The present study is to investigate the effect of post heat treatment on the microstructures and mechanical properties of a submerged-arc-welded 304 stainless steel. The base material consisted of austenite and long strips of delta-ferrite surrounded by Cr-carbide, and the welds consisted of delta ferrite and austenite matrix. For the heat treatment at 850 °C or lower, Cr-carbides were precipitated in the weld metal resulting in the reduction of elongation. The strength, however, was slightly reduced despite the presence of Cr-carbides and this could possibly be explained by the relaxation of internal stress and the weakening of particle hardening. In the heat treatment at 1050 °C, the dissolution of Cr-carbide and disappearance of delta ferrite resulted in the lower yield strength and higher elongation partially assisted from deformation-induced martensitic transformation. Consequently, superior property in terms of fracture toughness was achieved by the heat treatment at 1050 °C, suggesting that the mechanical properties of the as-weld metal can be enhanced by controlling the post weld heat treatment.

High-Mn austenitic steels have been recently developed for a storage or transportation application of liquefied natural gas (LNG) in cryogenic fields. Since the structural materials are subjected to extremely low temperature, it requires excellent mechanical properties such as high toughness strength. In case of high-Mn steels, twinning deformation during the cold-working process is known to increase strength yet may cause embrittlement of heavy deformed twin and anisotropic properties. In this study, a recrystallization process through appropriate annealing heat treatments after cold-working was applied to improve the impact toughness for high-Mn austenitic steels. Microstructure and mechanical properties were performed to evaluate the influence of cold-worked and annealed high-Mn austenitic steels. Mechanical properties, such as strength and impact toughness, were investigated by tensile and Charpy impact tests. The relationship between strength and impact toughness was determined by microstructure analysis such as the degree of recrystallization and grain refinement. Consequently, both elongation and toughness were significantly increased after cold-working and subsequent annealing at 1000 °C as compared to the as-received (hot-rolled) specimen. The cold-worked high-Mn steel was completely recrystallized at 1000 °C and showed a homogeneous micro-structure with high-angle grain boundaries.

This study investigated the mechanical properties of steel in flanges, with the goal of obtaining high strength and high toughness. Quenching was applied alone or in combination with tempering at one of nine combinations of three temperatures T_TEM and durations t_TEM. Cooling rates at various flange locations during quenching were first estimated using finite element method simulation, and the three locations were selected for mechanical testing in terms of cooling rate. Microstructures of specimens were observed at each condition. Tensile test and hardness test were performed at room temperature, and a Charpy impact test was performed at −46 °C. All specimens had a multiphase microstructure composed of matrix and secondary phases, which decomposed under the various tempering conditions. Decrease in cooling rate (CR) during quenching caused reduction in hardness and strength but did not affect low-temperature toughness significantly. After tempering, hardness and strength were reduced and low-temperature toughness was increased. Microstructures and mechanical properties under the various tempering conditions and CRs during quenching were discussed. This work was based on the properties directly obtained from flanges under industrial processes and is thus expected to be useful for practical applications.