What is heat treatment residual stress
What is heat treatment residual stress?
Table of Contents
Heat treatment residual stress refers to the residual stress of the workpiece after heat treatment, which has a very important impact on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it will cause the deformation of the workpiece. When it exceeds the strength limit of the material, it will crack the workpiece, which is its harmful side and should be reduced and eliminated.
However, under certain conditions, controlling the stress to make it reasonably distributed can improve the mechanical properties and service life of parts and turn harmful into beneficial.
Analyzing the distribution and change law of stress in steel during heat treatment and making it reasonably distributed 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 extensive attention.
Heat treatment stress of steel
During the heating and cooling process of the workpiece, due to the inconsistent cooling speed and time between the surface and the core, the temperature difference will lead to uneven volume expansion and contraction and stress, that is, thermal stress. Under the action of thermal stress, because the initial temperature of the surface layer is lower than that of the heart and the shrinkage is greater than that of the heart, the heart is pulled. At the end of cooling, because the final cooling volume shrinkage of the heart cannot be carried out freely, the surface layer is compressed and the heart is pulled. That is, under the action of thermal stress, the surface of the workpiece is finally compressed and the center 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 final residual stress.
On the other hand, due to the change of microstructure during heat treatment, i.e. the transformation from austenite to martensite, the increase of specific volume will be accompanied by the expansion of workpiece volume, and each part of workpiece will undergo phase transformation successively, resulting in inconsistent volume growth and structural stress. The final result of the change of tissue stress is the tensile stress on the surface and the compressive stress in the center, which is just opposite to the thermal stress. The microstructure stress is related to the cooling rate, shape and chemical composition of the workpiece in the martensitic transformation zone.
Practice has proved that thermal stress and microstructure stress will occur as long as there is phase transformation in any workpiece during heat treatment. However, the thermal stress has been generated before the tissue transformation, and the tissue stress is generated in the process of tissue transformation. In the whole cooling process, the result of the comprehensive action of thermal stress and tissue stress is the actual stress in the workpiece.
The result of the combined action of these two stresses is very complex, which is affected by many factors, such as composition, shape, heat treatment process and so on. In terms of its development process, there are only two types, namely thermal stress and tissue stress. When the action direction is opposite, they offset each other, and when the action direction is the same, they overlap each other. Whether offset or superimposed, the two stresses should have a dominant factor. When the thermal stress is dominant, the result is that the center of the workpiece is pulled and the surface is compressed. When the tissue stress is dominant, the result is the tension on the compressed surface of the workpiece center.
Classification of heat treatment stress
Heat treatment stress can be divided into thermal stress and tissue stress. The heat treatment distortion of workpiece is the result of the comprehensive action of thermal stress and tissue stress. The state of heat treatment stress in the workpiece and its effect are different. The internal stress caused by uneven heating or cooling is called thermal stress; The internal stress caused by the inequality of tissue transformation is called tissue stress. In addition, the internal stress caused by the uneven transformation of the internal structure of the workpiece is called additional stress. The final stress state and stress size of the workpiece after heat treatment depend on the sum of thermal stress, structural stress and additional stress, which is called residual stress.
The distortion and crack formed during heat treatment are the result of the combined action of these internal stresses. At the same time, under the action of heat treatment stress, sometimes one part of the workpiece will be in the state of tensile stress and the other part will be in the state of compressive stress. Sometimes, the distribution of stress state in each part of the workpiece may be very complex. This should be analyzed according to the actual situation.
1. Thermal stress
Thermal stress is the internal stress caused by uneven volume expansion and contraction due to different heating or cooling rates between the surface of the workpiece and the center or thin and thick parts during heat treatment. Generally, the faster the heating or cooling rate, the greater the thermal stress.
2. Tissue stress
The internal stress caused by the isochronous change of specific volume caused by phase transformation is called tissue stress, which is also called phase transformation stress. Generally, the greater the specific volume before and after the transformation of tissue structure and the greater the time difference between the transformation of each part, the greater the tissue stress.
3. Additional stress
During the heat treatment of the workpiece, in addition to the thermal stress and structural stress, the internal stress can also be formed due to the non-uniformity of the structure between the surface and the center of the workpiece and the inconsistency of the elastic distortion inside the workpiece, which is called additional stress. For example, carburization or decarburization of the surface layer of the workpiece, surface quenching or local quenching and other factors that can lead to uneven microstructure on the surface and center of the workpiece can produce stress near the heat treatment.
- (1) Additional stress formed during surface quenching or local quenching. During local quenching or surface quenching (such as induction quenching, flame quenching and laser quenching), martensitic structure is formed only in the quenched part, and the part without quenching is still the original structure, resulting in the difference in specific volume of the whole work piece. At this time, the expansion caused by the increase of specific volume caused by martensite on the surface of the workpiece is limited by the central part, so that the surface is subjected to compressive stress and the central part is subjected to tensile stress.
