UNS N08031 Seamless And Welded Pipe

UNS N08031 Seamless and Welded Pipe is the nitrogen-strengthened super-austenitic that chemical plant engineers reach for when the medium is hot sulfuric acid, wet-process phosphoric acid, or chloride-laden oxidizing service that pushes 6Mo grades past their limit.

 

Developed as the bridge between the 6Mo super-austenitics and the nickel-based alloys, it carries 27% chromium, 31% nickel, 6.5% molybdenum and a deliberate copper addition - a combination that delivers a PREN around 55, above 904L and the 6Mo grades, at a cost well below the nickel alloys. 

Where Alloy 31 sits in the selection ladder

The material ladder in chloride and acid service runs roughly 316L - 904L - 6Mo super-austenitics (AL6XN, 254SMO, 926) - Alloy 31 - nickel alloys.

 

Alloy 31 occupies the rung above the 6Mo family: its higher chromium and molybdenum plus the copper addition give it markedly better resistance in hot reducing acids and in oxidizing-chloride mixtures, while its nickel content at 30-32% keeps the structure stable and resists chloride stress-corrosion cracking.

 

In wet-process phosphoric acid and hot sulfuric acid - two services that regularly defeat the 6Mo grades - Alloy 31 is frequently the specification that survives.

 

The chemistry that buys the margin

Element	Composition limit, %
Ni	30.0 - 32.0
Cr	26.0 - 28.0
Mo	6.0 - 7.0
N	0.15 - 0.25
Cu	1.0 - 1.4
Fe	Balance

 

Four rows do the work. The chromium at 26-28% forms a dense passive film and carries the alloy in oxidizing media - it is the reason Alloy 31 outperforms the 6Mo grades in oxidizing-chloride service.

 

The molybdenum at 6-7% defends against reducing acids and chloride pitting. The copper at 1.0-1.4% is the acid specialist: copper improves resistance in sulfuric and phosphoric acid, which is exactly where the alloy earns its living.

 

And the nitrogen at 0.15-0.25% strengthens the austenite and raises pitting resistance, allowing the high chromium and molybdenum without a structural penalty. The result is a pitting resistance equivalent of about 55, with critical pitting temperatures in chloride testing far above the 6Mo grades.

 

Mechanical properties per ASTM B622

Property	Minimum value (ASTM B622)
Tensile strength	650 MPa (94 ksi)
Yield strength, 0.2% offset	280 MPa (41 ksi)
Elongation	40%

 

The nitrogen gives Alloy 31 a useful strength premium over the older austenitic grades while the elongation keeps fabrication ductile. In the annealed condition the alloy is fully austenitic, and it bends, rolls and machines with the shop practice of the high-alloy stainless family.

 

The acid duty table

The selection of Alloy 31 is a medium decision. The table below summarizes the services where the alloy is the standard answer:

 

Medium / duty	Verdict	Why
Hot sulfuric acid (concentration- and temperature-dependent)	Strong	Copper + high Ni/Mo resist the acid across the useful concentration band
Wet-process phosphoric acid (P2O5 with chloride impurities)	Strong	The alloy was developed for this service; resists the chloride-bearing acid
Oxidizing chloride media, hypochlorite, chlorine dioxide	Strong	High chromium carries the oxidizing load; molybdenum stops pitting
Seawater and brine, warm	Good	PREN ~55 with high crevice resistance
Chloride stress-corrosion cracking risk	Good	30-32% Ni keeps the structure SCC-resistant
Strong reducing acids with no oxidizer	Evaluate	Nickel alloys or molybdenum-rich grades may serve better

 

Typical applications

• Sulfuric acid plants - heat exchangers, piping, and acid distributors
• Wet-process phosphoric acid production - reaction, evaporation, and piping systems
• Flue gas desulfurization (FGD) scrubber piping and internals
• Chemical process piping in oxidizing-chloride service
• Heat exchanger tubing where 6Mo grades have a documented pitting history

 

Welding

Alloy 31 is welded with nickel-based over-alloyed filler so that the weld metal meets or exceeds the base corrosion resistance, with procedure qualification per ASME IX. No preheat is required, and the fully austenitic weld pool is managed with controlled heat input and interpass temperature to avoid hot cracking - the standard discipline for high-nickel austenitic alloys. The alloy fabricates with the shop practice of the super-austenitic family.

 

Sizes, standards and delivery forms

Parameter	Range
Standards	ASTM B622 (seamless), B619 (welded), B474 (large-diameter welded)
Outside diameter	6.0 mm to 610 mm (larger by enquiry)
Wall thickness	0.5 mm to 25.4 mm, schedules and custom
Length	Fixed 6 m / 12 m, or cut-to-length
Delivery condition	Solution annealed and pickled

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Ordering checklist

• Grade and standard: "Alloy 31 seamless pipe, UNS N08031, per ASTM B622"
• Size: OD x wall in millimetres, or NPS + schedule
• Service medium on the enquiry - concentration, temperature, chloride level - so the wall and grade are confirmed
• Form: seamless or welded, with examination level if welded
• Certificate: EN 10204 3.1 with heat chemistry (including nitrogen and copper) and tensile results

 

Alloy 31 is the specialist rung of the stainless ladder: when hot sulfuric or phosphoric acid service has a documented history of defeating 6Mo stainless, the specification conversation moves to N08031. It is not the cheapest alloy on the ladder - it is not meant to be - it is the alloy that survives the acid, which is the only cost that matters in process piping.

 

 

 

 

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