A comparative study of the wear behaviour of sintered and laser surface melted AISI M42 high speed steel diluted with iron

Powders of AISI M42 high-speed steel (HSS) were blended with different proportions of water-atomised iron powders. The powders were subsequently submitted to uniaxial pressing and then divided in three lots. The first was submitted to sintering, the second was submitted to sintering plus laser surface melting (LSM) and the third was submitted to sintering plus LSM plus double tempering at the secondary hardening peak temperature of M42 HSS. The objective of this procedure was to evaluate the processing route that leads to reduced porosity in AISI M42 HSS and to higher abrasive wear resistance. Therefore the samples, with different chemical compositions and microstructures, were submitted to a detailed microstructural characterisation followed by microscale hardness and abrasive wear tests. It was observed that LSM leads to almost complete elimination of residual porosity and to the dissolution of large brittle carbides that are present in the as-sintered samples, leading to a homogeneous and extremely fine microstructure. This microstructure is formed of saturated plate martensite and a small proportion of retained austenite. The double tempering treatment, carried out in the laser surface melted samples samples, leads to the elimination of retained austenite and to a decrease of the lattice parameters of martensite due to the precipitation of thin carbides within martensite. As a result, while the hardness of the material in the sintered condition is between 245 and 625 HV (depending on the proportion of dilution with iron), after LSM the hardness is higher than 820 HV in all the samples. Surprisingly, the abrasive wear resistance of the laser melted and of the laser melted and tempered samples is lower than that of the as-sintered ones. Observation of the wear craters by scanning electron microscopy shows that this result is due to the different wear mechanisms acting on the samples processed by different routes.