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Keep 304 stainless steel precision tube corrosion resistance carburization process

Keep 304 stainless steel precision tube corrosion resistance carburization process

Time:2022-04-13 Source:China Wuxi JiaNuo Special Steel Co.

The corrosion resistance of 304 stainless steel precision tube is mainly due to the addition of a certain concentration of chromium in the steel. On the one hand, when chromium reaches a certain content in the iron matrix, the electrode potential of iron will increase sharply; on the other hand, chromium forms a dense oxide film on the surface, which acts as a shield against corrosion in the environment. And carburizing is an effective surface strengthening method for 304 stainless steel precision tube, but the conventional carburizing process will reduce its corrosion resistance.

Why conventional carburizing treatment will destroy the corrosion resistance of austenitic stainless steel tubes? This is because at high temperatures, the chromium atoms in the steel tube are easily combined with carbon atoms to form chromium carbide, which first precipitates at the austenite grain boundaries of the carburized layer and forms a network-like distribution. Due to the large radius of Cr atoms, internal Cr is difficult to diffuse to the surface chromium-poor layer, resulting in a local lack of chromium on the surface, stainless steel dense Cr2O3 oxide film of the protective layer is also damaged.

Therefore, in order to ensure that 304 stainless steel precision tube carburization is not corroded, to ensure that the precipitation of carbide. Since chromium carbides are formed within a certain temperature range at high temperature range, carburizing must be carried out at an appropriate low temperature range to avoid carbide formation and precipitation.

At this temperature, carbon atoms can diffuse into the lattice of austenitic stainless steel and form solid solution after carburizing due to the small radius of carbon atoms and diffusion through the interstitial mechanism; Fe and Cr atoms have a larger radius and can only diffuse through the exchange mechanism. Without sufficient diffusion activation energy, the Fe and Cr atoms cannot move. This ensures that no chromium carbides are formed. Chromium carbides are formed at 550°C. Therefore, the low-temperature carburization treatment of 304 stainless steel precision tubes will be carried out at temperatures lower than 550°C in order to improve other properties such as surface strength without damaging the original corrosion resistance of the steel tube.

The main process steps are as follows: precision stainless steel tube surface pretreatment before treatment, known as alloy surface activation. The activation process uses a mixture of pure HCl and N2, which is maintained at 250°C for 2 h. HCl can effectively remove the structure of chromium oxide passivation film on the surface of austenitic stainless steel, and the purpose of adding N2 is to create a non-oxidizing environment at atmospheric pressure to ensure that the chromium atoms in the matrix are no longer oxidized and avoid regeneration of the passivation film.

Then, carburizing at 470°C for 20 hours results in a hardened layer with a depth of about 70 microns. Compared with the untreated material, the performance of the LTCSS-treated 316 stainless steel tube is significantly improved, with the surface hardness increasing from 400hv to 1000hv; the fatigue limit increasing from 200MPa to 325mpa; in terms of corrosion resistance, the anodic pitting potential increases from +140mv to +990mv in 0.6mol/l NaCl solution.

In summary, conventional carburizing will destroy the corrosion resistance of 304 stainless steel precision tube, but after low-temperature carburizing treatment, not only maintains the original corrosion resistance, but also greatly improves the surface hardness and wear resistance, effectively improving the reliability and service life of 304 stainless steel precision tube.