Common heat treatment processes include normalizing, annealing, solid-solution treatment, aging, quenching, tempering, stress relief annealing, carburizing, nitriding, tempering and normalization, spheroidizing, and brazing, among others.

1. Normalizing: A heat treatment process in which steel or steel components are heated to an appropriate temperature above the critical points AC3 or ACM, held at that temperature for a specified period, and then cooled in air to obtain a pearlitic microstructure.

2. Annealing: A heat treatment process in which a hypoeutectoid steel workpiece is heated to a temperature 20–40 degrees above AC3, held at that temperature for a certain period, and then slowly cooled in the furnace (or cooled by burying it in sand or lime) down to below 500 degrees before being air-cooled.

3. Solid-solution heat treatment: This heat treatment process involves heating the alloy to a high temperature within the single-phase region and holding it at that temperature long enough to ensure that any excess phases are fully dissolved into the solid solution. The alloy is then rapidly cooled to obtain a supersaturated solid solution.

4. Time Sensitivity: The phenomenon in which the properties of an alloy—after solid-solution heat treatment or cold plastic deformation—change over time when the alloy is kept at room temperature or slightly above room temperature.

5. Solution Treatment: This process ensures the complete dissolution of various phases within the alloy, thereby strengthening the solid solution and enhancing both toughness and corrosion resistance. It also relieves internal stresses and softens the material, making it easier for subsequent forming and machining operations.

6. Ageing Treatment: Heat and hold the material at a temperature conducive to the precipitation of strengthening phases, allowing these phases to precipitate and harden, thereby increasing the material’s strength.

7. Quenching: A heat treatment process in which steel is austenitized and then cooled at an appropriate cooling rate, causing the workpiece to undergo a transformation into unstable microstructures such as martensite throughout its cross-section or within a specified range.

8. Tempering: A heat treatment process in which a quenched workpiece is heated to an appropriate temperature below the critical point AC1, held at that temperature for a specified period, and then cooled using a method that meets the required specifications, thereby achieving the desired microstructure and properties.

9. Carbon-Nitrogen Co-diffusion in Steel: Carbon-nitrogen co-diffusion is a process in which both carbon and nitrogen are simultaneously diffused into the surface layer of steel. Traditionally, carbon-nitrogen co-diffusion is also referred to as cyaniding. Among these processes, medium-temperature gas carbon-nitrogen co-diffusion and low-temperature gas carbon-nitrogen co-diffusion (also known as gas soft nitriding) are widely used. The primary purpose of medium-temperature gas carbon-nitrogen co-diffusion is to enhance the hardness, wear resistance, and fatigue strength of steel. In contrast, low-temperature gas carbon-nitrogen co-diffusion focuses mainly on nitrogen diffusion, with its primary goal being to improve the wear resistance and galling resistance of steel.

10. Quenching and Tempering: The heat treatment process that combines quenching with high-temperature tempering is commonly referred to as quenching and tempering. This treatment is widely used for various critical structural components, especially those subjected to alternating loads, such as connecting rods, bolts, gears, and shafts. After quenching and tempering, the microstructure obtained is tempered sorbite, which exhibits superior mechanical properties compared to normalized sorbite of the same hardness. The hardness of the tempered sorbite depends on the high-temperature tempering temperature and is influenced by the steel’s tempering stability and the cross-sectional dimensions of the workpiece; typically, it ranges between HB200 and 350.

11. Brazing: A heat treatment process in which two workpieces are heated and melted together using a filler metal to achieve bonding.