Heat treatment

Heat treatment refers to a metal thermal processing process in which the material is in a solid-state through heating, heat preservation, and cooling to obtain the desired structure and properties. In the process from the Stone Age to the Bronze Age and Iron Age, the role of heat treatment is gradually recognized by people.

Similar Articles:

• Normalizing
• Quenching & Tempering
• Normalizing & Annealing
• Quenching and tempering(QT)
• Quenched and tempered steel
• Heat treatment quality inspection
• Heat treatment workshop work instructions
• Explanation of metallographic structure terms
• Factors Affecting Heat Treatment Deformation
• Common heat treatment process of steel castings
• The reasons and effects of steel parts needing quenching and tempering

Definition

Heat treatment

• Normalizing: The heat treatment process of heating the steel or steel parts to an appropriate temperature above the critical point AC3 or ACM for a certain period and then cooling in the air to obtain a pearlite structure.

• Annealing: The hypoeutectoid steel workpiece is heated to 20-40 degrees above AC3, and after holding for some time, it is slowly cooled with the furnace (or buried in sand or cooled in lime) to a heat treatment process of below 500 degrees and cooled in air.

• Solution heat treatment: heat treatment process in which the alloy is heated to a high temperature single-phase region and maintained at a constant temperature so that the excess phase is fully dissolved into the solid solution, and then rapidly cooled to obtain a supersaturated solid solution.

• Aging: After solution heat treatment or cold plastic deformation, the properties of alloys change with time when they are kept at room temperature or slightly higher than room temperature.

• Solution treatment: Fully dissolve various phases in the alloy, strengthen the solid solution, improve toughness and corrosion resistance, eliminate stress, and soften, to continue processing and forming.

• Aging treatment: Heating and maintaining the temperature at the precipitation temperature of the strengthening phase, so that the strengthening phase is precipitated, hardened, and the strength is improved.

• Quenching: A heat treatment process in which the steel is austenitized and then cooled at an appropriate cooling rate so that the workpiece can undergo martensite and other unstable microstructure transformations in all or a certain range of the cross-section.

• Tempering: A heat treatment process in which the quenched workpiece is heated to an appropriate temperature below the critical point AC1 for a certain period, and then cooled by a method that meets the requirements to obtain the required structure and properties.

• Carbonitriding of steel: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the surface of the steel. Traditionally, carbonitriding, also known as cyanidation, is widely used in medium-temperature gas carbonitriding and low-temperature gas carbonitriding (ie, gas soft nitriding). 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.

• Quenching and tempering: The heat treatment that combines quenching and high-temperature tempering is generally called quenching and tempering. Quenching and 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 treatment, the tempered sorbite structure is obtained, and its mechanical properties are better than the normalized sorbite structure with the same hardness. Its hardness depends on the high-temperature tempering temperature and is related to the tempering stability of the steel and the size of the workpiece section, generally between HB200-350.

Process characteristics

Metal heat treatment is one of the important processes in machinery manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but changes the microstructure inside the workpiece or changes the chemical composition of the workpiece surface. , to give or improve the performance of the workpiece.

It is characterized by improving the intrinsic quality of the workpiece, which is generally not visible to the naked eye. 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, the heat treatment process is 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. Therefore, 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.

Thermal process

Crafting process

The heat treatment process generally includes three processes heating, heat preservation, and cooling, and sometimes there are only two processes of heating and cooling. These processes are interconnected and uninterrupted. Heating is one of the important processes of heat treatment.

There are many heating methods for metal heat treatment. The earliest ones used charcoal and coal as heat sources, and more recently, liquid and gas fuels were used. The application of electricity makes heating easy to control and free of environmental pollution. These heat sources can be used for direct heating or indirect heating through molten salt or gold, 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 properties of the parts after heat treatment. Therefore, the metal should usually be heated in a controlled atmosphere or protective atmosphere, in molten salt and a vacuum, and can also be protected by coating or packaging methods.

The heating temperature is one of the important process parameters of the heat treatment process. The selection and control of the heating temperature are the main issues to ensure the quality of the heat treatment. The heating temperature varies with the metal material to be processed and the purpose of the heat treatment, but generally it is heated above the phase transition temperature to obtain a high-temperature structure. In addition, the transformation takes a certain amount of time, so when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain period to make the internal and external temperatures consistent and the microstructure changes completely. This period is called the holding time. When high-energy-density heating and surface heat treatment are used, the heating speed is extremely fast, and there is generally no holding time, while the holding time of chemical heat treatment is often longer.

