Shot peening is an ideal process to improve the fatigue strength of compression springs

A spring factory in Beijing is one of the largest manufacturers of cold-coiled springs in China. Its main products are automobile engine valve springs and various clamp, tension, torsion and compression springs. In recent years, the manufacturer has carried out large-scale technological transformations and introduced spring shot peening equipment from Wellbrite Group to improve the fatigue life reliability of valve springs and plunger springs, making its products leading the industry. status.
Compression springs are subjected to high alternating loads and the maximum stress occurs mostly on the surface of the spring material, so shot blasting/shot peening strengthening is also an ideal process to improve the fatigue strength of compression springs. The spring material causes tensile stress during rolling, drawing, coiling and compression. In addition to the high alternating load conditions after service, the coiling process itself will cause destructive tensile stress on the inner ring of the spring.

Shot blasting/shot peening introduces a counteracting compressive stress, which transforms the surface into a residual pressure layer with a strength of about 150 ksi (1035 MPa). This is 60% of the ultimate tensile strength (UTS) of the spring. As a result, the fatigue life of the spring has been extended to 500,000 load cycles without failure.

In addition to compression springs, shot blasting/shot peening is also applicable to the strengthening of other springs such as tension springs, torsion springs, suspension springs, leaf springs, etc. The fatigue failure point usually occurs in the place where the residual tensile stress and the alternating load stress are concentrated, so the strengthening position of different springs is different.

A New Technology and New Process for Modification of Ultra-cold Treatment Materials

The instability of metallic materials and non-metals (deformation of the structure and internal stress caused by processing) has caused instability in production, and over-reliance on workers has always plagued the industry. The company adheres to the spirit of “first-class technology, first-class efficiency, and first-class service” to ensure effective solutions to material deformation, ensure processing dimensional accuracy, increase wear resistance, increase elasticity, eliminate stress, and refine the organization, thereby improving quality. cut costs.
Ultra-cold technology is the only processing technology that can be used on the tools, knives, and parts that have been formed after the material is heat treated. It can stabilize the precise dimensions of the material, improve the wear resistance of the material, and restore the mechanical properties of the material.

The treatment of materials by ultra-cold technology is not limited to the surface treatment of the material, but also penetrates into the internal structure of the material, which reflects the overall effect, especially the regrind of the cutting tool, does not affect the organization structure, can be used repeatedly, and its reusable performance Significantly better than coating technology. The ultra-cold treatment technology can effectively reduce the quenching stress and enhance the dimensional stability of the workpiece.

The competitiveness of traditional industries is facing the transformation and upgrading of the industrial structure, and correct changes must be made. The guarantee of product quality will change from competitive conditions to survival conditions. The improvement of the efficiency of industrial products is a problem faced by industrialists. However, the basic engineering of metal materials is more important. The basic work of heat treatment makes the quality of industrial products not perfect. Although heat treatment gives life to metal materials, it does not give life and efficiency. In addition to the previous heat treatment, the complete basic project also includes the subsequent metal ultra-cold treatment, which is the basic work to ensure product quality. Deep Cryogenic Treatment will be the only choice for the quality of metal products.

Ultra-cold application industries include: precision stamping molds, nano materials, precision plastic molds, cutting tools, hobbing tools, aluminum alloy materials, cemented carbide cutting tools/fixtures, powder metallurgy molds, etc.

Ultra-cold treatment for high-speed steel

In the process of ultra-cold treatment, a large amount of retained austenite in the metal is transformed into martensite, which reduces the saturation of the supersaturated metastable martensite, reduces the microscopic stress, precipitates and disperses, and precipitates dispersed ultrafine carbides When the material is plastically deformed, it effectively hinders the dislocation movement, thereby effectively strengthening the matrix structure.

How to heat treatment of die casting mold parts

Heat treatment of die casting mold parts:
1. The quenching equipment is a vacuum quenching furnace with high pressure and high flow rate.

(1) Before quenching: heat balance method is adopted to improve the overall consistency of mold heating and cooling. All thin-walled holes, grooves, and cavities that affect this point must be filled and blocked, and the mold can be heated and cooled as much as possible; at the same time, pay attention to the furnace installation method to prevent the die-casting mold from being caused by high temperatures. Deformation caused by its own weight.

