Fracture behaviour in secondary hardening embrittlement of isothermally peak-aged 6W3Ni steel

Abstract

Secondary hardening embrittlement (SHE) in a 6W3Ni steel isothermally peak aged in the range 550–650°C was investigated by conducting impact testing in the range 20–250°C. In the 600 and 650°C peak-aged conditions the impact toughness underwent a brittle-to-ductile transition with increasing test temperature, although the ductile-brittle transition temperature in the latter condition was higher by 200°C than that in the former condition. In the 550°C peak-aged condition, however, the impact toughness was not recovered and remained at the lower shelf energy level, about 5 J, even at 250°C. The fracture mode was intergranular in the 550°C peak-aged condition, whereas it comprised mostly transgranular dimples in the 600°C peak-aged condition. In the 650°C peak-aged condition, where some intergranular area was observed, the area of transgranular cleavage changed to transgranular dimples at temperatures above 200°C. The fracture behaviour has been discussed in terms of the impurity segregation associated with W 2C precipitation, the presence of coarse cementite and the hardening behaviour. SHE is caused by: the easy occurrence of intergranular fracture due to impurity segregation in the 550°C peak-aged condition; a decrease in ductile fracture energy due to the presence of coarse cementite and intrinsic hardening in the 600°C peak-aged condition; and the combined effect of all three factors in the 650°C peak-aged condition.