Alloy 31
Alloy 31 (UNS N08031) is a high-alloy austenitic nickel-chromium-molybdenum material, also known commercially as Nicrofer 3127 hMo or by its EN designation 1.4562. They are provided in accordance with ASTM B622. It contains roughly 31% nickel, 27% chromium, and 6.5% molybdenum, giving it a PREN above 54. The main reason engineers specify Alloy 31 is its broad acid resistance in both sulphuric and phosphoric environments, combined with strong resistance to chloride stress corrosion cracking. It is widely used in fertiliser plants, phosphoric acid production, flue gas systems, and offshore process equipment where aggressive mixed-acid conditions are present.
1. Chemical Composition
The table below lists the chemical composition of Alloy 31 per ASTM B622 / UNS N08031, along with its proportion.
| Grade | C | Mn | P | S | Si | Cr | Ni | Mo | Cu | N | Fe |
| Alloy 31 | ≤ 0.015 | ≤ 2.00 | ≤ 0.020 | ≤ 0.010 | ≤ 0.30 | 26.00–28.00 | 30.00–32.00 | 6.00–7.00 | 1.00–1.40 | 0.15–0.25 | Balance |
2. Mechanical Properties
Alloy 31 meets the following minimum mechanical property requirements in the annealed condition per ASTM B622.
| Property | Condition | Tensile Strength Min | Yield Strength (0.2% Offset) Min | Elongation Min | Hardness Max | PREN | Service Temperature Range |
| Value | Solution Annealed | 690 MPa (100 ksi) | 310 MPa (45 ksi) | 35% | 100 HRB | > 54 | -196°C to +400°C (cryogenic to moderate high-temp) |
3. Equivalent Grade
Alloy 31 is provided under several designations depending on the region and specification system.
| Grade | UNS | GB | JIS | DIN / EN (W.Nr.) | GOST | Trade Name |
| Alloy 31 | N08031 | -- | -- | 1.4562 | -- | Nicrofer 3127 hMo |
3. Key Technical Advantages
Alloy 31 delivers specific performance characteristics that make it suitable for demanding corrosion environments.
- High PREN and Pitting Resistance: With PREN >54, Alloy 31 resists chloride pitting better than 316L and S32750. Its higher molybdenum and nitrogen make it suitable for seawater-cooling and brine systems where other alloys fail.
- Resistance to Sulphuric and Phosphoric Acid: Alloy 31 resists both oxidising and reducing acids. Copper improves sulphuric acid resistance. In impure wet-process phosphoric acid with chlorides and fluorides, it performs more reliably than 904L and 316L.
- Chloride Stress Corrosion Cracking (SCC) Resistance: With 31% nickel, Alloy 31 resists chloride stress corrosion cracking where 304 and 316 fail. It stays resistant at much higher temperatures and chloride levels, similar to C-276 in practice.
- Cost-Effective Alternative to C-276 and C-22: C-276 is often used where it isn’t needed. In most sulphuric and phosphoric acid duties below 60–70°C, Alloy 31 offers similar corrosion resistance at lower cost, making it a strong alternative when strong reducing acids or HCl aren’t present.
- Weldability and Fabrication: Alloy 31 welds well despite its high alloy content. Very low carbon reduces sensitisation during welding. It uses standard austenitic welding methods with matching filler, usually supplied and installed solution-annealed.
4. Common Manufacturing Standards
Alloy 31 seamless pipe is manufactured and tested to the following standards.
- ASTM B622: Main spec for seamless Ni-alloy pipe in UNS N08031
- ASME SB-622: ASME pressure vessel equivalent of B622
- DIN 17459 / EN 10216-5 : European spec for seamless Ni-alloy tubes
- JIS G3463 : Japanese spec for stainless & heat-resistant boiler tubes
- GB/T 14976 : Chinese spec for seamless stainless steel pipes
- ASTM B829 : General requirements for Ni-alloy seamless pipe
- ASTM E8 / E18 : Mechanical testing — tensile & hardness
5. Primary Applications
Alloy 31 seamless pipe is used across several industrial sectors where acid and chloride resistance are required simultaneously.
- Phosphoric Acid Production (Wet-Process Reactor and Piping Systems): Wet-process phosphoric acid contains chlorides, fluorides, and sulphates alongside the acid itself. Standard grades fail rapidly; Alloy 31 handles the mixed-acid environment without pitting or SCC.
- Sulphuric Acid Plants (Heat Exchangers and Transfer Lines): Used in the absorption and concentration sections where sulphuric acid concentrations and temperatures vary. The copper addition gives it useful resistance where other nickel alloys are less reliable.
- Flue Gas Desulphurisation (FGD): FGD systems handle hot, wet, chloride-containing gas streams. Alloy 31 is used in the piping and spray systems where condensate creates aggressive localised conditions.
- Offshore Oil and Gas (Produced Water Handling and Injection Systems): Produced water from offshore wells contains high chloride levels, CO₂, and H₂S. Alloy 31 provides resistance to pitting and SCC in these conditions, particularly in injection pump piping and manifold systems.
- Fertiliser Industry (Nitric and Mixed-Acid Process Piping): Fertiliser manufacturing involves nitric acid, phosphoric acid, and ammonium compounds at elevated temperatures. Alloy 31 is used for process piping, valves, and heat exchanger tubing in these circuits.