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Heat treatment of metals

Modern life can come into contact with a variety of metal products, such as steel bridges, car shells, kitchen knives, stainless steel guardrails, iron pots and pans, cans, iron hammers, etc.. We are accustomed to think that they are similar in performance, right? In fact, they have different properties, such as the high strength of the bridge steel; the hardness of the kitchen knife; the high heat resistance of the iron pot; the high ductility of the can; stainless steel corrosion resistance; the beautiful appearance of the car shell.

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Metal workpieces to have these properties to meet our different needs, in addition to by choosing the right alloy, almost all other rely on different metal heat treatment methods.

Why do metals need heat treatment?

When it comes to metal heat treatment, you can’t get around iron, which is the most abundant metal on our planet and the most widely used. Pure iron refers to the carbon content of less than % iron metal, is a flexible and ductile silver-white metal, has a good magnetic conductivity, mainly used in the core of the system of generators and motors. Steel is the general name of iron-carbon alloy, the carbon mass percentage between % and %, we usually use the iron metal workpiece, almost all are steel. Carbon content higher than % is called pig iron, it is brittle and hard, processing performance is poor, is to say that most of the weighing weights are made of pig iron.
Without heat treatment of iron, steel and pig iron strength, stiffness and surface hardness, corrosion resistance and other properties, are difficult to meet the different needs of our real life. So, people racked their brains and kept exploring, and in addition to the reasonable choice of materials and various forming processes, there was heat treatment. Therefore, heat treatment is to make the metal workpiece have the required mechanical, physical and chemical properties, and is an effective means to improve the performance of the metal workpiece.

What is metal heat treatment?

Metal heat treatment is a process of heating a metal or alloy workpiece in a certain medium to a suitable temperature and maintaining it in this temperature for a certain period of time, and then cooling it at different speeds, in different mediums, so as to change the surface or internal microstructure of the metal material 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 by changing the internal microstructure of the workpiece, or changing the chemical composition of the surface of the workpiece, 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 machinery manufacturing and is one of the important processes in the manufacturing process of mechanical parts and work molds. It can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the organization and stress state of the blank to facilitate various cold and hot processing.
For example, white cast iron after a long time annealing treatment can obtain malleable cast iron, improve plasticity; gear using the correct heat treatment process, the service life can be multiplied or dozens of times than the gear without heat treatment; In addition, cheap carbon steel by infiltrating certain alloy elements have some expensive alloy steel performance, can replace some heat-resistant steel, stainless steel; tooling is almost all need to go through heat treatment before use.

Metal micro metallographic organization is complex, and in different conditions metallographic will be different. Metallographic different, the performance will be different. And the metallographic organization can be changed and controlled by heat treatment. Steel is the most widely used material in the machinery industry, steel heat treatment is also the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also be changed through heat treatment of its mechanical, physical and chemical properties to obtain different use properties. The following figure is the main steel several metallographic organization (the steel will be sampled, after grinding, polishing, and finally with a specific corrosive agent corrosion display, the organization observed under the metallographic microscope).

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Four main metallographic structures of iron and steel

The classification of mechanical heat treatment

Metal heat treatment process can be broadly divided into three categories of overall heat treatment, surface heat treatment and chemical heat treatment, as follows.
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According to the different heating medium, heating temperature and cooling methods, each major category can be distinguished into a number of different heat treatment processes. The same metal using different heat treatment processes, can obtain different metallographic organization, and thus have different properties. Steel is the most widely used metal in industry, and the steel microstructure is also the most complex, so there are a variety of steel heat treatment processes.

Bulk Heat Treatment

Bulk heat treatment is a metal heat treatment process in which the workpiece is heated as a whole and then cooled at an appropriate rate to change its surrounding metallurgical structure and thus change the overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.

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Annealing is to heat the workpiece to the appropriate temperature, according to the material and the size of the workpiece using different holding time, and then slow cooling, the purpose is to make the metal internal organization to achieve or close to equilibrium, or to make the internal stress generated by the previous process to release, to obtain good process performance and performance, or for further quenching for organizational preparation. Take 45# steel as an example, the metallographic phase after annealing is austenite, which becomes too soft after annealing, and generally 45# steel is not annealed.
Professional explanation: sub-eutectoidal steel workpiece heated to AC3 (end of heating ferrite into austenite temperature) above 20-40 degrees, holding a period of time, with the furnace slowly cooled (or buried in sand or lime cooling) to below 500 degrees in the air cooling heat treatment process.


