Heat treatment refers to a metal thermal process in which the material is heated, held and cooled by means of heating in the solid state in order to obtain the desired organization and properties.
I. Heat Treatment
1, Normalizing: the steel or steel pieces heated to the critical point of AC3 or ACM above the appropriate temperature to maintain a certain period of time after cooling in the air, to get the pearlitic type of organization of the heat treatment process.
2, Annealing: eutectic steel workpiece heated to AC3 above 20-40 degrees, after holding for a period of time, with the furnace slowly cooled (or buried in sand or lime cooling) to 500 degrees below the cooling in the air heat treatment process.
3, Solid solution heat treatment: the alloy is heated to a high temperature single-phase region of constant temperature to maintain, so that the excess phase is fully dissolved into solid solution, and then cooled quickly to get a supersaturated solid solution heat treatment process.
4、Ageing：After solid solution heat treatment or cold plastic deformation of the alloy, when it is placed at room temperature or kept at a slightly higher temperature than room temperature, the phenomenon of its properties changing with time.
5, Solid solution treatment: so that the alloy in a variety of phases fully dissolved, strengthen the solid solution and improve the toughness and corrosion resistance, eliminate stress and softening, in order to continue processing molding.
6, Aging treatment: heating and holding at the temperature of the precipitation of the reinforcing phase, so that the precipitation of the reinforcing phase to precipitate, to be hardened, to improve strength.
7, Quenching: steel austenitization after cooling at an appropriate cooling rate, so that the workpiece in the cross-section of all or a certain range of unstable organizational structure such as martensite transformation of the heat treatment process.
8,Tempering: the quenched workpiece will be heated to the critical point of AC1 below the appropriate temperature for a certain period of time, and then cooled in accordance with the requirements of the method, in order to obtain the desired organization and properties of the heat treatment process.
9, Steel carbonitriding: carbonitriding is to the surface layer of steel at the same time infiltration of carbon and nitrogen process. Customary carbonitriding is also known as cyanide, medium temperature gas carbonitriding and low temperature gas carbonitriding (i.e. gas nitrocarburizing) is more 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 to nitriding-based, its main purpose is to improve the wear resistance of steel and bite resistance.
10, Tempering treatment (quenching and tempering): the general custom will be quenched and tempered at high temperatures in combination with heat treatment known as tempering treatment. Tempering treatment is widely used in a variety of important structural parts, especially those working under alternating loads of connecting rods, bolts, gears and shafts. Tempering after the tempering treatment to get tempered sohnite organization, its mechanical properties are better than the same hardness of normalized sohnite organization. Its hardness depends on the high temperature tempering temperature and steel tempering stability and workpiece cross-section size, generally between HB200-350.
11, Brazing: with brazing material will be two kinds of workpiece heating melting bonded together heat treatment process.
II.The characteristics of the process
Metal heat treatment is one of the important processes in mechanical manufacturing, compared with other machining processes, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but by changing the internal microstructure of the workpiece, or change the chemical composition of the surface of the workpiece, to give or improve the use of the workpiece properties. It is characterized by an improvement in the intrinsic quality of the workpiece, which is generally not visible to the naked eye. In order to make the metal workpiece with the required mechanical properties, physical properties and chemical properties, in addition to the reasonable choice of materials and a variety of molding process, heat treatment process is often essential. Steel is the most widely used materials in the mechanical industry, steel microstructure complex, 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 and other alloys can also be heat treatment to change its mechanical, physical and chemical properties, in order to obtain different performance.
III.The process
Heat treatment process generally includes heating, holding, cooling three processes, sometimes only heating and cooling two processes. These processes are connected to each other, can not be interrupted.
Heating is one of the important processes of heat treatment. Metal heat treatment of many heating methods, the earliest is the use of charcoal and coal as a heat source, the recent application of liquid and gas fuels. The application of electricity makes heating easy to control, and no environmental pollution. The use of these heat sources can be directly heated, but also through the molten salt or metal, to floating particles for indirect heating.
Metal heating, the workpiece is exposed to air, oxidation, decarburization often occurs (i.e., the surface carbon content of the steel parts to reduce), which has a very negative impact on the surface properties of the heat-treated parts. Therefore, the metal should usually be in a controlled atmosphere or protective atmosphere, molten salt and vacuum heating, but also available coatings or packaging methods for protective heating.
