Copper Precipitation Behavior and Mechanical Properties of Cu-Bearing Ferritic Stainless Steel with Different Cr Addition

The microstructure evolution and mechanical properties of Cu-bearing ferritic stainless steel with different Cr addition (Cr = 12, 15 and 17 wt%) were investigated. The phase transformation behavior under different cooling rate, Cu-rich precipitation behavior and its influence on the mechanical properties under different aging treatment are systematically characterized using dilatometry, differential scanning calorimeter (DSC) and transmission electron microscopy (TEM). The results indicated that the increase in Cr content narrowed the austenite phase region at high temperatures, affecting its microstructure under different cooling rates. The 12Cr-1.5Cu steel exhibited a fully austenitic phase region at high temperature and occurred apparent martensitic transformation after air cooling. Cooling rate significantly influenced the phase transition of the steels, and subsequently affected its mechanical properties. All three investigated steels showed higher strength and lower plasticity in air cooling condition compared to furnace cooling condition, due to the presence of martensite. After aging treatment, high number densities of Cu-rich precipitates were formed in steel matrix and the size of Cu-rich precipitates increased obviously with increasing aging temperature, while the tendency for number density was opposite. Fine and dispersed Cu-rich precipitates formed during low-temperature aging enhanced strength of the steels, while larger Cu-rich phases developed during high-temperature aging endowed greater ductility to the steels. Notably, the Cr content had no significant effect on the precipitation behavior of Cu-rich precipitation. These comprehensive results and analyses could provide a solid foundation for broader applications of Cu-bearing ferritic stainless steels.