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Quenching operation process of steel

Quenching of steel

The process of heating the steel to a certain temperature above (30-50) C of Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel) for a certain period of time and then cooling rapidly in water or oil is called quenching.
 The purpose of quenching is to improve hardness, strength and wear resistance to meet the performance of parts. Steel quenching is an important and widely used process in heat treatment process, such as tools, measuring tools, dies, bearings, springs and automobiles, tractors, diesel engines, cutting machine tools, pneumatic tools, drilling machinery, agricultural machinery, petroleum machinery, chemical machinery, textile machinery, aircraft and other parts are being quenched. Fire craftsmanship.

1. quenching heating temperature

Quenching and heating temperature is determined according to the composition, microstructure and performance requirements of the steel. Hypoeutectoid steel is AC3+ (30~50 C); eutectoid steel and hypereutectoid steel are AC1+ (30~50 C).
 If the quenching heating temperature of hypoeutectoid steel is lower than that of AC3, the steel is not yet fully austenitized, and some ferrite remains in the quenched structure after quenching. The hardness of ferrite is low, so that the hardness after quenching can not meet the requirements, but also affect other mechanical properties. If the hypoeutectoid steel is heated to a temperature much higher than that of AC3, the austenite grain will be significantly coarsened, thus destroying the properties of the quenched material. Therefore, the quenching heating temperature of hypoeutectoid steel is AC3 + (30 ~ 50 C), which not only ensures full austenitization, but also keeps the austenite grain fine.
 The quenching and heating temperature of hypereutectoid steel is generally recommended as AC1+ (30~50 C). In actual production, it is also appropriate to raise about 20 degrees according to the situation. When heated in this temperature range, the microstructure is fine grained austenite and some fine uniformly distributed undissolved carbides. Except for a few retained austenite after quenching, the microstructure is fine carbide particles uniformly distributed on the lamellar martensite matrix. Such an organization has high hardness, good wear resistance and relatively little brittleness.
 The quenching and heating temperature of hypereutectoid steel can not be lower than AC1 because the steel has not been austenitizing at this time. When heated to a little higher than AC1 temperature, the pearlite completely transforms into austenite, and a small amount of cementite dissolves into austenite. The grain size of austenite is fine and the mass fraction of carbon is slightly higher than that of eutectoid. If the temperature continues to rise, the secondary cementite will continue to dissolve into austenite, resulting in austenite grain growth, and its carbon concentration will continue to increase, which will lead to the increase of quenching deformation tendency, the increase of microstructure cracks and brittleness. At the same time, because of the high carbon content of austenite, the amount of retained austenite increases after quenching, which reduces the hardness and wear resistance of the workpiece. Therefore, the quenching heating temperature of hypereutectoid steel is much higher than that of AC1.
 When choosing the quenching heating temperature of the workpiece in production practice, besides abiding by the above general principles, the influence of the chemical composition, technical requirements, size and shape of the workpiece, original structure, heating equipment, cooling medium and many other factors should be considered, and the heating temperature should be adjusted appropriately. If the alloy steel parts are used, the upper limit is usually taken, and the lower limit for the complicated parts is taken.
 The quenching and heating temperature selected by the new toughening process is different from that of the common quenching temperature. If the hypoeutectoid steel is austenitized at temperatures slightly lower than AC3, it can improve toughness, reduce brittle transition temperature and eliminate tempering brittleness. Such as 45, 40Cr, 60Si2 and other materials made of workpiece sub temperature quenching heating temperature of AC3 - (5~10 C).
 More lath martensite or all lath martensite can be obtained by high temperature quenching. If 16Mn steel is quenched at 940 degrees, 5CrMnMo steel quenched at 890 degrees, and 20CrMnMo steel quenched at 920 degrees, the effect is better.
 The carbon content of austenite can be reduced and the toughness of high carbon steel can be improved by quenching high carbon steel at low temperature, fast and short time, reducing the quenching temperature properly, or adopting the methods of rapid heating and shortening the holding time.
 2. holding time
 In order to make the internal and external parts of the workpiece complete the transformation of microstructure, carbide dissolution and austenite composition homogenization, it is necessary to maintain a certain time at quenching heating temperature, that is, holding time.

