Simulated Stress-Induced Sensitization Study for the Heat-Affected Zone of the 304LN Stainless Steel Weld Using a Thermo
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TRODUCTION
AUSTENITIC stainless steel (ASS) has a wide range of applications in several industrial sectors, including paper and pulp, chemical, petrochemical and nuclear industries, where corrosion and safety play an important role in influencing the performance of a component. In ASS, precipitation of chromium-rich carbide and nitride takes place at grain boundaries, when they are held at the temperature range of 773 K to 1073 K (500 C to 800 C). The formation of carbide, known as the sensitization of stainless steel, results in depletion of chromium in the matrix adjacent to the grain boundary.[1] Such chromium-depleted locations possess relatively less resistance to corrosion and subsequently, become prone to intergranular corrosion (IGC) attack. Thus, the change in microstructure of the matrix that occurs with sensitization adversely affects the corrosion resistance and mechanical properties of ASS.[2] This is widely recognized as a potential cause for failure in stainless steel pipes and containers used for
NIKKI ARCHANA BARLA, PRAKRITI KUMAR GHOSH, and SOURAV DAS are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India. Contact e-mail: [email protected] VINOD KUMAR is with the R & D Centre for Iron & Steel, Steel Authority of India Ltd., Ranchi 834002, India. Manuscript submitted March 31, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
transportation and storage of various kinds of materials in chemical, petrochemical and nuclear plants.[3] It was further reported that the corrosion susceptibility of ASS at a given corrosive medium is primarily influenced by the presence of a favorable extent of (a) carbide former,[4] (b) grain size,[5] and (c) prior deformation[6–8] in the matrix. Introducing a stress prior to sensitization reduces the temperature for carbide nucleation and accelerates its precipitation at a normal sensitization temperature compared to that observed in practically stress-free ASS.[9] A prior deformation introduces dislocations adjacent to grain boundaries. The increased dislocation density near grain boundaries facilitates easier chromium carbide precipitation by enhancing the diffusivity of chromium in the matrix that may also result in IGC.[10] Carbide nucleation at the grain boundary is favored if prior deformation is low, but nucleation starts in the grain interior as well when the prior deformation is high.[11] The kinetics of sensitization in stainless steel is also accelerated by the presence of residual stresses[12–14] in the matrix. In the case of welding of 304 type ASS, chromium carbide nucleates significantly in certain locations of the weld heat-affected zone (HAZ) due to sensitization. Once chromium carbide is nucleated, it can grow even at relatively lower temperatures in the range of 573 K to 823 K (300 C to 550 C). Such sensitization of ASS can readily occur during welding and subsequently proceed at suitable service temperatures.[15] After a long service period of about 10 to 15 years at an oper
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