Basic knowledge of metal heat treatment
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Basic knowledge of metal heat treatment Metal heat treatment is a process in which metal workpieces are heated to a suitable temperature in a certain medium, maintained at this temperature for a certain period of time, and then cooled at different speeds. 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 workpiece, but gives or improves the performance of the workpiece by changing the microstructure inside the workpiece or changing the chemical composition of the workpiece surface. Its characteristic is to improve the intrinsic quality of the workpiece, which is generally not visible to the naked eye. In order to make metal workpieces have 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 indispensable. Steel is the most widely used material in the mechanical industry. The microstructure 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. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance properties. As we progressed from the Stone Age to the Bronze Age and Iron Age, the role of heat treatment became increasingly recognized. As early as 770 BC to 222 BC, the Chinese discovered in production practice that the properties of copper and iron will change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing agricultural tools. In the sixth century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, the quenching process was rapidly developed. Two swords and a halberd unearthed from Yanxiadu, Yi County, Hebei Province, China, have martensite in their microstructure, indicating that they have been quenched. With the development of quenching technology, people gradually discovered the impact of quenching agent on quenching quality. Pu Yuan, a native of Shu in the Three Kingdoms, once made 3,000 knives for Zhuge Liang in Xiegu, Shaanxi today. According to legend, he sent people to Chengdu to get water for quenching. This shows that China paid attention to the cooling capabilities of different water qualities in ancient times, and also paid attention to the cooling capabilities of oil and urine. The sword unearthed in China from the tomb of King Jing in Zhongshan during the Western Han Dynasty (206 BC to AD 24) has a carbon content of 0.15-0.4% in the heart, but a carbon content of more than 0.6% on the surface, indicating that the carburizing process has been applied. But at that time, as a secret of personal "craft", it was not allowed to be disclosed, so its development was very slow. In 1863, British metallographers and geologists demonstrated six different metallographic structures of steel under a microscope, proving that the internal structure of steel will change when it is heated and cooled. The high-temperature phase in the steel transforms into a harder phase during rapid cooling. The allotropy theory of iron established by the Frenchman Osmond and the iron-carbon phase diagram first formulated by the British Austin laid the initial theoretical foundation for modern heat treatment technology. At the same time, people have also studied methods to protect metals during the heating process of metal heat treatment to avoid oxidation and decarburization of metals during the heating process. From 1850 to 1880, there were a series of patents for the application of various gases (such as hydrogen, coal gas, carbon monoxide, etc.) for protective heating. From 1889 to 1890, the British man Lake obtained patents for bright heat treatment of various metals. Since the 20th century, the development of metal physics and the transplantation and application of other new technologies have enabled greater development of metal heat treatment processes. A significant development was the application of rotary drum furnaces for gas carburizing in industrial production from 1901 to 1925. ； In the 1930s, the dew point potentiometer appeared, which made the carbon potential of the furnace atmosphere controllable. Later, methods for further controlling the carbon potential of the furnace atmosphere were developed using carbon dioxide infrared meters, oxygen probes, etc. ； In the 1960s, heat treatment technology used the role of plasma fields to develop ion nitriding and carburizing processes. ； The application of laser and electron beam technology has enabled new surface heat treatment and chemical heat treatment methods for metals. Metal 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. The first was to use charcoal and coal as the heat source, and then to use liquid and gaseous fuels. 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. When metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of the steel part is reduced), 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 atmosphere or protective atmosphere, in molten salt, and in a vacuum. Protective heating can also be performed by coating or packaging methods. 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 with 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, so when the surface of the metal workpiece 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 microstructure transformation to be complete. This period of time is called the holding time. When high-energy-density heating and surface heat treatment are used, the heating speed is extremely fast and there is generally no holding time, while the holding time of 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. However, there are different requirements due to different steel types. For example, air-hardened steel can be hardened at the same cooling rate as normalizing. 