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Discussion on factors affecting heat treatment deformation

Issuing time:2020-11-27 09:12Author:momo


Causes of deformation

The main reason for the deformation of steel is the internal stress or externally applied stress in the steel. Internal stress is caused by uneven temperature distribution or phase change, and residual stress is also one of the reasons. The deformation caused by external stress is mainly due to the "collapse" caused by the weight of the workpiece. Under special circumstances, collision with the heated workpiece or the depression caused by the clamping of the clamping tool should also be considered. Deformation includes elastic deformation and plastic deformation.The dimensional change is mainly based on the transformation of the structure,so it shows the same expansion and contraction, but when the workpiece has holes or complex shapes, it will cause additional deformation. If a large amount of martensite is formed by quenching, it will expand, and if a large amount of retained austenite is produced, it will shrink accordingly. In addition, generally shrink during tempering, while the secondary hardening phenomenon of alloy steel expands. If cryogenic treatment is carried out, it will expand further due to the martensitization of retained austenite. The increase in carbon content increases, so the increase in carbon content also increases the dimensional change.

The main period of quenching deformation

Heating process: The workpiece deforms due to the gradual release of internal stress during the heating process.

Insulation process: mainly collapse and deformation under its own weight, that is, collapse and bend.

Cooling process: deformation due to uneven cooling and structural transformation.

Heating and deformation

When heating large workpieces, residual stress or uneven heating can cause deformation. There sidual stress mainly comes from the machining process. When these stressesexist, as the temperature rises, the yield strength of the steel gradually decreases, even if the heating is very uniform, very slight stress will caused eformation.

Generally, the residual stress at the outer edge of the workpiece is relatively high. When the temperature rise starts from the outside, the deformation at the outer edge isrelatively large. The deformation caused by the residual stress include selastic deformation and plastic deformation.

The thermal stress generated during heating and the desired change stress are both causes of deformation. The faster the heating speed, the larger the size of the workpiece, and the greater the cross-sectional change, the greater the heating deformation.Thermal stress depends on the degree of uneven temperature distribution and temperature gradient, which are all causes of differences in thermal expansion. If the thermal stress is higher than the high temperature yield point of the material, plastic deformation is caused, and this plastic deformation is expressed as"deformation".

The phase change stress is mainly due to the unequal time of the phase change, that is, when a part of the material undergoes a phase change, while the other part has not undergone aphase change. When heated, the structure of the material transforms into austenite and plastic deformation occurs when the volume shrinks. If all parts of the material undergo the same organizational transformation at the sametime, no stress will be generated. For this reason, slow heating can appropriately reduce heating deformation, and preheating is best.

In addition, there are many cases of "collapse" deformation due to its own weight during heating.The higher the heating temperature, the longer the heating time, and the more serious the "collapse" phenomenon.

Cooling and deformation

When the cooling is uneven,thermal stress will cause deformation. Due to the difference in cooling rate between the outer edge and the inside of the workpiece, the thermal stress is inevitable. In the case of quenching, the thermal stress and the structural stress are superimposed, and the deformation is more complicated. In addition,the unevenness of the structure and decarburization will also lead to differences in the phase transition point and the expansion of the phase   transition.

In short,"deformation" is caused by both phase transformation stress and thermal stress, but not all of the stress is consumed in deformation, but apart of it exists in the workpiece as residual stress. This stress is the cause of aging deformation and aging cracking.

The deformation caused by cooling takes the following forms:

At the initial stage of quenching, the quenched side is recessed and then turned into a bulge. As aresult, the faster-cooling side is bulging. In this case, the deformation caused by thermal stress is greater than the deformation caused by phase change.

The deformation caused by the thermal stress is that the steel material tends to be spherical, and the deformation caused by the phase transformation stress makes it tend to be spool-shaped. Therefore, the deformation caused by quenching and cooling is acombination of the two, which shows different deformations according to different quenching methods.

When only part of the inner hole is quenched, the inner hole shrinks. When the whole ring-shaped workpieceis heated and quenched as a whole, its outer diameter always increases, while the inner diameter expands and contracts according to the size. Generally, when the inner diameter is large, the inner hole expands, and when the inner diameteris small, the inner hole shrinks.

Cold treatment and deformation

Cold treatment promotes martensite transformation, the temperature is lower, and the deformation is smaller than quenching cooling, but the stress generated at this time is larger, and the superposition of residual stress, phase transformation stress and thermal stress can easily lead to cracking.

Tempering and deformation

In the process of tempering,due to the homogenization, reduction or even disappearance of internal stress,the deformation of the workpiece tends to be reduced due to changes in the structure, but at the same time, once the deformation occurs, it is difficult to correct. In order to correct this deformation, methods such as pressure tempering or shot peening are often used.

Repeated quenching and deformation

Under normal circumstances,the workpiece after a quenching is repeated quenching without intermediate annealing, which will increase the deformation. The deformation caused by repeated quenching, after repeated quenching, the deformation tends to be spherical and easy to produce cracks, but the shape is relatively stable, and no deformation is easy to occur. Therefore, the intermediate annealing should be increased before repeated quenching, and the number of repeated quenching should be less than Equal to 2 times (excluding the first quenching).

Residual stress and deformation

During the heating process,at about 450°C, the steel changes from an elastic body to a plastic body, so it is easy to exhibit upward plastic deformation. At the same time, the residual stress will disappear due to recrystallization when it is about above this temperature. Therefore, during rapid heating, due to the temperature difference between the inside and outside of the workpiece, the outside reaches 450°C and becomes a plastic zone, which deforms due to residual stress at the lower internal temperature. After cooling, this zone is the place where deformationoccurs. Since it is difficult to achieve uniform and slow heating in the actual production process, it is very important to perform stress relief annealing before quenching. In addition to stress relief through heating, vibration relief is also effective for large parts.

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