Company name: wuxi tianfeng metal products co. LTD
Address: no. 39, zhenze road, huazhuang street, economic development zone, wuxi city, jiangsu province (old factory area)
Address: no.210-2, chengnan road, wangzhuang industrial supporting area (new), new district, wuxi city, jiangsu province
Contact person: +86-1377 1199 895
How to deal with the deformation of forgings in heat treatment
In the process of forging heat treatment, it is hard to avoid and sometimes unpredictable deformation occurs, so do not panic when you encounter deformation, how to deal with it, the following will introduce the specific situation.
1. Causes of deformation
The main reason of steel deformation is the existence of internal stress or external stress. Internal stress is caused by uneven temperature distribution or phase change, and residual stress is also one of the reasons. Deformation caused by external stress is mainly due to the workpiece weight caused by "collapse", in special circumstances should also be considered collision heated workpiece, or clamping tools caused by the sag.
Deformation includes elastic deformation and plastic deformation. The size change is mainly based on the organizational transformation, so it shows the same expansion and contraction, but when the workpiece has holes or complex shape of the workpiece, it will lead to additional deformation. If a large amount of martensite is formed after quenching, it will expand, and if a large amount of residual austenite is produced, it will shrink correspondingly. In addition, when tempering, shrinkage generally occurs, while alloy steel with secondary hardening expands. If cryogenic treatment is carried out, it expands further due to martensite of residual austenite. The specific volume of these tissues increases with the increase of carbon content, so the increase of carbon content also increases the size change.
2. The main occurrence period of quenching deformation
1) heating process: during the heating process, the workpiece will be deformed due to the gradual release of internal stress.
2) thermal insulation process: the gravity collapse deformation is the main, that is, collapse and bending.
3) cooling process: due to non-uniform cooling and microstructure transformation, deformation occurs.
3. Heating and deformation
When large workpiece is heated, residual stress or uneven heating can produce deformation. The residual stress mainly comes from the machining process. When these stresses are present, because the yield strength of the steel gradually decreases as the temperature increases, even very slight stresses can cause deformation even if the heating is uniform.
Generally, the residual stress at the outer edge of the workpiece is high. When the temperature rise starts from the outside, the deformation at the outer edge is large. The deformation caused by the residual stress includes elastic deformation and plastic deformation.
Both the thermal stress and the deformation stress are the causes of deformation. The higher the heating speed, the larger the forging size and the larger the section change, the larger the heating deformation. Thermal stress depends on the degree of non-uniform distribution of temperature and temperature gradient. If the thermal stress is higher than the high temperature yield point of the material, it will cause plastic deformation, and the plastic deformation is expressed as "deformation".
The stress of phase change is mainly caused by the anisochrony of phase change, that is, when one part of the material has phase change and the other part has not. Plastic deformation occurs when the microstructure of the material changes into austenite and volume shrinkage occurs. If all parts of the material undergo the same structural transformation at the same time, no stress is produced. Therefore, slow heating can reduce the deformation of heating properly, using preheating.
In addition, there are many cases of "collapse" deformation due to dead weight during heating. The higher the heating temperature, the longer the heating time, and the more serious the "collapse" phenomenon.
4. Cooling and deformation
When the cooling is uneven, the thermal stress will be generated and the deformation will occur. Due to the difference in cooling speed between the outer edge and the inner part of the workpiece, the thermal stress is inevitable. In addition, the inhomogeneity and decarburization of the microstructure also lead to differences in phase transition points and different expansion amounts.
In a word, "deformation" is caused by both phase change stress and thermal stress, but not all of the stress is consumed in the deformation, but part of the residual stress exists in the workpiece, which is the cause of aging deformation and aging crack.
The deformation caused by cooling ii can be expressed in the following forms:
1) at the initial stage of quenching, one side of the quenching is depressed, and then it turns to be convex, and the side of the quenching is convex as a result. The deformation caused by thermal stress is greater than that caused by phase change.
2) the deformation caused by thermal stress is that the steel material tends to be spherical, while the deformation caused by phase change stress makes it tend to be wound like a spool. Therefore, the deformation caused by quenching and cooling is a combination of the two.
3) when the inner hole is only partially quenched, the inner hole shrinks. When the whole ring workpiece is heated and quenched as a whole, its outer diameter will always increase, while the inner diameter will shrink according to different sizes. Generally, when the inner diameter is large, the inner hole will expand and shrink when the inner diameter is small.
5. Cold treatment and deformation
Cold treatment promotes martensite transformation with lower temperature, resulting in less deformation than quenching cooling. However, the stress generated at this time is larger, and the superposition of residual stress, phase change stress and thermal stress can easily lead to cracking.
6. Tempering and deformation
In the tempering process, due to the homogenization, reduction or even disappearance of the internal stress and the change of the structure, the deformation tends to decrease, but at the same time, once the deformation occurs, it is difficult to correct. In order to correct this deformation, the methods such as pressure tempering or peening hardening are often adopted.
7. Repeated quenching and deformation
Under normal circumstances, the workpiece after a quenching without intermediate annealing and repeated quenching, will increase the deformation. The deformation caused by repeated quenching, after repeated quenching, the deformation accumulates and tends to be spherical, which is easy to produce cracking, but the shape is relatively stable, no longer easy to produce deformation, so before repeated quenching should increase the intermediate annealing, repeated quenching times should be less than or equal to 2 times (excluding the first quenching).
8. Residual stress and deformation
Heating process, at about 450 ℃, steel by elastomer into plastic body, making it easy to increase plastic deformation. At the same time, the residual stress will disappear due to recrystallization. Therefore, rapid heating, because the workpiece internal and external temperature difference and external reached 450 ℃ changed into the plastic zone, by which the lower part of the internal temperature of the residual stress and deformation, after cooling, the region is the place where appear deformation. Since it is difficult to achieve uniform and slow heating in the actual production process, it is important to conduct stress relief annealing before quenching. In addition to eliminating stress through heating, it is also effective to adopt vibration to eliminate stress for large parts.