02/11/2022
5 Factors affecting deformation in heat treatment
First, the cause of deformation
The main reason of steel deformation is the internal stress or external stress in steel. The internal stress is caused by uneven temperature distribution or phase transformation, and the residual stress is also one of the reasons. The deformation caused by external stress is mainly due to the "collapse" caused by the weight of the workpiece. In special cases, the impact of the heated workpiece or the depression caused by the clamping tool should also be considered. Deformation includes elastic deformation and plastic deformation. The size change is mainly based on the tissue transformation, so it exhibits the same expansion and contraction, but when the workpiece has holes or complex shape workpiece, additional deformation will be caused. If a large amount of martensite is formed by quenching, the expansion will occur, and if a large amount of residual austenite is produced, the corresponding shrinkage will occur. In addition, the general shrinkage occurs during tempering, while the alloy steel with secondary hardening phenomenon will expand. If cryogenic treatment is carried out, it will further expand due to martensitization of residual austenite.
Second, the main occurrence period of quenching deformation
1. Heating process: workpiece in the heating process, due to the gradual release of internal stress and deformation.
2. Heat preservation process: dead weight collapse deformation is the main deformation, namely collapse bending.
3. Cooling process: deformation due to uneven cooling and tissue transformation.
Three, heating and deformation
When heating large workpiece, there is residual stress or uneven heating, can produce deformation. The residual stress mainly comes from the machining process. In the presence of these stresses, even very mild stresses can cause deformation because the yield strength of the steel decreases gradually with increasing temperature.
Four, cooling and deformation
When the cooling is uneven, thermal stress will be generated 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 tissue stress are superimposed, and the deformation is more complicated. In addition to the uneven tissue, decarbonization, will also lead to differences in the phase transition point, the expansion of the phase transition is also different.
Five, cold treatment and deformation
The cold treatment promotes martensite transformation, and the temperature is lower, so the deformation is smaller than that of quenching cooling, but the stress generated at this time is larger, and the superposition of residual stress, phase transformation stress and thermal stress is easy to lead to cracking.
