The influence of nitriding temperature on the properties of austenitic stainless steel

Stainless steel has the characteristics of a beautiful surface, good corrosion resistance, good toughness, easy plastic processing, and good welding performance. It is widely used in industries such as petroleum, chemical engineering, food, medical care, and textile machinery. However, due to its low hardness and poor wear resistance, its application is limited in many situations. Especially under conditions where multiple factors such as corrosion, wear, and heavy loads coexist, the service life of stainless steel parts is significantly shortened. Enhancing the surface strength of stainless steel through ion nitriding and improving its wear resistance to extend its service life is an effective method. 

Austenitic stainless steel cannot be strengthened through phase transformation. Moreover, conventional ion nitriding, due to the high nitriding temperature (above 500℃), causes the formation of Cr nitrides in the nitrided layer, resulting in a chromium deficiency in the base material. At the same time, the surface hardness significantly increases while the surface corrosion resistance severely deteriorates, losing the characteristics of stainless steel. By using a direct current pulse ion nitriding equipment, low-temperature ion nitriding treatment is carried out on austenitic stainless steel. While the corrosion resistance remains basically unchanged, the surface hardness is increased, thereby improving its wear resistance. This is compared with the ion nitriding treatment samples under the conventional nitriding temperature. 

The experiment was conducted in a 30kW direct current pulse ion nitriding furnace. The parameters of the direct current pulse power supply were as follows: voltage adjustable from 0 to 1000V, duty cycle adjustable from 15% to 85%, and frequency 1kHz. The temperature measurement system was measured by an infrared thermometer IT-8. The material of the sample was 18Cr-12Ni-2.5Mo austenitic stainless steel, with its chemical composition (mass fraction, %) being: 0.06C, 19.23Cr, 11.26Ni, 2.67Mo, 1.86Mn, and the remainder being Fe; the sample size was Φ24mm × 10mm. Before the experiment, the sample was ground with water-sandpaper to remove oil stains, cleaned with alcohol, dried, and placed in the center of the cathode disk, with the vacuum being reduced to below 50Pa. 

Ion nitriding of austenitic stainless steel was carried out at both low temperature and conventional nitriding temperature. The microhardness of the nitrided layer in both cases reached above 1150HV0.05. The nitrided layer obtained by low-temperature ion nitriding was thinner and had a larger hardness gradient. After ion nitriding at low temperature, the wear resistance of austenitic stainless steel increased by 4 to 5 times, while the corrosion resistance remained basically unchanged. Under conventional nitriding temperature, although the wear resistance also increased by 4 to 5 times through ion nitriding, due to the precipitation of chromium nitrides on the surface of austenitic stainless steel, its corrosion resistance decreased significantly.