The effect of grain size on fracture behaviour in tempered martensite embrittlement for AISI 4340 steel

Tempered martensite embrittlement (TME) in AISI 4340 steel was studied for how variations in the test temperature and grain size affect the plastic flow.

The grain size was changed by varying the austenitizing temperature in the range of 870–1200°C. For the evaluation of TME with test temperature, Charpy impact testing was performed in the range of −196-23°C.

TME occurs because of an effective activation of intergranular brittle fracture in the 300°C tempered condition where grain boundary carbides are present, the ductile-brittle transition temperature (DBTT) increases with increasing grain size and the transition to brittle fracture is attributed to the occurrence of intergranular brittle fracture. This effect of grain size on the fracture behaviour indicates that the intergranular brittle fracture is controlled by the stress concentration susceptibility, i.e. the extent of dislocation pile-up at the grain boundaries, which increases with increasing grain size.

In the 300°C tempered condition (in the presence of grain boundary carbides), the DBTT is higher by 70–150°C, compared with the 200°C tempered condition (nearly devoid of grain boundary carbides) where the transition to brittle fracture results from transgranular brittle fracture. A critical test temperature below which intergranular TME can occur is reduced with decreasing grain size.

Therefore, intergranular TME can be produced by the occurrence of intergranular brittle fracture in the presence of grain boundary carbides, which can be more effectively activated as the stress concentration susceptibility increases with increasing grain size or with decreasing test temperature.