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-CH4 at mosphere relative to in a vacuum have been investigated. An increase i n temperature above the optimum for sintering (minimum temperature at which the theoretical density is reached) in a vacuum causes a signifi cant increase in the grain size, due to the dissolution of carbides an d the increased amount of liquid phase. In addition, this liquid phase transforms on cooling to produce undesirable microstructures containi ng continuous eutectic films. Conversely, oversintering in the N-2-H-2 -CH4 atmosphere results in only a small increase in the grain size and the absence of eutectic carbides. This different behaviour can be exp lained in terms of the formation, by reaction with the atmosphere, of very fine (< 1 mu m) vanadium carbonitrides, which are very stable and do not dissolve even at high temperatures, and also by the enlargemen t of the austenite + carbide + liquid phase region. The better mechani cal properties of high speed steels which have been oversintered in a N-2-H-2-CH4 atmosphere than those oversintered in a vacuum is due in p art to the finer grain sizes obtained, which can be explained in terms of the Zener drag exerted by the fine and stable carbonitrides, and i n part to the absence in the microstructure of eutectic carbides, due to the effect of nitrogen in enlarging the three phase region in the e quilibrium diagram, from which eutectic is formed during cooling.