We have already reported that die wall lubricated warm compaction followed by high temperature sintering enabled the high sintered density above 7.7 Mg/m
3 and the excellent tensile strength of 2200 MPa.
In order to improve the mechanical property of highly densified sintered steel, we have tried to control the microstructure by the combination of fine and coarse powders of different chemistry, and production of composite structure of ductile martensite as a matrix and hard martensite in the sinter-neck regions proved to be an effective method to increase the tensile strength.
In this paper, the effect of the microstructure on the rotating bending fatigue strength of the highly densified sintered steel was investigated. In case of bright quenching and tempering, the endurance limit of the composite structure was 580 MPa which was the same as the single-phase structure. The crack origin was the large residual pore which consisted of some adjacent residual pores. In case of carburized quenching and tempering, the endurance limit of the composite structure was 615 MPa less than the fatigue strength of wrought steel at 10
7 cycles, and the crack origin was the grain boundary near the specimen surface.
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