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, the same metal can obtain different structures and thus have different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes. Overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to change its overall mechanical properties. The overall heat treatment of steel generally includes four basic processes: annealing, normalizing, quenching and tempering. Annealing is to heat the workpiece to an appropriate temperature, use different holding times according to the material and workpiece size, and then slowly cool it. The purpose is to make the internal structure of the metal reach or contact * * equilibrium state to obtain good process performance and usability, or to prepare the structure for further quenching. Normalizing is to heat the workpiece 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 the material, and is sometimes used as the final heat treatment for some parts with low requirements. Quenching is to heat and preserve the workpiece and then quickly cool it in a quenching medium such as water, oil, other inorganic salts, and organic aqueous solutions. After quenching, the steel becomes hard but at the same time becomes brittle. 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. Annealing, normalizing, quenching, and tempering are the "four fires" in overall heat treatment. Among them, quenching and tempering are closely related and are often used together and are indispensable. “"Four Fires" have evolved different heat treatment processes with different heating temperatures and cooling methods. In order to obtain a certain strength and toughness, the process of combining quenching and high-temperature tempering is called quenching and tempering. After some alloys are quenched to form a supersaturated solid solution, they are kept at room temperature or a slightly higher temperature for a longer period of time to improve the hardness, strength or electromagnetic properties of the alloy. This heat treatment process is called aging treatment. The method of effectively and closely combining pressure processing deformation and heat treatment to obtain good strength and toughness of the workpiece is called deformation heat treatment. ； Heat treatment performed in a negative pressure atmosphere or vacuum is called vacuum heat treatment. It not only prevents the workpiece from oxidation and decarburization, keeps the surface of the workpiece smooth and clean after treatment, and improves the performance of the workpiece, but also allows chemical heat treatment by introducing a penetrating agent. Surface heat treatment is a metal heat treatment process that only heats the surface layer of the workpiece to change the mechanical properties of its surface layer. In order to only heat the surface layer of the workpiece without transferring too much heat into the interior of the workpiece, the heat source used must have a high energy density, that is, a large amount of heat energy is given to the workpiece per unit area, so that the surface layer or part of the workpiece can reach high temperatures in a short or instant. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. Commonly used heat sources include oxygen acetylene or oxygen propane and other flames, 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 workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface layer of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloying elements, and keep it warm for a long time, so that the surface of the workpiece can penetrate into 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. Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools and molds. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing. For example, after long-term annealing treatment of white cast iron, malleable cast iron can be obtained to improve the plasticity. ； Using the correct heat treatment process for gears, the service life can be doubled or dozens of times longer than that of gears without heat treatment. ； In addition, cheap carbon steel has certain properties of expensive alloy steel by being infiltrated with certain alloy elements, and can replace some heat-resistant steel and stainless steel. ； Almost all tools and molds need to be heat treated before they can be used. Classification of Steel Steel is an alloy with iron and carbon as its main components. Its carbon content is generally less than 2.11%. Steel is an extremely important metal material in economic construction. Steel is divided into two categories according to its chemical composition: carbon steel (carbon steel for short) 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 a small amount of manganese, silicon, sulfur, phosphorus and other impurities. Carbon steel has certain mechanical properties, good process properties, and is cheap. Therefore, carbon steel has been widely used. However, with the rapid development of modern industry and science and technology, the performance of carbon steel can no longer fully meet the needs, so various alloy steels have been developed. Alloy steel is a multi-component alloy obtained by adding certain elements (called alloying elements) purposefully to carbon steel. Compared