The microstructure changes in high-speed steels during continuous heating from the as-quenched state

Abstract

The paper presents the research of the influence of continuous heating from the as-quenched state (continuous tempering) on the microstructure of high-speed steels. Two widely known high speed steels HS18-0-1 and HS6-5-2 of a different content of W and Mo were used for the investigation. The results confirmed the following order of phase transitions during continuous tempering: ε carbide precipitation, cementite precipitation with a simultaneous transformation of a part of the retained austenite, independent nucleation of MC carbides followed by M2C carbides nucleation. The precipitated MC carbides are stable in the steel microstructure and exhibit low susceptibility to coarsen, whereas the precipitated M2C carbides transform "in situ" into M6C and M23C6 carbides. The ε carbides precipitate on dislocations within the martensite volume, while cementite initially nucleates independently at the preferential sites, i.e. on twin boundaries and between martensite plates, absorbing carbon mainly from the retained austenite, which leads to its destabilization. Such microstructure is formed within the temperature range of 250-300°C and its morphology is similar to that of upper bainite. It suggests that the precipitating cementite mainly causes temper brittleness of type I. It is additionally intensified by the transformation of a part of the retained austenite and dissolution of ε carbides.