Basic knowledge of heat treatment of metal materials
Basic knowledge of heat treatment of metal materials
Summary of the basic knowledge of heat treatment of steel pipes, steel and metal materials!
Heat treatment refers to a metal hot working process in which the material is in a solid state, through heating, heat preservation and cooling to obtain the desired structure and performance.
ONE.Heat treatment
1. Normalizing: heating the steel or steel parts to an appropriate temperature above the critical point AC3 or ACM for a certain period of time and cooling in air to obtain a heat treatment process of pearlite structure.
2. Annealing: heat the sub-eutectoid steel workpiece to 20-40 degrees above AC3, after a period of heat preservation, slowly cool with the furnace (or buried in sand or lime) to below 500 degrees in air cooling heat treatment process .
3. Solid solution heat treatment: The alloy is heated to a high temperature single-phase zone to maintain a constant temperature, so that the excess phase is fully dissolved into the solid solution, and then quickly cooled to obtain a supersaturated solid solution heat treatment process.
4. Aging: After the alloy undergoes solution heat treatment or cold plastic deformation, its performance changes with time when it is placed at room temperature or maintained slightly above room temperature.
5. Solid solution treatment: fully dissolve various phases in the alloy, strengthen solid solution and improve toughness and corrosion resistance, eliminate stress and softening, so as to continue processing and forming.
6. Aging treatment: heating and heat preservation at the temperature at which the reinforced phase is precipitated, so that the reinforced phase is precipitated and precipitated to be hardened and increase the strength.
7. Quenching: A heat treatment process that austenitizes the steel and cools it at an appropriate cooling rate, so that the workpiece undergoes a martensite and other unstable structural transformation in the entire cross-section or within a certain range.
8. Tempering: Heat the quenched workpiece to an appropriate temperature below the critical point AC1 for a certain period of time, and then cool it with a method that meets the requirements to obtain the required structure and performance of the heat treatment process.
9. Carbonitriding of steel: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the surface layer of steel. Traditionally, carbonitriding is also known as cyanidation. Medium temperature gas carbonitriding and low temperature gas carbonitriding (ie gas nitrocarburizing) are widely used. The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low-temperature gas carbonitriding is mainly nitriding, and its main purpose is to improve the wear resistance and seizure resistance of steel.
10. Quenching and tempering: It is generally used to combine quenching and high temperature tempering as heat treatment. Tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. After quenching and tempering, the tempered sorbite structure is obtained, and its mechanical properties are better than that of the normal hardness sorbite structure of the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of the steel and the cross-sectional size of the workpiece, generally between HB200-350.
11. Brazing: A heat treatment process in which two workpieces are heated, melted and bonded together with brazing material.
TWO. Process characteristics
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing techniques, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but by changing the microstructure inside the workpiece or changing the chemical composition of the surface , Give or improve the performance of the workpiece. Its characteristic is to improve the inherent quality of the workpiece, which is generally not visible to the naked eye. In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment processes are often essential. Steel is the most widely used material in the machinery industry. The microstructure of steel is complex and can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance.
Three. Process
The heat treatment process generally includes three processes of heating, heat preservation, and cooling, and sometimes there are only two processes of heating and cooling. These processes are connected to each other without interruption.
Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment, the earliest is the use of charcoal and coal as the heat source, and recently the use of liquid and gas fuel. The application of electricity makes heating easy to control without environmental pollution. These heat sources can be used for direct heating, or can be indirectly heated by molten salts or metals, or even floating particles.
When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of the steel part is reduced), which has a very adverse effect on the surface performance of the part after heat treatment. Therefore, the metal should usually be heated in a controlled or protective atmosphere, in a molten salt and in a vacuum, and can also be protected and heated by coating or packaging methods.
The heating temperature is one of the important process parameters of the heat treatment process. Selecting and controlling the heating temperature is the main problem to ensure the quality of the heat treatment. The heating temperature varies with the metal material being processed and the purpose of the heat treatment, but it is generally heated above the phase transition temperature to obtain a high-temperature structure. In addition, the transformation requires a certain time. Therefore, when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain period of time to make the internal and external temperatures consistent and complete the transformation of the microstructure. This period of time is called the holding time. When using high-energy density heating and surface heat treatment, the heating speed is extremely fast, generally there is no holding time, and the holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the heat treatment process. The cooling method differs depending on the process, mainly to control the cooling rate. Generally, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster. However, there are different requirements due to different steel types. For example, air-hardened steel can be hardened with the same cooling rate as normalizing.
