Abstract
In the previous report on the sintered low alloy steels (4600grade, 0.4mass%C) produced by MIM process varying Ni content from 0 to 8 mass%, the mechanical properties were improved significantly with increasing Ni content to 6mass%. This was attributed to the solution strengthening and the mezzo-heterogeneous microstructure which were consisted of Ni rich martensite surrounded by a network of tempered martensite. However, the tensile and fatigue strength of the steel added 8 mass% Ni were dropped by the increase of retained austenite. In this study, the effect of the different type of Ni powders on the mechanical properties of Fe-6Ni-0.5Mo-0.2Mn (mass%) was investigated to clarify and optimize the mezzo-heterogeneous microstructure.
The tensile properties of the heat treated steels using the spherical Ni powder were lower than those using the spiky Ni powder. This seems to be due to the segregated distribution considerably of Ni rich phase and the decrease of Ni diffusion rate.
