Microstructure, mechanical properties and corrosion behavior of NbC modified AISI 440C stainless steel by vacuum sintering and heat treatments

Different amounts of niobium carbide (NbC) powder (5, 10 and 15 wt%) are mixed and added to 440C stainless steel powder. The composite powders are sintered at 1260, 1270, 1280 and 1290 °C for 1 h, respectively. The experimental results show that the optimal sintering temperature for the 440C-NbC composites is 1270 °C. Meanwhile, the 440C specimens with 5% NbC addition possess the optimal transverse rupture strength (TRS) value of 1985.2 MPa, as well as the highest polarization resistance of 1.01 × 102 Ω cm2. While the 440C specimens contain or with 15 wt% NbC show the highest hardness value of HRA 80.9 after sintering at 1280 °C. Furthermore, the microstructural evaluation reveals that the rod-shaped M7C3 carbides located on the grain boundaries are gradually reduced after NbC particles are added, and the spherical-shaped M7C3 carbides are precipitated and dispersed in the matrix. After heat treatment, the primary M7C3 carbides are converted to M23C6 carbides, which results in a secondary hardening. The results clearly show that heat treatment effectively improves the particle size of the carbides and strengthens the matrix of the 440C-NbC composites.