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00 Introduction Many parts operate under alternating loads such as torsion and bending; sometimes they are also subjected to friction, impact, etc. Therefore, it is necessary for the part’s surface to have sufficient strength, hardness, and wear resistance. At the same time, the core of the part must possess adequate plasticity and toughness. This led to the development of surface heat treatment techniques. 01 Surface quenching: Surface quenching involves using rapid heating to austenitize only the surface of the workpiece, followed by quenching. It only alters the structure and properties of the surface layer, strengthening and hardening it, while the core retains its original structure and properties; it is commonly used in medium-carbon quenched and tempered steel. There are many heating methods for surface quenching, including induction heating, flame heating, laser heating, and ion beam heating. Here, induction heating is mainly introduced. Induction hardening involves placing the workpiece inside an inductor (coil). When an alternating current of a certain frequency is passed through the inductor, an alternating magnetic field is generated, which induces a closed induced current (i.e., eddy currents) within the workpiece. The distribution of the induced current across the cross-section of the workpiece is highly uneven; the current density is very high at the surface and decreases gradually as one moves inward. This phenomenon is known as the skin effect. The electrical energy of the high-density current in the surface layer of the workpiece is converted into thermal energy, raising the surface temperature and thereby achieving surface heating. The higher the current frequency, the greater the difference in current density between the surface and the interior of the workpiece, and the thinner the heating layer becomes. Surface quenching can be achieved by rapidly cooling (the quenching medium is usually water or an aqueous solution of polymers) once the temperature of the heating layer exceeds the critical temperature of the steel. Based on the frequency of the alternating current, induction hardening treatment can be divided into five categories: ultra-high frequency, high frequency, ultrasonic frequency, medium frequency, and power frequency. Ultra-high frequency induction quenching uses a current frequency as high as 27 MHz, resulting in an extremely thin quenched layer of only about 0.15 mm, which makes it suitable for surface quenching of thin layers on workpieces with complex shapes. The current frequency used for high-frequency induction hardening is typically 200 kHz to 300 kHz, with a hardening depth of 0.5 mm to 2 mm; it can be used for surface hardening of components such as gears, cylinder liners, cams, and shafts. The current frequency used for ultrasonic induction quenching is generally between 20 kHz and 40 kHz. Small-module gears are heated using this ultrasonic induction current, with the heated zone distributed roughly along the tooth profile, resulting in good mechanical properties after quenching. The current frequency used for medium-frequency induction hardening is generally between 2.5 kHz and 10 kHz, with a hardening depth of 2 mm to 8 mm. It is primarily used for the surface hardening of workpieces such as gears with large module numbers, shafts of large diameter, and cold-rolling rolls. The current frequency used for power-frequency induction hardening is 50Hz–60Hz, with a hardening depth of 10mm–15mm; it is primarily used for the penetrative heating of large-diameter steel materials and for workpieces that require a deeper hardened layer. 03 The main advantages of induction hardening are: ① No need for overall heating, resulting in minimal deformation of the workpiece and low power consumption. ②It has a fast heating rate, resulting in minimal oxidation and decarburization of the workpiece surface. ③The surface hardened layer can be adjusted as needed and is easy to control. ④Heating equipment can be installed on machining production lines, facilitating mechanization and automation. It helps reduce transportation needs, saves labor costs, and improves productivity. ⑤The martensite structure in the hardened layer is fine, resulting in high hardness, strength, and toughness. ⑥After surface quenching, the surface layer of the workpiece has high compressive internal stresses, resulting in improved fatigue resistance. The disadvantages of induction hardening are that the induction heating equipment is complex and has poor adaptability; it is difficult to ensure quality for parts with complex shapes. Induction hardening is widely used for surface hardening of components such as gears, shafts, crankshafts, cams, and rolls, with the aim of improving their wear resistance and fatigue resistance. The rear axle of the vehicle is subjected to induction hardening, which increases the number of fatigue cycles at the design load by approximately 10 times compared to quenching and tempering. The workpiece material for induction hardening is generally medium-carbon steel. To meet the special requirements of certain workpieces, low hardenability steels specifically designed for induction hardening have been developed. Workpieces made of high-carbon steel and cast iron can also be subjected to induction hardening.