Four types of heat treatment for pulleys?

Quenching is a type of heat treatment used to harden metal. In simple terms, it involves heating metal (such as steel) to a high temperature (800 degrees Celsius) and maintaining that temperature for a certain period of time to ensure uniform internal structure (reaching the austenitic state), followed by rapid cooling by immersing it in water, oil, or saltwater. This rapid cooling causes the metal to form a harder crystalline structure (martensitic ultra-hard structure), significantly increasing hardness and strength. Imagine a soft iron block being heated and then quickly immersed in ice water—it hardens instantly, becoming as hard as steel but too brittle to withstand stress. Therefore, it is slightly reheated (tempering) to achieve both hardness and toughness.

After quenching, 45 steel pulleys achieve higher hardness and wear resistance.

Tempering (Tempering) Metaphor: It's like “pulling back” the brittle, hard component after quenching, “calming it down,” so it's both hard and slightly ductile.

This involves reheating the quenched steel to a temperature below the critical point (typically 150°C to 650°C), holding it at that temperature for a certain period, and then cooling it down (typically air cooling).

Purpose: To eliminate or reduce internal stresses caused by quenching, preventing deformation and cracking.

To reduce brittleness, enhance toughness and ductility, and stabilize the microstructure and working dimensions.

Under the premise of maintaining a certain hardness and strength, significantly improve toughness and plasticity to obtain a stable microstructure with excellent comprehensive mechanical properties. Tempering is the final critical step in heat treatment and determines the final properties of the workpiece.

The relationship between quenching and tempering: Quenching and tempering are an inseparable combination process. Quenching is to increase hardness, while tempering is to eliminate the negative effects of quenching and adjust the properties. The combination of the two is commonly referred to as “quenching and tempering.”

What is annealing?

Annealing involves heating metal (such as steel) above its critical temperature, holding it at that temperature for a period of time to homogenize the internal structure (reaching the austenitic state), and then cooling it slowly (typically by furnace cooling, which involves closing the furnace door and allowing the workpiece to cool along with the furnace). The cooling rate must be very slow to form a stable, uniform structure, refine the crystals, eliminate internal stresses, reduce hardness and strength, improve ductility and toughness, enhance machinability, and reduce the risk of cracking during subsequent processing.

What is normalizing?

A metaphorical analogy: like scooping rice out of a pot and letting it cool naturally in the air until it reaches a warm temperature, resulting in a moderate texture.

This involves heating the metal (such as steel) above the critical temperature, holding it for a period of time to ensure uniform internal structure, with heating temperatures similar to or slightly higher than those used in annealing. After holding, the workpiece is removed from the furnace and cooled naturally in still or flowing air.

The key difference between normalizing and annealing lies in the cooling rate. Normalizing can be viewed as “annealing with a faster cooling rate,” resulting in a finer microstructure than annealing. Consequently, the strength, hardness, and toughness of the normalized state are slightly higher than those of the annealed state.