Study On Thermal Stability Of Low Temperature Nitrided Layer On Stainless Steel

Martensitic stainless steel is widely used in key components in fields such as aerospace,shipbuilding,ocean engineering,and petrochemicals.With the improvement of equipment performance,its hardness and wear resistance are severely tested.Conventional nitriding technology（temperature>500℃）can improve its hardness and wear resistance,but it will cause a decrease in its corrosion resistance.However,the new low-temperature nitriding technology can improve the hardness and wear resistance of stainless steel without affecting its inherent corrosion resistance.Since the nitrogen-expandedα-phase in the low-temperature nitriding layer is a metastable phase,the decomposition of the nitrogen-expandedα-phase will occur during service,affecting the performance of the low-temperature nitriding layer.Currently,the evolution of the microstructure and performance change laws of the low-temperature nitriding layer are not clear.In this study,AISI 630 martensitic precipitation-hardening stainless steel and AISI 430 ferritic stainless steel were studied using low-temperature plasma nitriding technology.By studying the microstructure and performance of the low-temperature nitriding layer,the optimal low-temperature nitriding process was obtained.Furthermore,by simulating the service process of the low-temperature nitriding layer in a hot environment through annealing treatment,the evolution laws of the microstructure and performance change trend of the low-temperature nitriding layer were analyzed.Combining the theoretical calculation results,the thermal stability mechanism of the low-temperature nitriding layer was explored.The experimental results show that a uniform low-temperature plasma nitriding layer can be prepared on the surfaces of AISI 430 and AISI 630 stainless steels by using low-temperature ion nitriding technology.After nitriding at 450℃for 12 h,the thickness of the nitriding layer can reach more than 30μm,mainly composed of nitrogen-expandedαphase(denoted asα_N)and a small amount of Fe_2-3N and Fe₄N phases.The corrosion resistance of the low-temperature nitriding layer is significantly better than that of the untreated stainless steel.The surface hardness of AISI 430 and AISI 630 stainless steels with the low-temperature nitriding layer can reach up to 1800 HV_0.1 and 1500 HV_0.1,respectively,which is more than three times that of the untreated stainless steel.The hardness of the low-temperature nitriding layer shows a gradient distribution,and the effective hardening layer thickness can reach more than 40μm.Compared with the untreated stainless steel,the friction coefficient of the low-temperature nitriding layer is reduced to below 0.5,and the wear rate can be reduced by up to two orders of magnitude,significantly improving the wear resistance.The low-temperature nitriding layer maintains good toughness compared with the conventional nitriding layer.After isothermal and isochronal annealing,Fe_2-3N and high-nitrogenα_N phase on the surfaces of the nitrided samples of AISI 430 and AISI 630 stainless steels decompose,and the nitrogen content on the surface of the samples decreases.Nitrogen atoms diffuse towards the interior of the samples,and the thickness of the nitrided layer significantly increases.The nitrided layer structure becomes more uniform and dense,and microcracks disappear.The low-temperature annealed nitrided layer mainly consists ofαN phase and Fe₄N phase,as well as a small amount of Fe₂O₃ and Fe_2-3N phases.In contrast,the high-temperature annealed nitrided layer not only contains the aforementioned phases but also Cr N phase.The hardness of the low-temperature annealed low-temperature nitrided layer decreases to 1000 HV_0.1,while that of the high-temperature annealed low-temperature nitrided layer decreases to 700 HV_0.1,but the effective hardening layer thickness increases significantly.The corrosion resistance of the annealed low-temperature nitrided layer slightly decreases.Except for the nitrided samples of AISI 430 stainless steel that undergo annealing at higher temperatures and for longer times,their corrosion resistance is still better than that of the untreated samples.The corrosion resistance of the annealed nitrided samples of AISI 630 stainless steel is better than that of the untreated samples.Annealing has no significant effect on the friction and wear behavior of the low-temperature nitrided layer,where the friction coefficient changes little or even decreases,and the wear rate can be further reduced.The wear mechanism of the nitrided samples before and after annealing is basically the same.The toughness of the low-temperature nitrided layer is significantly reduced after annealing.By combining theoretical calculations and experimental characterization of the surface microstructure before and after annealing of the low-temperature nitriding layer,it was found that the low-temperature nitriding layer on the surface of stainless steel consists of three parts:a compound layer on the surface,the middle is a dense nitrogen-containingα_N layer,and a diffusion layer close to the substrate.The stableα_N phases in the low-temperature nitriding layer are Fe₁₆N₂ and Fe₁₆N₁.The nitrogen content of theα_N phase in the low-temperature nitriding layer is different before and after annealing,where low-temperature annealing only results in a small amount of decomposition of Fe₁₆N₂ in the low-temperature nitriding layer,while high-temperature annealing causes a large amount of decomposition of Fe₁₆N₂ in the low-temperature nitriding layer.After annealing,the nitrided layer is mainly composed of the low-nitrogen concentrationα_N phase.