Heat Treatment of Metals Overview

Heat treatment is a processing technology that changes the physical, chemical, and mechanical properties of metal materials by heating, insulation, and cooling. This process can change the crystal structure, grain size, grain boundary morphology, chemical composition, and phase composition of the metal material by applying a controlled temperature and cooling rate to improve its mechanical properties, corrosion resistance, wear resistance and wear resistance weldability. Heat treatment is usually used to manufacture parts and tools with high precision, reliability, and durability.

What are the benefits involved in heat-treated metals

Heat treatment is a type of processing that exposes metal materials to high or low-temperature conditions, the main purpose of which is to change the physical and chemical properties of materials to achieve specific material performance requirements. Here are some of the benefits of heat treatment:

Improve strength and hardness: By controlling the time and temperature of heat treatment, the crystal structure and grain boundary morphology of the metal can be changed, thus improving the strength and hardness of the metal, making it more durable and resistant to wear.

Increase toughness and ductility: Proper heat treatment can eliminate internal stresses and defects in metal, thereby improving its toughness and ductility, making it more flexible and easy to work with.

Boost corrosion resistance: Heat treatment under specific temperatures and time conditions can form a dense surface layer, thereby improving the corrosion resistance of the metal, making it more durable and long life.

Enhance wear resistance: special crystal structure can be formed through heat treatment, to improve the wear resistance and friction coefficient of the metal so that it is more suitable for the manufacture of high-strength, high-wear-resistance parts.

Change color and appearance: Some metal materials will undergo changes in color and surface appearance under certain heat treatment conditions, which can make them more beautiful and decorative.

The disadvantages of heat treatment of metals

Heat treatment is a process in which the properties and structure of materials are changed by heating and cooling. Although heat treatment can improve the hardness, strength, and corrosion resistance of materials, there are also some potential disadvantages.

Dimensional variation

Heat treatment may cause dimensional changes in the material. This is due to the fact that the material expands during heating and contracts during cooling. If the temperature, time, or cooling rate is incorrect, it can cause the workpiece to deform, deform, or shrink. This can result in non-conforming workpiece dimensions, which can make the workpiece unable to meet its design objectives or difficult to process.

Brittleness increase

Some materials may become more brittle during heat treatment. This increased brittleness may lead to cracks and fractures in the parts during use. Brittleness is usually caused by changes in grain boundaries, grain size, and chemical composition. If the temperature and cooling rate is not correct, this brittleness can increase.

Residual stress relieving

Residual stresses may occur in the material during heat treatment. These stresses can cause problems such as deformation, cracking, and stress corrosion. This residual stress is usually caused by factors such as temperature, cooling rate, and chemical composition of the material. If these stresses are not managed properly, they can lead to material failure or part damage.

Easy oxidation

Some materials oxidize easily at high temperatures and form oxide skins. The properties of the materials will be negatively affected if these oxide skins are not treated properly. Oxide skin may lead to contamination and corrosion of the material surface, thereby reducing the corrosion resistance and mechanical properties of the material. To avoid this, the heat-treated material should receive an appropriate surface treatment, such as pickling or grinding, to remove the oxide skin.

Types of Heat Treatment for Precision Machined Parts

Hardening

Hardening is a type of heat treatment that increases the hardness and strength of the material by heating it at high temperatures and then cooling it quickly.

Hardening treatment is usually divided into three steps: heating, insulation, and cooling. During heating, the material is heated to high temperatures, usually within the heat treatment temperature range of the material. During the insulation phase, the material is kept at a high temperature in order to achieve an even temperature. During the cooling phase, the material is cooled rapidly in order to change its structure of the material, thereby increasing its hardness and strength.

Hardening treatments can be performed in different ways, the most common of which are water hardening and oil hardening. Water hardening involves placing heated material into cooled water to quickly cool the material. This method can produce high hardness and strength, but may also cause deformation and cracks in the material. Oil hardening involves placing materials into cooled oil to cool them more slowly than water hardening. This method produces lower hardness and strength, but also reduces the risk of deformation and cracking.

Hardening is an important metal processing technology, which has been widely used in many industries, such as aviation, automobile, construction, and medical devices. By hardening, the strength and hardness of materials can be greatly increased, making them more suitable for a variety of applications.

Tempering

Tempering is usually done after the hardening of metal materials such as steel to reduce the hardness of the material and increase its toughness.

Tempering is the process of reheating the already hardened material to the appropriate temperature, then holding it for a period of time, and finally allowing it to cool slowly. The heating temperature of tempering treatment is usually lower than that of quenching, generally between 200℃ and 700℃, depending on the type of material and the properties required. Tempering time is usually determined by factors such as thickness, size, and hardness of the material.

