2.5 Distortion and Residual Stress
Key Takeaways
- Welding distortion types: longitudinal shrinkage, transverse shrinkage, angular distortion, bowing, buckling, twisting
- Higher heat input, larger welds, thinner material, and poor fit-up all increase distortion
- Control techniques: pre-setting, balanced welding, back-step, intermittent welds, minimum weld size, mechanical restraint
- Residual stresses in the weld and near-HAZ are tensile; surrounding base metal is in compression
- Residual stress reduces fatigue life and contributes to HIC and stress corrosion cracking
- PWHT at 1,100–1,250°F for carbon steel relieves residual stress and tempers martensite
2.5 Distortion and Residual Stress
Welding introduces intense, localized heat that causes non-uniform expansion and contraction of the metal. This results in residual stresses (internal stresses locked into the structure) and distortion (visible dimensional changes). Understanding and controlling these effects is a core competency for welding inspectors.
Types of Welding Distortion
| Type | Description | Common Cause |
|---|---|---|
| Longitudinal shrinkage | Weld shortens along its length | Weld metal contraction along weld axis |
| Transverse shrinkage | Parts pull toward each other across the joint | Weld metal contraction perpendicular to weld axis |
| Angular distortion | Parts rotate around the weld axis (butterfly effect) | Non-symmetric weld cross-section (single-V groove) |
| Bowing (camber) | Plate curves along its length | Weld not on neutral axis of the cross-section |
| Buckling | Wavy distortion in thin plates | Compressive stresses exceed critical buckling load |
| Twisting | Rotational distortion | Asymmetric welding sequence |
Factors That Increase Distortion
| Factor | Effect |
|---|---|
| Higher heat input | More expansion/contraction cycle → more distortion |
| Larger weld size | More molten metal → more shrinkage |
| Thinner material | Less rigidity to resist distortion forces |
| Poor fit-up | Wider gaps require more weld metal → more shrinkage |
| Lack of restraint | Parts free to move during welding |
| Single-sided joints | Non-symmetric welds cause angular distortion |
Distortion Control Techniques
| Technique | Description |
|---|---|
| Pre-setting (pre-cambering) | Position parts so they distort into the desired final position |
| Balanced welding | Alternate welds on both sides of the joint (double-V, double-U) |
| Back-step welding | Weld short segments in reverse direction to overall progression |
| Intermittent welding | Use skip welds instead of continuous welds where code permits |
| Proper sequence | Weld from center outward; alternate sides; weld free ends before restrained |
| Minimum weld size | Use the smallest weld size that meets design requirements |
| Mechanical restraint | Use clamps, strongbacks, tack welds to resist movement |
| Lower heat input | Reduce amperage, increase travel speed where possible |
| Peening | Hammering the weld between passes to relieve stress (use with caution — not on final pass) |
Residual Stress
Residual stresses are internal stresses that remain in a structure after the external force (heat) is removed. In welding:
- The weld metal and near-HAZ are in tension (they want to shrink but are restrained)
- The surrounding base metal is in compression (balancing the tensile zone)
- Residual stresses can approach the yield strength of the material
Consequences of residual stress:
- Reduces fatigue life (tensile residual stress is additive with applied tensile stress)
- Contributes to hydrogen-induced cracking
- Can cause stress corrosion cracking (SCC) in certain environments
- Distorts parts during subsequent machining (stress relief)
Post-Weld Heat Treatment (PWHT) / Stress Relief
PWHT (typically 1,100–1,250°F / 595–675°C for carbon steel) reduces residual stress by allowing the metal to yield plastically at elevated temperature. It also tempers any martensite in the HAZ, improving toughness.
For the Exam: Understand the relationship between heat input, weld size, and distortion. Know the distortion control techniques, especially pre-setting, balanced welding, and back-step welding. Also know that residual stresses in the weld zone are tensile.
What is the nature of residual stress in the weld metal and near-HAZ after welding?
Which distortion control technique involves positioning parts so they distort into the desired final shape?