Alloy 31 (UNS N08031) - High-PREN Super Austenitic Stainless Steel Supplier of Bars, Tubes & Plates | Shanghai Hangbo Alloy Group
Date: 2026年8月4日 Categories: News Views: 272
By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012
Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360
Quick Answer: What Is Alloy 31?
Alloy 31 (UNS N08031 / W.Nr. 1.4562) is the highest-alloyed commercial super austenitic stainless steel, with 31% nickel, 27% chromium, 6.5% molybdenum, and deliberate nitrogen strengthening, achieving a PREN >=48 that surpasses 254SMO (PREN 43) and approaches nickel-base alloy territory. It is the premier wrought alloy for sulfuric acid service, phosphoric acid evaporators, and FGD scrubbers -- applications where 254SMO's corrosion margin is insufficient but C-276's cost is prohibitive. Shanghai Hangbo Alloy Group supplies Alloy 31 as round bars, seamless tubes, and plates.
Industry Pain Point: Phosphoric acid plant operators have long relied on graphite heat exchangers for concentrated acid service because no stainless steel could survive the Fe³⁺-contaminated 54% P₂O₅ at 95°C. Graphite is brittle, prone to thermal shock cracking, and limits plant throughput to modest heat transfer coefficients. Alloy 31 -- with its 27% Cr, 6.5% Mo, and deliberate copper addition -- survives this exact environment at corrosion rates <0.1 mm/year, enabling all-metallic heat exchangers that increase throughput by 30-50% over graphite with dramatically lower maintenance downtime.
Key Properties at a Glance
| Property | Value |
|---|---|
| UNS Number | N08031 |
| W.Nr. | 1.4562 |
| Alloy Family | Super Austenitic Stainless Steel (Fe-Ni-Cr-Mo-N) |
| Density | 8.00 g/cm3 (0.289 lb/in3) |
| Melting Range | 1330-1380C (2425-2515F) |
| Max Service Temp. | ~450C (~840F) |
| PREN (min) | >= 48 |
| Tensile Strength (RT, Annealed) | >= 650 MPa (>= 94 ksi) |
| Yield Strength 0.2% (RT, Annealed) | >= 275 MPa (>= 40 ksi) |
| Elongation (RT) | >= 35% |
| Key Standards | ASTM B625, B649, B622, DIN EN 1.4562 |
Product Overview
Alloy 31 was developed by VDM Metals (now Outokumpu) to close the performance gap between the 6% Mo super austenitics (254SMO, AL-6XN) and the nickel-base C-family alloys (C-276, C-22). The alloy achieves this through a metallurgical "stack" of corrosion-resistance mechanisms:
- 27% Chromium -- the highest chromium content of any commercially available wrought austenitic stainless or nickel alloy, providing a Cr₂O₃ passive film of exceptional stability and rapid re-passivation kinetics.
- 6.5% Molybdenum + 0.2% Nitrogen -- the Mo+N combination pushes PREN to >=48, substantially above 254SMO (43) and approaching Alloy 59 (PREN ~50).
- 1.2% Copper -- deliberately added for resistance to sulfuric acid in the intermediate concentration range (40-80% H₂SO₄) where the cathodic reaction shifts to hydrogen evolution and copper's noble potential reduces the corrosion rate.
- 31% Nickel -- stabilizes the fully austenitic FCC structure against sigma-phase precipitation despite the ultra-high Cr and Mo content. This is the key reason Alloy 31 can contain 27% Cr without forming embrittling intermetallics during welding or service.
The practical consequence: Alloy 31 resists environments that are simultaneously highly acidic AND highly oxidizing -- the combination that defeats both standard stainless steels (which fail in acid) and standard nickel alloys (which fail in strongly oxidizing media). This "mixed-environment" capability is the alloy's unique market position.
Shanghai Hangbo Alloy Group supplies Alloy 31 as round bars, seamless tubes, and plates, all with full EN 10204 3.1 certification and ready for immediate shipment.
