Effect of heat treatment on metal materials

Heat treatment is a process to artificially change the internal structure and structure of a metal or alloy by means of a certain thermal action (sometimes combined with mechanical, chemical or other actions), so as to obtain the required properties. The main functions and purposes of heat treatment in the production process of metal materials and products are as follows:

1. Improve process performance and ensure smooth process;

2. Improve the use performance and give full play to the potential of materials.

1、 Essence of Metal Heat Treatment

In the production process of various metal materials and products, in order to make metal workpieces have the required mechanical, physical and chemical properties, in addition to reasonable selection of materials and various forming processes, heat treatment is one of the indispensable important links. In order to obtain the required properties of metal materials, heat treatment technology plays an important role and is widely used in modern technology. Compared with other processing technologies, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but endows or improves the usability of the workpiece by changing the internal microstructure of the workpiece or changing the chemical composition of the workpiece surface. Its feature is to improve the internal quality of the workpiece, which is generally not visible to the naked eye.

The corresponding heat treatment can be carried out throughout the production process of metal to improve the performance of metal materials. Metal castings usually need to be annealed at low temperature to eliminate internal stress, or fully annealed, or normalized, and some also need to be tempered after quenching (aging). The heat treatment of metal ingots, and the heat treatment of finished products in the process of pressure processing, is one of the main processes in the manufacture of semi-finished products and machine parts in metallurgical enterprises and machinery plants. As an intermediate process, heat treatment can improve some processing properties (such as forging, cutting, etc.); As a final operation, it can endow metals and alloys with the required mechanical, physical and chemical properties to ensure that the products meet the specified quality requirements. Heat treatment is also more effective than machining or other treatments in influencing the depth and diversity of structural changes of metal materials. For example, various steels often need normalizing treatment to obtain fine and uniform structure and better mechanical properties. The quenched and tempered steel shall be quenched and tempered at high temperature to ensure good overall mechanical properties. In addition, heat treatment is an important part of the processing flow of semi-finished products and products of non-ferrous metals and their alloys. Aluminum alloys generally need aging strengthening to improve their strength to meet the required mechanical properties.

2、 Process of metal heat treatment

Heat treatment process generally includes three processes of heating, heat preservation and cooling, sometimes only two processes of heating and cooling. These processes are interconnected and uninterrupted. Heating is one of the important steps of heat treatment. There are many heating methods for metal heat treatment, the first is to use charcoal and coal as heat sources, and then use liquid and gas fuels. The application of electricity makes heating easy to control and free from environmental pollution. These heat sources can be used for direct heating, or indirect heating through molten salts or metals, or even floating particles.

During metal heating, when the workpiece is exposed to the air, oxidation and decarburization often occur (that is, the carbon content on the surface of steel parts decreases), which has a very adverse effect on the surface performance of parts after heat treatment. Therefore, metal should be heated in a controlled atmosphere or a protective atmosphere, molten salt and vacuum, or by coating or packaging.

Heating temperature is one of the important process parameters of heat treatment process, and the selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material being treated and the purpose of heat treatment, but generally it is heated above the phase change temperature to obtain the required structure. In addition, the transformation takes a certain time. Therefore, when the metal workpiece surface reaches the required heating temperature, it must be kept at this temperature for a certain time to make the internal and external temperatures consistent and the microstructure completely transformed. This time is called the holding time. When high-energy density heating and surface heat treatment are used, the heating speed is extremely fast, and generally there is no holding time or the holding time is very short, but generally it is heated above the phase change temperature to obtain the required structure.

Cooling is also an indispensable step in the heat treatment process. The cooling method varies with different processes, mainly controlling the cooling rate. Generally, it is heated above the phase change temperature to obtain the required structure. However, there are different requirements due to different steel grades. For example, air hardening steel can be hardened at the same cooling rate as normalizing.

Metal heat treatment process can be generally divided into overall heat treatment, surface heat treatment, local 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. Different heat treatment processes are used for the same metal, which can obtain different 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 kinds of steel heat treatment processes.

Overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to change its overall mechanical properties. There are four basic processes for the overall heat treatment of steel: annealing, normalizing, quenching and tempering.

Annealing → heat the workpiece to an appropriate temperature, adopt different holding time according to the material and workpiece size, and then slowly cool (the slowest cooling speed), so as to make the internal structure of the metal reach or approach the equilibrium state, obtain good process performance and service performance, or prepare for further quenching.

Normalizing → heating the workpiece to a suitable temperature and then cooling it in the air. The effect of normalizing is similar to that of annealing, except that the structure obtained is finer, which is often used to improve the cutting performance of materials, and sometimes used as the final heat treatment for some parts with low requirements.

Quenching → After the workpiece is heated and kept warm, it is cooled rapidly in water, oil or other inorganic salts, organic aqueous solutions and other quenching media. After quenching, the steel becomes hard but brittle at the same time. In order to reduce the brittleness of steel parts, the quenched steel parts shall be kept warm for a long time at an appropriate temperature higher than room temperature but lower than 710 ℃, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are the “four fires” in the overall heat treatment. The quenching and tempering are closely related and often used together.

With the difference of heating temperature and cooling mode, the “four torches” have evolved different heat treatment processes. Quenching and tempering are closely related and often used together. After some alloys are quenched to form supersaturated solid solution, they shall be kept at room temperature or slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloy. This kind of heat treatment process is called annealing similarity, but the obtained structure is thinner, which is often used to improve the cutting performance of materials, and sometimes used as the final heat treatment for some parts with low requirements. The method of combining the deformation of pressure processing with heat treatment effectively and closely to make the workpiece obtain a good combination of strength and toughness is called thermomechanical treatment; The heat treatment conducted in negative pressure atmosphere or vacuum is called vacuum heat treatment. It can not only prevent the workpiece from oxidation and decarburization, maintain the surface of the workpiece after treatment, improve the performance of the workpiece, but also conduct chemical heat treatment through infiltration agent.

Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change its mechanical properties. In order to only heat the surface of the workpiece without transferring too much heat into the interior of the workpiece, the heat source used must have a high energy density, that is, a large amount of heat energy is given to the workpiece per unit area, so that the surface or part of the workpiece can reach a high temperature in a short time or instantaneously. The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment. The common heat sources include oxyacetylene or oxypropane flame, induced current, laser and electron beam.

Chemical heat treatment is a metal heat treatment process by changing the chemical composition, structure and properties of the surface layer of the workpiece. 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 the medium (gas, liquid, solid) containing carbon, nitrogen or other alloy elements for a long time, so that the surface of the workpiece is infiltrated with carbon, nitrogen, boron, chromium and other elements. After the elements are infiltrated, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment include carburizing, nitriding, metallizing, and composite carburizing.

Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools. Generally speaking, 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 working.

For example, malleable cast iron can be obtained from white cast iron after long-term annealing treatment to improve plasticity; The gear adopts the correct heat treatment process, and its service life can be doubled or tens of times longer than that of the gear without heat treatment; In addition, cheap carbon steel has some valuable alloy steel properties by infiltrating some alloy elements, which can replace some heat-resistant steel and stainless steel; Tools and moulds can be used only after almost all of them have been heat treated.