What is heat treatment deformation? One article to help you understand!

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This post was last edited by sjytangtang on 2019-9-3 15:24. What is heat treatment deformation? One article to help you understand! I. Causes of deformation The main reason for steel deformation is the presence of internal stresses in the steel or external stresses applied to it. Internal stress is caused by uneven temperature distribution or phase changes, and residual stress is also one of the causes. Deformation caused by external stresses is mainly due to the \"collapse\" resulting from the weight of the workpiece; in special cases, factors such as collisions with the heated workpiece or indentations caused by clamping tools must also be taken into account. Deformation includes two types: elastic deformation and plastic deformation. Dimensional changes are primarily due to structural transformations, thus exhibiting the same expansion and contraction; however, when there are holes or complex shapes in the workpiece, this will result in additional deformation. If quenching results in a large amount of martensite, expansion occurs; if a large amount of residual austenite is formed, contraction takes place accordingly. Furthermore, shrinkage generally occurs during tempering, while alloy steels that exhibit secondary hardening expand. If deep cryogenic treatment is applied, further expansion takes place due to the martensitization of residual austenite. The specific volume of these microstructures increases as the carbon content rises, and therefore an increase in carbon content also leads to a greater change in size. II. Main periods when quenching deformation occurs 1. Heating process: During the heating of the workpiece, deformation occurs as a result of the gradual release of internal stresses. 2. Insulation process: Deformation is mainly caused by self-weight, that is, collapse bending. 3. Cooling process: Deformation occurs due to uneven cooling and structural transformation. III. Heating and Deformation: When heating large workpieces, residual stresses or uneven heating can both cause deformation. Residual stress mainly originates from the processing process. When these stresses are present, as the temperature rises the yield strength of steel gradually decreases, so even a slight stress can cause deformation, regardless of how uniform the heating is. Generally, the residual stress is higher at the outer edge of the workpiece. When the temperature rises starting from the outside, the deformation at the outer edge is greater. The deformation caused by residual stress includes both elastic and plastic deformation. Both thermal stress generated during heating and residual stress are causes of deformation. The faster the heating rate, the larger the workpiece size, and the greater the variation in cross-section, the greater the heating deformation will be. Thermal stress depends on the degree of uneven temperature distribution and temperature gradients, both of which are causes of differences in thermal expansion. If the thermal stress is higher than the material’s high-temperature yield point, plastic deformation occurs, and this plastic deformation is manifested as \"deformation\". Phase transformation stress arises mainly from the heterogeneity of phase transformation, that is, it occurs when one part of the material undergoes phase transformation while other parts have not yet done so. When heated, the microstructure of the material transforms into austenite; volumetric contraction can then lead to plastic deformation. If all parts of the material undergo the same structural transformation simultaneously, no stress is generated. To this end, slow heating can appropriately reduce heating deformation; preheating is the best approach. Furthermore, since \"collapsing\" deformation due to its own weight occurs very frequently during heating, the higher the heating temperature and the longer the heating time, the more severe this \"collapsing\" phenomenon becomes. IV. Cooling and Deformation: Uneven cooling generates thermal stress, leading to deformation. Due to the differences in cooling rates between the outer edge and the interior of the workpiece, such thermal stress is inevitable. During quenching, thermal stress combines with structural stress, resulting in more complex deformation. In addition, factors such as organizational heterogeneity and decarburization can also lead to variations in the phase transition points, as well as differences in the amount of expansion during phase transformation. In short, \"deformation\" is caused by both phase transformation stress and thermal stress; however, not all of this stress is used for deformation – a portion remains as residual stress in the workpiece. It is this residual stress that leads to aging deformation and aging cracks. Deformation caused by cooling manifests in the following forms: 1. At the initial stage of rapid cooling, the side that is cooled rapidly becomes concave, before turning convex; as a result, the side that undergoes rapid cooling becomes convex. This situation is due to thermal stress-induced deformation being greater than that caused by phase transformation. 2. The deformation caused by thermal stress leads the steel to assume a spherical shape (see Figure 1), whereas the deformation caused by phase transformation stress causes it to take on a wire-like shape (see Figure 2). Therefore, the deformation caused by quenching cooling manifests as a combination of the two (Figure 3), and it exhibits different deformations depending on the quenching method, as shown in Figure 4. 3. When only the inner hole is quenched, the inner hole contracts. When the entire ring-shaped workpiece is heated and quenched as a whole, its outer diameter always increases, while the inner diameter expands or contracts depending on the size; generally, when the inner diameter is large, the inner hole expands, and when the inner diameter is small, the inner hole contracts. V. Cryogenic treatment and deformation: Cryogenic treatment promotes the martensitic transformation; at lower temperatures, the deformation resulting is less than that caused by quenching, but the stresses generated are higher. The accumulation of residual stresses, phase transformation stresses, and thermal stresses can easily lead to cracking. VI. Tempering and Deformation: During the tempering process, internal stresses are homogenized, reduced, or even eliminated, and the microstructure changes, which leads to a reduction in deformation. However, once deformation occurs, it is very difficult to correct. To correct this deformation, methods such as pressurized tempering or shot peening hardening are commonly used. VII. Repeated quenching and deformation: Generally, subjecting a workpiece to repeated quenching without intermediate annealing after the first quenching will increase deformation. Figure 5 shows the deformation caused by repeated quenching. After repeated quenching, the deformation accumulates and the shape tends to become spherical; cracks are likely to occur, but the shape remains relatively stable and further deformation is less likely. Therefore, an intermediate annealing step should be added before repeated quenching, and the number of repeated quenchings should be 2 or fewer (excluding the initial quenching). VIII. Residual stress and deformation: During the heating process, at around 450°C, steel changes from an elastic material to a plastic one, which makes it prone to upward plastic deformation. At the same time, residual stress will also disappear due to recrystallization at temperatures slightly above this value. Therefore, during rapid heating, due to the temperature difference between the inside and outside of the workpiece, the outer part reaches 450°C and becomes a plastic zone; as a result, deformation occurs under the effect of residual stresses in the areas with lower internal temperatures. After cooling, this is the area where deformation appears. Since it is difficult to achieve uniform and gradual heating in the actual production process, it is very important to carry out stress-relief annealing before quenching. In addition to relieving stress through heating, vibration is also an effective method for reducing stress in large components.