Heat treatment strengthening technology of spring

In my country, the heat treatment of springs with wire less than 15mm, oil-quenched and tempered steel wire and toughened steel all use protective atmosphere heat treatment. Protective atmosphere heat treatment can eliminate surface decarburization and oxidation, and improve the surface quality of the material.
Protective atmosphere induction heating heat treatment is generally performed on the wire before the coil spring is formed. Some spring factories combine the heat treatment of the wire material with the spring production to reduce costs. The induction heating treatment has a good strengthening effect, and the induction heating speed is fast, which helps to refine the crystal grains and reduce surface decarburization, and can give full play to and improve the strength and toughness of the material.

Heat treatment of steel materials

Serial number name meaning

1 Small crystals with inconsistent appearance and the same internal lattice orientation formed after the crystallization of crystal grains and grain boundary metals are called crystal grains. The interface between the grain and the grain is called the grain boundary

2 Phases and phase boundaries In metals or alloys, all homogeneous components with the same composition, the same structure, and the interface separated from each other are called phases. The interface between phases is called phase boundary

3 Solid solution A solid phase formed by dissolving atoms of another element in the crystals of one metal element constituting the alloy is called a solid solution. Solid solution generally has higher strength, good plasticity, corrosion resistance, and higher resistance and magnetism

4 The solid phase with metallic characteristics formed by the interaction of the atoms of different elements in the metal compound alloy, the lattice type and performance are completely different from its constituent elements, is called a metal compound

5 Austenite Austenite is a solid solution in which carbon and other elements are dissolved in y-Fe. Austenite has face-centered cubic crystals with good plasticity and generally exists at high temperatures

6 Ferrite Ferrite is a solid solution in which carbon and other elements are dissolved in a-Fe. Ferrite has a body-centered cubic lattice and contains very little carbon. Its performance is very similar to that of pure iron. It is also called pure ferrite.

7 Cementite Cementite is a compound of iron and carbon, also known as iron carbide (№c), with a carbon content of 6.69% and a complicated character structure. Its performance is hard and brittle, almost no plasticity

8 Pearlite Pearlite is a lamellar structure between ferrite and cementite. It is named for its fingerprint-like pearly luster in its microstructure. Its performance is between ferrite and cementite, moderate strength, hardness, and good plasticity and toughness

9Sortenite, also known as fine pearlite, is a mixture of ferrite and cementite decomposed by austenite at a temperature lower than the formation temperature of pearlite. Its layers are thinner than pearlite and can only be distinguished under a high-power microscope. Hardness, strength and impact toughness are all higher than pearlite

10 Troostite, also known as ultrafine pearlite, is a mixture of ferrite and cementite decomposed by austenite at a temperature lower than that of pearlite. The layers are thinner than sorbite. Its hardness and strength are higher than sorbite

11 Bainite Bainite is a mixture of supersaturated ferrite and cementite. Bainite is divided into upper bainite and lower bainite. Formed at higher temperature is called “upper bainite”, which is feather-shaped; formed at lower temperature is called “lower bainite”, which is needle-shaped or bamboo leaf-shaped. Compared with upper bainite, lower bainite has higher hardness and strength, and maintains certain toughness and plasticity.

12 Martensite Martensite usually refers to a supersaturated solid solution of carbon in a-Fe. The hardness of martensite in steel increases with the increase of carbon content. High-carbon martensite has high hardness and brittleness, while low-carbon martensite has higher toughness. Martensite has the highest hardness among austenite transformation products

13 ledeburite is a kind of eutectic structure in carbon alloy. At high temperature, it is composed of austenite and cementite; at low temperature (below 727°C), it is composed of pearlite and cementite. The carbon content is 4.3%, and the structure contains a lot of cementite, so the hardness is high, the plasticity and toughness are low

14 Fracture inspection Fracture structure is one of steel quality marks. After notching or breaking the sample, check the fracture with the naked eye or 10 times magnifying glass, which is called fracture inspection. The defects of the metal can be seen from the fracture

15 tower turning hair

Pattern inspection: Turn the steel into a prescribed tower or stepped sample, and then use acid etching or magnetic particle method to inspect the hair pattern, referred to as tower inspection

Note: Carbon content refers to mass fraction

Table 2 General heat treatment of steel materials

Name Heat Treatment Process Heat Treatment Purpose

1. Annealing heats the steel to a certain temperature, keeps it for a certain period of time, and then slowly cools to room temperature ①Reduce the hardness of the steel and increase the plasticity to facilitate cutting and cold deformation processing

② Refine the grains, uniform the structure of the steel, improve the performance of the steel and prepare for the subsequent heat treatment

③ Eliminate internal stress in steel. Prevent deformation and cracking of parts after processing

