| Secondary hardening embrittlement (SHE) in the 6W-3Ni(WN3) and 6W-6Ni(WN6) steels isothermally aged in the range, 550-650℃, was studied to analyze the effects of isothermal aging conditions, which cause variations in carbide morphology, impurity segregation, and hardening or softening of matrix, on the impact toughness and fracture behavior. In underaged conditions, the impact toughness reached the upper shelf energy (USE) levels with increasing test temperature, and mainly ductile dimples were observed on the fracture surfaces. In overaged conditions, on the other hand, the impact toughness was not recovered but remained at the low levels, 2-5 J, even at 250℃, and the fracture surfaces showed intergranular mode. In peak-aged conditions, the impact toughness reached USE levels and fracture surfaces were mostly represented by dimples, similar to those of underaged conditions. However, their USE levels were lower than those of underaged conditions by 10-15 J, or the impact toughness was not recovered and the fracture surfaces exhibited the intergranular mode, similar to those of overaged conditions. In underaged and peak-aged conditions showing the recovery of impact toughness, the relatively easy occurrence of transgranular dimples in the USE regime was caused by the coarse cementite and the intrinsic hardening with fine W_2C precipitation. In contrast, in overaged and peak-aged conditions showing no recovery of impact toughness, the easy occurrence of intergranular fracture is believed to be caused by the impurity segregation at the grain boundaries. |
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