What is quenching and tempering
What is quenching and tempering
Table of Contents
Quenching and tempering is a heat treatment process to improve the comprehensive mechanical properties of iron and steel materials. That is, it is quenched and then tempered at high temperature. Its purpose is to make steel parts obtain a good combination of strength and toughness, which not only has high strength, but also has excellent toughness, plasticity and cutting performance. Some alloy steels or low alloy steels can obtain good comprehensive properties only after quenching and tempering. These steels can also be called quenched and tempered steels. The quenching and tempering treatment of nodular cast iron can not only improve the strength, but also improve the hardness and wear resistance.
Quenching and tempering treatment refers to the dual heat treatment method of quenching and high temperature tempering. Its purpose is to make the workpiece have good comprehensive mechanical properties. High temperature tempering refers to tempering between 500-650 ℃. Most quenched and tempered parts work under relatively large dynamic load. They bear the effects of tension, compression, bending, torsion or shear. Some surfaces also have friction, which requires certain wear resistance and so on. In short, the parts work under various composite stresses. Such parts are mainly structural parts of various machines and mechanisms, such as shafts, connecting rods, studs, gears, etc. they are widely used in manufacturing industries such as machine tools, automobiles and tractors. Especially for large parts in heavy machine manufacturing, quenching and tempering treatment is more used. Therefore, quenching and tempering treatment plays a very important role in heat treatment. Because of the different stress conditions of the quenched and tempered parts in mechanical products, the required performance is not exactly the same. Generally speaking, all kinds of quenched and tempered parts should have excellent comprehensive mechanical properties, that is, the appropriate combination of high strength and high toughness, so as to ensure the long-term smooth operation of the parts.
Tempered sorbite was obtained after quenching and tempering. Tempered sorbite is formed by martensite during tempering. It can be distinguished only when it is magnified more than 500 – 600 times under the optical metallographic microscope. It is a composite structure with carbide (including cementite) spheres distributed in the ferrite matrix. It is also a tempering structure of martensite and a mixture of ferrite and granular carbide. At this time, the ferrite has basically no carbon supersaturation, and the carbide is also a stable carbide. It is a kind of balanced tissue at room temperature.
Quenched and tempered steel includes carbon quenched and tempered steel and alloy quenched and tempered steel. Whether it is carbon steel or alloy steel, its carbon content is strictly controlled. If the carbon content is too high, the strength of the workpiece after quenching and tempering is high, but the toughness is not enough. If the carbon content is too low, the toughness is improved and the strength is insufficient. In order to obtain good comprehensive performance of quenched and tempered parts, the carbon content is generally controlled at 0.30 – 0.50%.
During quenching, tempering and quenching, the whole section of the workpiece is required to be quenched thoroughly, so that the workpiece can obtain the microstructure dominated by fine needle quenched martensite. Through high temperature tempering, the microstructure dominated by uniformly tempered sorbite was obtained. It is impossible for small factories to conduct metallographic analysis for each furnace. Generally, they only conduct hardness test, that is to say, the hardness after quenching must reach the quenching hardness of the material, and the hardness after tempering shall be checked according to the requirements of the drawing.
Steps and principles of quenching and tempering
Quenching is the first step. The heating temperature depends on the composition of steel, and the quenching medium is selected according to the hardenability of steel and the size of steel parts. After quenching, the internal stress of steel is large and brittle. Tempering must be carried out in order to eliminate stress, increase toughness and adjust strength. Tempering is the most important process to set the mechanical properties of quenched and tempered steel. The curve of mechanical properties of various steels with tempering temperature, also known as tempering curve of steel, can be used as the basis for selecting tempering temperature. For high temperature tempering of some alloy quenched and tempered steels, attention should be paid to prevent the second type of tempering brittleness to ensure the service performance of the steel.
Application of quenching and tempering
Quenching and tempering treatment is widely used in structural parts that require excellent comprehensive performance, especially under alternating load, such as automobile shaft and gear, aeroengine turbine shaft, compressor disk and so on. Structural steel parts requiring induction heating quenching are usually quenched and tempered before surface quenching to obtain fine and uniform sorbite, which is conducive to the surface hardening layer and good comprehensive mechanical properties at the core. The quenching and tempering treatment of nitrided parts before nitriding can improve the machinability of steel and prepare the microstructure for nitriding. In order to obtain a high finish of the measuring tool before quenching, eliminate the stress caused by rough machining, reduce quenching deformation, and make the hardness after quenching high and uniform, quenching and tempering treatment can be carried out before finishing. For tool steel with network carbide or coarse grain after forging, quenching and tempering treatment can be used to eliminate carbide network and refine grain, spheroidize carbide, improve machinability and prepare for final heat treatment.