Retreat

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Don’t (1) complete annealing to heat the steel to the critical temperature (the critical temperature of different steels is also different, generally 710-750℃, the critical temperature of individual alloy steels can reach 800-900℃) above 30-50℃, heat preservation for a certain period of time, then Slowly cooling with the furnace (or buried in sand) to refine grains, uniform structure, reduce hardness, and fully eliminate internal stress. Complete annealing is suitable for forgings or steel castings with carbon content (mass fraction) below 0.8%

(2) Spheroidizing annealing heats the steel to 20~30ºC above the critical temperature. After heat preservation, it is slowly cooled to below 500℃ and then air-cooled to reduce the hardness of the steel, improve the cutting performance, and prepare for subsequent quenching to reduce Deformation and cracking after quenching, spheroidizing annealing is suitable for carbon steel and alloy tool steel with carbon content (mass fraction) greater than 0.8%

(3) Stress-relief annealing heats the steel parts to 500~650ºC, keeps them for a certain period of time, and then slowly cools them (usually cooling with the furnace) to eliminate the internal stress generated during welding and cold straightening of steel parts, and eliminate the generation during cutting of precision parts Internal stress to prevent deformation during subsequent processing and use

Stress relief annealing is suitable for all kinds of castings, forgings, welded parts and cold extruded parts, etc.

2. Normalizing heat the steel parts to 40~60ºC above the critical temperature, keep it for a certain period of time, and then cool it in the air ①Improve the structure and cutting performance

② Normalizing is often used as the final heat treatment for parts with low requirements for mechanical properties

③ Eliminate internal stress

3. Quenching heats the steel parts to the quenching temperature, keeps them warm for a period of time, and then rapidly cools them in water, salt water or oil (individual materials are in the air) ① to make the steel parts have higher hardness and wear resistance

②Make the steel parts get some special properties after tempering, such as higher strength, elasticity and toughness, etc.

Quench

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(1) Single-liquid quenching heats the steel parts to the quenching temperature, and after heat preservation, they are cooled in a quenching agent

Single-liquid quenching is only suitable for carbon steel and alloy steel parts with simple shapes and low technical requirements. When quenching, for carbon steel parts with a diameter or thickness greater than 5-8mm, choose salt water or water cooling; alloy steel parts choose oil cooling

(2) Double-liquid quenching heats the steel to the quenching temperature. After heat preservation, it is quickly cooled to 300-400ºC in water, and then moved to oil for cooling

(3) Flame surface quenching uses acetylene and oxygen mixed combustion flame to spray to the surface of the part to quickly heat the part to the quenching temperature, and then immediately spray water to the surface of the part. Flame surface quenching is suitable for single-piece or small batch production, and the surface requires hard Large medium-carbon steel and medium-carbon alloy steel parts that are wear-resistant and can withstand impact loads, such as crankshafts, gears and guide rails, etc.

(4) Surface induction hardening

The fire puts the steel part in the inductor, the inductor generates a magnetic field under the action of a certain frequency of alternating current, and the steel part generates an induced current under the action of the magnetic field, so that the surface of the steel part is rapidly heated (2-10min) to the quenching temperature. Spray water on the surface of the steel immediately.

The surface induction hardened parts have hard and wear-resistant surfaces, while the core maintains good strength and toughness.

Surface induction hardening is suitable for medium carbon steel and alloy steel parts with medium carbon content

4. Tempering heats the quenched steel parts below the critical temperature, keeps it for a period of time, and then cools it in air or oil

Tempering is carried out immediately after quenching, and is also the last process of heat treatment① to obtain the required mechanical properties. Under normal circumstances, the strength and hardness of the parts after quenching are greatly improved, but the plasticity and toughness are significantly reduced, and the actual working conditions of the parts require good strength and toughness. After selecting the appropriate tempering temperature for tempering, the required mechanical properties can be obtained

②Stable organization, stable size

③ Eliminate internal stress

(1) Low-temperature tempering heats hardened steel parts to 150-50ºC, and keeps them at this temperature for a certain period of time, and then cools them in air. Low-temperature tempering is mostly used for cutting tools, measuring tools, molds, rolling bearings and carburized parts, etc. Eliminate internal stress caused by quenching of steel parts

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(1) Intermediate temperature tempering heats the quenched steel parts to 350-450%, and cools them down after holding for a period of time. It is generally used for various springs and hot die parts to obtain higher elasticity, certain toughness and toughness. hardness

(1) High temperature tempering heats the quenched steel parts to 500-650ºC, and then cools them after heat preservation. They are mainly used for important structural parts requiring high strength and high toughness, such as main shafts, crankshafts, cams, gears and connecting rods. Steel parts obtain better comprehensive mechanical properties, that is, higher strength, toughness and sufficient hardness, and eliminate internal stress caused by quenching of steel parts

5. Quenching and tempering The quenched steel parts are tempered at high temperature (500~600ºC), which are mostly used for important structural parts, such as shafts, gears, connecting rods, etc. Quenching and tempering are generally performed after rough machining to refine the grains. Make steel parts obtain higher toughness and sufficient strength, so that it has good comprehensive mechanical properties

6.

