Effect of N/C Ratio on Precipitation Behavior of (Cr,Fe)23C6 Carbide in Novel Cast Austenitic Heat-Resistant Steels during Directional Solidification

The precipitation of (Cr,Fe)(23)C-6 carbide could significantly degrade the mechanical properties of Nb-bearing cast austenitic heat-resistant steels, designed for exhaust components of automotive gasoline engines at 1000 degrees C. In the current research, the precipitation behavior of (Cr,Fe)(23)C-6 carbide in these alloys, with great variations in N/C (Nitrogen/Carbon) ratio, was investigated through the liquid metal cooling directional solidification method, combined with thermodynamic calculations. Microstructural characterization suggested that the (Cr,Fe)(23)C-6 carbide formed in the steady-state zone and the competitive zone, upon cooling to room temperature, after the solidification ended. It grew in the colony of the -ferrite, through the eutectoid reaction and showed different concentrations of C and Si from the delta-ferrite. Its precipitation temperature decreased significantly with increasing the N/C ratio, thereby retarding its precipitation. Therefore, the quantity of (Cr,Fe)(23)C-6 carbide could be limited though increasing the N/C ratio of this type of alloys.