Direct fabrication of high-performance high speed steel products enhanced by LaB6

Highlights: • A convenient method for manufacturing near-net-shaped PM HSS using mixed powders and vacuum sintering was proposed. • Near full densification (> 99.4%) was obtained via activated and reactive sintering. • The strength, toughness, and hot hardness of Ref. steels were obviously modified by LaB₆. • The purifying effect of LaB₆ was proved by a core-shell structure with LaB₆ as core and impurities as shell. A direct fabrication technology (DFT) without smelting has been developed for fabricating sophisticated high speed steel products with low pollution, near-net shaping and short process. The steel consisting of (wt.%): 6.4W, 5.0Mo, 4.2Cr, 3.1V, 8.5Co and 1.28C, was fabricated as exemplary material. The activated and reactive sintering of green compacts under vacuum with low activation energy, redox reaction enhanced diffusion and the construction of concentration gradient of alloying elements around pores, promotes the nearly full densification (> 99.40%). Also, the DFT steels show high purity and superior mechanical properties. Minor strengthening agent LaB₆ (0.1 wt.%), which is easily to be accurately introduced in DFT, obviously increases the hot hardness, temper resistance, bend strength and toughness of DFT M3:2. The strengthening effect of boron atoms and La-rich complexes are proposed to directly result in the high hot hardness and temper resistance of LaB₆ containing steel.