with carbon steel, the performance of alloy steel has been significantly improved, so it is increasingly used. Due to the wide variety of steel materials, steel materials must be classified in order to facilitate production, storage, selection and research. Steel can be divided into many categories according to its use, chemical composition, and quality.: one. Classification by use According to the use of steel, it can be divided into three categories: structural steel, tool steel, and special performance steel. structural steel: 1. Steel used as 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 A, B, special steel and ordinary low alloy steel in carbon steel. tool steel: Steel used to make various tools. According to different uses of tools, they can be divided into cutting tool steel, mold steel and measuring tool steel. Special performance steel: It is steel with special physical and chemical properties. It can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel, etc. two. Classification by chemical composition According to the chemical composition of steel, it can be divided into two categories: carbon steel and alloy steel. carbon steel: According to the carbon content, it can be divided into low carbon steel (carbon content ≤0.25%) ； Medium carbon steel (0.25%＜carbon content＜0.6%) ； High carbon steel (carbon content ≥0.6%). alloy steel: According to the alloying element content, it can be divided into low alloy steel (total alloying element content ≤5%) ； Medium alloy steel (total alloying element content = 5%--10%) ； High alloy steel (total alloying element content >10%). In addition, according to the main alloying elements contained in steel, it can also be divided into manganese steel, chromium steel, chromium-nickel steel, chromium-manganese-titanium steel, etc. three. According to quality classification, according to the content of harmful impurities phosphorus and sulfur in steel, it can be divided into ordinary steel (phosphorus content ≤0.045%, sulfur content ≤0.055% ； Or phosphorus and sulfur content are both ≤0.050%) ； High-quality steel (phosphorus and sulfur content ≤0.040%) ； High-grade high-quality steel (phosphorus content ≤0.035%, sulfur content ≤0.030%). In addition, according to the type of smelting furnace, steel is divided into open-hearth steel (acidic open-hearth furnace, alkaline open-hearth furnace), air converter steel (acid converter, alkaline converter, oxygen top-blown converter steel) and electric furnace steel. According to the degree of deoxidation during smelting, steel is divided into boiling steel (incomplete deoxidation), killed steel (relatively complete deoxidation) and semi-killed steel. When steel mills name steel products, they often combine the three classification methods of use, composition, and quality. For example, steel is called ordinary carbon structural steel, high-quality carbon structural steel, carbon tool steel, advanced 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 performance and usage performance. The so-called process performance refers to the performance of metal materials under specified cold and hot processing conditions during the manufacturing process of mechanical parts. The quality of the process performance of metal materials determines its adaptability to processing and forming during the manufacturing process. Due to different processing conditions, the required process properties are also different, such as casting performance, weldability, forgeability, heat treatment performance, cutting processability, etc. The so-called performance refers to the performance of metal materials under the conditions of use of mechanical parts, which includes mechanical properties, physical properties, chemical properties, etc. The performance of metal materials determines its range of use and service life. In the machinery manufacturing industry, general mechanical parts are used in normal temperature, normal pressure and non-strongly corrosive media, and during use, each mechanical part will bear different loads. The ability of metal materials to resist damage under load is called mechanical properties (or mechanical properties). The mechanical properties of metal materials are the main basis for the design and material selection of parts. Depending on the nature of the applied load (such as tension, compression, torsion, impact, cyclic load, etc.), the mechanical properties required for metal materials will also be different. Commonly used mechanical properties include: Strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit, etc. Each mechanical property is discussed separately below. 1． Strength Strength refers to the performance of a metal material to resist damage (excessive plastic deformation or fracture) under static load. Since the load acts in the form of tension, compression, bending, shearing, etc., strength is also divided into tensile strength, compressive strength, flexural strength, shear strength, etc. There is often a certain relationship between various strengths. In use, tensile strength is generally used as the most basic strength indicator. 2． Plasticity Plasticity refers to the ability of a metal material to produce plastic deformation (permanent deformation) without destruction under load. 3． Hardness Hardness is a measure of how hard or soft a metal material is. At present, the most commonly used method for measuring hardness in production is the indentation hardness method, which uses an indenter of a certain geometric shape to press into the surface of the metal material being tested under a certain load, and the hardness value is measured 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 indicators of the mechanical properties of metal under static load. In fact, many machine parts are operated under cyclic loading, and fatigue will occur in the parts under such conditions. 5. Impact toughness: The load acting on the machine parts at a very high speed is called impact load. The ability of metal to resist damage under the impact load is called impact toughness.
