Stress relief methods

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Stress problems are challenging issues that arise in the design, manufacturing, or construction of many engineering projects or products, as well as during their operation or use; they are also direct factors affecting the safety of such projects or products. In the previous issues, we shared stress simulation analysis and stress detection methods; this time, we will introduce methods for stress elimination. Stress relief is an important process aimed at reducing or eliminating residual stresses within a material, in order to improve its performance and stability. Residual stresses usually arise after materials undergo processes such as welding, casting, forging, or machining, and they can lead to dimensional instability, reduced strength, shortened fatigue life, and even issues such as cracks and stress corrosion. There are various methods for stress relief, including natural aging, thermal aging, vibration aging, sub-resonant aging, cold working and stretching, and ultrasonic shock. Each method has its own characteristics. Let’s analyze them one by one: Natural aging: This involves placing the material in the natural environment, where changes in climate and temperature are used to gradually relieve stress. This method is time-consuming, usually taking more than a year, and its stress-relief effect is limited, only reducing residual stress by 2–10%. Therefore, it is less used in actual production. Thermal aging: Utilizing the annealing technique in heat treatment, the workpiece is heated to 500 – 650°C, held at that temperature for a certain period of time, and then cooled slowly. This method can effectively eliminate most stress, but it may cause oxidation of the material surface and a decline in its properties. Vibration aging: It is an efficient material treatment technique with significant advantages, which make it the preferred choice for many applications. By applying a periodic external force to the workpiece to induce resonance, residual stresses can be homogenized, reduced, or eliminated; the strength and hardness of the material can be improved; the orientation of the crystals can be enhanced; the wear resistance of the material can be increased; and the crystal structure in the deeper layers of the material’s surface can become more uniform and dense, thereby reducing friction and fatigue damage. This method requires low energy and takes little time, and it is suitable for various types of workpieces, including large, medium, and small components. In components that have undergone vibration treatment, residual stresses can be reduced by 30% to 90%, which improves their strength and fatigue life while reducing stress corrosion. Sub-resonant aging: Stress is eliminated through sub-resonance; although it is environmentally friendly, it is complex to operate and can only remove a small portion of the stress, resulting in a limited range of applications. Cold working drawing: Plastic deformation is induced in the material through drawing or rolling, thereby adjusting the grain structure and eliminating internal stresses. This method can improve the performance and dimensional stability of the material, but it also has significant drawbacks; new stresses may arise during the stretching process, requiring additional treatment steps to eliminate them. If not controlled properly, it may cause excessive deformation of the material, affecting the dimensional accuracy and shape stability of the product. Furthermore, it is not suitable for all types of metal materials, especially those that are sensitive to cold working. Ultrasonic shock: It is a very effective method for stress relief, widely used in the processing and treatment of metal materials. It induces compressive plastic deformation on the metal surface through high-frequency impacts, thereby altering the stress field and introducing beneficial compressive stresses, while also enhancing the material’s strength and fatigue resistance. The working principle of ultrasonic impact is to use high-power energy to drive the impact head, which strikes the metal surface at a frequency of about 20,000 times per second. High frequency, high efficiency, and focused high energy cause compressive plastic deformation in the metal surface layer. This deformation alters the original stress field, generating beneficial compressive stresses. During the impact process, the temperature of the metal surface rises rapidly and then cools down quickly, causing changes in the structure of the surface layer of metal and strengthening the impacted area. Based on the Ostwald effect, ultrasonic oscillation is used to make the stress distribution within the material more uniform. It is particularly suitable for post-weld treatment of welded structures, and can effectively eliminate welding residual stresses. Improve the fatigue strength and service life of welded joints, including ordinary joints, load-bearing joints, and welded joints of dissimilar materials. Reduce the likelihood of stress corrosion cracking. Enhances the strength and resistance to brittle fracture of the material. Therefore, ultrasonic shock is widely used in fields such as mechanical equipment, vehicle engineering, petrochemical industry, aerospace, military industry, railways, automobiles, bridges, and so on. In summary, choosing the appropriate stress relief method can improve the quality and reliability of products and extend their service life. The selection method can be determined based on material properties, workpiece size and shape, as well as production requirements. Because different materials respond differently to various stress-relief methods. For example, certain alloy steels may require specific tempering treatments after quenching to eliminate residual stresses. Large structural components may be more suitable for thermal aging and vibration aging, while small and medium-sized parts can opt for vibration aging or ultrasonic shock. If production efficiency is the primary consideration, vibration aging or ultrasonic shock may be a better choice, as it takes less time and requires less investment in equipment.