On the Isothermal Martensitic Transformation in High Speed Steel at Subzero-Temperature

Abstract

By measuring the differential dilatation at subzero-temperature between an oil-hardened high speed steel and an annealed steel specimens, it was concluded that martensite both as isothermal and athermal components is formed over a range of temperature on cooling, the existence of the former component being demonstrated by isothermal holding, and that during subsequent heating from −196° to room temperature after the cooling from the temperature of is othermal holding, an abnormal dilatation corresponding to the isothermal component takes place over the temperature range from −125° to −50°. The dilatation curves showed that the amount of transformation during isothermal holding on subzero-cooling increases with a lowering of the isothermal holding temperature “t”, passes through a maximum at a certain temperature, which is dependent upon aging time prior to subzero-cooling after oil-quenching, and then falls to nearly zero at t below −150°. On the other hand, the amount of abnormal dilatation in subsequent heating diminishes with a lowering of t of prior holding, reaches a minimum at t=−90°, independent of the aging time prior to subzero-cooling, and then increases with further lowering of t.