Master Alloys to Obtain Premixed Hardenable Powder Metal Steels

Systems of alloys 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 atomized in nitrogen. Admixtures of master alloys to water-atomized, forging grade, pure iron powder were sintered at 1232°C (2250°F). After hot forging, these P/M steels exhibited hardenabilities which were 75%-90% of theoretical hardenability, as calculated from the factors for conventional steels. Alloying efficiency was further improved to 85%-100% of theoretical hardenability when additions of approximately 2% silicon and 1% rare earth misch-metal were made to the master alloys. The silicon and rare earth misch-metal additions were used to enhance diffusion and sintering. 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 three 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 which was prealloyed only with molybdenum to counteract the slowing down of manganese diffusion during sintering. Accordingly, an iron alloy powder containing 0.3% Mo admixed with a manganese-rich master alloy doped with silicon and rare earths produced a P/M steel which exhibited alloying efficiency approaching 100%. This led to the conclusion that liquid phase alloying is the best method for producing high density, warm-compacted P/M steels having good hardenability.