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Heat stress and its impact on
Heat stress and its impact on
Date：2013/10/23 10:14:43
Heat treatment is the residual force after heat treatment of the workpiece down to the final residual stress , the workpiece shape, size , and performance has very important influence . When it exceeds the yield strength of the material , they cause deformation of the workpiece , the ultimate strength of the material over time will cause the workpiece cracking, which is its harmful side, should be reduced and eliminated. However, under certain conditions, to make reasonable control of the stress distribution , we can improve the mechanical properties of parts and service life becomes harmful as beneficial . Analysis of steel during heat stress distribution and variation , so that a reasonable distribution on improving product quality has profound practical significance. For example, on the rational surface residual stress distribution affects the service life of parts problems have attracted widespread attention.
1, the heat treatment of steel stress
Workpiece in the heating and cooling process, because the central portion of the cooling surface and the speed and time of inconsistencies , the formation temperature , volume expansion and contraction can lead to uneven stress , the thermal stress. Under the effect of thermal stress , because the surface temperature is below the core part begins , shrinkage core portion by the central portion is greater than the tension , when the end of cooling , since the volume shrinkage of the cooling core section last leaving the surface pressure can not freely central portion tension . That is, in the final thermal stress under the action of the workpiece surface pressure while the heart of tension . This phenomenon is cooled rate , material composition and heat treatment process and other factors. When cooled , the faster , the higher the carbon content and the alloy composition , the cooling process under thermal stress generated in the non-uniform plastic deformation larger , the final form of the residual stress it gets. The other hand, during the heat treatment of steel changes in the organization that the austenite to martensite transition , will be accompanied by an increase in specific volume due to the volume expansion of the workpiece , the workpiece successively all parts of the phase change , resulting in inconsistent grown produce tissue volume stress . The final result of the stress variation is the surface tensile stress , the center of compressive stress , the thermal stress is just the opposite. The size of the stress in the workpiece martensite transformation zone cooling rate , shape, chemical composition and other factors.
Practice has proved that any workpiece during the heat treatment , as long as the phase change , thermal stress and stress occurs . Thermal stress in the organization just before the transition has produced , and the stress is in the process of organizational change resulting in the entire cooling process, thermal stress and the stress combined result is that stress actually exists in the workpiece . Both stress the combined result is a very complex , being affected by many factors, such as composition, shape , heat treatment and so on. During its development is only two types, namely, thermal stress and the stress , the direction opposite to the offset between the two , the direction of superposition of the two are the same . Whether or cancel each other mutual superposition of two should be a dominant factor in stress , thermal stress dominant role when the result is tension workpiece core part , the surface pressure. Organizations stress the dominant role when the result is pressure on the workpiece surface tension core part .
2 , heat treatment of quenching cracks stress
Exist in different parts of hardened parts can cause stress concentration factors ( including metallurgical defects included ) , for quenching cracks have catalytic role, but only in the tensile stress inside ( especially in the maximum tensile stress ) and will show out , if the floor has not promote stress cracking effect.
Quenching speed is a hardened mass can affect and determine the residual stress of the important factor, but also capable of quenching cracks impart a significant and even decisive factor. To achieve the purpose of quenching is usually necessary parts in the high temperature section accelerate the cooling rate , and make more than the critical quenching rate of steel to be martensite . In terms of the residual stress , this can increase the effect of offsetting the stress value of thermal stress , it can reduce the tensile stress on the surface and to inhibit longitudinal purposes. The effect will vary with the temperature accelerated cooling rate increases . Moreover, in the case to the hardenability , the larger the size of the workpiece cross-section , although the actual cooling speed is more slow , but instead the risk of cracking greater . All this is due to the thermal stress of such steel increases with the size of the actual cooling rate slows down, reducing thermal stress , organizational stress increases with the increase of size , and finally the formation of the stress -based tensile stress in the workpiece the role of surface characteristics caused . And with the smaller cooling slower traditional concept of stress differ . In terms of such steel , hardened under normal conditions of high hardenability steel can only be formed diastema . Avoid quenching crack reliable principle is to try to minimize the cross-section outside of unequal martensitic transformation . Just slow implementation of martensitic transformation zone cooling sufficient to prevent longitudinal formation. Generally only produce non- hardenability in arc parts crack, although the overall rapid cooling is required formation condition , but its real causes , not in the rapid cooling ( including martensite region ) itself , but partially hardened member position ( determined by the geometry ) , the area at high critical temperature significantly slow cooling rate , and therefore caused no hardened . Produced in a large non- hardenability and a longitudinal member of the cross-sectional split is determined by the thermal stress in the main component of the residual tensile stress in the hardened member center, and hardened in the quenching section of the center member at the end of the first crack is formed by expansion caused by the inside out . In order to avoid such cracks , often using a water - oil double quenching processes. In this process, the implementation of the rapid cooling the high temperature section , the sole purpose is to ensure that the outer metal be martensite , and from the perspective of internal stress , then rapid cooling detrimental . Second , slow cooling cooling late main purpose is not to reduce the rate of expansion of martensitic transformation and the stress values, and that to minimize the temperature difference and the cross-sectional center of the cross section contraction speed of the metal , so as to reduce the stress value and the final quenching crack suppression purposes.
