What is quenching?
Steel quenching is one of the steel is heated to a critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel) above temperature, heat preservation for a period of time, make all or part of the austenitizing, then at greater than the critical cooling rate of cold faster cold below Ms to martensite isothermal near (or Ms) or bainite transformation of heat treatment process. Generally also aluminum alloy, copper alloy, titanium alloy, toughened glass and other materials of the solid solution treatment or with a rapid cooling process of the heat treatment process known as quenching. Purpose of quenching: 1) to improve the mechanical properties of metal or parts. For example: improve the hardness and wear resistance of tools and bearings, improve the elastic limit of springs, improve the comprehensive mechanical properties of shaft parts. 2) to improve the material properties or chemical properties of some special steels. For example, improve the corrosion resistance of stainless steel, increase the permanent magnetism of magnetic steel. Quenching cooling, in addition to the rational selection of quenching medium, but also have the correct quenching method, commonly used quenching methods, mainly single liquid quenching, double liquid quenching, classification quenching, isothermal quenching, local quenching. The steel workpiece after quenching has the following characteristics: (1) obtained martensite, bainite, residual austenite and other unbalanced (that is, unstable) structure. There is a large internal stress. The mechanical properties cannot meet the requirements. Therefore, the steel workpiece after quenching generally after tempering.
What is tempering?
Tempering after quenching metal timber or parts of heating to a certain temperature, heat preservation, after a certain time in a certain way the cooling of the heat treatment process and tempering is followed by an operation, after quenching are also usually workpiece heat treatment at the end of the procedure, and thus have a combination of quenching and tempering process known as the final processing. The main purpose of quenching and tempering is: 1) to reduce the internal stress and reduce brittleness, the quenching part has a great stress and brittleness, such as not tempering in time will often produce deformation or even cracking. 2) adjust the mechanical properties of the workpiece. After quenching, the workpiece has high hardness and brittleness. In order to meet the different performance requirements of various workpiece, the hardness, strength, plasticity and toughness can be adjusted by tempering. 3) stabilize the workpiece size. The microstructure can be stabilized by tempering to ensure that it will not be deformed in the future use. 4) improve the cutting performance of some alloy steel. The role of tempering is: (1) improve the stability of the organization, so that the workpiece in the process of use no longer occur in the organization of the transformation, so that the workpiece geometric size and performance remain stable. To eliminate the internal stress, in order to improve the performance of the workpiece and stabilize the workpiece geometry. Adjust the mechanical properties of steel to meet the use requirements. Tempering has these effects, because when the temperature increases, the atomic activity capacity increases, iron, carbon and other alloy elements in the iron and steel atoms can spread quickly, to achieve the rearrangement and combination of atoms, so that the unstable unbalanced organization gradually into a stable equilibrium organization. The elimination of internal stress is also related to the decrease of metal strength when the temperature increases. When general steel temper, hardness and strength decline, plastic increase. The higher the tempering temperature, the greater the change of these mechanical properties. Some alloy steels with high alloying element content, when tempered at a certain temperature range, will precipitate some fine metal compounds, making the strength and hardness rise. This phenomenon is called secondary hardening. Tempering requirements: different purposes of the workpiece should be tempering at different temperatures to meet the requirements in use. Tools, bearings, carburized quenching parts, surface quenching parts are usually under 250℃ for low temperature tempering. After low temperature tempering, the hardness changes little, the internal stress decreases, the toughness increases slightly. (2) spring tempering at 350 ~ 500℃ in the middle temperature, can obtain higher elasticity and the necessary toughness. The parts made of carbon structural steel are usually tempered at 500 ~ 600℃ to obtain the right strength and toughness. When steel is tempered at about 300℃, its brittleness is often increased. This temperature range should not normally be tempered. Some medium carbon alloy structural steels also tend to become brittle if they are slowly cooled to room temperature after tempering at high temperature. This phenomenon is called the second temper brittleness. Adding molybdenum to steel or cooling in oil or water during tempering prevents the second type of tempering brittleness. This brittleness can be eliminated by reheating the second type of tempered brittle steel to its original tempering temperature. In production, the performance of the workpiece is often required. According to the heating temperature, tempering is divided into low temperature tempering, medium temperature tempering, and high temperature tempering. The heat treatment process of quenching and subsequent high temperature tempering is called tempering. 1, low temperature tempering: 150-250℃, M cycle, reduce internal stress and brittleness, improve plastic toughness, has a higher hardness and wear resistance. Used for making measuring tools, tools and rolling bearings. 2, medium tempering: 350-500℃, T cycle, with high elasticity, a certain degree of plasticity and hardness. Used for making spring, forging die, etc. 3. High temperature tempering: 500-650℃, S cycle, with good comprehensive mechanical properties. Used for making gears, crankshafts, etc.
What is normalizing?