Heating temperature is one of the important process parameters of the heat treatment process, the selection and control of the heating temperature, is to ensure the quality of heat treatment of the main issues. Heating temperature varies with the treated metal material and the purpose of heat treatment, but generally are heated above the phase transition temperature to obtain high temperature organization. In addition, the transformation requires a certain amount of time, so when the surface of the metal workpiece to achieve the required heating temperature, but also have to be maintained at this temperature for a certain period of time, so that the internal and external temperatures are consistent, so that the microstructure transformation is complete, which is known as the holding time. The use of high energy density heating and surface heat treatment, the heating rate is extremely fast, there is generally no holding time, while the chemical heat treatment of the holding time is often longer.
Cooling is also an indispensable step in the heat treatment process, cooling methods due to different processes, mainly to control the cooling rate. General annealing cooling rate is the slowest, normalizing the cooling rate is faster, quenching the cooling rate is faster. But also because of the different types of steel and have different requirements, such as air-hardened steel can be quenched with the same cooling rate as normalizing.
IV. Process classification
Metal heat treatment process can be roughly divided into the whole heat treatment, surface heat treatment and chemical heat treatment of three categories. According to the heating medium, heating temperature and cooling method of different, each category can be distinguished into a number of different heat treatment process. The same metal using different heat treatment processes, can obtain different organizations, thus having different properties. Iron and steel is the most widely used metal in industry, and steel microstructure is also the most complex, so there are a variety of steel heat treatment process.
Overall heat treatment is the overall heating of the workpiece, and then cooled at an appropriate rate, to obtain the required metallurgical organization, in order to change its overall mechanical properties of metal heat treatment process. Overall heat treatment of steel roughly annealing, normalizing, quenching and tempering four basic processes.
Process means:
Annealing is the workpiece is heated to the appropriate temperature, according to the material and the size of the workpiece using different holding time, and then slowly cooled, the purpose is to make the internal organization of the metal to achieve or close to the equilibrium state, to obtain good process performance and performance, or for further quenching for the organization of the preparation.
Normalizing is the workpiece is heated to the appropriate temperature after cooling in the air, the effect of normalizing is similar to annealing, only to get a finer organization, often used to improve the cutting performance of the material, but also sometimes used for some of the less demanding parts as the final heat treatment.
Quenching is the workpiece is heated and insulated, in water, oil or other inorganic salts, organic aqueous solutions and other quenching medium for rapid cooling. After quenching, the steel parts become hard, but at the same time become brittle, in order to eliminate the brittleness in a timely manner, it is generally necessary to temper in a timely manner.
In order to reduce the brittleness of steel parts, the quenched steel parts at a suitable temperature higher than room temperature and lower than 650 ℃ for a long period of insulation, and then cooled, this process is called tempering. Annealing, normalizing, quenching, tempering is the overall heat treatment in the “four fires”, of which the quenching and tempering are closely related, often used in conjunction with each other, one is indispensable. “Four fire” with the heating temperature and cooling mode of different, and evolved a different heat treatment process. In order to obtain a certain degree of strength and toughness, the quenching and tempering at high temperatures combined with the process, known as tempering. After certain alloys are quenched to form a supersaturated solid solution, they are held at room temperature or at a slightly higher appropriate temperature for a longer period of time in order to improve the hardness, strength, or electrical magnetism of the alloy. Such a heat treatment process is called aging treatment.
Pressure processing deformation and heat treatment effectively and closely combined to carry out, so that the workpiece to obtain a very good strength, toughness with the method known as deformation heat treatment; in a negative-pressure atmosphere or vacuum in the heat treatment known as vacuum heat treatment, which not only can make the workpiece does not oxidize, do not decarburize, keep the surface of the workpiece after treatment, improve the performance of the workpiece, but also through the osmotic agent for chemical heat treatment.
