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Abstract
In this contribution, originally published in 2002, it is shown that the microstructure of AISI M2 high-speed steel can be modified substantially by a vacuum heat treatment combined with a deep cryogenic treatment 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 semiempirical 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. 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.