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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. 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 aspect 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. As society progressed from the Stone Age to the Bronze Age and Iron Age, the role of heat treatment gradually became recognized. As early as 770–222 BC, the Chinese had already discovered in their practical manufacturing activities that the properties of copper and iron would change under the influence of temperature and deformation under pressure. The softening treatment of white cast iron is an important process for manufacturing agricultural tools. In the 6th century BC, steel weapons were gradually adopted, and the quenching process developed rapidly in order to increase the hardness of steel. The two swords and one halberd unearthed in Yanxiadu, Yi County, Hebei Province, China, all contained martensite in their microstructure, indicating that they had been quenched. With the development of quenching technology, people have gradually come to recognize the impact of quenching agents on the quality of quenching. Pu Yuan from the Shu state during the Three Kingdoms period once forged 3,000 swords for Zhuge Liang in what is now Xiaguo, Shaanxi; it is said that people were sent to Chengdu to fetch water for hardening the swords. This shows that China already in ancient times was aware of the cooling capabilities of different water qualities, as well as those of oil and urine. The swords found in the tomb of King Jing of Zhongshan from the Western Han Dynasty (206 BC–24 AD) in China have a carbon content of 0.15–0.4% in their core, while the carbon content on the surface exceeds 0.6%, indicating that carburizing techniques were used. But at that time, as a personal \"skill,\" its secrets were not shared, so its development was slow. In 1863, British metallographers and geologists demonstrated six different microstructural forms of steel under a microscope, proving that the internal structure of steel changes when it is heated and cooled, with the phases present at high temperatures transforming into a harder phase upon rapid cooling. The theory of iron allotropes established by the Frenchman Osmond, along with the iron-carbon phase diagram first developed by the Britishman Austin, laid the theoretical foundation for modern heat treatment processes. At the same time, methods for protecting metals during the heating process in metal heat treatment have also been studied, in order to prevent oxidation and decarburization of the metals during heating. Between 1850 and 1880, there was a series of patents related to the use of various gases (such as hydrogen, gas, carbon monoxide, etc.) for protective heating. Between 1889 and 1890, the Britishman Lake obtained patents for various methods of bright heat treatment for metals. Since the 20th century, the development of metal physics and the adoption of other new technologies have led to further advancements in metal heat treatment processes. A significant advancement was the use of rotary kilns for gas carburizing in industrial production from 1901 to 1925 ； In the 1930s, dew point potentiometers were developed, enabling the control of the carbon potential in the furnace atmosphere; later, methods such as carbon dioxide infrared detectors and oxygen probes were invented to further regulate the carbon potential in the furnace atmosphere ； In the 1960s, heat treatment techniques made use of the effects of plasma fields, leading to the development of ion nitriding and carburizing processes ； The application of laser and electron beam technologies has also provided metals with new methods for surface heat treatment and chemical heat treatment. Processes of metal heat treatment: Heat treatment processes generally include three stages: heating, holding at a constant temperature, and cooling; sometimes there are only two stages, namely 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 adverse effect 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 the 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 is slowest for annealing, faster for normalizing, and fastest for quenching. 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: 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, which is why there are a large variety of heat treatment processes for steel. 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. Annealing involves heating the workpiece to an appropriate temperature, maintaining that temperature for a period of time that varies depending on the material and size of the workpiece, and then cooling it slowly. 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. 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. Quenching involves heating a 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 becomes harder, but it also becomes more brittle. To reduce the brittleness of steel parts, the quenched steel parts are held at an appropriate temperature above room temperature but below 650°C for an extended period of time before being cooled; this process is known as tempering. 