A Study On Temper Embrittlement Sensitivity In 1.25Cr - 0.5Mo Weld Metal
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 – 550°C, 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.
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.
2.2 Preparation of Test Coupons
A small section of dimension 300×170× 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 325x40×8 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.
This heat treatment consists of heating the as-welded specimens that are equilibrated at 400°C to 690°C at a heating rate of 30° C / min and holding at 690°C for 2.30 h and 16 hours. This is followed by cooling at 40° 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– 120°h-1 ) / cooling (1 – 60° h-1) schedule and holding options. The temperature accuracy of the furnace is about ± 1°C. The screening test is also frequently monitored every hour for any unavoidable errors due to power shut down etc.
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 10×10×55 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.
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.
Hence the microstructural stability at elevated temperatures determines the component’s 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.
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 ΔCv Tr54sc < 10 °C (1) Where CvTr54 is the Charpy V-notch 54 J impact energy transition temperature of completely heat treated specimen without step cooling. ΔCvTr54sc 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° 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°C respectively. Hence it is very safe for long term applications.
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) ×104 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°C 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.
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–pearlitic 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°C 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.