The difference between annealing and tempering is:
Simply put, annealing means not having hardness, and tempering still retains a certain hardness.
Tempering:
The structure obtained by high temperature tempering is tempered sorbite. Generally, tempering is not used alone. The main purpose of tempering after parts quenching is to eliminate quenching stress and obtain the required structure. According to different tempering temperatures, tempering is divided into low temperature, medium temperature and high temperature tempering. Tempered martensite, troostite and sorbite were obtained respectively.
Among them, the heat treatment combined with high temperature tempering after quenching is called quenching and tempering treatment, and its purpose is to obtain comprehensive mechanical properties with good 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.
annealing:
Pearlite transformation occurs during the annealing process. The main purpose of annealing is to make the internal structure of the metal reach or approach the equilibrium state, and prepare for subsequent processing and final heat treatment. Stress relief annealing is an annealing process to eliminate the residual stress caused by plastic deformation processing, welding, etc. and existing in the casting. There is internal stress inside the workpiece after forging, casting, welding and cutting. If it is not eliminated in time, the workpiece will be deformed during processing and use, which will affect the accuracy of the workpiece.
It is very important to use stress relief annealing to eliminate the internal stress generated during processing. The heating temperature of stress relief annealing is lower than the phase transformation temperature, therefore, no structural transformation occurs during the entire heat treatment process. The internal stress is mainly eliminated naturally by the workpiece during the heat preservation and slow cooling process.
In order to eliminate the internal stress of the workpiece more thoroughly, the heating temperature should be controlled during heating. Generally, it is put into the furnace at a low temperature, and then heated to the specified temperature at a heating rate of about 100°C/h. The heating temperature of the weldment should be slightly higher than 600°C. The holding time depends on the situation, usually 2 to 4 hours. The holding time of the casting stress relief annealing takes the upper limit, the cooling rate is controlled at (20-50) ℃/h, and it can be cooled to below 300 ℃ before it can be air-cooled.
Aging treatment can be divided into two types: natural aging and artificial aging. Natural aging is to place the casting in the open field for more than half a year, so that it will occur slowly, so that the residual stress can be eliminated or reduced. Artificial aging is to heat the casting to 550~650℃ Perform stress relief annealing, which saves time compared with natural aging, and removes residual stress more thoroughly.
What is tempering?
Tempering is a heat treatment process that heats quenched metal products or parts to a certain temperature, and then cools them in a certain way after holding for a certain period of time. Tempering is an operation performed immediately after quenching, and is usually the last heat treatment of the workpiece. Therefore, the joint process of quenching and tempering is called final heat treatment. The main purpose of quenching and tempering is to:
1) Reduce internal stress and reduce brittleness. Quenched parts have great stress and brittleness. If they are not tempered in time, they will often deform or even crack.
2) Adjust the mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, it can be adjusted by tempering, hardness, strength, plasticity and toughness.
3) Stable workpiece size. The metallographic structure can be stabilized by tempering to ensure that no deformation will occur during future use.
4) Improve the cutting performance of some alloy steels.
In production, it is often based on the requirements for the performance of the workpiece. According to different heating temperatures, tempering is divided into low temperature tempering, medium temperature tempering, and high temperature tempering. The heat treatment process combining quenching and subsequent high-temperature tempering is called quenching and tempering, that is, it has good plasticity and toughness while having high strength. It is mainly used to handle machine structural parts with large loads, such as machine tool spindles, automobile rear axle shafts, powerful gears, etc.
What is quenching?
Quenching is a heat treatment process that heats metal products or parts above the phase transition temperature, and then rapidly cools at a rate greater than the critical cooling rate after heat preservation to obtain a martensitic structure. Quenching is to obtain martensitic structure, and after tempering, the workpiece can obtain good performance, so as to fully develop the potential of the material. Its main purpose is to:
1) Improve the mechanical properties of metal products or parts. For example: improving the hardness and wear resistance of tools, bearings, etc., increasing the elastic limit of springs, improving the comprehensive mechanical properties of shaft parts, etc.
2) Improve the material properties or chemical properties of some special steels. Such as improving the corrosion resistance of stainless steel, increasing the permanent magnetism of magnetic steel, etc.
When quenching and cooling, in addition to the reasonable selection of quenching medium, correct quenching methods are also required. The commonly used quenching methods mainly include single-liquid quenching, double-liquid quenching, graded quenching, isothermal quenching, and partial quenching.
The difference and connection between normalizing, quenching, annealing and tempering
Purpose and use of normalizing
① For hypoeutectoid steel, normalizing is used to eliminate the overheated coarse-grained structure and Widmanstatten structure of castings, forgings, and weldments, and the banded structure in rolled materials; refine grains; and can be used as pre-heat treatment before quenching.