Executive Standards
| Product Form | ASTM | DIN/EN |
|---|---|---|
| Plate, Sheet & Strip | ASTM B625 | DIN EN 1.4562 |
| Bar & Rod | ASTM B649 | EN 10088-3 |
| Seamless Pipe & Tube | ASTM B622 / B668 | EN 10216-5 |
| Welded Pipe | ASTM B619 / B626 | ISO 1127 |
| Forgings | ASTM B649 / B564 | EN 10088-3 |
Chemical Composition (wt.% per ASTM B625 - 100% OES Verified)
| Element | Min | Max | Role in Alloy 31 |
|---|---|---|---|
| Nickel (Ni) | 30.0 | 32.0 | Austenite stabilizer; raises sigma solvus to accommodate high Cr+Mo |
| Chromium (Cr) | 26.0 | 28.0 | Highest Cr of any wrought austenitic -- extreme passive film stability |
| Molybdenum (Mo) | 6.0 | 7.0 | Pitting/crevice corrosion resistance; primary PREN contributor |
| Copper (Cu) | 1.0 | 1.4 | Sulfuric acid resistance; noble potential shifts corrosion potential to passive region |
| Nitrogen (N) | 0.15 | 0.25 | Solid-solution strengthener; PREN booster (+16 per 1% N); austenite stabilizer |
| Iron (Fe) | Balance | -- | Matrix element |
| Carbon (C) | -- | 0.015 | Ultra-low -- prevents intergranular corrosion from sensitization |
| Manganese (Mn) | -- | 2.0 | Austenite stabilizer; deoxidizer |
| Silicon (Si) | -- | 0.3 | Kept low to avoid sigma-promoting effect |
| Phosphorus (P) | -- | 0.020 | Control for hot workability |
| Sulfur (S) | -- | 0.010 | Controlled for corrosion resistance |
PREN Calculation: PREN = %Cr + 3.3 x %Mo + 16 x %N = 26.0 + 3.3 x 6.0 + 16 x 0.15 = 48.2 minimum. With typical mid-range values (Cr 27.0, Mo 6.5, N 0.20), PREN reaches 51.7 -- placing Alloy 31 in the same class as Alloy 59 (PREN ~50) and significantly above 254SMO (PREN ~43).
Mechanical & Physical Properties
Room-Temperature Mechanical Properties (Solution-Annealed 1150-1180C/WQ)
| Property | Value | Standard |
|---|---|---|
| Tensile Strength (Rm) | >= 650 MPa (>= 94 ksi) | ASTM B625 |
| Yield Strength 0.2% (Rp0.2) | >= 275 MPa (>= 40 ksi) | ASTM B625 |
| Elongation (A5) | >= 35% | ASTM B625 |
| Hardness | <= 100 HRB | -- |
| Young's Modulus (20C) | ~195 GPa | -- |
Physical Properties
| Property | Value | Unit |
|---|---|---|
| Density | 8.00 | g/cm3 |
| Thermal Conductivity (20C) | ~12 | W/m-K |
| Specific Heat | ~460 | J/kg-K |
| Electrical Resistivity | ~0.95 | micro-ohm-m |
| Thermal Expansion (20-200C) | ~15.0 | micro-m/m-C |
| Magnetic Permeability | <= 1.005 (non-magnetic) | mu-r |
Heat Treatment
Solution Annealing
| Parameter | Specification |
|---|---|
| Temperature | 1150-1180C (2100-2155F) |
| Soak Time | 30-60 minutes depending on section thickness |
| Cooling | Water quench -- mandatory. Air cooling not acceptable for sections >=3 mm. |
Metallurgical objective: Dissolve all secondary phases (sigma, chi, Cr₂N) into the austenitic matrix. The solution temperature must be above the sigma solvus (~1050-1100C for this composition) to ensure complete dissolution. The rapid water quench is critical because the high Mo+Cr content makes Alloy 31 kinetically prone to sigma-phase precipitation during cooling through the 700-1000C range.
No precipitation hardening -- Alloy 31 is solid-solution strengthened only.
Post-weld heat treatment: Generally not required for Alloy 31 when welded with ERNiCrMo-13 (Alloy 59) or ERNiCrMo-3 (Alloy 625) over-alloyed filler. If service conditions are extremely aggressive (boiling sulfuric acid, Fe³⁺-contaminated phosphoric acid), post-weld solution annealing at 1150-1180C + water quench is recommended.
Production Process
1. Round Bars & Forgings
- Melting: VIM (Vacuum Induction Melting) for nitrogen control +/-0.02% and carbon <=0.010%. ESR (Electroslag Remelting) for premium bars >=100 mm to ensure centerline soundness and remove oxide inclusions.
