Residual Stress and Six Common Methods for Its Elimination

Residual stress is a type of inherent or internal stress. For certain parts or standards, stress relief is required to meet actual usage requirements. The main methods are as follows:

1. Hammering Method

This method uses a steel hammer to strike areas of the workpiece where residual stress is concentrated. The metal surface receiving the hammer blows undergoes compressive stress and local plastic deformation, thereby reducing the peak value of residual stress, improving and balancing the original residual stress distribution, and preventing brittle failure. This method is especially suitable for welded parts and is widely used in welding operations, but it is less common for stamped parts.

2. Vibration Aging Method

Using specialized equipment, the workpiece is subjected to periodic external forces to induce resonance. This causes microscopic grains within the workpiece to slip or twin, reducing the peak residual stress and balancing its distribution. This method can eliminate approximately 50% of residual stress or reduce the peak by 50% within one hour. It is widely used, efficient, and cost-saving, but it cannot completely eliminate concentrated residual stress inside the workpiece.

3. Heat Treatment Aging

Also known as artificial aging, this traditional method eliminates residual stress. Usually, annealing or tempering is employed. The workpiece is slowly and uniformly heated to around 600°C using heat treatment equipment, held at this temperature for 4–8 hours, then gradually cooled to below 120°C before being removed and cooled to room temperature. This method is highly effective, fast, and thorough in eliminating residual stress.

4. Natural Aging

The workpiece is left outdoors or in a static state, allowing residual stress to be released over time. This method is not suitable for industrial large-scale production. However, for high-value or high-precision critical components, a combination of artificial aging and natural aging is commonly used.

5. Welding Stress Relief

During welding, the weld area rapidly heats and expands, while the heat-affected zone (HAZ) remains cooler, impeding expansion. This generates compressive stress in the weld and tensile stress in the HAZ. Because the weld is in a plastic state, it partially relieves this stress. During cooling, the HAZ cools faster and enters an elastic state, while the weld is still plastic. As the weld contracts slower than the HAZ, compressive stress remains in the weld, but the plasticity of the weld relieves part of this stress.

6. Residual Stress Relief in Machining

After machining, certain treatments can adjust residual stress on the machined surface. Surface strengthening is currently one of the most common methods.

Surface strengthening processes induce cold plastic deformation on the workpiece surface to increase surface hardness, strength, and form residual compressive stress. Common processes include:

• Shot Peening: High-speed shot impacts the workpiece surface, causing plastic deformation and flow, creating a hardened surface layer and residual compressive stress. Suitable for irregular or complex surfaces like springs or connecting rods.

• Roller Burnishing: Rollers uniformly press on the workpiece surface to strengthen it and induce residual compressive stress. Suitable for regular surfaces like outer cylinders, holes, or planes, and can be performed on existing machines with added tools.

• Pre-Stress Machining: Pre-applies elastic stress to the workpiece before cutting. During cutting, elastic deformation occurs, and after stress release, residual compressive stress forms on the machined surface. This method does not require expensive equipment, does not increase surface hardness, and is highly promising for future development.