercially with a view to reach customer at an economical cost.

7.1 Mild Steel Electrodes

There are so many manufacturing units who are producing popularly known as E-6013 types. While on one hand, the technological advancement and the increasing number of manufacturing units is a welcome sign on the other hand there are several associated problems related to quality.

Therefore is the importance of making ISI certification [BIS License]. Efforts should be made to see that all the plants including those of the small scale units are certified by ISI.

7.2 Low fume electrodes

The effect of welding fumes on the welder and environment has been discussed at length in various seminars. Efforts have been made by the electrode manufacturers in reducing the fumes to a low level and many of the common types of electrodes are now formulated in order to produce minimum fumes.

On this subject it will be relevant to highlight the importance of other facilities like good ventilation, de-fuming/de-smoking apparatus etc. for providing a healthy environment to the welder. The formation and the activities of the OHS, Trichy are a welcome one in this direction.

7.3 Nil Ductility transition temperature data

Proper data to be generated for the consumables especially low hydrogen carbon steel & low alloy types even though still CVN toughness is still continuing. This data is required by off-shore drilling platforms, some of the nuclear components etc.

7.4 Creep Data

It has become essential to develop creep rupture data for the recent developmental consumables to ascertain its suitability. Co-coordinated efforts between user and manufacturer are important to carry out these tests since it is a time consuming test.

7.5 Step Cooling Data

As we know the temper embrittlement needs to be studied for the weldment. Enough literature is available on this subject and procedures are established at both fabricators\u2019 & consumable manufacturers\u2019 end. There is a challenge for manufacturers to generate data and submit to the fabricators for consideration. 

7.6 17-Class Stainless Steel Products

17 class electrodes is a modification of 16 class covering. 17 class produces spray arc and a finer rippled weld bead surface. The finish need not demand further machining. But it needs proper baking before use at 300oC for 1 hr to avoid starting porosity. This is another challenge at the users end.

Apart from development and improvement of SMAW consumables, it is equally important to educate how it is used at user\u2019s end. It can be said with certainty that the successful development of an electrode and its acceptance can only be possible if it is used carefully at fabricators end.

7.7 Users\u2019 support

The development and standardization of new and improved consumables is possible only with constant support of the users including consultants, inspection agencies etc. Unless the electrode manufacturers are given details of the requirement of the electrodes and also the feedback on the consumables developed, it will not be possible to develop suitable types.

The interest shown by the users in developing suitable indigenous consumables for their requirement has been excellent and thanks to their efforts which are responded by the electrode manufacturers who meet the challenge of developing suitable consumables. This coordinated effort must continue and Indian manufacturers should be given an opportunity and encouragement to develop suitable equivalents.

8.0 Conclusion

The SMAW consumables have been developed with a rapid pace which is primarily motivated by the development and use of new steels, improvement in quality standards, performance characteristics. Most of the applications are converted in to automation. With the increasing use of the automation higher productivity, the use of SMAW process is on the decline. The users also have to meet challenges in SMAW consumables in terms of their proper storage and use. A coordinated effort from the manufacturer as well as the users will go a long way in improving the standard, and achieving a better result.


TABLE-1: E7018-1 H4R with special requirements.

Element 

C

Mn

Si

Ni

Mo

Cr

V

S

P

Wt. % 

0.15

Max


1.60

Max


0.75

Max


0.30

Max


0.30

Max


0.20

Max


0.08

Max


0.02

Max


0.02

Max

Mn+Cr+Ni+Mo+V: 1.75 Max

Diffusible hydrogen: 4 ml/100gms of weld metal (Max)

Moisture: 0.3 as received or Conditioned

0.4 as exposed (27oC, 80 % RH & 9 Hours)

PWHT: SR at 600-640oC for 4 hrs



Property 

UTS  (MPa)

YS (MPa)

% El

(L=4d)

CVN Impact Strength

at minus 51oC (J)

Range 

520 Min

420 Min

26.0 Min

42 Min


TABLE-2: Offshore requirements

Element 

C

Mn

Si

Ni

Cr+Mo

Ni+Cu

S

P

Wt. % 



0.15

Max


1.40

Max


0.75

Max


0.20

Max


0.50

Max


0.50

Max


0.012

Max


0.015

Max


Property 



UTS

(MPa)


YS

(MPa)



% El

(L=4d)



CVN Impact Strength at minus 51oC (J)

Range 

490 Min

400 Min

22.0

27 Min (avg)


HIC Test : CSR \u2264 0.009 & CLR \u2264 10.00

SSCC Test : At 72 % of YS

\u201cTime to failure\u2019 shall not be less than 720 hrs\u201d

TABLE-3 : Special properties demanded by the fabricator

Element 

C

Mn

Si

V

Ni

Mo

Cr

S

P

Wt. % 



0.18 Max


1.3- 2.25 


0.60Max

0.05Max


1.75-2.50


0.30-0.55


0.30-1.5


0.03 Max

0.03 Max


* Ti, Cb+Ta, Co : To report

 Fe : Balance

Ti + Cb + Ta + Cu + Co : \u2264 0.25%

Diffusible hydrogen : 4ml/100 gms of weld metal

Preheat : 150oC

IPT : 250oC

PWHT : 552 \u00b110oC / 195 minutes

H/R : 55oC/ 1hr Max

C/R : 55oC/ 1hr Max


Property 



YS (MPa)