Quenching and tempering treatment process
The heat treatment process of steel includes annealing, normalizing, quenching, tempering and surface heat treatment. Tempering includes high temperature tempering, medium temperature tempering and
Low temperature tempering. Reheating the quenched steel to a certain temperature and then cooling it by a certain method is called tempering. The purpose is to eliminate the internal stress produced by quenching, reduce hardness and brittleness, and obtain the expected mechanical properties.
Quenching and tempering usually refers to the heat treatment process of quenching + high temperature tempering to obtain tempered sorbite. The method is quenching first. The quenching temperature is AC3 + 30 – 50 ℃ for hypoeutectoid steel; The hypereutectoid steel is AC1 + 30 – 50 ℃; Alloy steel can be slightly higher than carbon steel. Tempering at 500 – 650 ℃ after quenching.
What is quenched and tempered steel
The so-called quenched and tempered steel generally refers to medium carbon steel with carbon content of 0.3-0.6%. Generally, the parts made of this kind of steel are required to have good comprehensive mechanical properties, that is, they not only maintain high strength, but also have good plasticity and toughness. People often use modulation treatment to achieve this purpose, so people used to call this kind of steel quenched and tempered steel. It should be noted that sometimes quenched and tempered steel also refers to the process of adding manganese and silicon when smelting steel. Pay attention to the difference.
Quenching and tempering of 45# steel
45# steel is a medium carbon structural steel with good cold and hot workability, good mechanical properties, low price and wide source, so it is widely used. Its biggest weakness is that it is not suitable for workpieces with low hardenability, large section size and high requirements.
The quenching temperature of 45# steel is A3 + (30 – 50) ℃. In practice, the upper limit is generally taken. The higher quenching temperature can accelerate the heating speed of the workpiece, reduce the surface oxidation, and improve the work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is required. If the actual furnace loading is large, the holding time needs to be extended appropriately. Otherwise, insufficient hardness may occur due to uneven heating. However, if the holding time is too long, the disadvantages of coarse grain and serious oxidation decarburization will also appear, which will affect the quenching quality. We believe that if the charging amount is greater than that specified in the process documents, the heating and holding time needs to be extended by 1 / 5.
Because 45# steel has low hardenability, 10% saline solution with high cooling rate should be used. After the workpiece enters the water, it should be quenched thoroughly, but not cold thoroughly. If the workpiece is cold thoroughly in salt water, it may crack the workpiece, which is caused by the rapid transformation of austenite into martensite when the workpiece is cooled to about 180 ℃, resulting in excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature area, the slow cooling method should be adopted. As the outlet water temperature is difficult to master, it must be operated by experience. When the shaking of the workpiece in the water stops, the outlet water can be air cooled (oil cooling is better). In addition, the workpiece should move rather than static when entering the water, and make regular movement according to the geometry of the workpiece. Static cooling medium and static workpiece lead to uneven hardness and stress, resulting in large deformation and even cracking of workpiece. The quenched hardness of 45# steel quenched and tempered parts should reach hrc56 – 59, and the possibility of large section is lower, but not lower than hrc48. Otherwise, it means that the workpiece has not been completely quenched, and sorbite or even ferrite structure may appear in the structure. This structure is still retained in the matrix through tempering, which can not achieve the purpose of quenching and tempering.
45# steel is tempered at high temperature after quenching. The heating temperature is usually 560 – 600 ℃, and the hardness requirement is hrc22 – 34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawing has hardness requirements, the tempering temperature shall be adjusted according to the drawing requirements to ensure the hardness. For example, some shaft parts require high strength and high hardness; For some gears and shaft parts with keyway, the hardness requirements are lower because they have to be milled and inserted after quenching and tempering. The tempering holding time depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature and has little to do with the tempering time, but it must be re penetrated. Generally, the tempering holding time of the workpiece is always more than one hour.
Source: Network Arrangement – China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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