Project Description
Alloy 31 (UNS N08031, W.Nr. 1.4562), commercially known as Nicrofer 3127 hMo, is a nitrogen-strengthened iron-nickel-chromium-molybdenum superaustenitic stainless steel with a deliberate copper addition. Developed by VDM Metals to bridge the performance gap between high-alloy 6Mo stainless steels and expensive pure nickel alloys, it delivers exceptional resistance to sulfuric acid, phosphoric acid, chloride pitting, and stress corrosion cracking in highly aggressive mixed-acid environments. With a PREN ≥ 48 and over 31% nickel content, Alloy 31 is widely adopted in the petroleum industry as a CRA pipe material for sour gas production, deepwater subsea systems, produced water reinjection, and refinery acid gas removal where H₂S, CO₂, and chloride concentrations exceeding 100,000 ppm coexist at elevated temperatures. It is fully compliant with NACE MR0175 / ISO 15156 for sour service and certified for pressure vessel use from −196 °C to +550 °C. SHUNFU METAL sources prime Alloy 31 billets exclusively from ISO 9001-certified mills, manufacturing seamless pipe via vacuum induction melting (VIM) followed by electroslag remelting (ESR) to minimize sulfur, phosphorus, and non-metallic inclusion content. Every heat lot undergoes optical emission spectrometry verification, and only heats meeting the ASTM B622 composition window are released for hot extrusion or cold pilgering. SHUNFU METAL performs in-line ultrasonic testing per ASTM E213, eddy current inspection per ASTM E426, hydrostatic pressure testing per ASTM A999, and positive material identification on every single pipe before dispatch, guaranteeing zero-defect delivery to clients worldwide.
Oil & Gas Applications of Alloy 31
Sour gas wells with H₂S partial pressures exceeding NACE MR0175 Region 3 thresholds; resists sulfide stress cracking at depths > 5,000 m.
Deepwater tiebacks carrying multiphase fluids with chlorides > 100,000 ppm; resists under-deposit pitting at operating pressures to 15,000 psi.
High-pressure injection piping and manifold systems handling brine with dissolved CO₂ and H₂S at 40–80 °C.
Valve bodies, stems, and trim in HPHT wellhead assemblies requiring NACE MR0175 / ISO 15156 Level VII compliance.
Strain-hardened small-diameter tubes used in downhole intervention operations in H₂S-laden wellbores; high tensile with low diametral deformation.
Crude distillation overhead condensers and acid gas removal unit exchangers exposed to condensing sulfuric and naphthenic acids.
Amine unit piping and lean/rich solvent lines in AGR systems handling wet acid gas with entrained chlorides at 60–100 °C.
Offshore platform once-through cooling piping and plate-frame exchanger plates resistant to chloride crevice corrosion under biofouling deposits.
Corrosion inhibitor, scale inhibitor, and methanol injection tubing on subsea trees, manifold headers, and topside chemical dosing skids.
Chemical Composition of N08031 (ASTM B622)
The defining metallurgical feature of Alloy 31 is its intentional nitrogen addition at 0.15–0.25 wt%. Dissolved nitrogen occupies interstitial octahedral sites in the face-centered cubic austenite lattice, producing significant solid-solution strengthening that raises yield strength by 30–50 MPa over non-nitrogen-bearing superaustenitics. Simultaneously, nitrogen synergizes with chromium and molybdenum to elevate the PREN, shifting the critical pitting temperature substantially higher. The copper addition at 1.0–1.4% is equally critical: it stabilizes the passive film in reducing sulfuric acid environments where molybdenum alone provides insufficient protection. SHUNFU METAL verifies nitrogen content by inert gas fusion analysis and confirms full chemistry by optical emission spectrometry on every heat prior to release.
| C | Si | Mn | P | S | Cr | Ni | Mo | Cu | N | Fe |
|---|---|---|---|---|---|---|---|---|---|---|
| ≤ 0.015 | ≤ 0.30 | ≤ 2.00 | ≤ 0.020 | ≤ 0.010 | 26.0–28.0 | 30.0–32.0 | 6.0–7.0 | 1.0–1.4 | 0.15–0.25 | Bal. |
PREN = %Cr + 3.3×%Mo + 16×%N ≥ 48. Ultralow carbon (≤ 0.015%) eliminates chromium-carbide precipitation at grain boundaries during welding, preserving intergranular corrosion resistance without post-weld solution annealing. Copper (1.0–1.4%) extends passive film stability into the reducing-acid regime where other superaustenitics fail.
Mechanical Properties — Alloy 31 (Solution Annealed)
Alloy 31 is supplied exclusively in the solution-annealed condition at minimum 1,150 °C followed by rapid water quenching to room temperature. This thermal cycle fully recrystallizes the austenitic grain structure, dissolves all secondary and intermetallic phases, and homogenizes the alloying element distribution for maximum ductility, toughness, and uniform corrosion resistance. SHUNFU METAL batch-anneals pipe in computer-controlled gas-fired furnaces with multi-point thermocouple recording and redundant overtemperature protection. Post-anneal grain size is verified by optical microscopy per ASTM E112; typical results fall within ASTM grain size 4 to 6, providing an optimal balance of strength and ductility.
| Property | Metric (min.) | Imperial (min.) | Typical Value |
|---|---|---|---|
| Tensile Strength, Rm | 650 MPa | 94 ksi | 700 MPa |
| Yield Strength, Rp0.2 | 276 MPa | 40 ksi | 320 MPa |
| Elongation (2″ or 4D) | 40% | 40% | 45% |
| Hardness | ≤ 230 HB | ≤ 100 HRB | 190 HB |
| Impact, Charpy-V (−196 °C) | ≥ 140 J | ≥ 103 ft·lbf | 160 J |
| Density | 8.05 g/cm³ | 0.291 lb/in³ | — |
| Service Temperature | −196 °C to +550 °C (ASME pressure vessel certified) | ||
Properties per ASTM B622 / ASME SB-622 in solution-annealed + water-quenched condition. SHUNFU METAL certifies each pipe lot with a comprehensive mill test report covering tensile testing per ASTM E8, hardness per ASTM E18, flattening and flaring per ASTM B829, and intergranular corrosion susceptibility per ASTM A262 Practice C. Charpy impact testing at cryogenic temperature is available upon request.
