Factors affecting the deformation of workpieces

Thread Content

1. Causes of deformation The main causes of steel deformation are internal stresses within the steel or externally applied stresses. Internal stress is caused by non-uniform temperature distribution or phase changes; residual stress is also one of the reasons. 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. 2. The main periods when hardening deformation occurs: 1) Heating process: During heating, the workpiece deforms as internal stresses are gradually released. 2) Insulation process: Deformation is mainly due to self-weight collapse, that is, collapse bending. 3) Cooling process: Deformation due to uneven cooling and structural transformation. 3. Heating and deformation: When heating large workpieces, residual stresses or uneven heating can both lead to 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, despite uniform heating. 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 the thermal stress generated during heating and the phase transformation 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 temperature unevenness and the temperature gradient, 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 heterochrony of phase transformation, that is, it occurs when 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 \"collapse\" 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 the \"collapse\" phenomenon becomes. 4. 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, this thermal stress is inevitable; during quenching, thermal stress combines with structural stress, resulting in more complex deformation. In addition, factors such as uneven microstructure 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-change stress and thermal stress. However, not all of the stress is utilized for deformation; part of it remains in the workpiece as residual stress. This residual stress is what 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 sunken, before turning convex; as a result, the side that is cooled quickly 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 to the steel taking on a spherical shape, whereas the deformation caused by phase transformation stress causes it to assume a spindle-like shape; thus, the deformation resulting from quenching and cooling is a combination of these two effects. 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. 5. Cold treatment and deformation: Cold treatment promotes the martensitic transformation. At lower temperatures, the resulting deformation is less than that caused by quenching cooling. However, the stresses generated during this process are greater. The superposition of residual stresses, transformation stresses, and thermal stresses can easily lead to cracking. 6. Tempering and deformation: During the tempering process, due to the equalization, reduction, or even disappearance of internal stresses, as well as changes in the microstructure, deformation tends to decrease. 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. 7. Repeated quenching and deformation: Generally, when a workpiece that has been quenched once is subjected to repeated quenching without intermediate annealing, the deformation will increase. The deformation caused by repeated quenching accumulates over multiple cycles, eventually resulting in a spherical shape. This makes the material prone to cracking; however, its shape becomes relatively stable, and further deformation is less likely to occur. Therefore, an intermediate annealing process should be performed before repeated quenching, and the number of repeated quenching cycles should be limited to two or fewer (excluding the initial quenching). 8. Residual stress and deformation: During the heating process, at around 450°C, the steel changes from an elastic material to a plastic one, making 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, while the lower-temperature areas inside experience deformation as a result of residual stresses. After cooling, it is in these areas that deformation occurs. 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 eliminating stress through heating, using vibration to remove stress is also effective for large components.