What is Heat Treatment: Process, Types & Purposes

Heat treatment is one of the important processes in mechanical manufacturing, compared with other machining processes, heat treatment generally does not change the shape and overall chemical composition of material, but rather confers or improves the material’s usability by changing the internal microstructure of the material or by changing the chemical composition of the surface of the material. It is characterized by an improvement in the intrinsic quality, which is generally not visible to the naked eye. In order to make the metal, aterial with the required mechanical properties, physical properties and chemical properties, in addition to the reasonable choice of materials and a variety of forming processes, heat treatment process is often essential. Steel is the most widely used materials in the mechanical industry, steel microstructure complex, can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment. In addition, aluminium, copper, magnesium, titanium and its alloys can also be heat treatment to change its mechanical, physical and chemical properties, in order to obtain different performance.

Table of Content

• What Is Heat Treatment?
• Steps of Heat Treatment Process
• Types of Heat Treatment Processes
  • Annealing
  • Stress Relieving
  • Aging Treatment
  • Quenching
  • Normalizing
  • Tempering
  • Nitriding
  • Cyaniding
  • Induction Hardening
  • Flame Hardening
  • Carburisation
• What Does Heat Treatment Improve?
• Difference Between Heat Treatment and Annealing
• Heat Treatment of Stainless Steel
• Heat Treatment of Steel
• Heat Treatment of Aluminum Alloy
• FAQ

What Is Heat Treatment?

Heat treatment is a process that involves controlled heating and cooling of a material to modify its microstructure and enhance its mechanical properties such as tensile strength, hardness, toughness, and ductility. The primary objective of heat treatment is to modify the microstructure to achieve the optimal combination of properties for a particular application

Heat treatment can be used at different stages in the manufacturing process to change certain properties of that metal or alloy. For example, you might use heat treatment to make it stronger, harder, more durable, or more ductile, depending on what the material needs in order to perform properly. 

Steps of Heat Treatment Process

The heat treatment process generally includes heating, holding, cooling three processes, sometimes only two processes of heating and cooling. These processes are interconnected and cannot be interrupted. Heating is one of the important processes of heat treatment. Metal heat treatment of many heating methods, the earliest is the use of charcoal and coal as a heat source, and the recent application of liquid and gaseous fuels. The application of electricity makes heating easy to control, and no environmental pollution. The use of these heat sources can be directly heated, but also through the molten salt or gold, to floating particles for indirect heating.

Heating

Heating is one of the important processes of heat treatment. Metal heat treatment of many heating methods, the earliest is the use of charcoal and coal as a heat source, and the recent application of liquid and gaseous fuels. The application of electricity makes heating easy to control, and no environmental pollution. The use of these heat sources can be directly heated, but also through the molten salt or gold, to floating particles for indirect heating. Metal heating, the material is exposed to air, oxidation, decarburization often occurs (i.e., the surface carbon content of the steel parts to reduce), which has a very negative impact on the surface properties of the heat-treated parts. Therefore, the metal should usually be in a controlled atmosphere or protective atmosphere, molten salt and vacuum heating, but also available coatings or packaging methods for protective heating. Heating temperature is one of the important process parameters of the heat treatment process, the selection and control of the heating temperature, is to ensure the quality of heat treatment of the main issues. The heating temperature varies with the metal being treated and the purpose of heat treatment, but it is generally heated above the phase transition temperature to obtain high-temperature organisation. In addition, the transformation requires a certain amount of time, so when the surface of the metal to achieve the required heating temperature, but also have to be maintained at this temperature for a certain period of time, so that the internal and external temperatures are consistent, so that the microstructural transformation is complete, which is known as the holding time. The use of high energy density heating and surface heat treatment, the heating rate is extremely fast, there is generally no holding time, while the chemical heat treatment of the holding time is often longer.

Holding

The the soaking stage requires a certain amount of time, so when the surface of the metal to achieve the required heating temperature, but also have to be maintained at this temperature for a certain period of time, so that the internal and external temperatures are consistent, so that the microstructural transformation is complete, which is known as the holding time. The use of high energy density heating and surface heat treatment, the heating rate is extremely fast, there is generally no holding time, while the chemical heat treatment of the holding time is often longer.

