The four fires in heat treatment

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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. As some people say, machining is like surgery, while heat treatment is like internal medicine; it represents the **core competitiveness of manufacturing. Process: The heat treatment process generally includes three steps: heating, holding at a constant temperature, and cooling; sometimes it only involves two steps: heating and cooling. These processes are interconnected and must proceed continuously. When metals are heated, the workpieces are exposed to air, often resulting in oxidation and decarburization (i.e., a reduction in the carbon content on the surface of steel parts). This 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. Process classification: 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; as a result, there are numerous types 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 obtain the desired microstructure, thereby altering its overall mechanical properties. The overall heat treatment of steel mainly involves four basic processes: annealing, normalizing, quenching, and tempering – namely the “four fires” of heat treatment. Quenching: The quenching process for steel involves heating the steel to a temperature above its critical temperature Ac3 (for hypoeutectoid steel) or Ac1 (for hyper-eutectoid steel), holding it at that temperature for a certain period of time to allow full or partial austenitization, and then cooling it at a rate faster than the critical rate until the temperature drops below Ms (or maintaining it around Ms) in order to induce martensite (or bainite) transformation. Process: heating, holding temperature, cooling. The essence of quenching: it is the transformation of supercooled austenite into martensite or bainite, resulting in a martensitic or bainitic structure. Purpose of quenching: (1) To significantly improve the stiffness, hardness, wear resistance, fatigue strength, and toughness of steel, thereby meeting the various operational requirements of different mechanical parts and tools ； (2) Achieving special physical and chemical properties such as ferromagnetism and corrosion resistance in certain specialty steels through quenching. Application areas: The quenching process is most widely used. It is applied to parts such as tools, measuring instruments, molds, bearings, springs, as well as components in automobiles, tractors, diesel engines, machining machines, pneumatic tools, drilling equipment, agricultural machinery, petroleum machinery, chemical processing equipment, textile machinery, and aircraft. Quenching medium: The medium used for quenching and cooling workpieces is called a quenching medium (or quenchant). An ideal quenching medium should possess the property of enabling the workpiece to be quenched into martensite without inducing excessive quenching stress. Common quenching media include water, aqueous solutions, mineral oils, molten salts, and molten alkalis. Water: Water is a quenching medium with strong cooling capacity. Advantages: Wide availability, low price, and stable composition that does not easily deteriorate. Disadvantages: Unstable cooling capacity, which can easily cause the workpiece to deform or crack. In the “nose” region of the C curve (around 500–600°C), water is in the vapor film stage; if cooling is not fast enough, a \"soft spot\" will form” ； In the martensitic transformation temperature range (100–300°C), water is in a boiling state; rapid cooling can cause the martensitic transformation to occur too quickly, resulting in high internal stresses that may lead to deformation or even cracking of the workpiece. When the water temperature rises, an excess of gases in the water or the presence of insoluble impurities (such as oil, soap, mud, etc.) will significantly reduce its cooling capacity. Application: Suitable for the quenching and cooling of carbon steel workpieces with small cross-sectional dimensions and simple shapes. Saltwater and alkaline water: Appropriate amounts of salt and alkali are added to water. When high-temperature workpieces are immersed in this cooling medium, crystals of salt and alkali precipitate during the steam film stage and burst immediately, thereby breaking the steam film; the oxide scale on the surface of the workpieces is also shattered. This improves the cooling capacity of the medium in high-temperature areas, but the downside is that the medium is highly corrosive. Applications: Under normal circumstances, the concentration of salt water is 10%, while the concentration of sodium hydroxide solution ranges from 10% to 15%. It can be used as a quenching medium for carbon steel and low-alloy structural steel workpieces. The operating temperature should not exceed 60°C, and the parts should be cleaned promptly after