Increasing Sintering Gate and Avoiding Grain Growth in High Speed Steels by Sintering in Nitrogen Rich Atmospheres

The advantages of sintering high speed tool steels in a N 2 -H 2 -CH 4 atmosphere relative to in a vacuum have been investigated. An increase in temperature above the optimum for sintering (minimum temperature at which the theoretical density is reached) in a vacuum causes a significant increase in the grain size, due to the dissolution of carbides and the increased amount of liquid phase. In addition, this liquid phase transforms on cooling to produce undesirable microstructures containing continuous eutectic films. Conversely, oversintering in the N 2 -H 2 -CH 4 atmosphere results in only a small increase in the grain size and the absence of eutectic carbides