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. The process that combines quenching and high-temperature tempering to achieve certain strength and toughness is called quenching and tempering. After certain alloys are quenched to form a supersaturated solid solution, they are held at room temperature or an appropriately higher temperature for an extended period of time in order to enhance the alloy’s hardness, strength, or electrical and magnetic properties. Such a heat treatment process is called aging treatment. The method of effectively and closely combining pressure processing deformation with heat treatment to enable the workpiece to achieve an excellent balance between strength and toughness is known as deformation heat treatment ； Heat treatment carried out in a negative-pressure atmosphere or vacuum is called vacuum heat treatment. 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 penetrants. Surface heat treatment is a metal heat treatment process in which only the surface layer of the workpiece is heated to alter 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. 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 heating a workpiece in a medium containing carbon, nitrogen, or other alloying elements (gas, liquid, solid), and maintaining this temperature 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 converted 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 can thus replace certain heat-resistant steels and stainless steels ； Molds and dies almost all require heat treatment before they can be used. Classification of steel: Steel is an alloy whose main components are iron and carbon, with a carbon content generally below 2.11%. Steel is an extremely important metal material in economic development. Steel is classified into two main categories based on its chemical composition: carbon steel (abbreviated as carbon steel) and alloy steel. Carbon steel is an alloy obtained by smelting pig iron; in addition to iron and carbon as its main components, it also contains small amounts of impurities such as manganese, silicon, sulfur, and phosphorus. Carbon steel possesses certain mechanical properties, good processability, and is inexpensive. Therefore, carbon steel has found wide application. However, with the rapid development of modern industry and science and technology, the properties of carbon steel can no longer fully meet the requirements, leading to the development of various alloy steels. Alloy steel is a multi-element alloy obtained by purposefully adding certain elements (known as alloying elements) to carbon steel. Compared to carbon steel, alloy steel exhibits significantly improved properties, which leads to its increasing use. Due to the wide variety of steel grades, it is necessary to classify them in order to facilitate production, storage, selection, and research. Steel can be classified into many categories based on its purpose, chemical composition, and quality: 1. Classification by purpose: Based on their use, steels can be divided into three main categories: structural steel, tool steel, and steel with special properties. Structural steel: 1. Steel used for various machine parts. It includes carburized steel, quenched and tempered steel, spring steel, and rolling bearing steel. 2. Steel used in engineering structures. It includes grades A, B, and special grades of carbon steel, as well as ordinary low-alloy steel. Tool steel: Steel used to manufacture various tools. Based on their different uses, they can be classified into cutting tool steel, die steel, and measuring tool steel. Special performance steel: It is steel with special physicochemical properties. It can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel, etc. II. Classification by chemical composition: Based on the chemical composition of steel, it can be divided into two main categories: carbon steel and alloy steel. Carbon steel: It can also be classified into low-carbon steel (carbon content ≤ 0.25%) based on its carbon content ； Medium carbon steel (0.25% < carbon content < 0.6%) ； High-carbon steel (carbon content ≥0.6%). Alloy steel: It can also be classified as low-alloy steel based on the content of alloying elements (total alloying element content ≤ 5%) ； Medium alloy steel (total alloy content = 5%–10%) ； High-alloy steel (total alloy content > 10%). Furthermore, depending on the types of main alloying elements present in the steel, it can also be classified into manganese steel, chromium steel, chromium-nickel steel, chromium-manganese-titanium steel, etc. III. Classification by quality: Based on the levels of harmful impurities such as phosphorus and sulfur in the steel, it can be divided into ordinary steel (with phosphorus content ≤ 0.045% and sulfur content ≤ 0.055%) ； or the phosphorus and sulfur content is ≤0.050%) ； High-quality steel (phosphorus and sulfur content both ≤0.040%) ； High-grade high-quality steel (phosphorus content ≤ 0.035%, sulfur content ≤ 0.030%). In addition, steel is also classified by the type of smelting furnace into open-hearth steel (acidic open-hearth, basic open-hearth), converter steel (acidic converter, basic converter, oxygen top-blown converter steel), and electric furnace steel. Based on the degree of deoxidation during smelting, steel is classified into boiling steel (incomplete deoxidation), killed steel (relatively complete deoxidation), and semi-killed steel. When naming steel products, steel mills often combine three classification methods: application, composition, and quality. Steel can be classified as ordinary carbon structural steel, high-quality carbon structural steel, carbon tool steel, high-grade high-quality carbon tool steel, alloy structural steel, alloy tool steel, etc. Mechanical properties of metal materials Mechanical properties of metal materials: The properties of metal materials are generally divided into two categories: process properties and service properties. The so-called process properties refer to the characteristics exhibited by metal materials under specified cold and hot processing conditions during the manufacturing of mechanical parts. The processability of metal materials determines their ability to be shaped during the manufacturing process. Due to different processing conditions, the required process properties also vary, such as castability, weldability, forgeability, heat treatment properties, and machinability. The so-called service performance refers to the properties exhibited by metal materials under operating conditions in mechanical parts, and it includes mechanical properties, physical properties, chemical properties, etc. The quality of the performance of metal materials determines their range of application and service life. In the machinery manufacturing industry, mechanical parts are generally used in normal temperature and pressure conditions, in media that are not highly corrosive, and they are subjected to various types of loads during use. The ability of metal materials to resist failure under load is known as mechanical property (or mechanical strength). The mechanical properties of metal materials are the main basis for part design and material selection. Different properties of external loads (such as tension, compression, torsion, impact, cyclic loading, etc.) result in different mechanical property requirements for metal materials. Common mechanical properties include: strength, plasticity, hardness, impact toughness, resistance to repeated impacts, and fatigue limit. The various mechanical properties will be discussed separately below. 1. Strength: Strength refers to the ability of a metal material to resist failure (excessive plastic deformation or fracture) under static loads. Since loads can act in forms such as tension, compression, bending, and shear, strengths are also classified into tensile strength, compressive strength, bending strength, shear strength, etc. There is often a certain relationship between different strengths, and in practice, tensile strength is generally used as the most fundamental strength indicator. 2. Ductility Ductility refers to the ability of a metal material to undergo plastic deformation (permanent deformation) under load without breaking. 3. Hardness: Hardness is a parameter used to measure the softness or hardness of metal materials. The most commonly used method for measuring hardness in current production is the indentation hardness test. In this method, a indenter of a specific geometric shape is pressed into the surface of the metal material under a certain load, and its hardness value is determined based on the degree of indentation. Commonly used methods include Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC), and Vickers hardness (HV). 4. Fatigue: The strength, plasticity, and hardness discussed earlier are all mechanical property indicators of metals under static loads. In fact, many machine parts operate under cyclic loads, and under such conditions the parts experience fatigue. 5. Impact toughness: A load that acts on a component at high speed is called an impact load, and the ability of metal to resist destruction under such loads is known as impact toughness. Annealing–Quenching–Tempering Annealing---Quenching---Tempering I. Types of Annealing 1. Full annealing and isothermal annealing Full annealing, also known as recrystallization annealing, is commonly referred to simply as annealing. This type of annealing is mainly used for castings, forgings, and hot-rolled profiles made from various carbon steels and alloy steels with hypoeutectoid compositions; it is sometimes also used for welded structures. It is generally used as the final heat treatment for parts that are not subject to heavy processing, or as a preliminary heat treatment for certain parts. 