UTS (MPa)

%El

(L=4d)


CVN Impact

-51oC 

CVN Impact

-10oC

Range   

745-830

830 Min

18

27 J

80 J


Nil Ductility Transition Temperature

Specification:                                  ASTM E208

Specimen Type:                             P1

Testing Temperature:                 -46oC to +20oC

Fracture toughness test as per:  E1820

TABLE-4: WB-36 Material Composition and Its Properties

Element 

C

Mn

Si

Cu

Ni

Mo

Cr

Wt. % 

0.17 Max


0.8-1.2


0.25-0.50


0.50-0.80

1.00-1.30


0.25-0.50

0.30Max


Nb 

Al

S

P

0.015-

0.045

0.05

Max


0.02

Max


0.025

Max


Property 

YS (MPa)

UTS (MPa)

%El (L=5D)

Range 

440 Min

610-780

19 Min.

TABLE-5: P-91 consumable specification

Element 

C

Mn

Si

P

S

Cr

Ni

Mo

Wt. % 

0.08-

0.13


1.20

Max

0.30

Max


0.01

Max


0.01

Max

8.0-

10.5


0.80

Max


0.85-

1.20


Nb

Al 

Cu

0.15-0. 30 

0.02-0.10

0.02-0.07

  0.04 Max

0.25 Max

PWHT: 760oC for 2 hours

Property 

UTS (MPa)

YS (MPa) %

El (L=4d)

Range 

620 Min

530 Min

17 Min

TABLE-6: P-92

Element 

C

Mn

Si

P

S

Cr 

Ni

Mo



0.10-

0.14


0.90-

1.20

0.20-

0.50

0.02

Max


0.01

Max


9.0-

11.0


0.4-

0.8


0.95-

1.05



Nb

N

W

Al

0.18-0.25  

0.05-0.08

0.04-0.06

0.95-1.05

0.02 Max

PWHT: 760oC for 2 hours

Property 


UTS (MPa)

YS (MPa)

% El (L=4d)

CVN Impact at RT

Range 

650-850

520 Min

15 Min

27J Min

Table-7: Special Properties Specified By One Of The Customer

Element 

C

Mn

Si

Mo

Cr

V

S

P

Wt. % 


0.10-

0.15


1.0

Max

0.5

Max

0.9-

1.3


1.0 -

1.5


0.2 -

0.3

0.02

Max


0.02

Max



Heat treatment: SR at 690oC for 3 hrs

                               Normalizing at 940oC & Tempering at 720oC


Property 



YS

(MPa)


UTS

(MPa)


%El

(L=4d)


CVN Impact

At +20oC (in J)

Value  

529 Min

618 Min

16

27 Min


TABLE-8: 410NiMO (MSS Consumable requirements)

Element 

C

Mn

Si

Ni

Mo

Cr

S

P

Wt. % 



0.05

Max

0.6 -

0.9


0.6

Max

3.8-

5.5

0.40-

0.60

11.5-

14.0

0.03

Max


0.03

Max


Heat treatment: SR at 600oC for 3 hrs

Property 


YS

(0.2% offset)


UTS

(MPa)


%El

(L=4d)


CVN Impact

+20oC

CVN Impact

0oC

Value  

590 Min

790 Min

14

50 J

45 J


TABLE-9:                           INCONEL 182

Base metal:                      SA 387 Gr 11 Cl 1 + SA 240-410S

Application:                     Weld overlay and clad restoration

Preheat:                            150oC Min

IPT:                                     315oC Max

PWHT CYCLE

Rate of heating:               75oC Max for 1 hr

Rate of cooling:                95oC Max for 1 hr

Holding temperature:     705oC Min

Holding time:                   145 minutes Max

CVN at 0oC:                       60 J Min (avg)


Min acceptable for one specimen: 50 J

Hardness:                   241 BHN Max

Chromium Content: 13.5% Min

All other properties are as per ENiCrFe-3 classification


", "status": "P", "is_featured": false, "read_time": 8, "slug": "smaw-consumables-future-challenges", "meta_title": "SMAW Consumables and Future Challenges | D&H Secheron Electrodes", "meta_tags": "SMAW welding consumables, welding rod, electrode", "meta_description": "The SMAW process is widely used for the fabrication of a variety of materials right from carbon steels to Ni & Ni base alloys. Click here for more details.", "canonical_tags": "", "schema_tags": "", "image_alt_tag": "development of smaw electrode", "seo_keywords": "SMAW welding consumables, welding rod, electrode", "tag": 12}, {"id": 35, "category": {"id": 13, "title": "Research Articles", "slug": "research-articles"}, "date": "18 December 2021", "date_created": "2021-12-18T13:48:51.108840+05:30", "title": "Duplex Stainless Steel Weld Metals", "content": "

Influence of Core Wire Composition on the\r\nMechanical and Corrosion Properties of Duplex Stainless Steel Weld Metals

\r\n\r\n

 

1.0. INTRODUCTION


Duplex stainless steels are an alloy family with approximately equal proportions of ferrite and austenite. These materials are increasingly being used for several applications like down hole and well head tabular, well heads, flow lines, pipelines, manifolds, coolers, heat exchangers, valves and water pumps. The materials possess good resistance to general corrosion, pitting in chloride media together with good strength, typical two to three times higher than austenitic stainless steels & still maintains good ductility.


