Basic Knowledge of Metal Heat Treatment

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Metal heat treatment is a process in which metal workpieces are heated to an appropriate temperature in a certain medium, held at that temperature for a certain period of time, and then cooled at different rates. I. Metal Structure 1. Metal: A substance that is opaque, has a good metallic luster, exhibits thermal and electrical conductivity; its electrical conductivity decreases as the temperature rises, and it possesses properties such as ductility and malleability. Solids in which the atoms inside the metal are arranged in a regular pattern (i.e., crystals). 2. Alloys: Materials composed of two or more metals, or metals and non-metals, that possess metallic properties. 3. Phase: Components within an alloy that have the same composition, structure, and properties. 4. Solid solutions: These are solid metal crystals in which the atoms (or compounds) of one or several components are dissolved into the lattice of another component, while the lattice type of the other component remains unchanged. Solid solutions are divided into interstitial solid solutions and substitutional solid solutions. 5. Solid solution strengthening: As solute atoms enter the interstices or nodes of the solvent lattice, the lattice is distorted, which increases the hardness and strength of the solid solution; this phenomenon is known as solid solution strengthening. 6. Compounds: A chemical reaction occurs between the components of an alloy, resulting in a new crystalline solid structure with metallic properties. Mechanical mixture: An alloy composition made up of two crystal structures; although it consists of two types of crystals, it is considered a single component with independent mechanical properties. 7. Ferrite: A interstitial solid solution of carbon in a-Fe (iron with a body-centered cubic structure). 8. Austenite: An interstitial solid solution of carbon in γ-Fe (iron with a face-centered cubic structure). 9. Cementite: a stable compound formed from carbon and iron (Fe3C). 10. Pearlite: a mechanical mixture composed of ferrite and cementite (F+Fe3c, with 0.8% carbon content). 11. Ledeburite: a mechanical mixture of cementite and austenite (with 4.3% carbon). Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing methods, heat treatment generally does not change the shape of the workpiece or its overall chemical composition; instead, it enhances or improves the performance of the workpiece by altering its internal microstructure or the chemical composition of its surface. Its feature is improving the internal quality of the workpiece, which is generally not visible to the naked eye. To endow metal workpieces with the desired mechanical, physical, and chemical properties, in addition to the proper selection of materials and various forming processes, heat treatment processes are often essential. Steel is the most widely used material in the machinery industry. Its microstructure is complex, but it can be controlled through heat treatment; therefore, heat treatment of steel constitutes the main focus of metal heat treatment. Furthermore, metals such as aluminum, copper, magnesium, titanium, and their alloys can also have their mechanical, physical, and chemical properties altered through heat treatment to achieve different performance characteristics. II. Processes of metal heat treatment
Heat treatment processes generally consist of three stages: heating, holding, and cooling. Sometimes, there are only two stages: heating and cooling. These processes are interconnected and must continue without interruption. 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 through molten salts or metals, as well as floating particles. When metals are heated and the workpieces are 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 usually 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. Additionally, the transformation requires a certain amount of time. Therefore, when the surface of the metal workpiece reaches the desired heating temperature, it must be maintained at this temperature for a certain period to ensure that the internal and external temperatures are uniform and that the microstructural transformation is complete. This period is referred to 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 requirements also exist depending on the type of steel; for example, air-hardening steel can be hardened at a cooling rate similar to that used for normalizing. Metal heat treatment processes can be broadly divided into three categories: bulk 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; as a result, there are numerous types of heat treatment processes for steel. 1. Overall heat treatment is a metal heat treatment process in which the entire workpiece is heated and then cooled at an appropriate rate to alter its overall mechanical properties. The overall heat treatment of steel generally includes four basic processes: annealing, normalizing, quenching, and tempering. (1) Annealing involves heating the workpiece to an appropriate temperature; depending on the material and the 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. (2) 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. (3) 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 at the same time more brittle. (4) Tempering is performed to reduce the brittleness of steel components. After quenching, the steel components are held at an appropriate temperature above room temperature but below 650°C for a prolonged period, and then cooled. 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. (5) Quenching and tempering is a process that combines quenching and high-temperature tempering in order to achieve certain levels of strength and toughness. (6) 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. (7) Deformation heat treatment is a method that effectively and closely combines pressure processing deformation with heat treatment, thereby enabling the workpiece to achieve an excellent balance between strength and toughness. (8) Vacuum heat treatment is a type of heat treatment carried out in a negative-pressure atmosphere or vacuum. It not only prevents the workpiece from oxidizing or decarburizing, maintains a smooth surface on the workpiece after treatment, and improves its properties, but it also allows for chemical heat treatment by introducing infiltrants. 2. Surface heat treatment is a metal heat treatment process in which only the surface layer of the workpiece is heated in order to change its mechanical properties at that surface layer. In order to heat only the surface layer of the workpiece without allowing excessive heat to penetrate into its interior, the heat source used must have a high energy density, that is, it must deliver a large amount of thermal energy per unit area of the workpiece, enabling the surface layer or specific areas to reach high temperatures in a short period of time or instantaneously. The main methods of surface heat treatment include flame quenching and induction heating treatment, with common heat sources including oxygen-acetylene or oxygen-propane flames, induced currents, lasers, and electron beams. 3. Chemical heat treatment is a metal heat treatment process that alters the chemical composition, structure, and properties of the surface layer of a workpiece. Chemical heat treatment differs from surface heat treatment in that the latter alters the chemical composition of the workpiece’s surface layer. Chemical heat treatment involves placing a workpiece in a medium containing carbon, nitrogen, or other alloying elements—be it a gas, liquid, or solid—and heating it for an extended period of time, thereby allowing elements such as carbon, nitrogen, boron, and chromium to penetrate into the surface layer of the workpiece. After the infiltration of elements, other heat treatment processes such as quenching and tempering may also be required. The main methods of chemical heat treatment include carburizing, nitriding, and metal infiltration. Heat treatment is one of the important processes in the manufacturing of mechanical parts and molds. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the microstructure and stress state of the blank, facilitating various cold and hot processing operations. For example, white cast iron can be transformed into malleable cast iron through prolonged annealing, thereby increasing its plasticity ； Gears that undergo the proper heat treatment process can have a service life that is several times or even dozens of times longer than those that are not heat-treated ； Furthermore, inexpensive carbon steel can acquire certain properties of more expensive alloy steels by having certain alloying elements added to it, and thus can replace certain heat-resistant steels and stainless steels ； Almost all tools and molds must undergo heat treatment before they can be used.