Aging treatment (1) Artificial aging heats the quenched steel parts to 100~160℃, after a long time of heat preservation, and then cooling to eliminate internal stress, reduce part deformation, stabilize size, and is more important for parts with higher precision

(2) Naturally aging castings are placed in the open air; steel parts (such as long shafts, lead screws, etc.) are placed in sea water or suspended for a long time or lightly tapped. Parts that have undergone natural aging are preferably roughed first

7. Chemical heat treatment puts the steel parts in a chemical medium containing some active atoms (such as carbon, nitrogen, chromium, etc.), and makes certain atoms in the medium penetrate into the surface of the steel parts through heating, heat preservation, cooling and other methods , So as to change the chemical composition of the surface of the steel part, so that the surface of the steel part has a certain special performance

Transform

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Place

(1) The carbon infiltration of steel infiltrates carbon atoms into the surface of steel parts

Commonly used for wear-resistant and impacted parts, such as wheels, gears, shafts, piston pins, etc., so that the surface has high hardness (HRC60~65) and wear resistance, while the center still maintains high toughness

(2) Nitriding of steel infiltrates nitrogen atoms into the surface of steel parts

It is often used for important bolts, nuts, pins and other parts to improve the hardness, wear resistance, and

Corrosion resistance

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(3) The cyanidation of steel can infiltrate carbon and nitrogen atoms into the surface of steel parts at the same time. It is suitable for low-carbon steel, medium-carbon steel or alloy steel parts, and can also be used for high-speed steel tools to improve the hardness and wear resistance of steel parts.

8. Blackening Put the metal parts in a very concentrated alkali and oxidant solution to heat and oxidize, so that a magnetic ferroferric oxide film is formed on the surface of the metal parts. It is often used in low carbon steel and low carbon alloy tool steel.

Due to the influence of materials and other factors, the film color of the blackening layer is blue-black, black, red-brown, tan, etc., and its thickness is 0.6~O.8µm

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 various parts of the workpiece will change sequentially, resulting in inconsistent volume growth. Generate tissue 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 in the heat treatment process of any workpiece, as long as there is phase change, thermal stress and structural stress will occur. “It’s just that the thermal stress has been generated before the transformation of the organization, and the organizational stress is generated during the transformation of the organization.

During the entire cooling process, the result of the combined effect of the thermal stress and the organizational stress, & 127; is the actual workpiece Existing stress. 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. &127; When the tissue stress is dominant, the result is that the compression surface of the workpiece core is pulled.

2. The effect of heat treatment stress on quenching cracks

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 (&127; especially under the maximum tensile stress) It will show that &127; if there is no crack 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,

and the actual cooling rate decreases, and the thermal stress decreases, &127; the structural stress increases with the increase in size, and finally a tensile stress dominated by structural stress is formed. It is caused by the characteristics of the function on the surface of the workpiece. 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, only arc cracks can occur 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 the local position of the quenched part (determined by the geometric structure), the cooling rate in the high temperature critical temperature zone is significantly slowed down, so there is no hardening caused by. The transverse fractures and longitudinal splits generated 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 first form And caused by the 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 rapid cooling in the high temperature section is only to ensure that the outer layer of metal obtains the martensite structure, and from the point of view 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 structural 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.

3. The influence of residual compressive stress on the workpiece

Carburized surface strengthening is widely used as a method to improve the fatigue strength of workpieces. On the one hand, it can effectively increase the strength and hardness of the workpiece surface and improve the wear resistance of the workpiece. On the other hand, carburizing can effectively improve the stress distribution of the workpiece and obtain a larger residual compressive stress on the surface of the workpiece. &127;

Improve the fatigue strength of the workpiece. If austempering is carried out after carburizing, the residual compressive stress of the surface layer will be increased, and the fatigue strength will be further improved. Someone has tested the residual stress of 35SiMn2MoV steel after carburizing and austempering and quenching after carburizing and low temperature tempering.

Heat treatment process

Residual stress value (kg/mm2) After carburizing, 880-900 degrees salt bath heating, 260 degrees isothermal 40 minutes -65

After carburizing, 880-900 degrees salt bath heating and quenching, 260 degrees isothermal for 90 minutes-18

After carburizing, 880-900 degrees salt bath heating, 260 degrees isothermal for 40 minutes, 260 degrees tempering for 90 minutes -38

From the test results in Table 1, it can be seen that austempering has a higher surface residual compressive stress than the usual quenching low-temperature tempering process. Even if low temperature tempering is performed after austempering, the surface residual compressive stress is higher than that of low temperature tempering after quenching. Therefore, it can be concluded that the surface residual compressive stress obtained by austempering after carburizing is higher than that obtained by low-temperature tempering of common carburizing quenching.

