First, let’s understand what stress is. When a metal material is subjected to external force, an internal force opposing the external force is generated within the material. Its magnitude is equal to the external force, but the direction is opposite. This internal force per unit area is called stress.
During processes like hot or cold working, residual stress develops in metal workpieces, such as castings, forgings, and welded parts. High residual stress often has harmful effects, such as reducing the actual strength of components, lowering fatigue limits, and causing stress corrosion or brittle fractures. Therefore, reducing and eliminating residual stress in workpieces is a critical task, especially in applications like aerospace, aviation, marine, railway, and industrial production, where fatigue failure caused by residual stress can lead to significant losses and must not be overlooked.
Currently, methods for handling residual stress include natural aging and artificial aging (such as thermal aging, vibration aging, peening, and ultrasonic impact treatment).
1. Natural Aging
Natural aging is the oldest method for relieving stress. It is suitable for eliminating thermal stress (residual stress from casting and forging), cold stress (residual stress from machining), and welding stress (residual stress from welding processes). This method involves leaving the components outdoors, exposed to natural conditions. Over months of exposure to wind, sun, rain, and seasonal temperature fluctuations, repeated temperature stresses are applied, relaxing residual stress and stabilizing dimensional accuracy.
Although natural aging reduces only a small amount of residual stress, it stabilizes the workpiece’s dimensions effectively. This is because the workpiece undergoes slight plastic deformation over time, relaxing stress while strengthening parts of the base material. This enhances the stiffness in certain areas and increases resistance to deformation.
However, natural aging has several disadvantages, including a long production cycle, large space requirements, high capital investment, and difficulty in management. It also cannot promptly reveal internal defects in components, leading to its gradual phase-out.
2. Thermal Aging
Thermal aging is a traditional method for eliminating residual stress. It is suitable for removing thermal stress, cold stress, and welding stress. The process involves slowly and uniformly heating the component from room temperature to approximately 550°C, maintaining this temperature for 4-8 hours, and then carefully controlling the cooling rate to below 150°C before removing it from the furnace.
Thermal aging demands strict adherence to process requirements, such as maintaining a furnace temperature difference within ±25°C, a heating rate of no more than 50°C per hour, and a cooling rate of no more than 20°C per hour. The furnace temperature must not exceed 570°C, and the holding time should not be excessively long. Exceeding 570°C or prolonged holding times can lead to graphitization, reducing the component’s strength. If heating is too rapid, temperature differences between thin and thick sections can increase dramatically, creating additional thermal stress. If the combined stress exceeds the component’s strength limit, it may crack.
Improper cooling after thermal aging can significantly reduce its effectiveness, even introducing thermal stress similar to the original residual stress, undermining the achieved benefits of thermal aging.
3. Peening Aging
Peening aging was introduced early into the welding field and is particularly effective for addressing welding stress. Peening methods include manual hammering and electric hammering. Observations and analyses suggest that appropriate hammering can reduce and eliminate welding cracks. It is inferred that hammering can also alleviate residual stress, making it an effective treatment for preventing welding cracks.
4. Vibration Aging
Vibration aging (VSR) involves applying periodic excitation forces to a metal component using vibratory equipment. At a specific frequency, the component resonates, generating dynamic stress. This causes sub-resonant vibrations with tens of thousands of cycles of significant amplitude over time. The dynamic stress combines with residual stress, and if the combined stress exceeds the yield limit of the metal, slip occurs within the crystal lattice, resulting in minor plastic deformation. This reduces residual stress and strengthens the metal matrix.Vibration aging can now completely replace thermal aging in many cases.
These are the methods for eliminating stress in metal components.For more information on metal surface treatment processes, stay updated with Mulan Group Sheet Metal Fabrication, a leading sheet metal manufacturing service provider in China. We continuously update industry information for user reference.
