Prediction of Softening of High Speed Steel with Surface Nitriding for Hot Forging

Abstract

A model hot forging test was carried out using tools of matrix high-speed steel. Dynamic thermal loads were applied to tools subjected to ion nitriding on the surface layer of the tool, and the conditions of deterioration and thermal softening were examined by measuring Vickers hardness in the vicinity of the tool surface after the thermal load test. In addition, static tempering softening tests were carried out, and thermal softening equations are obtained using tempering parameter on the basis of the obtained master tempering curves. When the thermal load was applied under the same conditions to nitrided and heat-treated tools, the degree of surface deterioration and thermal softening was small for nitrided tools. Equations of softening for two cases, i.e., specimen subjected only to heat treatment and specimen subjected to both heat treatment and nitriding, were expressed using tempering parameter.