quenching and treated to prevent rust. Mineral oil is commonly used as the oil cooling medium, such as engine oil, transformer oil, and diesel. Engine oils generally come in grades 10, 20, and 30. The higher the grade, the greater the viscosity, the higher the flash point, the lower the cooling capacity, and accordingly, the higher the operating temperature allowed. Quenching method: Single-fluid quenching is a quenching process in which the austenitized workpiece is immersed in a certain quenching medium and cooled down to room temperature. Single-fluid quenching media include water, saltwater, alkaline water, oil, and specially formulated quenchants. Advantages: Simple to operate, conducive to mechanization and automation. Disadvantage: The cooling rate is limited by the cooling properties of the medium, which affects the quality of quenching. Application: Single-fluid quenching is only suitable for carbon steel in workpieces with relatively simple shapes. Two-fluid quenching involves first immersing the austenitized workpiece in a medium with high cooling capacity, removing it before it reaches the temperature of that quenching medium, and then immediately immersing it in another medium with lower cooling capacity for further cooling, such as using water followed by oil, or water followed by air. Two-fluid quenching reduces the tendency for deformation and cracking, but it is difficult to master properly and has certain limitations in terms of application. Martensitic staged quenching is a quenching process in which the austenitized workpiece is first immersed in a liquid medium (salt bath or alkali bath) at a temperature slightly higher or lower than the martensite transformation point of the steel. It is held there for an appropriate period of time; once both the inner and outer layers of the steel reach the temperature of the medium, the workpiece is taken out and cooled air-dry to obtain a martensitic structure. This process is also known as staged quenching. Advantages: Gradual quenching allows for air cooling after staying at the respective temperature levels until the temperature inside and outside the workpiece is uniform, which effectively reduces phase transformation stress and thermal stress, thereby minimizing the tendency for deformation and cracking during quenching. Applications: Suitable for alloy steel and high-alloy steel workpieces with high requirements for dimensional accuracy; it can also be used for carbon steel workpieces with small cross-sectional dimensions and complex shapes. Bainite isothermal quenching is a quenching process in which a steel part is austenitized and then rapidly cooled to the bainite transformation temperature range (260–400°C), where it is held at that temperature to allow the austenite to transform into bainite. It is sometimes also referred to as isothermal quenching; the holding time is generally between 30 and 60 minutes. Compound quenching involves rapidly cooling the workpiece below Ms to obtain 10%–20% martensite, followed by isothermal treatment in the lower bainite temperature range. This cooling method enables workpieces with larger cross-sections to obtain an M+B microstructure. The martensite formed during pre-quenching can promote bainitic transformation, and during isothermal treatment it causes the martensite to temper. Composite quenching is used for alloy tool steel workpieces; it can prevent Type I temper brittleness and reduce the amount of residual austenite, thereby decreasing the tendency toward deformation and cracking. Tempering, Tempering process: Tempering is a heat treatment process in which a workpiece that has been quenched is reheated to an appropriate temperature below the lower critical temperature; after being held at this temperature for a certain period of time, it is cooled to room temperature in air or in media such as water or oil. Purpose of tempering: (1) To eliminate the residual stresses generated in the workpiece during quenching, thereby preventing deformation and cracking ； (2) Adjust the hardness, strength, plasticity, and toughness of the workpiece to meet the requirements for its performance in use ； (3) Stabilize the structure and size to ensure precision ； (4) Improve and enhance processing performance. Tempering classification: Low-temperature tempering refers to the tempering of workpieces at temperatures between 150 and 250°C. Purpose: To maintain high hardness and wear resistance in quenched workpieces, and to reduce residual quenching stress and brittleness. Tempered martensite is obtained after tempering, referring to the structure formed when quenched martensite is tempered at low temperatures. Image (acicular martensite) Applications: cutting tools, measuring instruments, molds, rolling bearings, parts subjected to carburizing and surface quenching, etc. Medium-temperature