The Effect of Si on the Mo-Type High Speed Tool Steel

Abstract

Rod-shaped primary carbides, M₂C, which were aligned with rolling direction are frequently observed in high-carbon, high-molybdenum high-speed tool steel. It was known that the phase disappeared by silicon addition to the steel. Therefore, the effect of silicon on the carbide transformation and the heat treatment characteristics in AISI M7 was studied under various nitrogen content.
The results obtained are as follows:
(1) The carbide precipitated at the eutectic reaction was M₂C and not M₆C, and had a plate-like morphology.
(2) Silicon in the matrix accelerated the transformation of M₂C to M₆C by entering into M₂C during heating the ingot above 640-700°C. Considerable primary carbides were retained as M₂C even after hot working, when the silicon content in the steel was little.
(3) It was considered that the retained M₂C, having the plate-like morphology, became reorientated parallel to the hot working direction, without spheroidization, and therefore was microscopically observed as rod-shaped carbide.