Low temperature active screen plasma nitriding of 17–4 PH stainless steel

Abstract

© 2016 Elsevier B.V.In the present work, low temperature active screen plasma nitriding of a 17–4 PH precipitation-hardening stainless steel was investigated. The active screen technique has been used to avoid undesirable plasma concentration, edge effects and arching during D/C plasma nitriding. Solution treated (S) and Solution treated and aged (S + A) 17–4 PH stainless steel samples were Active Screen Plasma Nitrided (ASPN) for 20 h at low temperature (400 °C), in order to avoid undesirable precipitation of chromium nitrides. Formation of these nitrides impairs corrosion resistance of the stainless steel because they act as Cr sinks, reducing the overall amount of Cr available in the matrix. The main objective of this work was to characterize the effect of the starting conditions on the microstructure of the nitrided layers. Besides, the chemical gradients and hardness evolution during low-temperature, long term active screen plasma nitriding were also studied. A homogeneous nitrided layer was obtained along the entire surface after the nitriding process. Hardness measurements along the nitrided surface showed virtually constant hardness values, indicating that ASPN was effective to avoid edge effects. Moreover, the nitrided layer could not be etched by Villela reagent, suggesting that the corrosion resistance was not impaired. Nitrogen supersaturation after plasma nitriding was indicated for both starting conditions (S) and (S + A) by X-ray diffraction patterns, as the expanded martensite peaks were broadened and shifted to lower 2θ angles compared to the martensite peaks of the substrate. The nitrided layer of the (S + A) specimen was thicker (9.2 μm) than the nitrided layer of the (S) specimen (5.7 μm). Also, the maximum nitrogen content (measured by WDX) of the (S + A) specimen (3.7%) was greater than the maximum nitrogen content measured on the simply solution treated (S) specimen (1.9%). The difference in nitrogen pick-up was related to greater hardness values for the (S + A) specimens (max. Hardness 1130 HV0.01) in comparison with the (S) condition (max. hardness 950 HV0.01). The latter results were discussed in terms of the effect of Cu in the activity of Fe-N solid solutions. Thermo-Calc® simulations showed that when copper is in solid solution, it increases the nitrogen activity in iron-nitrogen alloys, decreasing the maximum nitrogen solubility in the steel. On the other hand, when copper is precipitated as nanoparticles in the matrix — as in the (S + A) condition — nitrogen activity decreases, implying a greater solubility of nitrogen. The substrate hardness after aging was not changed by the 400 °C/20 h nitriding treatment, indicating that the surface treatment can be carried out without affecting the bulk properties.