Master Alloys to Obtain Premixed Hardenable Powder Metallurgy Steels

Abstract

Systems for liquid phase alloying during sintering were investigated. The solidification range of alloys of Mn–Ni–Cr–Mo–Fe and Mn–Cu–Ni was determined. Alloys with the lowest and narrowest melting range were prepared and atomised in nitrogen. Admixtures of master alloys to water atomised, forging grade, pure iron powder were sintered at 1232°C. After hot forging, these PM steels exhibited harden abilities 75–90% of theoretical levels calculated from the factors for conventional steels. Alloying efficiency was further improved to 85–100% of theoretical hardenability when additions of approximately 2%Si and 1% rare earth mischmetal, used to enhance diffusion and sintering, were made to the master alloys. The steels obtained by premixing and sintering master alloys with pure iron powder were substantially homogeneous, had excellent microstructures, and exhibited very good tensile and impact properties. It was observed that molybdenum was the slowest diffusing element of those investigated and that manganese diffused about 3 times faster than molybdenum. However, it was also noted that the diffusion rate of manganese was slowed to the speed of molybdenum when both manganese and molybdenum were contained in the admixed alloy. It was decided to use a base iron powder prealloyed only with molybdenum to counteract the slowing of manganese diffusion during sintering. Accordingly, an Fe–0·3Mo alloy powder admixed with a manganese rich master alloy doped with silicon and rare earths produced a PM steel exhibiting alloying efficiency approaching 100%. This led to the conclusion that liquid phase alloying is the best method for producing high density, warm compacted PM steels having good hardenability. PM/0730