Influence of high-velocity oxygen fuel spraying and plasma nitriding on microstructure properties of iron-nickel-chromium alloy using hybrid surface heat treatment

In this investigation, an attempt has been made to develop the surface hardness of iron-nickel-chromium alloy (alloy20) and the analysis on the micro structure and mechanical properties were performed. The surface modification of alloy20 was performed using HVOF (High-Velocity Oxygen Fuel) coating, plasma nitriding (PN) processes and hybrid coating (HVOF combined with PN) on 2 mm thickness plate. The HVOF thermal spray coating was performed at 450 °C using Molybdenum and the PN process was done at 500 °C for 12 h to strengthen the surface material. The microstructure and mechanical properties were investigated by x-ray diffractometry (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) techniques, tensile and bending. The result showed that the hybrid surface treatment enhanced the surface hardness of base material by 18%, tensile strength reached to 826 MPa which is 21% greater than that of base material, residual stress and corrosion rate decreased by 5 and 6% respectively compared to base material. Further, a transformation/diffusion model has been used and correlated with experimental hardness value and concluded that both the experimental and theoretical values displayed similar trend. The present investigation concluded that the formation of expanded austenite reduced the development of Cr₂N and this led to the better results of hardness, tensile and bending strength. (paper)