What is the purpose of metal heat treatment

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What is the purpose of metal heat treatment

In modern life, you can come into contact with a variety of metal products, such as steel bridges, car shells, kitchen knives, stainless steel guardrails, iron pots, cans, iron hammers, etc. It’s common for us to assume that they are all about the same performance. In fact, they have different properties, such as the high strength of bridge steel; the hardness of kitchen knives; the high heat resistance of iron pots; the high ductility of cans; the strong corrosion resistance of stainless steel; For metal workpieces to have these properties that meet our different needs, in addition to selecting appropriate alloys, other almost all rely on different metal heat treatment methods.

1. Why do metals need to be heat treated?

When it comes to metal heat treatment, there is no escape from iron, which is the most abundant metal on our earth and the most widely used metal. Pure iron refers to iron metal with a carbon content of less than 0.02%. It is a silver-white metal with flexibility and good ductility. It has good magnetic permeability and is mainly used for making iron cores for generators and motors. Steel is a general term for iron-carbon alloys, and the carbon mass percentage is between 0.02% and 2.11%. The iron metal workpieces we usually use are almost all steel. The carbon content higher than 2.11% is called pig iron, it is brittle and hard, and the processing performance is poor, that is, the so-called weight is mostly made of pig iron. The strength, stiffness, surface hardness, corrosion resistance and other properties of unheated iron, steel and pig iron are difficult to meet our different needs in real life. Therefore, people racked their brains and kept exploring. In addition to the reasonable selection of materials and various forming processes, there was heat treatment. Therefore, heat treatment is to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, and it is an effective means to improve the performance of the metal workpiece.

2. What is metal heat treatment?

Metal heat treatment is to heat a metal or alloy workpiece to a suitable temperature in a certain medium, and after maintaining it at this temperature for a certain period of time, it is cooled in different media at different speeds, thereby changing the surface or interior of the metal material. A process in which the microstructure is used to control its properties.

Metal heat treatment is one of the important processes in machinery manufacturing. Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but changes the microstructure inside the workpiece or changes the chemical composition of the workpiece surface. , to give or improve the performance of the workpiece. Its characteristic is to improve the intrinsic quality of the workpiece. Therefore, it is a special process in mechanical manufacturing, and one of the important processes in the manufacturing of mechanical parts and tooling. It can guarantee and improve various properties of the workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing.

For example, after long-term annealing treatment of white cast iron, malleable cast iron can be obtained to improve plasticity; gears adopt the correct heat treatment process, and the service life can be doubled or dozens of times higher than that of gears without heat treatment; Some alloying elements have some expensive alloy steel properties and can replace some heat-resistant steels and stainless steels; almost all tools and dies need to be heat treated before they can be used.

The metal microstructure is complex, and the metallography will be different under different conditions. Different metallographic appearance, performance will be different. And the metallographic structure can be changed and controlled by heat treatment. Steel is the most widely used material in the machinery industry, and the heat treatment of steel is also the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance. The picture below shows the main metallographic structures of steel (the structure observed under the metallographic microscope after the steel is sampled, polished, polished, and finally corroded with a specific corrosive agent).

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3. Classification of machined metal heat treatment

Metal heat treatment process can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. 

According to the difference of heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal adopts different heat treatment processes to obtain different metallographic structures and thus have different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes.

Bulk Heat Treatment

The overall heat treatment is a metal heat treatment process that heats the workpiece as a whole, and then cools it at an appropriate speed to change its metallographic structure and thereby change the overall mechanical properties. The overall heat treatment of steel generally has four basic processes: annealing, normalizing, quenching and tempering.

1. (Annealing) annealing is to heat the workpiece to an appropriate temperature, adopt different holding times according to the material and workpiece size, and then slowly cool it, the purpose is to make the internal structure of the metal reach or close to a state of equilibrium, or to make the internal stress generated by the previous process. It can be released to obtain good process performance and performance, or to prepare for further quenching. Taking No. 45 steel as an example, the metallographic phase after annealing is austenite, which becomes too soft after annealing. Generally, 45 steel is not annealed.

Professional explanation: heat the hypoeutectoid steel workpiece to 20-40 degrees above AC3 (the final temperature at which ferrite transforms into austenite during heating), and after holding for a period of time, slowly cool with the furnace (or bury it in sand or lime Cooling) to 500 degrees below the heat treatment process of cooling in the air.

2. (Normalization) normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to that of annealing, but the obtained structure is finer. It is often used to improve the cutting performance of materials, and is sometimes used for some requirements. Tall parts as final heat treatment. The metallographic phase after normalizing on No. 45 is austenite + pearlite.

Professional explanation: heat the steel or steel parts to the critical point AC3 (for hypoeutectoid steel) or Accm (the final temperature at which the secondary cementite dissolves into austenite during heating, for hypereutectoid steel) 30℃—50 After normalizing, the heat treatment process of uniform cooling in free-flowing air is normalizing. After normalizing, the hypoeutectoid steel is F+S, the eutectoid steel is S, and the hypereutectoid steel is S+Fe3CII normal The main difference between fire and full annealing is that the cooling rate is faster, the purpose is to normalize the steel structure, also known as normalization.

