Introduction to Heat Treatment

1. Basic principle

The fundamental principle of heat treatment is to utilize phase transitions and structural changes in materials at different temperatures to alter their internal structure and properties. These changes typically involve the following steps:

Procedure :

1.Heating: The material is heated to a specific temperature to initiate changes in its internal structure.

2.Soaking: The material is held at this temperature for a certain period to allow the changes to proceed.

3.Cooling: The material is cooled at a controlled rate to achieve the desired properties.

2. Major Heat Treatment Methods

Annealing: The metal is heated to a certain temperature and then slowly cooled to eliminate internal stresses, increase ductility, and reduce hardness. Annealing is typically used to soften the material, making it easier to machine and form.
Normalizing: The metal is heated above its critical temperature and then allowed to cool naturally in air. This process results in a more uniform grain structure and improves the material’s mechanical properties.
Quenching: Rapid cooling of metal to increase its hardness and strength. However, quenched materials often become very hard but brittle, so tempering is often required afterward.
Tempering: After quenching, the metal is reheated to a lower temperature and held for some time to reduce brittleness and increase toughness.
Aging: The metal is heated and held at a lower temperature for a specific period to harden and stabilize certain alloys, commonly used in aluminum and stainless steel.

3. Benefits of Heat Treatment

Improved Material Properties: Heat treatment can significantly enhance the mechanical properties of metals, such as strength, hardness, and toughness.
Stress Control: It helps to reduce or eliminate internal stresses in the material, preventing cracking or deformation during subsequent processing.
Cost Optimization: Proper heat treatment can achieve the desired properties of the material, reducing the need for additional processing steps and lowering production costs.

4. Deformation problem

Heat treatment involves processes such as quenching, tempering, annealing, and normalizing, during which a phenomenon called “phase transformation” occurs. Phase transformation refers to the changes in the crystal lattice and microstructure (organization) of metals due to temperature variations, causing the metal to transition between solid and liquid states, and vice versa. When the microstructure changes due to phase transformation, the volume also changes, leading to deformation.

Quenching Cracks

This is a phenomenon of material cracking. It may also occur when cooling to below about 200°C after quenching, or the next day after quenching.

Application of Heat Treatment

Common Application

Heat treatment is an essential part of metalworking, as it can greatly improve the physical and mechanical properties of materials, enhancing the quality and lifespan of the final product.

Industrial bearings typically undergo quenching and tempering to enhance their hardness and wear resistance, which helps extend their service life. Bearings operate under high speeds and heavy loads, so surface hardening is essential to withstand friction and stress.

Gears are critical components in mechanical transmissions and are often treated with induction heat treatment to harden the surface layer selectively. This method increases the wear resistance and fatigue strength of the gear’s surface while maintaining the inner core’s toughness, preventing excessive brittleness.

Tool steel, used in the manufacturing of stamping dies and cutting tools, frequently employs vacuum quenching techniques. This treatment provides extremely high hardness and wear resistance while preventing surface oxidation and deformation, which is crucial for producing precision tools and molds.

Forged parts, such as crankshafts and connecting rods, often undergo normalizing to improve their internal structure and refine grain size. This treatment enhances the toughness and mechanical properties of the parts, making them more suitable for handling impact and fatigue loads.

Large shafts used in heavy machinery typically undergo tempering after quenching to reduce brittleness. This treatment ensures the components possess high strength and good toughness, allowing them to endure heavy loads and the stresses of continuous operation.

Industrial valves made from high-strength alloy steel are often annealed to relieve internal residual stress. This improves the material’s ductility, preventing the valves from cracking or failing under high-pressure conditions. Annealing also enhances corrosion resistance, making them suitable for high-temperature and high-pressure environments.