Surface heat treatment is only heating the surface layer of the workpiece to change the mechanical properties of the surface layer of the metal heat treatment process. In order to only heat the surface layer of the workpiece without excessive heat transfer into the workpiece, the use of the heat source must have a high energy density, that is, in the unit area of the workpiece to give a larger heat energy, so that the surface layer of the workpiece or localized can be a short period of time or instantaneous to reach high temperatures. Surface heat treatment of the main methods of flame quenching and induction heating heat treatment, commonly used heat sources such as oxyacetylene or oxypropane flame, induction current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process by changing the chemical composition, organization and properties of the surface layer of the workpiece. Chemical heat treatment differs from surface heat treatment in that the former changes the chemical composition of the surface layer of the workpiece. Chemical heat treatment is placed on the workpiece containing carbon, salt media or other alloying elements of the medium (gas, liquid, solid) in the heating, insulation for a longer period of time, so that the surface layer of the workpiece infiltration of carbon, nitrogen, boron and chromium and other elements. After infiltration of elements, and sometimes other heat treatment processes such as quenching and tempering. The main methods of chemical heat treatment are carburizing, nitriding, metal penetration.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. Generally speaking, it can ensure and improve the various properties of the workpiece, such as wear resistance, corrosion resistance. Can also improve the organization of the blank and stress state, in order to facilitate a variety of cold and hot processing.
For example: white cast iron after a long time annealing treatment can be obtained malleable cast iron, improve plasticity; gears with the correct heat treatment process, service life can be more than not heat-treated gears times or dozens of times; in addition, inexpensive carbon steel through the infiltration of certain alloying elements have some expensive alloy steel performance, can replace some heat-resistant steel, stainless steel; molds and dies are almost all need to go through heat treatment Can be used only after heat treatment.
Supplementary means
I. Types of annealing
Annealing is a heat treatment process in which the workpiece is heated to an appropriate temperature, held for a certain period of time, and then slowly cooled.
There are many types of steel annealing process, according to the heating temperature can be divided into two categories: one is at the critical temperature (Ac1 or Ac3) above the annealing, also known as phase change recrystallization annealing, including complete annealing, incomplete annealing, spheroidal annealing and diffusion annealing (homogenization annealing), etc.; the other is below the critical temperature of the annealing, including recrystallization annealing and de-stressing annealing, etc.. According to the cooling method, annealing can be divided into isothermal annealing and continuous cooling annealing.
1, complete annealing and isothermal annealing
Complete annealing, also known as recrystallization annealing, generally referred to as annealing, it is the steel or steel heated to Ac3 above 20 ~ 30 ℃, insulation long enough to make the organization completely austenitized after slow cooling, in order to obtain nearly equilibrium organization of the heat treatment process. This annealing is mainly used for sub-eutectic composition of various carbon and alloy steel castings, forgings and hot-rolled profiles, and sometimes also used for welded structures. Generally often as a number of not heavy workpiece final heat treatment, or as a pre-heat treatment of some workpieces.
2, ball annealing
Spheroidal annealing is mainly used for over-eutectic carbon steel and alloy tool steel (such as the manufacture of edged tools, gauges, molds and dies used in the steel). Its main purpose is to reduce the hardness, improve the machinability, and prepare for future quenching.
3, stress relief annealing
Stress relief annealing, also known as low-temperature annealing (or high-temperature tempering), this annealing is mainly used to eliminate castings, forgings, weldments, hot-rolled parts, cold-drawn parts and other residual stress. If these stresses are not eliminated, will cause steel after a certain period of time, or in the subsequent cutting process to produce deformation or cracks.
4. Incomplete annealing is to heat the steel to Ac1 ~ Ac3 (sub-eutectic steel) or Ac1 ~ ACcm (over-eutectic steel) between the heat preservation and slow cooling to obtain nearly balanced organization of the heat treatment process.
II.quenching, the most commonly used cooling medium is brine, water and oil.
Salt water quenching of the workpiece, easy to get high hardness and smooth surface, not easy to produce quenching not hard soft spot, but it is easy to make the workpiece deformation is serious, and even cracking. The use of oil as a quenching medium is only suitable for the stability of supercooled austenite is relatively large in some alloy steel or small size of carbon steel workpiece quenching.
III.the purpose of steel tempering
1, reduce brittleness, eliminate or reduce internal stress, steel quenching there is a great deal of internal stress and brittleness, such as not timely tempering will often make the steel deformation or even cracking.