Because of this the section thickness and hence over all weight of the component can be reduced considerably. Apart from this Duplex stainless steels also offer resistance to intergranular corrosion, erosion & abrasion; and its thermal expansion coefficient is intermediate between austenitic and carbon steel. The nominal compositions and mechanical properties of some duplex stainless steels are listed elsewhere. The experience of the welding duplex stainless steels shows that they have adequate weld-ability and the common welding processes being used are SMAW, GTAW & GMAW.


For welding these materials, nickel-rich matching compositions conforming to different AWS classifications are specified. But, nowhere the core wire to be used is specified. Therefore understanding its effect on weld metal is very much important. In this paper the influence of core wire variation on mechanical and corrosion properties of the weld metal are summarized.


2.0. EXPERIMENTAL DETAILS


As the purpose of the study, to investigate the effect of core wire chemistry, different batches were produced to meet E2209-16 class. Using these consumables all welds were prepared in 20 mm thick plates of IS: 2002 material with backing strip and 16 mm root gap. All these plates were buttered with three layers and beveled to 221\u20442o. The inter pass temperature was closely controlled with thermal chalks. [During welding, parameters used are shown in Table 1.] During preparation of weld coupons for both synthetic and non synthetic electrodes parameters are kept same to understand its effect on properties.


The chemical compositions of core wires used and their weld metals are shown in Table 2 and Table 3. All the weld metals were characterized for their microstructure with optical microscopy after preparing the metallographic finish using conventional mechanical polishing methods and etching in solution consists of 1 gram sodium meta-bi-sulphite, 20 ml concentrated HCL and 100 ml water. Rockwell hardness values of the weld metals were assessed with a 150Kgs load.


The ferrite content was estimated in terms of extended ferrite number using Fischer Feritscope. Impact testing was performed on 10x10x55 mm specimens with Charpy V-notch. Corrosion behavior of these weld metals was assessed by performing ASTM A262 Practice-C & ASTM G48A.


3.0 METALLOGRAPHIC STUDY


Weld metals produced with core wire A & B were studied. In weld metals with higher Creq / Nieq values, the ferrite to austenite transformation starts at lower temperatures and the equilibrium austenite content is also low, whereas in weld metals having lower Creq / Nieq values i.e., in weld metals enriched with austenite stabilizing elements such as Ni, N2, etc., transformation will start at much higher temperatures which helps the weld metal is achieving near equilibrium austenite level within the short duration for which it dwells at high temperatures. [Cr & Ni equivalents are evaluated using below formulae and tabulated in Table 6] [Also see: Fig. 1 Constant-iron (65%) section of Fe-Cr-Ni ternary diagram.] 

Creq: Cr + Mo + 0.7Nb 

Nieq: Ni + 35C + 20N +0.25Cu

PREN = %Cr + 3.3 %Mo + 16 %N 


4.0 MECHANICAL PROPERTIES


To establish repeatability of the results we have tested three coupons in each weld metal. The tensile values of the different weld metals are presented in table 4. The synthetic weld metal \u2018A\u2019 has a value 807 MPa and %El: 30. Between the weld metals non synthetic weld metal has higher 826 MPa UTS and marginally less elongation percentage reported as 28. A closed look into observed hardness values shows that 20-23 RC. This implies that a higher proportion of ferrite does not necessarily mean a higher hardness. Therefore the phase balance with nitrogen equalizing the hardness.


Since both weld metals have good impact toughness properties, it was compared at two different temperatures namely +20o C & -30oC. The impact properties of both the weld metals were estimated and presented in table 4.


It is evident from presented values that the impact behaviour of the weld metals made with non synthetic is superior when compared to the other synthetic weld metal \u2018A\u2019. The inferior impact strength observed with weld metal produced with \u2018A\u2019 core wire attributed to its relatively higher ferrite content. Ferrite suffers from loss in toughness due its ductile \u2013 brittle transition behaviour. Another reason for loss in impact toughness in synthetic weld metal is to be ascribed to their inclusion content.


5.0 CORROSION PROPERTIES


The corrosion behaviour of both the weld metals in \u2018HNO3 solution and FeCl3 solution are presented in table-5. In the case of ferric chloride test, it was tested at two temperatures namely 22oC & 50oC. In all these tests we have carried out the tests twice with two specimens and taken average of the values. From the table it is evident that non synthetic weld metal weight loss is very less.


This can be attributed to less percentage of carbon and inclusion content. ASTM 262A Practice-C results also show that the mpy (mils per year) value is less for Non synthetic electrodes as compared to Synthetic electrodes.


