What are quenching, tempering, normalizing and annealing? They are the four processes of heat treatment

What is quenching?

The quenching of steel is a heat treatment process that heats the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), holds it for a period of time, makes it fully or partially austenitized, and then rapidly cools it to below Ms (or isothermal near Ms) at a cooling rate greater than the critical cooling rate for martensite (or bainite) transformation. The solution treatment of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials or the heat treatment process with rapid cooling process is also called quenching. The purpose of quenching: 1) to improve the mechanical properties of metal products or parts. For example, improve the hardness and wear resistance of tools and bearings, improve the elastic limit of springs, and improve the comprehensive mechanical properties of shaft parts. 2) Improve the material properties or chemical properties of some special steels. Such as improving the corrosion resistance of stainless steel and increasing the permanent magnetism of magnetic steel. When quenching and cooling, in addition to reasonable selection of quenching medium, there should also be correct quenching methods. Common quenching methods include single liquid quenching, double liquid quenching, staged quenching, isothermal quenching, local quenching, etc. After quenching, steel workpieces have the following characteristics: ① Unbalanced (i.e. unstable) structures such as martensite, bainite and retained austenite are obtained. ② There is a large internal stress. ③ The mechanical properties cannot meet the requirements. Therefore, steel workpieces are generally tempered after quenching.


2 What is tempering?


Tempering is a heat treatment process that heats quenched metal products or parts to a certain temperature, keeps them warm for a certain time, and then cools them in a certain way. Tempering is an operation that is carried out immediately after quenching, and is usually a * * process for heat treatment of workpieces. Therefore, the combined process of quenching and tempering is called * * * final treatment. The main purposes of quenching and tempering are: 1) to reduce internal stress and brittleness. Quenched parts have great stress and brittleness. If not tempered in time, deformation and even cracking will often occur. 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 workpieces, the hardness, strength, plasticity and toughness can be adjusted by tempering. 3) Stabilize the workpiece size. The metallographic structure can be stabilized by tempering to ensure that no deformation will occur in the future use. 4) Improve the machinability of some alloy steels. The role of tempering is to: ① improve the stability of the structure, so that the structure of the workpiece will not change during use, so that the geometric dimensions and properties of the workpiece remain stable. ② Eliminate internal stress, so as to improve the working performance of the workpiece and stabilize the geometric dimensions of the workpiece. ③ Adjust the mechanical properties of steel to meet the service requirements. Tempering has these effects because when the temperature rises, the ability of atomic activity is enhanced, and the atoms of iron, carbon and other alloy elements in steel can be diffused quickly to achieve the rearrangement and combination of atoms, so that the unstable unbalanced structure gradually changes into stable balanced structure. The elimination of internal stress is also related to the decrease of metal strength when the temperature rises. In general, when steel is tempered, its hardness and strength decrease and its plasticity increases. The higher the tempering temperature, the greater the change of these mechanical properties. Some alloy steels with high content of alloy elements will precipitate some fine metal compounds when tempered at a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening. Tempering requirements: workpieces with different purposes shall be tempered at different temperatures to meet the requirements in use. ① Tools, bearings, carburized and quenched parts and surface quenched parts are usually tempered at a low temperature below 250 ℃. After low temperature tempering, the hardness changes little, the internal stress decreases, and the toughness slightly increases. ② When the spring is tempered at 350~500 ℃ at medium temperature, higher elasticity and necessary toughness can be obtained. ③ Parts made of medium carbon structural steel are usually tempered at 500~600 ℃ to obtain a good fit of strength and toughness. When steel is tempered at about 300 ℃, its brittleness is often increased, which is called * * * temper brittleness. Generally, tempering shall not be conducted in this temperature range. After tempering at high temperature, some medium carbon alloy structural steels are easy to become brittle if they are slowly cooled to room temperature. This phenomenon is called the second type of temper brittleness. Adding molybdenum to steel or cooling in oil or water during tempering can prevent the second type of temper brittleness. This kind of brittleness can be eliminated by reheating the steel with the second type of tempering brittleness to the original tempering temperature. In production, it is often based on the requirements for workpiece performance. Tempering is divided into low temperature tempering, medium temperature tempering and high temperature tempering according to different heating temperatures. The heat treatment process combining quenching and subsequent high temperature tempering is called quenching and tempering, which has high strength and good plasticity and toughness at the same time. 1. Low temperature tempering: 150-250 ℃, M cycles, reduce internal stress and brittleness, improve plastic toughness, and have high hardness and wear resistance. Used for making measuring tools, cutters, rolling bearings, etc. 2. Medium temperature tempering: 350-500 ℃, T-back, high elasticity, certain plasticity and hardness. Used for making springs, forging dies, etc. 3. High temperature tempering: 500-650 ℃, S-back, with good comprehensive mechanical properties. Used for making gears, crankshafts, etc.