From the viewpoint of the beneficial influence of the residual compressive stress of the surface layer on the fatigue resistance, the The carbon austempering process is an effective method to improve the fatigue strength of carburized parts. Why can the carburizing and quenching process obtain surface residual compressive stress? Why can carburizing austempering obtain greater surface residual compressive stress? There are two main reasons: one is that the surface layer of high-carbon martensite has a lower specific volume than the core.

The specific volume of martensite is large, the volume expansion of the surface layer is large after quenching, and the volume expansion of the low-carbon martensite in the core is small, which restricts the free expansion of the surface layer, and causes the stress state of the surface layer under tension in the core part. And another more important reason is the starting temperature (Ms) for the transformation of high-carbon supercooled austenite to martensite, which is lower than the starting temperature of the transformation of supercooled austenite to martensite ( Ms) low.

This means that in the quenching process, the martensite transformation of the core is often the first to cause the volume of the core to expand and be strengthened, and the surface has not yet cooled to its corresponding martensite starting point (Ms), so it is still in excess Cold austenite state, &127; has good plasticity, and will not play a serious pressing effect on the volume expansion of the core martensite transformation.

As the quenching cooling temperature continues to drop, the surface temperature drops below the (Ms) point, and the surface layer undergoes martensitic transformation, causing the volume of the surface layer to expand. But the core has already been transformed into martensite and strengthened at this time, so the core will have a great compression effect on the volume expansion of the surface layer, making the surface layer obtain the residual compressive stress. &127;

When austempering is performed after carburizing, when the isothermal temperature is above the martensite starting temperature (Ms) of the carburized layer, the martensite starting temperature (&127; Ms) point of the center is the appropriate temperature Austempering is better than continuous cooling and quenching to ensure the characteristics of the sequence of this transformation (&127; that is, to ensure that the surface martensite transformation only occurs in the cooling process after isothermal). &127; Of course,

the isothermal temperature and isothermal time of austempering after carburizing have a great influence on the residual stress of the surface layer. Someone has tested the surface residual stress of 35SiMn2MoV steel sample after carburizing at 260℃ and 320℃ for 40&127; minutes. The results are shown in Table 2. It can be seen from Table 2 that the surface residual stress at 260℃ is more than double the surface residual stress at 320℃.

It can be seen that the surface residual stress state is very sensitive to the isothermal temperature of carburizing austempering. Not only the isothermal temperature has an effect on the surface residual compressive stress state, but also the isothermal time has a certain effect. Someone tested the residual stress of 35SiMn2V steel at 310℃ for 2 minutes, 10 minutes, and 90 minutes. The residual compressive stress is -20kg/mm ​​after 2 minutes, -60kg/mm ​​after 10 minutes, and -80kg/mm ​​after 60 minutes. The residual stress changes little after 60 minutes and the isothermal time is extended.

The above discussion shows that the sequence of the carburized layer and the core martensite transformation has an important effect on the residual stress of the surface layer. Austempering after carburizing is of universal significance to further improve the fatigue life of parts. In addition, surface chemical heat treatment such as carburizing, nitriding, cyanidation, etc., which can reduce the surface martensite starting transformation temperature (Ms) point, provides conditions for causing surface residual compressive stress, such as the nitriding-quenching process of high carbon steel Due to the increase of the nitrogen content in the surface layer,

the martensite starting point (Ms) of the surface layer is reduced, and a higher surface residual compressive stress is obtained after quenching, which improves the fatigue life. Another example is that the cyanidation process tends to have higher fatigue strength and service life than carburizing, and it is also because the increase in nitrogen content can obtain higher surface residual compressive stress than carburizing.

In addition, from the point of view of obtaining a reasonable distribution of surface residual compressive stress, a single surface strengthening process is not easy to obtain an ideal surface residual compressive stress distribution, while a composite surface strengthening process can effectively improve the distribution of surface residual stress. . For example, the residual stress of carburizing and quenching is generally low in surface compressive stress, the maximum compressive stress appears at a certain depth from the surface, and the residual pressure layer is thicker.

The residual compressive stress on the surface after nitriding is very high, but the residual compressive stress layer is very thin and drops sharply inward. If the carburizing-&127; nitriding composite strengthening process is used, a more reasonable stress distribution state can be obtained. &127; Therefore, surface composite strengthening processes, such as carburizing-nitriding, carburizing-& 127; high-frequency quenching, etc., are all directions worthy of attention.

Vacuum heat treatment

Vacuum heat treatment is a metal heat treatment process that heats a metal workpiece under 1 atmosphere pressure (that is, under negative pressure).
In the late 1920s, with the development of electric vacuum technology, a vacuum heat treatment process appeared, which was only used for annealing and degassing at that time. Due to equipment limitations, this process has not made great progress for a long time. From the 1960s to the 1970s, air-cooled vacuum heat treatment furnaces, cold-wall vacuum oil quenching furnaces and vacuum heating high-pressure gas quenching furnaces were successively developed, which enabled new developments in vacuum heat treatment technology. Carburizing in a vacuum, the technological progress of carburizing, nitriding or infiltration of other elements under the action of a plasma field in a vacuum has further expanded the scope of application of vacuum heat treatment.

