Methods for correcting deformation caused by heat treatment

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Image: Mechanical correction method – This approach uses mechanical means or localized heating to induce slight localized plastic deformation in the deformed workpiece; simultaneously, residual internal stresses are released and redistributed, thereby achieving the purpose of correcting the deformation. Common mechanical correction methods include cold pressing correction, hot pressing correction before quenching and cooling to room temperature, pressure tempering correction, \"hot spot\" correction which involves locally heating the deformed workpiece using an oxygen-acetylene flame or high-frequency heat, and hammering correction. During use, handling, or finishing, parts that have undergone mechanical correction may partially revert to their original deformation and develop new deformations due to the relaxation and release of residual stresses. Therefore, for workpieces and precision parts under high load, mechanical correction is preferably avoided. When mechanical correction is necessary, the plastic strain achieved through such correction should exceed that resulting from heat treatment deformation; however, the amount of plastic deformation induced by correction must be kept within a very narrow range – generally, it should be more than 10 times the elastic limit strain and less than one-tenth of the conditional strength limit. Correction should be carried out as soon as possible after quenching, and residual stress relief treatment should be performed after correction. The correction of workpieces deformed during heat treatment requires skilled operators and is very time-consuming. Therefore, the automation of this correction process is an important task for heat treatment professionals. Image: Heat treatment correction method – For workpieces whose dimensions are out of spec due to expansion or contraction resulting from heat treatment, the deformation can be corrected by applying appropriate heat treatment methods again. Common heat treatment correction methods include rapid heating and cooling at the Ac1 temperature to shrink workpieces that have undergone expansion deformation. Since no change occurs in the specific volume of the material’s structure during this process, no structural stress is generated; only thermal stress results from the difference in thermal contraction between the core and the surface of the workpiece. During quenching, the rapid contraction of the workpiece surface exerts compressive stress on the core, which has a higher temperature and better plasticity; this causes the workpiece to undergo plastic shrinkage deformation along the direction of the dominant stress. This is the mechanism behind heat treatment-induced shrinkage. Due to their different chemical compositions, steels exhibit varying thermal conductivity and coefficients of thermal expansion. When heated to the Ac1 temperature, their plasticity and yield strength also differ, and the degree of plastic shrinkage that can be achieved through thermal stress varies as well. Generally, carbon steels and low-alloy steels show a more significant degree of shrinkage, while high-carbon and high-alloy steels exhibit less pronounced shrinkage. The heating temperature for the shrinkage treatment should be selected based on Ac1, with the principle that quenching should not occur upon cooling in water. For carbon steels with poor austenite stability, a temperature slightly higher than Ac1 can be used to take advantage of the phase transformation superplasticity in the phase transition region in order to achieve the greatest degree of shrinkage. The heating temperatures for various types of steel are as follows: Carbon steel: Ac1-20 to Ac1+20°C; Low-alloy steel: Ac1-20 to Ac1+10°C; Low-carbon high-alloy steel (such as 1Cr13, 2Cr13, 18Cr2Ni4WA, etc.): Ac1-30 to Ac1+10°C; Austenitic heat-resistant and corrosion-resistant steel: 850 to 1000°C. The heating time should be sufficient to ensure thorough heating of the workpiece, and quenching in brine is the best method. The heating and rapid cooling shrinkage treatment method at Ac1 temperature can be used to shrink various shapes of workpieces, such as the inner hole and outer diameter of ring-shaped workpieces, the holes, hole spacing, and overall dimensions of rectangular workpieces, the length of shaft-shaped workpieces, as well as certain workpieces that require local dimension reduction. The quenching expansion method is used to expand workpieces with shrinkage deformation—it is mainly applicable to workpieces with simple shapes. The principle involves utilizing the increase in specific volume that occurs during quenching when a martensitic phase transformation takes place on the surface layer of the workpiece. This creates tensile stress on the core, where no martensitic transformation has occurred or which remains unquenched. Through plastic tensile deformation of the core, the workpiece is caused to expand along the direction of the dominant stress. For workpieces made of low to medium carbon steels and low to medium carbon alloy structural steels, when water quenching is carried out at a temperature higher than the usual upper limit for quenching heating, the direction of the dominant stress can be increased by 0.20%-0.50% in cases where the workpiece is fully or partially quenched. Workpieces with simple shapes can be heated and normalized at a temperature of either below or slightly above Ac1, followed by 1–2 additional quenching cycles. For hypereutectoid alloy tool steel parts such as CrMn, 9CrSi, GCr15, and CrWMn, even when they were previously not fully hardened, they can be heated to the upper limit temperature specified in the conventional heat treatment specifications, so as to achieve maximum possible hardening or a deeper hardened layer. This causes the workpiece to expand by 0.15%–0.20% along the direction of the dominant stress. After quenching, it should be tempered at 240–280°C. The swelling deformation of such steels during quenching is primarily due to the increase in specific volume resulting from the martensite transformation; therefore, the amount of swelling deformation is limited, and there is a risk of cracking.