Main factors affecting heat treatment deformation

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I. Reasons for 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 “collapsing” resulting from the weight of the workpiece itself; in special cases, one must also consider impacts on heated workpieces or dents caused by clamping tools. 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 in the workpiece or the workpiece has a complex shape, 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. Additionally, contraction generally occurs during tempering; in alloy steels exhibiting secondary hardening, expansion takes place. When cryogenic treatment is applied, further expansion occurs due to the martensitization of residual austenite. The specific volume of these microstructures increases with increasing carbon content; therefore, an increase in carbon content also leads to a greater degree of dimensional change. II. Main periods when quenching deformation occurs
1. Heating process: During the heating process, the workpiece undergoes deformation due to the gradual release of internal stresses. 2. Insulation process: Primarily characterized by deformation due to self-weight collapse, namely 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 the thermal stress generated during heating and the 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 primarily 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. During heating, plastic deformation can occur when the microstructure of the material transforms into austenite and undergoes volume contraction. If all parts of a 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. 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. In the case of quenching, thermal stress combines with structural stress, resulting in more complex deformation. In addition, irregularities in the structure and decarburization can also lead to differences in phase transition temperatures, as well as variations 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 utilized for deformation – a portion remains as residual stress in the workpiece, and it is this stress that leads to aging deformation and aging cracks.