Tempering treatment can reduce the hardness of quenched material, and improve its toughness and machinability. Tempering also reduces internal stress, which reduces the risk of deformation and cracking. However, tempering treatment will reduce the hardness and strength of the material, so it is necessary to weigh and choose according to the required material properties in practical applications.

Tempering is usually used to manufacture parts that require high toughness and machinability, such as machine parts, tools, gears, etc. At the same time, tempering treatment can also be used to improve the corrosion properties of materials, reduce the internal residual stress, adjust the structure of materials, and other aspects.

Quenching

Quenching is a common method in heat treatment, which can make materials harder and more wear-resistant.

Quenching is the process of heating a material to a very high temperature and then cooling it quickly. This rapid cooling process changes the crystal structure of the material, which gives it greater hardness and strength. The quenching temperature and cooling rate can be adjusted according to the desired performance.

In quenching treatment, the material is usually first heated to a high temperature, usually higher than the critical temperature of the material, in order to achieve the complete dissolution of all alloying elements. The material is then quickly cooled to room temperature or close to it to create the desired organizational structure.

Quenching treatment can be performed in a variety of ways, the most common of which are water quenching and oil quenching. Water quenching involves placing materials into cooled water to rapidly reduce their temperature. This type of quenching can produce very hard materials, but it can also cause the material to become brittle or stressed. Oil is quenched relatively slowly, resulting in a material that is less hard but more ductile.

Quenching can be applied to a variety of metal materials, including steel, aluminum, copper, and nickel. In the quenching treatment of steel, the commonly used steel types are C45, C55, C60, C70, etc. The desired properties and hardness can be achieved by controlling the temperature and speed of the quenching treatment.

It is important to note that the quenching treatment does not apply to all materials. In some materials, quenching may cause deformation or cracking of the material. Therefore, careful analysis and testing of the material must be carried out before quenching is performed to ensure that quenching is feasible and that the desired properties can be obtained.

Passivation

Passivation is a heat treatment that improves the corrosion resistance of a metal by forming an oxide film on its surface. Passivation treatment is widely used in the manufacturing and processing of stainless steel, copper, aluminum, and other metals.

There are many methods of passivation, the most common of which are chemical passivation and electrochemical passivation.

Chemical passivation is a method of passivation by forming an oxide film on the surface of a metal. This method usually involves soaking the metal in a solution containing certain chemicals so that a thin film forms on its surface. This film can protect the metal from corrosion and has good wear resistance. Commonly used passivating agents include chromate, copper sulfate, nitric acid, etc.

Electrochemical passivation is a method of forming an oxide film on a metal surface through an electrochemical reaction. This method usually involves soaking the metal as an anode or cathode in an electrolyte and then forming a thin film on the surface of the metal by controlling an electric current. Such films are generally more uniform and dense than those produced by chemical passivation. Common methods of electrochemical passivation include anodic oxidation and anodic electrophoresis.

Passivation treatment can improve the corrosion resistance, wear resistance, and aesthetics of the metal. Stainless steel products are usually passivated to make the surface more smooth, beautiful, and enhances their corrosion resistance. Aluminum alloys are also commonly passivated to prevent oxidation and corrosion of their surfaces.

It should be noted that passivation treatment does not make the metal corrosion-free, but can only slow down the corrosion rate of the metal. Therefore, after passivation treatment, it still needs to be regularly maintained and cleaned to keep its surface smooth and beautiful and ensure its long-term corrosion resistance.

Precipitation Harding

Precipitation hardening is a heat treatment technique that enhances the mechanical properties of an alloy by forming a precipitate in it. During precipitation hardening, the alloy is heated to a temperature that causes the solute atoms in the solid solution to reach a sufficient concentration to form a precipitate. These deposits diffuse into grain boundaries and into grains, increasing the strength and hardness of the alloy.

Precipitation hardening treatment is usually divided into three steps: solid solution, quenching, and aging. In the solid solution step, the alloy is heated to a high temperature which causes the solute atoms to dissolve in the solid solution. A solid solution is then formed by rapid cooling (quenching). This prevents precipitates from forming at grain boundaries, thus maintaining the good toughness of the alloy.

In the aging step, the alloy is heated to the right temperature for the solute atoms to begin diffusing and forming a precipitate. These deposits form at grain boundaries and within grains. Through the formation of sediment, the hardness and strength of the alloy are improved.

The effect of precipitation hardening depends on the type and amount of precipitate, as well as the aging temperature and time. In general, the greater the amount of sediment, the higher the hardness and strength of the alloy. However, excessive amounts of sediment may cause the alloy to become brittle, which affects its toughness.

