456A Weld Quality Control — Questions and Answers
Question 1: What is the difference between a weld discontinuity and a weld defect?
- They are the same thing
- A discontinuity is any interruption in the normal weld structure; it becomes a defect only when it exceeds the acceptance criteria specified by the applicable code or standard (Correct answer)
- A discontinuity is always acceptable; a defect is always rejectable
- A discontinuity is visible; a defect is hidden
Correct answer: A discontinuity is any interruption in the normal weld structure; it becomes a defect only when it exceeds the acceptance criteria specified by the applicable code or standard
A discontinuity is any interruption in the typical structure of a weld (porosity, undercut, crack, inclusion, etc.). Not all discontinuities are defects. A discontinuity becomes a defect only when its size, type, or location exceeds the acceptance criteria of the governing code or standard (e.g., CSA W59, AWS D1.1).
Question 2: What is visual inspection (VT) and why is it the most important NDE method?
- Using a magnifying glass to examine welds after completion only
- A systematic examination of welds before, during, and after welding using the unaided eye or visual aids; important because it is the first line of quality control and can prevent defects rather than just finding them (Correct answer)
- Using cameras to photograph welds
- A pass/fail test based only on weld appearance
Correct answer: A systematic examination of welds before, during, and after welding using the unaided eye or visual aids; important because it is the first line of quality control and can prevent defects rather than just finding them
Visual inspection is the systematic examination of weld quality at all stages: before welding (fit-up, cleanliness, preheat), during welding (interpass temperature, technique, interpass cleaning), and after welding (size, profile, surface discontinuities). It is the most important NDE method because it can prevent defects and is required on every weld.
Question 3: What does radiographic testing (RT) detect in welds?
- Only surface cracks
- Subsurface (internal) discontinuities such as porosity, slag inclusions, lack of fusion, incomplete penetration, and cracks by passing X-rays or gamma rays through the weld (Correct answer)
- Only weld surface profile defects
- Hardness variations in the weld
Correct answer: Subsurface (internal) discontinuities such as porosity, slag inclusions, lack of fusion, incomplete penetration, and cracks by passing X-rays or gamma rays through the weld
Radiographic testing uses X-rays or gamma rays to create an image (film or digital) of the internal weld structure. It detects subsurface discontinuities including porosity, slag inclusions, lack of fusion, incomplete penetration, and cracks. It provides a permanent record (the radiograph) but requires radiation safety precautions and trained, certified interpreters.
Question 4: What is a Welding Procedure Specification (WPS) and why is it required?
- A description of the welder's qualifications
- A written document specifying all essential variables for a specific weld (process, filler metal, preheat, interpass temperature, technique, etc.) — required to ensure consistent, code-compliant welds (Correct answer)
- A list of approved welding equipment
- A purchase order for welding consumables
Correct answer: A written document specifying all essential variables for a specific weld (process, filler metal, preheat, interpass temperature, technique, etc.) — required to ensure consistent, code-compliant welds
A WPS is a written, qualified document that details all essential variables for producing a specific weld: welding process, base metal, filler metal type and size, shielding gas, preheat and interpass temperatures, polarity, amperage and voltage ranges, travel speed, joint design, and technique. Under CSA W47.1, all structural welds must be made to a qualified WPS.
Question 5: What is the purpose of preheat in welding and when is it required?
- To make the metal easier to cut
- To slow the cooling rate of the weld and HAZ, reducing the risk of hydrogen-induced cracking, reducing residual stresses, and improving fusion in thick or high-carbon/alloy steels (Correct answer)
- To increase welding speed
- To dry moisture from the electrode
Correct answer: To slow the cooling rate of the weld and HAZ, reducing the risk of hydrogen-induced cracking, reducing residual stresses, and improving fusion in thick or high-carbon/alloy steels
Preheat slows the cooling rate after welding, which: (1) reduces the risk of hydrogen-induced cracking by allowing hydrogen to diffuse out, (2) reduces hardness in the heat-affected zone (HAZ), (3) reduces residual stresses and distortion, and (4) improves wetting and fusion. CSA W59 specifies minimum preheat temperatures based on material thickness, grade, and hydrogen level.
Question 6: What is the purpose of a bend test in weld qualification?
- To test if the weld can be straightened
- To evaluate weld ductility, fusion, and soundness by bending a specimen to a specified radius and examining the outer surface for cracks or discontinuities exceeding the acceptance criteria (Correct answer)
- To test the hardness of the weld
- To determine the weld's tensile strength
Correct answer: To evaluate weld ductility, fusion, and soundness by bending a specimen to a specified radius and examining the outer surface for cracks or discontinuities exceeding the acceptance criteria
Guided bend tests evaluate weld quality by plastically deforming a specimen cut from a test weld. The convex (outer) surface is examined for cracks, lack of fusion, and other discontinuities. Face bends test the weld face side, root bends test the root side, and side bends test the cross-section. Under CSA W47.1, bend tests are part of welder qualification testing.
456A Weld Quality Control
What is the difference between a weld discontinuity and a weld defect?