② For hypereutectoid steel, normalizing can eliminate reticular secondary cementite and refine pearlite, which not only improves mechanical properties, but also facilitates subsequent spheroidizing annealing.
③ For low-carbon deep-drawing thin steel plates, normalizing can eliminate free cementite at grain boundaries to improve their deep-drawing properties.
④ For low-carbon steel and low-carbon low-alloy steel, use normalizing to obtain more fine-flaky pearlite structure, increase the hardness to HB140-190, avoid the phenomenon of “sticking knife” during cutting, and improve machinability . For medium carbon steel, when both normalizing and annealing can be used, it is more economical and convenient to use normalizing.
⑤ For ordinary medium-carbon structural steel, normalizing can be used instead of quenching and high-temperature tempering when the mechanical properties are not high, which is not only easy to operate, but also stabilizes the structure and size of the steel.
⑥ Normalizing at high temperature (150-200°C above Ac3) can reduce the composition segregation of castings and forgings due to the high diffusion rate at high temperature. Coarse grains after normalizing at high temperature can be refined by subsequent normalizing at a second lower temperature.
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 tempered at high temperature. It has good creep resistance when used at 400-550 °C.
⑧ In addition to steel parts and steel products, normalizing is also widely used in the heat treatment of ductile iron to obtain a pearlite matrix and improve the strength of ductile iron.
Since normalizing is characterized by air cooling, the ambient temperature, stacking method, airflow and workpiece size all have an impact on the structure and performance after normalizing. The normalized structure can also be used as a classification method of alloy steel. Generally, alloy steels are divided into pearlite steel, bainite steel, martensitic steel and austenitic steel according to the microstructure obtained by heating a sample with a diameter of 25 mm to 900 °C and air cooling.
Annealing is a metal heat treatment process in which the metal is slowly heated 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 detected by tensile test or hardness test. Many steel products are supplied in the state of annealing and heat treatment.
Rockwell hardness tester can be used to test the hardness of steel. For thinner steel plates, steel strips and thin-walled steel pipes, surface Rockwell hardness testers can be used to test 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 deformation and cracking of workpieces.
② Soften the workpiece for cutting.
③ Refining the grains and improving the structure to improve the mechanical properties of the workpiece.
④ Make organizational preparations for final heat treatment (quenching, tempering).
Commonly used annealing process
① Fully annealed. It is used to refine the coarse superheated structure with poor mechanical properties after casting, forging and welding of medium and low carbon steel. Heat the workpiece to 30-50°C above the temperature at which ferrite is completely transformed into austenite, keep it warm for a period of time, and then cool slowly with the furnace. During the cooling process, the austenite will transform again to 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°C above the temperature at which the steel begins to form austenite, and then slowly cooled after heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby 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 cutting. Generally, it is first cooled to the most unstable temperature of austenite at a faster rate, and kept for an appropriate time, the austenite will transform into troostite or sorbite, and the hardness can be reduced.
④ Recrystallization annealing. It is used to eliminate the hardening phenomenon (increase in hardness and decrease in plasticity) of metal wire and thin plate in the process of cold drawing and cold rolling. The heating temperature is generally 50-150°C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal softened.
⑤ Graphitization annealing. It is used to turn 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°C, keep it warm for a certain period of time and then cool it properly to decompose the cementite to form a group of flocculent graphite.
⑥ Diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve their performance. The method is to heat the casting to the highest possible temperature without melting, and keep it warm for a long time, and then cool slowly after the diffusion of various elements in the alloy tends to be evenly distributed.
⑦ Stress relief annealing. Used to eliminate the internal stress of steel castings and weldments. For iron and steel products heated to 100-200°C below the temperature at which austenite begins to form, cooling in air after heat preservation can eliminate internal stress.
Quenching, a heat treatment process for metals and glass. Heating alloy products or glass to a certain temperature, and then rapidly cooling in water, oil or air, generally used to increase the hardness and strength of the alloy. Commonly known as “dipping fire”. Metal heat treatment that reheats the quenched workpiece to an appropriate temperature lower than the lower critical temperature, and then cools it in air, water, oil and other media after holding it for a period of time.
Steel workpieces have the following characteristics after quenching:
① Unbalanced (that is, 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 generally have to be tempered after quenching.
The role of tempering
① Improve the stability of the structure, so that the workpiece will no longer undergo tissue transformation during use, so that the geometric size and performance of the workpiece will remain stable.
② Eliminate internal stress in order to improve the performance of the cnc parts and stabilize the geometric dimensions of the milled parts.
