Heat treatment knowledge summary, quickly put it away!

Heat treatment knowledge summary, quickly put it away

Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but by changing the microstructure within the workpiece, or changing the chemical composition of the workpiece surface, To give or improve the performance of the workpiece. It is characterized by improving the intrinsic quality of the workpiece, which is generally not visible to the naked eye.

Heat treatment process generally includes heating, heat preservation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and uninterrupted. Metal heat treatment process can be divided into overall heat treatment, surface heat treatment, local heat treatment and chemical heat treatment.

Overall heat treatment

The overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to change its overall mechanical properties. The overall heat treatment of steel is generally annealing, normalizing, quenching and tempering four basic processes. Annealing, normalizing, quenching, and tempering are the "four fires" in the overall heat treatment. Among them, quenching and tempering are closely related and often used in conjunction with each other.

Annealing

Is to heat the workpiece to the appropriate temperature, according to the material and the size of the workpiece with different holding time, and then slow cooling, the purpose is to make the metal internal organization to reach or close to equilibrium state, to obtain good process performance and performance, or for further quenching for organization preparation.

normalizing fire

Is to heat the workpiece to a suitable temperature after cooling in the air, the effect of normalizing is similar to annealing, but the organization is finer, often used to improve the cutting performance of low carbon materials, and sometimes used for some less demanding parts as the final heat treatment.

Quenching

After the workpiece is heated and insulated, it is rapidly cooled in quenching medium such as water, oil or other inorganic salts, organic aqueous solution, etc. After quenching, the steel becomes hard, but at the same time becomes brittle.

Tempering

In order to reduce the brittleness of the steel, the quenched steel is kept at an appropriate temperature higher than room temperature and lower than 650 ℃ for a long time, and then cooled.

"Four fire" with the heating temperature and cooling mode of different, but also evolved a different heat treatment process. In order to obtain a certain strength and toughness, the process of combining quenching and high temperature tempering is called quenching and tempering. After some alloys are quenched to form a supersaturated solid solution, they are kept at room temperature or a slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloy. Such a heat treatment process is called aging treatment.

The method of effectively and closely combining pressure deformation and heat treatment to make the workpiece obtain good strength and toughness is called thermomechanical treatment. Heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only prevent oxidation and decarburization of the workpiece, keep the surface of the workpiece smooth and clean after treatment, improve the performance of the workpiece, but also carry out chemical heat treatment through infiltrating agent.

surface heat treatment

A metal heat treatment process in which only the surface of the workpiece is heated to change the mechanical properties of the surface. In order to heat only the surface of the workpiece and not too much heat into the workpiece, the heat source used must have a high energy density, that is, in the unit area of the workpiece to give a larger heat energy, so that the surface of the workpiece or local can be short or instantaneous reach high temperature. The main methods of surface heat treatment are flame quenching and induction heating heat treatment, commonly used heat source oxygen acetylene or oxygen propane flame, induction current, laser and electron beam.

chemical heat treatment

Metal heat treatment process by changing the chemical composition, organization and performance of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in a medium containing carbon, nitrogen or other alloying elements (gas, liquid, solid) for a long time, so that the surface of the workpiece infiltrates carbon, nitrogen, boron and chromium. After the elements are infiltrated, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment are carburizing, nitriding and metal seepage.

Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools. It can control various properties of the workpiece, such as wear resistance, corrosion resistance, magnetic properties, etc. It can also improve the organization and stress state of the blank to facilitate various cold and hot processing. For example, white cast iron can be annealed for a long time to obtain malleable cast iron and improve plasticity. With the correct heat treatment process, the service life of gears can be doubled or tens of times higher than that of gears without heat treatment. In addition, cheap carbon steel has some expensive alloy steel properties by infiltrating some alloy elements, which can replace some heat-resistant steel and stainless steel. Almost all dies need heat treatment before they can be used.