Annealing--Quenching--Tempering Annealing---Quenching---Tempering 1. Types of annealing 1. Complete annealing and isothermal annealing Complete annealing is also called recrystallization annealing, generally referred to as annealing. This annealing is mainly used for castings, forgings and hot-rolled profiles of various carbon steels and alloy steels with hypoeutectoid composition, and is sometimes used for welded structures. It is generally used as the final heat treatment of some light workpieces, or as the preheat treatment of some workpieces. 2． Spheroidizing annealing Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel types used in manufacturing cutting tools, measuring tools, 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 called low-temperature annealing (or high-temperature tempering). This kind of annealing is mainly used to eliminate residual stress in castings, forgings, welded parts, hot-rolled parts, cold-drawn parts, etc. If these stresses are not eliminated, it will cause the steel parts to deform or crack after a certain period of time or during subsequent cutting processes. two. When quenching, the most commonly used cooling media are brine, water and oil. The workpiece quenched in salt water is easy to obtain high hardness and smooth surface, and is not prone to soft spots that are not hardened, but it is easy to cause serious deformation of the workpiece and even cracking. The use of oil as the quenching medium is only suitable for quenching some alloy steels or small-sized carbon steel workpieces where the stability of supercooled austenite is relatively large. three. The purpose of steel tempering 1. Reduce brittleness and eliminate or reduce internal stress. After quenching, steel parts will have great internal stress and brittleness. If they are not tempered in time, the steel parts will often deform or even crack. 2． To obtain the required mechanical properties of the workpiece, the workpiece has high hardness and high brittleness after quenching. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted through appropriate tempering to reduce the brittleness and obtain the required toughness and plasticity. 3． Stable workpiece size 4. For some alloy steels that are difficult to soften by annealing, high-temperature tempering is often used after quenching (or normalizing) to properly gather carbides in the steel and reduce the hardness to facilitate cutting. Selection of commonly used furnace types Selection of furnace types should be determined based on different process requirements and types of workpieces 1. For those that cannot be produced in batches, have unequal sizes of workpieces, and have many types, and require versatility and versatility in the process, box-type furnaces can be used. 2． When heating long shafts, long screw rods, pipes and other workpieces, deep well electric furnaces can be used. 3． For small batches of carburized parts, a well-type gas carburizing furnace can be used. 4． For the production of large quantities of automobile, tractor gear and other parts, a continuous carburizing production line or a box-type multi-purpose furnace can be selected. 5. When heating stamping plate blanks for mass production, it is best to use rolling furnaces and roller hearth furnaces. 6． For batches of shaped parts, push rod or conveyor belt resistance furnaces (push rod furnaces or cast belt furnaces) can be used for production. 7. Small mechanical parts such as: Screws, nuts, etc. can be selected from vibrating bottom furnace or mesh belt furnace. 8. The internal spiral rotary tube furnace can be used for heat treatment of steel balls and rollers. 9. Push rod furnaces can be used for mass production of non-ferrous metal ingots, while air circulation heating furnaces can be used for small non-ferrous metal parts and materials. Heating defects and control 1. Overheating phenomenon We know that overheating during heat treatment can easily lead to the coarsening of austenite grains, which will reduce the mechanical properties of parts. 1. General overheating: If the heating temperature is too high or the holding time at high temperature is too long, the coarsening of austenite grains is called overheating. Coarse austenite grains will reduce the strength and toughness of steel, increase the brittle transition temperature, and increase the tendency of deformation and cracking during quenching. The cause of overheating is that the furnace temperature instrument is out of control or the materials are mixed (often caused by people who do not understand the process). The overheated structure can be re-austenized under normal circumstances to refine the grains after annealing, normalizing or multiple high-temperature temperings. 