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Heat treatment of steel: It is a process in which solid steel is heated, held at a certain temperature, and cooled in appropriate ways to achieve the desired microstructure and properties. Heat treatment can be used not only to strengthen steel and improve the performance of mechanical parts, but also to enhance the processability of steel. Their common feature is that only the internal organizational structure is changed, while the surface shape and size remain unchanged. Heat treatment processes can significantly improve the mechanical properties of steel, enhancing the strength, toughness, and service life of components, as well as increasing their hardness and wear resistance. Therefore, important machine parts and tools must undergo heat treatment. Heat treatment can also improve the machinability of workpieces, thereby enhancing productivity and machining quality. Therefore, heat treatment plays a very important role in the machinery manufacturing industry. Here, 45 steel and 40Cr steel are taken as examples. In manufacturing, the process of quenching followed by high-temperature tempering is known as \"quenching and tempering\". Parts that undergo this treatment possess excellent overall mechanical properties, and they are widely used in various important structural components, especially those that are subjected to alternating loads such as connecting rods, bolts, gears, and shafts. However, its surface hardness is low, and it is not wear-resistant. The surface hardness of parts can be improved through quenching and tempering + surface hardening. 1. 45 steel – a high-quality medium-carbon structural steel. It is referred to as 45 steel in GB standards; in JIS standards it is called S45C, while in ASTM standards it is known as 1045 and 080M46. In DIN standards it is designated as C45 ； 45 steel is a high-quality carbon structural steel. Its chemical composition is as follows: the carbon (C) content ranges from 0.42% to 0.50%, the Si content is between 0.17% and 0.37%, the Mn content is 0.50% to 0.80%, and the Cr content