LASER SURFACE TREATMENT OF SINTERED M42 HIGH-SPEED STEEL DILUTED WITH IRON

摘要

In this work, powders of M42 high-speed steel were blended with water-atomised iron powders, in order to increase their compressibility, and then sintered. However, after sintering it was observed that some residual porosity, mainly superficial, still remains. To overcome this problem the sintered samples were submitted to laser surface melting (LSM) in order to homogenise the microstructure and to eliminate the residual porosity. It is shown that LSM enables for a complete elimination of residual porosity and the dissolution of large brittle M_7C_3 carbides, which are present in the sintered condition. Moreover, an homogeneous and extremely fine microstructure formed of saturated plate martensite and a small proportion of retained austenite is obtained after LSM. As a result, while the hardness of the material in the sintered condition lies between 400 and 800 HV (depending on the iron content), after LSM the hardness is higher than 900 HV. Surprisingly, the abrasive wear resistance of the laser-melted layers is 10 to 100 times lower than that of the as-sintered samples. Observation of the wear craters by scanning electron microscopy shows that this result is due to the different wear mechanisms acting on the samples submitted to LSM and on the as-sintered ones. The results are analysed and discussed on the basis of the influence of the microstructure on the abrasive wear behaviour of the material.