SINTERING BEHAVIOR OF GRADE-M WATER ATOMIZED HIGH-SPEED STEEL POWDERS UNDER VACUUM AND NITROGEN RICH ATMOSPHERE

The sintering behaviours of grade M (Px30) (composition similar to ASP 30) water atomised high speed steel powders under vacuum and an N2-H2-CH4 atmosphere are analysed. Sintering under the ps mixture allows high densities (99-100% theoretical) to be obtained at temperatures 30-45 K lower than for vacuum sintering of the same steel. Chemical analysis of gas sintered specimens has shown that a large amount of nitrogen is absorbed during atmosphere sintering (0.45-0.75%). Although MC and M6C primary particles are observed during vacuum sintering, in gas sintering the MC carbides change to MX carbonitrides. These particles are very fine (1 mum) and very resistant to coarsening during oversintering. The presence of these carbonitrides restricts the grain growth at and above the optimum sintering temperature. Sintering in the gas atmosphere allows significant over-sintering to be reached (40 K) without the formation of eutectic films or excessive grain growth.