Residual Stress: Definition, Causes, Measurement, and Management

Causes of Residual Stress

Residual stress originates from multiple sources during material processing. The most common causes include:

Mechanical Deformation

When materials undergo plastic deformation, residual stresses develop due to permanent changes in their internal structure. This can occur in:

• Cold Working – Processes like rolling, drawing, and hammering introduce stress into the material.
• Machining – Cutting operations create localized stress in surface layers due to friction and heat.
• Grinding and Polishing – High-speed material removal generates heat, leading to surface stress accumulation.

Inhomogeneous Structural Changes

Structural inconsistencies can develop due to variations in material properties, resulting in internal stress. These changes occur in:

• Casting and Forging – Differences in cooling rates create internal tension between different regions of a component.
• Welding – The heat-affected zone (HAZ) around a weld experiences expansion and contraction, leaving residual stress behind.
• Additive Manufacturing (3D Printing) – Layer-by-layer material deposition results in localized stress due to uneven thermal expansion.

Thermal Effects

Temperature changes create internal stress due to expansion and contraction. The larger the temperature gradient, the higher the residual stress. This occurs in:

• Welding and Heat Treatment – Rapid cooling (quenching) causes contraction stress, while slow cooling can create uneven stress distribution.
• Thermal Spraying and Coatings – Applying high-temperature coatings to cooler substrates results in differential contraction.
• Induction Hardening – Localized heating and rapid cooling produce a hardened surface layer while leaving the core relatively soft, causing stress gradients.

Phase Transformations

Metallurgical phase changes can cause material expansion or contraction, leading to residual stress. This occurs in:

• Austenite to Martensite Transformation – Common in hardened steels, this transition increases volume, generating internal tension.
• Precipitation Hardening – The formation of new phases at high temperatures alters material dimensions, inducing stress.
• Boronizing and Nitriding – These diffusion-based hardening processes alter the material’s microstructure, affecting residual stress distribution.

Types of Residual Stress

Residual stress can be categorized into three distinct types based on their scale and effect within a material: macrostress (Type I), microstress (Type II), and nanoscale stress (Type III). Each type has unique implications for material performance and requires different methods for measurement and management.

Macrostress (Type I)

Macrostress acts over multiple grains in a material and can be observed on a larger scale. It is commonly associated with visible deformations such as warping, cracking, or permanent shape distortion. Processes like welding, rolling, forging, and heat treatment often induce macrostress due to uneven expansion and contraction within a component.

• Common effects: Material warping, crack formation, and dimensional instability.
• Typical sources: Welding-induced thermal gradients, rolling operations, and large-scale mechanical processing.
• Measurement methods: Macrostress can be detected using X-ray diffraction (XRD), neutron diffraction, or strain-gauge techniques.

One of the critical concerns with macrostress is its impact on fatigue life. If a component is subjected to cyclic loading, tensile macrostress can accelerate crack initiation, leading to premature failure. Conversely, compressive macrostress, such as that introduced by shot peening, can be beneficial by delaying crack formation and improving fatigue resistance.

Microstress (Type II)

Microstress exists within individual grains but does not extend across multiple grains. It results from phase transformations, variations in microstructure, and local dislocation movements. Unlike macrostress, microstress is not visible to the naked eye but can significantly influence a material’s hardness and wear resistance.

• Often associated with martensitic transformation in steels, where changes in crystal structure introduce localized stress.
• Can contribute to increased surface hardness in hardened materials.
• Typically detected using advanced X-ray diffraction (XRD) techniques that analyze lattice strain.

Although microstress is often beneficial—improving mechanical properties and resistance to wear—it must be carefully controlled to prevent undesirable embrittlement, which can reduce toughness and ductility.

Nanoscale Stress (Type III)

Nanoscale stress, also called sub-micro residual stress, occurs at the atomic level within a single grain. It arises from crystalline defects such as dislocations, vacancies, and lattice distortions.

• Common effects: Alters material strength, influences fatigue resistance, and impacts crack propagation.
• Typical sources: Severe plastic deformation, diffusion-based hardening processes like boronizing, and deposition techniques.
• Measurement methods: Detected using electron microscopy, X-ray diffraction, or atomic force microscopy.

While nanoscale stress is generally not a direct cause of macroscopic failure, it plays a crucial role in defining a material’s mechanical behavior.

Measurement Methods for Residual Stress

Residual stress cannot be measured directly using conventional force-based techniques. Instead, indirect measurement methods are used to analyze deformations and material properties.

Non-Destructive Measurement Methods

These techniques assess residual stress without damaging the component, making them ideal for quality control in industries like aerospace and automotive.