- (2) Additional stress formed during carburizing quenching when carburizing workpiece is quenched, because its surface carbon content is high and its internal carbon content is low (the original carbon content of steel), the phase transformation temperature (i.e. MS point) of the surface and the center is different (the phase transformation temperature of the surface is lower than that of the center). Therefore, the internal organization changes and expands first. At this time, the surface structure is still austenite and still in plastic state. In the initial stage, the surface is subjected to tensile stress and the core is subjected to compressive stress. Due to the excellent plasticity of the surface layer, plastic distortion is easy to occur under the action of tensile stress, resulting in stress relaxation, that is, the stress value decreases. Then, when the surface layer of high carbon also expands due to martensitic transformation, the stress of the surface layer is just opposite to that of the center, that is, the surface is compressive stress and the center is tensile stress.
4. Residual stress
When heat treatment is accompanied by phase transformation, thermal stress and structural stress will be generated at the same time. The final stress state of the workpiece depends on the sum of thermal stress, structural stress and additional stress. The internal stress that remains after heat treatment is called residual stress. It is divided into residual tensile stress (expressed by “+” and residual compressive stress (expressed by “-“).
Effect of heat treatment stress on quenching crack
The factors existing in different parts of quenched parts that can cause stress concentration (including metallurgical defects) can promote the generation of quenched cracks, but they can only appear in the tensile stress field (especially under the maximum tensile stress), if they do not promote cracks in the compressive stress field.
Quenching cooling rate is not only an important factor that can affect quenching quality and determine residual stress, but also an important and even decisive factor on quenching cracks. In order to achieve the purpose of quenching, the cooling rate of parts in the high temperature section must be accelerated and exceed the critical quenching cooling rate of steel in order to obtain martensitic structure.
In terms of residual stress, this can increase the thermal stress value to counteract the effect of tissue stress, so it can reduce the tensile stress on the workpiece surface and achieve the purpose of restraining longitudinal crack. The effect will increase with the acceleration of high temperature cooling rate. Moreover, in the case of hardenability, the larger the cross-section size of the workpiece, although the actual cooling speed is slower, the greater the risk of cracking. All this is because the thermal stress of this kind of steel decreases with the increase of size, the actual cooling rate decreases, the thermal stress decreases, and the microstructure stress increases with the increase of size. Finally, the tensile stress dominated by microstructure stress acts on the surface of the workpiece. It is quite 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 quenched under normal conditions. The reliable principle to avoid quenching crack is to try to reduce the unequal time of martensite transformation inside and outside the section. Only slow cooling in the martensitic transformation zone is not enough to prevent the formation of longitudinal cracks. Generally, arc cracks can only occur in non hardenable parts. Although the overall rapid cooling is the necessary formation condition, its real formation reason is not in the rapid cooling (including martensitic transformation zone) itself, but in the local position of quenched parts (determined by geometric structure). The cooling speed in the high temperature critical temperature zone is significantly slowed down, so it is not caused by quenching.
The transverse and longitudinal splitting in large non hardenable parts are caused by the residual tensile stress with thermal stress as the main component acting on the center of the quenched part, and the crack first forms and expands from inside to outside at the section center of the quenched part at the end of hardening. In order to avoid such cracks, water oil dual 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 metal has martensitic structure. From the perspective of internal stress, rapid cooling is harmful and unhelpful. Secondly, the purpose of slow cooling in the later stage of cooling is not to reduce the expansion rate and microstructure stress value of martensitic transformation, but to minimize the section temperature difference and the shrinkage rate of metal in the center of the section, so as to reduce the stress value and finally inhibit quenching crack.
Influence of residual compressive stress on workpiece
Carburizing 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, obtain large residual compressive stress on the workpiece surface layer and improve the fatigue strength of the workpiece. If isothermal quenching is carried out after carburizing, the surface residual compressive stress will be increased and the fatigue strength will be further improved. Some people have tested the residual stress of 35simn2mov steel after isothermal quenching after carburizing and quenching and low temperature tempering after carburizing:
Table.1 Residual stress values of 135simn2mov steel after carburizing isothermal quenching and carburizing low temperature tempering
Residual stress value（kg/mm2)
After carburizing, heat in 880-900 ℃ salt bath and isothermal at 260 ℃ for 40 minutes.
After carburizing, it is heated and quenched in 880-900 ℃ salt bath and isothermal at 260 ℃ for 90 minutes.
After carburizing, heat in 880-900 ℃ salt bath, isothermal at 260 ℃ for 40 minutes, and return to fire at 260 ℃ for 90 minutes.
It can be seen from the test results in the above table that isothermal quenching has higher surface residual compressive stress than the usual quenching and low temperature tempering process. Even if low temperature tempering is carried out after isothermal quenching, the surface residual compressive stress is higher than that after quenching. Therefore, it can be concluded that the surface residual compressive stress obtained by isothermal quenching after carburizing is higher than that obtained by ordinary carburizing quenching and low-temperature tempering. From the point of view of the beneficial effect of surface residual compressive stress on fatigue resistance, carburizing isothermal quenching process is an effective method to improve the fatigue strength of Carburized parts.
