ASTM B514 and nickel-iron-chromium alloy UNS N08810 Welded Pipe
ASTM B514 is the standard specification for welded nickel-iron-chromium alloy pipe, covering UNS N08800 (nickel-iron-chromium alloy UNS N08800), N08810 (nickel-iron-chromium alloy UNS N08810), and N08811 (nickel-iron-chromium alloy UNS N08811). For nickel-iron-chromium alloy UNS N08810, the specification defines longitudinally welded pipe produced from cold-rolled strip or sheet, formed into a cylinder and joined by gas tungsten arc welding, gas metal arc welding, or plasma arc welding, either autogenously or with matching filler. The typical size range is 1/2 inch NPS to 24 inch NPS (12.7 mm to 610 mm nominal diameter) with wall thicknesses from Schedule 5S to Schedule 80S. After welding, the pipe must be solution annealed at 1150 to 1200 °C followed by rapid cooling, which dissolves the carbides in the heat-affected zone and develops the coarse grain structure, ASTM No. 5 minimum, that gives 800H its creep strength. Pickling is then commonly specified to restore a clean, corrosion-resistant surface.
Why Pickling Matters for 800H Pipe
Pickling is a chemical surface treatment using an acid solution, typically a nitric-hydrofluoric acid mixture, that removes the high-temperature oxide scale formed during solution annealing, surface contaminants such as embedded iron and grease, and weld discoloration. For nickel-iron-chromium alloy UNS N08810, the scale left by annealing has three layers: a loose, non-protective iron-rich outer layer, a brittle chromium-rich oxide middle layer, and a chromium-depleted alloy beneath, where the chromium content can fall from 20 percent to below 5 percent. If this scale is left in place, the depleted layer cannot form a protective Cr2O3 film in service, so carbon from the furnace atmosphere diffuses inward and forms internal chromium carbides, accelerating carburization and embrittlement. The brittle scale also spalls during thermal cycling, and each spall event removes metal, while embedded iron particles cause localized hot-spot attack. Pickling removes the depleted layer, restores the full 20 percent chromium at the surface, eliminates iron contamination, produces a uniform matte finish that allows visual inspection, and restores weld corrosion resistance by removing heat tint. The governing practices are ASTM A380 for cleaning, descaling, and passivation and ASTM B614 for nickel alloy surfaces.
Welded vs. Seamless Pipe Pickling Differences
Seamless nickel-iron-chromium alloy UNS N08810 pipe is covered by ASTM B407 and arrives at the pickle bath with a moderate, fairly uniform scale from hot extrusion, cold drawing, and annealing. Welded pipe to ASTM B514 carries additional challenges: the weld area has thicker scale and a blue-purple heat tint zone that can extend 5 to 10 mm on either side of the seam, the internal weld bead creates a crevice where acid can be trapped and cause localized over-pickling, and strip edges can carry burrs and rolling lubricants that produce pickle stains. Typical bath parameters for both products are 15 to 20 percent nitric acid with 3 to 5 percent hydrofluoric acid at 50 to 60 °C, but welded pipe needs longer immersion, about 15 to 45 minutes, to remove heat tint, and it requires a more thorough, high-pressure multi-stage rinse. For critical service the internal weld bead is ground flush before pickling, and the weld reinforcement is often ground smooth so that acid does not collect in undercuts. Intergranular attack in the HAZ is a risk if the solution anneal was inadequate, so proper annealing before pickling is essential.
Common Defects in Pickled Pipe
Pickled welded pipe can exhibit four categories of defects. Incomplete pickling leaves residual scale, visible as dark gray or black patches, blue-purple heat tint at the weld, or white pickle stain from dried acid salts; it is caused by insufficient time, a cold bath, or scale that is too thick, and it is rejected unless re-pickling or local abrasive cleaning restores the surface. Over-pickling causes general wall thinning, intergranular attack visible as crocodile-skin texture or grain dropout, and pitting where acid was trapped in crevices; wall must stay within tolerance, and any IGA or pits deeper than about 0.1 mm are cause for rejection. Weld-related defects are exposed by pickling, including undercut at the weld toe, incomplete penetration, porosity, and uneven weld reinforcement; acceptance follows ASME B31.3, with undercut limited to 0.4 mm depth and porosity limited in size and spacing, while cracks, incomplete fusion, and incomplete penetration are rejectable. Surface contamination after pickling includes rust spots from carbon steel tools or racks, oil and grease from cutting operations, and embedded iron from grinding with carbon steel wheels; rust spots and positive ferroxyl results are not allowed, and oil contamination is detected by the water break test.
Inspection, Testing, and Acceptance
Base specification testing per ASTM B514 includes tension testing to ASTM E8 with minimums near 515 MPa ultimate and 205 MPa yield for 800H, flattening and reverse flattening tests on each pipe, hydrostatic testing, and 100 percent radiography of the weld. Petrochemical service commonly adds surface examinations: 100 percent visual inspection for scale, heat tint, pitting, and iron contamination, the water break test, which passes only when a continuous water film drains uniformly, the ferroxyl test, which must show no blue color indicating free iron, dye penetrant examination of the weld seam to ASTM E165, hardness checks with limits near 90 HRB base metal and 95 HRB weld, grain size verification to ASTM E112 at No. 5 or coarser, and PMI by XRF on pipe ends. Intergranular corrosion testing may be specified to ASTM A262, and the acceptance criterion should be taken from the current revision of the standard for the practice selected. The documentation package should include the mill test certificate, RT and PT reports, a pickling certificate recording bath composition, temperature, time, and test results, a PMI report, and a dimensional report. After inspection, pickled surfaces must be dried thoroughly, protected with a removable rust preventive for storage, stored indoors away from carbon steel, and handled on plastic or stainless steel racks to preserve the passive surface.
Frequently Asked Questions
What is the difference between ASTM B514 and ASTM B407 for nickel-iron-chromium alloy UNS N08810? ASTM B514 covers longitudinally welded pipe made from strip or sheet, while ASTM B407 covers seamless pipe produced by hot extrusion and cold drawing. For the same grade, seamless pipe is preferred for critical high-temperature creep and carburization service, and welded pipe is a cost-effective choice for moderate service below about 600 °C.
Why must nickel-iron-chromium alloy UNS N08810 pipe be pickled after solution annealing? nnealing at 1150 to 1200 °C leaves a scale with a chromium-depleted layer beneath it. Pickling removes that layer and embedded iron, restores the protective chromium oxide-forming surface, allows inspection, and gives the pipe its full design life in high-temperature service.
Can mechanical cleaning replace pickling? No. Wire brushing leaves scale smeared and embeds iron, and grinding removes scale but can smear the depleted layer and leaves a non-uniform surface. Neither removes the chromium-depleted zone or iron contamination reliably, so pickling remains the specified method for critical service.
What does the water break test check? It checks surface cleanliness. A properly pickled and rinsed surface is wetted by a continuous water film that drains uniformly; if water beads up, residual scale, oil, or hydrophobic contamination remains and the surface fails the test.
Is intergranular attack in pickled pipe repairable? No. Intergranular attack indicates that grain boundaries were attacked, usually because the material was not fully solution annealed before pickling. IGA cannot be repaired and is cause for rejection.
What documents should accompany pickled 800H pipe? mill test certificate with chemistry, mechanical properties, heat treatment, and grain size, RT and PT reports, a pickling certificate with bath parameters and water break and ferroxyl test results, a PMI report, and a dimensional report. Specifiers should state the exact surface and testing requirements in the purchase order.
