Process of heat treatment of metal

The heat treatment process generally includes three processes of heating, heat preservation and cooling, and sometimes there are only two processes of heating and cooling. These processes are interconnected and uninterrupted. 

Heating is one of the important steps in heat treatment. There are many heating methods for metal heat treatment, the earliest is to use charcoal and coal as heat sources, and then use liquid and gas fuels. The application of electricity makes heating easy to control and free of environmental pollution. These heat sources can be used for direct heating or indirect heating through molten salts or metals, as well as floating particles. 

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

The heating temperature is one of the important process parameters of 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 to be treated and the purpose of the heat treatment, but generally it is heated above the phase transition temperature to obtain the desired structure. In addition, the transformation takes a certain amount of time, so when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain period of time to make the internal and external temperatures consistent and the microstructure changes completely. This period of time is called the holding time. When high-energy density heating and surface heat treatment are used, the heating speed is extremely fast, and generally there is no holding time or the holding time is very short, while the holding time of chemical heat treatment is often longer. 

Cooling is also an indispensable step in the heat treatment process. The cooling method varies with different processes, mainly controlling the cooling rate. Generally, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster. However, there are also different requirements due to different steel types. For example, hollow-hardened steel can be hardened with the same cooling rate as normalizing. 

The metal heat treatment process can be roughly 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. The same metal adopts different heat treatment processes to obtain different structures and thus have 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 types of steel heat treatment processes. 

The overall 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 generally has four basic processes: annealing, normalizing, quenching and tempering. 

Annealing → Heating the workpiece to an appropriate temperature, using different holding times according to the material and workpiece size, and then slowly cooling (the slowest cooling rate), the purpose is to make the internal structure of the metal reach or close to the equilibrium state, and obtain good process performance and use properties, or to prepare the structure for further quenching.

Normalizing → 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 sometimes used for some parts with low requirements. as a final heat treatment. 

Quenching → After the workpiece is heated and kept warm, it is rapidly cooled in a quenching medium such as water, oil or other inorganic salts and organic aqueous solutions. After quenching, the steel becomes hard, but at the same time becomes brittle. 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 710 ℃ 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 together, and neither is indispensable. 

"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 and not allow too much heat to be transferred into 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 or part of the workpiece can be short-term or instantaneous. reach high temperature. The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment. Commonly used heat sources include 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 latter changes the chemical composition of the workpiece surface. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloying elements for a long time, so that the surface 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, metal infiltration, composite infiltration, etc. 

Heat treatment is one of the important processes in the manufacturing process 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, which can improve plasticity; gears adopt the correct heat treatment process, and the service life can be doubled or dozens of times longer than those 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.