fracture behavior of intercritically treated complex structure in medium-carbon 6Ni steel

The fracture behavior of complex structure in 6Ni-0.3C steel in which the intercritical treatment in the range of 630 °C to 670 °C from the initial microstructure of coarse-grained martensite can control the amount and distribution of fibrous martensite has been investigated. The hardness increases, but the impact toughness decreases with increasing the temperature and time of the intercritical treatment. As the amount of martensite increases and the martensite coarsens with increasing the temperature and time, the fracture along the prior austenite grain boundaries becomes predominant and the fracture mode is changed from dimple type to low energy tear type. This fracture behavior is attributed to the increase in stress concentration susceptibility at the ferrite-coarser martensite interfaces along the prior austenite grain boundaries.