Abstract The relationship between the microstructures and cryogenic mechanical properties of the dissimilar laser welded joint between ultra-low carbon 5% Ni steel (SA645) and 304L stainless steel (AISI304L) was investigated to identify the crucial microstructural characteristics determining the low-temperature toughness of the joint. The microstructural constituents of the weld metal were changed by varying the laser beam offset towards AISI304L side. The low-temperature (−120 °C) impact energy of the weld without beam offset which consisted of full lath martensite was only 10.22 J. High-density tangled dislocations in lath martensite resulted in the embrittlement of the weld metal. With increase in the offset of laser beam, the cryogenic toughness of the weld with a dual microstructure of lath martensite and retained austenite (RA) was enhanced more than seven times (72.16 J). Areas with relatively low strain/stress caused by the strain-induced martensite transformation of RA were observed near the crack propagation path, which deflected crack propagation path and increased the resistance of crack propagation. However, the weld metal with a triplex microstructure of martensite, RA and δ-ferrite exhibited a lower cryogenic impact toughness (37.15 J) despite the high volume fraction of RA when the beam offset was high. The thermal stability of RA decreased significantly owing to the weakening of restrain effect of surrounding martensite. The improvement effect of RA on cryogenic toughness was attenuated due to the decreased thermal stability of RA and the occurrence of δ-ferrite. Research results indicate that the volume fraction and stability of RA and the existence of δ-ferrite with increasing the laser beam offset were confirmed as the pivotal microstructural factors determining the cryogenic impact toughness.

中文翻译：

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确定 SA645 和 AISI304L 异种接头低温冲击韧性的关键显微组织特征采用自熔光纤激光焊接

摘要 研究了超低碳 5% Ni 钢 (SA645) 和 304L 不锈钢 (AISI304L) 异种激光焊接接头的显微组织和低温力学性能之间的关系，以确定决定低温韧性的关键显微组织特征。关节。通过改变朝向 AISI304L 侧的激光束偏移来改变焊缝金属的微观结构成分。由全板条马氏体组成的没有光束偏移的焊缝的低温（-120°C）冲击能量仅为10.22 J。板条马氏体中的高密度缠结位错导致焊缝金属脆化。随着激光束偏移量的增加，具有板条马氏体和残余奥氏体 (RA) 双重显微组织的焊缝的低温韧性提高了 7 倍以上 (72.16 J)。在裂纹扩展路径附近观察到由 RA 的应变诱发马氏体转变引起的应变/应力相对较低的区域，这使裂纹扩展路径发生偏转并增加了裂纹扩展的阻力。然而，具有马氏体、RA 和 δ-铁素体三重显微组织的焊缝金属表现出较低的低温冲击韧性 (37.15 J)，尽管当光束偏移高时 RA 的体积分数很高。由于周围马氏体的抑制作用减弱，RA的热稳定性显着下降。由于RA的热稳定性降低和δ-铁素体的出现，RA对低温韧性的改善作用减弱。研究结果表明，随着激光束偏移的增加，RA的体积分数和稳定性以及δ-铁素体的存在被确认为决定低温冲击韧性的关键微观结构因素。