The technological process of metal heat treatment

Metal heat treatment is a process of heating, insulating and cooling the metal workpiece in a certain medium, and controlling its properties by changing the surface or internal structure of the metal material.

Heat treatment residual force 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 causes the deformation of the workpiece, exceeding the strength limit of the material will make the workpiece cracking, this is its harmful side, should be reduced and eliminated. But under certain conditions to control the stress to make it reasonable distribution, can improve the mechanical properties and service life of parts, become harmful to benefit. It is of great practical significance to analyze the distribution and change law of stress in the heat treatment process of steel and make it reasonably distributed. For example, the influence of the reasonable distribution of surface residual compressive stress on the service life of parts has been paid much attention.

(I) heat treatment stress of steel

During the heating and cooling process of the workpiece, due to the surface and the core of the cooling speed and time is not consistent, the formation of temperature difference, will lead to the volume expansion and contraction of uneven stress, that is, thermal stress. Under the action of thermal stress, because the surface temperature is lower than the core at the beginning and the shrinkage is greater than the core, the core will be pulled. When the cooling is finished, the surface will be compressed and the core will be pulled because the final cooling volume contraction of the core cannot proceed freely. That is, under the action of thermal stress, the surface of the workpiece is finally 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 speed is faster, the carbon content and alloy composition are higher, and the non-uniform plastic deformation under the thermal stress during the cooling process is larger, the residual stress is larger. On the other hand, in the process of heat treatment of steel, due to the change of microstructure, that is, the transformation of austenite to martensite, the increase of specific volume will be accompanied by the expansion of the workpiece volume, the various parts of the workpiece phase transition successively, resulting in the inconsistence of the volume growth and the generation of microstructure stress. The ultimate result of the change of tissue stress is the tensile stress on the surface and the compressive stress on the core, which is the opposite of the thermal stress. The microstructure stress is related to the cooling rate, shape and chemical composition of the workpiece in martensitic phase transition zone.

It has been proved by practice that thermal stress and microstructure stress will occur in any workpiece during heat treatment as long as there is phase transition. However, the thermal stress has been generated before the tissue transformation, while the tissue stress is generated during the process of 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 combined action of these two stresses is very complex, which is influenced by many factors, such as composition, shape, heat treatment process, etc. In terms of its development, there are only two types, namely thermal stress and tissue stress, which cancel out when the action direction is opposite, and superimpose on each other when the action direction is the same. Whether they cancel each other or superposition each other, the two stresses should have one dominant factor. When thermal stress dominates, the result is that the workpiece's core is under tension and the surface is under pressure. As a result, the surface tension of the workpiece under compression is the result of the predominant stress in the microstructure.

(ii) influence of heat treatment stress on quenching crack

The factors (including metallurgical defects) that can cause stress concentration in different parts of the quenchings can promote the generation of quenchings, but only in the tensile stress field (especially under the maximum tensile stress), if in the compressive stress field does not promote the cracking.

Quenching cooling rate is an important factor that can affect the quenching quality and determine the residual stress, and also a factor that can exert important and 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 make it exceed the critical quenching cooling rate of the steel in order to obtain martensite structure. As far as the residual stress is concerned, this can increase the thermal stress value to offset the effect of the stress on the tissue, and thus reduce the tensile stress on the surface of the workpiece to achieve the purpose of restraining the longitudinal crack. The effect will increase with the speed of high temperature cooling. Moreover, in the case of quenching, the larger the section size of the workpiece, although the actual cooling rate is slower, the greater the risk of cracking. All this is due to the fact that the thermal stress of this kind of steel slows down with the increase of the size of the actual cooling rate, the thermal stress decreases, the microstructure stress increases with the increase of the size, and finally the formation of the microstructure stress mainly tensile stress acting on the surface of the workpiece action characteristics. And contrary to the traditional idea that the slower the cooling, the less stress. For this kind of steel, only longitudinal crack can be formed in high hardenability steel under normal conditions. The reliable principle to avoid quenching is to try to minimize the time inequality of the transformation of martensite inside and outside the section. Slow cooling in the martensite transition zone alone is not enough to prevent the formation of longitudinal fractures. In general can only be produced in the hardenability of the crack, although the necessary forming conditions of the rapid cooling as a whole, but it's really the reasons, but not in rapid cooling (including the martensitic transformation zone) itself, but a partial quenching position (determined by the geometrical structure), in the critical temperature of high temperature zone significantly slow cooling rate, thus no hardening. The transverse and longitudinal splits produced in the large non-hardenable parts are caused by the residual tensile stress with the thermal stress as the main component acting on the center of the quenched part, and at the center of the section where the quenched part was last hardened, the crack first forms and expands from inside to outside. In order to avoid such cracks, water - oil double - liquid quenching process is often used. In this process, rapid cooling is carried out in the high temperature section only to ensure that the outer metal gets martensite structure, when rapid cooling is harmful from the point of view of internal stress. Secondly, the purpose of 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 temperature difference of the section and the contraction rate of the metal at the center of the section, so as to reduce the stress value and finally suppress the quenching.

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