SECONDARY HARDENING EMBRITTLEMENT IN W-NI STEELS

Secondary hardening embrittlement (SHE) in 6W3Ni (WN3) and 6W6Ni (WN6) steels was studied to determine how aging conditions which cause variations in impurity segregation, carbide morphology, and hardening or softening of the matrix affect the fracture behaviour. Tempering was conducted in the secondary hardening range 500-650°C. To evaluate SHE with test temperature, Charpy impact testing was performed in the range from -30 to 250 °C. SHE was observed in the averaged condition in spite of a decrease in hardness as well as in the underaged condition with an increase in hardness. The fracture surfaces contained mostly transgranular dimples in the underaged condition, but intergranular fracture in the overaged condition. While SHE is significantly affected by a reduction in ductile fracture energy caused by coarse cementite and/or by intrinsic hardening in the underaged condition, it is primarily associated with the easy occurrence of intergranular fracture caused by the impurity segregation in the overaged condition. © 1990.