tempering refers to the tempering of workpieces at temperatures between 350 and 500°C. Objective: To achieve high elasticity and yield strength, along with appropriate toughness. After tempering, tempered martensite is obtained, which refers to a multiphase structure formed during the tempering of martensite, characterized by a ferritic matrix in which extremely fine spherical carbides (or cementites) are distributed. Image (martensite) Applications: springs, forging dies, impact tools, etc. High-temperature tempering refers to the tempering of workpieces at temperatures above 500°C. Objective: To obtain comprehensive mechanical properties with good strength, plasticity, and toughness. After tempering, tempered sorbite is obtained; it refers to a multiphase structure in which fine spherical carbides (including cementite) are distributed within a ferrite matrix formed during the tempering of martensite. Image (Sorbite) Application scope: Widely used in various important load-bearing structural components, such as connecting rods, bolts, gears, and shaft parts. Normalizing: Normalizing is a metal heat treatment process in which a steel workpiece is heated to a temperature 30–50°C above the critical temperature (the temperature at which complete austenitization occurs). After being held at this temperature for an appropriate period, the workpiece is removed from the furnace and cooled in air, or by spraying water, mist, or blowing air. Purpose: (1) To refine the grain size and homogenize the distribution of carbides ； (2) Remove internal stresses from the material ； (3) Increase the hardness of the material. Image. Material: hypoeutectic tinplate. Condition: normalized. Microstructure: graphite brown, martensite light yellow, pearlite green and dark yellow, cementite tan.
Image. Material: A-299. Condition: normalized. Microstructure: brown ferrite; yellow, blue, and white pearlite with a brown tint.
Image. Material: 13MnNiMoNb. Condition: normalized. Microstructure: light brown pearlite grayish-brown; ferrite red, yellow, and blue.
Image. Material: 45 steel. Condition: normalized. Microstructure: light blue ferrite; pearlite exhibiting various colors.
Advantages: (1) The cooling rate during normalization is slightly faster than that during annealing; consequently, the interlamellar spacing of pearlite formed is smaller. The microstructure resulting from normalization is finer than that obtained through annealing, leading to higher hardness and strength ； (2) Off-furnace cooling after normalizing does not require any equipment, resulting in higher productivity. Applications: Suitable only for carbon steel and low- to medium-alloy steels, but not for high-alloy steels. Because the austenite in high-alloy steels is very stable, a martensitic structure will also be obtained upon air cooling. Specific applications: (1) For low-carbon steel and low-alloy steel, normalizing can increase their hardness, thereby improving their machinability ； (2) For medium-carbon steel, normalizing can replace quenching and tempering by preparing the microstructure for high-frequency quenching, and it can also reduce the deformation of the steel parts as well as lower processing costs ； (3) For high-carbon steel, normalizing can eliminate the network-like cementite structure, facilitating spheroidizing annealing ； (4) For large steel forgings or steel castings with abrupt changes in cross-section, normalizing can be used instead of quenching to reduce the tendency of deformation and cracking, or to prepare the microstructure for subsequent quenching ； (5) For steel parts that have been quenched and then repaired, normalizing can be used to eliminate the effects of overheating, thereby allowing them to be quenched again ； (6) Used for cast iron parts to increase the amount of pearlite in the matrix, thereby enhancing the strength and wear resistance of the castings. Annealing: The heat treatment process in which a metal or alloy is heated to an appropriate temperature, held at that temperature for a certain period of time, and then cooled slowly (usually by cooling within the furnace) is called annealing. The essence of annealing is to heat the steel to the austenitizing temperature and then induce pearlite transformation; the structure obtained after annealing is the equilibrium structure. Purpose of annealing: (1) To reduce the hardness of steel and improve its plasticity, thereby facilitating machining and cold deformation processing ； (2) Chemical composition and microstructure of uniform steel; refining grains to improve the properties of the steel or to prepare the microstructure for quenching ； (3) Eliminate internal stresses and work hardening to prevent deformation and cracking. Annealing method 01: Full annealing. Process: The steel is heated to 20–30°C above Ac3, held at that temperature for a