Heat Treatment Residual Force

Heat treatment residual force is the final residual stress of the workpiece after heat treatment, which has a very important effect on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it causes deformation of the workpiece, and 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.

But under certain conditions to control the stress so that the reasonable distribution, you can improve the mechanical properties of the parts and service life, turning harm into benefit. Analysis of steel in the heat treatment process of the distribution of stress and the law of change, so that the reasonable distribution of product quality has far-reaching practical significance. For example, the reasonable distribution of residual compressive stress on the surface layer of the impact on the service life of parts has attracted widespread attention. 


1. Heat Treatment Stress of Steel

Workpiece in the heating and cooling process, due to the surface layer and the heart of the cooling rate and time inconsistency, the formation of temperature differences, it will lead to volume expansion and contraction unevenly and produce stress, that is, thermal stress. Under the action of thermal stress, as the surface layer starts at a lower temperature than the heart, the contraction is also greater than the heart and the heart is pulled, and when the cooling ends, the surface layer is compressed and the heart is pulled due to the final cooling volume contraction of the heart that cannot be carried out freely. That is, under the action of thermal stress, the surface layer of the workpiece is finally compressed and the heart is pulled. This phenomenon is influenced by the cooling rate, material composition and heat treatment process.

When the faster the cooling rate, the higher the carbon content and alloy composition, the greater the uneven plastic deformation generated in the cooling process under the action of thermal stress, the greater the final formation of residual stress. On the other hand, the steel in the heat treatment process due to changes in the organization that is austenite to martensite transformation, because the increase in specific volume will be accompanied by the expansion of the volume of the workpiece, the workpiece parts successive phase changes, resulting in volume growth inconsistency and tissue stress. The final result of the change in tissue stress is that the surface layer is subject to tensile stress and the heart is subject to compressive stress, which is exactly the opposite of thermal stress. The magnitude of the tissue stress is related to the cooling rate of the workpiece in the martensitic phase transformation zone, the shape, the chemical composition of the material, etc.

Practice has proven that thermal and tissue stresses occur in any workpiece during heat treatment as long as there is a phase change. Only the thermal stress is generated before the tissue transformation, while the tissue stress is generated during the tissue transformation process. The result of the combined effect of thermal stress and tissue stress during the whole cooling process is the actual stress existing in the workpiece. The result of the combined action of these two stresses is very complex and is influenced by many factors, such as composition, shape, heat treatment process, etc. There are only two types of stresses, namely thermal stress and tissue stress, which cancel each other when they act in opposite directions and iterate each other when they act in the same direction. Regardless of whether they cancel each other or iterate each other, one of the two stresses should be the dominant factor, and the result of the action of thermal stress is that the heart of the workpiece is under tension and the surface is under pressure. When the tissue stress is dominant, the result of the action is that the workpiece is compressed at the heart and pulled at the surface.


2. Effect of Heat Treatment Stress on Quenching Cracks

Presence in different parts of the quenched parts can cause stress concentration factors (including metallurgical defects), the quenching crack generation have a catalytic effect, but only in the tensile stress field (especially under the maximum tensile stress) will show, if in the compressive stress field and no pro-cracking effect.

Quench cooling rate is an important factor that affects quench quality and determines residual stresses, and is also a factor that can exert an important and decisive influence on quench cracking. In order to achieve quenching, it is usually necessary to accelerate the cooling rate of the part in the high temperature section and to exceed the critical quenching cooling rate of the steel in order to obtain a martensitic structure. As far as residual stresses are concerned, this reduces the tensile stresses on the surface of the workpiece and inhibits longitudinal cracking because it increases the value of the thermal stresses that counteract the tissue stresses. The effect will increase with the speed of high temperature cooling.

Moreover, in the case of hardening, the larger the cross-sectional size of the workpiece, although the actual cooling rate is slower, the greater the risk of cracking. All this is due to the size of this type of steel with the increase in the actual cooling rate slows down, the thermal stress decreases, the tissue stress increases with the increase in size, and finally the formation of tissue stress is mainly the role of tensile stress on the surface of the workpiece caused by the characteristics of the action. And with the slower cooling stress smaller traditional concept is very different. For this type of steel parts, only longitudinal cracks can be formed in high hardenability steel parts quenched under normal conditions. Avoid quenching cracking principle is to try to minimize the section inside and outside the martensitic transformation is not equal time. Slow cooling in the martensitic transformation zone alone is not sufficient to prevent the formation of longitudinal cracks.

Generally can only be generated in the non-hardened parts of the arc crack, although the overall rapid cooling as a necessary condition for the formation, but the real reason for its formation, but not in the rapid cooling (including martensite transformation zone) itself, but the local location of the quenched parts (determined by the geometric structure), the cooling rate in the high temperature critical temperature zone is significantly slowed, and therefore not hardened due to. Generated in large non-hardened parts in the cross-sectional fracture and longitudinal splitting, is caused by the residual tensile stresses with thermal stresses as the main component acting in the center of the quenched part, and in the center of the quenched part at the end of the hardened section, the first formation of cracks and expansion from the inside out.

In order to avoid such cracks, a two-liquid quenching process with water and oil is often used. The purpose of rapid cooling in the high-temperature section of this process is simply to ensure that the outer layer of metal is martensitized; from the point of view of internal stresses, rapid cooling at this point is not beneficial. Secondly, the purpose of slow cooling in the late cooling stage is not to reduce the expansion rate of the martensite phase transformation and the tissue stress value, but to minimize the temperature difference in the cross-section and the contraction rate of the metal in the center of the cross-section, so as to reduce the stress value and ultimately inhibit quenching.