Induction hardening and tempering of cold work tool steels

Despite being in practical use for a long time, induction hardening of tool steels has recently gained significant interest. While the mechanisms and effects caused by short heat treatment cycles in non- and low-alloy steels for machinery parts are well investigated and understood, little is known about the behaviour of medium and high alloy steels, such as cold work tool steels. When processing these steels, austenitising temperatures times have to be carefully selected, with significantly longer durations needed to achieve uniform microstructures than those used with carbon steels. Usually, an inversion of time and temperature can be applied. However, with tempering, different tempering phenomena react differently to short heat treatment cycles. While the softening due to the transformation of tetragonal martensite into "cubic martensite" and transition carbides takes place in a very short time at slightly higher temperatures than usual, effects such as secondary hardening or the elimination of retained austenite can become difficult to achieve. These effects are more pronounced with higher concentrations of alloying elements. In addition to the explanation of these phenomena, this paper describes a newly developed method for the measurement of toughness properties of induction hardened and tempered material. As the results indicate, similar toughness values as those found after conventional treatment could be achieved for equivalent hardness values.