Microstructural Effects on J‐Integral Fracture Toughness of Welded High‐Mn Steels at 298 and 77 K

J-integral fracture toughness tests are conducted on welded high-Mn steels at room and cryogenic temperatures. The specimens are categorized into weld metal (WM) and heat-affected zone (HAZ) to consider the different microstructural effects. The microstructure of WM is composed of dendrites formed during solidification, while the microstructure of HAZ consists of coarsened grains from the welding process. Both specimens show twinning behavior after J-integral fracture toughness tests at 298 K and the superior K (JIc) fracture toughness of WM (389 MPa root m) due to the formation of plentiful twins at the crack tip. At 77 K, K (JIc) fracture toughness of HAZ (221 MPa root m) is lower than that of WM (264 MPa root m) due to different stacking fault energies (SFEs). Twinning still prevails in WM at 77 K, while epsilon-martensite is formed in HAZ by relatively low SFE. The interface between gamma-matrix and epsilon-martensite can act as a crack initiation site during the fracture toughness test, leading to the deterioration of J-integral fracture toughness in HAZ.