Abstract: (1) Overheating phenomenon We know that heating and superheating during heat treatment is the most likely to cause coarse austenite grains, which will reduce the mechanical properties of the parts. There are many theoretical controversies about the origin of fracture inheritance. It is generally believed that impurities such as MnS are dissolved into austenite and enriched in grain boundaries due to excessive heating temperature, and these inclusions are cooled.
(1) Overheating
We know that heating overheating during heat treatment is most likely to cause coarsening of austenite grains and degrade the mechanical properties of the parts.
1. General overheating: The heating temperature is too high or the holding time is too long at high temperature, causing the austenite grain coarsening to be called overheating. The coarse austenite grains lead to a decrease in the toughness of the steel, an increase in the brittle transition temperature, and an increase in the tendency of deformation cracking during quenching. The cause of overheating is that the furnace temperature meter is out of control or mixed (often not knowing the process). The superheated structure can be re-austenized under normal conditions to refine the grain after annealing, normalizing or multiple high temperature tempering.
2. Fracture inheritance: Steel with overheated structure, after reheating and quenching, although the austenite grains can be refined, sometimes large coarse fractals still appear. There are many theoretical controversies about the origin of fracture inheritance. It is generally believed that impurities such as MnS are dissolved into austenite and enriched in grain boundaries due to excessive heating temperature, and these inclusions will precipitate along the grain boundary during cooling. It is easy to break along the coarse austenite grain boundary when subjected to impact.
3. Inheritance of coarse tissue: When the steel with coarse martensite, bainite and weiss body is re-austenated, it is heated at a slow speed to the conventional quenching temperature, and even lower, its austenite crystal The granules are still coarse, a phenomenon known as tissue heredity. To eliminate the heritability of the large tissue, intermediate annealing or multiple high temperature tempering treatments can be used.
(two), over burning phenomenon
If the heating temperature is too high, not only the austenite grains are coarsened, but also the localized oxidation or melting of the grain boundaries causes the grain boundaries to weaken, which is called over-burning. After the steel is over-fired, the performance is seriously deteriorated, and cracks are formed during quenching. The burnt tissue cannot be recovered and can only be scrapped. Therefore, it is necessary to avoid over-burning during work.
(C), decarburization and oxidation
When the steel is heated, the carbon in the surface layer reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere), and the carbon concentration in the surface layer is reduced, which is called decarburization. The surface hardness, fatigue strength and resistance of the decarburized steel after quenching The abrasiveness is lowered, and the residual tensile stress is formed on the surface to form a surface network crack.
When heated, the phenomenon that iron and alloy of the steel surface layer react with oxygen, carbon dioxide, water vapor or the like in an element or medium (or atmosphere) to form an oxide film is called oxidation. At high temperatures (generally 570 degrees or more), the dimensional accuracy and surface brightness of the workpiece are deteriorated after oxidation, and the steel having poor hardenability of the oxide film is prone to quenching soft spots.
In order to prevent oxidation and reduce decarburization, there are: surface coating of workpiece, sealed and sealed with stainless steel foil, heated by salt bath furnace, heated by protective atmosphere (such as purified inert gas, controlled carbon potential in furnace), flame burning furnace (making the furnace gas reductive)
(4) Hydrogen embrittlement
The phenomenon that the high strength steel is reduced in plasticity and toughness when heated in a hydrogen-rich atmosphere is called hydrogen embrittlement. Hydrogen embrittlement can be eliminated by hydrogen removal treatment (such as tempering, aging, etc.), and hydrogen embrittlement can be avoided by heating in a vacuum, low hydrogen atmosphere or inert atmosphere.
Eight common heat treatment concepts
1. Normalizing: The steel or steel is heated to a suitable temperature above the critical point Ac3 or above for a certain period of time and then cooled in the air to obtain a heat treatment process of the pearlite-like structure.
2. Annealing: heat treatment of sub-eutectoid steel workpieces to 30-50 degrees above Ac3, after a period of heat preservation, cooling with air (or buried in sand or lime) to 500 degrees below air cooling
3. Solution heat treatment: heating the alloy to a high temperature single-phase zone to maintain the temperature, so that the excess phase is sufficiently dissolved at a rapid rate to obtain a supersaturated solid solution heat treatment process
4. Aging: The phenomenon that the properties of a alloy change with time after being subjected to solution heat treatment or cold plastic deformation at room temperature or slightly above room temperature.
