On 2018-08-08 14:48:54
Effect of heat treatment stress on quenching crack
Heat treatment refers to the material in the solid state, through the means of heating, insulation and cooling, in order to obtain the expected structure and performance of a metal thermal processing process. Metal heat treatment is one of the important processes in mechanical manufacturing, compared with other processing processes, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but by changing the microstructure of the workpiece, or change the chemical composition of the workpiece surface, give or improve the performance of the workpiece.
Metal heat treatment is a kind of process method that the metal workpiece is heated to the appropriate temperature in a certain medium and kept in the temperature for a certain period of time before being cooled at different speeds. Nantong heat treatment is characterized by the improvement of the inherent quality of the workpiece, which is generally not visible to the naked eye.
The influence of heat treatment stress on quenching crack is as follows:
The factors (including metallurgical defects) that can cause stress concentration in different parts of the quench parts can promote the generation of quench cracks, but only in the tensile stress field (especially under the maximum tensile stress) can be shown, if there is no crack promoting effect in the compressive stress field.
The quenching cooling rate of workpiece is an important factor that can affect the quenching quality and determine the residual stress. In order to achieve the purpose of quenching, it is usually necessary to accelerate the cooling speed of parts in the high temperature section and make it exceed the critical quenching cooling speed of steel to obtain martensite structure.
As far as residual stress is concerned, it can reduce the tensile stress on the surface of the workpiece and suppress longitudinal cracking because it can increase the thermal stress value which can offset the effect of the tissue stress. The effect will increase with the speed of high temperature cooling. Moreover, under the condition of being able to quench, the larger the section size of the workpiece, although the actual cooling speed is slower, the greater the risk of cracking. All of these are caused by the fact that the thermal stress of this kind of steel slows down with the increase of the size, the thermal stress decreases, the microstructure stress increases with the increase of the size, and finally the tensile stress, mainly the microstructure stress, ACTS on the surface of the workpiece. It is quite different from the traditional idea that the slower the cooling is, the less the stress is. For this type of steel, only longitudinal cracks can be formed in high hardenability steel quenched under normal conditions.
The reliable principle to avoid quenching and cracking is to try to minimize the unequal time of martensite transformation inside and outside the cross section. Slow cooling in the martensite transition zone alone is not enough to prevent the formation of longitudinal cracks. 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 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. At the center of the quenched section of the quenched part, cracks are first formed and spread from the inside out. In order to avoid this kind of crack, water - oil double - liquid quenching process is often used. Rapid cooling at high temperatures is carried out in this process only to ensure that the outer metal has a martensite structure, and from the point of view of internal stress, rapid cooling is detrimental. Secondly, the purpose of slow cooling in the later period of cooling is not to reduce the expansion rate and tissue stress value of martensitic phase transformation, but to minimize the temperature difference in the section and the contraction rate of the metal in the center of the section, so as to reduce the stress value and finally suppress the quenching and cracking.