2. Broken inheritance: Steel with a superheated structure can refine the austenite grains after reheating and quenching, but sometimes coarse granular fractures still appear. The theory of fracture inheritance is controversial. It is generally believed that impurities such as MnS were dissolved into austenite and enriched at the crystal interface due to excessive heating temperature. During cooling, these inclusions will precipitate along the crystal interface, and they will easily break along the coarse austenite grain boundaries when impacted. 3. Inheritance of coarse tissue: When steel parts with coarse martensite, bainite, and Wignisten structures are re-austenized and heated slowly to the conventional quenching temperature, or even lower, the austenite grains will still be coarse. This phenomenon is called tissue inheritance. To eliminate the inheritance of coarse tissue, intermediate annealing or multiple high-temperature tempering treatments can be used. 2. Over-burning phenomenon: The heating temperature is too high, which not only causes the austenite grains to become coarse, but also causes local oxidation or melting of the grain boundaries, resulting in weakening of the grain boundaries, which is called over-burning. After the steel is over-burned, its properties deteriorate severely and cracks form during quenching. Burned tissue cannot be recovered and can only be scrapped. Therefore, overheating should be avoided at work. 3. When decarburized and oxidized steel is heated, the carbon on the surface reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere), reducing the carbon concentration on the surface, which is called decarburization. After quenching, the surface hardness, fatigue strength and wear resistance of decarburized steel are reduced, and the residual tensile stress formed on the surface easily forms surface network cracks. When heated, the phenomenon in which the iron and alloys on the surface of steel react with elements and oxygen, carbon dioxide, water vapor, etc. in the medium (or atmosphere) to form an oxide film is called oxidation. After the workpiece is oxidized at high temperature (generally above 570 degrees), the dimensional accuracy and surface brightness deteriorate, and steel parts with poor hardenability with oxide films are prone to quenching soft spots. Measures to prevent oxidation and reduce decarbonization are: The surface coating of the workpiece should be sealed and heated with stainless steel foil packaging, salt bath furnace heating, protective atmosphere heating (such as purified inert gas, controlling the carbon potential in the furnace), flame burning furnace (making the furnace gas reductive). 4. Hydrogen embrittlement phenomenon The phenomenon of reduced plasticity and toughness of high-strength steel when heated in a hydrogen-rich atmosphere is called hydrogen embrittlement. Workpieces with hydrogen embrittlement can also be eliminated by hydrogen removal treatment (such as tempering, aging, etc.). Hydrogen embrittlement can be avoided by heating in a vacuum, low hydrogen atmosphere or inert atmosphere.
Several common heat treatment concepts Several common heat treatment concepts 1. Normalizing: A heat treatment process that heats steel or steel parts to an appropriate temperature above the critical point AC3 or ACM, maintains it for a certain period of time, and then cools it in the air to obtain a pearlite-like structure. 2． Annealing: The heat treatment process is to heat the hypoeutectoid steel workpiece to 20-40 degrees above AC3, keep it warm for a period of time, then slowly cool it in the furnace (or bury it in sand or cool in lime) to below 500 degrees and then cool it in the air. 3. Solution heat treatment: The heat treatment process is to heat the alloy to a high temperature in the single-phase zone and maintain it at a constant temperature to fully dissolve the excess phase into the solid solution, and then rapidly cool it to obtain a supersaturated solid solution. 4. aging: After solid solution heat treatment or cold plastic deformation, the properties of an alloy change with time when placed at room temperature or slightly above room temperature. 5. Solution treatment: Fully dissolve various phases in the alloy, strengthen the solid solution, improve toughness and corrosion resistance, eliminate stress and softening, and facilitate continued processing and shaping 6. aging treatment: Heating and holding at the temperature where the strengthening phase precipitates will cause the strengthening phase to precipitate, harden, and improve strength. 7. Quenching: A heat treatment process in which the steel is austenitized and then cooled at an appropriate cooling rate so that the workpiece undergoes unstable organizational structure transformation such as martensite in all or a certain range of the cross section 8. Tempering: Heat treatment process that heats the quenched workpiece to an appropriate temperature below the critical point AC1 for a certain period of time, and then cools it using a method that meets the requirements to obtain the required structure and properties. 9. Carbonitriding of steel: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the surface layer of steel. * Traditionally, carbonitriding is also called cyanidation. Currently, medium-temperature gas carbonitriding and low-temperature gas carbonitriding (i.e., gas soft nitriding) are widely used. 