Study on Low-temperature Gas Nitriding and Plasma Polishing of Stainless Steel AISI 304

AISI 304 stainless steel is one of the commonly-used steel among austenitic stainless steel. This stainless steel has excellent corrosion resistance thanks to its high concentration of chromium and nickel. The surface is coated with a passive film that helps protect the steel from oxidation. However, it shows low hardness because of its austenite phase, and it can not be strengthened with heat treatment such as quenching, giving rise to drawbacks such as being prone to wear, scratches, and abrasion. Thus, the method of solid solution strengthening is often used to improve the hardness of AISI 304. This study aims to strengthen the surface of austenitic stainless steel, and reduce the surface roughness. The experiments are divided into three parts: activation treatment, low temperature gas nitriding, and plasma polishing. In order to remove the oxidation film on the surface, the specimens will go through activation treatment using HCl gas in high temperature for a short period of time. After the treatment, the surface hardness is strengthened by low temperature gas nitriding. Having been through the processes above, the surface will become rougher due to side effects like etching and lattice expansion. In order to reduce the surface roughness, the method of plasma polishing is studied in the third part. Plasma polishing is a polishing method by treating the specimen as the positive electrode. The specimen will be immersed in low concentration electrolyte, and applied with hundreds of voltages. Then, the surface would become polished when the plasma film is formed in appropriate conditions. The main results of the experiments are: (1) With longer time span and higher temperature of gas nitriding, the nitrided layer grows thicker. The nitrided layer contains rich nitrogen and high hardness. However, overheating would cause a decrease of corrosion resistance. (2) With lower temperature and higher current density of plasma polishing, the polishing efficiency becomes higher. The surface will become uniform when the polishing efficiency is higher than corrosion rate. (3) The surface roughness is reduced when the polishing time is increased, but the roughness would maintain at a certain level after polished for a long time. (4) With adequate polishing efficiency, the surface roughness could be reduced in a short period of time while maintaining enough thickness of nitrided layer.