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The steel used to manufacture carburized parts is called carburizing steel. The main heat treatment steps for carburized steel are generally quenching and low-temperature tempering after carburization. The core of the treated part is a low-carbon martensite structure with sufficient strength and toughness, while the surface consists of hardened and wear-resistant tempered martensite along with a certain amount of fine carbide structures. Some structural components operate under conditions of severe impact and wear, such as the gearbox gears in cars and tractors, as well as the cams and piston pins in internal combustion engines. Depending on the working conditions, these parts are required to have high surface hardness and wear resistance, while the core is required to possess high strength and appropriate toughness; in other words, the workpiece is needed to have the property of being hard on the surface and tough inside. To achieve both of these properties, low-carbon steel can be used, with carburizing and quenching followed by low-temperature tempering. In this case, the core of the part has a quenched structure of low-carbon steel, ensuring high toughness and sufficient strength, while the surface layer (up to a certain depth) contains a high carbon content (0.85%–1.05%); after quenching, it achieves very high hardness (HRC>60), resulting in good wear resistance. 02 Composition characteristics of carburized steel: The carbon content in carburized steel is generally low (between 0.15% and 0.25%), classifying it as low-carbon steel. Such a carbon level ensures that the core of the carburized parts possesses good toughness and ductility. To improve the strength of the core portion of steel, a certain amount of alloying elements such as Cr, Ni, Mn, Mo, W, Ti, B, etc., can be added to the steel. Among them, alloying elements such as Cr, Mn, and Ni mainly serve to increase the hardenability of steel, thereby strengthening both the surface and core structures after quenching and low-temperature tempering. In addition, small amounts of carbide-forming elements such as Mo, W, and Ti can form stable alloy carbides, which help to refine the grain structure and prevent overheating of the steel during carburizing. Trace amounts of B (0.001%–0.004%) can significantly increase the hardenability of alloy carburized steel. Classification of carburizing steel: Based on differences in hardenability or strength grades, alloy carburizing steel is divided into three categories. 1) Low hardenability alloy carburizing steels, namely low-strength carburizing steels (tensile strength ≤ 800 MPa), such as 15Cr, 20Cr, 15Mn2, 20Mn2, etc. These steels have low hardenability; after carburizing, quenching, and low-temperature tempering, they exhibit low strength in the core area, as well as a poor balance between strength and toughness. It is mainly used to manufacture wear-resistant parts that are subject to low stress and have modest strength requirements, such as diesel engine cam shafts, piston pins, sliders, and pinions. When carburized, the grain size in the core of such steels tends to grow, especially in manganese steel. When higher performance is required, this type of steel is often subjected to a secondary quenching process after carburizing; that is, normalizing is performed first after carburizing to eliminate the overheated structure formed during carburizing, followed by reheating and quenching. 2) Medium hardenability alloy carburizing steels, namely medium-strength carburizing steels (tensile strength = 800–1200 MPa), such as 20CrMnTi, 12CrNi3A, 20CrMnMo, 20MnVB, etc. These steels contain approximately 4% in total of alloying elements; by adding Cr and Mn together to the steel, its hardenability and mechanical properties can be improved more effectively (tensile strength = 1000–1200 MPa). It is generally used to manufacture medium and small wear-resistant parts designed for heavy loads, as well as gears with a large module designed for moderate loads. Such as the gearboxes and rear axle gears of automobiles and tractors, gear shafts, cross-pin heads, spline bushings, valve seats, cam discs, etc. Due to the presence of Ti, V, and Mo, these steels exhibit a low tendency for austenite grain growth during carburizing; therefore, they can be quenched directly after pre-cooling to around 870°C at the self-carburizing temperature, and subsequent low-temperature tempering enables the parts to achieve good mechanical properties. 