Effect of sub-zero treatment at -120°C. and -150°C on properties of high speed steel W6Mo5Cr4V2 is studied by means of hardness and wear resistance tests. The microstructure and wear morphology are analyzed by means of scanning electron microscope (SEM). It is concluded that the hardness and wear resistance of the high speed steel increase by sub-zero treatment. With the sub-zero temperature decrease

the performance of high speed steel improves significantly. The performance of cycle sub-zero treatment sample is better than a long time sub-zero treatment sample. After sub-zero treatment for 3 times 1 h at -150 °C

the high speed steel can obtain the optimal comprehensive performance. The results show that the significant performance improvement of high speed steel is attributed to the transformation of retained austenite to martensite in the samples after sub-zero treatment. At the same time

the wear resistance performance improved is attributed to that a large amount of carbides precipitate from high speed steel work as the rigid particles during the friction and wear process. There is super-cooling degree that continuously exists during the cycle cryogenic treatment process. The transformation of retained austenite to martensite and the tiny carbides precipitated from matrix martemsite can be promoted by super-cooling each cycle to improve the performance of the high speed steel.