Generally, the carbon content of industrial cast iron is 2% - 4%. Carbon in cast iron mostly exists in the form of graphite, and sometimes in the form of cementite. In addition to carbon, cast iron also contains 1% - 3% silicon, as well as manganese, phosphorus, sulfur and other elements.
Alloy cast iron also contains nickel, chromium, molybdenum, aluminum, copper, boron, vanadium and other elements. Carbon and silicon are the main elements that affect the microstructure and properties of cast iron.
Cast iron can be divided into:
① gray cast iron. The carbon content is high (2.7% - 4.0%), and the carbon mainly exists in the form of flake graphite. The fracture is gray, which is referred to as gray iron for short. Low melting point (1145 ~ 1250 ℃), small shrinkage during solidification, compressive strength and hardness close to carbon steel, and good shock absorption. Used for manufacturing machine tool bed, cylinder, box and other structural parts.
② White cast iron. The content of carbon and silicon is low, carbon mainly exists in the form of cementite, and the fracture is silver white. Shrinkage is large during solidification, and shrinkage cavities and cracks are easy to occur. High hardness and brittleness, unable to bear impact load. It is often used as a blank of malleable cast iron and to make wear-resistant parts.
③ Malleable iron. Obtained from annealed white cast iron, graphite is distributed in flocculent form, which is called ductile iron for short. It has uniform microstructure and properties, wear resistance, good plasticity and toughness. It is used to manufacture parts with complex shape and can bear strong dynamic load.
④ Ductile iron. Gray cast iron is obtained by spheroidizing molten iron, and the precipitated graphite is spherical, which is called nodular iron for short. It has higher strength, better toughness and plasticity than ordinary gray cast iron. It is used for manufacturing internal combustion engines, auto parts and agricultural machinery.
⑤ Vermicular cast iron. Gray cast iron is obtained by vermiculation treatment, and the precipitated graphite is vermicular. The mechanical properties are similar to that of nodular cast iron, and the casting properties are between gray cast iron and nodular cast iron. Used to make auto parts.
⑥ Alloy cast iron. Ordinary cast iron is obtained by adding an appropriate amount of alloy elements (such as silicon, manganese, phosphorus, nickel, chromium, molybdenum, copper, aluminum, boron, vanadium, tin, etc.). Alloy elements change the matrix structure of cast iron, which has the corresponding characteristics of heat resistance, wear resistance, corrosion resistance, low temperature resistance or non-magnetic. It is used for manufacturing parts of mining, chemical machinery, instruments, meters, etc.
Steel for casting. A kind of casting alloy. Cast steel is divided into cast carbon steel, cast low alloy steel and cast special steel.
Cast carbon steel. Cast steel with carbon as the main alloying element and containing a small amount of other elements. Cast low carbon steel with carbon content less than 0.2%, cast medium carbon steel with carbon content of 0.2% - 0.5%, and cast high carbon steel with carbon content greater than 0.5%. With the increase of carbon content, the strength and hardness of cast carbon steel increase. Cast carbon steel has high strength, plasticity and toughness and low cost. It is used in heavy machinery to manufacture parts that bear heavy loads, such as rolling mill frame, hydraulic press base, etc; It is used in railway vehicles to manufacture parts that bear heavy stress and impact, such as bolster, side frame, wheel and coupler.
Cast low alloy steel. Cast steel containing manganese, chromium, copper and other alloy elements. The total amount of alloy elements is generally less than 5%, with greater impact toughness, and better mechanical properties can be obtained through heat treatment. Cast low alloy steel has better service performance than carbon steel, which can reduce the quality of parts and improve the service life.
Cast special steel. There are many kinds of alloy cast steel refined to meet special needs, which usually contain one or more high-volume alloy elements to obtain some special properties. For example, high manganese steel containing 11% - 14% manganese can resist impact wear, and is mostly used for wear-resistant parts of mining machinery and engineering machinery; Various stainless steels with chromium or chromium nickel as the main alloy element are used for parts that work under corrosive or high temperature conditions above 650 ℃, such as valve bodies for chemical industry, pumps, vessels, or turbine casings of large capacity power stations.
