Influence of PWHT on Toughness of High Chromium and Nickel Containing Martensitic Stainless Steel Weld Metals
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TRODUCTION
LOW carbon martensitic stainless steels containing 0.03 wt pct carbon and 5.0 wt pct nickel are widely used for many turbine components in power plants because of their good toughness and strength.[1–5] Presently, these steels find wide applications in oil and gas industries in place of expensive duplex stainless steels due to their good corrosion resistance and weldability.[1,4,6] Although these steels are weldable, commercially available consumables for welding are not exactly matching in composition with the base metal. Hence, often austenitic stainless steel filler wires are considered for welding, especially in the case of repair welding for which it is difficult to meet the requirements of post-weld heat treatment (PWHT) recommended for welding with martensitic stainless steel consumables. Repair welding with 410 filler wire is difficult not only because of the challenge in achieving the required temperature [1008 K (735 C)] during in situ PWHT but also in obtaining desired toughness in the weld metal. In situ PWHT of repaired turbine components made from this class of steels at a high temperature is an integrity concern, as nearby components will get heated up. Further, in spite of this high temperature PWHT, it M. DIVYA and R. PANDIAN, SO/Ds, CHITTA RANJAN DAS and S. MAHADEVAN, SO/Fs, S.K. ALBERT, Head, MTD, A.K. BHADURI, AD, MDTG, and T. JAYAKUMAR, Director, MMG, are with Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India. Contact e-mail: [email protected] SUJOY KUMAR KAR, Professor, is with the Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, India. Manuscript submitted May 27, 2014. Article published online March 21, 2015 2554—VOLUME 46A, JUNE 2015
is reported that the toughness of 410 weld metal cannot be enhanced beyond 25 to 30 J.[7] AISI 414 SS, a martensitic stainless steel is a modified version of AISI 410 SS with addition of Ni and Mo to it. Microstructure of this steel in normalized and tempered (N&T) conditions consists of lath martensite and retained austenite.[1–5] Two-stage tempering is employed to achieve suitable strength, toughness, and corrosion resistance in these steels.[1,2,8] Similarly, the matching welding consumables used for welding this steel E/ER 410NiMo requires two-stage PWHT in the range of ~ 853 K to 953 K (580 C to 680 C) to restore corrosion resistance and toughness.[2,6,9,10] In the first stage, the material is heated to a temperature just above the Ac1 transformation temperature and cooled to room temperature, and in the second stage, the same material is heated to a temperature just below the Ac1 transformation temperature and cooled to room temperature.[6] Studies on weld joints of this steel showed an increase in volume fraction of retained austenite during PWHT which in turn leads to an increase in the toughness.[1,3,8] The higher volume fraction of retained austenite observed in second stage of heat treatment (PWHT at a lower temperature) was attributed to be the cause for the improvement in toughne
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