Cementite, also known as iron carbide, is a compound of iron and carbon with the molecular formula of Fe3C (or Fe2C: Fe). According to the weight, it is composed of 6.67% carbon and 93.3% iron. Its lattice is orthogonal crystal structure, its plasticity and toughness are almost zero, and its brittleness is very large. It is common in ordinary carbon steel and cast iron. In white cast iron, it can be obtained directly after solution cooling. In carbon steel, it can be formed in Austenite Cooling and martensite tempering. Cementite has thermal instability and can eventually be transformed into ferritic iron and graphite
Graphite is composed of carbon and is an allotrope of elemental carbon (the same element has different physical and chemical properties due to different arrangement). Graphite can be considered the highest grade coal
Carbon in cast iron exists in the form of cementite or graphite. According to the existing forms of carbon, it can be divided into several types:
1) White cast iron refers to the cast iron in which carbon appears in the form of free carbide, and the fracture is white. Because of its high hardness and brittleness, it is processed by cutting, so it is rarely directly used to make mechanical parts, and it is often used as wear-resistant materials.
2) Gray cast iron refers to the cast iron whose carbon mainly appears in the form of graphite, and the fracture is gray. According to the form of graphite, gray cast iron can be divided into the following types:
Gray cast iron, carbon is mainly in the form of flake graphite.
Nodular cast iron, cast iron in which carbon occurs mainly in the form of spherical graphite.
Malleable cast iron, carbon is mainly in the form of flocculent graphite.
Vermicular graphite cast iron, carbon is mainly in the form of vermicular graphite.
3) Hemp cast iron refers to the cast iron whose carbon part appears in the form of cementite and part of graphite. The fracture is gray white. It is rarely used in industry because of its hardness and brittleness.
In addition, in order to make cast iron obtain special properties such as wear resistance, corrosion resistance and heat resistance, some alloy elements (such as chromium, copper, titanium, aluminum, dilute, etc.) are often added to cast iron, which is called alloy cast iron.