What is metal heat treatment? Explanation of the main types such as quenching, tempering, and annealing

Metal heat treatment is a processing technique that changes the properties of metal materials such as iron and steel, such as hardness, toughness, and wear resistance, by heating and cooling them. There are various processing methods for different applications, such as automotive parts, tools, and building materials, so correctly understanding the characteristics, applications, and effects of each heat treatment method will help you improve product performance and select the optimal processing method.

In this article, we will explain the mechanism of metal heat treatment and typical processing methods, along with the advantages and disadvantages of the processing.

What is metal heat treatment?

Metal heat treatment is a processing technique that changes the properties of metals, such as hardness, toughness, and strength, by combining heating and cooling. For example, about a quarter of the steel materials used in automobiles are heat treated, making it an essential technology in manufacturing.

Metal heat treatment, also known as "reddening and cooling," is an important process that greatly affects product performance, reliability, and lifespan. It is performed for the purposes of improving strength and workability, removing residual stress, etc.

Metal heat treatment processing method

Metal heat treatment can have a significant effect on the properties of the metal, depending on the heating and cooling methods used. Therefore, there are several heat treatment methods, depending on the combination of heating and cooling methods.
Here we will explain the general processing steps of metal heat treatment.

1. Heating method
2. Cooling method
3. Examples of steel materials used

Heating method

When heating, the furnace is heated to around 800 to 1,000°C.Ordinary steel undergoes a property change known as "transformation" at approximately 700°C, changing into a soft structure known as austenite.

There are the following transformation heating methods:

Cooling method

When heated metal is cooled to approximately 550°C, it changes from austenite to a hard structure called martensite.Martensite is hard and has a finer and more uniform metal structure than austenite.

The rates at which the metal is heated and cooled determine the metal's final properties. There are several types of cooling:

Examples of steel materials used

Below are some examples of steel materials for which metal heat treatment is used:

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Types of heat treatment

Heat treatment can be performed on the entire metal or just on the surface.Here we will explain the general classification of metal heat treatment, its purpose, and specific processing methods.

First of all, metal heat treatment can be broadly divided into "full heat treatment" and "surface heat treatment." Full heat treatment is a processing method in which heat treatment is applied to the entire metal, and there are general heat treatment and special heat treatment.

General heat treatment: Adjusting the hardness, toughness, and internal structure of metal

Special heat treatment: for specific applications

Surface heat treatment is a processing method that hardens or modifies only the surface of a metal. By keeping the inside soft, it aims to achieve both strength and workability.

Surface hardening heat treatment

Surface modification heat treatment

Quenching (hardening treatment)

Quenching is a heat treatment method that increases the hardness and wear resistance of metal by heating steel to around 850 to 950°C and then rapidly cooling it with water or oil.

It is often used with carbon steel and tool steel, and is widely used in high-load parts such as cutting tools, gears, and bearing parts.

Characteristics of hardening [Advantages]

• Strength is increased by 2 to 3 times.
• High hardness and high resistance to wear

Points to note when hardening [Disadvantages]

• There is a risk of distortion or cracking during rapid cooling.
• Depending on the material and processing method, toughness (tenacity) may decrease and the material may become brittle.

Tempering (to impart toughness)

Tempering is the process of reheating and cooling metal that has been hardened by quenching.This process reduces hardness to a moderate degree while restoring toughness (tenacity).It is generally performed in combination with hardening.

High-temperature tempering is a metal heat treatment performed by reheating at approximately 450 to 650°C and then cooling, while low-temperature tempering is a metal heat treatment performed by reheating at approximately 100 to 200°C and then cooling.

Characteristics of tempering [Advantages]

• The balance between strength and toughness can be adjusted
• It gives metal toughness and prevents cracks and chips.
• By removing internal stress, springback during processing is suppressed, improving dimensional stability.

Points to note about tempering [Disadvantages]

• Careful temperature control is required
• If the treatment is not done according to the material's conditions, the hardness may actually decrease.

Annealing (softening and stress relief)

Annealing involves heating the metal in a furnace at 500 to 750°C until it becomes austenite.This is a processing method in which the material is slowly cooled in a furnace to create a structure called pearlite.This is done to soften the metal and homogenize the metal structure, and is applied to materials before bending or drilling.

Characteristics of annealing [Advantages]

• Improved workability and longer tool life
• Residual stress is removed during cooling, making distortion less likely to occur.
• Improved accuracy in post-processing is expected

Points to note about annealing [Disadvantages]

• The strength and hardness of the material decreases.
• Furnace cooling increases processing time
• In the case of precision machining, reheating may be required.

Normalizing (uniform structure)

This is a heat treatment method to make the steel structure fine and uniform.Similar to annealing, but air-cooled rather than cooled to maintain a certain level of strength and toughness.

Characteristics of normalizing [Advantages]

• The crystal structure becomes uniform and the properties become stable.
• Air cooling makes the process faster than furnace cooling.

Points to note when normalizing [Disadvantages]

• Slightly less workable than annealed
• When the material is large, the cooling rate may differ, resulting in structural differences.
• In the case of precision machining, reheating may be required.

Age hardening (precipitation hardening)

This is a heat treatment method in which fine precipitates are formed by holding the material at a constant temperature for a long period of time, thereby increasing hardness and strength.It is used for parts that require light weight and high strength.

Characteristics of age hardening [Advantages]

• Strength and hardness improve over time
• It can be hardened without causing dimensional changes to processed parts.

Points to note about age hardening [Disadvantages]

• Processing time is required, which reduces productivity
• Can only be used with depositable metals
• Dimensional changes may occur due to changes in microstructure

Sub-zero Processing

Sub-zero treatment is a process in which hardened steel is exposed to extremely low temperatures, such as -100°C or below.This is a heat treatment method that transforms retained austenite into martensite.It is widely used for high-precision parts and tool steels and bearing steels that require long life.

Characteristics of sub-zero processing [Advantages]

• Improved dimensional stability due to removal of retained austenite
• Higher hardness than quenching alone

Points to note about sub-zero treatment [Disadvantages]

• Special cryogenic equipment such as liquid nitrogen is required, which increases the equipment costs.
• There is a risk of cracking due to thermal stress depending on the material and shape.

Carburizing and nitriding (surface hardening)

Carburizing and nitriding processes diffuse and penetrate carbon or nitrogen into the metal surface,This is a heat treatment method that hardens only the surface layer.It is used for machine parts that are subject to high loads, such as gears and shafts.

Characteristics of carburizing and nitriding treatment [Advantages]

• Improved surface wear resistance and fatigue strength
• The core maintains its toughness, making it resistant to impacts and bending.

Points to note about carburizing and nitriding treatment [Disadvantages]

• The core is not hardened
• Limited effect on thick parts
• Processing costs tend to rise

Summary of metal heat treatment

Metal heat treatment is an important technology that allows for flexible control of material properties such as strength, hardness, and toughness.In addition to basic treatments such as quenching, tempering, and annealing, we can combine sub-zero treatment and surface hardening treatment (carburizing and nitriding) to give products optimal mechanical properties.

Sanwa Needle Bearings' strength lies in ultra-precision processing, with grinding as its core technology.In order to process with an accuracy of ±0.0001mm, we select the appropriate heat treatment depending on the application and purpose.We can handle any material or industry, so please feel free to contact us if you have any metal processing problems.