Four, process classification
The metal heat treatment process can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to different heating media, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal adopts different heat treatment processes to obtain different structures and thus have different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes.
The overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools at an appropriate rate to obtain the required metallographic structure to change its overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.
Process means:
Annealing is to heat the workpiece to an appropriate temperature, use different holding times according to the material and the size of the workpiece, and then slowly cool down, the purpose is to make the internal structure of the metal reach or near the equilibrium state, to obtain good process performance and performance, or for further quench Prepare for organization.
Normalizing is to heat the workpiece to an appropriate temperature and cool it in the air. The effect of normalizing is similar to annealing, except that the resulting structure is finer. It is often used to improve the cutting performance of the material, and sometimes used for some parts that are not demanding. As the final heat treatment.
Quenching is to quickly cool the workpiece in the quenching medium such as water, oil or other inorganic salts, organic aqueous solution after heating and holding the workpiece. After quenching, the steel parts become hard, but at the same time they become brittle. In order to eliminate brittleness in time, it is generally necessary to temper in time.
In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature and lower than 650 ℃ for a long time, and then cooled. This process is called tempering. Annealing, normalizing, quenching, and tempering are the "four fires" in the overall heat treatment. Quenching and tempering are closely related, and they are often used together and are indispensable. "Four fires" evolved different heat treatment processes with different heating temperatures and cooling methods. 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 supersaturated solid solutions, they are kept at room temperature or a slightly higher appropriate temperature for a longer period of time to improve the hardness, strength or electrical magnetic properties of the alloy. This heat treatment process is called aging treatment.
The method of combining pressure processing deformation and heat treatment effectively and tightly to obtain a good strength and toughness of the workpiece is called deformation heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which not only can make The workpiece does not oxidize or decarburize, keep the surface of the workpiece clean after treatment, improve the performance of the workpiece, and can also be penetrated with a chemical heat treatment.
Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change the mechanical properties of the surface. In order to only heat the surface of the workpiece without excessive heat being transferred into the workpiece, the heat source used must have a high energy density, that is, a large thermal energy is given to the unit area of the workpiece, so that the surface or part of the workpiece can be short-term or instantaneous Reach high temperature. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. Commonly used heat sources are flames such as oxyacetylene or oxypropane, induction current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process that changes the chemical composition, structure and properties of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the former changes the chemical composition of the workpiece surface. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, salt medium or other alloy elements, and keep it warm for a long time, so that the surface of the workpiece is infiltrated with elements such as carbon, nitrogen, boron and chromium. After infiltrating the elements, sometimes other heat treatment processes such as quenching and tempering are performed. The main methods of chemical heat treatment are carburizing, nitriding and metalizing.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools. In general, it can guarantee and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing.
For example: white cast iron can obtain malleable cast iron through long-term annealing treatment to improve plasticity; gears adopt the correct heat treatment process, and the service life can be doubled or tens of times longer than gears without heat treatment; in addition, inexpensive carbon steel passes Infiltration of certain alloy elements has certain expensive alloy steel properties, which can replace some heat-resistant steels and stainless steels; almost all tools and molds require heat treatment before they can be used.
Complementary means
One. Types of annealing
Annealing is a heat treatment process that heats the workpiece to an appropriate temperature for a certain period of time and then slowly cools down.
There are many types of steel annealing processes, which can be divided into two categories according to heating temperature: one is annealing above the critical temperature (Ac1 or Ac3), also known as phase transformation recrystallization annealing, including complete annealing, incomplete annealing, and ball Annealing and diffusion annealing (homogenization annealing), etc .; the other is annealing below the critical temperature, including recrystallization annealing and stress relief annealing. According to the cooling method, annealing can be divided into isothermal annealing and continuous cooling annealing.
1. Complete annealing and isothermal annealing
Complete annealing is also called recrystallization annealing, which is generally referred to as annealing. It is to heat steel parts or steel to 20 ~ 30 ℃ above Ac3, and keep it warm for a long enough time, so that the structure is austenitized and slowly cooled to obtain near equilibrium Organization heat treatment process. This kind of annealing is mainly used for the casting of various carbon steels and alloy steels with sub-eutectoid composition, forgings and hot-rolled profiles, and sometimes for welding structures. Generally used as the final heat treatment of some non-heavy workpieces, or as the pre-heat treatment of some workpieces.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as the steel used for manufacturing cutting tools, measuring tools, and molds). Its main purpose is to reduce the hardness, improve the machinability, and prepare for the subsequent quenching.