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 mainly nitriding, and its main purpose is to improve the wear resistance and seizure resistance of steel. 10. quenching and tempering: generally * The heat treatment that combines quenching and high-temperature tempering is called quenching and tempering. Quenching and tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. After quenching and tempering treatment, the tempered sorbite structure is obtained, and its mechanical properties are better than those of the normalized sorbite structure with the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of the steel and the cross-sectional size of the workpiece, generally between HB200-350. 11. brazing: The heat treatment process of bonding two workpieces together with solder. Types and applications of tempering. According to the different performance requirements of the workpiece and the different tempering temperatures, tempering can be divided into the following types:: (1) Low temperature tempering (150-250 degrees) The structure obtained by low temperature tempering is tempered martensite. Its purpose is to reduce the internal stress and brittleness of quenched steel while maintaining its high hardness and high wear resistance to avoid cracking or premature damage during use. It is mainly used for various high-carbon cutting tools, measuring tools, cold stamping dies, rolling bearings and carburized parts, etc. The hardness after tempering is generally HRC58-64. (2) Medium temperature tempering (350-500 degrees) The structure obtained by medium temperature tempering is tempered troostite. The purpose is to obtain high yield strength, elastic limit and high toughness. Therefore, it is mainly used for the treatment of various springs and hot work molds. The hardness after tempering is generally HRC35-50. (3) High-temperature tempering (500-650 degrees) The structure obtained by high-temperature tempering is tempered sorbite. * Traditionally, the heat treatment that combines quenching and high-temperature tempering is called quenching and tempering treatment. Its purpose is to obtain comprehensive mechanical properties with better strength, hardness, plasticity, and toughness. Therefore, it is widely used in important structural parts of automobiles, tractors, machine tools, etc., such as connecting rods, bolts, gears and shafts. The hardness after tempering is generally HB200-330.
Chemical reaction between atmosphere and metal 1. Chemical reaction between atmosphere 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. Nitriding 2NH3→2[N]+3H2 Fe+[N]→FeN 2. Effects of various atmospheres on metals Nitrogen: Hydrogen will react with Cr, CO, Al.Ti at ≥1000 degrees: Can reduce copper, nickel, iron and tungsten. When the water content in hydrogen reaches 0.2-0.3%, the steel will decarburize.: ≥At 800 degrees, iron and steel are oxidized and decarburized, and carbon monoxide does not react with copper.: Its reducing properties are similar to hydrogen and can carburize steel. Effects of various atmospheres on resistor components: nickel-chromium wire, iron-chromium-aluminum: Sulfur-containing atmosphere is harmful to resistance wires Nitriding of steel and nitriding of carbonitriding steel (gas nitriding) Concept: Nitriding is the process of infiltrating nitrogen atoms into the surface layer of steel. Its purpose is to improve surface hardness and wear resistance, as well as improve fatigue strength and corrosion resistance. It uses ammonia gas to decompose active nitrogen atoms when heated. After being absorbed by the steel, it forms a nitride layer on its surface and diffuses toward the center. Nitriding is usually carried out using specialized equipment or pit carburizing furnaces. Suitable for various high-speed transmission precision gears, machine tool spindles (such as boring bars, grinder spindles), high-speed diesel engine crankshafts, valves, etc. Nitriding workpiece process route: Forging - annealing - rough machining - quenching and tempering - finishing - stress removal - rough grinding - nitriding - fine grinding or grinding. Since the nitride layer is thin and brittle, it requires a higher-strength core structure. Therefore, quenching and tempering heat treatment must be performed first to obtain tempered sorbite to improve the mechanical properties of the core and the quality of the nitride layer. After nitriding, the steel no longer needs to be quenched and has a very high surface hardness (greater than HV850) and wear resistance. The nitriding treatment temperature is low and the deformation is very small. Compared with carburizing and induction surface quenching, the deformation is much smaller than carbonitriding of steel.: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the surface layer of steel. * Conventionally, carbonitriding is also called cyanidation. At present, medium-temperature gas carbonitriding and low-temperature gas carbonitriding (i.e. gas soft nitriding) are widely used. 