Pipefitter Welding and Joining Techniques 4 — Questions and Answers
Question 1: What is the primary purpose of back purging with argon gas when welding stainless steel pipe?
- To cool the weld faster
- To prevent oxidation on the inside of the pipe (Correct answer)
- To increase weld penetration depth
- To reduce arc voltage
Correct answer: To prevent oxidation on the inside of the pipe
Back purging with argon displaces oxygen inside the pipe, preventing oxidation (sugaring) on the weld root, which is critical for corrosion resistance in stainless steel.
Question 2: In orbital welding, what does the term 'weld schedule' refer to?
- The timeline for completing a weld job
- A pre-programmed set of welding parameters for a specific pipe and material (Correct answer)
- The order in which welds are made on a piping system
- The inspection schedule after welding is complete
Correct answer: A pre-programmed set of welding parameters for a specific pipe and material
A weld schedule in orbital welding is a stored set of parameters (amperage, travel speed, pulse settings) programmed for a specific pipe diameter, wall thickness, and material.
Question 3: When using an oxyacetylene torch for pipe brazing, the correct flame adjustment for most copper alloy work is:
- Oxidizing flame
- Carburizing flame
- Neutral flame (Correct answer)
- Reducing flame
Correct answer: Neutral flame
A neutral flame (balanced oxygen and acetylene) is correct for brazing copper alloys as it neither adds carbon nor oxidizes the base metal.
Question 4: What is the minimum preheat temperature typically required for P91 chrome-moly alloy steel pipe welding?
- 100°F (38°C)
- 300°F (149°C)
- 400°F (204°C) (Correct answer)
- 600°F (315°C)
Correct answer: 400°F (204°C)
P91 (9Cr-1Mo) alloy steel requires a minimum preheat of 400°F (204°C) to prevent hydrogen-induced cracking and ensure proper martensitic transformation.
Question 5: Which welding discontinuity is most likely to result from improper interpass temperature control during multi-pass pipe welding?
- Undercut
- Lack of fusion
- Hot cracking (Correct answer)
- Overlap
Correct answer: Hot cracking
Excessive interpass temperatures increase the heat-affected zone and can cause hot cracking in materials susceptible to liquation cracking between passes.
Question 6: A pipefitter is joining a carbon steel pipe to a stainless steel pipe. Which filler metal is typically used for this dissimilar metal weld?
- ER70S-6
- ER308L
- ER309L (Correct answer)
- ER316L
Correct answer: ER309L
ER309L is the standard filler metal for dissimilar metal welds between carbon steel and stainless steel because its higher alloy content accommodates dilution from both base metals.
Question 7: What does PWHT stand for and when is it most commonly required in pipe welding?
- Pre-Weld Heat Treatment — applied before any welding begins
- Post-Weld Heat Treatment — required on thick wall or alloy pipe to relieve residual stress (Correct answer)
- Pipe Weld Hardness Test — a non-destructive examination method
- Partial Weld Heat Treatment — used on short weld segments only
Correct answer: Post-Weld Heat Treatment — required on thick wall or alloy pipe to relieve residual stress
PWHT (Post-Weld Heat Treatment) is applied after welding to relieve residual stresses and temper the heat-affected zone, commonly required by code on thick carbon steel or alloy pipe welds.
Pipefitter Welding and Joining Techniques 4
What is the primary purpose of back purging with argon gas when welding stainless steel pipe?