• X-ray Diffraction (XRD): Measures lattice strain to determine residual stress levels.
• Neutron Diffraction: Similar to XRD but penetrates deeper into the material, suitable for large components.
• Ultrasonic Methods: Uses sound waves to detect stress variations within a material.
• Magnetic Methods: Based on variations in magnetic permeability due to residual stress.

Semi-Destructive Measurement Methods

These methods cause minimal damage to the component and can often be repaired after testing.

• Hole-Drilling Method: A small hole is drilled into the material, and strain gauges measure the stress relaxation around it.
• Deep-Hole Drilling: A more advanced version of hole-drilling, suitable for detecting stress deep inside the material.
• Barkhausen Noise Analysis: Evaluates stress levels by analyzing noise generated from magnetic domain movements.

Destructive Measurement Methods

These techniques involve cutting or sectioning the material, permanently altering the component.

• Slitting Method: Cutting a thin slit in the material releases residual stress, allowing measurement of strain relief.
• Layer Removal Method: Sequentially removing thin layers and measuring deformation to determine internal stress distribution.
• Contour Method: Cutting the component in half and analyzing its deformation profile.

Managing and Reducing Residual Stress

Residual stress can be managed and reduced through various techniques that either redistribute the internal stresses or eliminate them altogether. The choice of method depends on the material type, application, and performance requirements. Proper residual stress management enhances component durability, reduces the risk of failure, and improves wear resistance.

Heat-Based Stress Relief

Thermal treatments are widely used to relieve residual stress by allowing atoms to realign in a more stable configuration. The level of heating required depends on the material and the desired outcome.

• Stress Relief Annealing: This process involves heating the material to a temperature range of 550-650°C for steel, allowing residual stresses to redistribute without significantly altering the material’s hardness. It is commonly used in welded, cast, or heavily machined components to prevent distortion or premature failure.
• Tempering: Applied after hardening processes, tempering reduces brittleness while maintaining strength. It helps relieve internal stresses caused by rapid cooling (quenching) and prevents cracking.
• Recrystallization Annealing: This method eliminates work-hardening effects in cold-worked metals by promoting the formation of new, stress-free grains. It is particularly useful for materials subjected to extensive deformation, such as rolled or drawn steel.

Mechanical Stress Relief

Mechanical treatments introduce controlled plastic deformation to redistribute residual stress. These methods are widely used in high-performance industries where material fatigue is a concern.

• Shot Peening: This technique involves bombarding the material’s surface with small, high-velocity metallic or ceramic beads. The impact induces compressive residual stress, which counteracts tensile stress and enhances fatigue resistance. Shot peening is widely used in aerospace, automotive, and tooling applications to improve component longevity.
• Deep Rolling: A process where hardened rollers apply compressive force to the material’s surface, strengthening it and increasing its resistance to fatigue and wear. This method is particularly effective for crankshafts, shafts, and high-stress components.
• Vibration Stress Relief (VSR): A non-thermal technique that subjects a component to controlled vibrations at low frequencies. The oscillations help redistribute residual stress, particularly in large welded structures where thermal treatment is impractical. Although not as effective as heat-based methods, VSR is useful for maintaining dimensional stability in precision components.

Surface Hardening and Residual Stress Management

Certain surface treatments not only improve wear resistance but also modify residual stress distribution in a way that benefits material performance.

• BORINOX® Treatment (Low-Temperature Diffusion Hardening): This proprietary low-temperature diffusion process introduces compressive residual stress into stainless steel components. Unlike traditional hardening methods, BORINOX® enhances wear resistance without reducing corrosion resistance, making it ideal for applications in the food, pharmaceutical, and chemical industries. Additionally, it prevents cold welding and galling, increasing the longevity of stainless steel parts.
• BOROCOAT® (Boronizing):Boronizing forms ultra-hard boride layers on the surface of a material while controlling residual stress. The diffusion process strengthens the component’s surface, significantly improving wear and corrosion resistance. Unlike coatings that may delaminate or chip, BOROCOAT® is integrally bonded to the substrate, maintaining its integrity even under extreme conditions. It is commonly used in oil and gas, automotive, and heavy machinery applications.

By selecting the appropriate stress relief method, engineers can enhance material performance, extend component lifespan, and prevent premature failures in demanding applications.

Conclusion

Residual stress plays a crucial role in determining the strength, durability, and failure resistance of materials. While it can be beneficial in some cases, improper residual stress management can lead to cracking, warping, and premature failure. Understanding its causes, measurement techniques, and mitigation methods allows engineers to optimize material performance across various industries.

By utilizing advanced stress relief methods and surface hardening technologies like BORINOX® and BOROCOAT®, manufacturers can enhance component longevity, reduce maintenance costs, and ensure higher reliability in demanding applications.