Effects of quenching temperature on the microstructure and properties of M4 powder metallurgy high speed steel

To investigated the effects of quenching temperature on the microstructures and properties of M4 powder metallurgy high speed steel (PM HSS), the metallographic structures of the PM HSS samples were observed by optical microscope, the grain sizes of the quenched microstructure were graded, and the composition and distribution of carbide in the tempered microstructure were analyzed. The hardness and bending strengthen of the PM HSS samples were tested by the Rockwell hardness tester and the universal testing machine, respectively. The results show that the hardness of the quenched samples first increases and then decreases with the increase of quenching temperature, reaching a maximum as HRC 62.9 at 1200 ℃. The grain size of the quenched samples declines with the increase of quenching temperature. After the triple tempering, the hardness of the tempered samples is higher than that of the quenched samples, showing the same trend with the increase of quenching temperature. The maximum hardness of the tempered samples reaches HRC 66.4 at the quenching temperature of 1190 ℃. With the increase of quenching temperature, the bending strength of the tempered samples gradually decreases, the carbides coarsen markedly, and the uniformity of carbide dimension deteriorates. According to the results, the optimal quenching temperature of M4 powder metallurgy high speed steel is 1180~1190 ℃.