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 connected and uninterrupted.
Heating is one of the important steps of heat treatment. There are many heating methods for metal heat treatment. The earliest was to use charcoal and coal as heat sources, and then to use liquid and gas fuels. The application of electricity makes heating easy to control without environmental pollution. These heat sources can be used for direct heating, or indirect heating by molten salt or metal, or even 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 of the surface of the steel parts is reduced), which has a very negative effect on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be heated 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 issue to ensure the quality of the heat treatment. The heating temperature varies with the metal material being processed and the purpose of the heat treatment, but it is generally heated above the phase transition temperature to obtain the required structure. In addition, the transformation takes a certain time. Therefore, 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 complete the microstructure transformation. This time is called the holding time. When using high-energy density heating and surface heat treatment, the heating speed is extremely fast, generally there is no holding time or the holding time is very short, and 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 from process to process, and the main thing is to control the cooling rate. Generally, annealing has the slowest cooling rate, normalizing cooling rate is faster, and quenching cooling rate is faster. However, there are different requirements due to different steel grades. For example, hollow hard steel can be quenched at the same cooling rate as normalizing.
Metal heat treatment processes can be roughly divided into overall heat treatment, surface heat treatment, partial 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 the industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes.
Overall heat treatment is a metal heat treatment process that heats the entire workpiece and then cools it at an appropriate speed to change its overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.
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 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 to quickly cool the workpiece in a quenching medium 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 reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature and lower than 710°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 kept 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 effectively and tightly combining pressure processing deformation and heat treatment to obtain good strength and toughness of the workpiece is called thermomechanical heat treatment; heat treatment in a negative pressure atmosphere or 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 penetrant 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 introducing excessive heat into the interior of 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 laser heat treatment, 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 latter 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, nitrogen or other alloying elements, and keep it for a long time, so that the surface of the workpiece can infiltrate elements such as carbon, nitrogen, boron and chromium. After infiltration of elements, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment are carburizing, nitriding, metalizing, composite infiltration and so on.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. Generally speaking, it can guarantee and improve 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 obtain malleable cast iron after long-term annealing treatment, which can improve plasticity; gears adopt the correct heat treatment process, and the service life can be doubled or tens of times longer than gears without heat treatment; in addition, cheap carbon steel can be penetrated Certain alloying elements have the properties of some expensive alloy steels, which can replace certain heat-resistant steels and stainless steels; almost all tools and molds require heat treatment before they can be used.