2. Spheroidizing annealing is mainly used for hypereutectoid carbon steels and alloy tool steels (such as the steels used in the manufacture of cutting tools, measuring instruments, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching. 3. Stress-relief annealing: Stress-relief annealing is also known as low-temperature annealing (or high-temperature tempering). This type of annealing is used primarily to eliminate residual stresses in castings, forgings, welded parts, hot-rolled products, and cold-drawn parts. If these stresses are not eliminated, they will cause the steel component to deform or develop cracks after a certain period of time, or during subsequent machining processes. II. During quenching, the most commonly used cooling media are brine, water, and oil. Workpieces quenched in salt water can easily achieve high hardness and a smooth surface, and are less prone to the formation of soft spots that do not harden; however, they are prone to severe deformation or even cracking. Using oil as a quenching medium is only suitable for quenching certain alloy steels with high stability of supercooled austenite or small-sized carbon steel workpieces. III. Purposes of steel tempering 1. To reduce brittleness and eliminate or minimize internal stresses. After quenching, steel parts have high levels of internal stress and brittleness; if not tempered in a timely manner, these parts may deform or even crack. 2. To achieve the mechanical properties required for the workpiece, quenching results in a high hardness but also high brittleness. In order to meet the different property requirements of various workpieces, the hardness can be adjusted through appropriate tempering, thereby reducing brittleness and obtaining the desired toughness and ductility. 3. Stabilize the dimensions of the workpiece. 4. For certain alloy steels that are difficult to soften through annealing, high-temperature tempering is often used after quenching (or normalizing) to cause the carbides in the steel to aggregate appropriately, thereby reducing its hardness and facilitating machining. Selection of Common Furnace Types The choice of furnace type should be determined based on different processing requirements and the type of workpieces. 1. For applications where mass production in fixed formats is not possible, where the workpieces vary in size and there are many different types, and where versatility and adaptability in processing are required, a box furnace can be used. 2. For heating workpieces with long axes, long screws, pipes, etc., a deep-chamber electric furnace can be used. 3. For small batches of carburized parts, a vertical gas carburizing furnace can be used. 4. For the mass production of parts such as car and tractor gears, a continuous carburizing production line or a box-type multi-purpose furnace can be used. 5. For the bulk production of heating stamping sheet blanks, it is best to use roller furnaces or shaft furnaces. 6. For batches of shaped parts, push-rod type or conveyor belt type resistance furnaces (push-rod furnaces or cast-band furnaces) can be used in production. 7. For small mechanical parts such as screws and nuts, bottom-vibrating furnaces or mesh-belt furnaces can be employed. 8. For the heat treatment of steel balls and rollers, a rotary tubular furnace with an internal spiral can be used. 9. For the mass production of non-ferrous metal ingots and billets, pusher-type furnaces can be used, while for small non-ferrous metal parts and materials, air-circulation heating furnaces are suitable. Heating Defects and Their Control I. Overheating Phenomenon We know that overheating during the heat treatment process can easily lead to the enlargement of austenite grains, thereby reducing the mechanical properties of the parts. 1. General overheating: Excessively high heating temperatures or prolonged exposure to high temperatures, which leads to the coarsening of austenite grains, is referred to as overheating. Large austenite grains lead to a decrease in the strength and toughness of steel, an increase in the brittle transition temperature, and an increased tendency to deformation and cracking during quenching. The cause of overheating is an out-of-control furnace temperature gauge or mixing (often due to a lack of understanding of the process). Overheated tissue can, under normal conditions, be re-austenitized through annealing, normalizing, or multiple high-temperature temperings to refine the grains. 2. Fracture morphology inheritance: In steel with overheated structure, reheating and quenching can refine the austenite grains, but coarse granular fracture patterns may still appear at times. There are many theoretical debates regarding the origin of fracture patterns. It is generally believed that excessive heating caused impurities such as MnS to dissolve into the austenite and accumulate at the grain boundaries; during cooling, these impurities precipitate along these grain boundaries, making the material prone to fracturing along the coarse austenite grain boundaries when subjected to impact. 