Welding and Joining Techniques Flashcards

When welding P91 (9Cr-1Mo-V) chrome-moly steel pipe, which post-weld heat treatment (PWHT) temperature range is specified by ASME B31.1 to achieve the required creep-resistant microstructure?

Answer: 1350°F–1400°F (732°C–760°C)

ASME B31.1 and applicable material specifications require P91 steel to undergo PWHT in the range of 1350°F–1400°F (732°C–760°C). This temperature range tempers the martensitic microstructure formed during welding, relieving residual stresses and restoring the creep-rupture strength critical for high-temperature, high-pressure service. Lower temperatures are insufficient to adequately temper the martensite, while exceeding ~1430°F risks re-austenitizing and degrading properties.

A pipefitter is socket-welding a 2-inch Class 3000 fitting onto carbon steel pipe using SMAW. Before making the fillet weld, the pipe is bottomed in the socket and then deliberately pulled back approximately 1/16 inch. What is the primary engineering reason for this gap?

Answer: To prevent hydraulic lock and allow thermal expansion of the pipe end during welding

The 1/16-inch gap (specified in ASME B16.11 and ASME B31.1/B31.3) prevents the pipe end from being fully bottomed against the socket shoulder. During welding, the pipe end expands thermally; if fully seated, this expansion has nowhere to go, creating high residual stress concentrations at the root of the socket that can cause cracking under cyclic thermal or pressure loading. The gap absorbs this expansion.

During orbital GTAW of stainless steel instrument tubing in a pharmaceutical clean-room piping system (BPE service), the weld joint shows a golden-to-light straw heat tint on the ID weld bead. According to ASME BPE standards, what corrective action is required?

Answer: The weld must be rejected and the spool re-welded with improved ID purge gas flow to achieve a bright, silver finish

ASME BPE (Bioprocessing Equipment) requires the ID weld bead in high-purity and pharmaceutical service to have a bright silver color — indicating a fully inert atmosphere during welding (typically pure argon purge). A golden, straw, or blue heat tint indicates chromium oxide formation, which compromises corrosion resistance and creates a surface that can harbor biofilm. Unlike structural piping codes, BPE does not permit even light tints; the joint must be rejected and remade with adequate purging.

A pipefitter must join a 6-inch Schedule 80 304L stainless steel pipe to a 6-inch Schedule 80 carbon steel pipe in a sulfuric acid dilution service. The design engineer specifies an Alloy 20 (UNS N08020) transition weld using ENiCrFe-3 filler metal. Why is a direct carbon steel-to-304L weld inadvisable, and why is ENiCrFe-3 chosen over E309L for this dissimilar metal joint?

Answer: E309L causes sigma phase embrittlement at the carbon steel fusion line, while ENiCrFe-3 provides a nickel-rich buffer that resists dilution cracking and sulfuric acid attack at the transition

E309L is a common dissimilar-metal filler for carbon-to-austenitic stainless joints in general service, but in sulfuric acid environments the carbon steel fusion zone and heat-affected zone remain vulnerable to acid attack. The Alloy 20 intermediate piece and ENiCrFe-3 filler (a nickel-chromium-iron electrode) provide a nickel-rich weld deposit with superior resistance to dilute sulfuric acid. Additionally, nickel-based fillers accommodate the significant coefficient of thermal expansion mismatch between carbon steel and 304L better than austenitic stainless fillers, reducing fatigue cracking risk at the dissimilar interface.

When performing a branch connection weld on a 12-inch run pipe using the set-on (o-let) method, an MT (magnetic particle) examination reveals a linear indication running longitudinally along the toe of the branch fillet weld, oriented parallel to the weld axis. Under ASME B31.3 Normal Fluid Service acceptance criteria, how must this indication be evaluated?

Answer: Linear indications of any length are rejectable regardless of orientation when detected at the weld toe by MT

ASME B31.3 Table 341.3.2 specifies that for magnetic particle examination of welds, all linear indications are rejectable — there is no minimum acceptable length for linear indications detected in welds (unlike PT/MT criteria for base metal). A linear indication at the weld toe is particularly concerning because the toe is a stress concentration zone prone to fatigue cracking. The orientation parallel to the weld axis does not make it 'non-relevant'; non-relevance applies only to indications caused by geometry (e.g., sharp corners, threads) that are verified to be false indications.

A pipefitter is preparing to make a full-penetration butt weld on 10-inch Schedule 120 P22 (2.25Cr-1Mo) alloy steel pipe using SMAW. The ambient temperature is 28°F (-2°C). The applicable WPS requires a minimum preheat of 300°F (149°C) and a maximum interpass temperature of 600°F (316°C). The root pass is completed at 350°F. When the pipefitter returns after a 45-minute break, the joint has cooled to 180°F. What is the correct procedure before depositing the next weld pass?

Answer: Re-preheat the entire joint uniformly back to at least 300°F before continuing to weld

When the interpass temperature drops below the minimum preheat requirement (300°F for P22), the entire weld area — including a minimum of 3 inches on each side of the joint — must be uniformly re-preheated to the minimum preheat temperature before welding resumes. P22 is a hydrogen-cracking-susceptible alloy; the preheat requirement exists to slow cooling rates and allow hydrogen to diffuse out of the weld HAZ. Localized heating only at the groove would create steep thermal gradients and high residual stresses. Continuing to weld below minimum preheat risks hydrogen-induced cold cracking, which in P22 can occur hours after welding.