Why is metal tempering needed?
Tempering is a technological process consisting of heat treatment of an alloy or metal hardened to martensite, in which the main processes are the decomposition of martensite, as well as polygonization and recrystallization.
Tempering is carried out in order to obtain higher ductility and reduce the fragility of the material while maintaining an acceptable level of its strength. To do this, the product is heated in an oven to a temperature from 150–260 °C to 370–650 °C, followed by slow cooling.
Low temperature holiday [edit | edit code]
Carry out at temperatures up to 250 °C. Hardened steel retains high wear resistance, but such a product (if it does not have a viscous core) will not withstand high dynamic loads. Cutting and measuring instruments made of carbon and low-alloy steels are subjected to this tempering.
Medium temperature holiday [edit | edit code]
It is carried out at temperatures of 350–500 °C and is used mainly for springs and leaf springs, as well as for dies. Diffusion processes accelerate, excess carbon atoms are released in the form of cementite, that is, martensite decomposes into a ferrite-cementite mixture.
After average tempering, the structure consists of equilibrium ferrite and dispersed inclusions of cementite; this structure is called granular tempering troostite. Such a vacation provides high limits of elasticity and endurance, as well as relaxation resistance.
Cooling after tempering is carried out at temperatures of 400–500 °C in water, after which compressive residual stresses arise, which increase the endurance limit of the springs.
High temperature tempering [edit | edit code]
Carry out at temperatures of 500–680 °C. At the same time, high strength and ductility, as well as maximum viscosity, remain. Parts that absorb shock loads (gears, shafts) are subjected to high tempering.
Staging [edit | edit code]
General principle: First, the decomposition of martensite ends, and then polygonization and recrystallization begin. Depending on the ratio of stages there are:
Tempering is the final stage of heat treatment of steel. Performed after hardening. The quality and service life of the part depends on it.
The task is to heat the steel billet to a temperature below the critical level, after which the value is maintained for a certain period of time and slowly (or quickly, depending on the specifics of the technical process) tempered to the desired value.
The following actions are performed:
- Possible stress in the steel workpiece is reduced or completely eliminated.
- The viscosity of the metal increases to the value required by operating conditions.
- The hardness of the workpiece decreases, this is important for its processing.
The main processes during the operation are: decomposition of martensite, subsequent polygonization, recrystallization.
The product is heated in an oven from 150-250 to 370-650 ºC, the value is controlled smoothly, sudden changes in indicators are unacceptable.
Types of steel tempering
Each type is characterized by its purpose, operating conditions, and prescribed temperature conditions, which are worked out depending on the subsequent operating conditions of the workpiece and the need to give it certain characteristics.
- Short. Heating temperature - 150-300 ºC;
- Average. With heating temperature - 300-450 ºC;
- High. Temperature - 450-650 ºC.
Short
The procedure is carried out taking into account heating in the oven to 150-250 ºC. Next, a long exposure is carried out, taking into account the temperature value; the final stage is cooling the workpiece in the open air.
When a steel billet is seasoned, martensite takes the form of tempering within a specified temperature range. The previously formed stress in the structure will be relieved, and the residual austenite will transform into martensite of a similar shape. If the steps are carried out correctly, the strength of the part is achieved, and it can be easily processed to obtain the required shape and dimensions.
Upon completion of the operation, the metal remains hard, but in some cases, the indicator increases. The result is achieved due to the decomposition of retained austenite. In parallel with the preservation of hardness, the brittleness of hardening is localized.
This type of operation is used in the manufacture of various products and cutting tools, provided that high hardness of the structure is ensured. Thanks to the transformation of martensite, the dimensions of the workpiece are stabilized.
This is relevant provided that the parameters of the measuring instrument, in the manufacturing process of which tool steel is used, are observed. When making an instrument, this type of operation is performed.
Source: https://MyTooling.ru/instrumenty/dlja-chego-nuzhen-otpusk-metalla
Steel tempering
Tempering consists of heating hardened steel to temperatures below A c1, holding it at a given temperature and then cooling it at a certain rate. Tempering is the final heat treatment operation, as a result of which the steel obtains the required mechanical properties.
In addition, tempering completely or partially eliminates the internal stresses that arise during hardening. These stresses are relieved more completely the higher the tempering temperature. For example, axial stresses in a cylindrical sample made of steel containing 0.3% C are reduced from 60 to 8 kgf/mm2 as a result of tempering at 550 °C.
Tangential and radial stresses are also greatly reduced.
The stresses decrease most intensively as a result of holding at 550 °C for 15–30 minutes. After holding for 1.5 hours, the stresses are reduced to the minimum value that can be achieved by tempering at a given temperature.
The cooling rate after tempering also has a great influence on the magnitude of residual stresses. The slower the cooling, the lower the residual stresses. Rapid cooling from 600 °C creates new thermal stresses.
For this reason, products with complex shapes should be cooled slowly to avoid warping after tempering at high temperatures, and products made of alloy steels prone to reversible temper embrittlement should in all cases be cooled quickly after tempering at 500–650 °C.
The main influence on the properties of steel is the tempering temperature. There are three types of vacation.
Low temperature tempering of steel
Low-temperature (low) tempering is carried out with heating to 150–200 °C, less often to 240–250 °C.
At the same time, internal stresses are reduced, quenched martensite is converted into tempered martensite, strength increases and toughness improves slightly without a noticeable decrease in hardness.
