Use metal surface heat treatment
First, the deformation of steel is mainly caused by internal stress or external stress. Internal stress is caused by non-uniform 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 steel rod. In special cases, the collision of the heated steel rod or the depression caused by the clamping tool should be taken into account. The deformation caused by the external stress is mainly due to the “collapse” of the steel rod due to its own weight. Deformation includes elastic deformation and plastic deformation. The size change is mainly based on the microstructure transformation, so it shows the same expansion and shrinkage. However, when the steel rod has holes or complex shape, 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, shrinkage usually occurs during tempering, while secondary hardening occurs in alloy steels. If cryogenic treatment is carried out, it will be further expanded due to martensization of retained austenite. The specific volume of these structures increases with the increase of carbon content, so the increase of carbon content also increases the size change.
Second, the main period of occurrence of quenching deformation is
1. Heating process: during the heating process, the steel rod deforms due to the gradual release of internal stress.
2. Heat preservation process: self-weight collapse deformation, namely collapse bending.
3. Cooling process: deformation due to uneven cooling and microstructure transformation.
Third, when heating and deformation of large steel bars, there is residual stress or non-uniform heating, which can result in deformation. The residual stress mainly comes from the machining process. When these stresses exist, the yield strength of the steel decreases gradually with the increase of temperature, even if the heating is uniform, the slight stress will lead to deformation. Generally, the residual stress of the outer edge of the steel rod is higher. When the temperature rises from the outside, the deformation of the outer edge is larger. The deformation caused by the residual stress includes two kinds of deformation: elastic deformation and plastic deformation. Both the thermal stress and the desired stress during heating are the causes of deformation. The faster the heating speed, the larger the size of the steel rod and the greater the cross-section change, the greater the heating deformation. The thermal stress depends on the non-uniform distribution of temperature and the 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, it causes the plastic deformation, which is called “deformation”.
Overseas manager: Tom Wang
Phone: 0086-13303078975(whatsapp, wechat,line)
Specialist of bar heat treatment furnace in China; Glad to be your business partner in induction heating field.
Post time: 04-29-2019