5. Solution treatment: fully dissolve various phases in the alloy, strengthen the solid solution and improve toughness and corrosion resistance, eliminate stress and soften, and continue processing
6. Aging treatment: heating and holding at the temperature of the strengthening phase, allowing the strengthening phase to precipitate and harden, improving strength
7. Quenching: heat treatment process in which the steel is austenitized and cooled at an appropriate cooling rate to cause the workpiece to be decomposed into the solid solution in all or a certain range of the cross section, and then the unstable microstructure structure such as fast martensite is transformed.
8. Tempering: heating the quenched workpiece to a suitable temperature below the critical point Ac1 for a certain period of time, followed by cooling in a satisfactory manner to obtain the desired microstructure and properties 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 into the surface layer of steel to improve surface hardness and wear resistance, as well as to improve fatigue strength and corrosion resistance.
It uses ammonia gas to decompose the active nitrogen atom when heated, and is absorbed by the steel to form a nitride layer on the surface and diffuse to the core.
Nitriding is usually carried out using specialized equipment or a well type carburizing furnace. Applicable to all kinds of high-speed transmission precision gears, machine tool spindles (such as masts, grinding machine spindles), high-speed diesel engine crankshafts, valves, etc.
Nitriding workpiece routing: forging - annealing - roughing - quenching - finishing - stress removal - rough grinding - nitriding - fine grinding or grinding.
Since the nitrided layer is thin and brittle, it requires a higher strength of the core structure, so the tempering heat treatment is first performed to obtain the tempered sorbite, and the mechanical properties of the core and the quality of the nitrided layer are improved.
After nitriding, steel no longer needs to be quenched to have high surface hardness and wear resistance.
The nitriding treatment has a low temperature and a small deformation, which is much less deformed than carburizing and induction surface quenching.
(2). Carbonitriding of steel: Carbonitriding is a process of simultaneously infiltrating carbon and nitrogen into the surface of steel. It is customary for carbonitriding to be called cyanidation. At present, the application of medium temperature gas carbonitriding and low temperature gas nitrocarburizing (ie, gas soft nitriding) is widely used. The main purpose of carbon monoxide in medium temperature gas is to improve the hardness, wear resistance and fatigue strength of steel. The 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: The heat treatment combined with quenching and high temperature tempering is called quenching and tempering. Quenching and tempering is widely used in a variety of important structural parts, especially those that work under alternating loads, such as connecting rods, bolts, gears and shafts. After quenching and tempering treatment, the tempered sorbite structure is obtained, and its mechanical properties are superior to the normal-fired sorbite structure of the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of the steel and the cross-sectional dimensions of the workpiece, generally between HB200 and 350.
11. Brazing: a heat treatment process in which two workpieces are bonded together with a brazing filler metal
Types and applications of nine tempering
According to the different performance requirements of the workpiece, depending on the tempering temperature, the tempering can be divided into the following types:
(1) Low temperature tempering (150-250 ° C)
The tissue obtained by low temperature tempering is tempered martensite. The purpose is to reduce the internal stress and brittleness of the quenched steel under the premise of maintaining the high hardness and high wear resistance of the quenched steel, so as to avoid cracking or premature failure during use. It is mainly used in various high carbon cutting tools, measuring tools, cold stamping dies, rolling bearings and carburizing parts. The hardness after tempering is generally HRC58-64.
(2) Medium temperature tempering (350-500 ° C)
The tissue obtained by medium temperature tempering is tempered troostite. Its purpose is to achieve high yield strength, elastic limit and high toughness. Therefore, it is mainly used for the treatment of various springs and hot working dies, and the hardness after tempering is generally HRC35-50.
(3) High temperature tempering (500-650 ° C)
The structure obtained by high temperature tempering is tempered sorbite. Conventionally, the heat treatment combining quenching and high temperature tempering is called quenching and tempering treatment, and the purpose is to obtain comprehensive mechanical properties with good strength, hardness, plasticity and toughness. Therefore, it is widely used in important structural parts of automobiles, tractors, machine tools, etc., such as connecting rods, bolts, gears and shafts. After tempering, the hardness is generally HB200-330.
