SINTERING BEHAVIOUR, MICROSTRUCTURE AND MECHANICAL PROPERTIES OF VANADIUM ENRICHED HIGH SPEED STEELS PROCESSED IN VACUUM AND NITROGEN RICH ATMOSPHERES

Water atomised high vanadium containing T42-type HSS powders, compositions containing 0.5-2.7 wt% C and 6-12 wt% V, and whose mean particle sizes ranged 58-140 mm, were sintered in vacuum and N2+7%H2 atmosphere by supersolidus liquid phase sintering (SLPS) mechanisms. SLPS densification was carried out within the g+ L+ carbides phase region. Sintering temperatures in vacuum were higher than in N2+7%H2 atmosphere. In fact, P68 (8V-2.4C), P77 (7V-2.7C) and P81 (6V-2.6C) vacuum sintering temperatures were 1260º, 1200º and 1200ºC, respectively, and when these compositions were sintered in N2+7%H2 atmosphere the respective sintering temperatures were 1200º, 1150º and 1150ºC. Sintering windows increased from about 10ºC in vacuum to about 30ºC in N2+7%H2 atmosphere. Vacuum sintered microstructures at optimum sintering temperature, in the case of P68 alloy were comprised of vanadium rich MC carbides embedded in a bainitic/martensitic matrix. P77 and P81 were found to be unsinterable in vacuum as microstructures though having MC carbides also showed deleterious grain boundary M2C eutectic carbides even in undersintered specimens. Bend strength of vacuum sintered P68 alloy was in the 2-2.4 GPa over a hardness range of 855-900 HV. N2+7%H2 atmosphere sintered specimens showed decreasing sinterability with decreasing carbon and increasing vanadium contents, due to the constriction of the g+L+carbide/carbonitride phase region. Microstructures of N2+7%H2 atmosphere sintered specimens comprised grain boundary MX carbonitrides along with grain boundary M6C and M3C carbides, and a pearlitic matrix. The MX carbonitride phase was the only one, which contained nitrogen. Compositions with carbon contents in excess of 3 wt% were found to be unsinterable, they distorted before attaining full density when sintered at 1110ºC, due to the formation of extensive amounts of M3C carbide. This was due to sintering involving heating and cooling through an invariant g+L+M6C+MX+M3C phase region. The presence of this region represented a compositional barrier limiting the range of alloys which could be sintered satisfactorily to full density. A comprehensive sintering mechanism was developed for the N2+7%H2 atmosphere sintering of the vanadium containing HSS: below solidus, nitrogen replaces carbon in the MC carbides which become in situ formed MX carbonitrides, the released carbon forming M3C carbides and decreasing solidus, then the sintering temperature; with increasing temperature, but still below solidus, in situ MX dissolve and grain boundary MX are formed, these later ones having higher vanadium content; on crossing the solidus M3C carbides liquate forming a Cr-rich liquid which is responsible for densification. Hardness of heat treated specimens sintered in N2+7%H2 atmosphere were higher, 885-997 HV, when compared to vacuum sintered material, 855-900 HV. Bend strength values obtained in the present research were in the range 2-2.4 GPa for P68 vacuum sintered composition. Nitrogen sintered P68, P77 and P81 compositions showed bend strength in the range 1.8-2.1 GPa