On 2018-08-08 14:48:54
Analysis of heat treatment deformation
Heat treatment deformation results in the scrap of batch precision steel parts, molds and gas cylinders, which brings economic losses to steel product manufacturing enterprises. Therefore, it is necessary to study the heat treatment deformation of steel products. This paper will start with the types and characteristics of heat treatment deformation, then explain the causes of heat treatment deformation, and finally put forward the control methods of heat treatment deformation for the causes of heat treatment deformation.
Analysis of heat treatment deformation
1、 Causes of deformation
The deformation of steel is mainly caused by internal stress or external stress. Internal stress is caused by uneven temperature distribution or phase transformation, and residual stress is one of the reasons. The deformation caused by external stress is mainly due to the "collapse" caused by the self weight of the workpiece. In special cases, it is also necessary to consider the impact of the heated workpiece, or the depression caused by the clamping tool. The deformation includes elastic deformation and plastic deformation. The change of size is mainly based on the change of organization, so it shows the same expansion and contraction, but when there are holes or complex shape workpiece on the workpiece, it will lead to 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, in general, shrinkage occurs during tempering, while the alloy steel with secondary hardening phenomenon expands. If cryogenic treatment is carried out, the martensitization of retained austenite will further expand, and the specific volume of these structures will increase with the increase of carbon content, so the increase of carbon content will also increase the size change.
2、 The main period of quenching deformation
Heating process: during the heating process, the workpiece deforms due to the gradual release of internal stress.
Heat preservation process: it is dominated by self weight collapse deformation, that is, collapse bending.
Cooling process: deformation due to uneven cooling and structural transformation.
3、 Heating and deformation
When heating large workpieces, there are residual stresses or uneven heating, which can produce deformation. The residual stress mainly comes from the machining process. When these stresses exist, the yield strength of the steel decreases with the increase of temperature. Even if the heating is very uniform, the slight stress will lead to deformation.
Generally, the residual stress of the outer edge of the workpiece is higher. When the temperature rise starts from the outside, the deformation of the outer edge is larger. The deformation caused by the residual stress includes elastic deformation and plastic deformation.
The thermal stress and strain stress are the causes of deformation. The faster the heating speed, the larger the workpiece size and the larger the cross-section change, the larger the heating deformation. The thermal stress depends on the uneven distribution of temperature and temperature gradient, which are the reasons for the difference of thermal expansion. If the thermal stress is higher than the high temperature yield point of the material, the plastic deformation will be caused, which is called "deformation".
The phase transformation stress is mainly due to the non isochronism of phase transformation, that is, the phase transformation occurs in some parts of the material, but not in other parts. When heated, the microstructure of the material changes into austenite, and plastic deformation occurs when volume shrinkage occurs. If all parts of the material undergo the same structural transformation at the same time, no stress will be generated. Therefore, slow heating can reduce heating deformation properly, and preheating is the best way.
In addition, there are many cases of "collapse" deformation due to self weight in heating. The higher the heating temperature is, the longer the heating time is, the more serious the "collapse" phenomenon is.
4、 Cooling and deformation
When cooling unevenly, thermal stress will be produced and deformation will occur. Due to the difference of cooling rate between the outer edge and the inner part 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 complex. In addition, the inhomogeneous structure and decarburization will lead to the difference of transformation point and the expansion of transformation.
In a word, "deformation" is caused by both transformation stress and thermal stress, but not all the stress is consumed in deformation, but part of it exists in the workpiece as residual stress, which is the cause of aging deformation and aging crack.
The deformation caused by cooling is shown in the following forms:
1. In the early stage of quenching, one side of the quenched part is sunken and then turned to bulge. As a result, one side of the quenched part is bulged. In this case, the deformation caused by thermal stress is greater than that caused by phase transformation.
2. The deformation caused by thermal stress is that the steel tends to be spheroidized, while the deformation caused by transformation stress tends to be wound. Therefore, the deformation caused by quenching and cooling is the combination of the two, which shows different deformation according to different quenching methods.
3. When the inner hole is partially quenched, the inner hole shrinks. When the whole ring workpiece is heated and quenched as a whole, its outer diameter always increases, while the inner diameter expands and shrinks according to different sizes. Generally, when the inner diameter is large, the inner hole expands and shrinks when the inner diameter is small.
5、 Cold treatment and deformation
Cold treatment promotes martensite transformation, and the temperature is lower, the deformation is smaller than quenching, but the stress is larger at this time, because of the superposition of residual stress, transformation stress and thermal stress, it is easy to cause cracking.
6、 Tempering and deformation
During the tempering process, due to the homogenization, reduction or even disappearance of internal stress, as well as the change of microstructure, the deformation tends to decrease, but at the same time, once the deformation occurs, it is difficult to correct. In order to correct this kind of deformation, pressure tempering or shot peening hardening are often used.
7、 Repeated quenching and deformation
In general, if the workpiece after one quenching is repeatedly quenched without intermediate annealing, the deformation will increase. For the deformation caused by repeated quenching, after repeated quenching, the deformation accumulates and tends to be spherical, easy to produce cracks, but the shape is relatively stable, no longer easy to produce deformation. Therefore, before repeated quenching, the intermediate annealing should be increased, and the number of repeated quenching should be less than or equal to 2 times (excluding the first quenching).
8、 Residual stress and deformation
During the heating process, the steel changes from elastomer to plasticity at about 450 ℃, so it is easy to take up plastic deformation. At the same time, when the residual stress is higher than this temperature, it will disappear because of recrystallization. Therefore, during rapid heating, due to the temperature difference between the inner and outer parts of the workpiece, the outer part reaches 450 ℃ and becomes a plastic zone, which is deformed due to the residual stress at the lower inner temperature. After cooling, the zone is where the deformation occurs. As it is difficult to realize uniform and slow heating in the actual production process, it is very important to carry out stress relief annealing before quenching. In addition to stress relief by heating, vibration stress relief is also effective for large parts.