Annealing is to heat the workpiece to an appropriate temperature, use different holding time according to the material and workpiece size, and then carry out slow cooling, the purpose is to make the metal internal structure to reach or close to the equilibrium state, to obtain good process and service performance, or to prepare the structure 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 that of annealing, except that the resulting structure is finer. It is often used to improve the cutting performance of the material, and sometimes used as the final heat treatment for some parts with low requirements.

Quenching is to heat and hold the workpiece, then cool it quickly in water, oil or other inorganic salts, organic aqueous solution and other quenching medium. 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 650℃, 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 are 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. 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 surface chemical composition 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 and metallizing. 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 to facilitate various cold and hot machining. For example, the malleable cast iron can be obtained by 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.