6.0 CONCLUSIONS:


(a) Both synthetic and non synthetic electrodes are meeting AWS: SFA 5.4: E2209 class requirements.

(b) Minor variations in the chemistry and micro-constituents in the weld metals seem to have little influence on the hardness and elongation.

(c) Strength and Sub zero Impact properties are superior in the case of non synthetic core wire weld metals.

(d) Good corrosion resistance observed with non synthetic core wire weld metals.

(e) Non synthetic core wire weld metals contain very less impurity level.


                 









", "status": "P", "is_featured": false, "read_time": 8, "slug": "duplex-stainless-steel-weld-metals", "meta_title": "Duplex Stainless Steel Weld Metals | D&H Secheron Electrodes", "meta_tags": "SS welding, stick welding, stainless steel", "meta_description": "Duplex stainless steels are an alloy family with approximately equal proportions of ferrite and austenite. Click here for more details.", "canonical_tags": "", "schema_tags": "", "image_alt_tag": "SS welding", "seo_keywords": "SS welding, stick welding, stainless steel", "tag": 12}, {"id": 23, "category": {"id": 13, "title": "Research Articles", "slug": "research-articles"}, "date": "21 December 2021", "date_created": "2021-12-21T11:56:03.705472+05:30", "title": "Development of MMAW Inconel Consumable", "content": "

Development of MMAW Inconel Consumable for Nuclear Steam Generator Applications


1.0 Introduction


As per the published literature, India aims to produce 20 000 MW nuclear power by 2020.

Several structural candidate materials are used for fabricating nuclear materials. Some of the materials which are specified are, austenitic base Inconel alloys of grade 600, 690, 625, 625, 718, 800 etc and stainless steels of 316LN, 304LN etc. Besides to this, ferritic base 9-12 wt. % Cr alloys having optimized additions of Mo, W, V, Nb, N etc have also been used. For most of the materials, welding consumables are available indigenously. 

But in case of Inconel category, we are still importing consumables from overseas to meet certain stringent requirements on specific forged materials for overlay applications. Therefore development of suitable MMAW consumables for overlay applications has become very much important for self reliance. In view of this, MMAW electrodes of \u2248 ENiCrFe-3 have been developed indigenously for weld overlaying and joining applications. In this paper, the welding of 20MnMoNi55 steel with \u2248 ENiCrFe-3 electrode by MMAW process is reported. 

The overlaid structural components find application in petrochemical, nuclear, oil & gas industries etc. For such applications, the components should possess good mechanical properties and corrosion resistance. Hence the objective of this present study is to evaluate the weldability characteristics and the essential properties of this weld metal for its suitability in overlay applications related to heat exchangers. The design requirement of this weld metal is tabulated in Table 1. It is clear from the table that strict control over the weld metal composition besides established welding parameters is necessary to achieve the specified properties.


2.0 Experimental Details


2.1 Cladding Assemblies Preparation


20MnMoNi55 plate of dimension 700\u00d7400\u00d720 mm has been used as a base material for cladding purpose. Cladding has been done with \u2248 ENiCrFe-3 electrode of three different sizes. The typical weld assemblies made using MMAW process is shown in Figure 1. The optimized welding procedure adopted during welding of these cladding assemblies is listed in Table 2.


2.2 Chemical Composition of welding consumable


The chemical composition of the Ni \u2013 based welding consumable has been optimized on the basis of experience without compromising on weldability characteristics. Following points are kept in mind while designing the product. (i) Effect of impurity elements such as P, B and S contents on solidification cracking of the weld (ii) effect of alloying elements on wetting characteristics to avoid micro-cracking (iii) effect of Si and Fe on formation of low melting laves phase (iv) optimization of Mn and Si to counteract the detrimental effects of S and P (v) addition of strengthening constituents such as Al and Ti contents.

Few trials have been taken after studying the core wire chemistry with different formulations. Then it was tested after depositing on a forge plate. Specimen taken from indicated location (CH1) in Figure 1. The chemical composition of the weld metal is analyzed by optical emission spectroscopy at five different locations of each weld pad. The locations are 5 mm and 6 mm height from the bottom of the weld pad. The required weld metal composition is identified from each set of weld pads that are prepared using different sizes of electrodes and the results of the optimized chemical compositions of the weld metal are listed in Table 3.


2.3 Non Destructive Evaluation of Weld


The surface of the prepared weld assemblies has been subjected to liquid penetrant test and ultrasonic test for surface and internal weld defect inspection. Ultrasonic examination has been performed to investigate the presence of any weld bead crack or bonding defects in the weld assemblies. As per the requirement, focused 70o angle beam is used for inspection of weld coupons.