Characteristics of vacuum heat treatment technology

The heat treatment of metal parts in vacuum can prevent oxidative decarburization and has a degassing effect, but the metal elements may evaporate.

• Prevent the heating chamber of the vacuum heat treatment furnace from oxidative decarburization in a near vacuum state during operation. Only trace amounts of carbon monoxide and hydrogen are present. They are reductive to the heated metal and do not undergo oxidative decarburization reactions; Reduce the formed oxide film, so the surface of the metal workpiece after heating can maintain the original metallic luster and good surface properties.

•Degassing effect When metal parts are heated in a vacuum environment, harmful gases in the metal, such as hydrogen and oxygen in titanium alloys, will escape at high temperatures, which helps to improve the mechanical properties of the metal.

•Evaporation of metal elements. Various elements have their own vapor pressure. If the pressure in the environment is lower than the vapor pressure of a certain element, this element will evaporate. During vacuum heat treatment, the vacuum degree or temperature during heating should be selected according to the vapor pressure of the alloy elements contained in the steel to avoid evaporation of the alloy elements.

Vacuum heat treatment process

Vacuum heat treatment can be used for annealing, degassing, solution heat treatment, quenching, tempering and precipitation hardening processes. After passing the appropriate medium, it can also be used for chemical heat treatment.

There are two types of quenching in vacuum: gas quenching and liquid quenching. Gas quenching is to fill the cooling chamber with high-purity neutral gas (such as nitrogen) for cooling after the workpiece is heated in vacuum. Suitable for gas quenching are high-speed steel and high-carbon high-chromium steel and other materials with low critical cooling rate of martensite. Liquid quenching is to heat the workpiece in the heating chamber, then move it to the cooling chamber to fill with high-purity nitrogen and immediately send it to the quenching oil tank for rapid cooling. If high surface quality is required, the tempering and precipitation hardening after vacuum quenching and solution heat treatment of the workpiece should still be carried out in a vacuum furnace.

Heat treatment process introduction-annealing process

Annealing is a heat treatment process in which metals and alloys are heated to an appropriate temperature, kept for a certain period of time, and then slowly cooled. After annealing, the structure of hypoeutectoid steel is ferrite plus lamellar pearlite; eutectoid steel or hypereutectoid steel is granular pearlite. In short, the annealed structure is a structure close to equilibrium.
Purpose of annealing

①Reduce the hardness of steel and increase plasticity to facilitate cutting and cold deformation processing.

② Refine the grains, eliminate the structural defects caused by casting, forging, and welding, uniform the structure and composition of the steel, improve the performance of the steel or prepare for the subsequent heat treatment.

③ Eliminate internal stress in steel to prevent deformation and cracking.

Types of annealing process

① Homogenization annealing (diffusion annealing)

Homogenization annealing is to reduce the segregation of the chemical composition of metal ingots, castings or forging billets and the unevenness of the structure. It is heated to a high temperature, maintained for a long time, and then slowly cooled, in order to homogenize the chemical composition and structure. The annealing process.

The heating temperature of homogenization annealing is generally Ac3 (150~200℃), that is, 1050~1150℃, and the holding time is generally 10~15h to ensure the full progress of diffusion and eliminate or reduce the unevenness of composition or organization. Because the heating temperature of diffusion annealing is high, the time is long, and the grains are coarse, for this reason, complete annealing or normalizing is performed after diffusion annealing to re-fine the structure.

②Fully annealed

Complete annealing, also known as recrystallization annealing, is an annealing process in which the iron-carbon alloy is completely austenitized and then slowly cooled to obtain a structure close to the equilibrium state.

Complete annealing is mainly used for hypoeutectoid steels, generally medium carbon steel and low and medium carbon alloy structural steel forgings, castings and hot-rolled sections, and sometimes also used for their welding components. Complete annealing is not suitable for hypereutectoid steels, because the complete annealing of hypereutectoid steels needs to be heated to above Acm. During slow cooling, cementite will precipitate along the austenite grain boundaries and distribute in a network shape, resulting in increased brittleness of the material. Leave hidden dangers to the final heat treatment.

The heating temperature for complete annealing is generally Ac3 (30~50℃) for carbon steel; Ac3 (500~70℃) for alloy steel; the holding time depends on the type of steel, the size of the workpiece, the amount of furnace installed, and the selected equipment model And other factors to determine. In order to ensure that the supercooled austenite undergoes the pearlite transformation completely, the cooling of the complete annealing must be slow, and the furnace is cooled to about 500°C and then air-cooled.