(2) The heating of the mold: During the heating process, it should be heated slowly (200℃/h heating), and a two-stage preheating method should be adopted to prevent the rapid heating from causing excessive temperature difference between the inside and outside of the mold, causing excessive thermal stress. At the same time, the phase change stress is reduced.

(3) Quenching temperature and holding time: the lower limit of the quenching heating temperature should be used, and the soaking time should not be too short or too long. The soaking time is generally determined by the wall thickness and hardness.

(4) Quenching and cooling: using pre-cooling, and by adjusting the air pressure and wind speed, the cooling rate is effectively controlled to maximize the ideal cooling. Namely: After pre-cooling to 850℃, increase the cooling rate and quickly pass the nose of the “C” curve. When the mold temperature is below 500℃, the cooling rate will gradually decrease. When the temperature is below the Ms point, the cooling method of approximately isothermal transformation is adopted to maximize Minimize quenching deformation. When the mold is cooled to about 150°C, turn off the cooling fan and let the mold cool naturally.

2. Annealing includes two parts: spheroidizing annealing after forging and stress relief annealing during mold making. Its main purpose is to improve the crystalline structure in the raw material stage; facilitate processing and reduce hardness; prevent deformation and quenching cracks after processing to remove internal stress.

(1) Spheroidizing annealing. After the die steel is forged, the internal structure of the steel becomes unstable crystals, and it is difficult to cut with high hardness. The steel in this state has high internal stress, is easily deformed and quenched after processing, and has poor mechanical properties. In order to crystallize carbides To become a spheroidized stable structure, spheroidizing annealing is required.

(2) Stress relief annealing. Machining die steel with residual stress will deform after machining. If there is still stress after machining, great deformation or quenching cracks will occur during quenching. To prevent these problems from occurring, stress relief annealing must be carried out.

Generally, three times of stress relief annealing are carried out during the mold making process:

(1) When cutting away more than 1/3 of the raw material volume or deep processing the raw material thickness 1/2, the machining allowance is left 5-10mm, and the first stress relief annealing is performed.

(2) When there is a margin (2-5mm) for finishing, perform the second stress relief annealing.

(3) After the mold test, the third stress relief annealing is carried out before quenching.

3. When the tempered and quenched mold is cooled to about 100°C, it must be tempered immediately to prevent further deformation or even cracking. The tempering temperature is determined by the working hardness, generally three times of tempering.

4. Nitriding treatment Generally, die-casting molds can be used after quenching and tempering (45~47HRC), but in order to improve the wear resistance, corrosion resistance and oxidation resistance of the mold, prevent mold sticking, and extend the life of the mold, it is necessary Carry out nitriding treatment. The depth of the nitride layer is generally 0.15 to 0.2 mm. After nitriding, it needs to be polished to remove the white layer (thickness about 0.01mm).

5. A few explanations

(1) The heat treatment deformation of the mold is caused by the combined action of phase transformation stress and thermal stress, and is affected by many factors. Therefore, under the premise of correct selection of materials, attention should be paid to the forging of the blank, and the method of six-sided forging should be adopted, and the upsetting should be repeated. At the same time, in the design stage of the mold, it is necessary to pay attention to make the wall thickness as uniform as possible (open process holes when the wall thickness is uneven); for molds with complex shapes, use the mosaic structure instead of the overall structure; for thin-walled molds , Sharp corners of the mold, it is necessary to use fillet transition and increase the fillet radius. During the heat treatment, it is necessary to make a good data record, the amount of deformation in each direction of length, width, and thickness, and the heat treatment conditions (furnace installation method, heating temperature, cooling rate, hardness, etc.), to accumulate experience for the heat treatment of the mold in the future.

(2) There are generally two technological processes for the processing of die-casting molds, both of which are determined according to actual conditions. The first type: general die-casting mold. Forging → spheroidizing annealing → rough machining → first stress relief annealing (leave a margin of 5~10mm) → rough machining → second stress relief annealing (leave a margin of 2~5mm) → finishing → third Secondary stress relief annealing (after mold trial, before quenching) → quenching → tempering → clamp repair → nitriding. The second type: particularly complex and easily deformed molds after quenching. Forging → spheroidizing annealing → rough machining → the first stress relief annealing (leave a margin of 5-10mm) → quenching → tempering → mechanical and electrical machining → the second stress relief annealing (leave a margin of 2-5mm )→Machine and electrical machining→Third stress relief annealing (after mold trial)→Clamping→Nitriding.