2, to obtain the required mechanical properties of the workpiece, the workpiece after quenching high hardness and brittleness, in order to meet the requirements of the different properties of a variety of workpieces, you can adjust the hardness through the appropriate tempering to reduce the brittleness of the required toughness, plasticity.
3、Stabilize the size of the workpiece
4, for annealing is difficult to soften certain alloy steels, in the quenching (or normalizing) is often used after high-temperature tempering, so that the steel carbide appropriate aggregation, the hardness will be reduced, in order to facilitate cutting and processing.
Supplementary concepts
1, annealing: refers to metal materials heated to the appropriate temperature, maintained for a certain period of time, and then slowly cooled heat treatment process. Common annealing processes are: recrystallization annealing, stress relief annealing, spheroidal annealing, complete annealing, etc.. The purpose of annealing: mainly to reduce the hardness of metal materials, improve plasticity, in order to facilitate cutting or pressure machining, reduce residual stresses, improve the organization and composition of the homogenization, or for the latter heat treatment to make the organization ready.
2, normalizing: refers to the steel or steel heated to or (steel on the critical point of temperature) above, 30 ~ 50 ℃ to maintain the appropriate time, cooling in still air heat treatment process. The purpose of normalizing: mainly to improve the mechanical properties of low carbon steel, improve the cutting and machinability, grain refinement, to eliminate organizational defects, for the latter heat treatment to prepare the organization.
3, quenching: refers to the steel heated to Ac3 or Ac1 (steel under the critical point of temperature) above a certain temperature, keep a certain time, and then to the appropriate cooling rate, to obtain martensite (or bainite) organization of the heat treatment process. Common quenching processes are single-medium quenching, dual-medium quenching, martensite quenching, bainite isothermal quenching, surface quenching and local quenching. The purpose of quenching: so that the steel parts to obtain the required martensitic organization, improve the hardness of the workpiece, strength and abrasion resistance, for the latter heat treatment to make good preparation for the organization.
4, tempering: refers to the steel hardened, then heated to a temperature below Ac1, holding time, and then cooled to room temperature heat treatment process. Common tempering processes are: low-temperature tempering, medium-temperature tempering, high-temperature tempering and multiple tempering.
Tempering purpose: mainly to eliminate the stress produced by the steel in the quenching, so that the steel has a high hardness and wear resistance, and has the required plasticity and toughness.
5, tempering: refers to the steel or steel for quenching and high-temperature tempering of the composite heat treatment process. Used in the tempering treatment of steel called tempered steel. It generally refers to medium carbon structural steel and medium carbon alloy structural steel.
6, carburizing: carburizing is the process of making carbon atoms penetrate into the surface layer of steel. It is also to make the low carbon steel workpiece has the surface layer of high carbon steel, and then after quenching and low temperature tempering, 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 plasticity of low carbon steel.
Vacuum method
Because the heating and cooling operations of metal workpieces require a dozen or even dozens of actions to complete. These actions are carried out within the vacuum heat treatment furnace, the operator can not approach, so the degree of automation of the vacuum heat treatment furnace is required to be higher. At the same time, some actions, such as heating and holding the end of the metal workpiece quenching process shall be six, seven actions and to be completed within 15 seconds. Such agile conditions to complete many actions, it is easy to cause the operator’s nervousness and constitute misoperation. Therefore, only a high degree of automation can be accurate, timely coordination in accordance with the program.
Vacuum heat treatment of metal parts are carried out in a closed vacuum furnace, strict vacuum sealing is well known. Therefore, to obtain and adhere to the original air leakage rate of the furnace, to ensure that the working vacuum of the vacuum furnace, to ensure the quality of the parts vacuum heat treatment has a very major significance. So a key issue of vacuum heat treatment furnace is to have a reliable vacuum sealing structure. In order to ensure the vacuum performance of the vacuum furnace, vacuum heat treatment furnace structure design must follow a basic principle, that is, the furnace body to use gas-tight welding, while the furnace body as little as possible to open or not open the hole, less or avoid the use of dynamic sealing structure, in order to minimize the opportunity for vacuum leakage. Installed in the vacuum furnace body components, accessories, such as water-cooled electrodes, thermocouple export device must also be designed to seal the structure.
