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Metal heat treatment process

Date: 2019-05-30 17:05
The heat treatment process generally includes three processes of heating, heat preservation and cooling, and sometimes only two processes of heating and cooling. These processes are connected to each other and cannot be interrupted.
    Heating is one of the important steps in heat treatment. There are many heating methods for metal heat treatment.  Wood and coal are used as heat sources, and liquid and gaseous fuels are applied. Electrical applications make heating easy to control and environmentally friendly. These heat sources can be used for direct heating, or by indirect heating of molten salts or metals, or floating particles.
    When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization (ie, the carbon content on the surface of the steel part) often occur, which has a detrimental effect on the surface properties of the parts after the heat treatment. Thus, the metal should generally be heated in a controlled or protective atmosphere, in a molten salt and in a vacuum, or protected by a coating or packaging process.
    Heating temperature is one of the important process parameters of heat treatment process. Selecting and controlling heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies depending on the metal material to be treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain the desired structure. In addition, the transformation takes a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain period of time, so that the internal and external temperatures are uniform, and the microstructure is completely transformed. This period of time is called the holding time. When high-energy density heating and surface heat treatment are used, the heating rate is extremely fast, generally there is no holding time or the holding time is short, and the heat treatment time of the chemical heat treatment is often long.
    Cooling is also an indispensable step in the heat treatment process. The cooling method varies from process to process, mainly to control the cooling rate. Generally, the cooling rate of annealing is slow, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster. However, there are different requirements depending on the type of steel. For example, an empty hard steel can be hardened at the same cooling rate as a normal fire.
    The metal heat treatment process can be roughly divided into an overall heat treatment, a surface heat treatment, a local heat treatment, and a chemical heat treatment. Depending on the heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal uses different heat treatment processes to obtain different microstructures and thus different properties. Steel is a metal that is widely used in industry, and the microstructure of steel is also complicated, so there are many kinds of steel heat treatment processes.
    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 overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.
    Annealing→heating the workpiece to the appropriate temperature, using different holding times according to the material and the workpiece size, and then slowly cooling (cooling rate*slow), in order to achieve or close to the equilibrium of the internal metal structure, and obtain good process performance and use. Performance, or preparation for further quenching.
Normalizing→The workpiece is heated to a suitable temperature and then cooled in the air. The effect of normalizing is similar to annealing, except that the obtained structure is finer, which is often used to improve the cutting performance of materials, and sometimes used for some less demanding parts. As * final heat treatment.
    Quenching → After heating and holding the workpiece, it is rapidly cooled in a quenching medium such as water, oil or other inorganic salts or organic aqueous solutions. After quenching, the steel becomes hard but becomes brittle at the same time. In order to reduce the brittleness of the steel, the quenched steel is heat-treated for a long time at a suitable temperature above room temperature and below 710 ° C, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are the "four fires" in the overall heat treatment. Among them, the quenching and tempering are closely related, and often used together, they are indispensable.
    The “four fires” evolved different heat treatment processes with different heating temperatures and cooling methods. In order to obtain a certain strength and toughness, the process of combining quenching and high-temperature tempering is called quenching and tempering. After quenching some alloys to form a supersaturated solid solution, it is kept at room temperature or a slightly higher temperature for a longer period of time to increase the hardness, strength or electrical magnetic properties of the alloy. Such a heat treatment process is called aging treatment. The method of effectively and tightly combining pressure processing deformation with heat treatment to obtain a good strength and toughness of the workpiece is called deformation heat treatment; the heat treatment in a vacuum atmosphere or vacuum is called vacuum heat treatment, which can not only make The workpiece is not oxidized, does not decarburize, keeps the surface of the workpiece smooth after treatment, improves the performance of the workpiece, and can also be subjected to chemical heat treatment by using an infiltrant.
    Surface heat treatment is a metal heat treatment process that only heats the surface layer of the workpiece to change its surface mechanical properties. In order to heat only the surface layer of the workpiece without excessive heat being introduced into the interior of the workpiece, the heat source used must have a high energy density, that is, to give a large amount of heat energy to the workpiece per unit area, so that the surface layer or local portion of the workpiece can be short-time or instantaneous. Reach high temperatures. The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment, commonly used heat sources such as aerobic acetylene or oxypropane, induction current, laser and electron beam.
    Chemical heat treatment is a metal heat treatment process that changes the chemical composition, structure and properties of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface layer of the workpiece. 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 penetrates into carbon, nitrogen, boron and chromium. After infiltration of the elements, other heat treatment processes such as quenching and tempering are sometimes performed. The main methods of chemical heat treatment include carburizing, nitriding, metal infiltration, and composite infiltration.
    Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tooling. In general, it can guarantee and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It also improves the microstructure and stress state of the blank to facilitate various cold and hot processing.
For example, white cast iron can obtain malleable cast iron after long-time annealing treatment to improve plasticity; the gear adopts the correct heat treatment process, and the service life can be doubled or several times higher than that of the gear without heat treatment; in addition, the inexpensive carbon steel passes through Some alloying elements have some expensive alloy steel properties, which can replace some heat-resistant steels and stainless steels; almost all of the tooling tools need to be heat treated before they can be used.
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