Hot-dip galvanizing post-treatment technology

The post-treatment process includes passivation, pre-phosphating and oiling. Passivation treatment can improve the surface structure and gloss of the galvanized layer, enhance the corrosion resistance of the galvanized layer, prolong its service life, and enhance the bonding force between the coating and the base metal. The current passivation treatment mainly uses chromate passivation. Add some activators, such as fluoride, phosphoric acid or sulfuric acid, etc. to the passivation solution to get a thicker chromate film after passivation. When there is fluoride in the passivation solution, it can weaken the surface tension of the strip steel, accelerate the film-forming reaction, and can enhance the chemical polishing effect to make the passivation film fine and bright. For hot-dip galvanized steel sheets for construction, Japan Steel Pipe Company has developed a special chromate passivation coating with excellent corrosion resistance and black rust resistance.
Over the years, relevant researchers have conducted a lot of research on non-toxic or low-toxic inorganic corrosion inhibitors as passivating agents. Molybdate is one of them, its toxicity is lower than that of chromate, but the corrosion resistance after passivation is only equivalent to low Cr passivation. A small amount of molybdate and phosphate are added to the non-toxic water-soluble acrylic resin to obtain a passivation solution instead of toxic chromate for passivation treatment. The results show that the hot-dip galvanized layer is passivated with the non-toxic passivation solution, which can delay the appearance of white rust on the galvanized layer, and its corrosion resistance is close to that of chromate passivation water. Although various chromium-free passivation processes have been reported in the literature, there is no chromium-free passivation process that can completely replace the chromate passivation process. Some chromium-free passivation processes are equivalent to chromate passivation in some aspects, but its market prospects, application scope and environmental protection effects need to be further studied. However, the replacement of chromate passivation by chromium-free passivation is a general trend.

At present, there are few hot-dip galvanizing units with pre-phosphating capacity. The pre-phosphorized film can not only solve the problem of rust prevention during storage and transportation of hot-dip galvanized sheets that are relatively easy to rust before, but also play a role as a solid lubricating film in the stamping and forming process of automobile sheets. Reduce the surface friction coefficient of the hot-dip galvanized sheet, and effectively reduce the defects such as zinc layer shedding and powdering that are easy to produce after the hot-dip galvanized sheet is rubbed with the mold. Therefore, hot-dip galvanized pre-phosphorized sheets have been widely promoted in some steel plants in Europe, the United States, Japan, etc., and have become an effective technology to solve the cracking problem of difficult stamping parts. Therefore, it is foreseeable that the unit online pre-phosphating has great development potential.

The control measures of spring steel reed brittleness and heat treatment crack defects

1. Brittleness is one of the common defects of reeds, which can be divided into heat treatment brittleness and hydrogen embrittlement after galvanizing. Microscopically, hydrogen embrittlement fractures have intergranular, dimples, secondary cracks, etc., and hair lines and hydrogen micropores can also be observed on the fracture surface.
1) Heat treatment brittleness

Spring steel has a tendency to heat sensitivity and temper brittleness, and its tempering temperature is often just right at the junction of the first type of temper brittleness and the second type of temper brittleness. If it cannot be tempered, kept warm and cooled in time, it will cause the spring The flakes are brittle.

Solution: Control the tempering temperature, and timely temper, heat preservation and cooling.

2) Hydrogen embrittlement

After hot-dip galvanizing, it is easy to increase the brittleness of the reed. This is because before galvanizing, pickling must be carried out to remove the oxide scale, which will cause a part of hydrogen to penetrate into the coating and the base metal, weakening the bond between the metal crystal atoms on the grain boundary Force and generate internal stress, making the reed brittle, that is, “hydrogen embrittlement.”

Solution: Control the pickling time and temperature, standardize the galvanizing process, and after galvanizing, keep it at 180°C for several hours before proceeding with the hydrogen removal process, which can reduce the brittleness of the coating and structure of the reed.

