Investigation of the Effects of La Addition on Microstructure and Friction Performance of High-Speed Steel Rolls

Refinement of the as-cast microstructure of high-speed steel (HSS) rolls is one of the most effective ways to improve the service performance significantly. In this paper, the effect of La from 0.25 to 1.0 wt pct added in HSS rolls on microstructure and wear resistance were studied by means of OM, XRD, SEM and TEM. The result shows that the as-cast microstructure of HSS mainly consists of pro-eutectic austenite (gamma-Fe), eutectic of MC (NbC and VC) + gamma-Fe, and eutectic of M6C + gamma-Fe. While, The addition of La in (HSS) rolls forms inclusions of La2O2S. The inclusions of La2O2S can become the nuclei of pro-eutectic austenite to refine as-cast microstructure, and the size of pro-eutectic austenite, MC + gamma-Fe and M6C + gamma-Fe eutectic colonies decreases, meanwhile, the eutectic carbides of MC and M6C change from rod shape to globular shape. However, the size of La2O2S is the smallest when the addition of La is 0.25 pct, and the size of La2O2S increases when the addition of La is 0.5 and 1 pct. As a result, the refining effect of HSS is the best when adding La is 0.25 pct. The heat treatment has little effect on eutectic carbides, while, mainly improves the microstructure uniformity and properties of the matrix. The wear resistance of HSS rolls after heat treatment is significantly improved by the addition of La, and the effect of refined microstructure on wear resistance was discussed.