Hardened steel (0.5–1.3% C) after low tempering retains a hardness within HRC 58–63, and therefore high wear resistance. However, such a product (if it does not have a viscous core) cannot withstand significant dynamic loads.
Therefore, cutting and measuring tools made of carbon and low-alloy steels, as well as parts that have undergone surface hardening, carburization, cyanidation or nitrocarburization, are subjected to low-temperature tempering. The duration of the tempering is usually 1–2.5 hours, and for products with large sections and measuring instruments, a longer tempering is prescribed.
Medium temperature tempering of steel
Medium temperature (medium) tempering is performed at 350–500 °C and is used mainly for springs and leaf springs, as well as for dies. Such tempering provides high elasticity limit, endurance limit and relaxation resistance. The structure of steel (0.45–0.8% C) after medium tempering is tempered troostite or troostomartensite with a hardness of HRC 40–50. The tempering temperature must be chosen in such a way as not to cause irreversible temper embrittlement.
Cooling after tempering at 400–450 °C should be carried out in water, which promotes the formation of compressive residual stresses on the surface, which increase the endurance limit of the springs.
High temperature tempering of steel
High temperature (high) tempering is carried out at 500–680 °C. The structure of steel after high tempering is sorbitol tempering. High tempering creates the best ratio of strength and toughness of steel.
Quenching with high tempering, compared with the normalized or annealed state, simultaneously increases the strength and yield limits, the relative contraction, and especially the impact strength (Table 1). Heat treatment, consisting of quenching and high tempering, is called improvement.
Medium-carbon (0.3–0.5% C) structural steels, which have high requirements for yield strength, endurance limit and impact strength, are subject to improvement. However, the wear resistance of improved steel due to its reduced hardness is not high.
Table 1 — The influence of heat treatment on the mechanical properties of carbon steel with 0.42% C* Heat treatmentσвσтδψAn,kgf m/cm2kgf/mm2%| Annealing at 880 °C | 55 | 35 | 20 | 59 | 9 |
| Quenching from 880 °C (cooling in water) and tempering at 300 °C | 130 | 110 | 12 | 35 | 3 |
| Quenching from 880 °C (cooling in water) and tempering at 600 °C | 62 | 43 | 22 | 55 | 14 |
| * Workpiece with a diameter of 12 mm. |
The improvement significantly increases the structural strength of steel, reducing sensitivity to stress concentrators, increasing the work of plastic deformation during crack movement (work of crack propagation) and reducing the temperature of the upper and lower thresholds of cold brittleness.
Tempering at 550–600 °C for 1–2 hours almost completely removes the residual stresses that arose during hardening. Most often, the duration of high tempering is 1–6 hours, depending on the overall dimensions of the product.
Source: http://weldworld.ru/theory/term-obrab/otpusk-stali.html
Steel tempering: types and characteristics, technology features and temper brittleness, heat treatment of alloys - Machine
Heat treatment of steel allows you to give products, parts and workpieces the required qualities and characteristics. Depending on the stage at which heat treatment was carried out in the manufacturing process, the workpieces’ workability increases, residual stresses are removed from the parts, and the parts’ performance qualities increase.
Steel heat treatment technology is a set of processes: heating, holding and cooling with the aim of changing the internal structure of the metal or alloy. In this case, the chemical composition does not change.
Thus, the molecular lattice of carbon steel at a temperature of no more than 910°C is a body-centered cube. When heated above 910°C to 1400°C, the lattice takes the shape of a face-centered cube. Further heating turns the cube into a body-centered one.
Heat treatment of steel
The essence of heat treatment of steels is a change in the grain size of the internal structure of the steel.
Strict adherence to temperature conditions, time and speed at all stages, which directly depend on the amount of carbon, alloying elements and impurities that reduce the quality of the material.
During heating, structural changes occur, which upon cooling occur in the reverse order. The figure shows what transformations occur during heat treatment.
Change in metal structure during heat treatment
Purpose of heat treatment
Heat treatment of steel is carried out at temperatures close to critical points. Here's what happens:
- secondary crystallization of the alloy;
- transition of gamma iron to the alpha iron state;
- transition of large particles into plates.
The internal structure of a two-phase mixture directly affects performance and ease of processing.
Formation of structures depending on cooling intensity
The main purpose of heat treatment is to give steels:
- In finished products:
- strength;
- wear resistance;
- corrosion resistance;
- heat resistance.
- In blanks:
- relief of internal stress after
- casting;
- stamping (hot, cold);
- deep drawing;
- increased plasticity;
- facilitating cutting.
- relief of internal stress after
Heat treatment is applied to the following types of steels:
- Carbon and alloyed.
- With varying carbon contents, from low carbon 0.25% to high carbon 0.7%.
- Structural, special, instrumental.
- Any quality.
Classification and types of heat treatment
The fundamental parameters affecting the quality of heat treatment are:
- heating time (speed);
- heating temperature;
- duration of holding at a given temperature;
- cooling time (intensity).
By changing these modes, you can obtain several types of heat treatment.
Types of heat treatment of steel:
- Annealing
- I – kind:
- homogenization;
- recrystallization;
- isothermal;
- removal of internal and residual stresses;
- II – kind:
- I – kind:
Heating temperature of steel during heat treatment
Vacation
Tempering in mechanical engineering is used to reduce the strength of internal stresses that appear during hardening. High hardness makes products brittle, so tempering is used to increase impact strength and reduce the hardness and brittleness of steel.