Ten atmosphere and metal chemical reaction
(One). Chemical reaction between atmosphere and steel
Oxidation
2Fe+O2→2FeO
Fe+H2O→FeO+H2
FeC+CO2→Fe+2CO
2. Restore
FeO+H2→Fe+H2O FeO+CO→Fe+O2
3. Carburizing
2CO→[C]+CO2
CH4→[C]+2H2
Fe+[C]→FeC
4. Nitriding
2NH3→2[N]+3H2
Fe+[N]→FeN
(two). The effect of various atmospheres on metals
Nitrogen: reacts with Cr, CO, Al.Ti at ≥1000 °C
Hydrogen: It can reduce copper, nickel, iron and tungsten. When the water content in hydrogen reaches 0.2-0.3%, the steel will be decarburized.
Water: ≥800 °C, oxidative decarburization of iron and steel, no reaction with copper
Carbon monoxide: its reducibility is similar to that of hydrogen, which can make steel carburize
(three). The effect of various atmospheres on the resistance components
Nickel-chromium wire, iron-chromium-aluminum: sulfur-containing atmosphere is harmful to resistance wires
Heat treatment of eleven bronze
Beryllium bronze is an extremely versatile precipitation hardening alloy. After solid solution and timely treatment, the strength can reach 1250-1500MPa (1250-1500 kg). The heat treatment characteristics are: good plasticity after solution treatment, and cold work deformation. However, after the aging treatment, it has an excellent elastic limit, and the hardness and strength are also improved.
1. Solution treatment of beryllium bronze
Generally, the heating temperature of the solution treatment is between 780 and 820 ° C. For the material used as the elastic component, 760-780 ° C is used, mainly to prevent the coarse grain from affecting the strength. The temperature uniformity of the solution treatment furnace should be strictly controlled at ±5 °C. The holding time can be generally calculated as 1 hour / 25 mm, and when the beryllium bronze is subjected to solution heat treatment in air or an oxidizing atmosphere, an oxide film is formed on the surface. Although it has little effect on the mechanical properties after aging strengthening, it will affect the service life of the tool during cold working. In order to avoid oxidation, it should be heated in a vacuum furnace or ammonia decomposition, an inert gas, a reducing atmosphere (such as hydrogen, carbon monoxide, etc.) to obtain a bright heat treatment effect. In addition, care should be taken to minimize the transfer time (when this is quenched), otherwise it will affect the mechanical properties after aging. Thin materials should not exceed 3 seconds, and general parts should not exceed 5 seconds. Quenching media generally use water (no heating requirements), of course, parts with complex shapes can also be used to avoid deformation.
2, aging treatment of bismuth bronze
The aging temperature of beryllium bronze is related to the content of Be, and the alloy containing Be less than 2.1% is suitable for aging treatment. For alloys with Be greater than 1.7%, the optimum aging temperature is 300-330 ° C and the holding time is 1-3 hours (depending on the shape and thickness of the part). For high conductivity electrode alloys with Be below 0.5%, the optimum aging temperature is 450-480 ° C and the holding time is 1-3 hours due to the increase of melting point. In recent years, two-stage and multi-stage aging has also been developed, that is, aging at a high temperature for a short period of time, and then aging for a long time at a low temperature, which has the advantage of improved performance but reduced deformation. In order to improve the dimensional accuracy of the beryllium bronze after aging, the clamp can be used for aging, and sometimes two separate aging treatments can be used.
3. De-stress treatment of beryllium bronze
é“Bronze stress relief annealing temperature is 150-200 ° C, holding time 1-1.5 hours, can be used to eliminate residual stress caused by metal cutting, straightening treatment, cold forming, etc., to stabilize the shape and dimensional accuracy of parts in long-term use. .
Twelve heat treatment stresses and their effects
The heat treatment residual force refers to the stress that remains after the heat treatment of the workpiece, 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 causes deformation of the workpiece. When the strength limit of the material exceeds the limit of the material, the workpiece is cracked. This is the harmful side and should be reduced and eliminated. However, under certain conditions, by controlling the stress to make it reasonably distributed, it is possible to improve the mechanical properties and service life of the parts, which is harmful. It is of far-reaching practical significance to analyze the distribution and variation of stress during the heat treatment process of steel, so that it can be rationally distributed to improve product quality. For example, the problem of the reasonable distribution of surface residual compressive stress on the service life of parts has attracted widespread attention.
