INFLUENCE OF ATMOSPHERE ON SINTERING OF T15 AND M2 STEEL POWDERS

The sintering behavior of T15 and M2 water-atomized high-speed steel powders under vacuum and N2-H2-CH4 atmosphere is analyzed. Sintering under the gas mixture allows high densities [99 to 100 pct total density (TD)] to be obtained at temperatures 45 to 50 K lower than for vacuum sintering for steel T15, but the high densities are reached at slightly higher temperatures (15K) than for vacuum sintering for steel M2. Chemical analysis of gas-sintered specimens has shown that a higher amount of nitrogen is absorbed during sintering of steel T15 (0.72 to 0.78 pct) than in the case of steel M2 (0.34 to 0.38 pct). 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-mu-m) and are very resistant to coarsening during oversintering. Sintering in the gas atmosphere allows important oversinterings (up to 50 K above the minimum sintering temperature for achieving a density higher than 99 pct of theoretical density) to be reached in the case of steel T15 without the formation of eutectic films. In the case of M2 steel, for gas sintering, the eutectic films appear for oversinterings smaller or equal to those for vacuum sintering.