Influence of carbon: Carbon is an element that strongly forms and stabilizes austenite and expands the austenite region in austenitic stainless steels. The ability of carbon to form austenite is about 30 times that of nickel, and carbon is an interstitial element that can significantly increase the strength of austenitic stainless steel through solid solution strengthening. Carbon can also improve the stress corrosion resistance of austenitic stainless steel in highly concentrated chloride (such as 42% MgCl2 boiling solution).
However, in austenitic stainless steel, carbon is often regarded as a harmful element, 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 steel in steel. Chromium forms high-chromium Cr23C6-type carbon compounds, which leads to the depletion of local chromium, which reduces the corrosion resistance of steel, especially the resistance to intergranular corrosion. Therefore, most of the newly developed chromium-nickel austenitic stainless steels since the 1960s are ultra-low carbon types with a carbon content of less than 0.03% or 0.02%. It can be known that as the carbon content decreases, the intergranular corrosion susceptibility of the steel decreases. Less than 0.02% has the most obvious effect, and some experiments also pointed out that carbon also increases the pitting corrosion tendency of chromium austenitic stainless steel. Due to the harmful effect of carbon, not only the carbon content should be controlled as low as possible in the smelting and smelting of austenitic stainless steel, but also in the subsequent hot, cold working and heat treatment processes to prevent the surface of stainless steel from increasing carbon, and chromium-free Carbide precipitation.