Low tempering is characterized by the internal structure of martensite, which, without reducing hardness, increases viscosity. Measuring and cutting tools are subjected to this heat treatment. Processing modes:
- Heating to a temperature of 150°C, but not higher than 250°C;
- holding time - one and a half hours;
- cooling - air, oil.
For medium tempering, transformation of martensite into trostite. Hardness decreases to 400 HB. Viscosity increases. Parts that operate under significant elastic loads are subjected to this tempering. Processing modes:
- heating to a temperature of 340°C, but not higher than 500°C;
- cooling - air.
3. High release
With high tempering, sorbitol crystallizes, which eliminates stress in the crystal lattice. Critical parts are manufactured that have strength, ductility, and toughness.
Annealing steel
Processing modes:
Heating to a temperature of 450°C, but not higher than 650°C.
Annealing
The use of annealing makes it possible to obtain a homogeneous internal structure without stress on the crystal lattice. The process is carried out in the following sequence:
- heating to a temperature slightly above the critical point, depending on the grade of steel;
- holding with constant temperature maintenance;
- slow cooling (usually cooling occurs together with the furnace).
1. Homogenization
Homogenization, otherwise known as diffusion annealing, restores the non-uniform segregation of castings. Processing modes:
- heating to a temperature from 1000°C, but not higher than 1150°C;
- exposure – 8-15 hours;
- cooling:
- oven – up to 8 hours, temperature reduction to 800°C;
- air.
Recrystallization, otherwise low annealing, is used after plastic deformation treatment, which causes hardening by changing the grain shape (hardening). Processing modes:
- heating to a temperature above the crystallization point by 100°C-200°C;
- holding – ½ – 2 hours;
- cooling is slow.
3. Isothermal annealing
Alloy steels are subjected to isothermal annealing to cause austenite decomposition. Heat treatment modes:
- heating to a temperature of 20°C - 30°C above the point;
- holding;
- cooling:
- fast - not lower than 630°C;
- slow – at positive temperatures.
4. Annealing to eliminate stress
Removal of internal and residual stresses by annealing is used after welding, casting, and machining. With the application of work loads, parts are subject to destruction. Processing modes:
- heating to a temperature of – 727°C;
- holding - up to 20 hours at a temperature of 600°C - 700°C;
- cooling is slow.
5. Complete annealing
Full annealing makes it possible to obtain an internal structure with fine grains, which contains ferrite and pearlite. Full annealing is used for cast, forged and stamped workpieces, which will subsequently be processed by cutting and subjected to hardening.
Complete annealing of steel
Processing modes:
- heating temperature – 30°C-50°C above point ;
- excerpt;
- cooling to 500°C:
- carbon steel – temperature decrease per hour is no more than 150°C;
- alloy steel – temperature decrease per hour is no more than 50°C.
6. Incomplete annealing
With incomplete annealing, lamellar or coarse pearlite is transformed into a ferrite-cementite grain structure, which is necessary for welds produced by electric arc welding, as well as tool steels and steel parts subjected to processing methods whose temperature does not provoke grain growth of the internal structure.
Processing modes:
- heating to a temperature above the point or, above 700°C by 40°C - 50°C;
- curing - about 20 hours;
- cooling is slow.
Hardening
Hardening of steels is used for:
- Promotions:
- hardness;
- strength;
- wear resistance;
- elastic limit;
- Reductions:
- plasticity;
- shear modulus;
- compression limit.
The essence of hardening is the fastest cooling of a thoroughly heated part in various environments. Heating is performed with and without polymorphic changes. Polymorphic changes are possible only in those steels that contain elements capable of transformation.
Steel hardening
Such an alloy is heated to a temperature at which the crystal lattice of the polymorphic element undergoes changes, due to which the solubility of alloying materials increases. As the temperature decreases, the lattice changes structure due to an excess of alloying element and takes on a needle-like structure.
The impossibility of polymorphic changes during heating is due to the limited solubility of one component in another at a rapid cooling rate. There is little time for diffusion. The result is a solution with an excess of undissolved component (metastable).
To increase the cooling rate of steel, the following media are used:
- water;
- water-based brine solutions;
- technical oil;
- inert gases.
Comparing the rate of cooling of steel products in air, cooling in water from 600°C occurs six times faster, and from 200°C in oil 28 times faster.
Dissolved salts increase the hardening ability. The disadvantage of using water is the appearance of cracks in places where martensite forms.
Industrial oil is used to harden alloy alloys, but it sticks to the surface.
Metals used in the manufacture of medical products should not have a film of oxides, so cooling occurs in an environment of rarefied air.
To completely get rid of austenite, which causes high brittleness in steel, products are subjected to additional cooling at temperatures from -40°C to -100°C in a special chamber. You can also use carbonic acid mixed with acetone. This processing increases the accuracy of parts, their hardness, and magnetic properties.
If parts do not require volumetric heat treatment, only the surface layer is heated using HDF (high-frequency current) installations. In this case, the depth of heat treatment ranges from 1 mm to 10 mm, and cooling occurs in air. As a result, the surface layer becomes wear-resistant, and the middle is viscous.
The hardening process involves heating and holding steel products at temperatures reaching about 900°C. At this temperature, steels with a carbon content of up to 0.7% have a martensite structure, which, during subsequent heat treatment, will transform into the required structure with the appearance of the desired qualities.
