Some Properties of High-Speed Tool Steel Powders Atomized with Water

Abstract

Martensite, residual austenite and, carbides in the powders of two kinds of high-speed tool steels, SKH 57 and SKH 9 (JIS) atomized with water were examined in as-atomized state and tempered states mainly by means of X-ray diffraction analysis. The main results are as follows:
(1) There is more residual austenite in the powders in as-atomized state than in conventionally solidified and quenched high-speed steel. The lattice parameters of both martensite and austenite in the powders are larger, and the tetragonality of martensite is smaller than that estimated from their carbon content, because of high supersaturation of alloying elements.
(2) By tempering of powders at 300°C, the lattice parameter and tetragonarity of martensite are decreased, but the residual austenite is still stable. By tempering at 550°C, alloy carbides in MC-type are precipitated and the amount of residual austenite is decreased.
(3) Internal stress measured by a line broadening in X-ray diffraction profile is not high in the as-atomized state, and increase with secondery hardening during tempering. It corresponds to the secondary hardening behaviour of the steels.
(5) Alloy carbides (MC+M₂C) are distributed in network in as-atomized powders. The network is gradually broken down by tempering at 550°C, and at 750°C the carbides are spheroidized and transformed into M₆C, while a small amount of MC is still remained.