Effects of decarburization on the incipient melting temperature of AISI 4140 steel

Abstract

The continuous heating to fracture test (CHF) developed at McGill was used to study the effect of decarburization on the incipient melting temperature. In the CHF test, a sample was deformed at a constant true strain rate, while its temperature is increased at a constant rate. This test allows the incipient melting temperature to be determined using a single test, instead of the several required by isothermal tensile testing. An isothermal decarburization step, to create a decarburized layer at the surface of the sample, was added prior to the CHF test.Analysis of the true stress vs temperature curves obtained by CHF testing allows the incipient melting temperature to be determined. Optical metallography was used to determine the effect of decarburization time on the observed depth of decarburization. The heat transfer characteristics of induction heating were studied, since this heating technique is known to create a significant temperature gradient at the surface of the piece being heated. Since the decarburized layer is in the high temperature zone, and also has a higher melting point due to its lower carbon content, it plays an important role in the melting process.A phenomenological model was derived to describe the melting process. It allows for both the temperature gradient due to induction heating and the melting point gradient due to decarburization. The hypothesis is advanced that melting takes place at the position, within the sample, where the temperature profile crosses the incipient melting temperature gradient. From this study, it appears that decarburization acts so as to limit the risk of hot shortness on a workpiece being forged at high temperatures. This is because decarburization raises the IMT, and in this way widens the temperature window of optimum workability