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Nine factors that affect heat treatment deformation (一)
- Dec 24, 2018 -

First, the reasons for the deformation


The main cause of deformation of steel is the presence of internal stress or externally applied stress in the steel. The internal stress is caused by uneven temperature distribution or phase transformation, and residual stress is also one of the causes. The deformation caused by the external stress is mainly caused by the "collapse" caused by the weight of the workpiece. In special cases, the workpiece to be heated by the collision or the depression caused by the clamping of the clamping tool should also be considered. The deformation includes elastic deformation and plastic deformation. Dimensional changes are primarily based on tissue transformations and therefore exhibit the same expansion and contraction, but when there are holes or complex shaped workpieces on the workpiece, additional deformation will result. If quenching forms a large amount of martensite, expansion occurs, and if a large amount of retained austenite is produced, it is correspondingly shrunk. In addition, shrinkage generally occurs during tempering, and alloy steel with secondary hardening phenomenon expands. If cryogenic treatment is carried out, it expands further due to martensite of retained austenite. The specific volume of these structures follows As the carbon content increases, the amount of carbon increases also increases the amount of dimensional change.


Second, 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: the main gravity collapse deformation, that is, collapse and bending.


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


Third, heating and deformation


When a large workpiece is heated, there is residual stress or uneven heating, and deformation can occur. The residual stress is mainly derived from the processing. When these stresses are present, the yield strength of the steel gradually decreases as the temperature rises, and even if the heating is uniform, a very slight stress causes deformation.


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


The thermal stress and the intended stress that are generated during heating are both causes of deformation. The faster the heating rate, the larger the workpiece size, and the larger the cross-section change, the greater the heating deformation. Thermal stress depends on the degree of uneven distribution of temperature and temperature gradient, which are both 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 appears as "deformation".


The phase transformation stress is mainly caused by the unequality of the phase transition, that is, when a part of the material undergoes a phase transition, and other parts have not undergone a phase change. Plastic deformation occurs when the structure of the material is transformed into austenite when it undergoes volume shrinkage upon heating. If the same tissue transition occurs simultaneously in all parts of the material, no stress is generated. For this reason, slow heating can appropriately reduce the heating deformation, and it is preferable to use preheating.


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


Fourth, cooling and deformation


When the cooling is uneven, thermal stress will be generated to cause deformation. The thermal stress is unavoidable due to the difference in cooling rate between the outer edge and the inner part of the workpiece. 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 organization, decarburization, etc., will also lead to differences in the phase transition point, and the amount of expansion of the phase change will also be different.


In short, "deformation" is caused by the combination of phase transformation stress and thermal stress, but not all of the stress is consumed in the deformation, but a part of the residual stress is present in the workpiece. This stress is the cause of the aging deformation and the ageing crack.


The deformation caused by cooling is manifested in the following forms:


1. In the initial stage of rapid cooling, the quenched side is sunken and then turned into a bulge. As a result, the cold side is convex. This case is caused by the deformation caused by thermal stress and the deformation caused by the phase change.


2. The deformation caused by thermal stress is that the steel tends to be spheroidized (see Figure 1), and the deformation caused by the phase transformation stress tends to be wound around the axis (see Figure 2). Therefore, the deformation caused by quenching cooling is a combination of the two (Fig. 3), and different deformations are shown in Fig. 4 according to the quenching method.


3. When the inner hole is partially quenched, the inner hole contracts. When the whole annular workpiece is heated and quenched, its outer diameter always increases, and the inner diameter increases and decreases according to the size. When the inner diameter is large, the inner diameter increases, the inner diameter is small, and the inner diameter shrinks.