Effects of the sub-zero treatment condition on microstructure, mechanical behavior and wear resistance of W9Mo3Cr4V high speed steel

The sub-zero treatment can improve mechanical properties and wear resistance of many metallic materials and the improvement is largely affected by the sub-zero treatment condition. In this study, samples of W9Mo3Cr4V (W9) high speed steel (HSS) were treated at various sub-zero temperatures before and after tempering, respectively. The main objective of this work is to investigate the effect of the sub-zero treatment condition on mechanical properties and wear resistance of the W9 HSS and correlate it to variations in microstructure. Microstructures of the treated samples were characterized with electron microscopy (SEM and TEM), and corresponding properties, including hardness, impact toughness and wear resistance, were evaluated. It was demonstrated that the mechanical properties of the cryogenically treated samples were superior to those of conventionally treated ones. The cryogenically treated samples with subsequent tempering performed better than cryogenically treated samples that experienced prior tempering. The lower temperatures for sub-zero treatment were more effective in improving the mechanical properties. The life of W9 HSS taps in cutting 30CrMnTi steel was assessed, which showed a trend similar to that observed during wear testing. Efforts are made to clarify possible mechanisms responsible for the improvements.