Effect of arc current on microstructure and wear characteristics of a Ni-based coating deposited by PTA on gray cast iron

The plasma transferred arc (PTA) technique is currently used to coat the edges of moulds for the glass industry with nickel-based hardfacing alloys. However the hardness and wear performance of these coatings are significantly affected by the procedure adopted during the deposition of coatings. The aim of the present investigation is to study the effect of arc current on the microstructure, hardness and wear performance of a nickel-based hardfacing alloy deposited on gray cast iron, currently used in molds for the glass industry. Microstructure, hardness and wear assessments were used to characterize the coatings. Electron probe micro analysis (EPMA) mapping, scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDAX) and X-ray diffraction (XRD) were used to characterize the microstructure of the deposits. The effect of post weld heat treatment (PWHT) on the microstructure and hardness was also studied. The typical microstructure of the coatings consists of dendrites of Ni–Fe, in the FCC solid solution phase, with interdendritic phases rich in Cr–B, Ni–Si and Fe–Mo–C. Increasing the arc current reduces the proportion of porosity and hardness of the coatings and modifies their composition due to the increasing dilution of the cast iron. The partial melted zone (PMZ) had a typical white cast iron plus martensite microstructure, while the heat affected zone (HAZ) had only a martensite structure. The wear tests showed decreasing wear resistance with decreasing hardness of the coatings. PWHT reduces the hardness of the PMZ and HAZ but does not significantly alter the hardness of the bulk coating.