Superhigh Strengthening and Net Shaping for Sintered Low Alloy Steels by Utilizing the Partial Constituent Gradient Microstructure in MIM Process

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Metal Injection molding (MIM) process is an advanced powder processing technique because of net shaping with shape complexity at low processing energy and 100% material utilization. This study has been performed to clarify and to optimize the relationship between the mechanical properties and the microstructures for the superhigh strengthening sintered low alloy steels (Fe-Ni system) by using MIM process. A 6%mass%Ni MIM steel showed excellent properties of 2000MPa tensile strength, 5% elongation, and 500MPa fatigue strength. The Ni rich phase consists of only the martensitic structure if the size of agglomerated Ni powders is below 18μm, whereas the retained austenite is formed in the center of Ni rich phases if the size of agglomerated Ni powders is above 18μm. If the Ni powders of approximately 20μm particle size is dispersed uniformly in the matrix, therefore, superior mechanical properties may be obtained by even the same composition of Fe-6Ni-0.5Mo-0.2Mn steels. Instead of Fe-Ni, moreover, the Fe-Mn low alloy steel was investigated to obtain high performance properties, because Ni has harmful influence upon the human body, for example, a Ni allergy.