Metal heat treating process

    Heat Treatment generally includes heating, insulation, cooling three processes, sometimes only two process heating and cooling. The process of convergence with each other, can not be interrupted.

Heating is an important step of heat treating. Metal heat treatment method of heating a lot, the first is the use of charcoal and coal as the heat source, and then the application of liquid and gaseous fuels. Application of electric heating is easy to control, and no environmental pollution. With these heat sources can be directly heated, it can also be a salt or molten metal, as well as indirect heating floating particles.
    When the metal is heated, the workpiece is exposed to the air, often oxidized, decarbonization (ie, reduce the carbon content of steel parts surface), which after heat treatment the surface properties of the parts have a very negative impact. Thus the metal is usually done in a controlled atmosphere or protective atmosphere, molten salt and vacuum heating, paint or packaging methods can also be used to protect heating.

    The heating temperature is one of the important parameters of the heat treatment process, selection and control of the heating temperature
It is to ensure the quality of heat treatment a major problem. With the purpose of heating temperature was treated metal material and heat treatment vary, but are generally heated to a temperature above the phase transition, in order to obtain tissue temperature. Further change will take some time, so when the metal surface to achieve the required heating temperature, must at this temperature for a certain time, so that the temperature inside and outside the same, so the microstructure of the transition is completed, this time called the incubation period. When using high-energy density and surface heat treatment heating, heating fast, generally no holding time, and holding time of chemical treatment is often longer.
    The cooling step is missing is not available during the heat treatment process, cooling method by processes differ mainly controlling the cooling rate. General annealing cooling slowest cooling faster normalizing, quenching cooling faster. But also because of different grades have different requirements, such as empty hard steel can be used as cooling rate normalizing hardened.

    Metal heat treatment process can be divided into the overall heat treatment, surface treatment and chemical heat treatment of three categories. Depending on the heating medium, heating temperature and cooling method, each category can be divided into a number of different heat treatment process. The same metal with different heat treatment process, available in different organizations, so as to have different properties. Steel is the most widely used industrial metal, but also the most complex microstructure of the steel, and therefore a wide range of steel heat treatment process.
    Overall heat treatment is heating the whole of the workpiece, and then cooled at a rate appropriate to change the metal heat treatment process its overall mechanical properties. Roughly the overall heat treatment of steel annealing, normalizing, quenching and tempering four basic processes.

    Annealing the workpiece is heated to a suitable temperature, depending on the material and size of the workpiece with different holding time, and then slowly cooled, the purpose of the metal within the organization at or near equilibrium, good process performance and performance, or for the further hardening make organizational preparations. After normalizing the workpiece is heated to a suitable temperature in the air cooled, with annealing normalizing effect similar to, but more fine organization was, generally used to improve the material cutting, but also sometimes used for some less demanding parts As a final heat treatment.

    Quenching the workpiece after heat insulation, rapid cooling in water, oil, or other inorganic salts, organic aqueous solution quenching medium. After hardening steel hardening, but brittle. In order to reduce the brittleness of the steel, after quenching the steel at above room temperature and below a suitable temperature for a long period of 650 ℃ heat, then cooling, this process is called tempering. Annealing, normalizing, quenching and tempering is the overall heat treatment of the "Four fire", which is closely related to quenching and tempering, often used in conjunction, are indispensable.
 
     "Four fire" with different heating temperatures and cooling, but also the evolution of the different heat treatment process
. In order to obtain a certain strength and toughness, the quenching and tempering combine technology, called quenching. After some alloys supersaturated solid solution hardening, placing it at the proper temperature is maintained at room temperature or slightly longer, in order to improve the alloy hardness, strength or electrical magnetism. Such a heat treatment process called aging.
    The pressure processing deformation and heat treatment effectively and closely combine the workpiece to obtain good strength, toughness method is called thermomechanical treatment; heat treatment in a vacuum atmosphere or a vacuum is called vacuum heat treatment, which not only make workpiece no oxidation, decarbonization, after keeping the surface smooth process, improve the performance of the work, but also through the infiltration of agents chemical treatment.

    Surface Heat is only heating the workpiece surface, metal heat treatment process to change its surface mechanical properties. In order to heat only the workpiece surface without causing excessive heat into the interior of the workpiece, the heat source used must have high energy density, which give greater heat per unit area of ​​the workpiece, the workpiece surface can be short-term or partial or transient reaches a high temperature. The method of heat treatment of the surface of the main flame quenching and induction heating heat treatment, conventional heat source oxyacetylene flame propane or oxygen, induced current, laser and electron beam.

    Chemical treatment is by changing the workpiece surface chemical composition, microstructure and properties of metal heat treatment process. Chemical treatment of the surface with different heat treatment is that the latter altered the chemical composition of the workpiece surface. Chemical treatment is the workpiece on media containing carbon, nitrogen or other alloying elements (gas, liquid, solid) of the heating, insulation for a long time, so that the workpiece surface infiltration of carbon, nitrogen, boron and chromium and other elements. After incorporating the elements, sometimes also for other thermal treatment processes such as quenching and tempering. The main method of chemical treatment of carburizing, nitriding, surface alloying.

    Heat treatment is one of the important processes of mechanical parts and mold manufacturing process. Broadly speaking, it can ensure and improve various properties of the workpiece, such as wear and corrosion resistance. It can also improve the organization and the stress state of the blank, in order to facilitate a variety of cold and hot processing.

    Such as white cast iron can be obtained after long annealing malleable iron, improve the plasticity
; Gears correct heat treatment process, life can be more than doubled without heat treatment of gears or several times to improve; in addition, inexpensive steel by infiltration of some alloying elements on steel properties have some of the price of expensive, can replace some heat-resistant steel, stainless steel; workers die almost all need to go through the heat treatment can be used.