AWS AWS Distortion Control & Residual Stress 5 — Questions and Answers
Question 1: Which post-weld treatment is most effective at redistributing and reducing residual stresses in carbon steel weldments?
- Post-weld heat treatment (PWHT) / stress relief annealing (Correct answer)
- Electropolishing the weld surface
- Applying a zinc-rich coating
- Shot peening the base metal away from the weld
Correct answer: Post-weld heat treatment (PWHT) / stress relief annealing
PWHT at appropriate temperatures (typically 1100–1200°F for carbon steel) allows plastic flow and creep to redistribute and reduce residual stresses.
Question 2: A fabricator is welding two plates butt-welded together and wants to minimize transverse shrinkage. Which approach is most effective?
- Minimize the weld cross-sectional area by using joint designs with the least required fill (Correct answer)
- Increase root opening to allow more penetration
- Use higher amperage to complete the weld faster
- Allow longer interpass cooling between passes
Correct answer: Minimize the weld cross-sectional area by using joint designs with the least required fill
Minimizing the volume of weld metal deposited directly reduces transverse shrinkage because less material contracts upon cooling.
Question 3: What is the effect of peening on residual stresses in a weld?
- Introduces compressive residual stresses at the surface, offsetting tensile residual stresses (Correct answer)
- Increases tensile residual stresses by work-hardening the surface
- Eliminates all residual stress throughout the weld cross-section
- Has no measurable effect on residual stress distribution
Correct answer: Introduces compressive residual stresses at the surface, offsetting tensile residual stresses
Peening stretches the weld surface plastically, introducing compressive stresses that counteract the tensile residual stresses left by solidification shrinkage.
Question 4: When welding a closed-section box structure, why is distortion particularly difficult to control compared to open sections?
- Rigid restraint of all four sides increases residual stress without allowing stress relief through distortion (Correct answer)
- Box sections have lower thermal conductivity trapping more heat
- Fillet welds used in box sections always produce more distortion than groove welds
- Shielding gas is harder to control inside box sections
Correct answer: Rigid restraint of all four sides increases residual stress without allowing stress relief through distortion
In closed box sections, all sides restrain each other, preventing visible distortion but causing very high residual stresses that can lead to cracking.
Question 5: Which distortion type is characterized by a rotation of the joint members about the weld axis, causing the plates to form a V-shape when viewed from the end?
- Angular distortion (Correct answer)
- Longitudinal bowing
- Transverse shrinkage
- Rotational distortion
Correct answer: Angular distortion
Angular distortion produces a V-shape across the weld cross-section because the top of the weld shrinks more than the root due to the larger weld cross-section area at the top.
Question 6: What is the relationship between joint restraint and residual stress magnitude after welding?
- Higher restraint produces higher residual stresses because the metal cannot freely contract (Correct answer)
- Higher restraint reduces residual stress by limiting thermal expansion
- Restraint has no effect on residual stress magnitude
- Lower restraint always results in higher residual stresses
Correct answer: Higher restraint produces higher residual stresses because the metal cannot freely contract
When a weld is highly restrained, shrinkage forces cannot be relieved through movement, so they remain as high residual stresses within the weldment.
Question 7: Which of the following welding variables, if increased, will most directly INCREASE longitudinal distortion (bowing) in a long weld on a beam?
- Total heat input per unit length (Correct answer)
- Shielding gas flow rate
- Number of tack welds along the joint
- Electrode diameter while keeping current constant
Correct answer: Total heat input per unit length
Higher heat input per unit length increases the volume of metal that undergoes thermal expansion and contraction, causing greater longitudinal shrinkage and bowing.
AWS AWS Distortion Control & Residual Stress 5
Which post-weld treatment is most effective at redistributing and reducing residual stresses in carbon steel weldments?