③Incomplete annealing

Incomplete annealing is an annealing process in which the iron-carbon alloy is heated to a temperature between Ac1 and Ac3 to achieve incomplete austenitization, followed by slow cooling.

Incomplete annealing is mainly suitable for medium and high carbon steel and low alloy steel forgings, etc. Its purpose is to refine the structure and reduce the hardness. The heating temperature is Ac1 (40-60) ℃, and the temperature is slowly cooled after heat preservation.

④ Isothermal annealing

Isothermal annealing is to heat the steel or blank to a temperature higher than Ac3 (or Ac1), and after keeping it for a proper time, it will quickly cool to a certain temperature in the pearlite temperature range and maintain it isothermally to transform austenite into pearlite. The organization is then cooled in the air by an annealing process.

The isothermal annealing process is applied to medium carbon alloy steel and low alloy steel, and its purpose is to refine the structure and reduce the hardness. The heating temperature of hypoeutectoid steel is Ac3 (30~50)℃, and the heating temperature of hypereutectoid steel is Ac3 (20~40)℃. Keep it for a certain period of time. After the furnace is cooled to slightly lower than the Ar3 temperature, it will undergo isothermal transformation, and then air-cooled out . The structure and hardness of isothermal annealing are more uniform than that of complete annealing.

⑤ Spheroidizing annealing

Spheroidizing annealing is an annealing process to spheroidize carbides in steel. The steel is heated to 20-30°C above Ac1, kept for a period of time, and then slowly cooled to obtain a structure of spherical or granular carbides uniformly distributed on the ferrite matrix.

Spheroidizing annealing is mainly suitable for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. These steels are air-cooled after rolling and forging, and the resulting structure is lamellar pearlite and network cementite. This structure is hard and brittle, not only difficult to cut, but also easily deformed and cracked in the subsequent quenching process. The spherical pearlite structure obtained by spheroidizing annealing, in which cementite is spherical particles, is dispersed on the ferrite matrix.

Compared with flaky pearlite, it has low hardness and is convenient for cutting and processing. When heated, the austenite grains are not easy to grow, and the deformation and cracking tendency of the workpiece is small when cooling. In addition, spheroidizing annealing can sometimes be used for some hypoeutectoid steels that need to improve cold plastic deformation (such as stamping, cold heading, etc.).

The heating temperature of spheroidizing annealing is Ac1 (20-40) ℃ or Acm-(20-30) ℃, and isothermal cooling or direct and slow cooling after heat preservation. During the spheroidizing annealing, the austenization is “incomplete”, but the flaky pearlite is transformed into austenite, and a small amount of excess carbide is dissolved. Therefore, it is impossible to eliminate network carbides. If the hypereutectoid steel has network carbides, normalizing must be carried out before spheroidizing annealing to eliminate them to ensure normal spheroidizing annealing.

There are many spheroidizing annealing processes, and the two most commonly used processes are ordinary spheroidizing annealing and isothermal spheroidizing annealing. Ordinary spheroidizing annealing is to heat the steel to 20-30°C above Ac1, hold it for a proper time, then slowly cool it with the furnace, and cool it down to about 500°C. Isothermal spheroidizing annealing is the same heating and holding process as the ordinary spheroidizing annealing process,

followed by the furnace cooling to a temperature slightly lower than Ar1 for isothermal, and the isothermal time is 1.5 times the heating and holding time. After isothermal, the furnace is cooled to about 500℃ and then air-cooled. Compared with ordinary spheroidizing annealing, spheroidizing annealing can not only shorten the cycle, but also make the spheroidized structure uniform, and can strictly control the hardness after annealing.

⑥ Recrystallization annealing (intermediate annealing)

Recrystallization annealing is a heat treatment process in which the cold-deformed metal is heated to above the recrystallization temperature and maintained for an appropriate time to recrystallize the deformed grains into uniform equiaxed grains to eliminate deformation strengthening and residual stress.

⑦ Stress relief annealing

Stress-relief annealing is an annealing process to eliminate residual stress caused by plastic deformation processing, welding, etc. and the residual stress in the casting.

There are internal stresses in the workpiece after forging, casting, welding and cutting. If it is not eliminated in time, the workpiece will be deformed during processing and use, which will affect the accuracy of the workpiece. It is very important to use stress relief annealing to eliminate internal stress generated during processing.

The heating temperature of stress relief annealing is lower than the phase transition temperature A1, therefore, no structural transformation occurs during the entire heat treatment. The internal stress is mainly eliminated by the workpiece in the process of heat preservation and slow cooling. In order to eliminate the internal stress of the workpiece more thoroughly, the heating temperature should be controlled during heating.

Generally, it enters the furnace at low temperature, and then heats to the specified temperature at a heating rate of about 100°C/h. The heating temperature of welding parts should be slightly higher than 600℃. The holding time depends on the situation, usually 2 to 4 hours. The holding time of stress relief annealing for castings is taken as the upper limit, the cooling rate is controlled at (20-50) ℃/h, and it can be air-cooled after cooling to below 300 ℃.