Cooling

Cooling is an indispensable step in the heat treatment process, the cooling method is different due to different processes, mainly to control the cooling rate. General annealing cooling rate is the slowest, normalising the cooling rate is faster, quenching the cooling rate is faster. But also because of the different types of steel and have different requirements, such as air-hardened steel can be quenched with the same cooling rate as normalising.

Quenching is when you rapidly cool metal in air, oil, water, brine, or another medium. Usually quenching is associated with hardening because most metals that are hardened are cooled rapidly with quenching, but it is not always true that quenching or otherwise rapid cooling results in hardening. Water quenching, for example, is used to anneal copper, and other metals are hardened with slow cooling. 

Types of Heat Treatment Processes

1. Annealing
2. Stress Relieving
3. Aging Treatment
4. Quenching
5. Normalizing
6. Tempering
7. Nitriding
8. Cyaniding
9. Induction Hardening
10. Flame Hardening
11. Carburisation

Annealing

Annealing is a heat treatment process to heat the material slowly to a certain temperature, holding it for a sufficient period of time, and then cooling it at a suitable rate. The purpose of annealing is to improve machinability while releasing stresses, increasing the ductility and toughness of the material, and producing a special microstructure.

Stress Relieving

Stress relieving is a heat treatment in which the metal is heated at a lower temperature than the recrystallization temperature after cold deformation in order to remove the internal stresses but still retain the cold work hardening effect.

Aging Treatment

Ageing treatment is a heat treatment process designed to eliminate internal stresses in metal or alloy material and stabilise their dimensions and properties. This treatment process usually consists of several steps, including solution treatment, heat treatment and aging treatment. Aging treatments can be divided into two types: natural aging and artificial aging. Natural ageing involves leaving the material at room temperature or under natural conditions for a longer period of time, after which the dimensions and properties of the material will stabilise; whereas artificial ageing involves heating the material to a higher temperature and holding it there for a period of time in order to accelerate the ageing process. In general, after aging, hardness and strength increase, while plastic toughness and internal stresses decrease.

Quenching

Quenching applies to both ferrous and non-ferrous metals, and the process generally involves heating and holding the material and then cooling it rapidly in a quenching media such as water, oil or inorganic salts. After quenching the metal material becomes hard, but at the same time will become brittle (for titanium alloys, because of the quenching of its hardness does not change much, so the quenching process is generally referred to as solid solution). The main characteristics of quenching in comparison with other heat treatments are rapid cooling, martensitic phase transformation and increased hardness.

Normalizing

The material is heated to a suitable temperature and then cooled in the air, the effect of normalizing is similar to annealing, only that the organisation obtained is finer, often used to improve the cutting performance of the material, but also sometimes used for some parts that do not require a high degree of as the final heat treatment.

The main feature of normalizing is that the cooling rate is faster than annealing and lower than quenching, which enables the crystalline grains of the steel to be refined in a slightly faster cooling process, resulting in satisfactory strength and a significant increase in toughness and a reduction in the tendency of the component to crack.

Tempering

Tempering is a common heat treatment process used to remove internal stresses in a material after quenching and to improve its hardness and toughness. Tempering is usually carried out after quenching, by heating the material to a certain temperature and holding for a period of time, and then cooling to obtain the desired properties. Tempering is usually divided into three types: low-temperature tempering, medium-temperature tempering and high-temperature tempering. Tempering is usually able to eliminate the residual stresses generated in the quenching process of the material and improve its mechanical properties such as hardness, toughness and thermal conductivity.

