Part 1: Introduction to Basic Knowledge     

Steel heat treatment is divided into ordinary heat treatment (annealing, normalizing, quenching, and tempering), and surface heat treatment (surface quenching: flame quenching, induction quenching; chemical heat treatment: carburizing, nitriding, carbonitriding, etc.). Heat treatment processes generally consist of three steps: heating, holding, and cooling.   

AnnealingIt is a heat treatment process in which steel is heated to an appropriate temperature, held for a certain time, and then slowly cooled (generally furnace cooling). Annealing reduces the hardness of parts, improves plasticity and toughness, and eliminates internal stress.      

NormalizingIt is a heat treatment process in which steel is heated to a temperature above the critical temperature for austenite transformation, held to ensure complete transformation to austenite, and then cooled in air. Normalizing refines the grain size, homogenizes the structure, and eliminates internal stress.      

QuenchingIt is a heat treatment process in which steel is heated to a temperature above the critical temperature for austenite transformation, held for a certain time, and then rapidly cooled at a rate greater than the critical cooling rate. Quenching increases the hardness and strength of parts and improves wear resistance.      

TemperingIt is a heat treatment process in which quenched steel is heated below the critical temperature for austenite transformation, held for a certain time, and then cooled to room temperature. Tempering is divided into low-temperature tempering, medium-temperature tempering, and high-temperature tempering, with tempering temperatures of150～250℃, 250～500℃, and 500～600℃, respectively. Tempering reduces internal stress in steel; as the tempering temperature increases, strength and hardness decrease, while plasticity and toughness increase.   

Surface QuenchingIt refers to a heat treatment process in which the steel surface is heated by flame or induction heating, generally used for medium carbon steel. The principle is shown in the simplified diagram below:

      Chemical Heat TreatmentIt refers to a heat treatment process in which steel is placed in an active medium at a certain temperature and held to allow one or more elements to diffuse into the surface of the steel, thereby changing its surface chemical composition, microstructure, and properties. The most common methods are carburizing and nitriding.     

Chemical heat treatment and surface quenching are the two main processes of surface heat treatment, both forhigh surface hardness, improved wear resistance and fatigue strength; low core hardness, good plasticity and toughness.     

 Part 2: Commonly Used Heat Treatment Processes for Tool Steel Parts     

Steel parts on power tools generally do not indicate hardness unless heat treatment is required; they usually only have the tempering hardness of the raw material. The most common heat treatment processes are overall quenching and tempering, carburizing quenching and tempering, and high-frequency induction quenching and tempering.   

Overall Quenchingand tempering heat treatment process, generally used for medium carbon steel or high carbon steel. Overall quenched and tempered parts generally have tensile stress on the surface, which is superimposed on the tensile stress caused by deformation, making them prone to fracture. The following are common overall quenching and tempering processes for commonly used materials. Due to many restrictive factors in heat treatment processes, the following table is for comparison only.     

There are also some overall quenching processes that control deformation or improve the metallographic structure level, such as vacuum overall quenching and salt bath quenching, which also have good applications in specific situations.      Carburizing Quenchingand tempering heat treatment process, generally used for low carbon steel, with commonly used materials including20Cr, 20CrMo, 20CrMnTi, 20CrNiMo, etc. Carburizing quenching and tempering parts generally have compressive stress on the surface, which offsets the tensile stress caused by deformation, reducing fracture; therefore, the fatigue resistance of carburizing quenching and tempering parts is generally superior to that of overall quenching and tempering parts.   The carburizing quenching and tempering process generally involves the chemical decomposition of an active medium at a certain temperature to precipitate active carbon atoms. For example, in gas carburizing, kerosene acts as an active medium, producing methane during high-temperature pyrolysis and releasing active carbon atoms. The concentration of carbon atoms determines the carbon potential in the furnace. The holding time is determined by the depth of the carburized layer.The following figure shows a typical carburizing quenching and tempering process route.   Nitriding follows the same principle, with ammonia acting as the active medium, releasing active nitrogen atoms during high-temperature pyrolysis. Nitriding can improve surface hardness, increase wear resistance, and improve fatigue resistance; microscopically, it refines the grain size and releases stress. Sometimes carburizing and nitriding are carried out simultaneously, i.e., carbonitriding.     

The inspection items required for carburizing quenched parts include surface hardness, core hardness, hardened layer depth (usually with HV550 as the critical point), and metallography (metallography is formulated according to different needs and different standards. This includes surface carbides, martensite, and retained austenite; core martensite and ferrite). In practice, more attention is paid to hardness, often neglecting the influence of the metallographic structure on strength.   

High-Frequency Induction Quenchingand tempering is a type of induction quenching and tempering, which is heated by an induction coil+ circulating water cooling heat treatment process. Induction quenching and tempering are divided into high-frequency induction quenching and tempering, medium-frequency induction quenching and tempering, and power frequency induction quenching and tempering, with corresponding frequencies and hardened layer depths: high frequency (200～300KHz, depth 0.5～2.5mm), medium frequency (0.5～10KHz, depth 2～10mm), power frequency (50Hz; depth 10～20mm). This process is simple, efficient, and has high surface hardness, but usually has a large amount of deformation, and is used for heat treatment of ordinary precision parts. The following is a schematic diagram of the heat treatment equipment.
In summary, in addition to the above-mentioned heat treatment processes for power tool parts, there are some less widely used heat treatment processes, such as solution treatment, aging treatment, and laser heat treatment. In practical applications, the appropriate material and heat treatment process should be selected based on performance requirements. In other words, once the material is selected, the heat treatment process is basically determined. Heat treatment is a very complex process, and the equipment and process levels vary greatly among different manufacturers. Choosing a good heat treatment plant will significantly improve efficiency.