The difference between annealing and tempering

The difference between annealing and tempering is: (in short, annealing is not hardness, tempering still retains a certain hardness).

Tempering:

The structure obtained by high temperature tempering is tempered sothosome. Tempering is generally not used alone. Tempering is carried out after quenching of parts. The main purpose is to eliminate quenching stress and obtain the required organization. Tempering is divided into low temperature, medium temperature and high temperature tempering according to different tempering temperatures. 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 strength, hardness and plasticity, toughness are better comprehensive mechanical properties. Therefore, it is widely used in automobiles, tractors, machine tools and other important structural parts, such as connecting rods, bolts, gears and shafts. Hardness after tempering is generally HB200-330.

Annealing:

The main purpose of annealing is to make the internal organization of the metal reach or close to equilibrium state, in preparation for subsequent processing and final heat treatment. Stress relief annealing is an annealing process performed to eliminate residual stress caused by plastic deformation processing, welding, etc., and existing in the casting. There is internal stress in 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 the stress relief annealing is lower than the phase transition temperature, so that no structural transformation occurs during the entire heat treatment process. Internal stress is mainly through the workpiece in the insulation and slow cooling process of natural elimination. In order to eliminate the stress in the workpiece more thoroughly, the heating temperature should be controlled during heating.

Generally, it is fed into the furnace at a low temperature and then heated to the specified temperature at a heating rate of about 100 ℃/h. The heating temperature of welding parts shall be slightly higher than 600 ℃. The holding time depends on the situation, usually 2~4h. The upper limit of holding time for stress relief annealing of castings is taken, the cooling rate is controlled at (20~50)℃/h, and the castings can be discharged from the furnace for air cooling only when the temperature is below 300 ℃. Aging treatment can be divided into natural aging and artificial aging. Natural aging is to place the casting in the open field for more than half a year, then it occurs slowly, so that the residual stress is eliminated or reduced. Artificial aging is to heat the casting to 550~650 ℃ for stress relief annealing, which saves time compared with natural aging and removes residual stress thoroughly.

What is tempering?

Tempering is a heat treatment process that heats the quenched metal or parts to a certain temperature, after holding for a certain period of time, and cools in a certain way. Tempering is an operation immediately after quenching, and it is usually the last process of 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:

1) to reduce the internal stress and reduce brittleness, quenching parts there is a lot of stress and brittleness, such as not timely tempering often produce deformation and even cracking.

2) Adjust the mechanical properties of the workpiece. After the workpiece is quenched, the hardness is high and the brittleness is large. In order to meet the different performance requirements of various workpieces, it can be adjusted by tempering, hardness, strength, plasticity and toughness.

3) Stabilize the workpiece size. By tempering, the metallographic structure can be stabilized to ensure that no deformation occurs during subsequent use.

4) Improve the cutting performance of certain alloy steels.

In production, often according to the requirements of the workpiece performance. According to the different heating temperature, the 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 high strength and good plasticity toughness. It is mainly used to deal with 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 in which metal materials or parts are heated to above the phase change temperature, and then cooled sharply at a rate greater than the critical cooling rate to obtain martensite tissue. Quenching is to obtain the martensite structure, and then after tempering, the workpiece to obtain good performance, in order to give full play to the potential of the material. Its main purpose is:

1) Improve the mechanical properties of metal products or parts. For example: improve the hardness and wear resistance of tools, bearings, etc., 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, increasing the permanent magnetism of magnetic steel.

Quenching cooling, in addition to the need for reasonable selection of quenching medium, but also the correct quenching method, commonly used quenching methods, mainly single liquid quenching, double liquid quenching, graded quenching, isothermal quenching, local quenching and so on.

The difference and connection of "four fires"

The normalizing fire has the following purposes and uses:

① For sub-co-analysis steel, normalizing is used to eliminate the overheated coarse crystal tissue and Wei's tissue of casting, forging and welding parts, the band tissue in the rolling material, refine the grain, and can be used as a pre-heat treatment before quenching.