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 mainly nitriding, and its main purpose is to improve the wear resistance and anti-seizure properties of steel. Heat Treatment of Beryllium Bronze Beryllium bronze is a very versatile precipitation hardening alloy. After solid solution and aging treatment, the strength can reach 1250-1500MPa (1250-1500kg). Its heat treatment characteristics are: It has good plasticity after solution treatment and can be deformed by cold working. However, after aging treatment, it has an excellent elastic limit, and its hardness and strength are also improved. (1) Solution treatment of beryllium bronze. Generally, the heating temperature for solution treatment is between 780-820℃. For materials used as elastic components, 760-780℃ is used, mainly to prevent coarse grains from affecting the strength. The temperature uniformity of the solution treatment furnace should be strictly controlled within ±5°C. The holding time can generally be calculated as 1 hour/25mm. When beryllium bronze is subjected to solid solution heating treatment in air or oxidizing atmosphere, an oxide film will be formed on the surface. Although it has little effect on the mechanical properties after age strengthening, it will affect the service life of the tool mold during cold working. To avoid oxidation, it should be heated in a vacuum furnace or ammonia decomposition, inert gas, reducing atmosphere (such as hydrogen, carbon monoxide, etc.) to obtain a bright heat treatment effect. In addition, attention should be paid to shortening the transfer time (during quenching) as much as possible, otherwise the mechanical properties after aging will be affected. Thin materials shall not exceed 3 seconds, and general parts shall not exceed 5 seconds. The quenching medium generally uses water (no heating required). Of course, oil can also be used for parts with complex shapes to avoid deformation. (2) Aging treatment of beryllium bronze The aging temperature of beryllium bronze is related to the Be content. All alloys containing Be less than 2.1% should be aged. For alloys with Be greater than 1.7%, the optimal aging temperature is 300-330°C, and the holding time is 1-3 hours (depending on the shape and thickness of the part). For highly conductive electrode alloys with Be less than 0.5%, due to the increased melting point, the optimal aging temperature is 450-480°C and the holding time is 1-3 hours. In recent years, dual-stage and multi-stage aging have also been developed, that is, short-time aging at high temperature, and then long-term heat preservation aging at low temperature. The advantage of this is that the performance is improved but the deformation is reduced. In order to improve the dimensional accuracy of beryllium bronze after aging, fixtures can be used for aging, and sometimes two separate stages of aging treatment can be used. (3) Stress relief treatment of beryllium bronze. The stress relief annealing temperature of beryllium bronze is 150-200℃ and the holding time is 1-1.5 hours. It can be used to eliminate residual stress caused by metal cutting, straightening, cold forming, etc., and stabilize the shape and dimensional accuracy of parts during long-term use.
Heat treatment stress and its effects Heat treatment residual stress refers to the final stress that remains after heat treatment of the workpiece. It has an extremely important impact on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it will cause deformation of the workpiece. When it exceeds the strength limit of the material, it will cause the workpiece to crack. This is its harmful side and should be reduced and eliminated. However, by controlling the stress to make it reasonably distributed under certain conditions, the mechanical properties and service life of the parts can be improved, turning harmful effects into beneficial effects. Analyzing the distribution and change patterns of stress during the heat treatment of steel and making it reasonably distributed has far-reaching practical significance for improving product quality. For example, the impact of reasonable distribution of surface residual compressive stress on the service life of parts has attracted widespread attention. 1. Heat treatment stress of steel During the heating and cooling process of the workpiece, due to the inconsistent cooling speed and time between the surface layer and the core, a temperature difference is formed, which will lead to uneven volume expansion and contraction and generate stress, that is, thermal stress. Under the action of thermal stress, since the initial temperature of the surface layer is lower than that of the core, the shrinkage is also greater than that of the core, causing the core to be stretched. When cooling is completed, the final cooling volume shrinkage of the core cannot proceed freely, causing the surface to be compressed and the core to be stretched. That is, under the action of thermal stress, the surface layer of the workpiece is eventually compressed and the core is stretched. This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. When the cooling rate is faster, the carbon content and alloy composition are higher, the uneven plastic deformation generated under the action of thermal stress during the cooling process is larger, and the final residual stress is larger. On the other hand, due to changes in the structure of steel during the heat treatment process, that is, when austenite transforms into martensite, the increase in specific volume will be accompanied by the expansion of the volume of the workpiece, and various parts of the workpiece will undergo phase transformation, resulting in inconsistent volume growth and tissue stress. The final result of tissue