Several factors affecting heat treatment deformation


Several factors affecting heat treatment deformation

1. Reasons for deformation

The main reason for the deformation of steel is the presence of 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" of the workpiece due to its own weight. 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-shaped workpieces, 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 contraction occurs during tempering, while the secondary hardening phenomenon of alloy steel expands. If cryogenic treatment is carried out, it will further expand due to the martensitization of retained austenite, and the specific volume of these structures will follow The increase in carbon content increases, so the increase in carbon content also increases the amount of dimensional change.

2. The main period of quenching deformation

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

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

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

Three, heating and deformation

When heating large workpieces, residual stress or uneven heating can cause deformation. The residual stress mainly comes from the machining process. When these stresses exist, as the temperature increases, the yield strength of the steel gradually decreases, even if the heating is uniform, a very slight stress will cause deformation.

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 is relatively large. The deformation caused by the residual stress includes elastic 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 distribution of temperature 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, it will cause plastic deformation, and this plastic deformation is expressed as "deformation".

  The phase change stress is mainly caused by 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 a phase 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 same time, 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 "collapsing" deformation due to its own weight during heating. The higher the heating temperature, the longer the heating time, and the more serious the "collapsing" phenomenon.

Fourth, cooling and deformation

   Uneven cooling will cause thermal stress to 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 amount of expansion of the phase transition is also different.

In short, "deformation" is caused by both phase transformation stress and thermal stress, but not all stress is consumed in deformation, but a part 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:

1. In the initial stage of quenching, the quenched side is recessed, and then turned into a bulge. As a result, the quenched side is raised. In this case, the deformation caused by thermal stress is greater than the deformation caused by phase change.

2. The deformation caused by thermal stress is that the steel tends to be spherical (see Figure 1), and the deformation caused by the phase transformation stress makes it tend to be spool-shaped (see Figure 2). Therefore, the deformation caused by quenching and cooling is a combination of the two (Figure 3). According to the different quenching methods, different deformations are shown in Figure 4.

3. When only part of the inner hole is quenched, the inner hole shrinks. When the entire ring-shaped workpiece is 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 diameter is small, the inner hole shrinks.

Five, 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.

Six, tempering and deformation

In the process of tempering, due to the homogenization, reduction or even disappearance of internal stress of the workpiece, and the change of the structure, the deformation tends to decrease, 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.

Seven, repeated quenching and deformation

Under normal circumstances, the workpiece after a quenching is repeatedly quenched without intermediate annealing, which will increase the deformation. The deformation caused by repeated quenching, after repeated quenching, its 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).

8. 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 the outside of the workpiece, the outside reaches 450°C and becomes a plastic zone, which deforms under the action of residual stress at the lower internal temperature. After cooling, this zone is the place where the deformation occurs. 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.

(Content source: material heat treatment)