We all know that solid metals and alloys are all crystals, that is, the atoms in them are arranged according to certain rules, and there are generally three arrangements: body-centered cubic lattice structure, surface The centered cubic lattice structure and the close-packed hexagonal lattice structure. Metals are composed of polycrystals, and their polycrystalline structures are formed during the crystallization of metals.
The iron constituting the iron-carbon alloy has two lattice structures: α-iron with a body-centered cubic lattice structure below 910 °C, and γ-iron with a face-centered cubic lattice structure above 910 °C. If carbon atoms are squeezed into the lattice of iron without destroying the lattice structure of iron, such a substance is called a solid solution.
The solid solution formed by dissolving carbon into α-iron is called ferrite, and its ability to dissolve carbon is extremely low, and the maximum solubility does not exceed 0.02%. The solid solution formed by dissolving carbon into γ-iron is called austenite, and its carbon-dissolving ability is high, up to 2%.
Austenite is the high temperature phase of iron-carbon alloys. The austenite formed by the steel at high temperature becomes unstable supercooled austenite when it is supercooled below 727℃. If it is supercooled to below 230°C at a very large cooling rate, the carbon atoms in the austenite have no possibility of diffusion, and the austenite will directly transform into a carbon-containing supersaturated α solid solution, called martensite. . Due to the supersaturation of carbon content, the strength and hardness of martensite are increased, the plasticity is decreased, and the brittleness is increased.
The corrosion resistance of stainless steel mainly comes from chromium. Experiments have shown that the corrosion resistance of steel will be greatly improved only when the chromium content exceeds 12%. Therefore, the chromium content in stainless steel is generally not less than 12%. Due to the increase of the chromium content, it also has a great influence on the structure of the steel. When the chromium content is high and the carbon content is low, chromium will balance the iron and carbon, and the γ phase region will shrink or even disappear. This stainless steel is a ferrite structure. The structure, which does not undergo phase transformation when heated, is called ferritic stainless steel.
When the chromium content is low (but higher than 12%) and the carbon content is high, when the alloy is cooled from high temperature, it is easy to form martensite, so this type of steel is called martensitic stainless steel.
Nickel can expand the γ phase region and make the steel have austenite structure. If the nickel content is high enough to make the steel also have austenitic structure at room temperature, the steel is called austenitic stainless steel.
