UNS N08031 (Alloy 31) Forging Parts | China Jiangyin Manufacturer

Oil and Gas — Sour Service Wellheads and Valve Components

N08031 qualifies under NACE MR0175/ISO 15156 for H₂S and high-chloride environments. In the solution-annealed condition, hardness falls at 160 – 200 HBW — below the 220 HBW limit required for sour service. The high Cr-Mo-N passive film resists sulfide stress cracking (SSC) at cathodic polarization conditions. We supply forged valve bodies, bonnets, stems, seats and spool bodies to API 6A and API 6D dimensional requirements. In 2020, we delivered 200+ wellhead forging sets to a UAE sour gas field project; the components have been in service for four years without material-related shutdowns.

Selection note: NACE MR0175 sour service requires both low hardness and adequate pitting resistance. N08031 in the solution-annealed condition meets both criteria without requiring cold work or precipitation hardening that would compromise one or the other.

Chemical Processing — Sulfuric and Phosphoric Acid Service

In H₂SO₄ and H₃PO₄ service, corrosion proceeds primarily by anodic dissolution. The 1.0 – 1.4% Cu in N08031 shifts the corrosion potential into the passive region in both acid types simultaneously; high-Mo grades without copper don't reliably achieve this above approximately 50% H₂SO₄ concentration. We manufacture forged tube sheets, reactor nozzles and pressure vessel shells for chemical plants, refineries and ore digestion facilities. In 2021, we supplied forged tube sheets and reactor nozzles to a German chemical plant's sulfuric acid ore digestion system, replacing 316L components that had corroded through in under 12 months. Ultrasonic inspection at 36 months showed no measurable wall loss on the N08031 parts.

Nuclear Power — Reactor Coolant Pump Components

Nuclear service requires full material traceability from ingot heat to finished part, maintained for the plant lifetime. We assign a heat code at ingot receipt, carry it through every forging, heat treatment and machining record, and deliver it on the EN 10204 3.2 MTC. In 2022, we supplied pump impellers and casings to a nuclear power plant project in Pakistan, qualified to ASME BPVC Section III and witnessed by TÜV Rheinland. The full documentation set ran from ingot certificate to final dimensional report in a single traceable package.

Power Generation — Flue Gas Desulfurization (FGD)

FGD scrubbing slurry contains 15 – 30% gypsum solids in a low-pH aqueous phase with chloride levels up to 100,000 ppm. Erosion, crevice corrosion and cycling between oxidizing and reducing conditions degrade most stainless steels within two to five years. N08031's PREN ≥ 48 provides crevice corrosion resistance at chloride levels where 254SMO (PREN ∼43) shows localized attack. We supply forged spray nozzle bodies, absorber headers and agitator shaft components for FGD systems at thermal power plants in Europe and Southeast Asia.

Marine and Offshore — Seawater Service

N08031 has a critical pitting temperature of approximately 45 °C in 6% FeCl₃ solution, giving reliable performance in seawater at bulk temperatures up to 35 – 40 °C. We supply forged tube sheet blanks, channel flanges, piping flanges and riser connectors for offshore platforms and desalination plants, with ASME B31.3 and NORSOK M-630 documentation available on request.

Valve and Pump Manufacturing

Body-to-bonnet and body-to-seat interfaces in valves create crevice geometries that concentrate corrosive species. Machining from a forged billet rather than a casting removes the porosity and shrinkage voids that initiate preferential attack at these crevices. We supply complete forging sets — body, bonnet, ball or gate, stem, seats — from a single ingot heat where feasible to keep hardness and composition matched across mating surfaces. Forged impellers and casings for pump service are dynamically balanced to G2.5 on our in-house equipment before dispatch.

Heat Treatment

All N08031 forgings are solution annealed at 1,149 – 1,177 °C (2,100 – 2,150 °F) and water quenched. Water quenching is required because at cooling rates below approximately 15 °C/s through the 900 – 650 °C range, sigma phase begins to precipitate at grain boundaries. Water quenching achieves 50 – 80 °C/s at the surface of typical cross-sections. For wall thickness above 150 mm, we record cooling rate via thermocouple in a matched test coupon processed alongside the production part. Soaking time is 1 hour per 25 mm of maximum cross-section, minimum 2 hours. The heat treatment chart and soak-time calculation ship with every EN 10204 3.1/3.2 certificate.

