Study on microstructure, phase transformation behavior of T15 high-speed steel and the effect of adding REEs

Rare earth elements (REEs) are a vital modifier in tool & die steel. Their effects on the phase transformation behavior, microstructure characteristics and property of as-cast T15 high-speed steel (HSS) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and dilatometry. The experimental results show that the as-cast microstructure of samples is refined and ameliorated after adding REEs, and the total quantity of eutectic carbides with hard and brittle properties is significantly reduced, which contribute to lower Rockwell hardness levels and higher bending strength. The morphology of grain boundaries carbides turns from lamellar to blocky polygonal. During the reheating process, REEs can reduce the thermal expansion coefficient of studied steel before austenitizing and promote the dissolution of alloying elements into austenite matrix. During cooling, REEs delays the occurrence of phase transformation, but it is favorable to the increase of the expansion amount of martensite transformation. Additionally, the microhardness value also increases with the increase of cooling rate. When the cooling rate is 20 °C/s, the full martensite transformation will occur and the microhardness of HSS1 reaches the maximum value of HV738, greater than that of RE-free HSS0 (HV701).