2.4 Heat Treatment of Weld Assemblies


As per the requirement, the clad assemblies have been subjected to a simulated heat treatment cycle, before carrying out any mechanical tests. The overview of the heat treatment procedure is mentioned in Table 1. It consists of heating the weld assemblies that is isothermally held at 300\u00b0C to 550\u00b0C at a rate of 30\u00b0 C h-1. At this temperature the weld assemblies is being soaked for over 40 h. This is followed by cooling the weld assemblies to 450\u00b0C at 30\u00b0 C h-1.Then, the assemblies are taken to 600\u00b0C at a rate of 30\u00b0 C h-1 and held at this temperature for 8h and cooled to 450\u00b0C. This particular heating, holding and cooling cycle (600\u00b0C/30\u00b0C h-1, 8h, and 450\u00b0C/ 30\u00b0C h-1) is repeated for three times before cooling to room temperature. This simulated heat treatment process is followed by mechanical evaluation of the weld metal. In general, the heat treatments that are subjected to weld assemblies are aimed at optimizing the properties as well as tempering and promoting stress relief.


2.5 Mechanical Test


2.5.1 Tensile Test

The tensile tests have been carried out at room temperature using an Amsler Universal

Testing Machine having a load capacity of 20kN. The two numbers tensile specimens

taken from indicated locations (T1 & T2) in Figure 1 of the prepared clad assemblies.

Tensile properties of the test specimens are presented in Table 4.


2.5.2 Charpy Impact Test


Charpy impact test is carried out to evaluate the toughness of the welding joints at 20\u00b0C.

Charpy tests are conducted on the machined specimens having a 2 mm notch positioned

at the centre of the weld. Impact specimens are machined from indicated locations

(IP 1-3) in Figure 1 of the prepared clad assemblies. Values for the test specimens are

presented in Table 5. An average value of 98-110 J is obtained in this present

investigation which is found to be well above the requirement specifications.


2.5.3 Bend Test (Side and Face)


Face and side bend test of the weld specimen has been carried out with Amsler Bend

Tester. Side bend test specimens are machined from indicated locations (SB 1-4) in

Figure 1 and face bend test specimens are machined from indicated locations (FB 1-2) in

Figure 1 of the prepared clad assemblies. The thickness of the weld specimen is

machined to about 1/4th of mandrel diameter. The test specimens are bent through an

angle of 180\u00b0 slowly to check for it soundness and nature of the defects introduced at the

bent side.


2.6 Hot Crack Test


Hot crack test specimens are machined from indicated locations (HC 1-3) in Figure 1 of the prepared clad assemblies. To check resistance to hot cracking, depositing a sequence of cross welded stringer beads on the HC 1-3, after simulated heat treatment. No preheating applied. The beads sequences are displayed in Figure 2. The welding parameters used same as used in the cladding. Liquid penetrant test conducted after grinding. After further grind in steps each of 0.5 mm such that the underlying layer is reached. Conducted liquid penetrant test for every steps and found satisfactory.


2.7 Hot Cracking Sensitivity Test (Thomas Schaeffler Test)


Four numbers of test specimens with a dimension of 45 mm \u00d7 45 mm \u00d7 25 mm have been

machined from the SS 347 base material. The schematic of the hot cracking test specimen is shown in Figure 3. This figure demonstrates that how the four pieces of test specimens (A, B, C and D) are arranged for the preparation of hot cracking test. The squarely arranged test specimens having 90 mm length and breadth are welded up to a length of only 50 mm in both directions. After joining, a single V groove is made on this test specimen, whose side view is shown in Figure 3. The side view of the grooved joint has a depth of 12.5 mm and angle 60\u00b0. After making this groove, the weld metal is deposited onto the groove in a clockwise direction by a continuous single pass. The specified discontinuous deposition procedure consists of depositing the weld from a particular point (X) marked on the test specimen to a certain distance (Y) and followed by cleaning and subsequent deposition of remaining portion of the groove from Y to X. The test assemblies are prepared as per the procedure is subjected to liquid penetrant test for crack inspection. The photograph of the grooved and weld deposited test specimen are displayed in Figure 3.


2.8 Metallography


The different microstructures that form during welding govern the toughness and other

mechanical properties of a material under investigation. Therefore, the knowledge of

compositional effects and welding parameters on micro-structural evolution is important for achieving good weld properties. In Figure 4, the optical micrographs of the weld metal and the HAZ portion of the base metal are shown. The etchants used for revealing the respective microstructure are 10% Oxalic acid for Inconel and 4% Picric acid with 1% Nitric acid for base metal respectively. The dendrite morphology of the weld is found to be composed of fine features of columnar and equiaxed grains. In general, the bright and dark dendrite regions are recognized in the solidification microstructure of the Inconel alloys is due to the segregation of low melting phases such as Nb-rich Laves phases and topologically close packed phases such as sigma, P and \u03bc phase. The investigations of the secondary phases in the present material are currently underway and hence a correct description of secondary phases is not dealt with this present paper. The HAZ regions of the base metal show finer as well as coarser features of ferrite + bainite. This may be due to the effect of maximum temperature reached and the cooling rate influenced by the HAZ region during multi pass welding. This observation suggests that the micro-structural features are not much influenced by the heat input utilized during welding. The typical optical micrograph of the base metal is also shown in Figure 4. The ferrite + bainite structure is clearly evident from this figure.


