Effect of Ni addition upon microstructure and mechanical properties of hot isostatic pressed 30CrMnSiNi2A ultrahigh strength steel

Hot isostatic pressing is an advanced powder metallurgical technology for the production of complex and special-shaped components made of ultrahigh strength steel. However, due to the low hardenability of 30CrMnSiNi2A steel, it is difficult to achieve the expected martensitic structure in the sintering process. In this study, the hardenability of the alloy is improved by adding Ni into 30CrMnSiNi2A steel powder. The microstructure, tensile properties and fracture behavior of the sintered 30CrMnSiNi2A-xNi steels (x = 0, 1.5 wt%, and 3.0 wt%) are systematically investigated. Experimental results show that with the increase in Ni addition, the matrix structure changes from granular bainite (0 wt% Ni) to lath bainite (1.5 wt% Ni) and lath martensite (3.0 wt% Ni); and the ultimate tensile strength increases from 1312 MPa to 1466 MPa and 1747 MPa. The improvement in strength is mainly related to the solid solution strengthening of C and Ni atoms in the matrix structure. In addition, the increase in dislocation density and grain boundary density in the matrix structure also promotes the improvement in strength, according to the kernel average misorientation and grain boundary analysis of EBSD. After the addition of Ni powder, the fracture mode changes from the transgranular quasi-cleavage to the interparticle debonding. This is mainly due to the increase in the oxide density at the particle boundaries and the matrix strength.