MICROSTRUCTURAL AND MICROHARDNESS CHARACTERISTICS OF LASER-SYNTHESIZED FE-CR-W-C COATINGS

The effects of laser-processing parameters on the microstructure and microhardness of Fe-Cr-W-G quaternary alloy coatings were investigated experimentally. The coatings were developed by laser processing a powder mixture of Fe, Cr, W, and C at a weight ratio of 10:5:1:1 on a low-carbon steel substrate using a 10 kW continuous wave CO2 laser. Depending on the processing parameters, either hypoeutectic or hypereutectic microstructures were produced. The hypoeutectic microstructures comprised primary dendrites of nonequilibrium face-centered cubic (fee) austenite gamma phase and eutectic consisting of pseudohexagonal close-packed (hcp) M(7)C(3) (M = Cr, Fe, W) carbides and fee gamma phase. The hypereutectic microstructures consisted of hcp M(7)C(3) primary carbides and eutectic similar to that in the hypoeutectic microstructures. The formation of hypoeutectic or hypereutectic microstructures was influenced by the alloy composition, particularly the C concentration, which depends on the amount of powder delivered into the melt pool and the extent of substrate melting. The enhancement of the lattice parameter of the gamma phase is associated with the significant dissolution of alloying elements and lattice strains resulting from rapid quenching. The higher hardness of the hypereutectic microstructures is principally attributed to the formation of hcp M(7)C(3) primary carbides. The relatively lower hardness of the hypoeutectic microstructures is related to the presence of gamma phase in the primary dendrites, excessive dilution from the base material, and relatively low concentrations of Cr and C. The results provide insight into the significance of laser-processing conditions on the composition and hardness of Fe-Cr-W-C alloy coatings and associated solidification characteristics.