Influence of Heat Treatment on the Microstructure and Hardness of 19% High-chromium Cast Irons

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Abstract
Although abrasion-resistant high-chromium cast irons are difficult to machine due to their high hardness and eutectic carbide content, this problem can be reduced by the use of annealing treatments to soften the matrix structures. After machining, the irons can then be hardened by destabilisation, air cooling and tempering prior to service as wear parts. The aim of the work described in this paper was to determine the optimum softening treatment for 19% chromium irons and to investigate the effects of annealing on the nature of the structures produced during subsequent destabilisation and hardening. The most effective treatment for reducing hardness was holding at 900–950°C followed by slow cooling at rates of less than 20°C per hour. Prior annealing had an influence on the hardened structures; fine annealed structures gave rise to fine distributions of secondary carbides after destabilisation, but coarse annealed structures resulted in coarse destabilised structures. The destabilisation of pre-annealed irons at temperatures above 1000°C produced sub-grain structures in the hardened matrix with coarse secondary carbide precipitation at the boundaries.