FORMATION AND OPTIMIZATION OF LOWER BAINITE IN POWDER SINTERED STEEL

This paper studies the formation and morphology of bainite structure of Fe-Mo-B-Cu-C and Fe-Mo-Ni-Cu-C sintered steel. The effect factors in forming bainite have been investigated, including alloy elements content, the adding ways of carbon and the heat treatment temperature. For Fe-Mo-B-Cu-C system,lower bainite can be obtained by heat treatment at temperature of 350℃ with 2% B, while for Fe-Mo-Ni-Cu-C system, the two ways of adding carbon (graphite powder and nickel-coat-graphite) can form acicular lower bainite with 4% Ni, heated treatment at temperature of 200℃ and 280℃ for adding graphite powder system, or heated treated at temperature of 280℃ for adding nickel-coat-graphite system. It is found that a large amount of bainite exist in the vicinity of solid-gas interfaces (such as pores surface), resulting from the inhomogeneity of carbon by losing carbon out off the interfaces. It is also observed that when carhon is added in form of nickel-coat-graphite, the acicular lower bainite tends to be more in amount, more homogeneous in distribution than those of graphite powder additions.