Unlock Peak Performance: Your Guide to Precision Heat Treatment Processes

Heat treatment is a metal hot working process where materials, in a solid state, are subjected to heating, holding (soaking), and cooling to achieve desired microstructures and properties.

I. Heat Treatment Processes

1.Normalizing: A heat treatment process where steel or steel parts are heated to a temperature above the critical point AC3 or ACM, held for a specific time, and then cooled in air to obtain a pearlite-type microstructure.

2.Annealing: A heat treatment process for hypoeutectoid steel workpieces. They are heated to 20-40°C above AC3, held for a period, then slowly cooled (in a furnace, sand, or lime) to below 500°C, followed by air cooling.

3.Solution Heat Treatment: An alloy is heated to a high-temperature single-phase region and held to fully dissolve excess phases into the solid solution, then rapidly cooled to obtain a supersaturated solid solution.

4.Aging: The phenomenon where the properties of an alloy change over time after solution heat treatment or cold plastic deformation, either at room temperature or slightly above.

5.Solution Treatment: Fully dissolves various phases in an alloy, strengthening the solid solution, improving toughness and corrosion resistance, relieving stress, and softening the material for further processing.

6.Aging Treatment: Heating and holding at a temperature where strengthening phases precipitate, causing hardening and increased strength.

7.Quenching: A process where steel is austenitized and then cooled at an appropriate rate, causing all or a specific portion of the workpiece's cross-section to transform into unstable structures like martensite.

8.Tempering: A process where a quenched workpiece is heated to an appropriate temperature below the critical point AC1, held for a time, and then cooled by a required method to obtain the desired structure and properties.

9.Carbonitriding of Steel: The simultaneous diffusion of carbon and nitrogen into the surface layer of steel. Commonly called cyaniding, with medium-temperature and low-temperature gas carbonitriding (gas soft nitriding) being widely used. Medium-temperature aims to increase hardness, wear, and fatigue resistance. Low-temperature primarily focuses on improving wear and seizure resistance.

10.Quenching and Tempering : Commonly refers to the combined process of quenching followed by high-temperature tempering. Widely used for critical structural components like connecting rods, bolts, gears, and shafts under alternating loads. It produces a tempered sorbite structure with superior mechanical properties compared to normalized sorbite of the same hardness. Hardness typically ranges from HB200–350, depending on tempering temperature, steel's tempering stability, and part dimensions.

11.Brazing: A heat treatment process where two workpieces are joined by melting a filler metal (brazing alloy).

II. Process Overview:
A heat treatment process generally includes three stages: heating, holding (soaking), and cooling, sometimes just heating and cooling. These stages are interconnected and continuous.

Heating is a critical step. When heated in air, workpieces often oxidize and decarburize (surface carbon loss in steel), adversely affecting post-treatment surface properties. Therefore, heating is usually done in controlled/protective atmospheres, molten salt, or vacuum, or using protective coatings/packaging.

Heating temperature is a key parameter, and its selection and control are major factors in ensuring quality. It varies with material and treatment goal but generally exceeds phase transformation temperatures to obtain high-temperature structures. Additionally, transformation requires time. Once the surface reaches the target temperature, it must be held there to equalize internal/external temperature and complete microstructural transformation—this is the soaking time. With high-energy-density or surface heat treatments, heating is extremely fast, often eliminating soaking. Chemical heat treatments typically require longer soaking times.

Cooling is an indispensable step, with methods varying by process, primarily controlling the cooling rate. Generally, annealing has the slowest cooling, normalizing is faster, and quenching is fastest. However, requirements differ by steel type; for example, air-hardening steels can achieve full hardness at cooling rates similar to normalizing.