Design of cost-effective powder consolidation towards highly alloyed tool steels with enhanced mechanical properties

In industrial production, highly alloyed tool steels (HATSs) are usually fabricated by powder consolidation using hot isostatic pressing (HIP) technology; HIP's primary disadvantage is very high production cost. However, a well-developed cost-effective powder consolidation technology, termed as vacuum hot-pressing (VHP) that is also able to produce HATSs in an industrial scale, has heretofore attracted few attentions, primarily attributable to nonuniform density distribution in VHP consolidated HATSs. In this paper, an innovative VHP processing approach was designed as follows, aiming not only to overcome the aforementioned VHP's disadvantage but also to enhance mechanical properties relative to those of its counterpart produced by HIP approach: first, inert gas-atomized prealloyed powders are consolidated via VHP purposely at a temperature lower than those used during HIP, in order to retain very fine carbides, but which likely yields partial densification with non-uniform density distribution and incomplete metallurgical bond between powders; then, identical full densification and complete metallurgical bond are achieved throughout the consolidated HATS by using hot-working at a temperature no higher than the VHP temperature, minimizing carbide growth. A HATS, T15 steel with nominal composition of Fe-1.5C–12W–4Cr–5V–5Co (wt%) as model material, was produced to assess and validate the designed VIP processing approach. The experimental results reveal submicrometric average carbide size in the as-VHP T15 steel smaller than that in its as-HIP counterpart, together with essentially unchanged carbides relative to those in the as-VHP T15 steel, identical full densification and complete metallurgical bond in the entire VHP plus hot-rolled (HR) T15 steel. After quenching and tempering (QT), the VHP + HR + QT T15 steel still retains submicrometric average remaining carbide size and displays refined average prior austenite grain size, with both smaller than, and exhibits bend fracture strength higher than, accompanied by hardness similar to, those in its counterpart produced by HIP approach. The mechanisms underlying microstructural evolutions associated with carbides, densification and metallurgical bond in the designed VHP processing approach were discussed. The aforementioned mechanical property characteristics were rationalized. VHP plus subsequent hot-working at lower temperatures provides a cost-effective commercialized avenue to manufacture HATSs with enhanced mechanical properties.