Decreasing Chromium Precipitation In Austenitic Stainless Steel During The Ionitriding Process

Glow discharge nitriding and nitrocarburising in AISI 304 austenitic stainless steel were approached in this paper. The aim of this paper was to improve the hardness and wear resistance of AISI 304 austenitic stainless steel and retain the corrosion resistance. The austenitic stainless steels are widely used in many fields such as chemical industry, oil industry, food processing and medical apparatus and instruments due to their well corrosion resistance, but they have low hardness and bad wear resistance. For further expanding the application of austenitic stainless steels, pulse current ionitriding was employed to improve their hardness and wear resistance. Because of the advantages of fast nitriding, small workpiece deformation and energy saving, ionitriding are widely used in different kinds of metal materials, especially the parts of stainless steel. The cathode sputtering effect of ionitriding can remove the passivating film on the surface of stainless steels, so it needn't do previously depassivation processing before ionitriding. But ionitriding can improve the surface hardness and wear resistance of austenitic stainless steel as well as decrease its corrosion resistance.The effects of nitriding temperature, time, pressure, voltage in ammonia gas and content of acetone, nitrocarburising temperature in ammonia and acetone mixture on the quality of nitrided layers were studied in this paper. Some measurements were used such as an optical microscope, a scanning electron microscope, an X-ray diffractometer, a Vickers microhardness tester and an electrochemical testing technique to study the metallurgical structure, thickness, phase composition, surface microhardness and polarization curves of the nitrided layers. The results were shown:The nitrided layers after ionitriding and the nitrocarburized layers after ionitrocarburising were similar; they all exhibit a diffusion layer with an obscure "white" appearance on it. The most influence on the quality of nitrided layers and nitrocarburized layers is nitriding temperature. As treatment temperature increasing, the content of CrN and the thickness of nitrided layers increase; the surface microhardness of the nitrided layers increases at first and then decreases; and the microhardness gradients are gradually gentle. The nitriding time mainly affects the thickness of the nitrided layers. As the nitriding time extending, the thickness of the nitrided layers apparently increases. The nitriding pressure affects the thickness and the surface microhardness of the nitrided layers. As the nitriding pressure increasing, the thickness of the nitrided layers increases; the surface microhardness of the nitrided layer increases at first and then decreases; and the microhardness gradients are gradually gentle. The nitriding voltage affects the content of CrN and surface microhardness of the nitrided layers. As the nitriding voltage increasing, the content of CrN on the surface of the nitrided layers decreases; and the surface microhardness of the nitrided layers increases. The content of acetone in ammonia and acetone mixture during ionitrocarburising affects the content of CrN on the surface of the nitrocarburized layers, the thickness of the nitrocarburized layers, rate of corrosion on the surface of the nitrocarburized layers and the microhardness gradient. The addition of acetone in the nitriding gas helps to decrease the content of CrN. As the content of acetone in the nitrocarburising gas increasing, the thickness of the nitrocarburized layers increases at first and then decreases.The methods to decrease chromium precipitation in austenitic stainless steel during the ionitriding process include decreasing the nitriding temperature, shortening the nitriding time, making the nitriding pressure moderately, controling the nitriding voltage a bit higher and addition acetone in the nitriding gas. Comprehensive analysis, the processing parameters of austenitic stainless steel during ionitriding are nitriding at the temperature of 500℃, at the pressure of 350 Pa, with the voltage of 900 V for 2 h, and nitrocarburising with the content of acetone at 2.5 %. AISI 304 austenitic stainless steel treated in the condition of those processing parameters can obtain the nitrided layer with both high hardness and well corrosion resistance.