stress changes is that the surface layer is subject to tensile stress and the core is subject to compressive stress, which is exactly the opposite of thermal stress. The size of the organizational stress is related to factors such as the cooling rate, shape, and chemical composition of the material in the martensitic transformation zone. Practice has proved that during the heat treatment of any workpiece, as long as there is a phase change, thermal stress and structural stress will occur. &127;It’s just that thermal stress has already been generated before the tissue transformation, while tissue stress is generated during the tissue transformation process. During the entire cooling process, the result of the combined effect of thermal stress and tissue stress is the actual stress in the workpiece. The result of the combined effect of these two stresses is very complex and is affected by many factors, such as composition, shape, heat treatment process, etc. In terms of its development process, there are only two types, namely thermal stress and tissue stress. When the direction of action is opposite, they cancel each other, and when the direction of action is the same, they superimpose each other. Regardless of whether they cancel each other or superimpose each other, the two stresses should have one dominant factor. When thermal stress dominates, the result is that the center of the workpiece is stretched and the surface is compressed. &127; When tissue stress dominates, the result is tension on the compressed surface of the center of the workpiece. 2. The influence of heat treatment stress on quenching cracks. Factors (including metallurgical defects) that can cause stress concentration in different parts of the quenched parts can promote the occurrence of quenching cracks, but they will only appear in the tensile stress field (especially under the maximum tensile stress). If there is no cracking effect in the compressive stress field. The quenching cooling rate is an important factor that can affect the quenching quality and determine the residual stress. It is also a factor that can have an important or even decisive influence on quenching cracks. In order to achieve the purpose of quenching, it is usually necessary to accelerate the cooling rate of the part in the high temperature section and make it exceed the critical quenching cooling rate of the steel to obtain the martensite structure. As far as residual stress is concerned, this can increase the value of thermal stress that offsets the effect of tissue stress, so it can reduce the tensile stress on the surface of the workpiece and achieve the purpose of inhibiting longitudinal cracks. Its effect will increase with the speed of high temperature cooling. Moreover, if it can be hardened, the larger the cross-sectional size of the workpiece, although the actual cooling rate is slower, the risk of cracking will be greater. All this is due to the fact that the actual cooling rate of thermal stress of this type of steel slows down with the increase in size, the thermal stress decreases, and the structural stress increases with the increase in size, and finally the tensile stress mainly composed of structural stress acts on the surface of the workpiece. And it is quite different from the traditional concept that the slower the cooling, the smaller the stress. For this type of steel parts, only longitudinal cracks can form in high-hardenability steel parts quenched under normal conditions. The reliable principle to avoid quenching cracking is to try to minimize the anisochrony of martensitic transformation inside and outside the section. Merely implementing slow cooling in the martensitic transformation zone is not enough to prevent the formation of longitudinal cracks. Under normal circumstances, arc cracks can only occur in non-hardenable parts. Although the overall rapid cooling is the necessary formation condition, the real cause of its formation is not the rapid cooling (including the martensite transformation zone) itself, but the local position of the quenched part (determined by the geometric structure). The cooling rate in the high-temperature critical temperature zone is significantly slowed down, so there is no hardening. Transverse and longitudinal splitting in large non-hardenable parts are caused by the residual tensile stress with thermal stress as the main component acting on the center of the quenched part, and at the center of the section of the quenched part that has not been hardened, cracks first form and expand from the inside out. In order to avoid such cracks, a water-oil double liquid quenching process is often used. The purpose of implementing rapid cooling in the high-temperature section in this process is only to ensure that the outer metal layer obtains a martensitic structure. However, from the perspective of internal stress, rapid cooling at this time is harmful and useless. Secondly, the purpose of slow cooling in the later stage of cooling is not mainly to reduce the expansion rate and structural stress value of martensitic transformation, but to minimize the cross-section temperature difference and the shrinkage rate of the metal in the center of the section, so as to reduce the stress value and ultimately inhibit quenching cracking.
It's too messy. Let's go back and sort it out ourselves. I'm just taking a look at it.^_^:lol