PSI - Issue 60

Cyril Reuben Raj et al. / Procedia Structural Integrity 60 (2024) 709–722 Cyril Reuben Raj / StructuralIntegrity Procedia 00 (2024) 000 – 000

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good weldability, high impact resistance and soundness of casting, higher resistance in stress corrosion cracking and high ductility finds adaptable in nuclear reactor [Sahin and Ubeli (2008)]. When austenitic stainless steel is in wrought conditions, it remains stable at austenite phase, but it transforms to duplex phase of austenite and ferrite during welding and due to the thermal aging phenomena occurs at the as-welded austenitic stainless steel region. In such a case, Similar Metal Weld (SMW) joints were considered in main heat transport system, which is rigorously exposed to harsh environmental conditions i.e. temperature in the range 300 to 350 ℃ and pressure upto 150 bar. Hence it is necessary to investigate the material properties at the weld zone to suit its feasibility in nuclear reactors as these joints were exposed to stress corrosion cracking, aging and other fabrication issues [Baddoo (2008), Chopra and Rao (2016)].

Abbreviation SS

Stainless Steel SMW Similar Metal Weld

SMAW Shielded Metal Arc Welding GTAW Gas Tungsten Arc Welding HV Vickers Hardness HAZ Heat Affected Zone YS Yield Strength UTS Ultimate Tensile Strength

Studies on thermal aging at certain hour depict the aging embrittlement of any material at a particular or range of temperatures. At initial stages, the ferrite phase will be augmented during Gas Tungsten Arc Welding (GTAW) or Shielded Metal Arc Welding (SMAW) of similar SS-SS 304LN austenitic stainless steels. However due to thermal aging at constant hours/temperature, the material transforms to duplex phase which is as common in ordinary stainless steels by Hong et al. (2018). The ferrite phase leads to low temperature embrittlement at the weld zone of the austenitic stainless steel pipe weldments. Upon further deep examination at microscopic level, Spinodal decomposition and G-phase precipitation is the root cause for the embrittlement of pipe weldments exposed to thermal aging. Very few researchers focused on the effect of heat input during welding process causes embrittlement in the weld joints [Chandra et al. (2012)]. The existence of delta-ferrite phase in the SS weldments deteriorates at higher scale during thermal aging and accelerated thermal aging depicts the durability or working life hours of the component. At service conditions, these pipe weldments were subjected to elevated temperature (350 ℃ ) for an extended period. This prolonged exposure of austenitic stainless steel pipe weldments at operating temperatures may result in the precipitation of various phases i.e. depends on ferrite content, heat treatment, thermo-mechanical environment and duration of operating hours [Lucas et al. (2016)]. Accelerated thermal aging studies are a recent point of interest among the researchers to deduct the root cause failure in pipe weldments. Accelerated thermal aging reduces the total time frame of the thermal aging experiments. Ideally, thermal aging conditions depend upon the total application life hours of plant operation, temperature and aging hours. Recent thermal aging study were reported based on various types of weld material and joints, such as SS-SS 316 L at 350 ℃ till 15,000 hrs and further accelerated to 400 ℃ till 8,000 hrs [Kim et al. (2022)], welded blocks of ER316L were thermal aged under muffle furnace at 400 ℃ up to 10,000 hrs [Chen et al. (2020)], ES316L 16 and YS316L welded blocks which are in SMW and GTAW zone with varying ferrite content of 9% and 15% were thermal aged at 350 ℃ for 5,000 and 8,000 hrs and accelerated to 400 ℃ for the same aging hours [Miura and Arai (2017)]. From the initial after effects of thermal aging studies, the researchers accelerated the thermal aging process by increasing its temperature and duration to 400 ℃ and 20,000 hrs . This offers enhanced crack propagation in the welded pipe joint with accelerated thermal aging in the specimen. Some of the few study depicts thermal aging influence on the mechanical i.e. tensile test and microstructure evolution of the pipe weldments. Suresh et al. (2018) revealed that the thermal aging imparted a softening effect on the weld metal, which leads to an increase in ductility in the weld joint zone. In addition, the increased ductility resulted in reduced stress factor/response in the aged SS-SS-316LN as compared to the as-welded pipe weldments. Similarly, Kumar et al. (2020) studied the effect of thermal aging on tensile tests and the results shows the reduction