3, the residual stress of the workpiece
Carburizing as surface hardening to improve the fatigue strength of a piece of application is very broad causes . On the one hand because it can effectively increase the strength and surface hardness, improved wear resistance of the workpiece , on the other hand carburizing the workpiece can effectively improve stress distribution in the surface layer of a larger residual compressive stress, improve the fatigue strength of the workpiece . If carburization quenching after isothermal surface residual compressive stress will increase , so that the fatigue strength is further improved. Someone 35SiMn2MoV after carburizing steel isothermal quenching after carburizing quenching and tempering with the residual stress tested its
1, the heat treatment of steel stress
Workpiece in the heating and cooling process, because the central portion of the cooling surface and the speed and time of inconsistencies , the formation temperature , volume expansion and contraction can lead to uneven stress , the thermal stress. Under the effect of thermal stress , because the surface temperature is below the core part begins , shrinkage core portion by the central portion is greater than the tension , when the end of cooling , since the volume shrinkage of the cooling core section last leaving the surface pressure can not freely central portion tension . That is, in the final thermal stress under the action of the workpiece surface pressure while the heart of tension . This phenomenon is cooled rate , material composition and heat treatment process and other factors. When cooled , the faster , the higher the carbon content and the alloy composition , the cooling process under thermal stress generated in the non-uniform plastic deformation larger , the final form of the residual stress it gets. The other hand, during the heat treatment of steel changes in the organization that the austenite to martensite transition , will be accompanied by an increase in specific volume due to the volume expansion of the workpiece , the workpiece successively all parts of the phase change , resulting in inconsistent grown produce tissue volume stress . The final result of the stress variation is the surface tensile stress , the center of compressive stress , the thermal stress is just the opposite. The size of the stress in the workpiece martensite transformation zone cooling rate , shape, chemical composition and other factors.
Practice has proved that any workpiece during the heat treatment , as long as the phase change , thermal stress and stress occurs . Thermal stress in the organization just before the transition has produced , and the stress is in the process of organizational change resulting in the entire cooling process, thermal stress and the stress combined result is that stress actually exists in the workpiece . Both stress the combined result is a very complex , being affected by many factors, such as composition, shape , heat treatment and so on. During its development is only two types, namely, thermal stress and the stress , the direction opposite to the offset between the two , the direction of superposition of the two are the same . Whether or cancel each other mutual superposition of two should be a dominant factor in stress , thermal stress dominant role when the result is tension workpiece core part , the surface pressure. Organizations stress the dominant role when the result is pressure on the workpiece surface tension core part .
2 , heat treatment of quenching cracks stress
Exist in different parts of hardened parts can cause stress concentration factors ( including metallurgical defects included ) , for quenching cracks have catalytic role, but only in the tensile stress inside ( especially in the maximum tensile stress ) and will show out , if the floor has not promote stress cracking effect.
Quenching speed is a hardened mass can affect and determine the residual stress of the important factor, but also capable of quenching cracks impart a significant and even decisive factor. To achieve the purpose of quenching is usually necessary parts in the high temperature section accelerate the cooling rate , and make more than the critical quenching rate of steel to be martensite . In terms of the residual stress , this can increase the effect of offsetting the stress value of thermal stress , it can reduce the tensile stress on the surface and to inhibit longitudinal purposes. The effect will vary with the temperature accelerated cooling rate increases . Moreover, in the case to the hardenability , the larger the size of the workpiece cross-section , although the actual cooling speed is more slow , but instead the risk of cracking greater . All this is due to the thermal stress of such steel increases with the size of the actual cooling rate slows down, reducing thermal stress , organizational stress increases with the increase of size , and finally the formation of the stress -based tensile stress in the workpiece the role of surface characteristics caused . And with the smaller cooling slower traditional concept of stress differ . In terms of such steel , hardened under normal conditions of high hardenability steel can only be formed diastema . Avoid quenching crack reliable principle is to try to minimize the cross-section outside of unequal martensitic transformation . Just slow implementation of martensitic transformation zone cooling sufficient to prevent longitudinal formation. Generally only produce non- hardenability in arc parts crack, although the overall rapid cooling is required formation condition , but its real causes , not in the rapid cooling ( including martensite region ) itself , but partially hardened member position ( determined by the geometry ) , the area at high critical temperature significantly slow cooling rate , and therefore caused no hardened . Produced in a large non- hardenability and a longitudinal member of the cross-sectional split is determined by the thermal stress in the main component of the residual tensile stress in the hardened member center, and hardened in the quenching section of the center member at the end of the first crack is formed by expansion caused by the inside out . In order to avoid such cracks , often using a water - oil double quenching processes. In this process, the implementation of the rapid cooling the high temperature section , the sole purpose is to ensure that the outer metal be martensite , and from the perspective of internal stress , then rapid cooling detrimental . Second , slow cooling cooling late main purpose is not to reduce the rate of expansion of martensitic transformation and the stress values, and that to minimize the temperature difference and the cross-sectional center of the cross section contraction speed of the metal , so as to reduce the stress value and the final quenching crack suppression purposes.
3, the residual stress of the workpiece
Carburizing as surface hardening to improve the fatigue strength of a piece of application is very broad causes . On the one hand because it can effectively increase the strength and surface hardness, improved wear resistance of the workpiece , on the other hand carburizing the workpiece can effectively improve stress distribution in the surface layer of a larger residual compressive stress, improve the fatigue strength of the workpiece . If carburization quenching after isothermal surface residual compressive stress will increase , so that the fatigue strength is further improved. Someone 35SiMn2MoV after carburizing steel isothermal quenching after carburizing quenching and tempering with the residual stress tested its