certain period of time, and then cooled slowly (along with the furnace) to obtain a balanced microstructure. This is a heat treatment process that results in complete austenitization. In actual production, in order to improve efficiency, the steel is cooled to around 500°C before being taken out of the furnace for air cooling. Purpose: To refine grains, homogenize the microstructure, eliminate internal stresses, reduce hardness, and improve the machinability of the steel. The microstructure of hypoeutectoid steel after complete annealing is F+P. Applications: Full annealing is mainly used for hypoeutectoid steels (wc=0.3~0.6%), typically medium-carbon steels as well as castings, forgings, and hot-rolled profiles of low- and medium-carbon alloy steels; it is sometimes also used for their welded joints. 02 Partial annealing – Image. Process: A heat treatment method in which steel is heated to Ac1~Ac3 (for hypoeutectoid steel) or Ac1~Accm (for hyper-eutectoid steel), held at that temperature for a certain period, and then cooled slowly to obtain a structure that is close to the equilibrium state. Applications: It is mainly used in hypereutectoid steels to obtain a spheroidal pearlite structure, thereby eliminating internal stresses, reducing hardness, and improving machinability. 03 Isothermal Annealing – Images: This is a heat treatment process in which the steel is heated to a temperature higher than Ac3 (or Ac1); after being held at that temperature for a certain period of time, it is cooled rapidly to a temperature within the pearlite region, where it is then held isothermally to allow the austenite to transform into pearlite. Finally, the material is cooled naturally to room temperature. Purpose: Similar to full annealing, the transformation is easier to control. Applications: Suitable for relatively stable steels: high-carbon steel (WC>0.6%), alloy tool steel, and high-alloy steel (total amount of alloying elements>10%). Isothermal annealing also helps to achieve a uniform microstructure and properties. However, it is not suitable for large-section steel components and large batches of furnace charge, as isothermal annealing makes it difficult to bring the interior of the workpieces or all the workpieces in a batch to the isothermal temperature. 04 Spheroidizing Annealing Images: A heat treatment process that spheroids the carbides in steel, resulting in granular pearlite. Heat to a temperature 20–30°C above Ac1; the holding time should not be too long, generally 2–4 hours is appropriate. The cooling method usually involves furnace cooling, or isothermal treatment at around 20°C below Ar1 for an extended period of time. Purpose: To reduce hardness, uniformize the structure, and improve machinability in order to prepare the structure for quenching. Applications: Primarily used for eutectoid and hypereutectoid steels, such as carbon tool steel, alloy tool steel, bearing steel, etc. Spheroidizing annealing yields spherical pearlite, in which cementite appears as small spherical particles that are dispersed throughout the ferritic matrix. Compared to flake pearlite, spherical pearlite not only has lower hardness and is thus easier to machine, but also results in austenite grains that do not tend to grow large during quenching heating, with less tendency to deform or crack during cooling. 05 Diffusion annealing (homogenization annealing): A heat treatment process in which steel ingots, castings, or forgings are heated to a temperature slightly below the solidus line and held at that temperature for an extended period of time, followed by slow cooling to eliminate variations in chemical composition. Purpose: To eliminate dendritic and regional segregation that occurs during the solidification of ingots, thereby homogenizing the composition and microstructure. Applications: Used in some high-quality alloy steels, as well as in alloy steel castings and ingots with severe segregation. The heating temperature for diffusion annealing is very high, usually 100–200°C above Ac3 or Accm; the exact temperature depends on the degree of segregation and the type of steel. The holding time is generally 10–15 hours. After diffusion annealing, full annealing and normalizing are required to refine the microstructure. 06 Stress-relief annealing – Images: The process involves heating the steel component to a temperature below Ac1 (usually 500–650°C), holding it at that temperature, and then cooling it slowly within the furnace. The stress-relief annealing temperature is below A1, therefore stress-relief annealing does not cause any changes in the microstructure. Purpose: To eliminate residual internal stress. Applications: It is mainly used to eliminate residual stresses in castings, forgings, welded parts, hot-rolled products, cold-drawn parts, etc. 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.