2. Heat treatment cracks

Heat treatment generally has a relatively small probability of crack defects in the reed. If cracks occur, the reasons are: improper heat treatment process, improper adjustment of tooling, and operator error. Improper heat treatment process means that the heating temperature of the reed is too high and the holding time is too long, which causes the austenite grains in the reed structure to be coarsened. After quenching, the martensite needles are coarse, resulting in the increase of internal stress and brittleness of the reed. Form heat treatment cracks.

Solution: Use a mesh belt furnace to process the reeds, control the heating temperature to 790℃, control the holding time, direct oil quenching, and temper in time after cleaning, which can effectively solve the heat treatment crack defects of the reeds.

Sangao mining machinery gold ore dressing equipment new technology improves grade

The new technology of Sangao Mining Machinery’s gold beneficiation equipment meets the different requirements of customers. It is equipped with a variety of gold beneficiation processes. The successfully developed and successfully developed gold ore flotation leaching desulfurization process has been well applied in practice and effectively improved the beneficiation grade. And the recovery rate can reach 98%.
The content of gold in the ore is extremely low. In order to extract gold, the ore needs to be crushed and finely ground and pre-enriched or separated from the ore by beneficiation methods. Gravity separation and flotation are frequently used in gold beneficiation. Gravity separation occupies a very important position in the production of placer gold. Flotation is a beneficiation method widely used in rock gold mines. At present, about 80% of rock gold mines in my country Using this method to select gold, the level of beneficiation technology and equipment has been greatly improved.

The amalgamation gold extraction process is an ancient gold extraction process, which is simple and economical, and is suitable for the recovery of coarse-grained monomer gold. Many gold mines in my country still use this method. With the development of gold production and the progress of science and technology, the gold extraction process of amalgamation method has been continuously improved and perfected. Due to the increasingly stringent environmental protection requirements, some mines have cancelled the amalgamation operation and replaced it with the gold extraction process by gravity separation, flotation and cyanidation.

In gold production, the gold extraction process by amalgamation still plays an important role, and there are application examples at home and abroad. Many gold mines such as Zhangjiakou in Hebei, Erdaogou in Liaoning, Jiapigou in Jilin and Yinan in Shandong have applied this process. The Erdaogou gold mine in Liaoning was originally a single flotation process. According to the nature of the ore, it was changed to a combined amalgamation and flotation process. The total recovery rate was increased by 7.81% (the amalgamation recovery rate was 64.6%), and the tailing grade was increased from 0.74g/t Reduced to 0.32g/t, the annual benefit is 1.58 million yuan. The key to the amalgamation gold extraction process is how to take protective measures to eliminate mercury poisoning pollution.

The Sangao gold beneficiation production line is composed of jaw crusher, ball mill, classifier, flotation machine, leaching adsorption tank, thickener and dryer, etc., and can be combined with feeder, hoist and conveyor to form a complete beneficiation production line. The production line has the advantages of high efficiency, energy saving, high processing capacity and reasonable economy.

The production process of the beneficiation production line is as follows: the mined ore is initially crushed by a jaw crusher, and after being crushed to a reasonable fineness, it is evenly sent to the ball mill through a hoist and a miner, and the ore is crushed and ground by the ball mill. The ore fines ground by the ball mill enter the next process: classification. The spiral classifier cleans and classifies the ore mixture based on the principle that the solid particles have different specific gravity and different precipitation speeds in the liquid. When the cleaned and graded mineral mixture passes through the magnetic separator, due to the different specific magnetic susceptibility coefficients of various minerals, the magnetic substances in the mixture are separated through magnetic and mechanical forces. After the initial separation of the magnetic separator, the mineral particles are sent to the flotation machine, and different drugs are added according to the characteristics of different minerals to separate the desired minerals from other substances. After the required minerals are separated, because they contain a large amount of water, they must be initially concentrated by a thickener and then dried by a dryer to obtain dry minerals.

Heat treatment stress and its influence

Heat treatment residual force refers to the residual stress of the workpiece after heat treatment, which has an extremely important influence on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it will cause deformation of the workpiece, and when it exceeds the strength limit of the material, it will crack the workpiece. This is its harmful side and should be reduced and eliminated. But under certain conditions, controlling the stress to make it reasonably distributed can improve the mechanical performance and service life of the parts, and turn harmful to beneficial. Analyzing the distribution and change law of stress in steel during heat treatment, and making it reasonable distribution has far-reaching practical significance for improving product quality. For example, the influence of the reasonable distribution of surface residual compressive stress on the service life of parts has attracted people’s attention.