3 What is normalizing?


Normalizing is a kind of heat treatment to improve the toughness of steel. Heat the steel member to 30 ° above the Ac3 temperature? After 50 ℃, the furnace shall be discharged for air cooling after holding for a period of time. The main feature is that the cooling rate is faster than that of annealing but lower than that of quenching. When normalizing, the crystalline grains of steel can be refined in a slightly faster cooling process, which can not only obtain satisfactory strength, but also significantly improve the toughness (AKV value) and reduce the cracking tendency of components 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, as well as the cutting properties



Normalizing has the following purposes and purposes: ① For hypoeutectoid steel, normalizing is used to eliminate the overheated coarse grain structure and widmanstatten structure of castings, forgings and weldments, and the banded structure in rolled products; Refine grains; It can also be used as pre heat treatment before quenching. ② For hypereutectoid steel, normalizing can eliminate the network secondary cementite and refine the pearlite, which not only improves the mechanical properties, but also facilitates the subsequent spheroidizing annealing. ③ For low carbon deep drawing steel sheet, normalizing can eliminate free cementite at grain boundary to improve its deep drawing property. ④ For low carbon steel and low carbon low alloy steel, more fine lamellar pearlite structure can be obtained by normalizing, which can increase the hardness to HB140-190, avoid the "sticking tool" phenomenon during cutting, and improve the machinability. For medium carbon steel, normalizing is more economical and convenient when normalizing and annealing are available. ⑤ For ordinary medium carbon structural steel, normalizing can be used to replace quenching and high temperature tempering in the case of low requirements for mechanical properties, which is not only simple to operate, but also stable in structure and size. ⑥ High temperature normalizing (150~200 ℃ above Ac3) can reduce component segregation of castings and forgings due to high diffusion rate at high temperature. The coarse grains after high temperature normalizing can be refined by the second lower temperature normalizing. ⑦ 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. They have good creep resistance when used at 400~550 ℃. ⑧ Besides steel parts and steels, normalizing is also widely used for heat treatment of nodular cast iron to obtain pearlite matrix and improve the strength of nodular cast iron. As the normalizing is characterized by air cooling, the ambient temperature, stacking mode, air flow and workpiece size all affect the structure and properties after normalizing. Normalized structure can also be used as a classification method of alloy steel. Alloy steels are generally divided into pearlitic steel, bainitic steel, martensitic steel and austenitic steel according to the structure obtained by air cooling after heating the sample with a diameter of 25 mm to 900 ℃.


4 What is annealing?


Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature, maintained for a sufficient time, and then cooled at an appropriate rate. The 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 the state of annealing heat treatment. Rockwell hardness tester can be used to test the HRB hardness of steels. For thinner steel plates, steel strips and thin-walled steel tubes, surface Rockwell hardness tester can be used to test the HRT hardness. The purpose of annealing is to: ① improve or eliminate various structural defects and residual stresses caused by steel casting, forging, rolling and welding, and prevent workpiece deformation and cracking. ② Soften the workpiece for machining. ③ Refine the grain and improve the structure to improve the mechanical properties of the workpiece. ④ Prepare the organization for the final heat treatment (quenching and tempering) of * * *.


The commonly used annealing processes are: ① complete annealing. It is used to refine the coarse overheating structure with poor mechanical properties of medium and low carbon steel after casting, forging and welding. The workpiece shall be heated to 30~50 ℃ above the temperature at which all ferrite is transformed into austenite, held for a period of time, and then slowly cooled with the furnace. During the cooling process, the austenite is transformed again, which can make the steel structure thinner. ② Spheroidizing annealing. It is used 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 heat preservation, it is slowly cooled. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thus reducing the hardness. ③ Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with high nickel and chromium content for machining. Generally, it is cooled to the unstable temperature of austenite * * * at a fast speed, and the hardness can be reduced if the austenite is transformed into troostite or sorbite after holding for a proper time. ④ Recrystallization annealing. It is used to eliminate the hardening phenomenon (hardness increase, plasticity decrease) of metal wire and thin plate during cold drawing and cold rolling. The heating temperature is generally 50~150 ℃ below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated to soften the metal. ⑤ Graphitizing annealing. It is used to change cast iron containing a large amount of cementite into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950 ℃, keep it warm for a certain time, and then cool it properly, so that the cementite can be decomposed into flocculent graphite. ⑥ Diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve their service performance. The method is to heat the casting to the highest possible temperature without melting, keep it warm for a long time, and slowly cool it after various elements in the alloy diffuse to uniform distribution. ⑦ Stress relief annealing. It is used to eliminate the internal stress of steel castings and weldments. For iron and steel products, the temperature below which austenite begins to form after heating is 100~200 ℃. After heat preservation, the internal stress can be eliminated by cooling in air.