Material Basis: UNS N10276 in Piping Service
Hastelloy C-276 is a nickel-molybdenum-chromium alloy with tungsten, classified as UNS N10276. ASTM B575 governs plate, sheet and strip; ASTM B622 governs seamless pipe and tube; ASTM B619 and ASTM B626 cover welded pipe and welded tube; ASTM B574 covers rod and bar; and ASTM B366 covers wrought fittings. Typical specified chemistry is molybdenum 15.0-17.0 %, chromium 14.5-16.5 %, iron 4.0-7.0 %, tungsten 3.0-4.5 %, cobalt 2.5 % maximum, carbon 0.010 % maximum and silicon 0.08 % maximum. The low carbon and silicon levels matter for welding, because they limit the precipitation that would otherwise reduce corrosion resistance in the heat-affected zone. Dimensional practice follows ASME B36.19M for pipe, ASME B16.5 and B16.47 for flanges and ASME B16.9 for fittings.
| Item | Specified value (UNS N10276) |
|---|---|
| Molybdenum | 15.0-17.0 % |
| Chromium | 14.5-16.5 % |
| Tungsten | 3.0-4.5 % |
| Iron | 4.0-7.0 % |
| Carbon | 0.010 % max |
| Silicon | 0.08 % max |
Fabrication Practices That Protect Corrosion Resistance
Cold and hot forming are both practical, but the alloy work-hardens quickly, so heavy reductions are split with intermediate anneals. All hot working and annealing operations must be followed by full descaling, because residual oxide holds contaminants that later initiate pitting. Carbon steel tooling, carbon steel wire brushes and grinding wheels previously used on stainless steel should not be used on C-276; embedded iron particles become corrosion sites. Marking paints and crayons must be sulphur-free and chloride-free, and any heat-tint or marking must be removed by grinding or by acid pickling in a nitric-hydrofluoric solution. Tube ends and pipe spools should be kept in clean, dry conditions with sulphur-free end caps.
Thermal cutting is acceptable if the cut edge is subsequently ground back and inspected; plasma and abrasive water-jet cutting produce a lower-contamination edge than oxy-fuel and are generally preferred for thick sections.
Welding Practice and Consumable Control
GTAW is the default process for root passes and for thin-wall pipe, with GMAW and SMAW used for fill and for larger diameter work. The correct consumables are ERNiCrMo-4 bare wire under AWS A5.14 and ENiCrMo-4 covered electrodes under AWS A5.11. Welding procedures must be qualified to ASME Boiler and Pressure Vessel Code Section IX, and the completed pipe system is normally built and examined to ASME B31.3 for process piping. The key variables are heat input, interpass temperature and cleanliness.
The alloy is sensitive to hot cracking, so a low heat input with a stringer or narrow weave technique is preferred, wide weaving is avoided and interpass temperature is typically capped below 150 °C. Preheating is not used. The weld pool must be shielded completely, since nitrogen and oxygen pick-up degrades toughness and corrosion resistance; trailing shields are used on the outside of thick joints and purge gas is required on the inside of pipe roots. Copper, zinc, lead and sulphur contamination causes cracking of nickel-molybdenum weld metal, so copper backing bars and zinc-rich marking are prohibited. Post-weld heat treatment is generally not required; where it is specified for a particular service, a full solution anneal followed by rapid cooling is used rather than a partial stress relief, which could leave harmful precipitates.
Examination, Testing and Cleaning After Welding
Radiographic or ultrasonic examination of butt welds and dye penetrant examination of the root and finish passes are the standard methods. Corrosion testing of weld procedure qualification coupons per ASTM G48 Method A is often required for critical service, as is hardness verification where the piping handles sour fluids under NACE MR0175 / ISO 15156. After all inspection, the system is cleaned with fresh water, dried and, where specified, pickled and passivated. Chloride-bearing cleaning agents and chlorine-based leak test water must be avoided; residual chlorides are the most common cause of pitting failures in completed systems.
Frequently Asked Questions
Q: What filler metal is used to weld UNS N10276 pipe?
A: ERNiCrMo-4 bare wire to AWS A5.14 for GTAW and GMAW, and ENiCrMo-4 covered electrodes to AWS A5.11 for SMAW. These match the base metal closely and give the best as-welded corrosion resistance.
Q: Is preheat required before welding C-276?
A: No. Preheat is not used. The controls that matter are low heat input, stringer bead technique and interpass temperature kept below about 150 °C to avoid hot cracking and precipitation.
Q: Why must the inside of the pipe be purged?
A: Nickel-molybdenum weld metal picks up oxygen and nitrogen readily. Without an inert purge, the root surface oxidises heavily, losing corrosion resistance and toughness. Purge oxygen is normally reduced to very low levels before the root pass is started.
Q: Which standard covers welded pipe and welded tube in this alloy?
A: ASTM B619 covers welded pipe and ASTM B626 covers welded tube. Both specify the filler metal class, weld joint type and nondestructive examination requirements.
Q: How are completed welds examined?
A: Butt welds are typically radiographed or ultrasonically examined, with dye penetrant testing of the root and cap. Where corrosion resistance is critical, weld qualification includes a ASTM G48 Method A corrosion test of the welded coupon.
Q: Can ordinary stainless steel brushes and grinding wheels be used on C-276?
A: No. Tooling previously used on carbon steel or stainless steel can embed iron and contaminants that become pitting sites. Dedicated, clean stainless or nickel-alloy tooling is used, and all heat tint is removed before service.
