When is heat treatment?

The heat treatment process generally includes three steps: heating, holding, and cooling; sometimes it only involves two steps: heating and cooling. These processes are interconnected and must proceed continuously. Heating is one of the important processes in heat treatment. There are many heating methods for metal heat treatment; initially, charcoal and coal were used as heat sources, and later liquid and gas fuels were employed. The use of electricity makes heating easy to control and causes no environmental pollution. These heat sources can be used for direct heating, or for indirect heating via molten salts or metals, as well as floating particles. When metals are heated and the workpiece is exposed to air, oxidation and decarburization often occur (that is, the carbon content on the surface of steel parts decreases), which has a very negative impact on the surface properties of the parts after heat treatment. Therefore, metals should generally be heated in a controlled or protective atmosphere, in molten salts, or in a vacuum; they can also be protected during heating using coatings or packaging methods. Heating temperature is one of the important process parameters in heat treatment processes. Selecting and controlling the heating temperature is a key issue in ensuring the quality of heat treatment. The heating temperature varies depending on the metal material being treated and the purpose of heat treatment, but it is generally raised above the phase transition temperature to obtain a high-temperature microstructure. Furthermore, the transformation requires some time; therefore, once the surface of the metal workpiece reaches the desired heating temperature, it must be held at that temperature for a certain period to ensure that the temperatures inside and outside are equal and that the microstructure undergoes complete transformation. This period is known as the holding time. When high-energy-density heating and surface heat treatment are used, the heating rate is extremely fast, and generally there is no holding time, whereas the holding time for chemical heat treatment is usually longer. Cooling is also an essential step in the heat treatment process; the cooling methods vary depending on the process, with the main focus being on controlling the cooling rate. Generally, the cooling rate for annealing is the slowest, that for normalizing is faster, and that for quenching is the fastest. However, different steel grades have different requirements; for example, air-hardening steels can be hardened at a cooling rate similar to that used for normalizing. Metal heat treatment processes can be broadly 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 major category can be further divided into several different heat treatment processes. The same metal can exhibit different microstructures, and thus different properties, when subjected to various heat treatment processes. Steel is the most widely used metal in industry, and its microstructure is also the most complex, which is why there are a large variety of heat treatment processes for steel. 2) Classification of overall heat treatment processes. Overall heat treatment is a metal heat treatment process in which the entire workpiece is heated and then cooled at an appropriate rate, in order to change its overall mechanical properties. The overall heat treatment of steel generally includes four basic processes: annealing, normalizing, quenching, and tempering. a. Annealing involves heating the workpiece to an appropriate temperature; depending on the material and size of the workpiece, a certain holding time is applied, followed by slow cooling. The purpose is to bring the internal structure of the metal to or near an equilibrium state, thereby achieving good mechanical and functional properties, or to prepare the structure for further quenching. b. Normalizing involves heating the workpiece to an appropriate temperature and then cooling it in air. The effect of normalizing is similar to that of annealing, except that the resulting microstructure is finer. It is often used to improve the machinability of materials, and sometimes as the final heat treatment for parts with less stringent requirements. c. Quenching involves heating the workpiece and holding it at that temperature before rapidly cooling it in a quenching medium such as water, oil, or other inorganic salts and organic aqueous solutions. After quenching, the steel parts become harder, but they also become more brittle. d. Tempering is a heat treatment process aimed at reducing the brittleness of steel parts; it involves holding the quenched steel parts at an appropriate temperature above room temperature but below 650°C for an extended period of time, before cooling them. Annealing, normalizing, quenching, and tempering are the “four key processes” in overall heat treatment. Among them, quenching and tempering are closely related and are often used together; neither can be omitted. “The “four fires,” depending on the heating temperature and cooling methods, give rise to various heat treatment processes. Quenching and tempering is a process that combines quenching with high-temperature tempering in order to achieve certain levels of strength and toughness. Aging treatment involves placing certain alloys, which have formed a supersaturated solid solution after quenching, at room temperature or a slightly higher appropriate temperature for an extended period of time, in order to increase the alloy’s hardness, strength, or electrical and magnetic properties. 3) Arrangement of heat treatment processes: a. Normalizing is commonly used to improve the machinability of materials, and it is sometimes also used as the final heat treatment for parts with modest requirements. So it is generally arranged before the machining of the parts. b. Quenching is intended to increase the hardness of the material; it is generally carried out after rough machining (such as turning, milling, etc.) and before finish machining (such as grinding, EDM, etc.). c. The purpose of tempering is to reduce the brittleness of steel components, and it is usually used in combination with the quenching process.

Simply put, it involves changing the internal structure of metal objects in order to achieve the desired mechanical properties. For example, improving hardness, oxidation resistance, and removing stress, etc ；

The three main functions of heat treatment are: removing stress from the workpiece； To process and modify the internal grains of the workpiece in order to achieve good machinability ； To achieve good hardness, they are generally subjected to heat treatment before/after initial processing to remove stress ； Before finishing machining, it is to achieve good hardness and mechanical properties

:) When the strength of our materials does not meet the requirements of the final design, heat treatment is considered, especially for transmission components.

Heat treatment is the process of heating a metal workpiece to a certain temperature and then cooling it at different rates. Objective: 1. Refine metal grains to improve hardness and mechanical strength ； 2. Eliminate internal stress ； 3. Change the original hardness to facilitate processing ； 4. Increase the surface hardness and wear resistance of the workpiece, thereby extending its service life ； Wait

You’ve learned it; in other words, it’s simple