On 2018-08-08 14:48:54
effective way to control the deformation of heat treatment
Heat treatment deformation will make the workpiece early processing precision suffered serious losses, sometimes even through complex, advanced shape modification technology (grinding, straightening, etc.) also difficult to recover the loss. This will directly affect the workpiece precision, strength, running noise, vibration, transmission power loss, and service life. So even if we have the world's most advanced machine tool, grinder, it is difficult to produce high precision, high added value products.
In order to reduce and control the deformation of heat treatment and improve market competitiveness, some advanced countries set up special funds. The importance of reducing and controlling heat treatment deformation can be seen.
Causes of heat treatment deformation:
Reducing and controlling the deformation of workpiece during heat treatment is the most concerned problem of material and heat treatment workers.
Scholars generally believe that the factors affecting the deformation of workpiece during heat treatment involve the design of workpiece, raw materials and many links in the whole process of processing. Many professionals believe that comprehensive consideration is important in processing, but the most important contradiction affecting the deformation of heat treatment is the reasonable control of heat treatment process temperature and cooling rate.
The research shows that with the reduction of heat treatment temperature, the accuracy loss caused by deformation of workpiece after heat treatment is reduced from 2 ~ 3 to below 1, which is far more significant than the tooth grinding, straightening and other shape modification technology in the later period.
The strength of the workpiece at 900℃ is very low; Although the heat treatment equipment is more and more advanced, but the workpiece in heating, cooling temperature changes are difficult to be completely consistent; When the workpiece is heated and cooled, the temperature variation of different parts will cause the thermal (expansion) stress of the workpiece and the change of the stress of the microstructure (volume changes when the workpiece is transformed). When the thermal stress, microstructure stress or a combination of the two is greater than the plastic resistance of a part of the workpiece at the instantaneous temperature, an irreversible deformation -- heat treatment deformation -- will occur at this part.
If the process temperature decreases, the high temperature strength loss of the workpiece decreases and the plastic resistance increases. In this way, the workpiece's resistance to stress deformation and high temperature creep (the workpiece is deformed due to dead weight or compression, and the large and thin wall pieces are more significant) will be enhanced and the deformation will be reduced.
If the process temperature is reduced, the temperature inconsistencies in various parts of the workpiece during heating and cooling will also be reduced, resulting in a relative reduction in thermal stress and microstructure stress, so that the deformation will be reduced.
The heating time of heat treatment is shortened, the high temperature creep time of the workpiece is reduced, and the deformation is also reduced.
Methods for reducing heat treatment temperature:
Reducing the process temperature and increasing the carburizing or carbonitriding speed have been the ideal goal of heat treatment experts at home and abroad for several decades, but due to the limitations of basic technical conditions and traditional heat treatment theory, it has been difficult to break through for many years.
At present, a new carburizing technology has been adopted by many domestic enterprises. This technology can achieve rapid carburizing or carbonitriding under the condition of lower process temperature, and the lowest can make the carburizing temperature to about 810℃, it also has the advantages of energy saving, environmental protection, high efficiency, high efficiency. This is a new heat treatment technology based on the traditional heat treatment theory and the latest modern chemical control principle.
The main factors that affect the deformation of heat treatment are that the high temperature strength loss of the workpiece decreases and the plastic resistance increases after the process temperature decreases. In this way, the comprehensive ability of the workpiece to resist stress deformation, quenching deformation and high temperature creep is enhanced, and the deformation will be reduced.
After the process temperature is lowered, the temperature range of heating and cooling of the workpiece will be reduced, and the temperature inconsistencies of various parts caused by this will be reduced. The thermal stress and microstructure stress caused by this will also be relatively reduced, so that the deformation will be reduced.
As the process temperature decreases and the heat treatment process time shortens, the high temperature creep time of the workpiece decreases and the deformation also decreases.
In order to ensure the final accuracy of the workpiece, a more ideal method is to find the deformation law of the workpiece heat treatment, during the processing to reserve a certain amount of deformation, so that the size of the workpiece after carburizing and quenching migration to the required range. This requires that the deformation of the workpiece has a good consistency, that is, the deformation law and the deformation range between the workpiece of the same batch and the workpiece of different batch are close to the same.
Although the deformation of heat treatment is difficult to control, but by reducing the process temperature, control the workpiece's pre-heat treatment conditions and quenching conditions of the workpiece for strict control.
The cooling rate and method determine the deformability of the part.
The selection of quenching medium with reasonable cooling rate can effectively reduce the deformation.
Choose the method of rapid cooling at high temperature (above 400℃) and slow cooling at below 350℃ to achieve the best effect; Such as: some salt bath, alkali bath and organic quenching agent.
Precooling and quenching.