Precipitation hardening is widely used in various alloys, including aluminum alloy, magnesium alloy, titanium alloy, and stainless steel. It can achieve different mechanical properties by changing heat treatment conditions, so as to meet different application requirements.

How Does a Metal Heat Treatment Process Work?

A metal heat treatment process typically involves heating a metal to a specific temperature and then cooling it down in a controlled manner. The process is designed to alter the microstructure of the metal in order to achieve specific mechanical, physical, or chemical properties.

Here is a general overview of how a typical metal heat treatment process works:

Heating

The metal is heated to a specific temperature using a furnace or other heating device. The temperature and heating time depending on the desired properties of the final product and the type of metal being treated.

Soaking

Once the metal has reached the desired temperature, it is held at that temperature for a specific amount of time to allow the heat to fully penetrate the metal and create the desired microstructure.

Cooling

The metal is then cooled down either rapidly or slowly, depending on the desired properties of the final product. Rapid cooling, or quenching, can result in a harder, more brittle metal, while slow cooling can result in a softer, more ductile metal.

Post-treatment processing

Depending on the specific heat treatment process used, additional steps may be necessary to further refine the metal’s properties. These steps may include tempering, annealing, or stress relieving.

It’s worth noting that different types of metals and different heat treatment processes may require different parameters and procedures. The specific heat treatment process and parameters should be selected and determined based on the specific situation.

Stages of Metal Heat Treatment Processes

Metal heat treatment processes are used to alter the physical and mechanical properties of metals through heating and cooling cycles. The stages involved in the metal heat treating process are:

Heating

The first stage of the process involves heating the metal to a specific temperature. The temperature is determined based on the metal’s composition, the desired properties, and the type of heat treatment being performed. The heating can be done using various methods such as the furnace, torch, or induction heating.

Soaking

Once the metal has reached the desired temperature, it is held at that temperature for a specific period of time to ensure that the entire piece is evenly heated. This stage is known as soaking, and the time required depends on the size and composition of the metal.

Cooling ProCESS

After soaking, the metal is cooled at a controlled rate to achieve the desired properties. Cooling can be done using various methods such as air cooling, oil quenching, or water quenching, depending on the type of heat treatment being performed.

Tempering

After the metal has been cooled, it is often tempered to improve its toughness and ductility. Tempering involves heating the metal to a lower temperature than the initial heating stage and then cooling it again. This process helps to reduce the hardness of the metal while improving its toughness and ductility.

Annealing Process

Annealing is a process used to soften the metal and improve its machinability. The metal is heated to a specific temperature and then cooled slowly to room temperature. This process helps to reduce internal stresses and improve the metal’s microstructure.

Stress relieving

Stress relieving is a process used to reduce residual stresses in metal that has undergone significant shaping or deformation. The metal is heated to a specific temperature and then cooled slowly to room temperature. This process helps to reduce the risk of cracking or failure due to residual stresses.

Overall, the stages involved in metal heat treatment processes depend on the type of heat treatment being performed and the specific properties desired for the metal.

Field of application of heat treatment processes

Heat treatment is a technology widely used in manufacturing that can be used to change the physical and mechanical properties of metals and alloy materials to meet the needs of different fields.

The following are some specific areas of heat treatment application:

Metal manufacturing PROCESSes

Heat treatment is a key process in the manufacture of many metal products, such as steel, aluminum, copper, zinc, magnesium, and other alloys. For example, heat treatment can improve the strength and toughness of steel, making it more suitable for manufacturing auto parts, aero-engine parts, construction steel, etc. In addition, heat treatment can also be used to change the hardness, strength, and corrosion resistance of stainless steel, copper, and aluminum alloys.

Machining

Heat treatment can give better workability to metals, such as in cutting, drilling, and milling processes. In these processes, the metal needs to be hard and strong enough, but at the same time ductile and machinable enough to ensure the accuracy and life of the parts.

Manufacturing PROCESS

Heat treatment can be used to manufacture many different types of products, such as automobile parts, ship, and aircraft parts, machinery parts, electronic equipment parts, etc. In these applications, heat treatment improves the performance and life of materials and ensures that parts have the required strength and durability.

Construction

Heat treatment can also be used to make building materials such as steel bars and steel plates. These materials need to be strong and durable enough to withstand the loads and environmental changes of the building.

Medical instruments

Heat treatment can also be used to manufacture medical instruments, such as surgical instruments and artificial joints. In these applications, heat treatment improves the biocompatibility and mechanical properties of the material, thus ensuring adequate strength and longevity of the instrument.