Ways to improve heat treatment performance

For aviation (typically, Nadcap and AMS2750 US military standards) and automotive (TS16949 test device) customers, process consistency is the main issue. Compatibility with higher acceptable standards is building confidence between heat treatment equipment manufacturers and customers . This is especially true when the heat treatment furnace design matches the design of the expert process control system. Eurotherm already has a unique knowledge of aviation and automotive heat treatment specifications. This knowledge and specially designed products and systems help ensure that the controlled parameters are kept within acceptable limits at all times.

Modern control system design mainly considers three issues:

① The gain and thermal hysteresis of most modern heat treatment furnaces with low-calorie material isolation are becoming the main factors of load size and set point, rather than heat treatment furnace structure or its own parts.

② Special consideration is given to the temperature of the actual workload. Customers are looking for them in their control algorithms. Therefore, more heat treatments use specific workload temperature optimization methods.

③ Acceptable heat treatments are generally approved by heat treatment furnaces defined by specific classifications. In order to obtain the maximum efficiency of production planning and the maximum value of workload capacity, it is critical that heat treatment equipment has been working at the most appropriate level.

The customer’s recurring problems are excessive work load temperature drift, which occurs when starting or trajectory slope, even when the controlled area is very close to the required thermal trajectory. Because many workpieces have different cross-sectional areas, annoying temperature drift can lead to destructive consequences, especially in thin-section parts under load. These drifts can cause failure audits of heat treatment furnaces. Because of this, many users turn to the rather crude method of excessive elimination and gain optimization, which is added to the heat treatment cycle time.

Strengthening technology of spring

(1) Heat treatment strengthening process technology of spring
1) Protective atmosphere heat treatment. In our country, the heat treatment of springs with wires less than 15mm, oil-quenched and tempered steel wire and toughened steel all use protective atmosphere heat treatment. Protective atmosphere heat treatment can eliminate surface decarburization and oxidation, and improve the surface quality of the material.

2) Induction heating or protective atmosphere induction heating heat treatment. This process is generally carried out on the wire before the coil spring is formed. Some spring factories combine the heat treatment of the wire material with the spring production to reduce costs. Induction heating treatment has a good strengthening effect. The induction heating speed is fast, which helps to refine the crystal grains and reduce surface decarburization, and can give full play to and improve the strength and toughness of the material.

3) Surface nitriding heat treatment technology. In recent years, high-stress valve springs or other high-stress clutch springs have also adopted surface nitriding technology in order to achieve reliable fatigue life. Now the more advanced technology is low-temperature gas nitriding technology, and the general nitriding temperature is (450~470) ℃, the gas nitriding time is (5-20)h.

(2) Shot peening process of spring

1) Combined shot peening technology. Combined shot peening is generally called multiple shot peening process. Most economical processes use secondary shot peening. This is achieved by shot peening with pellets of different diameters. For the first time, larger pellets were used to obtain residual compressive stress and surface finish.

2) Stress shot peening process. The stress shot peening process is also a relatively classic shot peening process, just because it is difficult to apply to mass production, but in recent years, due to the rapid development of stress shot peening equipment, it has gained a lot in the mass production of high-stress automobile suspension springs. development of.

Especially the combined application of stress-strengthening shot peening and other shot peening processes has a good strengthening effect. The pre-stress of stress shot peening is generally set at (700-800) MPa. After stress blasting, the peak value of the residual stress can reach (1200-1500) MPa, thereby obtaining high fatigue strength.

(3) The hot pressure process of the spring The hot pressure process is mainly applied to the coil springs that require high permanent deformation resistance, as an advanced anti-permanent deformation stabilization process. In addition to significantly improving the resistance to permanent deformation, the hot pressing process can also increase the fatigue life.

Selection skills of quenching medium in heat treatment of fasteners

Fasteners are widely used in machinery, automobiles, aviation, aerospace, construction, transportation, communications, electronic appliances and many other industries. Various industries have different requirements for fasteners, including different comprehensive mechanical properties. . The comprehensive mechanical properties of fasteners are mainly determined by the selected materials and heat treatment quality, so it is imperative to select materials strictly and reasonably and improve the level and quality of heat treatment.
Heat treatment is divided into two processes, heating and cooling, and the ideal heat treatment quality can be obtained by scientifically controlling the two processes. Reasonable selection and scientific use of quenching medium is the essence of controlling the cooling process.

1. Bolts, studs, screws, nuts and special-shaped non-standard parts mainly based on ML35 (32, 40) etc. Due to the problem of material hardenability, the quenching medium with faster cooling speed should be used in the heat treatment process of this kind of parts , So as to meet the requirements of the hardness, metallography and mechanical properties of parts quenching.