Cooling is also an indispensable step in the heat treatment process. The cooling method varies with different processes, mainly controlling 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 also different requirements due to different steel types. For example, hollow-hardened steel can be hardened with the same cooling rate as normalizing.

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 the different heating mediums, heating temperature, and cooling methods, 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 has 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 it at an appropriate rate to obtain the required metallographic structure to change its overall mechanical properties. The overall heat treatment of steel generally has four basic processes: annealing, normalizing, quenching, and tempering.

Craftsmanship

Annealing is to heat the workpiece to an appropriate temperature, adopt different holding times according to the material and workpiece size, and then slowly cool it, the purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and service performance, or for further quenching Prepare for an organization.

Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to that of annealing, but the obtained structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for some parts with low requirements. as the final heat treatment.

Quenching is to rapidly cool the workpiece in a quenching medium such as water, oil, or other inorganic salts and organic aqueous solutions after heating and keeping the workpiece warm. After quenching, the steel becomes hard, but at the same time becomes brittle. To eliminate the brittleness in time, it is generally necessary to temper in time. To reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature but lower than 650 ° C 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. Among them, quenching and tempering are closely related and are often used together, and neither is indispensable. The “four fires” have evolved different heat treatment processes with different heating temperatures and cooling methods. To obtain a certain strength and toughness, the process of combining quenching and high-temperature tempering is called quenching and tempering.

After some alloys are quenched to form a supersaturated solid solution, they are kept at room temperature or a slightly higher appropriate temperature for a long time to improve the hardness, strength, or electrical and magnetic properties of the alloy. Such a heat treatment process is called aging treatment.

The method of combining pressure working deformation and heat treatment effectively and closely, so that the workpiece can obtain a good combination of strength and toughness, is called deformation heat treatment. Heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment. It can not only make the workpiece not oxidized and decarburized, keep the surface of the workpiece smooth after treatment, and improve the performance of the workpiece but also can be chemically heat treated by introducing an infiltrating agent.

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. To only heat the surface layer of the workpiece without allowing too much heat to pass into the inside of the workpiece, the heat source used must have a high energy density, that is, a larger amount of heat energy is given to the workpiece per unit area so that the surface or part of the workpiece can be short-term or instantaneous. reach a high temperature. The main methods of surface heat treatment are flame quenching and induction heating heat treatment. Commonly used heat sources are flames such as oxyacetylene or oxy propane, induced current, laser, and electron beam. Chemical heat treatment is a metal heat treatment process that changes the chemical composition, structure, and properties of the workpiece surface.

The difference between chemical heat treatment and surface heat treatment is that the former changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, salt, or other alloying elements, and keep it for a long time so that the surface layer of the workpiece is infiltrated with elements such as carbon, nitrogen, boron, and chromium. After the elements are infiltrated, other heat treatment processes such as quenching and tempering are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding, and metalizing.

Heat treatment is one of the important processes in the manufacture of mechanical parts and tools. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing. For example, after long-term annealing treatment of white cast iron, malleable cast iron can be obtained, which can improve plasticity; gears adopt the correct heat treatment process, and the service life can be doubled or dozens of times longer than those without heat treatment; Some alloying elements have some expensive alloy steel properties and can replace some heat-resistant steels and stainless steels; almost all tools and dies need to be heat treated before they can be used.

Vacuum method

Because the heating and cooling of the metal workpiece require dozens or even dozens of actions to complete. These actions are carried out in the vacuum heat treatment furnace, which cannot be approached by operators, so the requirements for the automation degree of the vacuum heat treatment furnace are relatively high. At the same time, some actions, such as after heating and heat preservation, the quenching process of metal workpieces requires six or seven actions and should be completed within 15 seconds. In such agile conditions to complete many actions, it is easy to cause operator tension and constitute misoperation. Therefore, only higher automation can accurately and timely coordinate according to the program.

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 maintaining the original air leakage rate of the furnace and ensuring the working vacuum degree of the vacuum furnace is of great significance to ensure 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.

