Development of robust processing routes for powder metallurgy high speed steels

This paper reviews progress made in understanding the factors which control the supersolidus liquid phase sintering of high speed steel powders to ful l density. The correlation between alloy composition and sintering behaviou r is discussed for a number of alloy systems. Realising that for complete d ensification it is necessary for sintering to take place in the liquid +gam ma+M6C+MC (or MX) phase region, two approaches have been developed to exten d this critical phase field. This enables a scientific development of alloy s that are more robust to process variations than currently sintered high s peed steels of standard (for wrought materials) compositions. The new alloy systems possess wider process or sintering windows and have lower optimum sintering temperatures. The first approach relies on computer aided alloy d esign: vacuum sintering windows extending to 30-40 K at temperatures of 117 0-1200 degrees C have been achieved for novel Fe-C-4Cr-14Mo(-8Co) systems. The second approach involves sintering vanadium enriched high speed steels (HSSs) in nitrogen rich atmospheres. Such processing promotes the formation of MX carbonitrides in place of the more massive MC carbides, The solidus is lowered and sintering windows of similar to 30 K at temperatures of 1140 -1150 degrees C have been achieved. Compared with wrought HSSs, directly si ntered materials have uniform, coarser microstructures. The low levels of r esidual porosity achieved enable attainment of metal and wood cutting prope rties comparable to those achieved with wrought and hipped HSSs of similar compositions. MST/4512.