3. Stress relief annealing
Stress relief annealing is also called low temperature annealing (or high temperature tempering). This kind of annealing is mainly used to eliminate the residual stress of castings, forgings, welded parts, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, it will cause deformation or cracks in the steel parts after a certain time, or in the subsequent cutting process.
4. Incomplete annealing is a heat treatment process that heats the steel to Ac1 ~ Ac3 (hypoeutectoid steel) or Ac1 ~ ACcm (hypereutectoid steel), and then slowly cools down after heat preservation to obtain a nearly balanced structure.
Two. During quenching, the most commonly used cooling media are brine, water and oil.
The workpiece quenched with salt water is easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that are not hardened, but it is easy to deform the workpiece seriously and even crack. The use of oil as the quenching medium is only suitable for the quenching of some alloy steels or small-sized carbon steel workpieces with relatively large austenite stability.
Three. The purpose of steel tempering
1. Reduce brittleness, eliminate or reduce internal stress. After quenching, there is a lot of internal stress and brittleness. If not tempered in time, the steel will often deform or even crack.
2. Obtain the required 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, the hardness can be adjusted by suitable tempering to reduce brittleness and obtain the required Toughness and plasticity.
3. Stable workpiece size
4. For some alloy steels that are difficult to soften by annealing, high temperature tempering is often used after quenching (or normalizing), so that the carbides in the steel are properly aggregated, and the hardness is reduced to facilitate cutting.
Complementary concept
1. Annealing: refers to the heat treatment process of heating the metal material to an appropriate temperature for a certain period of time and then slowly cooling it. Common annealing processes include: recrystallization annealing, stress relief annealing, spheroidizing annealing, and complete annealing. The purpose of annealing: mainly to reduce the hardness of the metal material, improve the plasticity, facilitate cutting or pressure processing, reduce residual stress, improve the homogeneity of the organization and composition, or prepare the organization for subsequent heat treatment.
2. Normalizing: refers to the heat treatment process of heating steel or steel parts above or (the upper critical point temperature of steel), and maintaining it at 30 ～ 50 ℃ for an appropriate time, then cooling in still air. The purpose of normalizing: mainly to improve the mechanical properties of low-carbon steel, improve machinability, refine grains, eliminate structural defects, and prepare the tissue for subsequent heat treatment.
3. Quenching: refers to heating the steel to a certain temperature above Ac3 or Ac1 (the lower critical temperature of the steel) for a certain period of time, and then obtaining the martensite (or bainite) structure at an appropriate cooling rate Heat treatment process. Common quenching processes include single-media quenching, dual-media quenching, martensite graded quenching, bainite isothermal quenching, surface quenching and local quenching. The purpose of quenching: to obtain the required martensite structure of the steel parts, to improve the hardness, strength and wear resistance of the workpiece, to prepare for the organization of the subsequent heat treatment.
4. Tempering: refers to a heat treatment process in which steel parts are hardened and then heated to a certain temperature below Ac1 for a certain period of time, and then cooled to room temperature. Common tempering processes are: low temperature tempering, medium temperature tempering, high temperature tempering and multiple tempering.
The purpose of tempering: mainly to eliminate the stress generated by the steel parts during quenching, so that the steel parts have high hardness and wear resistance, and have the required plasticity and toughness.
5. Tempering: refers to the composite heat treatment process of quenching and tempering steel or steel parts. The steel used for quenching and tempering is called quenched and tempered steel. It generally refers to medium carbon structural steel and medium carbon alloy structural steel.
6. Carburization: Carburization refers to the process of infiltrating carbon atoms into the steel surface layer. It is also to make the low-carbon steel workpiece have a high-carbon steel surface layer, which is then quenched and tempered at a low temperature, so that the surface layer of the workpiece has high hardness and wear resistance, while the center part of the workpiece still maintains the toughness and low carbon steel Plasticity.
Vacuum method
Because the heating and cooling operations of metal workpieces require more than a dozen or even dozens of actions to complete. These actions are carried out in the vacuum heat treatment furnace, the operator can not access, so the vacuum heat treatment furnace has a higher degree of automation. At the same time, some actions, such as six or seven actions for the quenching process of the metal workpiece after the end of heating and heat preservation, must be completed within 15 seconds. To complete many actions under such agile conditions, it is easy to cause the operator's tension and constitute a misoperation. Therefore, only a high degree of automation can accurately and timely coordinate procedures.
