The behavior of densification, austenite grain growth and carbide precipitation during sintering of high speed steel powder compacts were investigated. A water-atomized 6W-5Mo-4Cr-2V steel powder was annealed, compacted, sintered, quenched and then submitted to the observation and quantitative measurements of the microstructures.
After the formation of liquid phase along grain boundaries during sintering, the liquid penetrates into solid grain boundaries and disintegrates the solid skeleton into individual crystal grains, causing rapid densification through the rearrangement of the grains. The microstructure during sintering consists mainly of austenite solid grains and a high carbon liquid phase and resembles that of liquid phase sintered heavy alloys. The solid grains grow presumably by a solution-reprecipitation mechanism and the size of the solid grains at the end of the liquid phase sintering corresponds to the final austenite grain size after sintering.
Such microstructures which maintain the configurations during liquid phase sintering can only be observed when the specimens are cooled extremely rapidly after sintering. If the cooling rate is moderate, the liquid phase decomposes into carbide and austenite phases during cooling and the typical microstructure with dispersed carbides is obtained.