② For over-eutectoid steel, normalizing can eliminate the mesh secondary carburizing body and refine the pearlite, which not only improves the mechanical properties, but also facilitates the subsequent spheroidization annealing.

③For low-carbon deep-drawing thin steel plate, normalizing can eliminate the free cementite at the grain boundary to improve its deep-drawing performance.

The low carbon steel and low carbon low alloy steel, the use of normalizing, can get more fine flake pearlite organization, the hardness increased to HB140-190, to avoid cutting "stick" phenomenon, improve the machinability. For medium carbon steel, it is more economical and convenient to use normalizing when both normalizing and annealing are available.

⑤ For ordinary medium carbon structural steel, in the case of low mechanical properties, normalizing can be used instead of quenching and high temperature tempering, which is not only easy to operate, but also makes the structure and size of the steel stable.

The high temperature normalizing (Ac3 above 150~200 ℃) due to the high diffusion rate at high temperature, can reduce the composition of castings and forgings segregation. The coarse grains after high temperature normalizing can be refined by subsequent 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, and then tempered at high temperature to have good creep resistance at 400~550 ℃.

⑧ In addition to steel parts and steel, 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 have an impact on the organization and performance after normalizing. The normalizing structure can also be used as a classification method of alloy steel. Generally, alloy steels are divided into pearlite steel, bainitic steel, martensitic steel and austenitic steel according to the structure obtained by air cooling after a sample with a diameter of 25mm is heated to 900°C.

Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature, held for a sufficient time, and then cooled at a suitable 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 testing or hardness testing. Many steels are supplied in the state of annealing heat treatment. Rockwell hardness tester can be used to test HRB hardness. For thinner steel plates, steel strips and thin-walled steel pipes, surface Rockwell hardness tester can be used to detect HRT hardness.

The purpose of annealing is:

① improve or eliminate the steel in the casting, forging, rolling and welding process caused by a variety of organizational defects and residual stress, to prevent deformation and cracking of the workpiece.

② soften the workpiece for cutting.

The grain refinement, improve the organization to improve the mechanical properties of the workpiece.

④ Prepare for the final heat treatment (quenching and tempering).

Commonly used annealing processes are:

① Complete annealing. It is used to refine the coarse overheated structure of medium and low carbon steel after casting, forging and welding. The workpiece is heated to ferrite all transformed into austenite above the temperature of 30~50 ℃, holding for a period of time, and then slowly cooled with the furnace, in the cooling process of austenite transformation again, you can make the steel structure fine.

② Spheroidizing annealing. Used to reduce the tool steel and bearing steel after forging high hardness. The workpiece is heated to 20~40 ℃ above the temperature at which the steel begins to form austenite, and slowly cooled after holding heat. 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 cutting. Generally, it is cooled to the most unstable temperature of austenite at a faster rate, and the hardness can be reduced by holding the temperature for an appropriate time, and the austenite is transformed into a torus or sorbite.

④ Recrystallization annealing. To eliminate the hardening phenomenon of metal wire and sheet in the process of cold drawing and cold rolling (hardness increases and plasticity decreases). The heating temperature is generally 50~150 ℃ below the temperature at which the steel begins to form austenite. Only in this way can the metal soften by eliminating the work hardening effect.

⑤ graphitization annealing. It is used to turn cast iron containing a large amount of carburizing into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950 degrees C, insulation for a certain period of time after the appropriate cooling, so that the carburizing body decomposition to form a group of floc graphite.

⑥ diffusion annealing. 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 slowly cool after the diffusion of various elements in the alloy tends to be evenly distributed.

⑦ Stress relief annealing. To eliminate the internal stress of steel castings and weldments. For steel products after heating began to form austenite temperature below 100~200 ℃, heat preservation after cooling in the air, you can eliminate the internal stress.

