The corrosion resistance of 304 stainless steel precision steel pipe is mainly due to the addition of a certain concentration of chromium to the steel. On the one hand, when the chromium content in the iron matrix reaches a certain level, the electrode potential of the iron will increase sharply; on the other hand, chromium forms a dense oxide film on the surface, which shields the corrosion in the environment. Carburizing is an effective surface strengthening method for 304 stainless steel precision steel pipes, but the traditional carburizing process will reduce its corrosion resistance.
Why does conventional carburizing treatment destroy the corrosion resistance of austenitic stainless steel pipes? This is because, at high temperatures, chromium atoms in the steel pipe easily combine with carbon atoms to form chromium carbide, which first precipitates at the austenite grain boundaries of the carburized layer and forms a network distribution. Due to the large radius of Cr atoms, it is difficult for the internal Cr to diffuse into the surface chromium-poor layer, resulting in local chromium deficiency on the surface, and the protective layer of the dense Cr2O3 oxide film of stainless steel is also damaged.
Therefore, on the premise of ensuring that the 304 stainless steel precision steel pipe is not corroded by carburizing, it is necessary to ensure that carbides are not precipitated. Since chromium carbides are formed within a certain temperature range within the high-temperature range, carburizing must be carried out within an appropriate low-temperature range to avoid the formation and precipitation of carbides.
At this temperature, due to the small radius of carbon atoms and diffusion through the interstitial mechanism, carbon atoms can diffuse into the lattice of austenitic stainless steel and form a solid solution after carburizing; Fe and Cr atoms have larger radii and can only diffuse through the exchange mechanism. Without sufficient diffusion activation energy, iron and chromium atoms cannot move. This ensures that chromium carbides will not be formed. Chromium carbides are formed at 550°C, so the low-temperature carburizing treatment of 304 stainless steel precision steel pipes will be carried out at a temperature below 550°C to improve its surface strength and other properties without damaging the original corrosion resistance of the steel pipe.
The main process steps are as follows: The surface of the precision stainless steel pipe is pretreated before treatment, which is called alloy surface activation. The activation process uses a mixed gas of pure HCl and N2 at 250°C for 2 hours. HCl can effectively remove the structure of the chromium oxide passivation film on the surface of austenitic stainless steel. The purpose of adding N2 is to create a non-oxidizing environment under atmospheric pressure to ensure that the chromium atoms in the matrix are no longer oxidized and avoid the regeneration of the passivation film.
Then, carburizing at 470℃ for 20 hours can obtain a hardened layer with a depth of about 70 microns. Compared with the untreated material, the performance of 316 stainless steel pipe treated with LTCSS is significantly improved, the surface hardness is increased from the original 400hv to 1000hv; the fatigue limit is increased from 200MPa to 325mpa; in terms of corrosion resistance, in 0.6mol/l NaCl solution, the anodic pitting potential increases from +140mv to +990mv.
In summary, conventional carburizing will destroy the corrosion resistance of 304 stainless steel precision steel pipes, but after low-temperature carburizing treatment, not only the original corrosion resistance is maintained, but also the surface hardness and wear resistance are greatly improved, which effectively improves the reliability and service life of 304 stainless steel precision steel pipes.
Post time: Jun-18-2024