(1) Heat treatment stress of steel

In the process of heating and cooling of the workpiece, due to the inconsistency of the cooling rate and time between the surface and the core, a temperature difference is formed, which will cause uneven volume expansion and contraction to produce stress, that is, thermal stress. Under the action of thermal stress, because the surface temperature is lower than the core part, and the contraction is greater than the core part, the core part is stretched. When the cooling is over, the final cooling volume contraction of the core part cannot proceed freely and the surface layer is compressed. Stretched. That is, under the action of thermal stress, the surface of the workpiece is compressed and the core is pulled. This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. When the cooling rate is faster, the carbon content and alloy composition are higher, and the uneven plastic deformation under the action of thermal stress during the cooling process is larger, and the residual stress formed in the end is larger. On the other hand, due to the change of the structure of the steel during the heat treatment process, that is, the transformation of austenite to martensite, the increase in specific volume will be accompanied by the expansion of the volume of the workpiece, and the phases of each part of the workpiece will change sequentially, resulting in inconsistent volume growth and structure. stress. The final result of the change of tissue stress is the tensile stress on the surface and the compressive stress on the core, which is just the opposite of the thermal stress. The size of the structure stress is related to the cooling rate, shape, and chemical composition of the material in the martensite transformation zone.

Practice has proved that as long as there is phase change in any workpiece during heat treatment, thermal stress and structural stress will occur. It’s just that the thermal stress has been generated before the structure transformation, and the structure stress is generated in the process of the structure transformation. During the entire cooling process, the result of the combined effect of the thermal stress and the structure stress is the actual stress in the workpiece. The result of the combined effect of these two stresses is very complex, and is affected by many factors, such as composition, shape, heat treatment process, etc. As far as its development process is concerned, there are only two types, namely thermal stress and tissue stress. When the direction of action is opposite, the two cancel out, and when the direction of action is the same, the two are superimposed. Regardless of whether they are mutually offset or superimposed on each other, the two stresses should have a dominant factor. When the thermal stress is dominant, the result is that the core of the workpiece is pulled and the surface is compressed. The result of the effect when the tissue stress is dominant is the tension of the compressed surface of the workpiece core.

(2) The effect of heat treatment stress on quenching cracks

The factors that can cause stress concentration (including metallurgical defects) in different parts of the quenched part have a promoting effect on the generation of quenching cracks, but only in the tensile stress field (especially under the maximum tensile stress). Come out, if there is no promoting effect in the compressive stress field.

Quenching cooling rate is an important factor that can affect the quality of quenching and determine the residual stress, and it is also a factor that can have an important and even decisive influence on quenching cracks. In order to achieve the purpose of quenching, it is usually necessary to accelerate the cooling rate of the parts in the high temperature section and make it exceed the critical quenching cooling rate of steel to obtain the martensite structure. As far as the residual stress is concerned, this can increase the thermal stress value that offsets the effect of the structural stress, so it can reduce the tensile stress on the surface of the workpiece to achieve the purpose of suppressing longitudinal cracks. The effect will increase as the high temperature cooling rate increases. Moreover, in the case of hardening, the larger the cross-sectional size of the workpiece, although the actual cooling rate is slower, the risk of cracking is greater. All this is due to the fact that the thermal stress of this type of steel slows down with the increase in size, the thermal stress decreases, and the structural stress increases with the increase in size, and finally the tensile stress mainly formed by the structural stress acts on the workpiece. Caused by the characteristics of the surface. And it is very different from the traditional concept that the slower the cooling, the smaller the stress. For this kind of steel parts, only longitudinal cracks can be formed in high hardenability steel parts that are quenched under normal conditions. The reliable principle to avoid quench cracking is to try to minimize the unequal time of martensite transformation inside and outside the section. Merely implementing slow cooling in the martensite transformation zone is not enough to prevent the formation of longitudinal cracks. Under normal circumstances, cracks can only be generated in non-hardenable parts. Although the overall rapid cooling is the necessary formation condition, the real reason for its formation is not the rapid cooling (including the martensite transformation zone) itself, but It is the local position of the quenched part (determined by the geometric structure), and the cooling rate in the high temperature critical temperature zone is significantly slowed down, so there is no hardening. The transverse fractures and longitudinal splits produced in large non-hardenable parts are caused by residual tensile stress with thermal stress as the main component acting on the center of the quenched part, and at the center of the quenched section of the quenched part, cracks are first formed and caused by Caused by expansion from the inside out. In order to avoid such cracks, the water-oil double liquid quenching process is often used. In this process, the purpose of implementing rapid cooling in the high temperature section is only to ensure that the outer layer of metal obtains the martensite structure. From the perspective of internal stress, rapid cooling is harmful and useless at this time. Secondly, the purpose of slow cooling in the later stage of cooling is not to reduce the expansion rate of martensite transformation and the value of organizational stress, but to minimize the temperature difference of the section and the shrinkage rate of the metal in the center of the section, so as to reduce the stress value and finally The purpose of suppressing cracking.

