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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 the equilibrium state, obtain good process performance and performance, or for further quenching Prepare for organization. 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 parts with low requirements. as the final heat treatment. Quenching is to rapidly cool the workpiece in a quenching medium such as water, oil or other inorganic salts and organic aqueous solutions after heating and keeping the workpiece warm. After quenching, the steel becomes hard, but at the same time becomes brittle. In order to eliminate the brittleness in time, it is generally necessary to temper in time. In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature but lower than 650 ° C for a long time, and then cooled. This process is called tempering.
Annealing, normalizing, quenching, and tempering are the "four fires" in the overall heat treatment. Among them, quenching and tempering are closely related and are often used in conjunction, and both are indispensable. The "four fires" have 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 some alloys are quenched to form a supersaturated solid solution, they are kept at room temperature or a slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical and magnetic properties of the alloy. Such a heat treatment process is called aging treatment. The method of combining pressure deformation and heat treatment effectively and closely to make the workpiece obtain good strength and toughness is called deformation heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which not only makes The workpiece is not oxidized or decarburized, the surface of the workpiece after treatment is kept smooth, and the performance of the workpiece is improved. 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 layer or local area 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. 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 former changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, salt or other alloying elements, and keep 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 elements are infiltrated, 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. Heat treatment is one of the important processes in the manufacture of mechanical parts and tools. Generally speaking, it can ensure 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.