Normalization is to heat the workpiece to a suitable temperature after cooling in the air, the effect of normalization is similar to annealing, but the organization obtained is finer, often used to improve the cutting properties of the material, but also sometimes used for some parts with low requirements as the final heat treatment. Normalized to 45 after the metallographic phase of austenite + pearlite. Professional explanation: the steel or steel parts are heated to the critical point AC3 (for sub-eutectoidal steel) or Accm (the end temperature of the secondary carburizing body dissolved into austenite when heated, for over-eutectoidal steel) above 30 ℃ -50 ℃, after holding appropriate time, in the free flow of air uniform cooling heat treatment process for normalizing. Normalized to produce sub-eutectoid steel for F + S, eutectoid steel for S, over-eutectoid steel for S + Fe3CII normalized and fully annealed the main difference is the faster cooling rate, the purpose is to normalize the steel organization, also known as normalization treatment.


Quenching is the workpiece is heated and insulated, and then rapidly cooled in water, oil or other inorganic salt solutions, organic aqueous solutions and other quenching and cooling media. After the steel parts become hard, but at the same time become brittle. Take 45# steel as an example, it is rarely quenched alone because it is difficult to get the desired hardness. Professional explanation: the steel austenitized steel parts with appropriate cooling rate, so that the workpiece in the cross-section of all or a certain range of unstable tissue structure such as martensite transformation of heat treatment process.


Tempering is to reduce the brittleness of the steel parts, the quenched steel parts in a certain appropriate temperature above room temperature and below 650 ℃ for a longer period of holding, and then cooled, this process is called tempering. Professional explanation: the quenched workpiece will be heated to the critical point AC1 (heating pearlite to austenite transformation start temperature) below a suitable temperature hold for a certain period of time, followed by cooling in accordance with the required method to obtain the required organization and properties of the heat treatment process. The following is an analysis of the results of 45# steel, several heat treatments.

Serial number Material Heat treatment process Etchant Metallography
1 45# steel 860 ° C furnace cooling (annealing) 3% nitric acid wine semen Pearlite + ferrite
2 45# steel 860 ° C air cooling (normalizing) 3% nitric acid wine semen Sorbite + ferrite
3 45# steel 860 ° C oil quenching (normalizing) 3% nitric acid wine semen Martensite + troostite
4 45# steel 860 ° C quenching + 200 ° C tempering 3% nitric acid wine semen Tempered martensite
5 45# steel 860 ° C quenching + 400 ° C tempering 3% nitric acid wine semen Tempered martensite 
6 45# steel 860 ° C quenching + 600 ° C tempering 3% nitric acid wine semen Tempered sorbite

After the temperature rises to the critical point, different heat treatment processes evolve with different heating temperatures and cooling methods. Different cooling rates will precipitate different metallic phases. V1 will precipitate ferrite and pearlite; V2 will precipitate ferrite and sorite; V3 will precipitate ferrite and quartzite. When the cooling rate reaches V4, very little reticulated ferrite and keratite are precipitated, and austenite is mainly transformed into martensite and keratite. When the cooling rate exceeds V5, it is all transformed into martensite.

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Annealing, normalizing, quenching, tempering is the overall heat treatment of the “four fires”, they are often combined with each other, of which the quenching and tempering are closely related, often used in conjunction.


In order to obtain a certain strength and toughness, the quenching and high-temperature tempering process combined. In 45# steel, for example, quenching is followed by tempering to obtain martensite, followed by tempering to obtain sothernite. In this way, the material can get high strength, but also has excellent toughness, plasticity, cutting properties.

Aging Treatment

Some alloys are also called solid solution (in solid state conditions, a component dissolved in other components and the formation of a single, homogeneous crystalline solid metal, usually stainless steel is a typical solid solution) after quenching to form a supersaturated solid solution, it will be placed at room temperature or a slightly higher appropriate temperature for a longer period of time to improve the hardness, strength or electrical magnetic properties of the alloy. For example, in order to eliminate the change in size and shape of precision gauges or molds and parts during long-term use, aging treatment is required.


The pressure processing deformation and heat treatment effectively and closely combined to carry out, so that the workpiece to obtain a very good strength, toughness fit method, also equivalent to hot forging. The most common example is the old-fashioned blacksmith store for iron.

Vacuum Heat-Treatment

Heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which not only makes the workpiece not oxidized, not decarburized, keep the surface of the treated workpiece bright and clean, and improve the performance of the workpiece. After vacuum heat treatment, the part has small distortion and high quality, and the process itself is flexible in operation and harmless. Therefore, vacuum heat treatment is not only the necessary means of heat treatment of certain special alloys, but also in the heat treatment of general engineering steel, especially tools, molds and precision coupling parts, etc. After vacuum heat treatment, the service life is greatly improved compared with the general heat treatment.


