Evaluation of Microstructure at Interfaces of Welded Joint Between Low Alloy Steel and Stainless Steel
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INTRODUCTION
WELDED components find wide application in the nuclear industry. For these assemblies, stringent safety is the prime requirement. The transition joint between stainless steel pipe and low alloy steel nozzle of the pressure vessel is one of such joints. SA 312 type 304LN stainless steel (304LN SS) has been used as a piping material owing to its excellent resistance to corrosion and embrittlement. Low alloy steel ASTM A508 Grade 3 Class 1 (LAS) has been used for the steam drum due to its adequate strength, toughness, and weldability.[1] Joints are produced with predefined parameters; however, these joints often fail during service exploitation due to degeneration/disbonding.[2] A review was carried out by Lundin[3] on the welded assembly between low alloy steel and stainless steel. Physical and microstructural heterogeneities were
K. RAVIKIRAN is with the Advanced Materials Processing Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli 620015, India. G. DAS and M. GHOSH are with the Material Engineering Division, CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. Contact e-mail: [email protected] SURANJIT KUMAR and P.K. SINGH are with the Reactor Safety Division, Bhaba Atomic Research Centre, Mumbai 400085, India. K. SIVAPRASAD is with the Department of Metallurgical and Materials Engineering, National Institute of Technology. Manuscript submitted June 16, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
reported for the joints. These occurred due to the variation in carbon concentration, differences in the coefficient of thermal expansion between virgin alloys, undesirable thermal stress, and phase transformation near the interface. Moreover, a composition variation across the interface was created.[4] These drawbacks were mitigated to some extent with the use of austenitic alloys as buttering material and weld metal. The feasibility of using Inconel and stainless steel as a filler alloy for joining LAS and stainless steel was studied by Sireesha et al.[5] They reported that Inconel became more effective in reducing thermal stresses and microstructural heterogeneity with respect to stainless steel for welded joints. Inconel alloys can also restrict carbon diffusion to a greater extent with respect to austenitic stainless steel.[6] The most commonly used nickel-base alloys are Inconel 82 and 182. However, joints fabricated with Inconel 82/182 exhibited susceptibility to stress corrosion cracking due to the presence of chromium.[7] On the other hand, it has been summarized by another group of researchers that welded joints fabricated with austenitic stainless steel exhibited superior mechanical properties at room temperature compared to joints produced with Inconel 182.[8] Joining of austenitic stainless steel to low alloy steel using IN82/182 developed dendritic structure and segregation of secondary phases within dendritic arms. Signature of partial recrystallization and extensive grain boundary migration within weld metal were
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