DEVELOPMENT OF CONSUMABLES FOR SUPERCRITICAL GRADE STEEL CASTINGS
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°C. The specification of G17CrMoV5-10 casting is shown in Table-1.
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.
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.
3.1 Preparation of Test Coupons
A small section of dimension 300×170× 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 325x40×8 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.
3.4 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 impact test is accomplished in compliance to ASTM E23 standard to determine the toughness of the material.
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.
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.
• Suitable SMAW process consumables were developed for welding ASTM
A217M-C12A & GX 12CrMoWVNbN 10-11grade castings.
• 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.
• Consumables with less amount of P, S, As, Sn, and Sb give the good toughness & crack resistance.
• Alloyed core wire yielded better toughness properties.
• 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.