Room-Temperature Mechanical Properties

Typical Rp0.2 runs 310 – 370 MPa rather than at the 276 MPa specification minimum. Many pressure vessel and sour service purchase orders include a mill over-spec requirement; achieving it consistently requires holding the solution anneal above 1,149 °C to fully relieve residual cold work from the final forging reduction.

Elevated-Temperature Tensile Properties

N08031 is approved for pressure-retaining service to approximately 500 °C under most pressure vessel codes. Values below are representative of solution-annealed specimens; forging cross-section position effects are typically ±5% on Rp0.2 and ±3% on Rm. Test reports at specific temperatures are available on request at order stage.

Physical Properties

Values are for solution-annealed material. Thermal expansion coefficients are relevant for mixed-alloy tube-and-shell heat exchanger designs where differential expansion at operating temperature must be accounted for.

Filler Metal Selection for Welding UNS N08031

Use an overalloyed filler, not a matching one. GTA welds dilute approximately 20 – 30% with base metal. A matching N08031-composition filler will produce a deposit with lower Cr, Mo and N than the parent material, reducing the deposit PREN by 3 – 5 points. In chloride or acid service, this creates a preferential corrosion site at the weld. The recommended filler is ERNiCrMo-13 (Alloy 59, UNS N06059), which deposits 22 – 24% Cr and 15 – 16% Mo. After dilution, deposit PREN still exceeds that of the N08031 base metal.

Filler Metal Options

Key Process Parameters

• Interpass temperature: ≤ 150 °C, measured. Use a contact thermometer or IR gun on the adjacent weld bead. Excess heat accelerates nitrogen loss from the weld pool and risks sigma-phase precipitation in previous-pass HAZ.
• Heat input: 0.5 – 1.5 kJ/mm for GTAW root passes. Lower heat input reduces HAZ width and limits Cr depletion adjacent to any carbides.
• Shielding gas: Ar or Ar + 2 – 5% N₂. Nitrogen addition compensates for weld pool nitrogen loss. CO₂-containing mixtures destabilize the austenite and are not suitable.
• Preheat: not required for standard sections. For sections > 75 mm, 50 – 75 °C warm-up reduces thermal shock at arc strike.
• PWHT: Full solution anneal at 1,149 – 1,177 °C + water quench is required for nuclear-qualified welds and recommended for sour service welds on sections above 50 mm wall. Not mandatory for general pressure service if interpass temperature and heat input are controlled.

Machining Parameters

N08031 work-hardens at a similar rate to 254SMO and faster than 316L. Surface hardness can increase from 160 HBW to over 300 HBW in a single pass with a worn tool, causing dimensional error on subsequent cuts. Our in-house CNC department uses:

• Cutting speed: 18 – 35 m/min for turning with ISO K20 coated carbide. This is 30 – 60% of the speed used for 316L and keeps the cutting zone below the threshold for rapid insert cratering.
• Feed rate: 0.15 – 0.35 mm/rev for roughing. Continuous positive feed is more important than speed; dwelling or zero-feed contact hardens the surface for the next pass.
• Depth of cut: ≥ 2 mm for roughing to cut below the work-hardened layer from the previous pass.
• Coolant: flood at full concentration (6 – 8% water-soluble oil). Inadequate coolant is the most common cause of premature insert failure on this grade.
• Tool changes: on condition, not on a fixed interval. Change at first chatter or surface roughness change; a worn insert accelerates work hardening on N08031 faster than on most austenitics.

Our in-house CNC equipment handles turning to 5,000 mm length × 3,000 mm swing. As-machined dimensional reports are standard with all fully machined orders.

Cold Working

Cold reduction increases yield strength approximately 50% at 20% reduction and up to 100% at 40% reduction. Cold working above 30% reduction builds residual stress sufficient to increase susceptibility to SCC in chloride service, even at PREN ≥ 48. N08031's work-hardened microstructure is also less stable than 316L's due to the lower stacking fault energy from higher Mo and N, increasing the likelihood of deformation-induced martensite. Intermediate annealing at 1,149 – 1,177 °C is required after each 30% reduction increment. Parts intended for sour or acid service should receive a final solution anneal regardless of any intermediate anneals during forming.