Effect of heat treatment on the microstructure and passive behavior of 316L stainless steel fabricated by selective laser melting

Selective laser melting (SLM), a rising additive manufacturing technology, has extensive application potential because of its advantage in fabricating components with individual and complex shapes. During the SLM progress, the laser molten pool cools very quickly, leading to non-equilibrium microstructure formation and high thermal residual stress in the SLM components. Therefore, a suitable heat treatment is required to reduce the residual thermal stress and obtain excellent mechanical properties after the SLM process. In this paper, the 316L stainless steel fabricated by SLM (SLM-316L SS) is first heat-treated at 900 ℃ for 0, 0.5, 1.0, 3.0, and 5.0 h. Then, the microstructures of SLM-316L SS treated at different times are investigated using SEM, TEM, and EDS, and their corrosion resistance is estimated through electrochemical measurements. Finally, the microstructure evolution of SLM-316L SS during heat treatment at 900 ℃ is discussed based on the experimental data, and the effect of the microstructure on the formation kinetics and properties of the passive film on SLM-316L SS is explained. The results of the microstructure analysis reveal that the dislocations and sub-grain boundaries in SLM-316L SS disappeared with increasing holding time, accompanied by the precipitation of MnS inclusions, carbides, and σ phases along the grain boundaries. The potentiodynamic polarization in the buffer solution with 0.1 mol·L^-1 NaCl reveals that a sample with a longer holding time shows a more negative pitting potential. According to the EIS test results, the shape of the curve in Nyquist diagram is not completely circular. The calculated film thicknesses decrease with increasing holding time. The potentiostat polarization under 0.1, 0.2, 0.3, 0.4, and 0.5 V vs SCE was used to form passive films on SLM-316L SS after heat treatments. By fitting the Mott-Schottky curves, the negative slopes demonstrate that the passive films formed on the samples are an n-type semiconductor, and the calculated point defect densities in the sample increase with the holding time. In addition, a logarithmic relationship holds between the carrier densities and the formation potentials of the passive films, and, using this relationship, the calculated diffusion coefficient of point defects across the passive film of SLM-316L SS increases with the holding time. A theoretical model related to the energy band structure and space charge layer is obtained based on the Mott-Schottky results to explain the electrochemical reaction on the passive film/solution interface.