During the heat treatment of metal parts spring sleeves, controlling residual stress is crucial for ensuring their mechanical properties, fatigue life, and dimensional stability. Residual stress primarily arises from nonuniform expansion or contraction caused by temperature gradients during heat treatment, as well as volume changes caused by phase transformations. Optimizing the residual stress distribution in metal parts spring sleeves requires comprehensive control across multiple dimensions, including process parameter optimization, cooling medium selection, subsequent treatment, and vibration aging.
Temperature control during the initial heat treatment phase is crucial. During heating, temperature gradients can easily form between the surface and interior of a metal parts spring sleeve due to temperature differences, leading to tensile stress on the surface and compressive stress inside. To reduce these stresses, isothermal heating techniques are employed. This involves slowly increasing the temperature to uniformize the internal temperature of the material and avoid localized overheating. Furthermore, the heating rate must be kept within a reasonable range to prevent plastic deformation caused by thermal stress accumulation. For example, controlling the heating rate between a few degrees Celsius and tens of degrees Celsius per minute can effectively reduce the impact of thermal stress on the residual stress distribution.
The quenching process is a key step in residual stress control. During quenching, metal parts spring sleeves are rapidly cooled from high temperatures. This difference in cooling rates creates significant temperature gradients between the surface and interior, which in turn induces residual stresses. To optimize stress distribution, the appropriate cooling medium must be selected based on the material's properties. For steels with good hardenability or metal parts spring sleeves with complex shapes, oil cooling significantly reduces thermal stresses due to its slower cooling rate and more uniform workpiece temperature change. For materials with poor hardenability, a combination of water and oil cooling or the addition of coolants to the water can be used to adjust the cooling rate. Furthermore, the quenching temperature must be selected based on the material's chemical composition and phase transformation characteristics to ensure phase transformation occurs at a uniform temperature and minimize the negative impact of temperature gradients on residual stresses.
Tempering is a critical step in relieving quenching residual stresses. After quenching, high residual tensile stresses exist within the metal parts spring sleeve, requiring tempering to reduce these stress levels. Precise control of tempering temperature and time is crucial for optimizing residual stress distribution. Typically, the tempering temperature is set within a range that balances material toughness and strength, and residual stresses are gradually released through heating and holding. For example, the tempering temperature of medium-carbon steel spring sleeves is typically controlled within several hundred degrees Celsius, and the holding time must be sufficiently long to ensure temperature uniformity and avoid stress redistribution due to local overheating. After tempering, the residual stress in the metal spring sleeve is significantly reduced, and the structural stability is improved, thereby extending its service life.
Vibration aging technology provides a new solution for residual stress control. By applying additional dynamic stress to the metal spring sleeve, when the combined dynamic stress and residual stress exceed the material's yield point, the workpiece undergoes microscopic or macroscopic plastic deformation, thereby reducing and equalizing the residual stress. Vibration aging treatment can largely eliminate residual stress in elastomers, with a higher percentage being eliminated in areas of high tensile stress. This technology prevents microcracks caused by heat treatment or machining, improving the deformation resistance and dimensional accuracy of the metal spring sleeve. In practical applications, vibration aging is often combined with heat treatment to form a composite process of "heat treatment + vibration aging" or "vibration aging + heat treatment" to further enhance stress control.
Surface hardening techniques such as shot peening and rolling can improve the stress distribution of metal parts spring sleeves by introducing a residual compressive stress layer. Shot peening uses high-speed projectiles to impact the workpiece surface, forming a dense compressive stress layer and improving fatigue resistance and stress corrosion resistance. Rolling, on the other hand, applies static pressure to the surface through a rolling head, inducing plastic deformation, refining the surface structure, and increasing surface hardness and residual compressive stress. While these techniques may increase surface roughness, subsequent finishing processes can ensure surface quality and achieve a coordinated optimization of stress distribution and surface properties.
