Pipefitter Pipe Fabrication Methods 2 — Questions and Answers

Question 1: When making a 90-degree offset using two 45-degree elbows, what is the travel distance if the offset is 18 inches?

18.00 inches
21.97 inches
25.46 inches (Correct answer)
12.73 inches

Correct answer: 25.46 inches

The travel distance for a 45-degree offset equals the offset multiplied by the constant 1.414 (square root of 2). For an 18-inch offset: 18 x 1.414 = 25.45 inches.

When routing a pipe around an obstacle using two 45-degree elbows, the diagonal run between the elbows is longer than the actual perpendicular offset distance. The relationship comes from trigonometry: the hypotenuse of a 45-45-90 right triangle equals the leg multiplied by the square root of 2 (1.41421). So travel = offset x 1.414. For 18 inches: 18 x 1.414 = 25.452 inches. Pipefitters subtract the fitting allowances (take-out) for each 45-degree elbow before cutting the spool piece. This calculation is fundamental to the UA apprenticeship math curriculum.

Question 2: What is the correct process for post-weld heat treatment (PWHT) of a carbon steel weld to reduce residual stress?

Cool rapidly with water after welding
Heat the weldment uniformly to 1100 to 1200 degrees F (593 to 649 C) and hold for a specified time, then cool slowly (Correct answer)
Apply preheat to 400 degrees F and immediately cool in still air
Heat to 1500 degrees F and quench in oil

Correct answer: Heat the weldment uniformly to 1100 to 1200 degrees F (593 to 649 C) and hold for a specified time, then cool slowly

PWHT for carbon steel requires heating the weld zone to 1100 to 1200 degrees F, holding at temperature for approximately 1 hour per inch of wall thickness, then cooling slowly in still air to relieve residual stresses.

Residual stresses in welds result from the rapid heating and cooling cycle of welding. PWHT relieves these stresses by heating the metal into a temperature range where the steel can creep and relax internally (1100 to 1200 degrees F for carbon steel per ASME B31.3 Table 331.1.1). The soak time allows the temperature to equalize through the wall thickness. Controlled slow cooling prevents reintroduction of thermal stresses. PWHT also tempers any hard zones in the HAZ (heat-affected zone), reducing susceptibility to hydrogen-induced cracking in sour service.

Question 3: What is the minimum preheat temperature required by ASME B31.3 for welding carbon steel pipe with a wall thickness greater than 1 inch?

50 degrees F (10 C)
175 degrees F (79 C) (Correct answer)
300 degrees F (149 C)
400 degrees F (204 C)

Correct answer: 175 degrees F (79 C)

ASME B31.3 Table 330.1.1 requires a minimum preheat of 175 degrees F (79 C) for carbon steel (P-Number 1) with base metal thickness greater than 1 inch, to slow cooling and prevent hydrogen cracking.

Preheat requirements under ASME B31.3 depend on material P-number and base metal thickness. For P-1 carbon steel: thickness 1 inch or less requires no preheat (or 50 degrees F minimum ambient); thickness greater than 1 inch requires 175 degrees F minimum preheat. The preheat must be maintained throughout welding and is measured 3 inches from the weld joint. Preheat slows the cooling rate, reduces hardness in the HAZ, allows hydrogen to diffuse out of the weld zone, and prevents cold cracking, which is critical for thick-wall pressure piping.

Question 4: What is the purpose of back-purging with argon gas when TIG welding stainless steel pipe?

To cool the weld faster and reduce distortion
To prevent oxidation (sugaring) on the pipe ID weld root, maintaining corrosion resistance (Correct answer)
To increase weld penetration depth
To pre-heat the base metal evenly before welding

Correct answer: To prevent oxidation (sugaring) on the pipe ID weld root, maintaining corrosion resistance

Back-purging displaces oxygen from the pipe interior so the weld root is shielded from oxidation. Without purging, stainless steel forms a porous, chromium-depleted 'sugared' root bead that severely reduces corrosion resistance.

When TIG-welding stainless steel, the weld root on the pipe inside diameter (ID) is exposed to heat and can react with atmospheric oxygen to form chromium oxide scale, called sugaring or oxidation. This depletes chromium from the metal surface, destroying the passive layer that gives stainless its corrosion resistance. Back-purging with argon (or sometimes nitrogen) keeps the ID atmosphere inert during welding. Purge dams are installed each side of the weld, argon is flowed in to displace air, and oxygen content is measured with an analyzer to confirm it is below 50 to 100 ppm before welding begins.

Question 5: When using a pipe wrap-around to mark a cutline on a pipe, what is the tool ensuring?

The mark is at the correct elevation for installation
The cut line is perfectly perpendicular (square) to the pipe axis (Correct answer)
The mark matches the isometric drawing pipe length
The mark locates the weld centerline for NDE inspection

Correct answer: The cut line is perfectly perpendicular (square) to the pipe axis

A pipe wrap-around or scribe marker ensures the cutline is exactly 90 degrees to the pipe axis (square cut), preventing angled cuts that would create gap variations and poor weld fit-up.

A square cut (perpendicular to the pipe centerline) is essential for proper weld fit-up. If a pipe is cut at even a slight angle, the gap at the joint varies around the circumference: one side is too tight, the opposite side has an excessive gap. This leads to inconsistent root bead penetration, potential burn-through on the tight side, and undercut on the wide side. A pipe wrap-around (a flexible band designed to wrap snugly around the pipe OD) provides a perfectly square reference line for the grinder or saw cutoff. NCCER pipefitting curricula emphasize this as a fundamental fabrication skill.

Question 6: What type of pipe joint is best suited for connecting dissimilar metals to prevent galvanic corrosion in a piping system?

Butt weld joint
Threaded coupling
Dielectric union or insulating flange kit (Correct answer)
Socket weld coupling

Correct answer: Dielectric union or insulating flange kit

A dielectric union or insulating flange kit uses non-conductive gaskets, sleeves, and washers to electrically isolate dissimilar metals, preventing galvanic current flow that causes accelerated corrosion.

When two dissimilar metals are in contact in the presence of an electrolyte (process fluid or moisture), galvanic corrosion attacks the more anodic metal. Common examples include connecting copper water lines to galvanized steel, or titanium heat exchangers to carbon steel headers. Dielectric unions (for small bore, low pressure) and insulating flange kits (for large bore/high pressure) interrupt the electrical path by using phenolic or PTFE gaskets, Mylar sleeves around bolt holes, and neoprene washers under bolt heads, ensuring current cannot flow between the dissimilar metals.

Pipefitter Pipe Fabrication Methods 2

When making a 90-degree offset using two 45-degree elbows, what is the travel distance if the offset is 18 inches?