Heat treatment process generally includes heating, heat preservation, cooling three processes, sometimes only heating and cooling two processes.These processes are interconnected and uninterruptible. 

Heating is one of the important steps of heat treatment.There are many heating methods for metal heat treatment, * early is the use of charcoal and coal as a source of heat, and then the application of liquid and gas fuel.The application of electricity makes heating easy to control and free from environmental pollution.These sources of heat can be heated directly or indirectly through molten salts or metals, 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 decreases), which has a very adverse effect on the surface performance of the parts after heat treatment.As a result, the metal should normally be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging methods. 

The heating temperature is one of the important technological parameters in the heat treatment process. The selection and control of the heating temperature is the main problem to ensure the quality of the heat treatment.The heating temperature varies with the metal material being treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain the desired tissue.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 time, so that the internal and external temperatures are consistent and the microstructure changes completely. This period of time is called the heat preservation time.When high-energy density heating and surface heat treatment are adopted, the heating speed is very fast, and generally there is no or very 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 from process to process, mainly controlling the cooling rate.Generally, the cooling rate of annealing is * slow, while the cooling rate of normalizing is faster, and the cooling rate of quenching is faster.But there are also different requirements depending on the type of steel. For example, air-hardened steel can be hardened 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 microstructure can be obtained by different heat treatment process for the same metal, so it has different properties.Iron and steel is widely used in industry, and the microstructure of iron and steel is also complex, so the heat treatment technology of iron and steel is various. 

Integral heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate 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, different holding time according to the material and workpiece size, and then slow cooling (cooling speed * slow), the purpose is to make the metal internal structure to reach or close to the equilibrium state, to obtain good process performance and service performance, or to prepare the structure for further quenching.

Normalizing → Heating the workpiece to a suitable temperature and then cooling it in the air, the effect of normalizing is similar to annealing, except that the resulting tissue is finer. It is often used to improve the cutting performance of the material, and is sometimes used as the final heat treatment for some parts with low requirements. 

Quenching → After the workpiece is heated and kept warm, it is cooled quickly in quenched medium such as water, oil or other inorganic salts and organic aqueous solution.After quenching the steel becomes hard but brittle at the same time.In order to reduce the brittleness of the steel piece, the quenched steel piece is kept warm for a long time at an appropriate temperature above room temperature but 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 which quenching and tempering are closely related and often used together without one. 

"Four fires" evolve into different heat treatment processes with different heating temperatures and cooling methods.In order to obtain a certain strength and toughness, the process of quenching and tempering at a high temperature is called tempering.After some alloys are quenched to form supersaturated solid solution, they are kept at room temperature or a little higher for a longer time to improve the hardness, strength or electrical magnetism of the alloys.Such a heat treatment process is called aging treatment.The method of combining pressure working deformation with heat treatment effectively and closely to obtain good strength and toughness of the workpiece is called deformable heat treatment.The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment, it can not only make the workpiece does not oxidize, do not decarburize, keep the surface of the processed workpiece clean, improve the performance of the workpiece, but also through the infiltration agent for chemical heat treatment. 

Surface heat treatment is a metal heat treatment process that only heats the surface of workpiece to change its surface mechanical properties.In order to heat only the surface layer of the workpiece without causing too much heat into the workpiece interior, the heat source used must have a high energy density, that is, give greater heat energy on the workpiece per unit area, so that the surface or part of the workpiece can reach high temperature for a short time or instantaneous.The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment, common heat sources such as flame such as oxygen acetylene or oxygen propane, induced current, laser and electron beam, etc.

Chemical heat treatment is a metal heat treatment process by changing the surface chemical composition, structure and properties of 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 put the workpiece in the medium containing carbon, nitrogen or other alloying elements (gas, liquid, solid) heating, heat preservation for a long time, so that the surface layer of the workpiece infiltrated carbon, nitrogen, boron, chromium and other elements.After infiltration of elements, sometimes other heat treatment processes such as quenching and tempering.The main methods of chemical heat treatment are carburizing, nitriding, metal carburizing and compound carburizing. 

Heat treatment is one of the important processes in the manufacture of mechanical parts and dies.Generally speaking, it can guarantee and improve various properties of the workpiece, such as wear resistance, corrosion resistance and so on.It can also improve the microstructure and stress state of the blank, so as to facilitate various cold and hot machining. 

For example, white cast iron can obtain malleable cast iron after annealing for a long time to improve its plasticity.Gear using the correct heat treatment process, the service life can be compared with the gear without heat treatment multiplied or dozens of times to improve;In addition, the low price of carbon steel through the infiltration of some alloy elements have some valence alloy steel properties, can replace some heat resistant steel, stainless steel;Almost all molds need to be heat treated before they can be used.