The corrosion resistance of 304 stainless steel precision tubes is mainly due to the addition of a certain concentration of chromium. On one hand, when chromium reaches a certain content in the iron matrix, the electrode potential of the iron increases sharply; on the other hand, chromium forms a dense oxide film on the surface, acting as a shield against corrosion from the environment. Carburizing is an effective surface strengthening method for 304 stainless steel precision tubes, but traditional carburizing processes can reduce their corrosion resistance.

Why does conventional carburizing treatment damage the corrosion resistance of austenitic 304 stainless steel precision tubes? This is because at high temperatures, chromium atoms in 304 stainless steel precision tubes easily combine with carbon atoms to form chromium carbides. Chromium carbides first precipitate at the austenite grain boundaries of the carburized layer and form a network distribution. Due to the large atomic radius of Cr, internal Cr has difficulty diffusing to the chromium-depleted surface layer, resulting in localized chromium deficiency on the surface, and the protective layer of the dense Cr2O3 oxide film of the stainless steel is also damaged.

Therefore, to ensure that the 304 stainless steel precision tube does not corrode during carburizing, it is necessary to ensure that carbides do not precipitate. Since chromium carbides form within a specific temperature range within a high-temperature range, carburizing must be carried out at an appropriate low temperature to avoid carbide formation and precipitation.

At this temperature, due to the small atomic radius of carbon atoms in 304 stainless steel precision tubes, carbon atoms can diffuse into the austenitic stainless steel lattice through interstitial diffusion, forming a solid solution after carburizing. Fe and Cr atoms, with larger atomic radii, can only diffuse through exchange mechanisms. Without sufficient diffusion activation energy, iron and chromium atoms cannot move. This ensures that chromium carbides do not form in the 304 stainless steel precision tube. Chromium carbides form at 550℃; therefore, low-temperature carburizing of 304 stainless steel precision tubes will be carried out at temperatures below 550℃ to improve the surface strength and other properties of the 304 stainless steel precision tube without compromising its original corrosion resistance.

The main process steps are as follows: The surface of the 304 stainless steel precision tube undergoes pretreatment before processing, called alloy surface activation. The activation process uses a mixture of pure HCl and N2 gas, held at 250℃ for 2 hours. HCl effectively removes the chromium oxide passivation film from the surface of austenitic stainless steel. The addition of N2 creates a non-oxidizing environment at atmospheric pressure, ensuring that chromium atoms in the matrix of the 304 stainless steel precision tube are no longer oxidized, thus preventing passivation film regeneration. Then, carburizing at 470℃ for 20 hours yields a hardened layer approximately 70 micrometers deep. Compared to untreated material, LTCSS-treated 316 stainless steel tubes exhibit significantly improved performance: surface hardness increases from 400 HV to 1000 HV; fatigue limit increases from 200 MPa to 325 MPa; and in terms of corrosion resistance, the anodic pitting potential in 0.6 mol/L NaCl solution increases from +140 mV to +990 mV.

In conclusion, conventional carburizing compromises the corrosion resistance of 304 stainless steel precision tubes. However, low-temperature carburizing not only maintains the original corrosion resistance but also significantly improves surface hardness and wear resistance, effectively enhancing the reliability and service life of the 304 stainless steel precision tubes.