The vacuum heat treatment of metal parts is carried out in a closed vacuum furnace, and strict vacuum sealing is well known. Therefore, obtaining and adhering to the original gas leakage rate of the furnace and ensuring the working vacuum of the vacuum furnace have a very important meaning for ensuring the quality of the vacuum heat treatment of the parts. Therefore, a key problem of the vacuum heat treatment furnace is to have a reliable vacuum sealing structure. In order to ensure the vacuum performance of the vacuum furnace, a basic principle must be followed in the structural design of the vacuum heat treatment furnace, that is, the furnace body should be gas-tight welding, and at the same time, the furnace body should be opened or not opened as little as possible, and the dynamic seal should be used or avoided. Structure to minimize the chance of vacuum leaks. The components and accessories installed on the vacuum furnace body such as water-cooled electrodes and thermocouple lead-out devices must also be designed with a sealed structure.
Most heating and insulation materials can only be used under vacuum. The heating and heat-insulating lining materials of the vacuum heat treatment furnace work under vacuum and high temperature, so these materials are required to have high temperature resistance, good radiation results, and low thermal conductivity. Anti-oxidation performance is not high. Therefore, vacuum heat treatment furnaces widely use tantalum, tungsten, molybdenum and graphite as heating and heat insulation materials. These materials are easily oxidized under atmospheric conditions, therefore, ordinary heating furnaces cannot use these heating and insulation materials.
Water cooling device: the furnace shell, furnace cover, electric heating element, water cooling electrode, intermediate vacuum heat insulation door and other components of the vacuum heat treatment furnace all work under vacuum and heat. Working under such extremely unfavorable conditions, it is necessary to ensure that the structure of each component is not deformed or damaged, and the vacuum sealing ring is not too hot or burned. Therefore, each component should be equipped with a water cooling device according to different conditions to ensure that the vacuum heat treatment furnace can operate normally and have a sufficient service life.
Adopt low voltage and large current: in the vacuum container, when the vacuum space is within the range of several torr to lxlo-1 torr, the conducting conductor in the vacuum vessel will generate glow discharge phenomenon at a higher voltage. In the vacuum heat treatment furnace, severe arc discharge will burn the electric heating elements, heat insulation layer, etc., causing major accidents and losses. Therefore, the working voltage of the electric heating element of the vacuum heat treatment furnace generally does not exceed 80 to 100 volts. At the same time, effective methods should be adopted in the design of the electric heating element structure, such as avoiding the parts with sharp points as far as possible, and the distance between the electrodes should not be too small to prevent the occurrence of glow discharge or arc discharge.
Tempered
According to the different performance requirements of the workpiece, according to the different tempering temperature, the tempering can be divided into the following types:
(1) Low temperature tempering (150-250 degrees)
The structure obtained by low temperature tempering is tempered martensite. The purpose is to reduce the internal stress and brittleness of the quenched steel while maintaining the high hardness and high wear resistance of the quenched steel, so as to avoid cracking or premature damage during use. It is mainly used for various high-carbon cutting tools, measuring tools, cold punching dies, rolling bearings and carburized parts. The hardness after tempering is generally HRC58-64.
(2) Tempering at medium temperature (250-500 degrees)
The structure obtained by tempering at medium temperature is tempered troostite. The purpose is to obtain high yield strength, elastic limit and high toughness. Therefore, it is mainly used for the treatment of various springs and hot working molds. The hardness after tempering is generally HRC35-50.
(3) High temperature tempering (500-650 degrees)
The structure obtained by high temperature tempering is tempered sorbite. It is customary to call the heat treatment combining quenching and high temperature tempering as quenching and tempering. 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, such as connecting rods, bolts, gears and shafts. The hardness after tempering is generally HB200-330.
Deformation prevention
The causes of deformation of precision and complex molds are often complicated, but as long as we master the deformation rules, analyze the causes, and use different methods to prevent mold deformation, it can be reduced and controlled. In general, the following methods can be used to prevent heat treatment deformation of precision and complex molds.
(1) Reasonable selection of materials. For precision and complicated molds, choose good-deformation mold steel (such as air quenched steel), and forge mold carbide with severe segregation of carbides for reasonable forging and quenching and tempering heat treatment. For large and unforgeable mold steels, solid solution double Refine heat treatment.
(2) The mold structure design should be reasonable, the thickness should not be too wide, and the shape should be symmetrical. For the mold with large deformation, the deformation rule should be mastered, and the processing allowance should be reserved. For large, precise and complex molds, a combined structure can be used.
