March 15, 2024
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The significance of S-phase infiltration layer technology of austenitic stainless steel
Austenitic stainless steel has good corrosion resistance, excellent toughness and processability, and is widely used in many fields. However, its friction and wear resistance and fatigue resistance are poor, which seriously affects the service life of stainless steel parts.
Tests have proven that stainless steel is quenched after high-temperature nitriding, that is, nitrogen is dissolved on the surface of the stainless steel workpiece in a vacuum furnace at 1050 to 1150°C, and then quickly cooled down so that the nitride has no time to precipitate, thus forming a nitrogen-containing solid solution on the surface of the workpiece. Strengthened austenitic nitrided layer. After high-nitrogen surface treatment, not only the strength, hardness and wear resistance of the austenitic stainless steel surface are improved, but the core structure and performance of the solid solution treatment are still maintained. Because the lattice parameters of this nitrided layer are different from those of the γ phase, it is called "S phase". In the S phase, nitrogen atoms are solidly dissolved inside the austenite lattice and inhibit the precipitation of chromium nitride at the grain boundaries. Therefore, the corrosion resistance of austenitic stainless steel is significantly improved without reducing the corrosion resistance of austenitic stainless steel. surface hardness. The S-phase modification technology on the surface of stainless steel thus developed has become an important milestone in the development of stainless steel surface treatment technology. Experiments have shown that introducing carbon-containing gas into the atmosphere of ion treatment instead of nitrogen can also obtain an S-phase hardened layer similar to the nitriding layer.
However, although traditional nitriding and carburizing technologies improve the surface hardness, wear resistance and fatigue strength of stainless steel parts, due to the high nitriding and carburizing temperatures, precipitation phases of nitrides and carbides are formed, sacrificing the quality of stainless steel. Corrosion resistance. At the same time, due to the formation of a dense oxide film on the surface of stainless steel, it hinders the penetration and diffusion of nitrogen and carbon atoms. These factors have seriously restricted the development, promotion and application of stainless steel nitriding and nitriding surface treatment technology.
Different from traditional nitriding and carburizing technology, S-phase carburizing technology is a low-temperature nitriding/carburizing technology. For example, low-temperature ion nitriding technology lowers the nitriding temperature to below 450°C. The infiltrated nitrogen forms solid solution austenite, which significantly improves the hardness of austenitic stainless steel. It also suppresses the precipitation of chromium nitrides during the nitriding process, maintaining the Improve the corrosion resistance of stainless steel. Low-temperature ion nitriding technology can obtain a single-phase nitrogen-containing expanded austenite phase of tens of microns. Low-temperature ion carburizing has the advantages of uniform carburizing layer, good toughness, strong load-bearing capacity, gentle hardness gradient, and high carburizing efficiency. It is also reported that by fluoriding the surface of stainless steel, the oxide film on the surface is eliminated, and a fluorinated film is formed on the surface of the sample. This fluorinated film improves the adsorption and diffusion infiltration of active nitrogen, which can make the nitriding of stainless steel The temperature is reduced to 300°C.
Nitriding austenitic stainless steel can provide a stronger sub-surface layer to support the oxide film formed during dry sliding, and can withstand higher loads than non-nitrided samples. The wear of nitrided steel is an oxidative wear mechanism, while the wear of non-nitrided steel is an adhesive and plastic deformation mechanism.
Through low-temperature nitriding/carburizing, austenitic stainless steel obtains a nitrogen/carbon solid solution saturated diffusion layer, that is, an S-phase infiltration layer, which not only improves the surface hardness of the stainless steel, but also improves the corrosion resistance of the stainless steel. For example, after ion nitriding at 400°C for 4 hours, the corrosion potential of AISI304 stainless steel in a 5.5% NaCl solution increases three times, and the S-phase corrosion resistance in a 3.5% NaCl solution can be increased by 75%. Low-temperature nitriding improves the corrosion resistance of stainless steel, thereby extending the service life of stainless steel parts. For example, the service life of austenitic stainless steel control rods in nuclear reactors is extended from one year to more than three years.