3. Geneticity of coarse microstructures: When steel parts with coarse martensite, bainite, or Wei’ site microstructures are re-austenitized by heating slowly to conventional quenching temperatures, or even lower, their austenite grains remain coarse; this phenomenon is known as structural geneticity. To eliminate the heredity of coarse structures, intermediate annealing or multiple high-temperature tempering treatments can be employed. II. Overheating phenomenon: Excessively high heating temperatures not only cause the austenite grains to become larger, but also lead to oxidation or melting at the grain boundaries, resulting in weakened grain boundaries; this is known as overheating. The properties of steel deteriorate severely after over-sintering, and cracks form during quenching. Overheated tissue cannot be restored and must be discarded. Therefore, it is necessary to avoid overheating in the work process. III. Decarburization and oxidation: When steel is heated, the carbon in its surface layer reacts with oxygen, hydrogen, carbon dioxide, water vapor, and other elements present in the medium (or atmosphere), resulting in a decrease in the carbon concentration on the surface. This process is known as decarburization. After quenching, decarburized steel exhibits reduced surface hardness, fatigue strength, and wear resistance; moreover, residual tensile stress forms on the surface, which can lead to the development of network-like cracks. When heated, the reaction between the iron and alloys in the surface layer of steel, along with various elements, with oxygen, carbon dioxide, water vapor, and other substances present in the medium (or atmosphere) to form an oxide film is known as oxidation. At high temperatures (usually above 570 degrees), oxidation of workpieces leads to a deterioration in dimensional accuracy and surface finish. Steel parts with poor hardenability of their oxide films are prone to develop soft spots after quenching. Measures to prevent oxidation and reduce decarburization include coating the workpiece surface, sealing and heating it with stainless steel foil, using a salt bath furnace for heating, heating in a protective atmosphere (such as purified inert gases with controlled carbon levels in the furnace), and using flame combustion furnaces to maintain a reducing atmosphere in the furnace. IV. Hydrogen embrittlement: The phenomenon of reduced plasticity and toughness in high-strength steels when heated in a hydrogen-rich atmosphere is known as hydrogen embrittlement. Workpieces affected by hydrogen embrittlement can have this issue eliminated through dehydrogenation treatments such as tempering or aging; heating in a vacuum, low-hydrogen atmosphere, or inert atmosphere can also prevent hydrogen embrittlement. Of course, in practical applications, some people take advantage of this phenomenon to serve human needs (such as in the crushing of alloys, etc.). Several common heat treatment concepts: 1. Normalizing: A heat treatment process in which steel or steel components are heated to an appropriate temperature above their critical points AC3 or ACM, held at that temperature for a certain period of time, and then cooled in air, resulting in a pearlitic structure. 2. Annealing: A heat treatment process in which a hypoeutectoid steel workpiece is heated to 20–40 degrees above AC3, held at that temperature for a certain period of time, and then cooled slowly within the furnace (or by burying it in sand or lime) until its temperature drops below 500 degrees, after which it is cooled in air. 3. Solution heat treatment: A heat treatment process in which an alloy is heated to its high-temperature single-phase region and held there to allow excess phases to dissolve fully into the solid solution; subsequent rapid cooling is then carried out to obtain a supersaturated solid solution. 4. Aging: The phenomenon whereby the properties of an alloy change over time when it is left at room temperature or at a temperature slightly above room temperature, after undergoing solution heat treatment or cold plastic deformation. 5. Solution treatment: This process ensures that all phases in the alloy are fully dissolved, strengthening the solid solution and improving its toughness and corrosion resistance; it also eliminates stresses and softens the material, making it possible to proceed with further processing. 6. Aging treatment: The material is heated to the temperature at which strengthening phases precipitate and held there for a certain period of time, allowing these phases to form and thereby hardening the material and increasing its strength. 7. Quenching: This is a heat treatment process in which steel is first austenitized and then cooled at an appropriate rate, causing unstable structural transformations such as martensite to occur throughout or in certain areas of the workpiece’s cross-section. 8. Tempering: The quenched workpiece is heated to an appropriate temperature below the critical point AC1 and held there for a certain time; afterward, it is cooled using suitable methods to achieve the desired structure and properties. 