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Heat treatment Heat treatment

Heat treatment refers to a metal hot working process in which materials are heated, insulated and cooled to obtain desired structure and properties in a solid state.

Heat treatment

1. Normalizing:
Heat treatment of pearlitic structures by heating steel to a critical point, AC3 or an appropriate temperature and cooling it after a certain period of time.

2. Annealing:
A heat treatment process for cooling in air. Heat the workpiece of hypoeutectoid steel to 20 -- 40 degrees above AC3. After a period of heat preservation, cool slowly (or be buried in sand or lime) to less than 500 degrees

3. Solid solution heat treatment:
The alloys are heated to a high temperature single-phase zone at constant temperature, allowing the excess phase to be fully dissolved into the solid solution, which is then rapidly cooled to get a supersaturated solid solution.

4. Aging:
The properties of alloys change with time when they are placed at room temperature or slightly above room temperature after solid solution heat treatment or cold plastic deformation.

5. Solid solution treatment:
To dissolve all components in the alloy, strengthen the solid solution, improve toughness and corrosion resistance, eliminate stress and soften, so as to continue processing and molding.

6. Aging treatment:
Raise the temperature, enable the enhanced phase precipitation to precipitate, can harden, improve the intensity.

7. Quenching:
The austenite of steel is cooled at an appropriate cooling rate, so that the workpiece is transformed into martensite in the cross section or within a certain range.

8. Tempering:
The hardened workpiece is heated to an appropriate temperature below the critical point AC1 for a certain period of time, which is then cooled by a method that meets the requirements to obtain the desired microstructure and properties.

9. Carburization of steel
Carburization is a process of simultaneous infiltration of carbon and nitrogen to the surface of steel. Traditionally, carbonitriding is also called cyanation.  Carbonitriding of medium temperature gas and low temperature gas is widely used. The main purpose of carbon nitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low temperature gas carbonitriding mainly nitriding. Its main purpose is to improve the wear resistance and anti-bite of steel.

10. Quenching and tempering
The heat treatment combined with quenching and tempering is commonly used to be called quenching and tempering. It is widely used in all kinds of important structural parts, especially the linkages, bolts, gears and shafts which work under alternating loads. The mechanical properties of tempering sorbite are better than that of normalizing sorbite with the same hardness. Its hardness depends on the tempering temperature and is related to the tempering stability of steel and the sectional dimension of workpiece, generally between HB200 and 350.

Process characteristics
Metal 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 the workpiece. By changing the microstructure inside the workpiece, or changing the chemical composition on the workpiece surface, it imparts or improves the workpiece performance. It is characterized by improving the internal quality of the workpiece, which is generally not visible to the naked eye.

In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to reasonable selection of materials and various forms, heat treatment process is often necessary. Iron and steel is the most widely used material in the mechanical industry. With complex microstructure, iron and steel can be controlled through heat treatment. So the heat treatment of iron and steel is the main content of metal heat treatment. In addition, the mechanical, physical and chemical properties of aluminum, copper, magnesium, titanium and other alloys can be changed through heat treatment to obtain different performance. [1]

Thermal process

The technological process

Heat treatment process generally includes three processes of heating, heat preservation and cooling, sometimes only two processes of heating and cooling. These processes are interlinked and uninterrupted. Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. Charcoal and coal were first used as heat sources, and liquid and gas fuels were used recently. The application of electricity makes heating easy to control and free of environmental pollution. These heat sources can be heated directly, or indirectly by molten salt or gold, and floating particles. When metal is heated, the workpiece is exposed to air, and oxidization and decarburization (i.e. reduction of carbon content on the surface of steel parts) often occur, which has a negative impact on the surface performance of the parts. As a result, the metal is usually heated in a controlled or protective atmosphere, molten salt and vacuum. They can also be heated with the protect of paint or packaging. The heating temperature is one of the important technological parameters of heat treatment process. Selecting and controlling the heating temperature is the key to guarantee the heat treatment quality. The heating temperature varies with the metal material and the purpose of the heat treatment, but is generally heated above the phase change temperature to obtain the high-temperature tissue. In addition, it takes a certain time to transform, so when the surface’s temperature of the metal workpiece reaches the requirement, it must maintain a certain time at this temperature to make the internal and external temperatures consistent and make the microstructure completely transformed. This period of time is called the holding time. When high energy density heating and surface heat treatment are adopted, the heating speed is extremely fast, and there is generally no heat preservation time, while the heat preservation time of chemical heat treatment is usually longer.