Nitriding

Nitriding is a chemical heat treatment process in which nitrogen atoms penetrate into the surface layer of the material in a certain medium at a certain temperature. Commonly, there are liquid nitriding, gas nitriding, ion nitriding. The traditional gas nitriding is to put the material into a sealed container, through the flow of ammonia and heating, insulation for a long time, the ammonia thermal decomposition of reactive nitrogen atoms, constantly adsorbed to the surface of the material, and diffusion of infiltration into the surface layer of the material, so as to change the surface layer of the chemical composition and organisation, to obtain excellent surface properties. If in the nitriding process at the same time infiltration of carbon to promote the diffusion of nitrogen, it is called nitrogen-carbon co-infiltration. Commonly used are gas nitriding and ion nitriding.

Cyaniding

Cyaniding is a surface treatment method that allows nitrogen atoms to penetrate into the steel surface through an unstable chemical reaction in a cyanide agent after heating the steel material to a certain temperature. A thin film is formed on the surface of the treated steel, which can effectively improve the hardness, wear resistance and corrosion resistance of the steel.

Heat treatment cyaniding is suitable for a wide range of steel materials. Low carbon steel, medium carbon steel, high carbon steel, alloy steel, cast iron and other materials can be heat treated cyanide. Of these, mild and medium carbon steels perform best

Induction Hardening

Induction Hardening is the use of the principle of electromagnetic induction, so that the parts in the alternating magnetic field cutting magnetic lines of force, the surface of the generation of induced currents, but also according to the skin effect of alternating current, in the form of eddy currents will be the surface of the parts of the rapid heating, and then cooled sharply quenching methods.

Flame Hardening

Flame hardening is a heat treatment method in which the surface of a metal is heated by means of a high-temperature flame and subsequently cooled rapidly to harden the metal surface. This method usually uses an acetylene-oxygen flame or a gas-oxygen flame, and the flame temperature can be as high as 2000-3100 degrees Celsius. Flame hardening is suitable for localized surface treatment of metals without affecting the overall properties, and is characterized by high precision and efficiency.

Carburization

Carburization is a common heat treatment process for metal materials, which can make the surface of the carburized material obtain high hardness and improve its wear resistance. The traditional process mainly includes: low-temperature tempering, pre-coolingdirect quenching, primary heating and quenching, carburizing high temperature tempering, secondary quenching and cold treatment, carburizing after induction heating and other processes.

What Does Heat Treatment Improve?

• to increase strength, hardness, and wear resistance
• to increase ductility and softness
• to increase toughness
• to obtain fine grain size
• to remove internal stresses induced by differential deformation by cold working, nonuniform cooling from high temperature during casting and welding
• to improve machinability
• to improve cutting properties of tool steels
• to improve surface properties
• to improve electrical properties
• to improve magnetic properties

Difference Between Heat Treatment and Annealing

1. Definition and Purpose
   • Heat treatment is a machining process to achieve the desired organisation and properties of a material by means of heating, holding and cooling. Its main purpose is to improve the mechanical, physical and chemical properties of the material.
   • Annealing is a specific process of heat treatment, which refers to heating a metal slowly to a certain temperature, holding it for a sufficient period of time, and then cooling it at a suitable rate. The main purpose of annealing is to reduce hardness, improve machinability, release stress, increase material ductility and toughness, produce special microstructures, etc.
2. Types of Processes
   • Heat treatment includes a variety of processes such as annealing, normalizing, quenching and tempering. Each process has a different purpose and method and is suitable for different materials and needs.
   • Annealing processes include complete annealing, spheroidal annealing, isothermal annealing, graphite annealing, diffusion annealing, stress relief annealing, incomplete annealing, etc.
3. Application
   • Heat treatment is widely used in various industrial fields, such as iron and steel, non-ferrous metals, plastics, etc., for improving the properties of materials to meet different needs of use.
   • Annealing treatment is commonly used in the processing of metal materials, especially after the manufacture of blanks and before rough machining, in order to improve the processing properties and organisational structure of materials.

