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.