Summary of Basic Knowledge of Metal Heat Treatment
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the steel product, but changes the micro-structure inside the item or changes the chemical composition of the surface, thus endowing or improving the performance of the product. The purpose of metal heat treatment is to improve the intrinsic quality of the metal so that it has the required mechanical properties, physical properties and chemical properties. In addition to the reasonable selection of materials and various forming processes, heat treatment processes are often essential. Steel is the most widely used material in the mechanical industry. The micro-structure of steel is complex and can be controlled through heat treatment. Therefore, the heat treatment of steel is the main content of metal heat treatment.
Heat Treatment Process
The heat treatment process generally includes three processes: heating, insulation, and cooling. Sometimes there are only two processes: heating and cooling. These processes are interconnected and cannot be interrupted.
Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. Charcoal and coal were first used as heat sources, and liquid and gaseous fuels were recently used. The application of electricity makes heating easy to control and has no environmental pollution. These heat sources can be used for direct heating, or indirect heating through molten salt or metal, or even floating particles.
Heating temperature is one of the important process parameters of the heat treatment process. Selection and control of the heating temperature are the main issues to ensure the quality of heat treatment. The heating temperature varies depending on the metal material being processed and the purpose of the heat treatment, but it is generally heated to above the phase transformation temperature to obtain a high-temperature structure. In addition, the transformation requires a certain amount of time. Therefore, when the surface of the metal 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 micro-structure transformation to be complete. This period of time is called insulation. When using high-energy-density heating and surface heat treatment, the heating speed is extremely fast and there is generally no insulation time, while the insulation time for chemical heat treatment is often longer.
Cooling is also an indispensable step in the heat treatment process. The cooling methods vary depending on the process, mainly controlling the cooling rate. Generally, annealing has the slowest cooling rate, normalizing has a faster cooling rate, and quenching has a faster cooling rate than normalizing. But there are also different requirements due to different steel types.
Process Classification
Metal heat treatment processes can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. Depending on the heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Using different heat treatment processes for the same metal can obtain different structures and thus have different properties.
Overall heat treatment is a metal heat treatment process that heats the metal 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 generally includes four basic processes: annealing, normalizing, quenching and tempering.
(1) Annealing is to heat the metal to an appropriate temperature, adopt different insulation times according to the material and metal size, and then slowly cool it. The purpose is to make the internal structure of the metal reach or approach an equilibrium state, obtain good process performance and usability.
(2) Normalizing is to heat the metal to a suitable temperature and then cool it in the air. The effect of normalizing is similar to that of annealing, except that the obtained structure is finer. It is often used to improve the cutting performance of materials, and is sometimes used as final heat treatment for parts with low requirements.
(3) Quenching is to heat and preserve the metal and then rapidly cool it in a quenching medium such as water, oil or other inorganic salts or organic aqueous solutions. After quenching, the steel becomes hard but at the same time becomes brittle.
(4) In order to reduce the brittleness of steel parts, the quenched steel parts are kept for a long time at an appropriate temperature above room temperature and below 650°C, and then cooled. This process is called tempering.
Surface heat treatment is a metal heat treatment process that only heats the surface layer of the metal to change the mechanical properties of the surface layer. In order to only heat the surface of the metal without transferring too much heat into the interior of the metal, the heat source used must have a high energy density, that is, a large amount of heat energy is given to the metal per unit area, so that the surface or part of the metal can reach high temperatures in a short time. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. Commonly used heat sources include oxygen acetylene, oxygen propane, 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 surface of the metal. Chemical heat treatment is to heat the metal in a medium (gas, liquid, solid) containing carbon, salt medium or other alloying elements, and keep it warm for a long time, so that the surface of the metal can penetrate elements such as carbon, nitrogen, boron and chromium. After the elements are infiltrated, other heat treatment processes such as quenching and tempering are sometimes performed. The main methods of chemical heat treatment include carburizing, nitriding, and metalizing.
Overall speaking, metal heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools and molds. It can ensure and improve various properties of the metal, such as wear resistance, corrosion resistance, etc. On the other hand, it can also improve the structure and stress state of the blank to facilitate various cold and hot processing.
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