Why can surface residual compressive stress be obtained by carburizing and quenching process? Why can higher surface residual compressive stress be obtained by carburizing isothermal quenching?
There are two main reasons:
One reason is that the specific volume of high-carbon martensite on the surface is larger than that of low-carbon martensite in the center. After quenching, the volume expansion of the surface is large, while the volume expansion of low-carbon martensite in the center is small, which restricts the free expansion of the surface, resulting in the stress state of compression on the surface and tension on the center.
Another more important reason is that the starting temperature (MS) of high carbon undercooled austenite to martensite is lower than that of undercooled austenite to martensite with low carbon content in the center. That is to say, in the quenching process, the martensite transformation in the core is often the first to cause the volume expansion of the core and obtain strengthening, while the surface has not cooled to its corresponding martensite transformation point (MS), so it is still in the supercooled austenite state, has good plasticity, and will not seriously suppress the volume expansion of the martensite transformation in the core.
With the continuous decrease of quenching cooling temperature, the surface temperature drops below the (MS) point, and martensitic transformation occurs on the surface, resulting in the expansion of surface volume. However, the core has already transformed into martensite and strengthened at this time, so the core will play a great role in suppressing the volume expansion of the surface layer, so that the surface layer can obtain residual compressive stress. When isothermal quenching is carried out after carburizing, when the isothermal temperature is above the Martensite Start transformation temperature (MS) of the carburized layer and the appropriate temperature below the Martensite Start transformation temperature (MS) of the core, isothermal quenching can better ensure the sequential characteristics of this transformation than continuous cooling quenching (that is, ensure that the surface martensite transformation only occurs in the cooling process after isothermal).
Of course, the isothermal temperature and time of isothermal quenching after carburizing have a great influence on the magnitude of surface residual stress. Some people have tested the surface residual stress of 35SiMn2MoV steel sample after carburizing and isothermal at 260 ℃ and 320 ℃ for 40 minutes. It is found that the surface residual stress isothermal at 260 ℃ is more than twice that isothermal at 320 ℃. It can be seen that the surface residual stress state is very sensitive to the isothermal temperature of carburizing and isothermal quenching.
Not only the isothermal temperature has an effect on the surface residual compressive stress state, but also the isothermal time has a certain effect. Some people have tested the residual stress of 35simn2v steel isothermal 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 when the isothermal time is extended after 60 minutes.
The above discussion shows that the sequence of carburized layer and core martensite transformation has an important influence on the magnitude of surface residual stress. Isothermal quenching after carburizing is of universal significance to further improve the fatigue life of parts.
In addition, surface chemical heat treatments that can reduce the initial transformation temperature (MS) of surface martensite, such as carburizing, nitriding and cyanidation, provide conditions for causing surface residual compressive stress, such as nitriding quenching process of high carbon steel, which reduces the initial transformation point (MS) of surface martensite due to the increase of nitrogen content, After quenching, higher surface residual compressive stress is obtained, and the fatigue life is improved. For another example, cyanidation process often has higher fatigue strength and service life than carburizing, which is also because the increase of nitrogen content can obtain higher surface residual compressive stress than carburizing.
In addition, from the point of view of obtaining the reasonable distribution of surface residual compressive stress, a single surface strengthening process is not easy to obtain the ideal distribution of surface residual compressive stress, while the 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 on the surface, the maximum compressive stress appears at a certain depth from the surface, and the residual pressure layer is thick. The surface residual compressive stress after nitriding is very high, but the residual compressive stress layer is very shallow and decreases sharply inward. If the carburizing nitriding composite strengthening process is adopted, a more reasonable stress distribution state can be obtained. Therefore, surface composite strengthening processes, such as carburizing nitriding, carburizing high-frequency quenching, are worthy of attention.
- 1. The stress produced during heat treatment is inevitable and often harmful. However, we can control the heat treatment process and try to make the stress distribution reasonable, so as to reduce the harmful degree to the minimum, or even turn the harmful into favorable.
- 2. When the thermal stress is dominant, the stress distribution is the tension of the heart and the compression of the surface. When the tissue stress is dominant, the stress distribution is the tension of the compression of the heart and the surface.
- 3. Longitudinal cracks are easy to form in high hardenability steel parts, arc cracks are often formed in non hardenability workpieces, and transverse and longitudinal splitting are easy to form in large non hardenability workpieces.
- 4. Carburizing reduces the initial transformation temperature (MS) of martensite on the surface layer, which can lead to the reversal of martensite transformation order during quenching. Martensite transformation first occurs in the center and then spread to the surface, so as to obtain the residual compressive stress on the surface layer and improve the fatigue strength.
- 5. Isothermal quenching after carburizing can ensure that the martensite transformation in the core can be fully carried out, and then the surface microstructure transformation can be carried out. The surface residual compressive stress of the workpiece is greater than that of direct quenching, which can further improve the fatigue strength of carburized parts.
- 6. The composite surface strengthening process can make the distribution of residual compressive stress on the surface more reasonable and significantly improve the fatigue strength of the workpiece.
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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