Quenching, a heat treatment process for metals and glass. The alloy products or glass are heated to a certain temperature, and then rapidly cooled in water, oil or air, generally used to improve the hardness and strength of the alloy. Commonly known as "dipped in fire". The quenched workpiece is reheated to an appropriate temperature below the lower critical temperature, and the metal heat treatment is cooled in air or water, oil and other media after holding for a period of time. The steel workpiece has the following characteristics after quenching:

① Unbalanced (I. e., unstable) structures such as martensite, bainite, and residual austenite were obtained.

② There is a large internal stress.

The mechanical properties can not meet the requirements. Therefore, steel workpieces are generally tempered after quenching.

The role of tempering is:

① Improve the stability of the organization, so that the workpiece no longer occurs during the use of the organization, so that the geometry and performance of the workpiece remain stable.

Eliminate internal stress in order to improve the performance of the workpiece and stabilize the geometry of the workpiece.

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 atomic activity ability is enhanced, and the atoms of iron, carbon and other alloying elements in steel can be diffused quickly to realize the rearrangement and combination of atoms, so that the unstable unbalanced organization is gradually transformed into a stable balanced organization. The elimination of internal stress is also related to the decrease of metal strength when the temperature increases. 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 steel with high alloy element content will precipitate some fine metal compounds when tempering 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 Tools, bearings, carburizing and quenching parts, and surface quenching parts are usually tempered at a low temperature below 250°C. After low temperature tempering, the hardness changes little, the internal stress decreases, and the toughness increases slightly.

The spring in the 350~500 ℃ temperature tempering, can obtain high elasticity and the necessary toughness.

The parts made of medium carbon structural steel are usually tempered at 500~600 ℃ to obtain a good match of suitable strength and toughness.

The heat treatment process of quenching and high temperature tempering is generally called quenching and tempering.

When steel is tempered at about 300 ℃, its brittleness is often increased, which is called the first type of temper brittleness. Generally, tempering should not be carried out in this temperature range. Some medium carbon alloy structural steels are also easy 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 1. steel

Concept: The steel is heated and slowly cooled after heat preservation to obtain a process close to the equilibrium organization.

1. Complete annealing

Process: heating Ac3 above 30-50 ℃ → heat preservation → cooling to below 500 ℃ with furnace → air cooling to room temperature.

Objective: To refine the grain, uniform structure, improve plastic toughness, eliminate internal stress, and facilitate mechanical processing.

2. Isothermal annealing

Process: heating above Ac3 → heat preservation → fast cooling to pearlite transformation temperature → isothermal residence → transformation to P → air cooling out of the furnace;

Purpose: Ibid. But the time is short, easy to control, deoxidation, decarburization is small. (Applicable to undercooled A relatively stable alloy steel and large carbon steel parts).

3, spheroidizing annealing

Concept: It is the process of spheroidizing carburizing in steel.

Object: Co-analysis steel and over-co-analysis steel.

Process:

(1) Isothermal spheroidizing annealing heats 20-30 degrees above Ac1 → 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 heats 20-30 degrees above Ac1 → 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 mesh cementite into granular (spherical)

Note: When annealing is heated, the organization 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.

Objective: To eliminate the residual internal stress of castings, forgings, weldments, etc., and to stabilize the size of the workpiece.

Tempering of 2. steel

Process: The quenched steel is reheated to a certain temperature below A1, and then cooled (generally air cooled) to room temperature.

Objective: To eliminate the internal stress caused by quenching, stabilize the size of the workpiece, reduce brittleness and improve cutting performance.

Mechanical properties: with the increase of tempering temperature, hardness, strength decreased, plastic toughness increased.

1. Low temperature tempering: 150-250 ℃ ,M back, reduce internal stress and brittleness, improve plastic toughness, and have higher hardness and wear resistance. Used in the production of measuring tools, tools and rolling bearings.

2. Medium temperature tempering: 350-500 ℃ ,T back, with 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.