Normalization
Normalization produces a fine grain structure. For low-carbon steels this is a ferrite-pearlite structure, for alloyed steels it is a sorbitol-like structure. The resulting hardness does not exceed 300 HB. Hot-rolled steels are subjected to normalization. At the same time, they increase:
- fracture resistance;
- processing performance;
- strength;
- viscosity.
Steel normalization process
Processing modes:
- heating occurs to a temperature of 30°C-50°C above the point ;
- maintaining in a given temperature range;
- cooling - in the open air.
Benefits of Heat Treatment
Heat treatment of steel is a technological process that has become a mandatory step in obtaining sets of parts made of steel and alloys with specified qualities. This can be achieved by a wide variety of modes and methods of thermal exposure. Heat treatment is used not only for steels, but also for non-ferrous metals and alloys based on them.
Steels without heat treatment are used only for the construction of metal structures and the manufacture of non-critical parts, the service life of which is short. There are no additional requirements for them. Everyday operation, on the contrary, dictates stricter requirements, which is why the use of heat treatment is preferable.
In thermally untreated steels, abrasive wear is high and proportional to its own hardness, which depends on the composition of chemical elements. Thus, non-hardened die matrices are well combined when working with hardened punches.
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Steel tempering: types and purposes
Tempering is the final stage of heat treatment of steel. Performed after hardening. The quality and service life of the part depends on it.
The task is to heat the steel billet to a temperature below the critical level, after which the value is maintained for a certain period of time and slowly (or quickly, depending on the specifics of the technical process) tempered to the desired value.
The following actions are performed:
- Possible stress in the steel workpiece is reduced or completely eliminated.
- The viscosity of the metal increases to the value required by operating conditions.
- The hardness of the workpiece decreases, this is important for its processing.
The main processes during the operation are: decomposition of martensite, subsequent polygonization, recrystallization.
The product is heated in an oven from 150-250 to 370-650 ºC, the value is controlled smoothly, sudden changes in indicators are unacceptable.
Average
A temperature of 300-500 ºC is required. The hardness at the last stage rapidly decreases, but the viscosity value increases. It is possible to obtain tempered troostite, the hardness of the metal increases to 43 HRC.
It is used in the production of springs, leaf springs, special technological tools, which are characterized by high strength and elasticity.
In this case, the hardness is set at an average level, this will allow the workpiece to be processed and given the desired characteristics.
High
It is carried out taking into account the temperature regime of 500-600 ºC. The main purpose is to obtain maximum toughness with an optimal combination of strength and elasticity of the steel structure. In practice, this is used in the manufacturing process of parts made from structural grades. While performing work, they are exposed to high voltage. This is relevant when the metal structure is exposed to shock loads during casting.
During the manufacture of parts designed for the use of various types of mechanisms and machine tools, it is customary to use heat treatment. The essence is to harden the workpiece with further high tempering. It is carried out taking into account the preservation of temperature, which ensures the production of sorbitol, excellent ductility and strength of the metal. The processing process is called “improving the characteristics of the metal.”
Heating in the metal may also be provided. It is performed exclusively in furnaces used in production when carrying out other methods of processing the workpiece. It will be necessary to ensure uniform temperature throughout the entire stage, while simultaneously accurately monitoring the condition of the metal.
Tempering brittleness
In parallel with the increase in the tempering temperature, the impact strength increases; cooling does not affect the characteristics. For certain steel grades, a decrease in this indicator is typical; the defect is called “temper brittleness.”
There are two types of phenomena, each of which is distinguished by the specifics of its formation and subsequent result. Pay attention to the features of each of them; the development of the technological process for creating the workpiece depends on this.
Type 1 temper brittleness
Occurs when the temperature range passes 300 ºC. This is not related to the cooling parameters of the workpiece at the final stage of processing. This manifestation is caused by the difference in the levels of martensite transformation in the workpiece being created. The measured value of fragility is irreversible; even if this element is heated again, it will not appear, therefore, the structure remains in a stable state.
Tempering brittleness 2nd kind
The phenomenon manifests itself in the structure of alloy steels when they are slowly cooled. The temperature is set to 450-650 ºC. When a high tempering takes place during the casting of a workpiece, the separation of dispersed inclusions of carbides is noted along the boundaries of the metal. Upon examination, the border zone is united due to the presence of alloying components.
When smooth cooling occurs, diffusion is formed; it manifests itself more sharply towards the grain boundaries. Parts of the structure in the border region are enriched in phosphorus. This manifestation will reduce the level of toughness as well as strength.
It is noted as a reversible process; with secondary heating, smooth cooling to the desired value, if an interval dangerous for the indicators is set, the defect has every chance of reoccurring.
Steels that tend to develop this type of brittleness in their structure cannot be heated to 650 ºC.
A decision is made to conduct a vacation of one type or another, depending on the characteristics of the workpiece, performance indicators, as well as the needs of the production process. It is important to maintain the temperature and subsequently carry out natural cooling of the workpiece, which will allow you to achieve an impressive result. There is nothing complicated in the process if you work out a map of technological operations in advance.
Source: https://prompriem.ru/metalloobrabotka/otpusk-stali.html
High metal tempering
High tempering of steel is a heat treatment method in which the metal is heated to a temperature not higher than the lower threshold of the transformation range Ac1, maintained at it and cooled slowly or quickly.