3.0 Discussions


In order to design a Nickel based welding consumable, it is important to gather systematic

information on the metallurgical evolution of weld with respect to composition. As per the requirements, optimized welding consumable variables such as composition, flux and

electrode size have been established as a result of enormous laboratory tests. The process parameters such as voltage, current, speed are optimized to achieve good quality weld and all-weld joint. The micro-structural changes occurred across the weld assemblies as a result of welding process is not found to differ much and this subsequently improves the all-weld properties. In addition to this, the optimization of heat input is an important factor in determining the solidification microstructure of nickel base welds. To avoid coarser and columnar dendritic grains besides segregation in the solidification microstructure, the welding parameters must be carefully controlled, since the morphology of dendrites has a major influence on tensile and hardness properties. The Charpy toughness of the weld metal is found to be well above the required value and this confirms that the steel-diluted Ni-Cr-Fe weld metal is not significantly affected. The weld metal microstructure stability is also found to be good after prolonged exposure at elevated temperature and thermal cycling treatment. From the above study, it is concluded that the weld and all weld metal properties meet the requirement for heat exchangers applications.


4.0 Conclusions


Major conclusions that are drawn from the present study is as follows,

1. Indigenous development of Inconel base welding consumable is developed successfully to meet steam generator applications.

2. Optimization of composition is based on choosing core wire and flux formulations.

3. Control over heat input avoids modification of microstructure in the diluted region.

4. The stability of weld microstructure at elevated temperatures is important for achieving adequate mechanical properties.

5. The weld metal microstructure is free from hot cracking.


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", "status": "P", "is_featured": false, "read_time": 15, "slug": "development-of-mmaw-inconel-consumable", "meta_title": "Development of MMAW Inconel Consumable | D&H Secheron Electrodes", "meta_tags": "MMAW", "meta_description": "The development of suitable MMAW consumables for overlay applications has become very much important for self-reliance. Click here for more details.", "canonical_tags": "", "schema_tags": "", "image_alt_tag": "Nuclear reactor welding study", "seo_keywords": "Welding study", "tag": 12}, {"id": 19, "category": {"id": 13, "title": "Research Articles", "slug": "research-articles"}, "date": "21 December 2021", "date_created": "2021-12-21T12:00:05.051448+05:30", "title": "Temper Embrittlement Sensitivity", "content": "

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A Study On Temper Embrittlement Sensitivity\r\nIn 1.25Cr - 0.5Mo Weld Metal


1. Introduction

Cr-Mo steels are considered as a candidate structural material for critical applications in petrochemical, power plants and nuclear industries etc. Mostly the fabrication of the components such as Boilers, Heaters, Heat exchangers, Reactors, Steam generators and hydro crackers component involves welding.

Various welding consumables with specified compositions are being used for welding purposes. However, the electrode should meet the requirements when it comes to specific applications. For example, when temper embrittlement criteria have to be achieved for fabricating creep resistant steel components, the control over the (Mn + Si) and strict control on impurity elements in the weld metal is very important to avoid temper embrittlement. The established temperature ranges for performing tempering treatments of these steels are around 370 \u2013 550\u00b0C, where they are prone to temper embrittlement due to the segregation of tramp elements present in weld metal. In view of this, few batches were produced with High purity MS wires and alloyed core wires with different flux formulations having very less amount of tramp elements and taken up for temper embrittlement sensitivity studies.

These electrodes are designed in such a way to perform sufficient usability in all conventional welding positions. The weld assemblies with optimized joint design have been prepared by SMAW process and subsequently evaluated for metallurgical and mechanical properties to ascertain temper embrittlement phenomena. The details of the results and analysis of weld specimens subjected to temper embrittlement are systematically presented in this paper.

The specification of SMAW E8018-B2 class consumables required to meet the temper embrittlement criteria is listed in Table 1. This table specifies the composition of the electrode to be attained, post weld heat treatment details of the weld, tensile Strength at room temperature and at elevated temperatures, temper embrittlement screening test, acceptance criteria using charpy energy etc.


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2. Experimental Procedure

The developmental work aiming at achieving high performance weld to meet the requirement of high temperature strength and low temperature toughness after step cooling treatment is described in the following paragraphs.


2.1 Chemical Composition

Several trials have been taken and established the weld metal chemistry to improve the resistance to Temper Embrittlement phenomenon. Over and above AWS class requirements, tramp elements are restricted to a very low level and further reduce the Mn & Si content of the weld metal. The weld metal composition meeting the requirement is confirmed by optical emission spectroscopy. The all weld test coupons were prepared by these electrodes. These test coupons are taken up for characterization and mechanical propertyevaluations. The optimized chemical composition of the weld metal is listed in Table 2.  


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2.2 Preparation of Test Coupons

A small section of dimension 300\u00d7170\u00d7 20mm machined from IS 2062 material is used as a base material for joining purpose. A single V groove having 10 degree bevel angle and 16 mm root gap distance is made on the base plate. The V joint is supported with a backing strip made of mild steel having dimension 325x40\u00d78 mm. A total of 7 layers have been made with this SMAW electrode to join the base metal. In order to avoid dilution, buttering is also made before welding. Since the material property is extremely sensitive to the welding parameters such as heat input, preheat, interpass temperatures, care has been taken to get good quality weld with enhanced properties by optimizing the welding parameters.


