Effects of residual carbon content on sintering shrinkage, microstructure and mechanical properties of injection molded 17-4 PH stainless steel

Carbon contamination from the thermoplastic binder is an inherent problem with the metal powder injection molding process. Residual carbon in the compacts after debinding has a strong impact on the sintering process, microstructure, and mechanical properties. In this study, injection molded 17-4 PH stainless steel was debound to two levels of residual carbon, 0.203 ± 0.014 wt% and 0.113 ± 0.008 wt%, by elevating the debinding temperature from 450°C to 600°C. Dilatometry in H2 atmosphere shows that the 600°C-debound compacts shrink much faster than those debound at 450°C when the sintering temperature rises to over 1200°C. Density measurements for tensile bars sintered between 1260°C and 1380°C confirm the beneficial effect of low residual carbon content on sintering shrinkage. Quantitative metallography reveals that more δ-ferrite forms along austenite grain boundaries during sintering of the 600°C-debound compacts. In both samples, density gradients across the compact section are correlated with the residual carbon content and corresponding δ-ferrite formation. Finally, tensile tests show that the 600°C-debound compacts have lower tensile strength but higher ductility than those debound at 450°C. The relevant mechanisms are discussed with a focus on the effects of residual carbon content, δ-ferrite amount, and porosity.