456A Blueprint Reading, Weld Symbols & Joint Preparation 4 — Questions and Answers
Question 1: What is the maximum allowable root face (land) dimension for a standard single-V groove joint when welding with SMAW using a 3.2 mm electrode?
- 1.5–3 mm root face to support the root pass without burn-through while allowing complete penetration (Correct answer)
- 0 mm — no root face is required for SMAW
- 6 mm to prevent burn-through on thick plate
- Root face must equal the electrode diameter (3.2 mm minimum)
Correct answer: 1.5–3 mm root face to support the root pass without burn-through while allowing complete penetration
A root face of 1.5–3 mm (approximately 1/16–1/8 in.) provides adequate support for the molten root pass without excessive risk of burn-through, while still allowing complete penetration with the arc. Too thick a root face prevents full penetration; too thin (or zero) increases burn-through risk. The root face dimension is specified in the WPS and on the joint preparation drawing.
Question 2: What is the purpose of tack welds in joint assembly and what special requirements apply to structural tack welds per CSA W59?
- Tack welds temporarily hold joint alignment; structural tacks must be made by qualified welders using the approved WPS and must be of adequate size and quality to avoid cracking or inclusion when covered by the final weld (Correct answer)
- Tack welds are only used for fit-up and must always be removed before final welding
- Tack welds can be made by anyone on the jobsite as they are not structural
- Tack welds must be twice the leg size of the final fillet weld
Correct answer: Tack welds temporarily hold joint alignment; structural tacks must be made by qualified welders using the approved WPS and must be of adequate size and quality to avoid cracking or inclusion when covered by the final weld
Tack welds that will be incorporated into the final weld must be made by qualified welders using the applicable WPS (same preheat, low-hydrogen process if required). Tacks must be free of cracks and discontinuities. Cracked tacks must be removed, never welded over. Tack welds at the end of joints that will not be consumed may be removed or feathered (ground) before final welding.
Question 3: What is 'hi-lo' (mismatch) in pipe or plate joint fit-up and what is the maximum allowed per CSA W59 for butt joints in structural steel?
- Vertical offset between the two base metal surfaces at the joint; maximum 3 mm or 25% of the thinner member thickness, whichever is less (Correct answer)
- Horizontal misalignment of the joint gap; maximum 5 mm for all applications
- The difference in groove angle between the two bevels; maximum 5°
- Root opening variation along the joint length; maximum 6 mm
Correct answer: Vertical offset between the two base metal surfaces at the joint; maximum 3 mm or 25% of the thinner member thickness, whichever is less
Hi-lo (offset or mismatch) is the vertical offset between the surfaces of two base metal pieces at a butt joint. Excessive hi-lo causes stress concentration, unequal penetration, and notch effects. CSA W59 limits hi-lo to 3 mm or 25% of the thinner member thickness, whichever is less. On pressure piping (CAN/CSA Z662), limits are typically 1.6 mm for pipe wall thicknesses up to 12.7 mm.
Question 4: What is the purpose of a 'weld run-off tab' at the end of a weld joint?
- Allows the weld to be started and terminated on temporary extensions, keeping weld defects (craters, start porosity) off the base metal joint (Correct answer)
- Provides a backing support for the root pass at the joint end
- Acts as a temporary tack weld to maintain joint alignment at the end
- Increases the effective weld length beyond the structural requirement
Correct answer: Allows the weld to be started and terminated on temporary extensions, keeping weld defects (craters, start porosity) off the base metal joint
Run-off tabs (extension bars) are temporary pieces of matching base material tack-welded to the ends of the joint. The weld starts on the run-on tab, proceeds through the joint, and ends on the run-off tab, keeping arc starts and stops (which are prone to porosity, craters, and inclusion) off the structural weld. Tabs are removed and the base metal ends dressed flush after welding.
Question 5: What is the maximum root opening (gap) allowed for a fillet-welded T-joint under CSA W59 before a larger weld size must be specified?
- 5 mm — root openings above 5 mm require increasing the weld leg size by the amount of the gap (Correct answer)
- No root opening is permitted in fillet-welded T-joints
- 10 mm — root openings to 10 mm are acceptable with no size adjustment
- 2 mm — any gap above 2 mm requires a groove weld instead
Correct answer: 5 mm — root openings above 5 mm require increasing the weld leg size by the amount of the gap
CSA W59 allows root openings in fillet-welded T-joints up to 5 mm (3/16 in.). For root openings up to 5 mm, the weld leg size must be increased by the amount of the gap to maintain the required effective throat. Root openings above 5 mm are generally not permitted without an engineering review or joint preparation change.
Question 6: What is the purpose of 'back gouging' a weld and what NDT method is typically used to verify the gouge depth is adequate?
- Removes the root pass from the back side to ensure defect-free base metal is present before depositing back-side weld; verified by visual inspection or dye penetrant testing of the gouge cavity (Correct answer)
- Increases the groove angle for better access on the second pass
- Removes slag from between weld passes using a gouging electrode
- Back gouging is only required for welds made with FCAW, not SMAW
Correct answer: Removes the root pass from the back side to ensure defect-free base metal is present before depositing back-side weld; verified by visual inspection or dye penetrant testing of the gouge cavity
Back gouging (by air carbon arc gouging, grinding, or chipping) removes the root of the first-side weld from the back. The gouge must extend into sound, fully fused weld metal to ensure the back-side weld will achieve complete fusion. The gouged surface is visually inspected and, for critical joints, dye penetrant (PT) or magnetic particle (MT) tested to confirm no cracks or lack of fusion remain before back welding.
456A Blueprint Reading, Weld Symbols & Joint Preparation 4
What is the maximum allowable root face (land) dimension for a standard single-V groove joint when welding with SMAW using a 3.2 mm electrode?