The influence of carbon is mainly manifested in two aspects. On the one hand, it is an element that stabilizes austenite and has a great effect, which is equivalent to 30 times that of nickel; on the other hand, due to the strong affinity of carbon and chromium, it can form with chromium. A series of complex carbides, the composition of which varies with the chromium content in the steel. When the chromium content is less than 10%, it is mainly cementite carbide (Fe, Cr)₃C; in high-chromium steel, it is formed. Complex carbides (Cr, Fe)₇C₃ or (Cr, Fe)₂₃C₆.
Therefore, the higher the carbon content in steel, the lower its corrosion resistance. For stainless steel, the main purpose is to require corrosion resistance, so the carbon content of stainless steel is generally low, most of which is only 0.1%-0.2%, generally not more than 0.4%. Only in a few cases, such as rolling bearings , springs and cutting tools, due to the high hardness and wear resistance, the carbon content is increased to 0.85%-0.95% (such as 9Cr18 steel). However, in order to maintain a certain corrosion resistance, the chromium content of this type of steel is correspondingly higher.