(3) Precise and complex molds should be heat-treated in advance to eliminate residual stresses generated during mechanical processing.
(4) Reasonably select the heating temperature and control the heating speed. For precision and complex molds, slow heating, preheating and other balanced heating methods can be used to reduce mold heat treatment deformation.
(5) On the premise of ensuring the hardness of the mold, try to adopt the pre-cooling, stepped cooling quenching or warm quenching process.
(6) For precise and complex molds, vacuum heating quenching and cryogenic treatment after quenching should be used as far as possible.
(7) For some sophisticated molds, pre-heat treatment, aging heat treatment, tempering and nitridation heat treatment can be used to control the precision of the mold.
(8) When repairing defects such as die blisters, blowholes, and abrasion, use cold welding machines and other repair equipment with small thermal impact to avoid deformation during the repair process.
In addition, the correct heat treatment process operations (such as plugging holes, binding holes, mechanical fixation, suitable heating methods, correct selection of the cooling direction of the mold and the direction of movement in the cooling medium, etc.) and reasonable tempering heat treatment process also reduce precision and complexity Effective measures for mold deformation.
Surface quenching and tempering heat treatment is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardened layer depth. Hardness testing can use Vickers hardness tester, Rockwell or surface Rockwell hardness tester can also be used. The choice of test force (scale) is related to the depth of the effective hardened layer and the hardness of the workpiece surface. There are three kinds of hardness testers involved.
1. Vickers hardness tester is an important method to test the hardness of the surface of the heat-treated workpiece. It can use a test force of 0.5 to 100 kg to test the surface hardened layer as thin as 0.05 mm. Its accuracy is the highest and can distinguish the surface of the heat-treated workpiece. Small difference in hardness. In addition, the effective depth of the hardened layer must also be tested by a Vickers hardness tester. Therefore, it is necessary to equip a unit that performs surface heat treatment or a large number of surface heat treatment workpieces with a Vickers hardness tester.
2. the surface Rockwell hardness tester is also very suitable for testing the hardness of surface quenched workpieces, the surface Rockwell hardness tester has three scales to choose from. It can test all kinds of surface hardened workpieces with effective hardening depth exceeding 0.1mm. Although the accuracy of the surface Rockwell hardness tester is not as high as that of the Vickers hardness tester, it has been able to meet the requirements as a test method for quality management and conformity inspection of heat treatment plants. Moreover, it also has the characteristics of simple operation, convenient use, low price, rapid measurement, and direct reading of the hardness value. The surface Rockwell hardness tester can be used for rapid and non-destructive piece-by-piece inspection of batches of surface heat-treated workpieces. This is of great significance for metal processing and machinery manufacturing plants.
3. When the surface heat treatment hardened layer is thick, Rockwell hardness tester can also be used. When the thickness of the heat-treated hardened layer is 0.4-0.8mm, the HRA ruler can be used, and when the thickness of the hardened layer exceeds 0.8mm, the HRC ruler can be used.
The three hardness values of Vickers, Rockwell and Surface Rockwell can be easily converted to each other and converted into standards, drawings or hardness values required by users. Corresponding conversion tables are given in the international standard ISO, American standard ASTM and Chinese standard GB / T.
Local hardening
If the local hardness of parts is required to be high, induction quenching and other methods can be used for local quenching heat treatment. Such parts are usually marked on the drawing with the location of local quenching heat treatment and local hardness value. The hardness of the parts should be tested in the designated area. The hardness testing instrument can use Rockwell hardness tester to test HRC hardness value. If the heat treatment hardened layer is shallow, surface Rockwell hardness tester can be used to test HRN hardness value.
Chemical heat treatment
Chemical heat treatment is to infiltrate the surface of the workpiece with atoms of one or several chemical elements, thereby changing the chemical composition, organization and performance of the workpiece surface. After quenching and low temperature tempering, the surface of the workpiece has high hardness, wear resistance and contact fatigue strength, and the core of the workpiece has high strength and toughness.
According to the above, the detection and recording of temperature during heat treatment are very important. Poor temperature control has a great impact on the product. Therefore, the temperature detection is very important, and the temperature change trend in the whole process is also very important, resulting in the temperature change must be recorded during the heat treatment process, which can facilitate future data analysis, and you can also see what time the temperature is in the end Did not meet the requirements. This plays a very important role in improving the subsequent heat treatment.