2.3 Dye Penetrant and Radiography Test

The weld deposits are analyzed with Dye penetration and X-ray radiography for the evaluation of any presence of crack and inclusions.

2.4 Post weld heat treatment

The details of the post weld heat treatment subjected to the weld specimen are listed in Table 3.

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This heat treatment consists of heating the as-welded specimens that are equilibrated at 400\u00b0C to 690\u00b0C at a heating rate of 30\u00b0 C / min and holding at 690\u00b0C for 2.30 h and 16 hours. This is followed by cooling at 40\u00b0 C / min. The weld specimen subjected to 2.30 h holding time is termed as minimum predicted post weld heat treatment (MPHT) whereas the one held for 16 h is termed as maximum attainable post weld heat treatment (MAHT). This type of special heat treatment is normally suggested by the customers to ascertain its suitability. However if customers do not mention, it is welding consumable supplier responsibility to achieve the requirement by their technology. Since the heat treatment details are already mentioned, we have not undertaken any trial and error heat treatment procedures to fix this MPHT and MAHT schedules.


2.4.1 Step Cooling

The step cooling heat treatment is generally performed to investigate the embrittlement phenomena. The typical step cooling treatment adopted in this present study to investigate the sensitivity of weld specimen to temper embrittlement is shown in Fig. 1. The screening test has been conducted in a well calibrated high quality box furnace having PID controlled heating (1\u2013 120\u00b0h-1 ) / cooling (1 \u2013 60\u00b0 h-1) schedule and holding options. The temperature accuracy of the furnace is about \u00b1 1\u00b0C. The screening test is also frequently monitored every hour for any unavoidable errors due to power shut down etc.

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2.5 Metallography Studies

The optical studies have been carried out using AXIOVERT 100A Optical microscope. Metallographic specimens have been prepared by adopting standard method of polishing procedures using various grades of emery sheets and cloth impregnated with fine alumina particles. This is followed by cleaning with distilled water and methanol. The etchant used for observing the microstructure is made of aqueous solution containing 2% Nitric Acid (Nital).


2.6 Tensile Studies

The tensile property of the weld specimen is analyzed using AMSLER Universal Tensile Testing Machine with a load capacity of 200 kN. The tensile measurements have been conducted at room temperature (RT) and at 454 deg C respectively. For tensile testing, the round specimens of diameter 12.5 mm and guage length 50 mm is prepared as per the ASTM standard A370 and E21. The tensile data of the weld specimens are analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (et) and reduction in area.


2.7 Charpy Impact Testing

For charpy impact testing, the specimens used are cut across the welded joints having dimensions of 10\u00d710\u00d755 mm and type V- notched, with 2mm of depth. The charpy transition curves are obtained from room temperature to sub-zero temperatures. The charpy impact test is accomplished in compliance to ASTM E23 standard to determine the ductile to brittle transition temperature.

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In Fig. 2, the optical micrograph of weld Specimens taken at 500 X, that are subjected to MPHT, MAHT, MPHT+SC and MAHT+SC for evaluating the temper embrittlement phenomena are shown. The acicular ferrite structure is clearly revealed from these micrographs and its presence seems to correlate with improved toughness in the weld. In addition to acicular ferrite, random formation of pearlite is also noticed in the micrographs. A thin black film is also noticed between the interfaces of acicular ferrite, which may be due to presence of both pearlite and precipitates. With subsequent step cooling treatment the coarsening of the acicular ferrite with carbides are also evident from the figure. In general, the microstructure of the 1.25Cr-0.5Mo alloy depends on the composition and cooling rate employed. 

It may be of ferrite + pearlite type, ferrite + pearlite + bainite type or ferrite + bainite type. The effect of microstructure on the temper embrittlement phenomenon is well studied and is an essential factor in determining the temper embrittlement resistance. Under service conditions, gradual changes in the performance of these components occur due to the influence of temperature / stress which determines its life time. In other words, the attainment of the equilibrium microstructure takes place slowly by various processes such as

 (i) Decomposition of ferrite/pearlite areas or errite/bainite areas

 (ii) Chemical segregation of impurities to interfaces and grain boundaries

 (iii) Formation of various precipitates

 (iv) Changes in chemical composition.

