II. Fastener Manufacturing Process (4)

2.2.8 Heat Treatment

Heat treatment is a comprehensive technological process in which materials are heated, held at temperature, and cooled within a specific medium to control their properties by altering the microstructure either on the surface or throughout the material.

The primary purpose of heat treatment for fasteners is to impart a good combination of mechanical properties to the workpiece through quenching and tempering (quenching + high-temperature tempering).

Annealing involves heating the workpiece to an appropriate temperature, holding it for a duration based on the material and workpiece size, and then cooling it slowly. The aim is to bring the internal microstructure of the metal to or near an equilibrium state, achieving favorable processing and service properties, or to prepare the microstructure for subsequent quenching.

Normalizing involves heating the workpiece to a suitable temperature and then cooling it in air. The effect is similar to annealing, but the resulting microstructure is finer. It is often used to improve the machinability of materials and sometimes serves as the final heat treatment for less demanding components.

Quenching involves heating the workpiece to temperature, holding it, and then rapidly cooling it in a quenching medium such as water, oil, or other inorganic salt or organic aqueous solutions. After quenching, the steel part becomes harder but also more brittle.

Tempering is performed to reduce the brittleness of steel parts. It involves heating quenched steel parts to an appropriate temperature above room temperature but below 650°C, holding them for an extended period, and then cooling them. This process is called tempering.

Annealing, normalizing, quenching, and tempering are the "four essential processes" of bulk heat treatment. Among them, quenching and tempering are closely related and are often applied together; both are indispensable.