Analysis of Residual Stress in Pressure Vessel Surface Layer

Pressure vessels are critical components designed to withstand high temperatures, pressures, and complex loads. The residual stress characteristics of their surface layers are crucial for ensuring operational safety, durability, and performance. Proper analysis and control of residual stress can significantly enhance the lifespan and reliability of pressure vessels. This article systematically explores the characteristics, impact, and optimization methods of residual stress in pressure vessel surface layers, focusing on machining errors, surface roughness, work hardening, corrosion resistance, and fatigue strength.

The Relationship Between Machining Errors and Residual Stress

Machining errors significantly influence the distribution of residual stress in the surface layers of pressure vessels. These errors often result from factors such as equipment precision, machining techniques, and tool wear.

1. Constant Systematic Errors

Constant systematic errors primarily affect the position of the dimensional distribution curve without altering its shape. If the center of the distribution curve deviates from the tolerance zone, uneven residual stress distribution may occur, impairing the vessel's mechanical performance. Calibrating equipment and adjusting process parameters can minimize such errors.

2. Variable Systematic and Random Errors

Variable systematic errors and random errors distort the shape of the dimensional distribution curve, often deviating from a normal distribution. These errors are usually caused by tool or grinding wheel wear, leading to uneven stress distribution that compromises sealing performance and fatigue resistance. Optimizing tool maintenance and machining parameters is critical to addressing these issues.

The Impact of Surface Roughness on Performance

Surface roughness is a key indicator of manufacturing quality, directly influencing wear resistance, corrosion resistance, and fatigue strength.

1. Surface Roughness and Wear Resistance

Surface roughness affects the distribution of contact pressure and friction performance:
Excessive Roughness: Reduced contact area leads to localized pressure concentration, causing premature failure due to elastic or plastic deformation and wear.
Overly Smooth Surface: Difficulty forming a stable lubrication film worsens lubrication conditions, also accelerating wear.
Selecting an appropriate surface roughness based on operating conditions helps balance wear resistance and lubrication performance, extending the vessel's service life.

2. Surface Roughness and Corrosion Resistance

Surface roughness significantly influences the accumulation of corrosive media:
High Roughness: Corrosive agents can easily accumulate in surface depressions, accelerating corrosion and reducing resistance.
Smooth Surface with Residual Compressive Stress: Helps seal micro-cracks, preventing penetration of corrosive substances and enhancing corrosion resistance.

3. Surface Roughness and Fatigue Strength

Rough surfaces create stress concentration points under cyclic loads, acting as initiation sites for fatigue cracks. Studies show reducing surface roughness from 63μm to 16μm can improve fatigue strength by about 25%. Enhancing surface quality is especially beneficial for high-strength materials.

The Influence of Work Hardening

Work hardening occurs due to plastic deformation during machining or use, strengthening the metal surface layer. This phenomenon affects multiple performance aspects of pressure vessels.

1. Improving Wear Resistance

Work hardening reduces elastic and plastic deformation in the surface layer, enhancing wear resistance. However, excessive hardening can lead to brittle microstructures, increasing the risk of micro-cracks or peeling. Controlling the degree of work hardening is essential in surface processing.

2. Work Hardening and Residual Stress

Work hardening typically induces residual compressive stress:
Residual Compressive Stress: Improves surface integrity, wear resistance, and crack resistance.
Residual Tensile Stress: Accelerates crack initiation and propagation, reducing long-term stability.

Residual Stress and Corrosion Resistance

The corrosion resistance of pressure vessels depends on surface roughness and residual stress.

Residual Compressive Stress: Seals micro-cracks, blocks penetration of corrosive media, and enhances corrosion resistance.
Residual Tensile Stress: Increases susceptibility to corrosion, especially under high-temperature, high-pressure, or cyclic load conditions.
Processes like shot peening and laser strengthening can introduce beneficial residual compressive stress, improving resistance.

Residual Stress and Fatigue Strength

Pressure vessels endure complex cyclic loads, making fatigue strength closely related to residual stress.

Positive Role of Compressive Stress: Residual compressive stress counteracts tensile stress from cyclic loads, delaying fatigue crack initiation and growth, and significantly improving fatigue strength.

Negative Role of Tensile Stress: Residual tensile stress exacerbates crack formation under cyclic loading, accelerating fatigue failure and potentially causing sudden vessel failure. Reducing residual tensile stress is essential during manufacturing and operation.

Optimizing Residual Stress in Pressure Vessels

Optimizing residual stress in pressure vessel surface layers is key to enhancing performance and extending service life.

Improved Machining Processes: Precision techniques such as ultra-precision grinding and polishing reduce surface roughness, minimizing the adverse effects of machining errors.

Surface Strengthening Treatments: Surface treatments like shot peening, laser quenching, and surface rolling introduce beneficial residual compressive stress, enhancing fatigue and corrosion resistance.

Material Selection: High-quality fine-grained materials not only improve overall strength but also enhance the interplay between surface roughness, fatigue strength, and corrosion resistance.

Conclusion

The characteristics of residual stress in pressure vessel surface layers play a pivotal role in overall performance. From machining errors and surface roughness to work hardening, fatigue strength, and corrosion resistance, controlling residual stress is crucial for improving reliability and longevity. By optimizing machining processes, strengthening surface treatments, and selecting suitable materials, pressure vessel performance can be effectively enhanced, ensuring safe and efficient operation. Further research into residual stress control technologies will provide stronger support for industry advancement.