3. (Quenching) Quenching is to rapidly cool the workpiece in a quenching medium such as water, oil or other inorganic salt solutions, organic aqueous solutions, etc. after heating and maintaining the workpiece. After the steel parts become hard, but at the same time become brittle. Taking 45 steel as an example, it is rarely quenched alone because it is difficult to obtain the desired hardness.

Professional explanation: The heat treatment process of cooling the austenitized steel at an appropriate cooling rate to make the workpiece undergo martensite and other unstable microstructure transformations in all or a certain range of the cross section.

4. (Tempering) Tempering is to reduce the brittleness of steel parts. The quenched steel parts are kept at an appropriate temperature higher than room temperature and lower than 650 ° C for a long time, and then cooled. This process is called tempering. fire.

Professional explanation: The quenched workpiece is heated to an appropriate temperature below the critical point AC1 (the starting temperature of pearlite to austenite transformation during heating) for a certain period of time, and then cooled by a method that meets the requirements to obtain the required structure and performance heat treatment process.

Annealing, normalizing, quenching, and tempering are the “four fires” in the overall heat treatment. They are often combined with each other. The quenching and tempering are closely related and are often used together.


Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change the mechanical properties of the surface. In order to only heat the surface layer of the workpiece without allowing too much heat to pass into the inside of the workpiece, the heat source used must have a high energy density, that is, a larger amount of heat energy is given to the workpiece per unit area, so that the surface or part of the workpiece can be short-term or instantaneous. reach high temperature. The main methods of surface heat treatment are flame quenching and induction heating heat treatment. Commonly used heat sources are flames such as oxyacetylene or oxypropane, induced current, laser and electron beam. For example, some shafts, gears and parts subject to changing loads have high wear resistance on the surface, while the interior requires good toughness and strength. It is possible to meet the overall performance requirements of the workpiece through surface heat treatment.

Thermo-Chemical Treatment

Chemical heat treatment is a metal heat treatment process that changes the chemical composition, structure and properties of the workpiece surface. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the workpiece surface. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloying elements for a long time, so that the surface of the workpiece is infiltrated with elements such as carbon, nitrogen, boron and chromium. After infiltration of elements, other heat treatment processes such as quenching and tempering are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding, and metalizing. 

Surface modified technique: It is a combination of chemical heat treatment and physical methods. A type of heat treatment technology that changes the chemical composition or structure of the surface of a material or workpiece to improve the performance of machine parts or materials. It includes chemical heat treatment (nitriding, carburizing, metalizing, etc.); surface coatings (low-pressure plasma spraying, low-pressure arc spraying, laser remelting composite and other thin-film coatings, physical vapor deposition, chemical vapor deposition, etc.) and non-metallic coatings technology, etc. These technologies used to strengthen the surface of parts or materials endow parts with various new properties such as high temperature resistance, corrosion resistance, wear resistance, fatigue resistance, radiation protection, electrical conductivity, and magnetic permeability. It can improve the reliability and prolong the service life of the parts that work in the environment of high speed, high temperature, high pressure, heavy load and corrosive medium. The most common is the non-stick pan at home. In addition to special industries, special-purpose metal products (such as gears, castings, non-stick pans, etc.), most mechanical design and manufacturing do not require additional heat treatment, because the steel mill has replaced the design party for overheating, so that the mechanical use The metal raw material is in a heat-treated state. Mechanical design engineers only need to choose.

Process method and development of metal heat treatment

Having said so much about metal heat treatment, let’s take a look at the development of metal smelting and heat treatment in the world. Human beings continue to explore, from using stone tools, to discovering metals, refining metals, and then to heat treatment of metals, they have gone through a long process.

Up to now, each heat treatment step has its own advanced heat treatment equipment for thermal annealing, normalizing, tempering, quenching, carburizing, nitriding, etc. Like annealing furnace, normalizing furnace, tempering furnace, quenching furnace, carburizing furnace, carburizing furnace, vacuum furnace, nitrate furnace, aging furnace, aluminum alloy heat treatment furnace, etc. They can not only rise and cool accurately, but also have great development in measurement and control technology, mainly in temperature measurement and control, atmosphere measurement and control and process program control, and constantly adopt new technologies. Advanced and high-quality components and instruments have been developed one after another, and are widely used in heat treatment equipment.

The application and control of computer and network technology is extended to almost all fields of advanced heat treatment equipment. With many years of exploration and accumulated rich experience in the field of induction heating and combustion control, Siemens Industrial Control has already become a benchmark in the field of metal heat treatment. Control methods such as sequential pulse combustion control, sequential pulse combustion control, expanding flame and interruptible ignition provide combustion equipment manufacturers with reliable technologies and solutions. Moreover, combustion equipment and induction heating are the heat sources required for heat treatment. In addition, Siemens also provides high-quality components for the heat treatment process, and a wide range of mechanical, electronic and various sensors are used in the heat treatment field.

The future of metal heat treatment What is the future of metal heat treatment development? Unpredictable. With the emergence and progress of 3D printing technology, it is also possible to directly print metal objects with different and specific metallographic structures that meet different performance requirements. It is even easy to manufacture metal objects with different metallographic structures and different properties in different parts of the same workpiece. This is also the current technical bottleneck of 3D printing and the development direction of 3D printing.

Author: Mose Li

Author: Mose Li

Director of Project Engineering at 3Q Machining

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