The cooling rate depends on the complexity of the shapes of the parts; in the case of alloy steels, the factor of temper brittleness is important. Tempering is the final stage of heat treatment of metals. The final quality of the finished part largely depends on the correctness of its implementation.
When high tempering is carried out after hardening, the term “steel improvement” is applicable.
Purpose of high metal tempering
The main goal of processing metal using the high tempering method is to give it maximum toughness while maintaining sufficient elasticity and tensile strength of the metal. In the process of high tempering, steel acquires the most advantageous combination of mechanical properties with toughness and ductility. Also, under the condition of slow cooling, the internal stresses of the metal that arise after hardening are almost completely eliminated. Used to relieve stress after straightening.
Heat treatment in the form of high tempering is used for parts made of medium-carbon and structural steels - they are subject to increased requirements regarding the limits of impact strength, endurance and fluidity. The structural strength of steel increases, the upper and lower thresholds of cold brittleness decrease, and the possibility of crack development is minimized.
The steel improvement technique is usually used for parts of various machine tools and machines.
High holiday mode
The temperature regime under which complete tempering is possible is above 500 ° C and below point Ac1 - the lower limit of the transformation range. Thus, the temperature range is in the range of 500 - 600 ° C, for alloy steels - up to 700 ° C. The holding time after reaching the required temperatures is 0.5 - 1 hour.
Air and oil media can be used as cooling media. As for the cooling rate, parts of complex shapes are cooled slowly to avoid warping; products made of alloy steels are cooled quickly to avoid brittleness.
The cooling rate also significantly affects the removal of internal stresses in the metal - the slower the cooling process, the less internal stress remains.
High tempering of steel at our plant is carried out in a modern chamber tempering electric furnace, equipped with a forced atmosphere circulation system inside the furnace. Among the advantages of our equipment:
- temperature control
- convection
- high precision adjusting devices
- uniform temperature distribution at different points of the furnace
- graphs-modes of heat treatment of products
The process of heat treatment of metal using such equipment takes place under conditions of strict adherence to technological requirements. Products that have been highly tempered on our equipment are distinguished by high quality metal and improved characteristics.
Structural changes as a result of high steel tempering
During high annealing, a process of recrystallization occurs (bringing the substance to a state of greater thermodynamic stability) in combination with spheroidization of cementite. Cementite particles acquire a round shape ranging in size from 0.5 to 2 microns.
, the structure of tempered sorbitol with a granular form is acquired. Tempering sorbitol gives steel increased toughness. Alloy steels acquire a granular pearlite structure. Structural stability is ensured and internal stress is relieved.
Technological processes at our plant are carried out in modern computerized equipment under the control of qualified personnel. This helps to achieve the highest performance in the field of chemical-thermal processing of metals. We practice an individual approach to each client and each order.
You can order a service for high metal tempering using the feedback form or by calling our website.
Source: http://pzto.pro/services/visokiy_otpusk.html
Tempering of steels
Tempering is a heat treatment process consisting of heating hardened steel to temperatures below the Ac1 point in order to obtain an equilibrium structure and a given set of mechanical properties.
After hardening, the steel has a structure based on martensite with a tetragonal distorted crystal lattice and retained austenite, the amount of which depends on the chemical composition of the steel. When hardened steel is heated, phase transformations occur in its structure, which can be shown in the form of a diagram.
Scheme of phase transformations during steel tempering
Low steel tempering
Low tempering of steel is done at temperatures up to 250°C. During this process, some of the excess carbon is released from martensite to form tiny carbide particles (ε-carbides). ε-carbides precipitate in the form of plates or rods and they are coherently bonded to the martensite lattice.
The decomposition of retained austenite during low tempering occurs according to the mechanism of bainite transformation: a heterogeneous mixture of low-carbon martensite crystals and dispersed carbides is formed.
The product of low tempering is tempered martensite, which differs from quenched martensite by its lower carbon concentration and the presence of carbides (ε-carbides) in it, which are coherently bonded to the martensite lattice.
At a temperature of about 250°C, the transformation of carbide into cementite begins; in this case, the coherence of the lattices of the α-solid solution of martensite and carbides is disrupted.
Iron-carbon tool materials (cutting and measuring tools), as well as steels that have been carburized or nitrocarburized, are subjected to low tempering. Low tempering is often done for steels after heat treatment with high frequency currents.
Average holiday
Average tempering is carried out at temperatures of 350–400 °C. In this case, all excess carbon is released from martensite with the formation of cementite particles. The tetragonality (degree of tetragonality) of the iron lattice decreases, it becomes cubic.
As a result, ferrite remains instead of martensite. Such a ferrite-cementite mixture is called tempering troostite, and the process leading to such changes is called medium-temperature tempering.
With medium tempering, the dislocation density decreases and the internal stresses in the steel decrease.
Medium tempering is used for heat treatment of elastic parts: leaf springs, springs, etc.
High holiday
During high tempering (450-550°C and above), structural changes occur in carbon steels that are not associated with phase transformations: the shape, size of carbides and ferrite structure change. With increasing temperature, coagulation occurs - the enlargement of cementite particles. The shape of the crystals gradually becomes spherical - this process is called spheroidization.
Coagulation and spheroidization of carbides begin to occur more intensely at a temperature of 400°C. The ferrite grains become large and their shape approaches equiaxial. The ferrite-carbide mixture, which is formed after tempering at a temperature of 400–600 ° C, is called tempering sorbitol. At a temperature close to point A1, a fairly coarse ferrite-cementite mixture is formed - pearlite.
https://www.youtube.com/watch?v=0vueOUKzTe4
High tempering at temperatures of 450-550°C is used for most structural steels. It is widely used in the heat treatment of various bushings, supports, fasteners operating in tension-compression and other products that experience static loads.