Operating procedure
1. Clean up the operation site, check whether the power supply, measuring instruments and various switches are normal, and whether the water source is smooth.
2. The operator should wear labor protection equipment, otherwise there will be danger.
3. Turn on the universal switch of the control power supply, and increase and decrease the temperature in stages according to the technical requirements of the equipment to extend the life of the equipment and the equipment in good condition.
4. Pay attention to the temperature of the heat treatment furnace and the speed regulation of the mesh belt, be able to master the temperature standards required for different materials, ensure the hardness of the workpiece, the flatness of the surface and the oxide layer, and do a good job in safety.
5. Pay attention to the temperature of the tempering furnace and the speed adjustment of the mesh belt, turn on the exhaust air, and make the workpiece reach the quality requirements after tempering.
6. Adhere to your post in your work.
7. It is necessary to configure the necessary fire-fighting appliances, and be familiar with the use and maintenance methods.
8. When the machine is stopped, it is necessary to check that all control switches are in the off state, and then turn off the universal switch.
Overheat
From the rough mouth of the bearing parts of the roller accessories, the microstructure after quenching can be overheated. But to accurately determine the degree of overheating, the microstructure must be observed. If coarse needle-shaped martensite appears in the quenched structure of GCr15 steel, it is a quenched superheated structure. The reason may be the overall overheating caused by the excessively high quenching temperature or too long heating and holding time; or it may be due to the serious ribbon carbide in the original structure, and the formation of local martensite needles in the low carbon zone between the two zones Caused by local overheating. The residual austenite in the superheated structure increases, and the dimensional stability decreases. Due to the overheating of the quenching structure and the coarseness of the steel crystals, the toughness of the parts will be reduced, the impact resistance will be reduced, and the life of the bearings will also be reduced. Serious overheating may even cause quenching cracks.
Underheated
If the quenching temperature is low or the cooling is not good, the microstructure will produce a more than standard sintered structure called the underheated structure, which reduces the hardness and sharply reduces the wear resistance, which affects the life of the roller bearing.
Quenching crack
The cracks formed by the internal stress during the quenching and cooling of the idler bearing parts are called quenching cracks. The causes of such cracks are: due to the high temperature of quenching or too rapid cooling, the tissue stress when the thermal stress and metal mass volume change is greater than the fracture resistance of the steel; the original defects on the working surface (such as surface micro cracks or scratches) Marks) or internal defects in the steel (such as slag inclusions, serious non-metallic inclusions, white spots, shrinkage cavities, etc.) forming stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient tempering after part quenching Or not tempered in time; too much cold punching stress caused by the previous process, forged folds, deep turning tool marks, sharp edges of oil grooves, etc. In short, the cause of quenching cracks may be one or more of the above factors. The existence of internal stress is the main reason for the formation of quenching cracks. The quenching crack is deep and slender, the fracture is straight, and the fracture surface has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring; the shape of the bearing steel ball is S-shaped, T-shaped or ring-shaped. The structure characteristic of quenching crack is that there is no decarburization on both sides of the crack, which is obviously different from forging crack and material crack.
Heat treatment deformation
NACHI bearing parts have thermal stress and tissue stress during heat treatment. This internal stress can be superimposed or partially cancelled, which is complicated and variable, because it can vary with heating temperature, heating speed, cooling method, cooling speed, parts The shape and size change, so heat treatment deformation is inevitable. Recognizing and mastering its changing rules can make the deformation of bearing parts (such as the ellipse of the ring, the size increase, etc.) within a controllable range, which is conducive to the production. Of course, mechanical collisions during the heat treatment process can also deform parts, but this deformation can be reduced and avoided with improved operations.
Surface decarburization
If the bearing parts of the idler parts are heated in an oxidizing medium during the heat treatment, the surface will undergo oxidation to reduce the mass fraction of carbon on the surface of the parts, resulting in decarburization of the surface. The depth of the surface decarburization layer exceeds the final processing allowance will make the parts scrapped. The determination of the depth of the surface decarburization layer can be used in the metallographic examination of the metallographic method and micro hardness method. The arbitration criterion can be made based on the measurement method of the surface layer microhardness distribution curve.
Soft spot
Due to insufficient heating, poor cooling, improper quenching, and other reasons, the local hardness of the surface of the roller bearing parts is not sufficient. This phenomenon is called quenching soft spot. Like decarburization of the surface, it can cause a serious drop in surface wear resistance and fatigue strength.
