Research On Corrosion-resistant Hardening Process Under Liquid Nitriding For Austenitic Stainless Steels

Austenitic stainless steels have excellent corrosion resistance, are widely used in the chemical, marine, petrochemical, instrumentation, manufacturing, food biotechnology, medicine and other industries. The use accounts for about 65-70% of stainless steels production.Common austenitic stainless steels have the properties of low surface hardness,poor wear resistance and fatigue resistance. Conventional nitriding treatment can effectively improve the surface hardness of austenitic stainless steel, but the generated segregation of Cr and CrN results in loss of corrosion properties of austenitic stainless steels. Low-temperature nitriding treatment can inhibit the precipitation of CrN to form S phase with fairly hardness of conventional nitrided layer, so that the corrosion resistance of austenitic stainless steels will not reduce. At home and abroad, low-temperature nitriding of austenitic stainless steels is being studied with the process of gas or plasma, however, the liquid method has only been studied in Japan.To carry out liquid nitriding austenitic stainless steels study, the paper with two kinds of ideas develops low temperature nitriding salts,which can be used below 450℃, and has achieved good results. CNO-concentrations of both salts are more than 47%.Using self-developed low-temperature nitriding salt, the paper examines some aspects as below:the liquid nitriding temperature on the diffusion layer compositions, morphologies, structure and properties; time on the impact of layer thickness at 430℃and 440℃temperature of liquid nitriding; hydrogen fluoride and plating methods for removing the passive film on the impact of layer thickness; comparison of low temperature plasma nitriding and liquid nitriding on different microstructures and phases; more discussions in-depth on some test results.Austenitic stainless steels at 430℃low temperature of liquid nitriding, have no precipitation of CrN; nitriding layer growth increases over time,but the speed gets slower after 12h; in 3% NaCl solution, liquid nitriding samples have higher passive current, but higher pitting potential, and get pitting corrosion resistance enhanced; 5% hydrochloric acid solution immersion tests show that samples liquid-nitrided at low temperature and untreated samples have fairly corrosion resistance, but corrosion resistance of the former in 6%FeCl3+0.05mol/LHCl solution is weaker than untreated sample; ring-block wear test shows that the low temperature samples liquid-nitrided and conventional nitrided samples have similar wear scars,and both coefficients of friction are lower than the untreated sample, thus may reduce the wear weight loss greatly; low-temperature liquid and low temperature plasma nitrided surfaces have a similar structure of layer, cross-section metallographies show that the former gets one more layer of unknown phase, which probably results from carbon solution.