Microstructure and corrosion resistance of selective laser melted 17-4PH steel after heat treatment

Utilizing the advantages of selective laser melting (SLM) for fabricating complex and precise components, this study systematically investigates the influence of solution (T4) and solution aging (T6) heat treatments on the microstructure and corrosion resistance of SLM-processed 17-4PH high-strength steel in 3.5 wt% NaCl solution. Microstructural characterization is carried out by optical microscopy, scanning electron microscopy with energy dispersive spectroscopy and transmission electron microscopy, and the corrosion behavior is evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy. The results reveal that T4 treatment transforms the SLM microstructure into a lath martensitic matrix with a small amount of retained austenite, but pores and microstructural inhomogeneity inherited from the SLM process remain. In contrast, T6 treatment promotes grain refinement and reduces surface pore defects through the generation and uniform distribution of nanoscale Cu rich phases and NbC precipitates. These precipitates are more effective than the T4 condition in mitigating grain boundary corrosion susceptibility and stabilizing the passive film. Electrochemical analysis shows that aging at 620 °C markedly increases the polarization resistance and pitting potential and reduces the corrosion current density by about 76%, indicating the formation of a more compact and stable passive film on the SLM 17-4PH high-strength steel. This work clarifies the correlation between heat treatment, defect and precipitate evolution, passive film characteristics and corrosion response, and demonstrates that an optimized T6 treatment can convert SLM 17-4PH steel into a state with significantly improved corrosion resistance. The findings provide a practical processing guideline for designing SLM 17-4PH steel components for service in chloride containing environments such as marine and energy related applications.