Effect Of Surface Pretreatment On Plasma Nitriding Of 304 Stainless Steel

This paper takes304austenitic stainless steel as the research object, which is widely used among the many industries of national economic construction, for its excellent comprehensive performance. However, the further application of austenitic stainless steel was restriced by its shortcomings of low hardness and poor abrasion resistance. Researchers have been studying the surface strengthening technology of austenitic stainless steel and discovered that the plasma nitriding processing is more effective at present. But plasma nitriding has one problem that high temperature can cause corrosion resistance decrease, due to the Cr element will precipitate with N generated compounds with the high temperature and Cr content reduce led to the decrease of the corrosion. Only under the condition of low temperature (450℃), the material can generate S phase hardening layer, which has both high hardness and corrosion resistance. But, at the same time, due to the low nitriding temperature, the permeability layer is very thin, and hardness gradient is larger.Surface nano processing technology can effectively improve the problem, for the refined grains has the advantages of both active and large volume fraction of grain boundary, which has a significant role in promoting plasma nitriding on low temperature.This paper combined the surface pretreatment and plasma nitriding technology, studied and analyzed the the influence of surface pretreatment on the304austenitic stainless steel plasma nitriding. Based on the theoretical of nano surface, the surface pre-treatment make material surface refine grain effectively by means of plastic deformation. The surface refining tissue has a good role in promoting the plasma nitriding, especially to the low temperature plasma nitriding. It is able to get a fine effect which the conventional ion nitriding cannot reach.The surface shot peening and surface mechanical attrition treatment (SMAT) was used as tow ways of surface pretreatment to304austenitic stainless steel in the paper. The metallographic microscopic analysis, SEM and XRD analysis are used for the two kinds of treatment on the deformation mechanism of304stainless steel. And it is determined that the surface shot peening for20min and SMAT treatment for30min is the optimal process on austenitic stainless steel surface refinement treatment.Take two groups of samples which has the best pretreatment effect to low temperature ion nitriding and traditional ion nitriding test. Make use of metallographic microscopic analysis. XRD, EDS to deserve nitriding layer structure and microhardness, electrochemical corrosion test for performance analysis. The influence of the surface pretreatment on304stainless steel plasma nitriding temperature and time was studied.In the metallographic microscopic analysis, it can be observed clearly that the nitriding layer of after pre-treatment is more thicker than the samples which were untreated. Under520℃temperature for6hours, the surface layers of sample after pre-treatment obtainsed about80micrometers, and the sample without pretreatment gained only about50micrometers. And when plasma nitriding experiments was under low temperature (350℃) in the short time (2hours) and, samples without pretreatment almost had no nitriding layer, while the other two groups of specimens were obtained certain thickness nitriding layer.In the test of the performance of nitriding layer, the surface layer of samples through the surface pretreatment had a higher microhardness than that of untreated samples. When ion nitriding test at420℃for6hours, the nitriding surface hardness of the specimen was as high as1337HV0.05. while the sample without the pre-treatment was only1169HV0.05. Put of sample after nitriding at solution for performance test. The results of electrochemical corrosion test, which has3.5wt%NaCl, showed that anode polarization curve of the corrosion potential of the samples after pre-treatment was higher than untreated samples, which showed that pre-treatmnet sample’s corrosion resistance is better than sampes untreated.Through the analysis of the above test results, it was finally concluded that both surface shot peening treatment for20min and SMAT treatment for30min on304austenitic stainless steel has a significant role in promoting plasma nitriding. Although under the condition of low temperature, it can also obtain a nitriding layer of high hardness, fine corrosion resistance, and large thickness.