Effect of Boron Addition and Sintering Atmosphere on Precipitation in Sintered Fe-Mo-C Steels

Abstract

Sintered steels, with and without boron addition, were prepared from powder compacts of pre-alloyed Fe-1.5Mo powder mixed with varied amounts of graphite (0, 0.1, 0.2, 0.3, and 0.4 wt.%) and hexagonal boron nitride (0 and 0.5 wt.%). Sintering was performed either in hydrogen or in vacuum atmosphere at 1280 °C for 45 minutes. The post-sintering cooling was performed in a furnace that was equivalent to 0.1 °C/s. The sintered boron-free steels showed dual-phase microstructure consisting polygonal ferrite and precipitate-containing grains. Each precipitate-containing grain contained packets, each of which was characterized by lamellar structure with alternating fibrous particles and ferritic laths, when carbon contents were in the range 0.1-0.3 wt.%. All the grains containing fine needle particles decorating ferritic lath boundaries were observed in the sintered Fe-Mo-0.4C steels. Boron addition caused some effects on sintered steels. The action of boron was the formation of grain boundary boride in the vacuum-sintered steels although it was hardly observed in case of hydrogen-sintered steels. Boron also promoted precipitation inside polygonal ferrite grains and along ferritic lath boundaries. The precipitate particle shape was fine needle-like in the sintered boron-containing steels. Due to liquid phase sintering, as a result of eutectic melting, the associated grain growth was observed. Disappearance of grain boundary boride was evidenced in the hydrogen-sintered steels. Without boron addition, hardening of the sintered steels strongly depended on carbon content. With boron addition, all the sintered steels showed high tensile strength and hardness even in the case of no graphite addition. Hardening action by carbon in the sintered boron-containing steels was weaker than that in the sintered boron-free steels.