Factors Affecting Heat Treatment Deformation

The reason for the deformation

The main reason for the deformation of steel is the existence of internal stress or externally applied stress in the steel. Internal stress is caused by uneven temperature distribution or phase transformation, and residual stress is also one of the reasons. The deformation caused by external stress is mainly caused by the “collapse” caused by the weight of the workpiece. In special cases, the collision with the heated workpiece or the depression caused by the clamping tool should also be considered. Deformation includes elastic deformation and plastic deformation. Dimensional changes are primarily based on tissue transitions, and therefore exhibit the same expansion and contraction, but additional deformations will result when the workpiece has cavities or complex-shaped workpieces. Expansion occurs if a large amount of martensite is formed by quenching, and the corresponding shrinkage occurs if a large amount of retained austenite is formed. In addition, shrinkage generally occurs during tempering, while the alloy steel with secondary hardening phenomenon expands. If cryogenic treatment is performed, it will further expand due to martensiteization of retained austenite. The specific volume of these structures increases with The increase in carbon content increases, so the increase in carbon content also increases the amount of dimensional change.

The main occurrence period of quenching deformation

Heating process: During the heating process, the workpiece is deformed due to the gradual release of internal stress.
Insulation process: mainly collapse and deform under its weight, that is, collapse and bend.
Cooling process: deformation due to uneven cooling and tissue transformation.

Heating and deformation

When heating large workpieces, there is residual stress or uneven heating, which can cause deformation. Residual stress mainly comes from the machining process. When these stresses are present, very slight stresses can cause deformation even if the heating is uniform, since the yield strength of the steel gradually decreases with increasing temperature.

Generally, the residual stress at the outer edge of the workpiece is high. When the temperature rise starts from the outside, the outer edge is deformed greatly. The deformation caused by the residual stress includes elastic deformation and plastic deformation.

Both the thermal stress and the imaginary stress generated during heating are the causes of deformation. The faster the heating speed, the larger the size of the workpiece, and the larger the change of the section, the greater the heating deformation. Thermal stress depends on the degree of temperature inhomogeneity and temperature gradients, both of which are responsible for differences in thermal expansion. If the thermal stress is higher than the high-temperature yield point of the material, it will cause plastic deformation, which is manifested as “deformation”.

The phase transition stress is mainly due to the anisochronous nature of the phase transition, that is, a part of the material undergoes a phase transition, while other parts have not yet undergone a phase transition. When heated, the microstructure of the material transforms into austenite and plastic deformation occurs when volume shrinks. If the same structural transformation occurs in all parts of the material at the same time, no stress occurs. For this reason, slow heating can appropriately reduce heating deformation, preferably preheating.

In addition, there are many cases of “collapse” deformation due to self-weight during heating. The higher the heating temperature and the longer the heating time, the more serious the “collapse” phenomenon.

Cooling and deformation

Uneven cooling will generate thermal stress and lead to deformation. Due to the difference in cooling rate between the outer edge and the interior of the workpiece, thermal stress is unavoidable. In the case of quenching, the thermal stress and the structural stress are superimposed, and the deformation is more complicated. In addition, the inhomogeneous structure, decarburization, etc. will also lead to differences in the phase transition point, and the amount of expansion of the phase transition is also different.

In a word, “deformation” is caused by the combination of phase transformation stress and thermal stress, but not all the stress is consumed in the deformation, but part of it exists in the workpiece as residual stress, which is the cause of aging deformation and aging cracks.

Deformation due to cooling takes the following forms:

In the early stage of quenching, the quenched side is concave, and then turned into a convex, and the result is that the fast-cooled side is convex. This situation belongs to the deformation caused by thermal stress greater than the deformation caused by phase transformation.

The deformation caused by thermal stress is that the steel tends to spheroidize, while the deformation caused by phase transformation stress makes it tend to be spool-like. Therefore, the deformation caused by quenching and cooling is a combination of the two, and different deformations are shown according to the different quenching methods.

When only the inner hole is partially quenched, the inner hole shrinks. When the whole annular workpiece is heated and quenched as a whole, its outer diameter always increases, while the inner diameter expands and shrinks at the same time 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.

Cold treatment and deformation

The cold treatment promotes martensitic transformation, the temperature is lower, and the resulting deformation is smaller than quenching cooling, but the stress generated at this time is larger, and cracking is easily caused due to the superposition of residual stress, transformation stress, and thermal stress.

Tempering and deformation

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

Repeated quenching and deformation

Usually, the workpiece after one-time quenching is repeatedly quenched without intermediate annealing, which will increase the deformation. The deformation is caused by repeated quenching, after repeated quenching, the deformation accumulates and tends to be spherical, which is prone to cracks, but the shape is relatively stable, and it is no longer easy to deform. Therefore, intermediate annealing should be added before repeated quenching, and the number of repeated quenching should be less than Equal to 2 times (excluding the first quench).

Residual stress and deformation

During the heating process, at around 450°C, the steel is transformed from an elastomer to a plastic body, so it is easy to show upward plastic deformation. At the same time, the residual stress will also disappear due to recrystallization 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 is deformed by residual stress at a lower internal temperature. After cooling, this area is where the deformation occurs. Since it is difficult to achieve a 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 by heating, vibration relief is also effective for large parts.

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