③ Adjust the mechanical properties of steel to meet the requirements of use.
*The reason why tempering has these effects is that when the temperature rises, the activity of atoms increases, and the atoms of iron, carbon and other alloying elements in steel can diffuse quickly to realize the rearrangement of atoms, thus making them unstable. The unbalanced organization gradually transforms into a stable balanced organization. The relief of internal stress is also related to the decrease in metal strength as the temperature increases. Generally, when steel is tempered, the hardness and strength decrease, and the plasticity increases. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with high content of alloying elements will precipitate some fine-grained metal compounds when tempered in a certain temperature range, which will increase the strength and hardness.
This phenomenon is called secondary hardening.
Tempering requirements: workpieces with different uses should be tempered at different temperatures to meet the requirements in use.
① Cutting tools, bearings, carburized and quenched parts, and surface quenched parts are usually tempered at a temperature below 250°C. After low-temperature tempering, the hardness does not change much, the internal stress decreases, and the toughness improves slightly.
② The spring is tempered at a medium temperature at 350-500°C to obtain high elasticity and necessary toughness.
③ Parts made of medium carbon structural steel are usually tempered at a high temperature of 500-600 ° C to obtain a good combination of strength and toughness.
The heat treatment process of quenching and high temperature tempering is collectively called quenching and tempering.
When steel is tempered at around 300°C, its brittleness often increases. This phenomenon is called the first type of temper brittleness. Generally, it should not be tempered in this temperature range. Some medium carbon alloy structural steels are also prone to become brittle if they are slowly cooled to room temperature after high temperature tempering. This phenomenon is called the second type of temper brittleness. The addition of molybdenum to the steel, or cooling in oil or water during tempering, can prevent the second type of temper brittleness. This brittleness can be eliminated by reheating the second type of temper brittle steel to the original tempering temperature.
Annealing of steel
Concept: The steel is heated, kept warm and then cooled slowly to obtain a process close to the equilibrium structure.
1. Fully annealed
Process: heating Ac3 above 30-50°C → heat preservation → cooling down to below 500°C with the furnace → air cooling at room temperature.
Purpose: to refine grains, uniform structure, improve plastic toughness, eliminate internal stress, and facilitate machining.
2. Isothermal annealing
Process: Heating above Ac3 → heat preservation → rapid cooling to the pearlite transition temperature → isothermal stay → transformation into P → air cooling out of the furnace;
Purpose: Same as above. But the time is short, easy to control, and the deoxidation and decarburization are small. (Applicable to alloy steel and large carbon machining steel parts with relatively stable supercooling A).
3. Spheroidizing annealing
Concept: It is the process of spheroidizing cementite in steel.
Objects: Eutectoid and hypereutectoid steels
Process:
(1) Isothermal spheroidizing annealing heating above Ac1 to 20-30 degrees → heat preservation → rapid cooling to 20 degrees below Ar1 → isothermal → cooling to about 600 degrees with the furnace → air cooling out of the furnace.
(2) Ordinary spheroidizing annealing heating Ac1 above 20-30 degrees → heat preservation → extremely slow cooling to about 600 degrees → air cooling out of the furnace. (Long cycle, low efficiency, not applicable).
Purpose: to reduce hardness, improve plasticity and toughness, and facilitate cutting.
Mechanism: Make sheet or network cementite into granular (spherical)
Explanation: When annealing and heating, the structure is not completely A, so it is also called incomplete annealing.
4. Stress relief annealing
Process: heating to a certain temperature below Ac1 (500-650 degrees) → heat preservation → slow cooling to room temperature.
Purpose: Eliminate the residual internal stress of castings, forgings, weldments, etc., and stabilize the size of the customized machining parts.
Steel tempering
Process: Reheat the quenched steel to a temperature below A1 and keep it warm, then cool (generally air-cooled) to room temperature.
Purpose: Eliminate internal stress caused by quenching, stabilize workpiece size, reduce brittleness, and improve cutting performance.
Mechanical properties: As the tempering temperature increases, the hardness and strength decrease, while the plasticity and toughness increase.
1. Low temperature tempering: 150-250℃, M times, reduce internal stress and brittleness, improve plastic toughness, have higher hardness and wear resistance. Used to make measuring tools, knives and rolling bearings, etc.
2. Tempering at medium temperature: 350-500°C, T time, with high elasticity, certain plasticity and hardness. Used to make springs, forging dies, etc.
3. High temperature tempering: 500-650℃, S time, with good comprehensive mechanical properties. Used to make gears, crankshafts, etc.
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Post time: May-15-2023