3) High hardenability alloy carburizing steels, namely high-strength carburizing steels (tensile strength > 1200 MPa), such as 12Cr2Ni4, 18Cr2Ni4WA, etc. These types of steel have an overall alloy content of ≤7.5%. Due to the high presence of Cr and Ni elements, they can **significantly improve the hardenability of the steel; in particular, the addition of a substantial amount of Ni not only enhances strength but also gives the steel good toughness. This type of steel can be used for important large-scale components that are subject to heavy loads and severe wear, such as the driving traction gears in diesel locomotives, diesel engine crankshafts, connecting rods, and precision bolts for cylinder heads. Due to the high content of alloying elements, the C curve shifts significantly to the right; as a result, a martensitic structure can be obtained even when cooled in air ； Furthermore, its martensite transformation temperature also drops sharply, causing the carburized surface layer to retain a large amount of residual austenite after quenching. To reduce the amount of residual austenite after quenching, high-temperature tempering can be performed prior to quenching to spheroidize the carbides, or cold treatment can be used after quenching. 03 Several heat treatment methods for carburizing. The heat treatment processes for carburized steel include preparatory heat treatment and carburizing quenching processes, with the heat treatment options comprising normal normalization, isothermal normalization, normalization + tempering, and isothermal annealing. Carburizing and quenching mainly involves direct quenching after pre-cooling following carburizing, single quenching after air cooling following carburizing, or double quenching after air cooling following carburizing; tempering is then performed after carburizing and quenching. The currently commonly used standard for gear steel is JB/T7516-1994 \"Heat Treatment Processes and Quality Control for Gear Gas Carburizing\". During the carburizing process, by controlling factors such as the surface carbon content, the morphology and distribution of carbides and residual austenite in the microstructure, the surface hardness gradient, and the depth of the effective carburized layer, it is possible to achieve the best quality of the carburized layer and minimize deformation, thereby improving the quality of the gears. Carburizing can only alter the chemical composition of the surface of a part; to achieve a structure that is hard on the outside and tough on the inside, carburizing treatment must be followed by quenching and low-temperature tempering in order to improve the strength and toughness of the steel as well as to stabilize the dimensions of the part. Depending on the composition, shape, and mechanical properties of the workpiece, the following heat treatment methods are commonly used after carburizing. 1) Direct quenching + low-temperature tempering: The part is taken out of the heat treatment furnace for direct quenching, followed by tempering to achieve the desired hardness in the surface. There are two conditions for direct quenching: the austenite grain size after carburizing heat treatment must be at level 5-6 or higher ； There are no obvious networked or massive carbides in the carburized layer. Steels such as 20CrMnTi are mostly quenched directly after carburizing. 2) Pre-cooling followed by direct quenching + low-temperature tempering. The purpose of pre-cooling is to reduce part deformation and to decrease the residual austenite on the surface due to the precipitation of carbides. Pre-cooling followed by direct quenching results in a slight increase in surface hardness, but no change in the grain structure. The pre-cooling temperature should be higher than Ar3 to prevent the formation of ferrite in the core; however, if the temperature is too high, it affects the precipitation of carbides during pre-cooling, leading to an increase in residual austenite, as well as greater quenching deformation. 3) Single heating quenching + low-temperature tempering: The carburized part is rapidly cooled to room temperature and then reheated for quenching and low-temperature tempering; this method is suitable for parts that require high strength and good toughness in their core area after quenching. 4) High-temperature tempering + quenching + low-temperature tempering: After high-temperature tempering, the residual austenite decomposes, and carbon and alloying elements in the case precipitate in the form of carbides; this facilitates mechanical processing while reducing the amount of residual austenite. This method is mainly used for Cr-Ni alloy steel components. 5) Secondary quenching + low-temperature tempering: After cooling the workpiece to room temperature, it is quenched twice, followed by low-temperature tempering. This is a heat treatment method that ensures high performance both in the core and on the surface; two quenching steps help reduce the amount of residual austenite on the surface. 6) Secondary quenching + cold treatment + low-temperature tempering is also known as a heat treatment method for reducing the amount of residual austenite in the surface layer of high-alloy steels; it is commonly used for gears and shaft components.