Influence of deep cryogenic treatment on microstructure, mechanical properties and dimensional changes of vacuum heat-treated high-speed steel

The microstructure of AISI M2 high-speed steel can be modified substantially by a vacuum heat treatment combined with a deep cryogenic treatment in order to optimise the ratio between hardness and fracture toughness. This ratio is affected significantly by the volume fractions of retained austenite and undissolved eutectic carbides, as well as by the mean distance between these carbides. Values calculated using a newly-developed semi-empirical equation prove that, for given vacuum heat treatment conditions, the volume fraction of retained austenite and the hardness have a significant effect on the fracture toughness K-Ic of the high-speed steel. Analysis of vacuum heat-treated modified "Navy C" ring specimens confirms that the dimensional changes can be controlled. The main factors that influence the dimensional changes are the volume fraction of retained austenite and the steel's hardness.