Heat Treatment of Stainless Steel

The purpose of heat treatment of stainless steel is to improve mechanical properties and corrosion resistance by changing its organisational structure. Stainless steel can be divided into ferrite, austenite, martensite, ferrite – austenite duplex and precipitation hardening stainless steel and other types according to its organisational structure, and different types of stainless steel have different effects and applications after heat treatment. 12

• Ferritic stainless steel: because of its stable organisation, no phase change occurs during heating and cooling, so the mechanical properties cannot be adjusted by heat treatment methods. The main purpose of heat treatment is to reduce embrittlement and improve resistance to intergranular corrosion.
• Austenitic stainless steel: contains high Cr and Ni elements, heating and cooling without phase change, the main purpose of heat treatment is to maintain the stability of the austenitic organisation, thereby improving corrosion resistance.
• Martensitic stainless steel: mechanical properties can be adjusted in a wide range by heat treatment methods to meet the needs of different conditions of use. Different heat treatment methods also have different effects on corrosion resistance.
• Ferritic – austenitic duplex stainless steel: with higher strength and plasticity than austenitic and ferritic stainless steels, while showing higher resistance to pitting corrosion, crevice corrosion and stress corrosion in cl- media and seawater.
• Precipitation hardening stainless steel: mechanical properties are adjusted by means of heat treatment, but the strengthening mechanism is different from that of martensitic stainless steel.

Heat treatment processes include quenching, tempering, annealing and other steps, the specific choice depends on the type of stainless steel and application requirements. For example, martensitic stainless steel is usually quenched and tempered to adjust its hardness and toughness to meet specific engineering needs.

Heat Treatment of Steel

Heat treatment of steel is a process method of heating, holding and cooling steel in the solid state in an appropriate manner to change its internal organisation in order to obtain the desired properties. Through heat treatment can eliminate defects in parts blanks (such as castings, forgings, etc.), improve the processing performance of steel, more importantly, heat treatment can significantly improve the mechanical properties of steel, give full play to the potential of steel, save production costs, improve the performance of the material, and extend the service life of the product. Therefore, heat treatment occupies a very important position in machinery manufacturing.

Heat Treatment of Aluminum Alloy

Aluminium alloy heat treatment technology is a technology to change the structural organization of aluminium alloy by controlling the heating rate, holding time and cooling rate. The main purpose of aluminium alloy heat treatment is to improve the mechanical properties of the alloy, enhance corrosion resistance, improve processing performance and obtain dimensional stability. Through heat treatment, the internal organisational structure of aluminium alloy can be optimised so as to enhance its overall performance.

The main heat treatment processes for aluminium alloys include the following:

Annealing

1. Purpose: To reduce hardness, eliminate internal stresses, and improve plasticity and machinability.
2. Process: Aluminium alloys are heated to a specific temperature (usually between 300°C and 500°C), held for a certain period of time and then slowly cooled.
3. Application: Suitable for improving the formability and machinability of aluminium alloys.

Solution Heat Treatment

• Purpose: To dissolve the alloying elements uniformly in the aluminium matrix in preparation for aging treatment.
• Process: The aluminium alloy is heated to a high solid solution temperature (approx. 450°C to 550°C), held for a period of time and then rapidly cooled (e.g. by water cooling).
• Effect: Increased plasticity and ductility of the material.

Aging Treatment

• Divided into: natural ageing and artificial ageing.
• Purpose: To promote the precipitation of reinforcing phases of alloying elements in the aluminium matrix to improve the strength and hardness of the material.
• Natural aging: carried out at room temperature for a longer period of time.
• Artificial aging: Heating at medium temperatures (about 100°C to 200°C) and holding for several hours to tens of hours.
• Effect: Significantly improves the mechanical properties of aluminium alloys.

Hardening (Quenching)

• Purpose: Rapid cooling to maintain the solid solution structure and prevent precipitation of elements.
• Process: Rapid cooling of aluminium alloys after solid solution treatment, generally using water or oil cooling.
• Application: To maintain high strength and good plasticity of the material.

Tempering

• Purpose: Eliminate stresses caused by quenching and improve toughness.
• Process: After quenching, the aluminium alloy is heated to a lower temperature, held for a period of time and then cooled.
• Effect: maintain the hardness of the material and improve the toughness.

FAQ