Process classification and means of heat treatment

1. Process classification

Metal heat treatment processes can be roughly divided into three categories: overall heat treatment, surface 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 uses 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 complicated, so there are many types of steel heat treatment processes.

Overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to obtain the required metallographic structure to change its overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.

2. Craftsmanship:

Annealing is to heat the workpiece to an appropriate temperature, use different holding time according to the material and workpiece size, and then slowly cool it, the purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and use performance, or for further quenching Prepare for the organization.

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, except that the resulting structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for parts that are not demanding. As the final heat treatment.

Quenching is the rapid cooling of the workpiece in quenching media such as water, oil, other inorganic salts, and organic aqueous solutions after heating and holding the workpiece. After quenching, the steel parts become hard, but at the same time become brittle. In order to eliminate the brittleness in time, it is generally necessary to temper in time.

In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature and lower than 650°C 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. Quenching and tempering are closely related and are often used in conjunction with each other. The "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 placed at room temperature or a slightly higher temperature for a longer period of time to improve the hardness, strength, or electrical magnetism of the alloy. Such a heat treatment process is called aging treatment.

The method of combining pressure processing deformation and heat treatment effectively and closely to obtain a good strength and toughness of the workpiece is called thermomechanical heat treatment; heat treatment performed in a negative pressure atmosphere or in a vacuum is called vacuum heat treatment, which can not only make The workpiece is not oxidized or decarburized. It can keep the surface of the workpiece smooth and clean after treatment and improve the performance of the workpiece. It can also be injected with a penetrating agent for chemical heat treatment.

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 heat only the surface of the workpiece without causing too much heat to be transferred into 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 or part of the workpiece can be short-term or instantaneous Reach high temperatures. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. Commonly used heat sources include flames such as oxygen acetylene or oxypropane, induction current, laser, and electron beam.

Chemical heat treatment is a metal heat treatment process that changes the chemical composition, organization and performance of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the former changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, salt or other alloying elements, and keep it for a long time, so that the surface of the workpiece is infiltrated with elements such as carbon, nitrogen, boron, and chromium. After the element has been infiltrated, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment are carburizing, nitriding and metalizing.

Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. Generally speaking, it can guarantee and improve the various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing.

For example: white cast iron can be annealed for a long time to obtain malleable cast iron and improve its plasticity; gears adopt the correct heat treatment process, and their service life can be doubled or tens of times longer than gears without heat treatment; in addition, cheap carbon steel passes Infiltration of certain alloying elements will have certain expensive alloy steel properties, which can replace certain heat-resistant steels and stainless steels; almost all of the tools and molds require heat treatment before they can be used.

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