Application of hardenability concepts in heat treatment of steel

Abstract

New information on hardenability is available to the heat treater, in the form of a critical review of existing hardenability data, more accurate predictions of hardenability from chemical composition, and new data on austenite phase transformation kinetics. This paper summarizes this new information and provides examples as well as references to more comprehensive coverage of several subjects. These subjects include 1) correlation of end-quench hardenability bar cooling rates with cooling rates in fully immersed rounds, 2) documentation of the strong influence of carbon on the effectiveness of alloying elements on hardenability, 3) alloy interaction effects, and 4) various aspects of austenite conditioning. Examples of the use of hardenability in metallurgical design emphasize the use of actual heat treating practice to establish hardenability requirements, the need to consider both core and case properties when heat treating carburized steels, and the role of hardenability in induction and flame-hardening.

Extensive work is in progress in Europe and North America to devise and perfect systems for predicting hardenability, microstructure and mechanical properties from chemical composition, section size and heat treatment parameters. These systems hold great promise, but important considerations are the limitations of such systems because of variability in composition due to inherent segregation in steel ingots and in the products of those ingots, variability in heat treating conditions, and, of course, the limitations imposed by the empirical nature of the predictive systems. Nevertheless, such systems provide steel producers, heat treaters and machine designers with useful new tools.