Most heating and insulation materials can only be used under vacuum. Vacuum heat treatment furnace heating and thermal insulation lining is in the vacuum and high temperature work, so these materials put forward the high temperature resistance, radiation results, thermal conductivity and other requirements. The requirements for oxidation resistance are not high. Therefore, the vacuum heat treatment furnace widely used tantalum, tungsten, molybdenum and graphite for heating and thermal insulation materials. These materials are very easy to oxidize in the atmospheric state, therefore, ordinary heat treatment furnace can not use these heating and insulation materials.
Water-cooled device: vacuum heat treatment furnace shell, furnace cover, electric heating elements, water-cooled electrodes, intermediate vacuum heat insulation door and other components, are in a vacuum, under the state of heat work. Working under such extremely unfavorable conditions, it must be ensured that the structure of each component is not deformed or damaged, and the vacuum seal is not overheated or burned. Therefore, each component should be set up according to different circumstances water-cooling devices to ensure that the vacuum heat treatment furnace can operate normally and have sufficient utilization life.
The use of low-voltage high-current: vacuum container, when the vacuum vacuum degree of a few lxlo-1 torr range, the vacuum container of the energized conductor in the higher voltage, will produce glow discharge phenomenon. In the vacuum heat treatment furnace, serious arc discharge will burn the electric heating element, insulation layer, causing major accidents and losses. Therefore, the vacuum heat treatment furnace electric heating element working voltage is generally not more than 80 a 100 volts. At the same time in the electric heating element structure design to take effective measures, such as try to avoid having the tip of the parts, electrode spacing between the electrodes can not be too small, in order to prevent the generation of glow discharge or arc discharge.
Tempering
According to the different performance requirements of the workpiece, according to its different tempering temperatures, can be divided into the following types of tempering:
(a) low-temperature tempering (150-250 degrees)
Low temperature tempering of the resulting organization for the tempered martensite. Its purpose is to maintain the high hardness and high wear resistance of quenched steel under the premise of reducing its quenching internal stress and brittleness, so as to avoid chipping or premature damage during use. It is mainly used for a variety of high-carbon cutting tools, gauges, cold-drawn dies, rolling bearings and carburized parts, etc., after tempering hardness is generally HRC58-64.
(ii) medium temperature tempering (250-500 degrees)
Medium temperature tempering organization for tempered quartz body. Its purpose is to obtain high yield strength, elastic limit and high toughness. Therefore, it is mainly used for a variety of springs and hot work mold processing, tempering hardness is generally HRC35-50.
(C) high temperature tempering (500-650 degrees)
High-temperature tempering of the organization for the tempered Sohnite. Customary quenching and high temperature tempering combined heat treatment known as tempering treatment, its purpose is to obtain strength, hardness and plasticity, toughness are better overall mechanical properties. Therefore, widely used in automobiles, tractors, machine tools and other important structural parts, such as connecting rods, bolts, gears and shafts. The hardness after tempering is generally HB200-330.
Deformation prevention
Precision complex mold deformation causes are often complex, but we just master its deformation law, analyze its causes, using different methods to prevent the mold deformation is able to reduce, but also able to control. Generally speaking, the heat treatment of precision complex mold deformation can take the following methods of prevention.
(1) Reasonable material selection. Precision complex molds should be selected material good microdeformation mold steel (such as air quenching steel), the carbide segregation of serious mold steel should be reasonable forging and tempering heat treatment, the larger and can not be forged mold steel can be solid solution double refinement heat treatment.
(2) Mould structure design should be reasonable, thickness should not be too disparate, the shape should be symmetrical, for the deformation of the larger mold to master the deformation law, reserved processing allowance, for large, precise and complex molds can be used in a combination of structures.
(3) Precision and complex molds should be pre-heat treatment to eliminate the residual stress generated in the machining process.
(4) Reasonable choice of heating temperature, control the heating speed, for precision complex molds can take slow heating, preheating and other balanced heating methods to reduce the mold heat treatment deformation.
(5) Under the premise of ensuring the hardness of the mold, try to use pre-cooling, graded cooling quenching or temperature quenching process.