Temper brittleness phenomenon
When tempering some steels, processes may occur that reduce the impact strength of the steel without changing other mechanical properties. This phenomenon is called temper brittleness and is observed in the tempering temperature ranges at 250–400ºС and 500–550ºС.
The first type of brittleness is called type 1 temper brittleness and is irreversible, so tempering steels at these temperatures should be avoided. This type is inherent in almost all steels alloyed with chromium, magnesium, nickel and their combination, and is due to the heterogeneous precipitation of carbides from martensite.
The second type of temper brittleness, temper brittleness of the ΙΙth kind, is reversible. Temper brittleness of the ΙΙ-th kind manifests itself when alloy steel is slowly cooled at a temperature of 500–550°C. This brittleness can be eliminated by repeated tempering at a high cooling rate (in water or oil).
In this case, the cause of this brittleness is eliminated—the precipitation of carbides, nitrides, and phosphides along the boundaries of former austenite grains. It is possible to eliminate the temper brittleness of alloy steels by introducing small additions of molybdenum (0.2–0.3%) or tungsten (0.5–0.7%) into them.
Graphically, these types of fragility look as shown in the figure.
Manifestation of temper brittleness in steels during tempering
Almost all steels obey the law: an increase in tempering temperature leads to a decrease in strength characteristics and an increase in plasticity, as shown in the figure below.
Effect of tempering temperature on the mechanical properties of steel
This pattern does not apply to high-speed tool steels alloyed with carbide-forming elements.
Tempering of high-speed tool steels
The main alloying elements of high-speed steels (P18, P6M5, etc.) are tungsten, molybdenum, cobalt and vanadium - elements that provide heat resistance and wear resistance during operation. High-speed steels belong to the carbide (ledeburite) class. For hardening, these steels are heated to a temperature above 1200°C (P18 to a temperature of 1270°C, P6M5 to a temperature of 1220°C).
High quenching temperatures are necessary for more complete dissolution of secondary carbides and obtaining austenite highly alloyed with chromium, molybdenum, tungsten, and vanadium. This ensures that heat-resistant martensite is obtained after quenching. Even at very high heat, only part of the carbides dissolves.
These steels are characterized by the preservation of fine grains at high heating temperatures.
Iron and “quick-cut” alloying elements have very different thermal conductivity properties, therefore, when heating, temperature stops should be made to avoid cracks. Typically at 800 and 1050°C. When heating a large instrument, the first exposure is made at 600°C. The holding time is 5-20 minutes.
Holding at the quenching temperature should ensure the dissolution of carbides within the limit of their possible solubility. Cooling of the instrument is most often done in oil. To reduce deformation, stepwise hardening is used in molten salts at a temperature of 400-500°C.
The structure of “quick cuts” after quenching consists of highly alloyed martensite containing 0.3-0.4% C, undissolved excess carbides and retained austenite. The higher the quenching temperature, the lower the position of points Mn, Mk and the more retained austenite.
In R18 steel there is approximately 25-30% retained austenite, in R6M5 steel - 28-34%. To reduce austenite, cold treatment can be done, but as a rule this is not required.
After quenching, tempering follows at 550 - 570°C, causing the transformation of retained austenite into martensite and dispersion hardening due to the partial decomposition of martensite and the release of dispersed carbides of alloying elements. This is accompanied by an increase in hardness (secondary hardness).
During the holding process during tempering, carbides are released from the retained austenite, which reduces its alloying, and therefore, upon subsequent cooling, it undergoes a martensitic transformation (Mn ~ 150°C). During a single tempering process, only part of the retained austenite is transformed into martensite. In order for all the austenite to transform into martensite, two and three times tempering is used.
The holding time is usually 60 minutes.
When assigning a regime, it is necessary to take into account the chemical properties of the elements and the frequency of carbide release depending on temperature. For example, the maximum hardness of R6M5 steel is obtained through 3-stage tempering. The first tempering is at a temperature of 350°C, the next two at a temperature of 560-570°C.
At a temperature of 350°C, cementite particles are released, evenly distributed in the steel. This contributes to the uniform release and distribution of special M6C carbides at a temperature of 560-570°C.
Source: https://HeatTreatment.ru/otpusk-stalej
What steel tempering technologies exist?
When metals are hardened, internal stress is generated. If it is not eliminated, the finished product will have a high fragility rating. Plasticity will be significantly below normal. To eliminate these problems, steel tempering is used. It is one of several heat treatment processes for metals.
What is a vacation?
Metal tempering is a thermal process that is used for all hardened parts. Many novice craftsmen do not understand how important the set of heat treatment stages is for a material. Heat treatment of metals improves the characteristics of a metal part. During such processing, the structure of the steel changes. Because of this, individual properties of the material deteriorate or improve.
This heat treatment allows you to relieve the internal stress that forms after hardening the steel. If this is not done, the material will be fragile and will not withstand serious loads. In addition to relieving internal stress, this process increases the hardness of the steel. This is an important feature in the manufacture of tools and parts for industrial equipment.
The temperature regime is selected depending on what grade of material will be processed. Based on this, the metal can be cooled in different solutions:
- in containers filled with molten alkali;
- in baths filled with saltpeter;
- in containers with oil or water.