Several common heat treatment concepts

1. Normalizing: The heat treatment process of heating steel or steel parts to an appropriate temperature above the critical point Ac3 or Acm for a certain period of time and then cooling in the air to obtain a pearlite structure.

2. Annealing: heat the hypoeutectoid steel workpiece to 30-50 degrees above Ac3, after holding for a period of time, slowly cool with the furnace (or be buried in sand or lime) to below 500 degrees and cool in the air. Craft

3. Solution heat treatment: heat the alloy to a high-temperature single-phase zone to maintain a constant temperature, so that the excess phase is fully dissolved and rapidly cooled to obtain a supersaturated solid solution heat treatment process

4. Aging: After solution heat treatment or cold plastic deformation, the performance of alloy changes with time when it is placed at room temperature or kept slightly higher than 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 soften, so as to continue processing and forming

6. Aging treatment: heat and keep warm at the temperature at which the strengthening phase precipitates, so that the strengthening phase precipitates out, hardened and improved strength

7. Quenching: After austenitizing the steel, it is cooled at an appropriate cooling rate to make the workpiece dissolve into the solid solution in all or a certain range in the cross section, and then heat treatment for the transformation of unstable structure such as martensite. Craft

8. Tempering: heat the quenched workpiece to an appropriate temperature below the critical point Ac1 for a certain period of time, and then use a method that meets the requirements to cool it to obtain the required structure and performance of the heat treatment process

9. Nitriding and carbonitriding of steel

(1). Nitriding of steel (gas nitriding)

Concept: Nitriding is a process of infiltrating nitrogen atoms into the surface layer of steel. Its purpose is to improve surface hardness and wear resistance, as well as fatigue strength and corrosion resistance.

It uses ammonia gas to decompose active nitrogen atoms when heated, and forms a nitride layer on the surface after being absorbed by the steel, while diffusing to the center.

Nitriding is usually carried out using specialized equipment or pit carburizing furnace. Suitable for various high-speed transmission precision gears, machine tool spindles (such as boring bars, grinder spindles), high-speed diesel engine crankshafts, valves, etc.

Process route of nitriding workpiece: forging-annealing-rough machining-quenching and tempering-finishing-stress removal-rough grinding-nitriding-fine grinding or grinding.

Because the nitrided layer is thin and brittle, it requires a high-strength core structure. Therefore, it is necessary to perform quenching and tempering heat treatment first to obtain tempered sorbite and improve the mechanical properties of the core and the quality of the nitrided layer.

After nitriding, the steel no longer needs to be quenched and has high surface hardness and wear resistance.

The nitriding treatment temperature is low and the deformation is small. Compared with carburizing and induction surface quenching, the deformation is much smaller

(2) Carbonitriding of steel: Carbonitriding is a process of simultaneously infiltrating carbon and nitrogen into the surface of the steel. Carbonitriding is customarily called cyanidation. At present, medium temperature gas carbonitriding and low temperature gas nitrocarburizing (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 the heat treatment of quenching and high temperature tempering as 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, the tempered sorbite structure is obtained, and its mechanical properties are better than the normalized 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 size of the workpiece section, generally between HB200-350.

