Effect of Activated Flux on the Microstructure, Mechanical Properties, and Residual Stresses of Modified 9Cr-1Mo Steel W
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ED 9Cr-1Mo steel designated as grade P91 was developed by modifying the plain 9Cr-1Mo ferritic steel (grade 91) with alloying addition of niobium, vanadium, and nitrogen. P91 steel is intended primarily for high-temperature applications including steam generators, gas turbine, chemical plants, and power plants. High thermal conductivity and low thermal expansion coefficient coupled with enhanced resistance to stresscorrosion cracking and less susceptibility to the thermal fatigue properties offer higher operating temperatures and pressures, which result in improved thermal efficiency of the power plants.[1,2]Modified 9Cr-1Mo steel in normalized and tempered condition is the material of the construction of the steam generator of 500 MWe prototype fast breeder reactors (PFBR), which is in advanced stage of construction in India.[3] During V. MADURAIMUTHU, Scientific Officer ‘‘E’’, M. VASUDEVAN, Head, AWPM and MP, A.K. BHADURI, Associate Director, and T. JAYAKUMAR, Director, are with the Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: [email protected] V. MUTHUPANDI, Professor, is with the Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirapalli 620 015, India. Manuscript submitted February 7, 2011. Article published online August 31, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
tempering heat treatment, a dispersion of Nb(C, N) and M23C6 is precipitated along the subgrain and prior austenite grain boundaries. The formation of these carbides and nitride precipitates and their stability in the ferrite matrix enhances the mechanical properties and make this material suitable for application in steam generator up to 873 K (600 C).[4] Fusion welding, such as manual metal arc welding and tungsten inert gas welding, are the principal welding processes employed for fabricating structural components made of modified 9Cr-1Mo steels in the power plant industry.[5] Hot-wire narrow-gap welding has been employed for the fabrication of the steam generator of PFBR. TIG welding is characterized by improved weld quality, generally free from defects with good surface finish and excellent strength. Tungsten inert gas (TIG) welding produces joints as good as base metal in terms of chemical composition, strength and service properties. Major difficulties arise in conventional TIG process when the thickness of the joints increases. This is because of insufficient penetrability of the arc, which necessitates either (1) application of edge preparation and use of filler wire of appropriate composition to make multipass weld for bridging the gap or (2) use of high-currentimmersed arc resulting in lowering of weld quality.[6] The depth of penetration achievable in single pass autogeneous conventional TIG welding is a major VOLUME 43B, FEBRUARY 2012—123
problem and is limited to approximately 3 mm in stainless steel and lesser for Cr-Mo steel. Poor productivity in conventional TIG welding results from a combination of low welding speed and dep
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