In production, metal parts are cooled in ovens.
In this case, a forced ventilation system is installed on the equipment. REMOVAL OF STEEL THE SIMPLE METHOD
Kinds
The steel tempering temperature is considered the most important parameter when carrying out this technological process. There are three types of tempering heat treatment. Features of the technological process depend on the type of heat treatment.
Heat treatment of tool alloys
Tool alloys or high-speed metals used for the manufacture of wear-resistant tools must be heat treated. As temperatures increase, their ductility does not increase and strength does not decrease.
To improve the characteristics of tool alloys, alloying additives are added to their composition - tungsten, molybdenum, vanadium or cobalt. Next, the workpieces are hardened at a temperature of 1200 degrees.
Tempering is considered one of the key stages of heat treatment. It allows you to relieve internal stress and increase the strength of the metal. It is important to choose the correct temperature regime and cooling rate of the workpiece. For cooling, containers with various solutions are used.
Source: https://metalloy.ru/obrabotka/termo/otpusk-stal
Metal release
Tempering is a heat treatment operation consisting of heating hardened steel to a temperature below the critical point AC1, holding at this temperature, and then cooling.
Depending on the heating temperature, two types of tempering are distinguished:
Low Vacation
Low tempering is characterized by heating in the range of 120-200°, holding and subsequent cooling in air. This type of tempering is used for tools and precision parts made from tool steel, for which high hardness and dimensional stability are important.
The cutting tool is subjected to low tempering at temperatures of 160-200°.
As a result of tempering, the steel retains high hardness, and sometimes increases it due to the decomposition of retained austenite.
Measuring tools and precision parts are subjected to low tempering at temperatures of 120-160°. After such a tempering (sometimes called artificial aging ), the dimensions of the product do not change.
After low tempering, steel retains high strength properties, but acquires low plastic properties.
Steel tempering: types and characteristics, technology features and temper brittleness, heat treatment of alloys
Metal tempering is the technological process of heat treatment of a hardened steel alloy. It makes it possible to complete phase transformations in the microstructure (martensite), which acquires the most stable state.
The fact is that during the hardening process, internal stresses arise in the metal - axial, radial, tangential.
To eliminate their negative consequences such as fragility and low ductility, products are heated in ovens at different temperatures (from 250 °C to 650 °C), kept for a specified time (from 15 minutes to 1.5 hours), and then slowly cooled.
The complex of these measures leads to the release of excess carbon, restructuring and ordering of the metal structure, and the elimination of defects in its crystalline structure. The processed materials acquire a given set of mechanical properties, among which the main ones are an increase in ductility and a decrease in fragility while maintaining a sufficient level of strength.
Products – Tekhmashholding – group of companies, official website
- To understand what the title is talking about, we will have to at least superficially study what hardness and strength are, as well as hardening of steel.
What is the difference between hardness and strength, how are they related and why is it difficult to get both at once?
What do we want from a finished steel product (in our case, a knife)? Of course, strength and hardness. But in everyday life, we hardly think about the fact that there is a serious difference between these concepts in physics. Let's figure it out. Imagine a sheet of cast iron. It is a very hard, but brittle material: this means that it can withstand enormous compressive pressure, but is completely unsuitable for impact or bending loads. In other words, with high hardness it lacks strength. On the contrary, tank armor plate has excellent strength to withstand bullets and shells. But you can’t make a knife out of it: the tough armor won’t hold an edge. How does this apply to steel? Simple: when hardening (a procedure aimed at increasing hardness), steel inevitably loses strength. Before hardening, a tempered, strong and tough steel plate cannot be broken or pierced, but can be scratched or bent; after hardening, it can scratch other steels, but now it breaks almost with your hands. What should I do? It was to solve this issue that steel tempering was invented.
What is steel tempering and why is it needed?
Vacation is not exactly re-hardening, although in some ways these procedures are similar. Steel for hardening is heated to the optimal temperature for it, and not to which the forge is, in principle, capable of “accelerating”, and then cooled - in oil, water or other media. But they release it after cooling. To do this, it is reheated at a much lower temperature, after which it is cooled again in a quenching medium. How are the quenching and tempering temperatures determined? For ordinary grades of steel, the temperatures are usually already known to the master. Hardening furnaces are equipped with special temperature sensors that allow you to adjust the required temperature. If you work in a forge without sensors, then you can focus on visual signs: roughly speaking, when the workpiece reaches a white color, the steel is already overheated and is no good, but a light yellow color indicates that it has reached the very extreme temperature that we need for hardening . The color of the steel changes, and an experienced craftsman can determine the required heating temperature “by eye.” With tempering, the situation is similar - the so-called “tarnish color” characterizes the required tempering temperature, which is selected according to the purpose of the product. Low-temperature tempering will preserve hardness at the expense of ductility, and high-temperature tempering will do the opposite. As for knives, here we most often talk about low-temperature tempering, up to 250 ° C, and less often about medium-temperature tempering. The higher the tempering temperature, the more resistant the steel will be to impact loads, and this is usually necessary for parts of moving mechanisms. For the steel from which the knife is made, the balance between hardness and strength is more important: the blade must be moderately wear-resistant* and at the same time be resistant to impact loads. Most often, the color of the heat, as professionals call it, will vary from cherry to yellow, but , we repeat, you cannot focus only on color: there is simply no universal tempering temperature for different steels and for products for different purposes.* Why don’t we set ourselves the task of making sure that the knife steel does not wear out at all? This is a throwback to the Stone Age: flint tools were very hard and therefore extremely fragile and inconvenient to work with. In addition, the impossibility of abrading steel would lead to the impossibility of sharpening it, because both dulling and sharpening are based on the same process - mechanical wear of the alloy.