I. The product size is M4-Ml 6, you can choose 3-5% KR6480 polymer water-soluble quenching agent, if you choose 35#, 35A, 35S can be relaxed to M18–M20.

II. The product size is M10~M24, you can choose 8~10% KR7280 water-soluble quenching agent.

III. If the product size is larger than M24, you can choose 10-15% KR7280 water-soluble quenching agent.

IV, the product is mainly 45#, you can choose KR6480 or KR7280 according to the size of your product.

2. Bolts, bolts, screws, nuts and special-shaped non-standard parts based on 35CrMo, 40Cr, 20MnTiB, 35VB, etc.

I. If your product is mainly 35CrMo, 40Cr, you can choose KRll8 quick quenching oil or 5-10% KR6480 polymer water-soluble quenching agent.

Ⅱ. If your products are mainly 20MnTiB and 35VB, you can choose KR7280 or KR6480.

3. Shallow carburized self-tapping screws and pins based on low-carbon steel 10#, 15#, 20#, 20Cr, 1018, 1022, 10B21, etc. can be selected as KR6480 or KRll8

4. Spring washers and retaining rings mainly made of 65Mn, 60Si2Mn, 70, etc. can be selected with KRl18 quick quenching oil.

5. There are many types of products, and KRll8 or KR6480 (which can meet the quenching and carburizing and quenching of all alloy steels and carbon steels of a certain size) can be selected for larger size spans. The internal quality of the workpiece depends on the heat treatment quality. Quenching medium plays a decisive role in heat treatment, but it is the smallest piece of heat treatment production cost. As long as you pay a little attention to the quenching medium and choose professional and reliable products, you can enjoy professional consultation, after-sales service, and To achieve unexpected results, and greatly reduce the overall production cost.

Remarks:

1. KR6480 is a highly versatile water-soluble quenching agent

2. KR7280 is currently the world’s fastest cooling water-soluble quenching agent and is widely used in Europe.

Some common sense about heat treatment of spring steel

1. Types of annealing
1. Fully annealed

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

2. Spheroidizing annealing: Spheroidizing annealing is mainly used for hypereutectoid carbon 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.

2. Quenching medium

When quenching, 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. The purpose of steel 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. 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 through appropriate tempering cooperation 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) to appropriately aggregate carbides in the steel and reduce the hardness to facilitate cutting.

Stainless steel heat treatment technology

Chromium is a factor of non-corrosion in such materials. In the past, it was found that the chromium content must be above 12% to form a dense surface oxide film to achieve corrosion protection. Therefore, the heat treatment of any stainless steel must take into account the chromium content Has it caused any changes?
(1) Matian loose iron stainless steel:

This type of stainless steel has a body-centered cubic structure (BCC) that can be attracted by magnets. It is made from the rapid cooling of Osten temperature. This has the best corrosion resistance, but the material is hard but brittle, and then tempered to increase Ductility, but corrosion resistance will be reduced, especially the tempering between 450°C and 650°C, will cause the carbon atoms in the crystal lattice gap to diffuse and precipitate chromium carbides that form a network with chromium, causing chromium elements in the adjacent area Consumption reduces the chromium content, prevents the formation of a protective film, and loses corrosion resistance, so special attention is required. The following is the heat treatment temperature of various Matian loose iron stainless steel materials.

(a) The temperature of 403, 410, 416se is 650-750℃.

(b) The temperature of 414 is 650-730℃.

(c) The temperature of 431 is 6.

(d) The temperature of 440-A, 440-B, 440-C, 420 is 680-750℃.

(2) Ferrous iron stainless steel:

This kind of stainless steel has a body-centered cubic structure (BCC) that can be attracted by magnets and is usually used in the automobile industry or the chemical industry. The strength will not be changed by heat treatment, but it can be cold-worked to increase the strength.

(3) Ostian iron stainless steel:

This type of stainless steel has a face-centered cubic structure (FCC) and does not work on magnets. As mentioned above, this type of material is easy to process, so after processing, different heat treatments can be applied to eliminate the residual stress of the material.

(4) Precipitation hardening stainless steel:

This kind of stainless steel is quenched at a high temperature and then heat treated at a low temperature. The aluminum or copper contained in the material precipitates along the slip surface or grain boundaries of the differential to form inter-metallic compounds, which can increase its strength or hardness. The commonly used precipitation hardening stainless steel is 17-4 PH, and there are 17-7 PH, PH15-7MO, AM-350, AM-355, etc.

(5) Heat treatment after welding of various stainless steels:

The chromium element contained in stainless steel, after welding, will often diffuse and precipitate and combine with carbon to form chromium carbide in the high-temperature area (heat-affected zone), resulting in partial reduction of the chromium component and failure to form a protective film. Corrosion conditions such as perforation are often It occurs in these heat-affected zones. In order to remedy this situation, the industry often heat-treats the object after welding. The function is to diffuse the chromium element in other areas to this chromium-deficient area to achieve a protective effect.