(1) Heat treatment stress of steel
During the heating and cooling process, due to the inconsistency between the cooling rate and time of the surface layer and the core, a temperature difference is formed, which causes volume expansion and uneven shrinkage to generate stress, that is, thermal stress. Under the action of thermal stress, since the surface layer starts to be lower than the core, the contraction is also larger than the core and the core is pulled. When the cooling is finished, the core is compressed due to the final cooling volume shrinkage of the core. Pulled. That is, under the action of thermal stress, the surface of the workpiece is finally pressed and the core is pulled. This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. The faster the cooling rate, the higher the carbon content and alloy composition, the greater the uneven plastic deformation caused by thermal stress during cooling, and the greater the residual stress formed. On the other hand, in the process of heat treatment, when steel changes to austenite to martensite, the increase of specific volume is accompanied by the expansion of the volume of the workpiece, and the parts of the workpiece are successively transformed, resulting in inconsistent volume growth and tissue formation. stress. The final result of the change in tissue stress is the tensile stress on the surface layer and the compressive stress on the core, just opposite to the thermal stress. The magnitude of the tissue stress is related to the cooling rate, shape, and chemical composition of the workpiece in the martensitic transformation zone.
Practice has shown that any workpiece in the heat treatment process, as long as there is a phase change, thermal stress and tissue stress will occur. However, the thermal stress has already occurred before the tissue transformation, and the tissue stress is generated during the tissue transformation. During the whole cooling process, the combined effect of thermal stress and tissue stress is the actual stress existing in the workpiece. The result of the combination of these two stresses is very complex and is influenced by many factors such as composition, shape, heat treatment process, and the like. There are only two types in terms of its development process, namely thermal stress and tissue stress. When the action direction is opposite, the two cancel each other. When the action direction is the same, the two are superimposed. Whether it is offsetting or superimposing 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 pressed. The result of the action of the tissue stress is that the surface of the workpiece is stressed.
(B), the effect of heat treatment stress on quenching crack
Factors that can cause stress concentration in different parts of the quenching part (including metallurgical defects) can promote the generation of quenching cracks, but only in the tensile stress field (especially under the maximum tensile stress) Come out, if there is no cracking effect in the compressive stress field.
The quenching cooling rate is an important factor that can affect the quenching quality and determine the residual stress. It is also a factor that can make an important or even decisive influence on the quenching crack. In order to achieve the purpose of quenching, it is usually necessary to accelerate the cooling rate of the part in the high temperature section and exceed the critical quenching cooling rate of the steel to obtain the martensite structure. In terms of residual stress, this can reduce the tensile stress on the surface of the workpiece and reduce the longitudinal cracking because it can increase the thermal stress value that counteracts the stress of the tissue. The effect will increase as the high temperature cooling rate increases. Moreover, in the case of hardenability, the workpiece having a larger cross-sectional dimension has a higher risk of cracking, although the actual cooling rate is slower. All of this is due to the increase of the thermal stress of the steel with the increase of the size, the actual cooling rate is slowed down, the thermal stress is reduced, the tissue stress increases with the increase of the size, and finally the tensile stress mainly caused by the tissue stress acts on the workpiece. The characteristics of the surface are caused by the characteristics. And it is quite different from the traditional concept that the slower the cooling, the less stress. For such steel parts, only longitudinal cracks can be formed in high hardenability steels quenched under normal conditions. The reliable principle of avoiding quenching is to try to minimize the unequality of martensite transformation inside and outside the section. It is not enough to implement slow cooling in the martensite transformation zone to prevent the formation of longitudinal cracks. In general, cracks can only be produced in non-hardenable parts. Although the overall rapid cooling is a necessary forming condition, its true formation is not in the rapid cooling (including the martensite transformation zone itself). It is the local position of the quenching part (determined by the geometry), and the cooling rate in the high temperature critical temperature zone is significantly slowed down, so there is no hardening. The transverse and longitudinal girdles produced in large non-hardenable parts are caused by the residual tensile stress with thermal stress as the main component acting at the center of the quenching part, and at the center of the hardened section at the end of the quenching part, the crack is first formed and Caused by internal and external expansion. In order to avoid such cracks, a water-oil two-liquid quenching process is often used. In this process, rapid cooling in the high temperature section is carried out, only to ensure that the outer layer metal obtains martensite structure, and from the viewpoint of internal stress, then rapid cooling is harmful. Secondly, the purpose of slow cooling in the late stage of cooling is not to reduce the expansion speed and the tissue stress value of the martensitic transformation, but to minimize the temperature difference of the section and the shrinkage speed of the metal at the center of the section, thereby reducing the stress value and finally The purpose of suppressing quenching.
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