Compaction and vacuum sintering of Kh18N15 stainless steel powder

Abstract

1. A study was made of the main properties, cold pressing behavior, melting, and isothermal vacuum sintering of Kh18N15 steel powder prepared by the reduction of oxides with calcium hydride. 2. Compared with the fine fractions of this powder, the coarse fractions have a much greater specific surface and smaller apparent density, which is due to the complex shape and high porosity of their particles. Lubricant additions substantially reduce and equalize the apparent density of the powder. 3. The cold compressibility of the powder improves with decrease in particle size. Compacts from the −45μ fraction with lubricant (2 wt.% of paraffin wax) have a density of 85–89% after pressing under a pressure of 8–15 tons/cm2. 4. Kh18N15 steel powder melts in a vacuum in the range 1375–1400°C, and consequently it may be recommended that its vacuum sintering should be performed at 1300°C. 5. In vacuum sintering at 1300°C, final specimen densities in the range 70–97% may be attained by varying the particle size from −150+100 to −45μ, the pressure from 1.6 to 15.2 tons/cm2, and the holding time from 1 to 8 h. 6. In the isothermal sintering of compacts from Kh18N15 steel powder, the effect of pressure on the final density of the material and its grain size is greater than that of the other factors. For the recrystallization of this material, the critical nominal pressure lies between 4 and 6 tons/cm2. 7. Prolonged vacuum sintering close to the solidus point has very little effect on the composition of powder metallurgical Kh18N15 steel, the main change being a slight loss of chromium due to evaporation.