Effect of grain size and carbide thickness on impact transition temperature of low carbon structural steels

Abstract

A model previously developed by Petch enables an estimate of the impact transition temperature (ITT) of low carbon structural steels to be made from measurements of grain size and carbide thickness. In the present work, various laboratory heat treated and commercially processed structural steels were assessed in order to ascertain the validity of the Petch model. Laboratory heat treated steels, with carbon contents of <0.1 wt-%, with a range of grain and carbide sizes, showed that accurate predictions of ITT could be made provided that the original Petch model was modified. The revised model incorporated a measure of the largest grains in the material rather than an average grain size, and necessitated a change to the expression used by Petch for the variation in yield stress with temperature at high strain rates appropriate to a Charpy test. The present work also showed that cleavage is associated with the largest carbides in the steel. The modified model also successfully estimated, to within ±10 K, the ITT of some commercially processed structural steels, of similar carbon content to the laboratory treated steels, but containing much thicker carbides. The revised model was slightly less successful in predicting the impact behaviour of a laboratory processed structural steel having a slightly higher carbon content (0.16 wt-%). It is believed that this is primarily because pearlite plays a more significant role in the cleavage fracture process in this steel compared to the lower carbon steels, but its role is ignored in the model.