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Hence the microstructural stability at elevated temperatures determines the component\u2019s life time; the knowledge about the role of initial microstructures on the microstructural stability is also an important factor. Among these various degradation processes, the segregation of impurities to grain boundaries which introduces temper embrittlement has been considered as a main factor responsible for the loss of toughness and premature failure of the components. The segregation of impure elements (phosphorus, antimony and tin) to grain boundaries considerably weakens the grain boundary leading to premature failure of the components. To predict the lifetime of the components of particular industrial relevance, some established simulation treatments (accelerated degradation) are subjected known as accelerated embrittlement tests. For carrying out accelerated embrittlement test, the step cooling treatment has been devised by American Petroleum Institute. This heat treatment operation is said to be equivalent to approximately 100 000 h of isothermal aging. This is explained as follows,


3.2 Accelerated Temper Embrittlement

Test: The accelerated temper embrittlement test has been conducted by subjecting the MPHT and MAHT weld specimens to step cooling treatment mentioned in section 2.4.1. After carrying out the step cooling treatment, all the heat treated weld specimens (MPHT, MAHT, MPHT+SC, MAHT+SC) have been subjected to Charpy Impact Test. The purpose of carrying out charpy testing is to establish the fracture toughness of this material after optimum heat treatment and after an embrittlement heat treatment cycle. Because this may give an idea about the material behavior before itself about the performance of the material after the long-term service.

In Fig 3, the charpy energy curve generated for the MPHT and MPHT + step cooled weld specimens with respect to various sub-zero temperatures is shown. Similarly the charpy curve generated for MAHT and MAHT + step cooled weld specimens is shown in Fig 4.


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In general, the charpy energy curve consists of three regions namely, the upper shelf region, lower shelf region and transition region. The fracture mode is ductile for upper shelf region whereas brittle for lower shelf region. The transition zone is supposed to have both ductile and brittle behavior. Considering Fig 3 and Fig 4, the MAHT + step cooled specimen energy curve is lower than the simple MAHT specimen curve. The toughness reduction is attributed due to the segregation of impurities to grain boundaries after a long term treatment. The difference or shift in the temperature between the step cooled and un-step cooled specimen at a particular energy value is used to derive the acceptance criteria for a particular application. In Table 4, the details of the toughness values of the heat treated specimens are listed.

CvTr54 + 2.5 \u0394Cv Tr54sc < 10 \u00b0C (1) Where CvTr54 is the Charpy V-notch 54 J impact energy transition temperature of completely heat treated specimen without step cooling. \u0394CvTr54sc is the shift in Charpy V-notch 54 J impact energy transition temperature of completely heat treated specimens after step cooling. Once the temperature corresponding to 54J is determined for step cooled and un- step cooled specimens, the values are substituted in expression and the resultant value is obtained. The value thus obtained should not exceed more than 10\u00b0 C. If it exceeds by 10 degree after step cooling, it is categorized as not accepted. In the present study, the value obtained after subjecting MAHT and MPHT weld specimens to step cooling treatment is found to be -30 and -15.5\u00b0C respectively. Hence it is very safe for long term applications.

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The details of the analysis results of temper embrittlement test are tabulated in Table 5. Referring to Table 1, the specification required for temper embrittlement susceptibility is fulfilled by the weld metal. Additionally, the tendency to embrittlement an also be predicted from the concentration of elements present in the material. The famous empirical expression used by Bruscato and Watanabe to predict embrittlement is given as X (ppm) = (10P+5Sb + 4Sn + As)/100  J-Factor = (Mn+Si) (P+Sn) \u00d7104  The value of the Bruscato factor X indicates the sensitivity of steel to temper embrittlement. For a particular industrial relevance, the X-factor is suggested by fabricators. In general if the value is larger, the material is prone to temper embrittlement. As per requirements, the material should have a value < 12 ppm. Application of X factor expression for the present electrode yields X-factor of about 10.89 ppm. This value is lower than the specified value and this suggests that the electrode composition is strictly controlled and meeting the laid down specification. Similarly the J-factor value, calculated is found to be 107 and it is within the established temper embrittlement range 100-400.


3.3 Tensile Studies

In Fig 5, the tensile response of the MAHT weld specimen tested at 454\u00b0C under the application of load is portrayed. The tensile parameters such as ultimate tensile strength (MPa), yield strength (MPa), percentage elongation, reduction in area have been determined from this plot. The tensile parameters are listed in Table 1. In addition to this, the room temperature tensile parameters of the MPHT and MAHT specimen are also listed.

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It is clear from Table 6 that the tensile strength of the weld and other tensile parameters are well above the specified limit of AWS: SFA 5.5 Section II, Part C and required specification. It is summarized as the E8018-B2 electrode has been developed successfully in-house for meeting temper embrittlement criteria. The optimization of the composition of welding consumable is made with appropriate core wire and flux formulations. The weld assembly made with the optimized electrode composition has been metallurgically examined. The as-welded structure is found to be of ferritic\u2013pearlitic type. The accelerated temper embrittlement test conducted on the post weld heat treated weld specimens suggests that the developed welding consumable has superior resistance against temper embrittlement. 

The tensile properties of the weld at room temperature and at 454\u00b0C are found to be well above the specified values & the hardness value (10 HRC) of MPHT specimen determined is found to be less than the specified limit, refer Table 1. The radiography and dye penetrant test ensures that the weld is free from inclusions and crack. The screening test has been repeated thrice to check the reproducibility of the developed weld against temper embrittlement and it is found that it meets the required criteria. As a result, the performance of the weld metal against temper embrittlement suggests that it is highly recommended for reactor component fabrication applications.


4. Conclusion

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