Effect of post-heat treatment on the microstructure and microhardness of a plasma sprayed Fe-C-Mn-Cr-W-V coating

The article describes the effect of post-heat treatment (soaking at 950°C for 80 min with consequent oil quenching) on the microstructure and microhardness of a pulse-plasma deposited coating using heterogeneous cathodes (steel T1 and high-chromium (28 wt. % Cr) cast iron) at voltage of 4 kV and current of 10 kA. The coating was deposited by 10 pulses using electrothermal axial plasma accelerator with alternating cathode materials which provided two-layers coating of 120 m width. Optical and electron scanning microscopy, energy dispersive spectroscopy, microhardness measurement were employed in this work. It has been shown that after the deposition the coating structure is austenite/martensite matrix with minor amount of M3C carbides and total microhardness of 500-650 HV. In the course of high-temperature soaking the supersaturated solid solution decomposes with the precipitation of dispersed particles of W-rich carbides М6С, М2С and Cr-rich carbides М7С3, М23С6, М3С2. Carbides precipitate along the grain boundaries making continuous carbide network. The formation of carbides is mainly completed within 40 minutes of holding. Carbides precipitation is accompanied by 2-3 fold increase in the microhardness of the coating. The chromium rich layer possesses higher carbides volume fraction which results in higher microhardness (1265-1750 HV) as compared with W-rich layer (1000-1300 HV)