Organization and Properties of Nitriding Layer for AISI 300 Series Austenitic Stainless Steel

This advanced surface strengthening technology can improve the anti-wearing properties of stainless steel without altering the original material properties of the matrix. The work aims to prepare a single S-phase nitriding layer on the surface of AISI 300 series austenitic stainless steel by low-temperature plasma nitriding technology (LTPNT) to improve the hardness and anti-wearing properties of stainless steel and compare the wear mechanisms of stainless steel before and after nitriding. The effect of the nitriding layer on the hardness, anti-wearing properties, and wear mechanism of stainless steel was investigated. Three widely used Cr-Ni commercial steels (AISI 304L, AISI 316L, and AISI 321) were sliced into 15 mm X 15 μm X 5 μm rectangular cubes, which were mechanically ground and polished. Nitrided samples at 380 °C for 12 hours were the major process parameters for LTPNT. The nitrided samples were mechanically polished and etched with marble mordents (Cu 2 SO 4 +HCl+distilled water). A metallographic microscope (AxioCam MRc5 ZEISS) was used to observe the cross-sectional morphology of the nitrided samples. The elemental distribution and phase composition of the nitriding layer were analyzed by EPMA and XRD (D/max-2400). The tribological properties of nitriding layer were characterized by specific wear rate and corrosion morphology. The wear microstructures of the samples were observed by a scanning electron microscope (QUATA FEG450). The corrosion resistance of samples was evaluated by an electrochemical experiment (CHI600E electrochemical workstation). In addition, wear tests were performed on material surface with performance comprehensive tester (CFT-I), and the surface and cross-sectional hardness were measured with a vicker microhardness meter (MVC-1000JMT1). The nitriding layer was uniform and dense, with a thickness of 15 μm approximately. The primary phase of the nitriding layer was the S-phase, but the Cr-N phase was not precipitated in the layer, indicating that nitriding did not deteriorate the corrosion resistance of the steel matrix. 304L nitriding layer consisted of supersaturated austenite (S-phase), a small amount of hard phase (γ′-Fe 4 N), whereas the nitriding layers of 316L and 321 consisted of S-phase. The surface hardness of the nitrided samples was about 1 100HV, which was about 5 times higher than that of untreated samples. The wear mechanism of three stainless steels changed from abrasive and adhesive wear to oxidation wear and micro-grooves. The specific wear rate of the steel matrix was about 20 times higher than that of nitrided samples, indicating that LTPNT could significantly improve the wear resistance. The corrosion current density of the three steels after nitriding was higher than that of the matrix slightly, which indicated a decrease in free corrosion potential after nitriding. The corrosion resistance of steels decreased to varying degree. Nitriding changed the corrosion mechanism of steels from uniform corrosion to local corrosion. Therefore, the enhancement of surface strengthening techniques for different materials was different, so a suitable surface strengthening process and parameters were required. The nitriding layer with high surface hardness and binding strength can be formed on the steel matrix with LTPNT. The metallograph of the nitriding layer is stable and dense. S-phase in the nitriding layer can significantly improve the surface hardness and anti-wearing properties of the steel and meanwhile it decreases the friction coefficient and specific wear rate. In summary, LTPNT can positively extend the service life of stainless steel. © 2023, Chongqing Wujiu Periodicals Press. All rights reserved.