3. quenching medium
 The medium used for quenching the workpiece is called quenching medium (or quenching medium). The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress. This requires that the temperature above the nose of the C curve should be slowly cooled to reduce the thermal stress caused by quenching; that the cooling rate at the nose should be greater than the critical cooling rate to ensure that the undercooled austenite does not undergo Non-martensitic transformation; and that the cooling rate should be as small as possible below the nose, especially at the temperature below the MS point, to reduce the cooling rate. The stress of tissue transformation.
 The commonly used cooling medium in production are water, water solution, oil, and molten salt and molten alkali.
 (1) Water: It has the advantages of high cooling capacity in the range of 650-550 ~C, safety, low cost, less pollution to the environment, easy control and automation; its disadvantage is that the cooling rate is still very fast in the range of 300-200 ~C, and easy to cause steel quenching cracking.
 (2) Salt solution: commonly used 5% ~ 15% NaCl solution, the advantage is that it can increase the cooling range of 650 ~ 550 C, and basically does not change the cooling capacity of 300 ~ 200 C, can avoid quenching soft spots, so that the hardness is uniform, is the common quenching medium in the factory.
 (3) Alkali aqueous solution: 5% ~ 15% NaOH aqueous solution is commonly used, the advantage is that it can increase the cooling rate in the range of 650 ~ 550 C, basically does not change the cooling capacity of 300 ~ 200 C, the disadvantage is that it is corrosive, poor chemical stability, easy to deteriorate.
 (4) Oil: The advantage is that the cooling process is very slow, and the workpiece is not easy to crack, no matter at 650 ~ 550 (?) C or 300 ~ 200 (?) C. The disadvantage is flammability, the use of properties will gradually change, the price is high.
 Generally speaking, the carbon steel is quenched with water and the alloy steel is quenched with oil cold.
 4. the cooling method is widely used in production practice. The classification of quenching is divided by different cooling methods. There are mainly single liquid quenching, double liquid quenching, graded quenching and isothermal quenching.
 Special parts are also compressed air quenching, spray quenching and jet quenching.
 (1) A single-liquid quenching method in which the heated workpiece is continuously cooled to room temperature in a quenching medium. Advantages and Disadvantages: Simple operation, easy to achieve mechanization and automation, suitable for simple shape workpiece, but this method of water-cooled deformation, oil-cooled hardening, can be combined with oil-water two-cooled two-liquid quenching as follows.
 (2) The heated carbon steel is cooled in water or brine first, and then cooled to (300-400). This method is called dual-liquid quenching. Advantages and Disadvantages: It can not only harden the workpiece, but also reduce the internal stress of quenching, effectively prevent the occurrence of quenching cracks. It is mainly used for high carbon tool steel with complex shapes, such as taps, plate teeth and so on. The disadvantage is that it is difficult to operate and skilled in technology.
 (3) The stepwise quenching method is to put the heated workpiece into the salt bath or alkali bath with temperature slightly higher than the MS point to cool rapidly for a period of time, and then take out the air-cooling after the surface and the heart reach the medium temperature, so that martensite transformation occurs. Advantages and disadvantages: compared with two liquid quenching, the stress and deformation are further reduced, and the operation is easy. But because the cooling capacity of salt bath and alkali bath is small, it is only suitable for work with complex shape and small size.
 (4) Isothermal quenching is similar to stepwise quenching except that the holding time in salt bath or alkaline bath is long enough to make the supercooled austenite isothermally transform into the lower bainite with high strength and toughness, and then take out air cooling.
 Advantages and Disadvantages: Because of the small internal stress in quenching, it can effectively prevent deformation and cracking, but the disadvantage of this method is long production cycle and certain equipment, often used in thin, thin and complex shape size requirements accurate, and high strength and toughness workpieces, such as forming tools, molds and springs.
 5. hardenability
 Hardenability refers to the ability of materials to obtain the depth of hardened layer.
 It is generally stipulated that the depth from the surface layer of the workpiece to the semi-martensite zone (where half of the martensite and half of the non-martensite textures are respectively easy to determine the hardness) is the depth of the hardened layer. The deeper the hardened layer is, the better the hardenability of the surface steel is. If the hardened layer reaches the center, it indicates that the steel is all hardened.
 Hardenability of steel has a great influence on mechanical properties, but not all mechanical parts must be fully hardened. For example, shaft parts subjected to bending and torsional stresses, parts subjected to surface heat treatment, etc., need only a certain depth of hardening layer to meet the use requirements.
 The hardenability of steel is mainly determined by the critical cooling rate. The smaller the critical cooling rate, the better the hardenability of steel. On the contrary, the hardenability of steel is reduced. Except for Co, most alloying elements can significantly improve the hardenability of steel.
It should be noted that hardenability and hardenability are two different concepts and must not be confused. Hardening is the high hardness obtained after quenching, mainly depending on the carbon content in martensite. The steel with good hardenability is not necessarily high in hardenability. For example, compared with low carbon alloy steel, the former has higher hardenability but lower hardenability, while the latter has lower hardenability but higher hardenability.

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