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Correction of heat treatment deformation

April 24, 2024

Correction of heat treatment deformation - the heat treatment deformation of a workpiece can be controlled and reduced to a certain extent, but can't be completely avoided.


Mechanical correction method - using mechanical or local heating methods to cause local micro plastic deformation of the deformed workpiece, accompanied by the release and redistribution of residual internal stress to achieve the purpose of correcting deformation. The commonly used mechanical correction methods include cold pressing correction, hot pressing correction before quenching and cooling to room temperature, pressure tempering correction, "hot spot" correction using oxygen acetylene flame or high-frequency local heating of deformed workpieces, hammering correction, etc. Mechanical corrected parts may partially recover their original deformation and generate new deformation due to the attenuation and release of residual stress during use, placement, or precision machining. Therefore, it is best not to perform mechanical calibration on workpieces and precision parts that are subjected to high loads. When mechanical correction is necessary, the plastic strain achieved by correction should exceed the plastic strain caused by heat treatment deformation, but the amount of corrected plastic deformation must be controlled within a very small range, generally greater than 10 times the elastic limit strain and less than one tenth of the conditional strength limit. Calibration should be carried out as soon as possible after quenching, and residual stress should be eliminated after calibration. The calibration of deformed workpieces during heat treatment requires operators to have proficient skills and is time-consuming. Therefore, calibration automation is an important task for heat treatment workers.


Heat treatment correction method - for workpieces with dimensions exceeding the tolerance due to expansion or contraction deformation caused by heat treatment, appropriate heat treatment methods can be used again to correct their deformation.


The commonly used heat treatment correction method is the heating and rapid cooling method at Ac1 temperature to shrink the swollen and deformed workpiece - the workpiece does not undergo a phase change with a specific volume change in structure, so it does not generate tissue stress, only thermal stress formed due to different heat shrinkage amounts in the center and surface. During rapid cooling, the surface of the workpiece shrinks rapidly, exerting compressive stress on the core with higher temperature and better plasticity, causing the workpiece to undergo plastic shrinkage deformation along the dominant stress direction. This is the mechanism of heat treatment shrinkage treatment. The chemical composition of steel is different, and its thermal conductivity and thermal expansion coefficient are different. After heating at Ac1 temperature, the plasticity and yield strength of steel are also different. The plastic shrinkage deformation effect that can be achieved by thermal stress is not the same. Generally, the shrinkage effect of carbon steel and low alloy steel is more obvious, while the shrinkage effect of high carbon and high alloy steel is relatively poor.


The heating temperature for shrinkage treatment should be selected based on Ac1, ensuring that it does not harden during quenching in water. For carbon steel with poor austenite stability, a temperature slightly higher than Ac1 can be used to maximize the shrinkage effect by utilizing the phase transition superplasticity in the phase transition temperature zone. The heating temperature for various types of steel is:


Carbon steel Ac1-20- Ac1+20C

Low alloy steel Ac1-20- Ac1+10C

Low carbon high alloy steel (1Cr13, 2Cr13, 18Cr2Ni4WA, etc.) Ac1-30- Ac1+10C

Austenitic heat-resistant and corrosion-resistant steel 850-1000C


The heating time should ensure sufficient heat penetration of the workpiece, and the best cooling method is salt water quenching. The Ac1 temperature heating and rapid cooling shrinkage treatment method can shrink and treat various shapes of workpieces, such as the inner and outer holes of circular workpieces, the hole spacing and outer dimensions of flat square workpieces, the length of axial workpieces, and some workpieces that require local size shrinkage.


The quenching expansion method is used to expand the shrinkage deformation of workpieces, mainly suitable for workpieces with simple shapes. The principle is to use the increase in specific volume when martensitic transformation occurs on the surface of the workpiece during quenching, apply tensile stress to the core that has not undergone martensitic transformation or has not been quenched, and achieve the purpose of the workpiece expanding along the dominant stress direction through tensile plastic deformation of the core. For workpieces made of low and medium carbon steel and low and medium carbon alloy structural steel, when using the upper limit of conventional quenching heating temperature for water quenching, the dominant stress direction can expand by 0.20-0.50% in the case of workpiece quenching or semi quenching. Simple shaped workpieces can be heated and normalized at a temperature slightly higher than Ac1, and then quenched 1-2 times. Overeutectoid alloy tool steel parts such as CrMn, 9CrSi, GCr15, CrWMn, etc., can be heated according to the upper limit heating temperature specified in conventional heat treatment specifications when they have not been quenched before, and can be quenched as much as possible or obtain a deeper hardening layer, which can cause the workpiece to expand by 0.15-0.20% along the dominant stress direction. After quenching, it should be tempered at 240-280C, and the quenching of this type of steel the expansion deformation mainly depends on the increase in specific volume of martensitic transformation during quenching, so the amount of expansion deformation is limited and there is a risk of quenching cracking.