CWI Weld Discontinuities and Defects 4 — Questions and Answers
Question 1: Which non-destructive testing method is BEST suited for detecting subsurface planar discontinuities parallel to the weld surface in a thick plate?
- Visual testing (VT)
- Liquid penetrant testing (PT)
- Ultrasonic testing (UT) (Correct answer)
- Magnetic particle testing (MT)
Correct answer: Ultrasonic testing (UT)
Ultrasonic testing is most effective for detecting internal planar discontinuities such as laminations and cracks, especially those parallel to the surface in thick sections.
Question 2: What is the primary difference between a 'discontinuity' and a 'defect' in welding inspection terminology?
- Discontinuities are surface only; defects are internal only
- A defect is a discontinuity that exceeds the acceptance criteria of the applicable code (Correct answer)
- Defects are found by NDE; discontinuities are found visually
- There is no difference; the terms are interchangeable
Correct answer: A defect is a discontinuity that exceeds the acceptance criteria of the applicable code
A discontinuity is any interruption in the material's typical structure, while a defect is a discontinuity that fails to meet the acceptance criteria of the applicable welding code.
Question 3: Arc strikes outside the weld zone are considered defects primarily because they:
- Increase the overall heat input of the weld
- Create hard, brittle zones that can initiate cracking in the base metal (Correct answer)
- Contaminate the shielding gas zone
- Reduce the weld reinforcement height
Correct answer: Create hard, brittle zones that can initiate cracking in the base metal
Arc strikes cause rapid localized heating and quenching of base metal, creating hardened zones with potential cracking that can initiate fatigue failure.
Question 4: A weld cross-section shows a narrow, incomplete fusion line between the weld metal and base metal on one side of the groove. This is BEST described as:
- Lack of penetration
- Lack of fusion (LOF) / incomplete fusion (Correct answer)
- Undercut
- Overlap
Correct answer: Lack of fusion (LOF) / incomplete fusion
Lack of fusion (incomplete fusion) occurs when the weld metal fails to fuse completely with the base metal or adjacent weld passes at the fusion interface.
Question 5: Which factor MOST influences the formation of hot cracks in austenitic stainless steel welds?
- High carbon equivalent
- High sulfur and phosphorus content promoting low-melting boundary films (Correct answer)
- Excessive preheat temperature
- Hydrogen dissolved in the weld metal
Correct answer: High sulfur and phosphorus content promoting low-melting boundary films
Hot cracking in austenitic stainless steels is primarily caused by sulfur and phosphorus segregating to grain boundaries and forming low-melting-point films that crack during solidification.
Question 6: When inspecting fillet welds, 'insufficient throat' is a defect because it:
- Creates a stress riser at the weld toe
- Reduces the effective weld area, lowering the joint's load-carrying capacity (Correct answer)
- Causes hydrogen cracking in the root
- Indicates improper shielding gas coverage
Correct answer: Reduces the effective weld area, lowering the joint's load-carrying capacity
Insufficient throat reduces the effective throat dimension of the fillet weld, directly decreasing the weld cross-sectional area and its load-carrying capacity.
Question 7: Which discontinuity would be MOST detrimental in a weld subjected to cyclic fatigue loading?
- Shallow porosity cluster less than 1mm diameter
- Rounded slag inclusion at the weld center
- Sharp planar crack at the weld toe (Correct answer)
- Slight surface roughness on weld cap
Correct answer: Sharp planar crack at the weld toe
Sharp planar cracks, especially at the weld toe, are most detrimental under fatigue because they act as stress concentrators with essentially zero tip radius, drastically reducing fatigue life.
CWI Weld Discontinuities and Defects 4
Which non-destructive testing method is BEST suited for detecting subsurface planar discontinuities parallel to the weld surface in a thick plate?