Enhancing corrosion fatigue performance of 17-4 PH stainless steel by simultaneous aging and plasma nitriding

This study investigated the effects of simultaneous aging and low-temperature plasma nitriding on the corrosion fatigue performance of 17-4 PH stainless steel. Plasma nitriding was conducted under two temperatures of 400 and 500 degrees C and, for comparison, some other specimens were subjected to aging treatment at the same temperatures and times. Microstructural characterization by scanning electron microscope (FESEM) and electron backscatter diffraction (EBSD) confirmed similar structures at the core in the nitrided and aged specimens, thereby, the influence of nitrogen at the surface of 17-4 alloy could be carefully investigated. X-ray diffraction and grazing incidence X-ray diffraction (GIXRD) identified expanded martensite as the matrix within the nitrided layer, accompanied by CrN, Fe3N, and Fe4N phases. X-ray photoelectron spectroscopy (XPS) analysis revealed high nitrogen concentration, in regions very close to the surface, after nitriding at 500 degrees C. The N1s spectra shifted towards lower binding energies, indicating interaction between nitrogen and the matrix atoms, and the formation of nitrides in the nitrided layer. The nitrided thicknesses experienced close values at 400 degrees C (10 h) and 500 degrees C (5 h), provided at 400 degrees C the layer was mostly a supersaturated solid solution but at 500 degrees C it was composed of iron nitrides and chromium nitride. High hardness values of above 1600 HV0.1 were obtained after optimum plasma nitriding as the result of combined effects of solution strengthening induced by nitrogen atoms and the presence of dispersed nitride phases. EBSD results indicated residual stress within the nitrided layer, which would affect the corrosion fatigue resistance. Experimental findings demonstrated that both aging and plasma nitriding substantially enhanced the corrosion fatigue resistance of 17-4 PH stainless steel in 3.5 wt% NaCl. Moreover, simultaneous aging and plasma nitriding proved superior behavior in corrosion fatigue, primarily due to delayed crack initiation and improved fatigue resistance, without the risk of overaging which is detrimental to mechanical properties.