Kinetics of structure transformation in pulsed plasma high-Cr coatings under post-heat treatment

The kinetics of the change in the microstructure and hardness of the coating obtained by pulse-plasma deposition of 28 % Cr cast iron during consequent heat treatment was investigated. After deposition, the coating is found to have a non-equilibrium structure of supersaturated gamma and alpha solid solutions without eutectic carbides. Holding at 950 degrees C for up to 120 min ensures the phase transformations in the coating, which are associated with precipitation of carbides M7C3 and M23C6 followed by the transformation of depleted austenite into martensite. This results in a 2.5-fold increase in the microhardness of the coating. It was established that the carbide precipitation proceeds in decelerating kinetics with a maximum precipitation rate during the first 5-15 min holding and with the consequent formation of about 63 vol. % of carbides after 120 min holding. The carbides are found to be depleted with chromium and enriched with iron due to the saturation of the plasma with carbon released during erosion of the inner wall of the plasma accelerator.