The following provides a detailed introduction to these four heat treatment processes:

1.  Annealing: The workpiece is heated above its critical temperature, held at that temperature for a

period, and then slowly cooled. Its primary purposes are to reduce hardness, improve machinability, refine grain structure, homogenize the microstructure, and eliminate residual stresses to stabilize dimensions. Annealing also prepares the microstructure for subsequent heat treatments.

2. Normalizing: The workpiece is heated above its critical temperature, held, and then cooled in air. Its effects include refining the microstructure, increasing the material's strength, hardness, and toughness, eliminatingoverheated structures and Widmanstätten structures, improving machinability. It can serve as either a final heat treatment or a preparatory treatment.

3.Quenching:The workpiece is heated above its critical point, held, and then rapidly cooled. Quenching significantly increases the material's hardness and wear resistance, improves its strength and toughness, and can impart specific physical and chemical properties. Materials after quenching usually require tempering.

4.Tempering: After quenching, the workpiece is heated to a temperature below its critical temperature, held, and then cooled. The purposes of tempering are to reduce brittleness, eliminate internal stresses, adjust mechanical properties, and stabilize the microstructure and dimensions.Depending on the tempering temperature, it can be categorized as low-temperature, medium-temperature, or high-temperature tempering to meet different performance requirements.

Summary:

Annealing: A heat treatment process where the workpiece is slowly heated in a furnace to a temperature above the critical point, held for a time, and then slowly cooled with the furnace (Furnace cooling).

Normalizing: Involves heating the workpiece and then cooling it in air after removing it from the furnace (Air cooling).

Quenching: Involves heating the workpiece to the quenching temperature (typically 30-50°C above the critical point), holding it for a period, and then rapidly cooling it by immersing it in a quenching medium (e.g., water, oil, polymer).

Tempering: A process performed after quenching, involving heating the workpiece to a lower temperature (below the critical point) followed by cooling.

Quenching and Tempering (Q&T / Conditioning):This process involves performing high-temperature tempering (typically 500~600°C) on a steel part after quenching. It is widely used for critical structural components like shafts, gears, connecting rods, etc. Q&T is generally performed after rough machining. Purpose: To refine the grain structure, enabling the steel part to achieve high toughness combined with sufficient strength, resulting in good overall mechanical properties.