SINTERING BEHAVIOUR, MICROSTRUCTURE AND MECHANICAL PROPERTIES OF VANADIUM ENRICHED HIGH SPEED STEELS PROCESSED IN VACUUM AND NITROGEN RICH ATMOSPHERES
Creators: name: Mascarenhas, João
Publisher: name: Zenodo
Year: 2023
Subjects: High Speed Steels, Vanadium Enriched, Powder Metallurgy, Sintering Mechanisms, Sintering Atmospheres, Microstructure, Mechanical Properties

description: 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 N<sub>2</sub>+7%H<sub>2</sub> 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 N<sub>2</sub>+7%H<sub>2</sub> 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 N<sub>2</sub>+7%H<sub>2</sub> atmosphere the respective sintering temperatures were 1200º, 1150º and 1150ºC. Sintering windows increased from about 10ºC in vacuum to about 30ºC in N<sub>2</sub>+7%H<sub>2</sub> 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 M<sub>2</sub>C 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. N<sub>2</sub>+7%H<sub>2</sub> 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 N<sub>2</sub>+7%H<sub>2</sub> atmosphere sintered specimens comprised grain boundary MX carbonitrides along with grain boundary M<sub>6</sub>C and M<sub>3</sub>C 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 M<sub>3</sub>C carbide. This was due to sintering involving heating and cooling through an invariant g+L+M<sub>6</sub>C+MX+M<sub>3</sub>C 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 N<sub>2</sub>+7%H<sub>2</sub> atmosphere sintering of the vanadium containing HSS: below solidus, nitrogen replaces carbon in the MC carbides which become <em>in situ</em> formed MX carbonitrides, the released carbon forming M<sub>3</sub>C carbides and decreasing solidus, then the sintering temperature; with increasing temperature, but still below solidus, <em>in situ</em> MX dissolve and grain boundary MX are formed, these later ones having higher vanadium content; on crossing the solidus M<sub>3</sub>C carbides liquate forming a Cr-rich liquid which is responsible for densification. Hardness of heat treated specimens sintered in N<sub>2</sub>+7%H<sub>2</sub> 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.