What is the effect of carbon element on austenitic stainless steel?

Carbon is an element that strongly forms and stabilizes austenite and expands the austenite region of austenitic stainless steel. The ability of carbon to form austenite is about 30 times that of nickel. Carbon is an interstitial element. The strength of austenitic stainless steel can be significantly improved through solid solution strengthening. Carbon can also improve the stress corrosion resistance of austenitic stainless steel in high concentrations of chloride (such as 42% magnesium chloride boiling solution).

However, in austenitic stainless steel, carbon element is generally considered a harmful element. This is mainly due to the fact that under certain conditions in stainless steel and corrosion-resistant applications (such as welding or heating at 450~850°C), carbon will interact with steel Chromium forms a high chromium Cr23C6 type carbon compound, which leads to local chromium depletion, which leads to a reduction in the corrosion resistance of steel, especially the intergranular corrosion resistance. Therefore, since the 1960s, most of the newly developed chromium-nickel austenitic stainless steels are ultra-low carbon type with a carbon content of less than 0.03% or 0.02%.

It can be seen that the intergranular corrosion sensitivity of steel decreases as the carbon content decreases. When the carbon content is less than 0.02%, the effect is most obvious. Some experiments with pearlite also pointed out that carbon also increases the pitting tendency of chromium austenitic stainless steel. Due to the harmful effects of carbon, it is not only used in the smelting and smelting of austenitic stainless steel, but also prevents carbon enrichment on the surface of stainless steel, and precipitates chromium-free carbides during subsequent hot working, cold working, and heat treatment.