Pipefitter Pipe Fabrication and Joining 2 — Questions and Answers

Question 1: What is the minimum preheat temperature required by AWS D1.1 when welding carbon steel with a carbon equivalent greater than 0.40?

150°F (65°C) (Correct answer)
32°F (0°C)
400°F (205°C)
75°F (24°C)

Correct answer: 150°F (65°C)

AWS D1.1 and ASME B31.3 require a preheat of at least 150°F when the carbon equivalent exceeds 0.40 to prevent hydrogen-induced cracking.

Carbon equivalent (CE) is calculated from the chemical composition of the steel. When CE exceeds 0.40, the steel is more hardenable and susceptible to hydrogen-induced cracking (cold cracking) in the heat-affected zone. Preheating to at least 150°F (65°C) slows the cooling rate after welding, allows hydrogen to diffuse out, and reduces residual stresses. The required preheat increases with higher CE values and thicker materials.

Question 2: When making a socket weld joint, what is the standard gap that should be left between the pipe end and the socket bottom?

1/16 inch (1.6 mm) (Correct answer)
No gap — pipe must bottom out
1/4 inch (6.4 mm)
1/8 inch (3.2 mm)

Correct answer: 1/16 inch (1.6 mm)

ASME B31.3 requires a 1/16" gap between the pipe end and the socket shoulder before welding to allow for thermal expansion.

ASME B31.3 Para. 328.5.2 requires that socket weld fittings be assembled by inserting the pipe to the bottom of the socket, then withdrawing it approximately 1/16" (1.6 mm) before welding. This gap allows for thermal expansion of the pipe during welding and operation. Without this gap, the pipe would bottom out as it expands, potentially cracking the socket or the weld. Forgetting this gap is a common error.

Question 3: What type of pipe joint is made by heating the end of a pipe and forming a conical seating surface that mates with a flared fitting?

A flared joint (Correct answer)
A compression joint
A mechanical joint
A press-fit joint

Correct answer: A flared joint

A flared joint is made by flaring the pipe end into a cone shape that mates with the angled seat of a flare fitting, creating a metal-to-metal seal.

Flared joints are made with a flaring tool that expands and forms the soft pipe (copper, aluminum, or soft steel tubing) end into a 37° (JIC) or 45° (SAE) conical flare. The flare seats against the matching cone angle of the fitting body, creating a metal-to-metal seal. A flare nut compresses the flare against the fitting. This joint type is used in hydraulic systems, refrigeration lines, and instrument tubing where re-makeable, high-pressure connections are needed.

Question 4: What is meant by 'back-purging' during stainless steel pipe welding?

Flowing inert gas inside the pipe to protect the weld root from oxidation (Correct answer)
Reversing the welding direction to fill undercut
Cleaning the weld joint with back-hand motion
Using back-pressure to prevent weld burn-through

Correct answer: Flowing inert gas inside the pipe to protect the weld root from oxidation

Back-purging means flowing argon or nitrogen through the inside of the pipe to displace oxygen and protect the weld root from oxidation (sugaring).

When welding austenitic stainless steel (300 series), the backside of the weld root must be protected from atmospheric oxygen. Without back-purging, the root oxidizes and forms a rough, sugary deposit that is chrome-depleted and susceptible to corrosion. Argon is the preferred purge gas; nitrogen can be used for some applications. The purge must achieve oxygen levels below 50-100 ppm before welding begins, verified with an oxygen analyzer.

Question 5: In threaded pipe joints per ASME B1.20.1, what is the standard taper of NPT (National Pipe Taper) threads?

1/16 inch per inch (1 in 16) (Correct answer)
1/8 inch per inch (1 in 8)
1/32 inch per inch (1 in 32)
1/4 inch per inch (1 in 4)

Correct answer: 1/16 inch per inch (1 in 16)

NPT threads have a taper of 1/16 inch per inch (3/4 inch per foot) as specified in ASME B1.20.1.

NPT (National Pipe Taper) threads per ASME B1.20.1 have a taper of 1/16 inch per inch of thread length (equivalent to 3/4 inch per foot or approximately 1°47'). This taper causes the threads to wedge together as the joint is tightened, creating the primary seal. Thread sealant (PTFE tape or pipe dope) is applied to fill any remaining voids between threads. NPTF (Dryseal) threads are a tighter-tolerance version designed to seal without sealant.

Question 6: What is the primary advantage of a 'butt weld' joint over a 'socket weld' joint for larger pipe sizes?

Butt welds provide full penetration with no crevice for corrosion or trapped fluid (Correct answer)
Butt welds are faster and easier to make
Socket welds are only available up to 2-inch pipe size
Butt welds do not require post-weld heat treatment

Correct answer: Butt welds provide full penetration with no crevice for corrosion or trapped fluid

Butt welds join pipe end-to-end with full penetration, eliminating the crevice between pipe and socket that can trap corrosive fluids or cause fatigue cracking.

Butt welds provide full weld penetration with no internal crevice, making them ideal for: (1) corrosive services where the socket crevice could trap fluid and cause crevice corrosion, (2) high-cycle fatigue applications where the socket crevice is a stress concentration point, (3) services requiring radiographic examination (RT) of the full weld cross-section, and (4) larger pipe sizes (typically 2" NPS and above per ASME B31.3). Socket welds are limited to 2" NPS and below.

Pipefitter Pipe Fabrication and Joining 2

What is the minimum preheat temperature required by AWS D1.1 when welding carbon steel with a carbon equivalent greater than 0.40?