9. Carburitriding of steel: Carburitriding is a process in which both carbon and nitrogen are introduced simultaneously into the surface layer of steel. *Carburitriding, also known as cyanidation, is currently widely used in the forms of medium-temperature gas carburitriding and low-temperature gas carburitriding (i.e., gas soft nitriding). The main purpose of medium-temperature gas carbonitriding is to improve the hardness, wear resistance, and fatigue strength of steel. Low-temperature gas carbonitriding is primarily nitridation, and its main purpose is to improve the wear resistance and anti-galling properties of steel. 10. Quenching and tempering: The heat treatment that generally involves combining quenching with high-temperature tempering is referred to as quenching and tempering. Quenching and tempering is widely used in various important structural components, especially those such as connecting rods, bolts, gears, and shafts that operate under alternating loads. After quenching and tempering, a tempered sorbite structure is obtained, whose mechanical properties are superior to those of the normalized sorbite structure with the same hardness. Its hardness depends on the high-temperature tempering temperature, as well as the tempering stability of the steel and the cross-sectional size of the workpiece; it generally ranges from HB200 to HB350. 11. Brazing: A heat treatment process in which two workpieces are bonded together using a filler metal. Types and applications of tempering Depending on the performance requirements of the workpieces and their respective tempering temperatures, tempering can be classified into the following types: (i) Low-temperature tempering (150–250 degrees). The microstructure resulting from low-temperature tempering is tempered martensite. Its purpose is to reduce the internal stresses and brittleness of quenched steel, while maintaining its high hardness and wear resistance, thereby preventing it from cracking or suffering premature damage during use. It is mainly used in various high-carbon cutting tools, measuring instruments, cold stamping dies, rolling bearings, and carburized parts, etc. The hardness after tempering is generally HRC 58–64. (II) Medium-temperature tempering (350–500 degrees): The microstructure obtained through medium-temperature tempering is tempered martensite. Its purpose is to achieve high yield strength, elastic limit, and high toughness. Therefore, it is mainly used for treating various springs and hot-working molds, with a hardness of generally HRC35–50 after tempering. (III) High-temperature tempering (500–650 degrees): The microstructure obtained through high-temperature tempering is tempered sorbite. *The heat treatment that combines quenching with high-temperature tempering is known as quenching and tempering, and its purpose is to achieve comprehensive mechanical properties with good strength, hardness, as well as plasticity and toughness. Therefore, they are widely used as important structural components in automobiles, tractors, machine tools, etc., such as connecting rods, bolts, gears, and shafts. The hardness after tempering is generally HB200–330. Chemical reactions between atmospheres and metals I. Chemical reactions between atmospheres and steel 1. Oxidation: 2Fe + O2 → 2FeO; Fe + H2O → FeO + H2; FeC + CO2 → Fe + 2CO 2. Reduction: FeO + H2 → Fe + H2O; FeO + CO → Fe + O2 3. Carburization: 2CO → [C] + CO2; Fe + [C] → FeC; CH4 → [C] + 2H2 4. Nitridation: 2NH3 → 2[N] + 3H2; Fe + [N] → FeN II. Effects of various atmospheres on metals Nitrogen: Reacts with Cr, CO, Al, Ti at temperatures ≥1000 degrees. Hydrogen: Can reduce copper, nickel, iron, and tungsten. When the water content in hydrogen reaches 0.2–0.3%, it can cause decarburization of steel. Water: At temperatures of ≥800 degrees, it causes oxidation and decarburization of iron and steel; it does not react with copper. Carbon monoxide: Its reducing properties are similar to those of hydrogen, and it can cause carburization of steel. III. The effect of various atmospheres on resistive elements: Nickel-chromium wires and iron-chromium-aluminum wires: Sulfur-containing atmospheres are harmful to these resistive wires. Heat treatment of brass: Ordinary brass is a copper-zinc alloy; based on its microstructure, it can be divided into simple brass (also known as α-brass), with a Cu content of 100%-62.4%, and two-phase brass (α+β brass), with a Cu content of 56.5%-62.4%. The solubility of Zn in Cu increases as humidity decreases, hence there is no heat treatment strengthening effect. Annealing is often used to modify the cold working properties of brass. The mechanical properties and cold deformation properties of brass gates after finishing annealing depend mainly on the grain size. The intermediate annealing temperatures for cold working of brass are as follows: Material grade Thickness(δ)>5mm Thickness(δ)=1-5mm Thickness(δ)=0.5-1mm Thickness(δ)