456A Weld Quality Inspection, Testing & Occupational Safety 2 — Questions and Answers
Question 1: What is a 'guided bend test' and what does it evaluate in a welded joint?
- A test coupon is bent around a mandrel of specified diameter; evaluates ductility and soundness of the weld and HAZ by exposing internal defects as cracks on the bent surface (Correct answer)
- A tensile test where the weld is bent under axial load to measure yield strength
- A fatigue test where the weld is bent repeatedly until fracture to measure fatigue life
- A hardness test performed on a bent specimen to evaluate HAZ hardness distribution
Correct answer: A test coupon is bent around a mandrel of specified diameter; evaluates ductility and soundness of the weld and HAZ by exposing internal defects as cracks on the bent surface
Guided bend tests (face bend, root bend, and side bend) are used for welding procedure qualification and welder performance qualification. The specimen is bent through 180° around a mandrel of diameter specified by the code (e.g., 4× base metal thickness). Any crack or open discontinuity above 3 mm constitutes failure. The test evaluates fusion quality and ductility of the weld/HAZ — defects open up during bending and become visible.
Question 2: What does a 'macro-examination' (macro-etch) of a weld cross-section reveal?
- The weld cross-sectional profile showing fusion line geometry, pass sequence, HAZ size, root penetration, and any gross discontinuities visible at low magnification after etching (Correct answer)
- The microstructural phase constituents (ferrite, martensite, pearlite) at high magnification
- The chemical composition of the weld metal at each cross-section
- The hardness gradient across the weld using microindentation
Correct answer: The weld cross-sectional profile showing fusion line geometry, pass sequence, HAZ size, root penetration, and any gross discontinuities visible at low magnification after etching
A macro-examination involves cutting a cross-section of the weld, grinding and polishing the surface, then etching with acid (typically 5–15% nitric in alcohol for steel) to reveal the weld structure. It shows: weld pass sequence and geometry, fusion line shape, HAZ extent, root penetration, and any cracks, porosity clusters, lack of fusion, or slag inclusions visible at low power (up to 10×). It is a required test in many procedure qualifications.
Question 3: What is the acceptance criterion for undercut in structural welds under CSA W59 for statically loaded structures?
- Undercut depth not exceeding 1 mm (1/32 in.) for welds transverse to the primary stress direction; 2 mm for welds parallel to the primary stress (Correct answer)
- No undercut is permitted under any circumstances
- Undercut up to 3 mm is acceptable for non-critical structural welds
- Undercut is only evaluated by UT; visual inspection is not used for acceptance
Correct answer: Undercut depth not exceeding 1 mm (1/32 in.) for welds transverse to the primary stress direction; 2 mm for welds parallel to the primary stress
CSA W59 (following AWS D1.1) allows limited undercut in statically loaded structures: up to 1 mm depth for welds transverse to tensile stress and up to 2 mm for welds parallel to stress (where the notch is less critical). For dynamically loaded structures (Category C), undercut is more tightly limited. Undercut exceeding limits must be repaired by grinding to a smooth transition or by depositing additional weld metal.
Question 4: What is 'phased array ultrasonic testing' (PAUT) and what advantage does it offer over conventional UT?
- Uses multiple piezoelectric elements electronically controlled to steer and focus the sound beam, providing real-time cross-sectional imaging (S-scan) and better defect characterization than single-element UT (Correct answer)
- Uses multiple conventional UT probes placed simultaneously around the weld
- An ultrasonic method that uses phased sound waves to test thin-wall piping only
- A calibration technique for conventional UT that uses phase comparison
Correct answer: Uses multiple piezoelectric elements electronically controlled to steer and focus the sound beam, providing real-time cross-sectional imaging (S-scan) and better defect characterization than single-element UT
PAUT uses an array of small piezoelectric elements that are electronically triggered in timed sequences to electronically steer and focus the beam without moving the probe. This generates sector scans (S-scans) showing a real-time cross-sectional view of the weld. Advantages over conventional UT: faster inspection, better defect detection and sizing, permanent digital record (data file), and the ability to inspect complex geometries. PAUT is now widely accepted by CSA Z662 and other Canadian codes.
Question 5: What is the purpose of a 'weld hardness test' (Vickers, Brinell, or Rockwell) on a procedure qualification record?
- Verifies that the HAZ hardness does not exceed specified limits (e.g., 350 HV max for most carbon steel), confirming adequate toughness and resistance to hydrogen-induced cracking (Correct answer)
- Measures the surface finish quality of the weld bead face
- Determines the minimum preheat temperature for production welding
- Evaluates the tensile strength of the weld metal by converting hardness to UTS
Correct answer: Verifies that the HAZ hardness does not exceed specified limits (e.g., 350 HV max for most carbon steel), confirming adequate toughness and resistance to hydrogen-induced cracking
Hardness testing of weld procedure qualification coupons (typically traversing from base metal through HAZ to weld metal) verifies that no excessively hard zones (martensite) were produced by the procedure. Maximum HAZ hardness limits (e.g., 350 HV for carbon steel, 250 HV for sour service per NACE MR0175) ensure adequate toughness and resistance to hydrogen-induced and sulfide stress cracking.
Question 6: What is a 'nick break test' and what weld defects does it expose?
- A forced fracture test where the weld metal is notched and broken to expose the weld cross-section, revealing porosity, slag inclusions, and incomplete fusion on the fractured surface (Correct answer)
- A test that measures the notch toughness of the weld at low temperatures
- A bending test used to evaluate root pass ductility in pipe welds
- A hardness measurement technique using a special notched indenter
Correct answer: A forced fracture test where the weld metal is notched and broken to expose the weld cross-section, revealing porosity, slag inclusions, and incomplete fusion on the fractured surface
The nick break test (used for pipeline and structural qualification) involves sawing notches into the sides of a fillet or groove weld specimen and then breaking it in tension or by hammer blow. The fractured surface exposes the interior of the weld metal, revealing any porosity, slag inclusions, incomplete fusion, or lack of penetration that may not be detectable from the surface. The exposed area is evaluated against code acceptance criteria.
456A Weld Quality Inspection, Testing & Occupational Safety 2
What is a 'guided bend test' and what does it evaluate in a welded joint?