Influence of carbon: Carbon is an element in austenitic stainless steel that strongly forms and stabilizes austenite and expands the austenite zone. The ability of carbon to form austenite is about 30 times that of nickel. Carbon is an interstitial element that passes through Solid solution strengthening can significantly improve the strength of austenitic stainless steel. Carbon can also improve the stress corrosion resistance of austenitic stainless steel in highly concentrated chlorides (such as 42% MgCl2 boiling solution).
However, in austenitic stainless steel, carbon is often regarded as a harmful element. This is mainly because under some conditions in stainless steel and corrosion-resistant applications (such as welding or heating at 450~850°C), carbon can interact with the steel in the steel. Chromium forms high-chromium Cr23C6 carbon compounds, which leads to local depletion of chromium and reduces the corrosion resistance of steel, especially intergranular corrosion resistance. Therefore, most of the newly developed chromium-nickel austenitic stainless steels since the 1960s have carbon content less than 0.03% or 0.02% ultra-low carbon type. It can be known that as the carbon content decreases, the intergranular corrosion sensitivity of steel decreases. When the carbon content is lower than 0.02%, the obvious effect will be achieved. Some experiments also pointed out that the carbon also increases the tendency of pitting corrosion in chromium austenitic stainless steels. Due to the harmful effects of carbon, not only should the carbon content be controlled as low as possible during the smelting and processing of austenitic stainless steel, but also during the subsequent hot, cold working and heat treatment processes, the surface of the stainless steel should be prevented from being carburized, and chromium-free Carbide precipitation.

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