(6) For precision and complex molds, under the conditions permit, try to use vacuum heating quenching and deep cooling treatment after quenching.
(7) For some precision and complex molds can be used pre-heat treatment, aging heat treatment, tempering nitriding heat treatment to control the accuracy of the mold.
(8) In the repair of mold sand holes, porosity, wear and other defects, the use of cold welding machine and other thermal impact of the repair equipment to avoid the repair process of deformation.
In addition, the correct heat treatment process operation (such as plugging holes, tied holes, mechanical fixation, suitable heating methods, the correct choice of the cooling direction of the mold and the direction of movement in the cooling medium, etc.) and reasonable tempering heat treatment process is to reduce the deformation of precision and complex molds are also effective measures.
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 hardening layer depth. Hardness testing can be used Vickers hardness tester, can also be used Rockwell or surface Rockwell hardness tester. The choice of test force (scale) is related to the depth of the effective hardened layer and the surface hardness of the workpiece. Three kinds of hardness testers are involved here.
First, Vickers hardness tester is an important means of testing the surface hardness of heat-treated workpieces, it can be selected from 0.5 to 100kg of test force, test the surface hardening layer as thin as 0.05mm thick, and its accuracy is the highest, and it can distinguish the small differences in the surface hardness of heat-treated workpieces. In addition, the depth of the effective hardened layer should also be detected by the Vickers hardness tester, so for surface heat treatment processing or a large number of units using surface heat treatment workpiece, equipped with a Vickers hardness tester is necessary.
Second, the surface Rockwell hardness tester is also very suitable for testing the hardness of surface hardened workpiece, surface Rockwell hardness tester has three scales to choose from. Can test the effective hardening depth of more than 0.1mm of various surface hardening workpiece. Although the surface Rockwell hardness tester precision is not as high as the Vickers hardness tester, but as a heat treatment plant quality management and qualified inspection means of detection, has been able to meet the requirements. Moreover, it also has a simple operation, easy to use, low price, rapid measurement, can directly read the hardness value and other characteristics, the use of surface Rockwell hardness tester can be a batch of surface heat treatment workpiece for rapid and non-destructive piece-by-piece testing. This is important for metal processing and machinery manufacturing plant.
Third, when the surface heat treatment hardened layer is thicker, can also be used Rockwell hardness tester. When the heat treatment hardened layer thickness of 0.4 ~ 0.8mm, can be used HRA scale, when the hardened layer thickness of more than 0.8mm, can be used HRC scale.
Vickers, Rockwell and surface Rockwell three kinds of hardness values can be easily converted to each other, converted to the standard, drawings or the user needs the hardness value. The corresponding conversion tables are given in the international standard ISO, the American standard ASTM and the Chinese standard GB/T.
Localized hardening
Parts if the local hardness requirements of higher, available induction heating and other means of local quenching heat treatment, such parts usually have to mark the location of local quenching heat treatment and local hardness value on the drawings. Hardness testing of parts should be carried out in the designated area. Hardness testing instruments can be used Rockwell hardness tester, test HRC hardness value, such as heat treatment hardening layer is shallow, can be used surface Rockwell hardness tester, test HRN hardness value.
Chemical heat treatment
Chemical heat treatment is to make the surface of the workpiece infiltration of one or several chemical elements of atoms, so as to change the chemical composition, organization and performance of the surface of the workpiece. After quenching and low temperature tempering, the surface of the workpiece has high hardness, wear resistance and contact fatigue strength, while the core of the workpiece has high toughness.
According to the above, the detection and recording of temperature in the heat treatment process is very important, and poor temperature control has a great impact on the product. Therefore, the detection of temperature is very important, the temperature trend in the whole process is also very important, resulting in the process of heat treatment must be recorded on the temperature change, can facilitate future data analysis, but also to see which time the temperature does not meet the requirements. This will play a very big role in improving the heat treatment in the future.
Operating procedures
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、Operators should wear good labor protection protective equipment, otherwise it will be dangerous.
3, open the control power universal transfer switch, according to the technical requirements of the equipment graded sections of the temperature rise and fall, to extend the life of the equipment and equipment intact.
