Metal heat treatment of operation process

Heat treatment process generally includes three processes: heating, heat preservation and cooling, sometimes only heating and cooling. These processes are interlinked and uninterrupted. Heating is one of the important steps of heat treatment.

There are many heating methods for metal heat treatment, the first one is to use charcoal and coal as heat source, and then use liquid and gas fuel. The application of electricity makes heating easy to control and no environmental pollution. These heat sources can be used to heat directly, or indirectly through molten salt or metal, or even floating particles.

When the metal is heated, the workpiece is exposed to the air, which often occurs oxidation and decarburization (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse effect on the surface performance of parts after heat treatment. Therefore, the metal should be heated in controllable or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging.

Heating temperature is one of the important process parameters of heat treatment process. 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 it is generally heated above the phase change temperature to obtain the required structure. In addition, the transformation needs 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 time, so that the internal and external temperatures are the same, and the microstructure is completely transformed, which is called the holding time.

When using high energy density heating and surface heat treatment, the heating speed is very fast, generally there is no heat preservation time or short heat preservation time, while the heat preservation time of chemical heat treatment is often longer. 

Cooling is also an indispensable step in the process of heat treatment. The cooling method varies with the process, mainly controlling the cooling speed.

Generally, the cooling speed of annealing is the slowest, that of normalizing is faster, and that of quenching is faster. But there are different requirements due to different kinds of steel. For example, air hardening steel can be quenched at the same cooling rate as normalizing. Metal heat treatment process can be divided into overall heat treatment, surface heat treatment, local heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Different heat treatment processes can be used for the same metal to obtain different structures and 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 heat treatment process. The whole heat treatment is a metal heat treatment process that heats the whole workpiece and cools it at a proper speed to change its whole mechanical properties. There are four basic heat treatment processes of iron and steel: annealing, normalizing, quenching and tempering. Annealing is to heat the workpiece to a proper temperature, adopt different holding time according to the material and workpiece size, and then cool it slowly, so as to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and use performance, or make organization preparation for further quenching.

Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the microstructure obtained is finer, which is often used to improve the cutting performance of materials, and sometimes used for the final heat treatment of parts with low requirements. Quenching is the rapid cooling of workpiece in water, oil or other inorganic salt, organic water solution and other quenching medium after heating and heat preservation. 

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 are kept warm for a long time at a proper 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 whole heat treatment, in which quenching and tempering are closely related and often used together. With the different heating temperature and cooling mode, the "four fire" has evolved different heat treatment processes.

In order to obtain a certain strength and toughness, the process of combining quenching and high temperature tempering is called tempering. After some alloys are quenched to form supersaturated solid solution, they are kept at room temperature or a higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloy. Such heat treatment process is called aging treatment. The effective and close combination of pressure processing deformation and heat treatment is called deformation heat treatment, which can make the workpiece obtain good strength and toughness matching. The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only make the workpiece not oxidized and decarburized, keep the surface of the workpiece after treatment clean and clean, improve the performance of the workpiece, but also penetrate the permeating agent for chemical heat treatment Li.

Surface heat treatment is a metal heat treatment process that only heats the surface of workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece without too much heat entering the interior of the workpiece, the heat source used must have a high energy density, that is, to give a larger heat energy to the workpiece per unit area, so that the surface layer or part of the workpiece can reach a high temperature in a short time or instantaneously. 

The main methods of surface heat treatment are laser heat treatment, flame quenching and induction heating heat treatment. The commonly used heat sources are flame, induction current, laser and electron beam, etc. Chemical heat treatment is a metal heat treatment process by changing the chemical composition, structure and performance of the workpiece surface.

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 infiltrates carbon, nitrogen, boron, chromium and other elements. After infiltration of elements, other heat treatment processes such as quenching and tempering are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding, metallizing and compound nitriding. 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 workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the structure and stress state of the blank, so as to facilitate all kinds of cold and hot processing.

For example, white cast iron can obtain malleable cast iron after long-time annealing treatment to improve the plasticity; gear adopts correct heat treatment process, and its service life can be doubled or dozens of times longer than that of gear without heat treatment; in addition, low-cost carbon steel has some alloy steel properties by permeating some alloy elements, which can replace some heat-resistant steel and stainless steel; tooling is almost complete It can only be used after heat treatment.

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