11. Brazing: a heat treatment process for bonding two workpieces together with brazing filler metal


(1). Types of annealing

1. Complete annealing and isothermal annealing

Complete annealing is also called recrystallization annealing, generally referred to as annealing. This annealing is mainly used for casting, forging and hot-rolled sections of various carbon steels and alloy steels with hypoeutectoid compositions, and sometimes also used for welded structures. Generally used as the final heat treatment of some unimportant workpieces, or as the pre-heat treatment of some workpieces.

2. Spheroidizing annealing

Spheroidizing annealing is mainly used for hypereutectoid carbon steels and alloy tool steels (such as steel grades used in manufacturing cutting tools, measuring tools, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching.

3. Stress relief annealing

Stress relief annealing is also called low temperature annealing (or high temperature tempering). This annealing is mainly used to eliminate residual stress in castings, forgings, welded parts, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, the steel parts will be deformed or cracked after a certain period of time or in the subsequent cutting process.

(Two) quenching

In order to improve the hardness, the main form is heating, heat preservation, and rapid cooling. The most commonly used cooling media are brine, water and oil. Salt water quenched workpieces are 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 cause serious deformation of the workpiece and even cracks. 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 stability of undercooled austenite.

(3) Tempering

1. Reduce brittleness and eliminate or reduce internal stress. There is a large internal stress and brittleness of steel parts after quenching. If not tempered in time, the steel parts 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 appropriate tempering to reduce the brittleness and obtain the required Toughness, 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.

Metal heat treatment 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 and uninterrupted.

Heating is one of the important steps of heat treatment. There are many heating methods for metal heat treatment. The earliest was to use charcoal and coal as heat sources, and then to use liquid and gas fuels. The application of electricity makes heating easy to control without environmental pollution. These heat sources can be used for direct heating, or indirect heating by molten salt or metal, 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 of the surface of the steel parts is reduced), which has a very negative effect on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be heated 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 is the main issue 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 the required structure. In addition, the transformation takes 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 microstructure transformation. This 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 or the holding time is very short, 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 varies from process to process, and the main thing is to control the cooling rate. Generally, annealing has the slowest cooling rate, normalizing cooling rate is faster, and quenching cooling rate is faster. However, there are different requirements due to different steel grades. For example, hollow hard steel can be quenched at the same cooling rate as normalizing.

Metal heat treatment processes can be roughly divided into overall heat treatment, surface heat treatment, partial heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal uses different heat treatment processes to obtain different structures and thus have different properties. Steel is the most widely used metal in the industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes.

Overall heat treatment is a metal heat treatment process that heats the entire workpiece and then cools it at an appropriate speed to change its overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.

Annealing is to heat the workpiece to an appropriate temperature, use different holding time 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 use performance, or for further quenching Prepare for 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 annealing, except that the resulting structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for parts that are not demanding. As the final heat treatment.

Quenching is to quickly cool the workpiece in a quenching medium such as water, oil, other inorganic salts, and organic aqueous solutions after heating and holding the workpiece. After quenching, the steel parts become hard, but at the same time become brittle. 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 710°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. Quenching and tempering are closely related and are often used in conjunction with each other.

The “Four Fires” have 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 a supersaturated solid solution, they are kept at room temperature or a slightly higher temperature for a longer period of time to improve the hardness, strength, or electrical magnetism of the alloy. Such a heat treatment process is called aging treatment. The method of effectively and tightly combining pressure processing deformation and heat treatment to obtain good strength and toughness of the workpiece is called thermomechanical heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only make The workpiece is not oxidized or decarburized. It can keep the surface of the workpiece smooth and clean after treatment and improve the performance of the workpiece. It can also be injected with a penetrant for 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 heat only the surface of the workpiece without introducing excessive heat into the interior of the workpiece, the heat source used must have a high energy density, that is, to give a larger heat energy to the workpiece per unit area, so that the surface or part of the workpiece can be short-term or instantaneous Reach high temperatures. The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment. Commonly used heat sources include flames such as oxygen acetylene or oxypropane, induction current, laser and electron beam.

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

Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. Generally speaking, 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 after long-term annealing treatment, which can 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, cheap carbon steel can be penetrated Certain alloying elements have the properties of some expensive alloy steels, which can replace certain heat-resistant steels and stainless steels; almost all tools and molds require heat treatment before they can be used.