4, to pay attention to the heat treatment furnace temperature and mesh belt speed regulation, can master the temperature standards required for different materials, to ensure the hardness of the workpiece and the surface straightness and oxidation layer, and seriously do a good job of safety.
5、To pay attention to the tempering furnace temperature and mesh belt speed, open the exhaust air, so that the workpiece after tempering to meet the quality requirements.
6, in the work should stick to the post.
7, to configure the necessary fire apparatus, and familiar with the use and maintenance methods.
8、When stopping the machine, we should check that all the control switches are in the off state, and then close the universal transfer switch.
Overheating
From the rough mouth of the roller accessories bearing parts can be observed after quenching microstructure overheating. But to determine the exact degree of overheating must observe the microstructure. If in the GCr15 steel quenching organization in the appearance of coarse needle martensite, it is quenching overheating organization. The reason for the formation of quenching heating temperature may be too high or heating and holding time is too long caused by the full range of overheating; may also be due to the original organization of the band carbide serious, in the low carbon area between the two bands to form a localized martensite needle thick, resulting in localized overheating. Residual austenite in the superheated organization increases, and dimensional stability decreases. Due to the overheating of the quenching organization, the steel crystal is coarse, which will lead to a reduction in the toughness of the parts, impact resistance is reduced, and the life of the bearing is also reduced. Severe overheating can even cause quenching cracks.
Underheating
Quenching temperature is low or poor cooling will produce more than the standard Torrhenite organization in the microstructure, known as the underheating organization, which makes the hardness drop, wear resistance is sharply reduced, affecting the life of the roller parts bearing.
Quenching cracks
Roller bearing parts in the quenching and cooling process due to internal stresses formed cracks called quenching cracks. Causes of such cracks are: due to quenching heating temperature is too high or cooling is too rapid, thermal stress and metal mass volume change in the organization of the stress is greater than the fracture strength of steel; work surface of the original defects (such as surface cracks or scratches) or internal defects in the steel (such as slag, serious non-metallic inclusions, white spots, shrinkage residue, etc.) in the quenching of the formation of stress concentration; severe surface decarburization and Carbide segregation; parts quenched after tempering insufficient or untimely tempering; cold punch stress caused by the previous process is too large, forging folding, deep turning cuts, oil grooves sharp edges and so on. In short, the cause of quenching cracks may be one or more of the above factors, the presence of internal stress is the main reason for the formation of quenching cracks. Quenching cracks are deep and slender, with a straight fracture and no oxidized color on the broken surface. It is often a longitudinal flat crack or ring-shaped crack on the bearing collar; the shape on the bearing steel ball is S-shaped, T-shaped or ring-shaped. The organizational characteristics of quenching crack is no decarburization phenomenon on both sides of the crack, clearly distinguishable from forging cracks and material cracks.
Heat treatment deformation
NACHI bearing parts in heat treatment, there are thermal stress and organizational stress, this internal stress can be superimposed on each other or partially offset, is complex and variable, because it can be changed with the heating temperature, heating rate, cooling mode, cooling rate, the shape and size of the parts, so heat treatment deformation is inevitable. Recognize and master the rule of law can make the deformation of bearing parts (such as the oval of the collar, size up, etc.) placed in a controllable range, conducive to the production. Of course, in the heat treatment process of mechanical collision will also make the parts deformation, but this deformation can be used to improve the operation to reduce and avoid.
Surface decarburization
Roller accessories bearing parts in the heat treatment process, if it is heated in an oxidizing medium, the surface will be oxidized so that the parts surface carbon mass fraction is reduced, resulting in surface decarburization. The depth of the surface decarburization layer more than the final processing of the amount of retention will make the parts scrapped. Determination of the depth of the surface decarburization layer in the metallographic examination of the available metallographic method and microhardness method. The microhardness distribution curve of the surface layer is based on the measurement method, and can be used as an arbitration criterion.
Soft spot
Due to insufficient heating, poor cooling, quenching operation caused by improper surface hardness of roller bearing parts is not enough phenomenon known as quenching soft spot. It is like surface decarburization can cause a serious decline in surface wear resistance and fatigue strength.
Post time: Dec-05-2023