Thermolizing is a metal heat treatment process that heats only the surface layer of a workpiece to change its surface mechanical properties. In order to heat only the surface layer of the workpiece without excessive heat transfer into the workpiece, the heat source used must have a high energy density, that is, in the unit area of the workpiece to give a large amount of heat energy, so that the surface layer of the workpiece or local can reach high temperatures for a short time or instantaneously. The main methods of surface heat treatment are flame quenching and induction heating heat treatment, commonly used heat sources such as oxyacetylene or oxypropane flame, induction current, laser and electron beam, etc.. For example, some shafts, gears and parts subjected to variable loads, the surface has a high resistance to wear, while the internal need for good toughness and strength. It can be possible to make the overall performance requirements of the workpiece by surface heat treatment.

Thermo-Chemical Treatment

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

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Surface modified technique

Surface modification technique is a combination of chemical heat treatment and physical methods. Change the chemical composition or structure of the surface of the material or workpiece to improve the performance of machine parts or materials, a class of heat treatment technology. It includes chemical heat treatment (nitriding, carburizing, metalizing, etc.); surface coating (low-pressure plasma spraying, low-pressure arc spraying, laser remelting compound and other thin film coating, physical vapor deposition, chemical vapor deposition, etc.) and non-metallic coating technology. These technologies used to strengthen the surface of the parts or materials to give the parts high temperature resistance, corrosion resistance, wear resistance, fatigue resistance, radiation protection, electrical and magnetic conductivity and other new characteristics. So that the original in the high speed, high temperature, high pressure, heavy load, corrosive media environment of the work of the parts, improve reliability and extend the service life. Nothing is more common than the non-stick pans at home. Except for 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 mills have already carried out heat treatment on behalf of the designer, leaving the raw metal for machinery in a heat-treated state. The machine designer just needs to choose the material.

How do I find the right machining manufacturer?

If you own a business that requires machining services, then you will want to make sure that you find the best possible machining solution for your needs. A good way to go about doing this is by researching the various manufacturers in this field and finding out which one can provide your company with effective products at an affordable price.
The world of machining is a highly specialized field, and manufacturers of all stripes require the best machinists to create their products.
But machining is a highly specialized field, and manufacturers of all stripes require the best machinists to create their products. Whether it’s a large corporation or a small business, companies need machinists who can meet specific demands in order to produce quality materials at a competitive price.
The world of machining is vast, encompassing everything from raw materials to finished products. It covers many different types of manufacturing activities, including:

  • Turning (which includes lathe work)
  • Milling (also called boring)
  • Drilling (including broaching)
  • Grinding

You can find out who has been working in the field of machining for some time and who has a good reputation among other manufacturers. While there are many ways to do this, one of the most common ways is by looking at their website and social media accounts. You should also check if they have any reviews on their site or elsewhere online; if so, look at them carefully to see what people think of the company as well as what services they provide customers with.
If you’re looking for specific certifications (such as ISO), this is another thing that you can search for on your own time. If no results come up after searching through multiple websites, try calling around until you find someone who knows about this certification or contact someone directly at each company that claims they have it already implemented within their business structure—this will help prevent any confusion later down the road when discussing what kinds of jobs need done versus what kind don’t need done due solely based upon whether or not each provider has received these types.
Finally, make sure that all machines used in production processes meet safety standards set forth by local governments; otherwise it may be hard for someone else working alongside those machines without protective gear like goggles or glasses because he/she might get hurt from debris flying off into his/her eyes when cutting metal parts etcetera.”
Another way to learn more about the company is by asking their references and how long they have been using the services of that particular company.
You can also ask them what they would rate the quality of work provided by that company.
Another good strategy is to research their website and contact them if you have any questions about their services.
You can also research a company’s website to see if they have any testimonials, photos or videos of their work, and a list of references.

You should ask about the type of machinery they use, the materials they work with and if there are any restrictions on what types of jobs they do (for example, only doing large-scale production work).

Make sure that you know what exactly it is the company does before you hire them for your project.
The first thing you should do is find out what exactly the company does. If you don’t know what they do, how will you be able to see if they are a good fit for your project? You need to make sure that the machining manufacturer specializes in the kind of work that you’re looking for before hiring them.
The next step is to make sure that there’s a good fit between your goals and those of the company. Make sure that their goals align with yours, because this can affect both how quickly they get the job done and how much money it ends up costing both parties involved in this transaction.
Finding the right solution for your machining needs can be a daunting task; however, it is crucial that you do so, as this will ensure that your business will be able to provide effective results.
Machining services can be used in many different ways. For example, they may help you manufacture parts or components for your company’s products or services. You may also use them to create prototypes of products that you are developing and testing before going into production with a full order of finished goods.
In order for this process to work effectively, it is important that you choose a machining provider who has experience in providing these kinds of solutions—and then select one based on how well their services align with yours (and vice versa). Here are some tips on how to find such providers:
As you can see, finding the right machining manufacturer is not an easy task. However, if you take the time to research your options and find a solution that meets all of your needs, then it will be worth it in the end. We hope this article has helped inspire some ideas for finding the right fit for your project!
Source: China machining solutions provider – Yaang Pipe Industry (



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