Normalizing is a heat treatment to improve the toughness of steel. Heat the steel member above Ac3 temperature 30? After 50℃, heat preservation for a period of time out of the air cooling. The main feature is that the cooling speed is faster than annealing and lower than quenching. When normalizing, the steel crystal grain can be refined in a slightly faster cooling, which can not only obtain a satisfactory strength, but also significantly improve the toughness (AKV value) and reduce the cracking tendency of the members. After normalizing some low alloy hot rolled steel plates, low alloy steel forgings and castings, the comprehensive mechanical properties of the materials can be greatly improved, and the cutting performance can also be improved.
Normalizing has the following purposes and USES: (1) for subeutectic steel, normalizing to eliminate the casting, forging, welding parts of the superheated coarse crystal structure and weidner's structure, rolling in the strip structure; Refine grain; And can be used as a pre-heat treatment before quenching. (2) for eutectoid steel, normalizing can eliminate the network of secondary cementation, and make the pearlite fine, not only improve the mechanical properties, and is conducive to the future of the spheroidization annealing. (3) for low carbon deep blanking steel plate, normalizing can eliminate the free cementite of grain boundary, in order to improve its deep blanking performance. (4) for low carbon steel and low carbon low alloy steel, using normalizing, can get more fine sheet pearlite structure, make the hardness increased to hb140-190, to avoid cutting "stick" phenomenon, improve the machinability. For medium carbon steel, normalizing is more economical and convenient when both normalizing and annealing are available. (5) for ordinary carbon structure steel, in the case of low mechanical properties, can be used to replace quenching and high temperature tempering normalizing, not only easy to operate, and make the steel structure and size stable. 6 high temperature normalizing (Ac3 above 150 ~ 200℃) due to high temperature diffusion rate, can reduce the composition of castings and forgings segregation. The coarse grain after high temperature normalizing can be refined by following the second lower temperature normalizing. For some low and medium carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain bainite structure, and then tempering at high temperature, for 400 ~ 550℃ with good creep resistance. In addition to steel and steel, normalizing is also widely used in heat treatment of nodular cast iron, to make it obtain pearlite matrix, improve the strength of nodular cast iron. Because normalizing is characterized by air cooling, the ambient temperature, stacking mode, airflow and workpiece size have an impact on the structure and performance after normalizing. Normalizing structure can also be used as a classification method for alloy steel. Alloy steels are usually divided into pearlite, bainitic, martensitic, and austenitic steels according to the microstructure obtained by air cooling after the specimen with diameter of 25 mm is heated to 900℃.
What is annealing?
Annealing is a metal heat treatment process in which the metal is heated slowly to a certain temperature, kept for a sufficient time, and then cooled at an appropriate rate. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile test or hardness test. Many steels are supplied in annealed condition. Hardness of steel can be tested by rockwell hardness tester to test HRB hardness, and for thinner steel plates, steel belts and thin-walled steel pipes, surface rockwell hardness tester can be used to test HRT hardness. The purpose of annealing is to: (1) improve or eliminate the steel in the process of casting, forging, rolling and welding caused by various tissue defects and residual stress, to prevent the workpiece deformation, cracking. (2) to soften the workpiece for cutting. (3) fine grain, improve the structure to improve the mechanical properties of the workpiece. (4) preparation for the final heat treatment (quenching, tempering).
Common annealing process is: (1) complete annealing. Used to refine the thick superheated structure of medium and low carbon steel with poor mechanical properties after casting, forging and welding. The workpiece is heated to the temperature of ferritic transformation to austenite all above 30 ~ 50℃, insulation for a period of time, and then with the furnace slowly cooling, in the cooling process of austenite transformation again, can make the steel fine. (2) spherification annealing. To reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20 ~ 40℃ above the temperature at which the steel begins to form austenite. After insulation, it is cooled slowly. During the cooling process, the lamellar cementite in the pearlite turns into a ball, thereby reducing the hardness. Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with high content of nickel and chromium. Generally, it is cooled to the most unstable temperature of austenite at a faster speed, and the hardness can be reduced after the austenite is transformed into tortolite or sostenite after proper insulation time. Recrystallization annealing. Used to eliminate metal wire, thin plate in the process of cold drawing, cold rolling hardening phenomenon (hardness increased, plastic decline). The heating temperature is generally 50 ~ 150℃ below the temperature at which steel begins to form austenite. Only in this way can the working hardening effect be eliminated and the metal be softened. (5) graphitization annealing. Used to make cast iron with a lot of cementite into malleable iron with good plasticity. The process operation is to heat the casting to about 950℃, hold the heat for a certain period of time after the appropriate cooling, so that the cementite decomposition into a mass of flocculent graphite. 6. Diffusion annealing. It is used to homogenize the chemical composition of alloy casting and improve its performance. The method is to heat the casting to the highest possible temperature without melting, and keep it warm for a long time, after the diffusion of various elements in the alloy tends to be evenly distributed. De-stress annealing. To eliminate the internal stress of steel castings and welding parts. For iron and steel products after heating began to form the austenite temperature below 100 ~ 200℃, after heat preservation in the air cooling, can eliminate the internal stress.