Pipefitter Welding and Joining Techniques 2 — Questions and Answers

Question 1: What welding process is designated by AWS process abbreviation GTAW?

Gas metal arc welding (MIG)
Flux-cored arc welding
Gas tungsten arc welding (TIG) (Correct answer)
Shielded metal arc welding (Stick)

Correct answer: Gas tungsten arc welding (TIG)

GTAW stands for Gas Tungsten Arc Welding, commonly called TIG (Tungsten Inert Gas) welding. It uses a non-consumable tungsten electrode and a separate filler rod, shielded by an inert gas such as argon.

AWS (American Welding Society) designates Gas Tungsten Arc Welding as GTAW. The process uses a non-consumable tungsten electrode to create an arc with the base metal; filler metal is fed separately by hand (or mechanically) as a rod or wire. The weld pool and electrode are shielded from atmospheric contamination by an inert gas (argon, helium, or a mixture). GTAW produces the highest quality welds with precise heat input control, making it mandatory for root passes on critical stainless steel and alloy piping.

Question 2: What is the maximum allowable root opening (gap) for a single-V butt weld in pipe per typical ASME Section IX workmanship requirements?

1/16 inch (1.6 mm)
3/16 inch (4.8 mm) (Correct answer)
1/4 inch (6.4 mm)
3/8 inch (9.5 mm)

Correct answer: 3/16 inch (4.8 mm)

ASME IX and most company WPS documents limit root opening for a standard single-V butt weld to a maximum of 3/16 inch (4.8 mm), with typical ideal root gap being 1/16 to 1/8 inch.

Root opening (gap) in a single-V groove weld directly controls root bead penetration and the risk of burn-through. Too small a gap produces an incomplete root; too large allows the weld pool to fall through. ASME Section IX qualified WPS documents typically specify root opening of 0 to 3/16 inch (0 to 4.8 mm) for standard carbon steel pipe welding. If the actual gap exceeds the WPS maximum during fit-up, the welder must reject the fit-up and require the fitter to re-fit to specification. The WPS essential variable for root opening is a binding requirement on certified welders.

Question 3: When welding in the 6G position, what does the '6' indicate?

6-inch pipe diameter
Inclined fixed pipe (45 degrees from horizontal) (Correct answer)
Horizontal fixed pipe
Vertical fixed pipe

Correct answer: Inclined fixed pipe (45 degrees from horizontal)

The 6G position is a fixed-pipe position with the pipe axis inclined at 45 degrees from horizontal. It is considered the most challenging qualification position because the welder must continuously change technique as they weld around the pipe.

AWS and ASME welding position codes for pipe: 1G = horizontal rolled (pipe horizontal, rotated); 2G = vertical fixed (pipe vertical, weld horizontal); 5G = horizontal fixed (pipe horizontal, welder goes around); 6G = inclined fixed at 45 degrees. The 6G qualification is particularly demanding because the 45-degree angle creates an ever-changing weld puddle behavior. A welder who qualifies in 6G is qualified for all pipe positions except 6GR (restricted, with backing ring), making it the most sought-after pipe welding qualification for high-pressure service.

Question 4: What is the purpose of an interpass temperature limit during multi-pass welding of alloy steel?

To ensure the weld pool stays molten between passes
To prevent overheating the base metal, which can cause grain coarsening and reduction in toughness and creep strength (Correct answer)
To keep the welder from burning the rod too fast
To maintain the pipe at preheat temperature throughout

Correct answer: To prevent overheating the base metal, which can cause grain coarsening and reduction in toughness and creep strength

Interpass temperature maximum limits prevent the cumulative heat from multiple weld passes from raising the base and weld metal into a temperature range that causes grain coarsening and loss of impact toughness.

Interpass temperature is the temperature of the weld zone immediately before depositing the next pass. For alloy steels (P4, P5, P91, etc.) and austenitic stainless steels, excessive interpass temperature causes grain coarsening (larger grains equal lower toughness), sensitization in stainless (chromium carbide precipitation at grain boundaries causes intergranular corrosion), and reduction in creep strength for high-temperature alloys like P91. ASME B31.1/B31.3 and the applicable WPS specify maximum interpass temperatures. Welders must measure interpass temperature with a contact pyrometer before each pass.

Question 5: What is the primary purpose of a weld coupon bend test in ASME Section IX welder qualification?

Measure the weld's tensile strength only
Evaluate weld ductility and detect surface and subsurface discontinuities in the weld and HAZ (Correct answer)
Measure the weld bead width and reinforcement
Test the weld for pressure tightness

Correct answer: Evaluate weld ductility and detect surface and subsurface discontinuities in the weld and HAZ

The guided bend test evaluates weld ductility and reveals discontinuities (cracks, porosity, lack of fusion, incomplete penetration) in the weld and heat-affected zone that might not be visible on the surface.

ASME Section IX requires guided bend tests (face, root, and/or side bends depending on thickness) to qualify welders. The test coupon is bent over a mandrel to a specified radius. A ductile, sound weld will bend without cracking; a defective weld will reveal cracks, porosity, incomplete fusion, or incomplete penetration as the material is strained. The acceptance criterion per QW-163 requires that any single crack after bending must not exceed 1/8 inch (3.2 mm) measured in any direction. Side bends are used for wall thickness 3/8 inch or greater instead of face/root bends.

Question 6: In threaded pipe connections, what type of sealant is used on a steam service line?

PTFE (Teflon) tape only
Thread sealant compound rated for high-temperature steam service (e.g., Rectorseal No. 5 or equivalent) (Correct answer)
No sealant is needed; steam connections seal by thread engagement alone
Standard petroleum-based pipe dope

Correct answer: Thread sealant compound rated for high-temperature steam service (e.g., Rectorseal No. 5 or equivalent)

Steam service requires a high-temperature thread sealant compound rated for steam temperature and pressure. Standard PTFE tape and petroleum-based pipe dope can break down and contaminate the steam system at elevated temperatures.

Thread sealants must be compatible with the service fluid, temperature, and pressure. For steam service, standard white PTFE tape breaks down at temperatures above approximately 500 degrees F and can shred into the system. Petroleum-based dopes also degrade in steam. High-temperature rated compounds such as Rectorseal No. 5 or equivalent products designed for steam/gas/petroleum service maintain their sealing integrity. The plant's piping specification or fitting manufacturer will specify the approved sealant for each service class.

Pipefitter Welding and Joining Techniques 2

What welding process is designated by AWS process abbreviation GTAW?