2.5 Distortion and Residual Stress
Key Takeaways
- Distortion types: longitudinal/transverse shrinkage, angular, bowing/camber, buckling, twisting
- Higher heat input, larger welds, thinner material, poor fit-up, and asymmetry all increase distortion
- Controls: pre-setting, balanced welding, back-step, intermittent welds, minimum weld size, mechanical restraint, lower heat input
- Restraint reduces distortion but raises residual stress and cracking risk — the two must be balanced
- Residual stress in the weld/near-HAZ is tensile (up to yield); surrounding base metal is in compression
- PWHT for carbon steel soaks ~1,100–1,200°F (below A1) for ~1 hour per inch, relieving stress and tempering martensite
Why Welding Distorts Metal
Welding pours intense, localized heat into a small region while the surrounding metal stays cool. The hot metal tries to expand but is restrained by the cooler metal around it, so it is squeezed (yields in compression) while hot. As it cools and shrinks, that same metal now pulls inward against the restraint and locks in tension. The visible, permanent shape change is distortion; the internal stresses that remain after everything cools are residual stresses. A CWI must be able to identify distortion types, understand their causes, and recognize the control techniques a fabricator should be using.
Types of Welding Distortion
| Type | Description | Common Cause |
|---|---|---|
| Longitudinal shrinkage | Weld shortens along its length | Contraction along the weld axis |
| Transverse shrinkage | Parts pull toward each other across the joint | Contraction perpendicular to the weld |
| Angular distortion | Parts rotate about the weld axis ("butterflying") | Non-symmetric weld cross-section (single-V groove) |
| Bowing / camber | Member curves along its length | Weld placed off the member's neutral axis |
| Buckling | Wavy distortion in thin plate | Compressive residual stress exceeds buckling load |
| Twisting | Rotational distortion | Asymmetric welding sequence |
| Rotational distortion | Plates rotate in-plane during welding | Thermal expansion ahead of the arc |
Angular distortion is the one most often illustrated on the exam: a single-V groove deposits more weld metal at the top of the joint than the root, so the top shrinks more and the plates rotate upward toward each other.
Factors and Control Techniques
Factors That Increase Distortion
| Factor | Effect |
|---|---|
| Higher heat input | More expansion/contraction → more distortion |
| Larger weld size | More molten metal → more shrinkage |
| Thinner material | Less stiffness to resist movement |
| Poor fit-up / wide gaps | More weld metal needed → more shrinkage |
| Lack of restraint | Parts free to move during welding |
| Single-sided / non-symmetric joints | Off-axis shrinkage → angular distortion |
Distortion Control Techniques
| Technique | How It Works |
|---|---|
| Pre-setting (pre-cambering) | Position parts opposite to expected distortion so they pull into the correct final shape |
| Balanced (symmetric) welding | Alternate welds on both sides of the neutral axis (double-V, double-U) to cancel angular pull |
| Back-step welding | Deposit short segments in the reverse direction of overall progression |
| Intermittent (skip) welding | Use stitch welds where code/design permits to reduce total weld metal |
| Proper sequence | Weld from the center outward; weld free ends before restrained ends |
| Minimum weld size | Use the smallest weld that meets design requirements |
| Mechanical restraint | Clamps, strongbacks, fixtures, and tack welds hold parts in position |
| Lower heat input | Reduce amperage / increase travel speed where permitted |
| Peening (intermediate passes only) | Mechanically work the weld to offset shrinkage — never on the root or final cover pass, and only when permitted |
Note the trade-off an inspector watches for: restraint reduces distortion but raises residual stress and cracking risk. Clamping a joint rigidly keeps it straight but locks in more tensile stress, which feeds hydrogen-induced cracking. Distortion control and cracking control therefore have to be balanced, not maximized independently.
Residual Stress and Post-Weld Heat Treatment
Residual stresses are internal stresses that remain after the heat source is gone. Their distribution is predictable and frequently tested:
- The weld metal and near-HAZ are in tension — they wanted to shrink but were restrained — and the tension can approach the material's yield strength.
- The surrounding base metal is in compression, balancing the tensile core so the body is in overall equilibrium.
Consequences of tensile residual stress:
- Reduces fatigue life (it adds directly to applied tensile service stress)
- Provides the stress "leg" required for hydrogen-induced cracking
- Enables stress-corrosion cracking (SCC) in susceptible environments
- Causes parts to move/distort when later machined (the locked stress redistributes)
Post-Weld Heat Treatment (PWHT) / stress relief is the primary cure. The part is heated uniformly so that the metal's yield strength drops below the residual stress level, letting it relax by local plastic flow, then cooled slowly and uniformly.
| Parameter | Typical Value (carbon steel, P-No. 1) |
|---|---|
| Soak temperature | 1,100–1,200°F (595–650°C) — below the A1 lower critical |
| Holding (soak) time | 1 hour per inch (25 mm) of thickness; ~15 min minimum |
| Heating/cooling rate | Controlled and uniform to avoid new thermal-gradient stress |
Besides relieving stress, PWHT tempers any martensite in the HAZ — restoring ductility and toughness — and drives out residual hydrogen, so it simultaneously attacks two of the three legs of hydrogen-induced cracking. The soak stays below A1 (~1,333°F) so the steel does not re-austenitize.
For the Exam: Residual stress in the weld and near-HAZ is tensile (up to yield); the surrounding base metal is in compression. Know the distortion types and controls (pre-setting, balanced welding, back-step), and that PWHT for carbon steel soaks at ~1,100–1,200°F for about 1 hour per inch of thickness, relieving stress and tempering martensite.
What is the nature of the residual stress in the weld metal and near-HAZ after the joint has cooled?
Which distortion control technique positions the parts opposite to the expected distortion so they pull into the correct final shape?
PWHT (stress relief) of carbon steel is typically performed at approximately what temperature and hold time?