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 welded with airtightness, and at the same time, the furnace body should be opened as little or no holes as possible, and the dynamic seal should be used less 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 insulating materials can only be used in a vacuum. The heating and insulating linings of the vacuum heat treatment furnace work under vacuum and high temperatures, so these materials are required to be resistant to high temperatures, have good radiation performance, and have small thermal conductivity. Antioxidative properties are not required. Therefore, the vacuum heat treatment furnace widely uses tantalum, tungsten, molybdenum, and graphite as heating and heat insulation materials. These materials are easily oxidized in the atmospheric state, therefore, these heating and insulating materials cannot be used in ordinary heat treatment furnaces.

Water-cooling device: The furnace shell, furnace cover, electric heating element, water-cooled electrode, intermediate vacuum insulation door, and other components of the vacuum heat treatment furnace all work in a vacuum and heated state. Working under such extremely unfavorable conditions, it is necessary to ensure that the structure of each component is not deformed or damaged and that the vacuum sealing ring is not overheated 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 sufficient service life.

Using low voltage and high current: In the vacuum container, when the vacuum space is within the range of several Torr to lxlo-1 Torr, the energized conductor in the vacuum container will produce glow discharge at a higher voltage. In the vacuum heat treatment furnace, severe arc discharge will burn down the electric heating element, heat insulation layer, etc., resulting in major accidents and losses. Therefore, the working voltage of the electric heating element of the vacuum heat treatment furnace generally does not exceed 80-100 volts. At the same time, effective measures should be taken in the structural design of the electric heating element, such as avoiding sharp-edged parts as much as possible, and the distance between the electrodes should not be too small to prevent the generation of glow discharge or arc discharge.

Sub craft

Annealing Heat Treatment Sulfurization Heat Treatment Hardening Heat Treatment for Stress Relief Heat Treatment.

Surface hardening

Case hardening and tempering heat treatment are usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness, and effective hardened layer depth. Vickers hardness tester can be used for hardness testing, and Rockwell or surface Rockwell hardness tester can also be used. The selection of the test force (scale) is related to the depth of the effective hardened layer and the surface hardness of the workpiece.

There are three durometers involved here.

• Vickers hardness tester is an important means to test the surface hardness of heat-treated workpieces. It can use a test force of 0.5-100kg to test the surface hardened layer as thin as 0.05mm thick. Its accuracy is yes, and it can distinguish the surface hardness of heat-treated workpieces. small differences. In addition, the depth of the effective hardened layer is also detected by a Vickers hardness tester. Therefore, it is necessary to have a Vickers hardness tester for units that perform surface heat treatment processing or use a large number of surface heat treatment workpieces.

• The surface Rockwell hardness tester is also very suitable for testing the hardness of surface quenched workpieces. There are three scales for the surface Rockwell hardness tester to choose from. Various case-hardened workpieces with an effective hardening depth of more than 0.1mm can be tested. 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 detection method for quality management and qualification inspection of heat treatment plants. Moreover, it also has the characteristics of simple operation, convenient use, low price, rapid measurement, and direct reading of hardness values. Using the surface Rockwell hardness tester, batches of surface heat-treated workpieces can be quickly and non-destructively tested piece by piece. This has important implications for metalworking and machine-building plants.

• When the surface heat treatment hardening layer is thick, the Rockwell hardness tester can also be used. When the thickness of the heat treatment hardened layer is 0.4-0.8mm, the HRA scale can be used, and when the thickness of the hardened layer exceeds 0.8mm, the HRC scale can be used. The three hardness values ​​of Vickers, Rockwell, and superficial Rockwell can be easily converted to each other and converted into standard, drawings or hardness values ​​required by users. The corresponding conversion table has been given in the international standard ISO, American standard ASTM, and Chinese standard GB/T.

Partial quenching

If the local hardness requirements of the parts are high, the local quenching heat treatment can be carried out through induction heating. For such parts, the location of the local quenching heat treatment and the local hardness value is usually marked on the drawings. The hardness test of the parts should be carried out in the designated area. The hardness testing instrument can use the Rockwell hardness tester to test the HRC hardness value. If the heat treatment hardening layer is shallow, the surface Rockwell hardness tester can be used to test the HRN hardness value.

Chemical heat treatment

Chemical heat treatment is to make the surface of the workpiece infiltrate the atoms of one or several chemical elements, thereby changing the chemical composition, structure, and properties 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, and the core of the workpiece has high toughness.

