Cause Analysis Of Deformation In Heat Treatment Process

1. Reasons for 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 corresponding shrinkage occurs if a large amount of retained austenite is formed. In addition, shrinkage generally occurs during tempering, while 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 The increase in carbon content increases, so the increase in carbon content also increases the amount of dimensional change.

2. The main occurrence period of quenching deformation:
1. Heating process: During the heating process, the workpiece is deformed due to the gradual release of internal stress.
2. Insulation process: mainly collapse and deform under its own weight, that is, collapse and bend.
3. Cooling process: deformation due to uneven cooling and tissue transformation.

3. 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 relatively 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.

4. 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, the 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.

The deformation caused by cooling is manifested in the following forms: 1. In the initial stage of quenching, the side of quenching is concave, and then turns to be convex, and the result is that the side of rapid cooling is convex, which belongs to the deformation caused by thermal stress is greater than the phase transition deformation caused. 2 The deformation caused by thermal stress is that the steel tends to be spherical, while the deformation caused by the transformation stress makes it tend to be spool-shaped. 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. 3 When only the inner hole is partially quenched, the inner hole shrinks. When the entire 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.

5. Cold treatment and deformation
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 by the superposition of residual stress, transformation stress and thermal stress.

6. Tempering and deformation
During the tempering process of the workpiece, the deformation tends to be reduced due to the homogenization, reduction or even disappearance of the internal stress, and the change of 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.

7. Repeated quenching and deformation
Usually, the workpiece after one-time quenching is repeatedly quenched without intermediate annealing, which will increase the deformation. Figure 5 shows the deformation 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 no longer prone to deformation. Therefore, intermediate annealing should be added before repeated quenching and repeated quenching. The number of times should be less than or equal to 2 times (excluding the first quenching).

8. 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 about 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 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 by heating, vibration relief is also effective for large parts.

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