Differences between carbon steel and stainless steel

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Carbon steel refers primarily to steel whose mechanical properties depend on the carbon content in it, with no large amounts of alloying elements added; it is sometimes also called plain carbon steel or carbon steel. Carbon steel, also known as carbonaceous steel, refers to an iron-carbon alloy with a carbon content WC of less than 2%. In addition to carbon, carbon steel generally also contains small amounts of silicon, manganese, sulfur, and phosphorus. Based on their application, carbon steel can be divided into three categories: carbon structural steel, carbon tool steel, and free-cutting structural steel. Carbon structural steel is further divided into building structural steel and machinery manufacturing structural steel ； Based on the smelting method, it can be divided into open-hearth steel, converter steel, and electric-arc furnace steel ； Based on the deoxidization method, it can be divided into boiling steel (F), killed steel (Z), semi-killed steel (b), and specially killed steel (TZ) ； Based on carbon content, carbon steel can be divided into low-carbon steel (WC ≤ 0.25%), medium-carbon steel (WC 0.25%-0.6%), and high-carbon steel (WC > 0.6%) ； Based on phosphorus and sulfur content, carbon steel can be classified into ordinary carbon steel (with higher levels of phosphorus and sulfur), high-quality carbon steel (with lower levels of phosphorus and sulfur), ultra-high-quality steel (with even lower levels of phosphorus and sulfur), and special-grade high-quality steel. In general, the higher the carbon content in carbon steel, the greater its hardness and strength, but its ductility is lower. Stainless steel, also known as acid-resistant steel, is composed of two main types: stainless steel and acid-resistant steel. In simple terms, steel that can resist corrosion caused by the atmosphere is called stainless steel, while steel that can resist corrosion caused by chemical substances is called acid-resistant steel. Stainless steel is a high-alloy steel with an iron content of over 60%, to which alloying elements such as chromium, nickel, and molybdenum are added. When the chromium content in steel exceeds 12%, the steel is less prone to corrosion and rusting in air and dilute nitric acid. The reason is that chromium can form a very dense layer of chromium oxide on the surface of steel, effectively protecting it from corrosion. The chromium content in stainless steel is generally over 14%, but stainless steel is not completely rust-proof. In coastal areas or places with severe air pollution, where the chloride content in the air is high, the surfaces of stainless steel exposed to the atmosphere may develop some rust spots; however, these spots remain on the surface only and do not penetrate into the internal structure of the stainless steel. 　 Generally speaking, steels with a chromium content of Wcr greater than 12% possess the properties of stainless steel. Based on their microstructure after heat treatment, stainless steels can be divided into five categories: ferritic stainless steels, martensitic stainless steels, austenitic stainless steels, austenitico-ferritic stainless steels, and precipitate-hardening stainless steels. Stainless steels are generally classified according to their microstructure as follows: 1. Ferritic stainless steels. Contains 12%–30% chromium. Its corrosion resistance, toughness, and weldability increase as the chromium content rises, and its resistance to chloride stress corrosion is superior to that of other types of stainless steel. 2. Austenitic stainless steel. It contains more than 18% chromium, as well as about 8% nickel and small amounts of elements such as molybdenum, titanium, and nitrogen. It has excellent comprehensive performance and can resist corrosion by various media. 3. Austenite-ferrite duplex stainless steel. It combines the advantages of austenitic and ferritic stainless steels, as well as superplasticity. 4. Martensitic stainless steel. It has high strength, but poor plasticity and weldability.