Temperature pressure

According to the above, it is very important to detect and record the temperature during the heat treatment process, and poor temperature control has a great impact on the product. Therefore, the detection of temperature is very important, and the trend of temperature change in the whole process is also very important. As a result, the temperature change must be recorded during the heat treatment process, which can facilitate data analysis in the future and can also check which time the temperature is. did not meet the requirements. This plays a very important role in improving the subsequent heat treatment.

Operating procedures

• Clean up the operation site, and check whether the power supply, measuring instruments and various switches are normal and whether the water source is unobstructed.

• The operator should wear protective equipment, otherwise there will be a danger.

• Turn on the universal transfer switch of the control power supply, and step up and cool down according to the technical requirements of the equipment, to prolong the life of the equipment and keep the equipment in good condition.

• Pay attention to the furnace temperature and mesh belt speed regulation of the heat treatment furnace, be able to master the temperature standards required for different materials, ensure the hardness, surface flatness, and oxide layer of the workpiece, and do a good job of safety work.

• Pay attention to the furnace temperature and mesh belt speed regulation of the tempering furnace, and open the exhaust air to make the workpiece meet the quality requirements after tempering.

• You should stick to your post at work.

• It is necessary to configure the necessary fire-fighting equipment, and be familiar with the use and maintenance methods.

• When shutting down, check that all control switches are turned off, and then turn off the universal transfer switch.

Common problem

Overheating of the microstructure after quenching can be observed from the rough mouth of the bearing parts. But to accurately judge the degree of its overheating must observe the microstructure. If coarse acicular martensite appears in the quenched structure of GCr15 steel, it is a quenched superheated structure. The reason for the formation may be the overall overheating caused by the quenching heating temperature being too high or the heating and holding time being too long; it may also be due to serious banded carbides in the original structure, forming local martensitic needle-like thick in the low-carbon area between the two bands, localized overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases. Due to the overheating of the quenched structure and the coarse crystals of the steel, the toughness of the parts will be reduced, the impact resistance will be reduced, and the life of the bearing will also be reduced. Severe overheating can even cause quenching cracks.

If the underheated quenching temperature is too low or the cooling is poor, a tortenite structure exceeding the standard will be produced in the microstructure, which is called underheated structure.

High quenching cracks or too rapid cooling, the thermal stress and the structural stress of the metal mass and volume change are greater than the fracture strength of the steel; the original defects of the working surface (such as surface microcracks or scratches) or the internal defects of the steel (such as slag inclusions), serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) form stress concentration during quenching; severe surface decarburization and carbide segregation; parts are insufficiently tempered or not tempered in time after quenching; cold caused by previous processes Excessive punching stress, forging folding, deep turning tool marks, sharp edges and corners of oil grooves, etc.

In short, the cause of quenching cracks may be one or more of the above factors, and 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 annular. The organizational characteristics of quenching cracks are that there is no decarburization on both sides of the cracks, which is different from forging cracks and material cracks.

Heat treatment deformation. During heat treatment of bearing parts, there are thermal stress and tissue stress. This internal stress can be superimposed or partially offset, which is complex and changeable because it can vary with heating temperature, heating speed, cooling method, and cooling speed. , The shape and size of the parts change, so heat treatment deformation is inevitable. Knowing and mastering its changing law can make the deformation of bearing parts (such as the ellipse of the ferrule, the size increase, etc.) in a controllable range, which is beneficial to the production. Of course, the mechanical impact during the heat treatment will also deform the part, but this deformation can be reduced and avoided with improved operations.

Surface decarburization, if the bearing parts are heated in an oxidizing medium during the heat treatment process, the surface will be oxidized to reduce the mass fraction of carbon on the surface of the parts, resulting in surface decarburization. The depth of the surface decarburization layer exceeds the allowance of final machining and the part is scrapped. Determination of the depth of the surface decarburization layer can be used in metallographic examination metallographic method and microhardness method. The measurement method of the microhardness distribution curve of the surface layer shall prevail, which can be used as the arbitration criterion. Insufficient soft spot heating, poor cooling, improper quenching operation, and other reasons, the phenomenon of insufficient local hardness on the surface of roller bearing parts is called quenching soft spot. Like surface decarburization, it can cause a serious decrease in surface wear resistance and fatigue strength.

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