When an unwanted vacation occurs and how to avoid it
ForgingFirst of all, of course, we are talking about thermomechanical processing of steel. After hardening, steel is especially vulnerable - it is very hard, but can be broken with a snap of your fingers, since it has lost strength during the hardening process. To make the workpiece material functional, tempering is required. It is important here not to overdo it. It was found experimentally that repeated hardening (that is, bringing it to extreme temperatures) will not lead to anything good and should not be done; but heating to lower temperatures is what is needed. But if the master simply missed the right moment, high-temperature tempering occurs, in other words, that same unwanted re-hardening, during which the steel again loses its strength. However, too low a tempering of the workpiece is also not indicated: this can negatively affect the properties of the finished blade, which as a result will turn out to be fragile. Sharpening The second point, which is much more dangerous, is metalworking operations and sharpening on grinding wheels. Forging is still done by professionals, and there the risk of overheating the workpiece is, in principle, minimal. But finishing operations (including sharpening, which is most often done by the owner and only occasionally by a professional) pose many dangers to the knife, including distortion of the geometry of the blade and much more. But now we are interested in spontaneous tempering. On a grinding wheel, due to the high rotation speed, the contact zone between the wheel and the workpiece quickly reaches critically high temperatures. Since steel is thermally conductive, the knife blade also overheats in the contact zone. It doesn’t just overheat, it actually gets hot – just try to touch the tool that you just took away from the grinding wheel. At the point of contact, the color of the steel changes to brown. Can you guess what's happening? The temperature there is much higher than normal for a vacation; it easily crosses the mark that is safe for hardening the blade. Undesirable additional tempering occurs, which, as we remember, leads to a loss of hardness and elasticity of the steel. When sharpening a knife, the temperature increases, and it is impossible to control it without experience and skills. If it exceeds the permissible norm, spontaneous tempering occurs and the cutting edge loses its properties. Therefore, both when sharpening and when performing locksmith operations, it is so important to regularly cool the blade, placing it, for example, in a container of water. What to do? Some experts recommend cooling the blade, and sometimes the sharpening tool itself, with regular running water. But this solves only part of the problem, since cooling reduces the speed of work and is used only when sharpening. Therefore, the second thing we do to perform locksmith operations to remove triggers is to reduce the rotation speed of the wheel to 60 rpm, and this is the maximum. This sharpening method is still not very suitable for good knives. On a regular grinding wheel you can sharpen scythes, axes, inexpensive utility and kitchen knives, but not more or less worthwhile ones. Many other sharpening methods have been invented for them, including equally fast mechanized ones - for example, sharpening machines with water stones. How are they different from ordinary ones? Firstly, they have a different material for the abrasive tool, which is more gentle on the surface of the blade and does not remove too much steel. Secondly, the design has a special reservoir that not only supplies water in a way that is safe for electrical parts, but also wets the surface of the disk, as a result of which a suspension is formed on it, which additionally polishes the knife. That is, with such a machine, not only accurate sharpening is done, but also the finishing of the cutting edge (if you choose a wheel of the correct grain size). And, of course, the rotation speed here is much lower, so the abrasive tool is more gentle on the knife. Such machines are specially designed for sharpening knives: thanks to the use of special water circles that produce a suspension, water cooling and low rotation speed, they do not harm the knife during the sharpening process . The second and third photos are a grinding wheel with a #1000 grid and a finishing wheel with a #6000 grid***So, what did we find out? For raw steel, the correct temperature regime of hardening imparts hardness, but takes away strength; Strength can be restored using the correct tempering mode - low-temperature heating followed by cooling. If sharpening is not done carefully on the grinding wheel, the temperature conditions may be exceeded, resulting in spontaneous tempering and the knife blade simply losing its working properties. Special machines for sharpening knives help to avoid this situation.www.tojiro.ru
how to release steel
2 years ago I’ll show you how easy it is to temper steel and harden it without any special conditions. 5 years ago TEMPERING STEEL AFTER HARDENING JUST DO IT AT HOME THE HOLIDAYS ARE A SIMPLE WAY My affiliate program I recommend! 5 years ago Annealing a quick cutter in a regular fire. The purpose of annealing is to reduce hardness to increase machinability, improve 1 year ago
Source: https://pellete.ru/stal/kak-otpustit-stal.html
Vacation became types and purpose of vacation
Metal tempering is the technological process of heat treatment of a hardened steel alloy. It makes it possible to complete phase transformations in the microstructure (martensite), which acquires the most stable state.
The fact is that during the hardening process, internal stresses arise in the metal - axial, radial, tangential.
To eliminate their negative consequences such as fragility and low ductility, products are heated in ovens at different temperatures (from 250 °C to 650 °C), kept for a specified time (from 15 minutes to 1.5 hours), and then slowly cooled.
The complex of these measures leads to the release of excess carbon, restructuring and ordering of the metal structure, and the elimination of defects in its crystalline structure. The processed materials acquire a given set of mechanical properties, among which the main ones are an increase in ductility and a decrease in fragility while maintaining a sufficient level of strength.
