Welding and Joining Techniques Flashcards

When performing a socket weld on a 2-inch Schedule 80 carbon steel pipe, what is the correct gap that must be maintained between the pipe end and the bottom of the socket before welding?

Answer: 1/16 inch (1.6 mm)

ASME B31.3 and B16.11 require a 1/16-inch (1.6 mm) gap between the pipe end and the socket bottom before welding. This gap allows for thermal expansion during welding and in service, preventing stress concentration and potential cracking at the root of the weld. Bottoming out the pipe eliminates this expansion space and can cause joint failure.

A pipefitter is joining two dissimilar metals: a P-No. 1 carbon steel pipe to a P-No. 8 austenitic stainless steel pipe. Which filler metal classification is most appropriate for this weld per ASME Section IX guidelines?

Answer: ER309L

ER309L is specifically designed for welding dissimilar metals, particularly carbon steel to austenitic stainless steel. Its higher chromium and nickel content compared to ER308L provides a dilution buffer against the carbon steel's composition. ER308L is used for 304 SS to 304 SS, ER316L for 316 SS to 316 SS, and ER70S-6 is a carbon steel filler inappropriate for stainless applications.

During a GTAW root pass on a 6-inch diameter pipe in the 6G position, the welder notices the weld puddle is pulling away from the trailing edge of the puddle (undercutting the toes). What is the MOST likely cause?

Answer: Travel speed is too fast for the amperage used

Undercutting at the toes of the weld in GTAW is classically caused by excessive travel speed relative to the heat input (amperage). When the welder moves too fast, the arc erodes the base metal at the edges of the puddle faster than the filler metal can fill it in, leaving a groove at the toe. Shielding gas issues typically manifest as porosity, not undercutting. Electrode size and preheat affect penetration and cracking, not this specific defect mode.

A brazed joint in a copper refrigerant piping system fails a pressure test repeatedly at the same fitting. Inspection reveals the braze alloy has only wetted the outer 1/4 inch of a 3/4-inch overlap joint. What process variable is the MOST probable root cause?

Answer: The torch was applied to the fitting cup rather than the pipe body, overheating the fitting and preventing capillary draw

Proper silver brazing technique requires heating the pipe body (the male member) so that capillary action draws the molten filler from the pipe surface into and through the joint gap toward the heat source. When excessive heat is applied directly to the fitting cup, the fitting overheats relative to the pipe — the alloy melts at the entrance but the temperature gradient runs the wrong direction, stopping capillary draw and producing only a shallow surface weld at the socket mouth. This is the classic 'cold joint that looks good from outside' failure mode.

Per ASME B31.3, when a weld on a High Pressure Fluid Service (Category M) piping system is rejected by radiographic examination and a repair weld is made, what is the minimum examination requirement for the completed repair?

Answer: Radiographic examination of the full weld length, including the repair area

ASME B31.3 requires that for Category M fluid service welds, any repaired weld must be re-examined over its full length by the same method used originally (radiography in this case), not just the repaired area. This is because the excavation and repair process can introduce new defects adjacent to the repair zone, and the heat cycle from the repair weld can affect the metallurgy of the surrounding original weld material. Examining only the repair area would leave the balance of the weld uninspected after the thermal re-cycling.

A pipefitter is performing an orbital GTAW weld on a 3-inch sanitary (hygienic) stainless steel tube for a pharmaceutical process line requiring full internal purging. The internal bead exhibits consistent brown and blue oxide discoloration ('sugar') along the entire weld length. Which corrective action should be taken FIRST?

Answer: Verify that the purge gas dew point is below −40°F (−40°C) and check all fittings and backing dams for leaks

Brown and blue oxide on the internal bead indicates oxygen contamination of the purge atmosphere — the weld pool was exposed to oxygen levels far above the acceptable threshold (typically <50 ppm for pharmaceutical-grade welds per ASME BPE). The first corrective action is to verify dew point of the purge gas and inspect for leaks at purge dams, fittings, and connections, since even a small leak can introduce enough oxygen to cause heavy oxidation despite adequate flow rate. Simply increasing flow rate will not help if contaminated atmospheric air is entering through a leak. Hydrogen forming gas is effective but addresses an already-correct setup, and changing travel speed does not address the root cause of oxygen ingress.