Manufacturing Route for nickel-molybdenum alloy B Welded Pipe
Welded pipe of nickel-molybdenum alloy B is produced from hot-rolled or cold-rolled strip and plate supplied in the solution-annealed condition, per ASTM B333 for the plate, sheet and strip forms. The strip edges are prepared by milling or grinding, then the strip is progressively roll-formed into a tubular shape; large diameters may be U-ed and O-ed in a press. The longitudinal seam is welded by gas tungsten arc welding, usually automated with orbital or longitudinal seam equipment, using matching filler metal and full argon shielding with back purging to protect the root pass. The internal weld bead is leveled or removed where a smooth bore is required, and the external bead may be dressed flush. The pipe is then solution annealed at about 1060°C to 1120°C (1940°F to 2050°F) and rapidly water quenched. This step is mandatory for corrosion service: it dissolves carbides and intermetallic phases precipitated during welding, recrystallizes the weld and heat-affected zones, relieves forming and welding stresses, and restores uniform chemistry, so that the seam becomes metallurgically equivalent to the parent metal.
Why the Weld Seam Is the Critical Zone
In hydrochloric acid, the corrosion resistance of the nickel-molybdenum alloy B family comes primarily from its high molybdenum content, 27 to 32 percent. Any factor that locally depletes molybdenum produces accelerated attack, so the weld seam is the controlling feature of a welded pipe. In the as-welded condition the two grades behave differently: B-2 is prone to heat-affected-zone embrittlement through ordering, while B-3 resists ordering and gives a wider processing window. After proper solution annealing both grades perform excellently. The matching filler metals are ERNiMo-7 for B-2 and ERNiMo-10 for B-3, formulated to match or slightly overmatch the base metal in molybdenum. Verification relies on ASTM G28 Method A corrosion testing of samples that include the weld seam, with acceptance based on comparing the weld-zone corrosion rate with the base metal, plus metallographic examination of the weld cross-section and hardness traverses, which for annealed B-3 typically read 85 to 95 HRB.
Essential Quality Controls
A purchase specification for nickel-molybdenum alloy B welded pipe in acid service should require: 100 percent radiographic or ultrasonic examination of the longitudinal seam, typically per ASTM E94; dimensional inspection to ASTM B619 tolerances; hydrostatic testing of every pipe per ASTM B619; ASTM G28 corrosion testing on weld-inclusive samples; positive material identification by XRF or OES on each pipe end; a hardness traverse across the weld with a maximum of 100 HRB; and dye-penetrant examination of the weld cap and root. The common defects to reject are lack of fusion, porosity, undercut, oxidation of the root, and heat-affected-zone cracking. Because B-3 tolerates the manufacturing window better, it is the preferred grade for welded pipe.
Welded vs Seamless: Cost and Engineering Trade-offs
For large-diameter lines, welded pipe is significantly more economical: for a nominal 8-inch schedule 40 line, welded pipe typically costs 60 to 70 percent of seamless, with lead times of 8 to 12 weeks versus 16 to 20 weeks and overall savings of roughly 30 to 40 percent. Welded pipe is the right choice for large diameters above about 6 inches, long runs, moderate pressures, steady-state non-cyclic service and lines accessible for inspection, provided the seam is 100 percent radiographed and the pipe is solution annealed after welding. Seamless remains the conservative choice for cyclic fatigue or vibration service, pressures near the code limits, critical safety zones and buried or permanently insulated lines where the seam cannot be inspected.
Code Compliance and Field Welding
Under ASME B31.3, welded nickel-molybdenum alloy B pipe supplied to ASTM B619 with general requirements per ASTM B775 is an accepted product, and filler metals must conform to AWS A5.14. The longitudinal joint quality factor E is 1.00 when the seam receives 100 percent radiography, which allows the full allowable stress, and 0.85 for spot radiography; this directly affects wall thickness through the B31.3 pressure design formula. Hazard classification drives examination: Category M toxic service and severe cyclic conditions require 100 percent radiography, while Category D allows visual examination. For field welding, GTAW with DCEN, argon shielding at 15 to 20 CFH, argon back purging until the root is at least 1/8 inch thick, heat input below about 15 kJ/in, interpass temperature below 100°C (212°F), stringer beads and dedicated stainless-steel tools are the standard requirements. B-3 is far more forgiving in the field than B-2, and for B-3 field welds no post-weld heat treatment is normally required, although pickling of heat tint may be specified for severe service.
Frequently Asked Questions
Is welded nickel-molybdenum alloy B pipe as corrosion resistant as seamless in HCl? Yes, when the pipe is supplied in the solution-annealed condition after welding. Annealing dissolves weld precipitates and recrystallizes the weld zone, making the seam metallurgically equivalent to the parent metal.
Why is nickel-molybdenum alloy B-3 preferred over B-2 for welded pipe? B-2 is prone to heat-affected-zone embrittlement through ordering when welded, while B-3 resists ordering, gives a wider processing window and is far more forgiving in both shop and field welding.
What filler metal is used for welding nickel-molybdenum alloy B-3? Matching filler ERNiMo-10 per AWS A5.14 is used for B-3, and ERNiMo-7 for B-2, formulated to match or slightly overmatch the base-metal molybdenum content.
How much can be saved by specifying welded pipe instead of seamless? For large diameters, welded pipe typically costs 60 to 70 percent of seamless, with lead times of 8 to 12 weeks instead of 16 to 20 weeks, giving overall savings of roughly 30 to 40 percent.
What tests should be specified for welded pipe in acid service? 100 percent radiography of the longitudinal seam, hydrostatic testing per ASTM B619, ASTM G28 corrosion testing on weld-inclusive samples, positive material identification, a hardness traverse and dye-penetrant examination.
Can nickel-molybdenum alloy B welded pipe be joined in the field? Yes, with GTAW, argon back purging, heat input below about 15 kJ/in, interpass temperature below 100°C and strict cleanliness. B-3 field welds normally need no post-weld heat treatment.
