Improved wear resistance through laser alloying and transformation hardening

Abstract

Laser surface alloying and transformation hardening approaches were used to improve the wear resistance of Fe-5Cr, Fe-0.2C and Fe-2Ni-0.75C steels. Alloying was achieved by thermal spraying of Ni-Cr, and Ni-Cr-WC powders, followed by surface melting with laser. The maximum power density and hence, the maximum heat input were dependent upon the spot size and thus, the distance of the specimen surface from the focal plane. The dwell time, which was a function of the scan rate, determined the nominal incident heat energy. Since the heat input, in turn, determined the depth of penetration, the alloy element concentration and the cooling rates in the fusion zone, the resulting structure, hardness and wear resistance were all controlled by varying these factors. The wear resistance, in this study, was determined by hardness, coating type, substrate material and microstructure. The solidification structure consisted of a fusion zone, with a predominantly dendritic structure (the spacings of which were determined by the cooling rate) and a heat affected zone, with a martensitic structure. In the fusion zone, the concentration of the alloy element determined the type of microstructure and thus, the hardness. The effect of the alloy element was to suppress the martensite start (Ms) temperature. For the Fe-5Cr and the medium carbon steels, this resulted in an austenite/martensite duplex structure, while for the low carbon steel, the martensite transformation was complete. In all cases, the wear resistance of the non-laser treated surfaces was the lowest, followed by Ni-Cr laser alloyed surfaces, Ni-Cr-WC laser alloyed surfaces and unalloyed, transformation hardened surfaces. The last category yielded the highest fusion zone hardness and hence, resulted in a nearly two-fold increase in wear resistance over the non-laser treated surfaces. Although the hardness of the alloyed surfaces was lower than that of the corresponding non-alloyed surfaces, the wear resistance was still higher due to the duplex austenite/martensite structure.Laser surface alloying and transformation hardening approaches were used to improve the wear resistance of Fe-5Cr, Fe-0.2C and Fe-2Ni-0.75C steels. Alloying was achieved by thermal spraying of Ni-Cr, and Ni-Cr-WC powders, followed by surface melting with laser. The maximum power density and hence, the maximum heat input were dependent upon the spot size and thus, the distance of the specimen surface from the focal plane. The dwell time, which was a function of the scan rate, determined the nominal incident heat energy. Since the heat input, in turn, determined the depth of penetration, the alloy element concentration and the cooling rates in the fusion zone, the resulting structure, hardness and wear resistance were all controlled by varying these factors. The wear resistance, in this study, was determined by hardness, coating type, substrate material and microstructure. The solidification structure consisted of a fusion zone, with a predominantly dendritic structure (the spacings of which were determined...