Welding Thermal and Residual Stresses

Welding involves localized heating and cooling of materials, which results in thermal stresses and residual stresses that can affect the performance, structural integrity, and lifespan of welded joints. Managing these stresses is critical in welding engineering, especially for applications in construction, pipelines, pressure vessels, and other load-bearing structures. Here’s an in-depth look at the causes, effects, and control methods of thermal and residual stresses in welding.

Download PDF: Learn to welding basics handbook

1. Thermal Stresses in Welding:

Thermal stresses arise due to the expansion and contraction of metal when exposed to heat from welding. During welding, the material in the immediate vicinity of the weld (known as the Heat-Affected Zone or HAZ) is exposed to high temperatures and then cools rapidly, leading to uneven expansion and contraction across the weld zone.

Causes of Thermal Stresses:

- Non-uniform Heating and Cooling: Welding involves localized heating, which leads to rapid expansion of metal near the weld and slower expansion in the surrounding areas.

- Temperature Gradient: The temperature difference between the HAZ and the unaffected base material creates a thermal gradient, causing differential expansion and contraction.

- Material Properties: Materials with high thermal expansion coefficients, such as stainless steels, are more prone to thermal stresses.

Effects of Thermal Stresses:

- Distortion: Non-uniform thermal expansion and contraction can distort the workpiece, leading to dimensional inaccuracies.

- Cracking: Severe thermal stresses may result in cracks, particularly if the material has low ductility or is sensitive to thermal shock.

- Stress Concentration: High thermal stresses can lead to stress concentrations around welds, increasing the risk of fatigue failure under cyclic loads.

Download PDF: A practical guide to welding solutions

2. Residual Stresses in Welding:

Residual stresses are the stresses that remain in a material after the welding process is completed and the weld has returned to room temperature. They are “locked in” due to the plastic deformation that occurs during thermal expansion and contraction. Residual stresses can be tensile (pulling the material apart) or compressive (pushing the material together).

Causes of Residual Stresses:

- Thermal Expansion and Contraction: The HAZ and weld metal expand when heated and then contract upon cooling, resulting in residual stresses if the metal undergoes plastic deformation.

- Restraint during Welding: The degree to which the surrounding structure restrains the welded part affects residual stress levels. Higher restraint leads to higher residual stresses as the material cannot freely expand and contract.

- Weld Sequence and Joint Design: Complex joint configurations and welding sequences can create uneven stress distribution, leading to residual stresses.

- Material Properties: Materials with high yield strength retain more residual stresses since they resist plastic deformation during contraction.

Effects of Residual Stresses:

- Reduction in Fatigue Life: Residual tensile stresses can reduce the fatigue resistance of welded components, making them susceptible to fatigue cracking under repeated loading.

- Stress Corrosion Cracking (SCC): Residual tensile stresses can lead to SCC, especially in corrosive environments, by making the material more susceptible to crack propagation.

- Dimensional Stability: High residual stresses can cause parts to warp or distort over time, especially when additional machining or heat treatment is applied after welding.

Download PDF: Welding symbols on drawings handbook

- Brittle Fracture: In materials with low fracture toughness, residual tensile stresses can increase the risk of brittle fracture, particularly in low-temperature applications.

3. Types of Residual Stresses:

Residual stresses are categorized into three types based on their scale and distribution within the welded material:

- Type I (Macro-Residual Stresses): These are long-range stresses that extend across the entire component or significant regions within it.

- Type II (Micro-Residual Stresses): Stresses that occur at the grain or phase level, particularly between different microstructures in the weld and HAZ.

- Type III (Sub-Micro-Residual Stresses): Localized stresses at the atomic or molecular scale, occurring within individual grains due to lattice distortions.

4. Controlling Thermal and Residual Stresses:

Mitigating thermal and residual stresses is essential to improving weld quality and structural integrity. Here are some common techniques:

Preheating:

- Description: Heating the base material to a certain temperature before welding.

- Effect: Reduces the thermal gradient between the weld and surrounding material, minimizing rapid expansion and contraction, thereby lowering thermal and residual stresses.

- Application: Commonly used for welding thick sections or materials prone to cracking, such as high-carbon steels.

Download PDF: Underwater welding pdf document

Post-Weld Heat Treatment (PWHT):

- Description: Heating the welded structure to a specific temperature after welding and then cooling it slowly.

- Effect: PWHT helps relieve residual stresses by allowing plastic deformation and realignment of the microstructure, particularly in steel structures.

- Application: Frequently used in pressure vessels, pipelines, and other critical structures as required by standards like ASME and API.

Weld Sequence Control:

- Description: The order and direction in which welds are applied.

- Effect: A controlled sequence can reduce the buildup of residual stresses and minimize distortion by distributing heat more evenly.

- Application: Used in large or complex weldments where multiple welds are applied.

Peening:

- Description: Mechanical treatment that involves hammering or applying pressure to the weld area.

- Effect: Introduces compressive stresses on the weld surface, which can offset tensile residual stresses and improve fatigue resistance.

- Application: Often applied to fillet welds, thick plates, and critical areas prone to fatigue cracking.

Download PDF: Welding notes pdf document

Clamping and Fixturing:

- Description: Using clamps or fixtures to hold parts in place during welding.

- Effect: Reduces distortion by restraining movement and ensuring even thermal expansion across the weld.

- Application: Common in welding assemblies where distortion control is crucial, such as automotive and aerospace parts.

Heat Sinks:

- Description: Materials or components that absorb heat and prevent excessive temperature buildup in the weld zone.

- Effect: Helps distribute heat away from the weld area, reducing thermal gradients and associated stresses.

- Application: Used in thin sheet welding or components where heat input must be carefully controlled.

Low-Heat Input Welding Techniques:

- Description: Techniques such as pulsed welding, laser welding, and friction stir welding that use controlled heat input.

- Effect: Reduced heat input limits the extent of the HAZ, minimizing thermal stresses and residual stresses.

- Application: Suitable for heat-sensitive materials like aluminum, stainless steels, and titanium.

Download PDF: Welding inspection and metrology pdf document

5. Measuring Residual Stresses:

Accurate measurement of residual stresses helps in understanding their impact on welded structures and improving welding procedures. Common measurement methods include:

- X-ray Diffraction (XRD): Used to measure residual stresses at the surface level by analyzing changes in atomic lattice spacing.

- Ultrasonic Stress Measurement: Employs ultrasonic waves to assess stress distributions within the material.

- Hole-Drilling Method: Involves drilling a small hole in the material and measuring the strain relief, useful for evaluating stresses in specific areas.

- Neutron Diffraction: A non-destructive method that uses neutron beams to measure stress deep within thick sections, although it requires specialized equipment.

6. Standards and Codes Related to Thermal and Residual Stresses:

Various welding codes and standards provide guidance on managing residual stresses:

- ASME Section VIII and API 510: Require post-weld heat treatment to reduce residual stresses in pressure vessels and piping.

- AWS D1.1: Includes guidelines for preheating and PWHT to mitigate residual stresses in structural steel.

- ISO 17663: Provides recommendations for heat treatment processes applied after welding to reduce residual stresses in metallic materials.

For more information follow Innovative Engineers Network

Conclusion:

Understanding and controlling thermal and residual stresses are essential for producing high-quality welds with minimal defects and distortion. By implementing proper welding techniques, preheat and post-weld treatments, and using advanced stress-relief methods, engineers can enhance the structural integrity, durability, and safety of welded structures across a wide range of industries.