Steel, also known as iron carbon alloy, is a binary alloy composed of iron and carbon. The most commonly used type of iron-based materials - carbon steel and cast iron - is an industrial iron-carbon alloy material. The reason for the wide range of application of steel materials is firstly due to the wide range of available components, from nearly carbon free industrial pure iron to cast iron containing about 4% carbon. Within this range, the phase structure and microstructure of the alloy undergo significant changes; In addition, various hot working processes, especially metal heat treatment techniques, can be used to significantly change the microstructure and properties of a certain component of the alloy. Steel testing items: The formation of alloy phases in iron-carbon alloys is related to the crystal structure of pure iron and the form of carbon present in the alloy. Pure iron has three isomeric states: below 912 ℃, it has a body centered cubic crystal structure, called α- Fe; 912-1394 ℃ is a face centered cubic crystal structure, known as γ- Fe; Above 1394 ℃, also in a body centered cubic structure, known as δ- Fe. In the liquid state, carbon and iron can completely dissolve in the range of less than 7% carbon; In solid state, the dissolution of carbon in iron is limited, and its solubility depends on the crystal structure of iron (solvent). Corresponding to the three isomers of iron, there are three solid solutions formed by carbon in iron: α Solid solution (ferrite) γ Solid solution (austenite) and δ Solid solution (8 ferrite). The spatial distribution of iron atoms in these solid solutions is related to α- Fe γ- Fe and δ- Fe is consistent, and the size of carbon atoms is much smaller than that of iron atoms. In the solid solution, it is located in the gap position of the lattice, causing lattice distortion. Carbon in γ- The solubility in Fe is the highest, but not exceeding 2.11%; Carbon in α- The solubility in Fe shall not exceed 0.0218%; And in the δ Not exceeding 0.09% in 6-Fe. When the carbon content of an iron-carbon alloy exceeds its solubility in iron, the excess carbon can exist in the form of iron carbides or in a elemental state (graphite) in the alloy, forming a series of carbides. Among them, Fe3C (cementite, 6.69% C) is a metastable phase, which is a interstitial compound with complex structures. Graphite is a stable equilibrium phase of iron carbon alloys, with a simple hexagonal structure. Fe3C may decompose into stable phases of iron and graphite, but this process is extremely slow at room temperature. Steel inspection standard: Mild steel with a content of less than 0.25% C, especially 08F, 08Al, etc. with a carbon content of less than 0.10%, is widely used for deep drawing parts such as automobiles and tank making due to its excellent deep drawing and weldability. 20G is the main material for manufacturing ordinary boilers. In addition, low-carbon steel is also widely used as carburized steel in the mechanical manufacturing industry