Cooling is also an indispensable step in the heat treatment process. The cooling method varies with different processes, controlling the cooling rate. Generally, the cooling speed of annealing is the slowest, the cooling speed of normalizing is faster, and the cooling speed of quenching is faster. However, there are different requirements for different types of steel. For example, hollow hard steel can be hardened with normal cooling speed.

Process classification

Metal heat treatment process can be divided into three major categories: integral heat treatment, surface heat treatment and chemical heat treatment. According to different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same kind of metal USES different heat treatment process, may obtain different tissue, thus the workpiece has the different performance. Iron and 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. Integral heat treatment is a metal heat treatment process in which the workpiece is heated whole and then cooled at an appropriate rate to obtain the required metallographic structure to change its overall mechanical properties. Generally speaking, there are four basic processes: annealing, normalizing, quenching and tempering.

Technology means

Annealing is to heat the workpiece to the appropriate temperature, adopt different heat preservation time according to the material and workpiece size, and then slowly cool the workpiece. The purpose is to make the inner structure of the metal reach or close to the equilibrium state and obtain good technical performance and service performance, or prepare the tissue for further quenching. Normalizing is to cool the workpiece in the air after heating it to a suitable temperature. The effect of normalizing is similar to that of annealing, but the obtained tissue is finer. It is often used to improve the cutting performance of materials, and is sometimes used for the final heat treatment of some parts with less requirements. Quenching refers to the rapid cooling of the workpiece in water, oil, inorganic salt, organic water solution and other quenching media after heating and keeping the workpiece. After quenching, the steel becomes hard, but at the same time becomes brittle. To eliminate brittleness in time, it generally need to be tempered in time. In order to reduce the brittleness of steel, keep the hardened steel at certain temperature between room temperature and 650 ℃ for a long time, then cool it again. This process is called tempering.

Annealing, normalizing, quenching and tempering are the "four torches" in the whole heat treatment, among which quenching and tempering are closely related and often used together. With different heating temperature and cooling mode, the "four torches" developed different heat treatment process. In order to make the workpiece have a certain strength and toughness, the process of combining quenching and high temperature tempering is called tempering. After some alloys are hardened into a supersaturated solid solution, they are kept at room temperature or slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism, etc. Such heat treatment process is called ageing treatment. Pressure machining deformation and heat treatment are combined effectively and closely, so that the workpiece can get a good combination of strength and toughness. Such heat treatment process is called deformation heat treatment. The heat treatment in the vacuum atmosphere or vacuum is called vacuum heat treatment, which can not only make the workpiece not oxidize, not decarbonization, keep the surface of the workpiece clean and improve the performance of the workpiece, but also conduct chemical heat treatment through the infiltrating 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 workpiece surface without inputting too much heat into the workpiece, the heat source used must have a high energy density, that is, to give a larger amount of heat energy to the workpiece per unit area, so that the workpiece surface or local energy can reach high temperature for a short time or instantaneous. The main methods of surface heat treatment are flame quenching and induction heat treatment. The commonly used heat source are oxyacetylene or oxypropane, induct current, laser and electron beam, etc.

Chemical heat treatment is a metal heat treatment process by changing the chemical composition, structure and performance of workpiece surface. Chemical heat treatment differs from surface heat treatment in that the former changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to put the workpiece in the medium containing carbon, salt or other alloy elements (gas, liquid, solid) and heat preservation, so that the surface of the workpiece can be infiltrated into carbon, nitrogen, boron, chromium and other elements. After infiltrating elements, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment are carburizing, nitriding and metal carburizing. Heat treatment is one of the important processes in the manufacturing of mechanical parts and moulds. Generally speaking, it can guarantee and improve various properties of workpiece, such as wear resistance, corrosion resistance. The structure and stress state of the blank can also be improved to facilitate various cold and hot processing. For example, white cast iron can be made malleable after annealing for a long time and its plasticity can be improved. Gear with correct heat treatment process, the service life can be doubled or dozens of times than the gear without heat treatment. In addition, the cheap carbon steels have the properties of certain valence alloy steels by infiltrating some alloying elements. In this way, the cheap carbon steels can replace some heat resistant steels and stainless steels. TIn addition, almost all the moulds need to be heat treated before use.


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