Effect of delta ferrite on impact properties of low carbon 13Cr–4Ni martensitic stainless steel

The effect of delta ferrite on impact properties of low carbon 13Cr–4Ni martensitic stainless steel was studied by instrumented Charpy impact, SEM fractographic analysis and EDS. The experimental results indicated the presence of delta ferrite did not change the upper and lower shelf energy of the steel apparently, while lowered the impact energy remarkably in the transition temperature range and raised the ductile-to-brittle transition temperature of the material. At 0 °C, which is in the ductile-to-brittle transition temperature range, the presence of delta ferrite lowered both the crack initiation and propagation energy during the impact progress and changed the fracture mode from dimple to quasi-cleavage as the delta ferrite content increases to 7.8%. It is suggested that in the ductile-to-brittle transition temperature range the cracks are initiated in the soft delta ferrite region, which lowers the maximum load and crack initiation energy. Additionally, when the cracks initiating in delta ferrite extend to the boundaries between the delta ferrite and the matrix, they trigger brittle fracture in the tempered martensite matrix, which decreases the crack propagation energy dramatically. In the upper shelf temperature range, the delta ferrite phase has a good plastic property, which counteracts the deleterious effect caused by its lower strength during the impacting. In the lower shelf temperature range, both delta ferrite and martensite fracture in cleavage mode, thus the presence of delta ferrite cannot change the impact energy significantly.