- Hot Forging: Billet heated to 1180-1220C, forged with minimum 4:1 reduction. Finishing temperature >=980C to prevent sigma-phase precipitation during deformation.
- Solution Annealing: 1150-1180C, water quench. Hardness check <= 100 HRB confirms complete solutioning.
- UT & Finishing: 100% ultrasonic testing per ASTM A388. PMI on every bar -- Cr (26-28%) and Mo (6-7%) are the rapid grade verification criteria.
2. Seamless Tubes
- Hollow Preparation: Hot-extruded at 1180-1220C.
- Cold Pilgering: Multi-pass cold reduction (30-50% per pass) with intermediate solution anneals between passes.
- Final Anneal: 1150-1180C bright anneal in hydrogen (for tubes <=OD 50 mm) or open-air + water quench + pickle + passivate.
- Testing: 100% hydrostatic or eddy current. ASTM G48 Method A (pitting) and ASTM A262 Practice C (intergranular) on request.
Industry Applications
| Industry | Typical Component | Why Alloy 31? |
|---|---|---|
| Phosphoric Acid | Evaporator tubes, acid heaters, piping | Survives Fe³⁺-contaminated 54% P₂O₅ at 95C -- replaces graphite |
| Sulfuric Acid | Coolers, piping for 40-80% H₂SO₄ at elevated temperatures | Copper alloying provides resistance in the mid-concentration range |
| FGD Scrubbers | Absorber inlet quench zones, outlet ducting | Resists condensing H₂SO₄ + high chlorides at the wet/dry interface |
| Chemical Processing | Acetic acid + halide mixtures, mixed-acid processes | Combined oxidizing + reducing acid resistance |
| Seawater & Brine | Plate heat exchangers, desalination brine heaters | PREN >=48 resists crevice corrosion at elevated temperatures |
Quality Assurance: 7-Stage Zero-Defect Inspection
- Raw Material Purity: Incoming Cr, Ni, Mo, Cu analyzed by OES. Nitrogen-bearing ferrochrome selected for precise N control.
- VIM Melt Chemistry: Cr +/-0.3%, Mo +/-0.15%, Cu +/-0.05%, N +/-0.02%. Carbon <=0.010% internal target.
- Hot Working: IR pyrometry, reduction >=4:1. Finishing temperature >=980C verified.
- Solution Anneal: Digital chart +/-5C. Quench delay <=30 sec. Hardness <=100 HRB verification.
- Destructive Testing: Room-temp tensile per ASTM E8. ASTM G48 Method A pitting test (50C minimum) on lot sample.
- NDE: 100% UT for bars >=50mm; 100% ET/hydro for tubes. PMI on every piece.
- Documentation: EN 10204 3.1 MTC with PREN calculation, G48 results, and full trace element report. SGS/TUV/BV witness available.
Frequently Asked Questions
Q1: What is Alloy 31 used for?
Phosphoric acid evaporators, sulfuric acid coolers, FGD scrubber quench zones, mixed-acid chemical processing, and seawater heat exchangers -- anywhere the combined challenge of strong acid + oxidizing agents exceeds the capability of 254SMO but doesn't justify the cost of C-276.
Q2: What is the density of Alloy 31?
8.00 g/cm3 -- comparable to standard stainless steels, lighter than nickel alloys.
Q3: What is the PREN of Alloy 31?
PREN >=48, with typical values 50-52. This exceeds 254SMO (PREN 43), AL-6XN (PREN 45), and approaches nickel-base alloys like Alloy 59 (PREN ~50).
Q4: How does Alloy 31 compare to 254SMO?
Alloy 31 has higher Cr (27% vs 20%), higher Ni (31% vs 18%), and higher PREN (>=48 vs >=43). The critical advantage is in sulfuric acid: Alloy 31's copper addition (1.2%) enables resistance in the 40-80% H₂SO₄ concentration range where 254SMO is attacked.
Q5: How does Alloy 31 compare to Hastelloy C-276?
Alloy 31 has similar PREN (48 vs 50) but significantly lower molybdenum (6.5% vs 16%). C-276 is superior in pure reducing acids (HCl), while Alloy 31 is superior in oxidizing acids (HNO₃) and mixed oxidizing/reducing environments.
Q6: What is the tensile strength?
>=650 MPa (>=94 ksi) in the solution-annealed condition.
Q7: What is the yield strength?
>=275 MPa (>=40 ksi). Strength derives from solid-solution hardening, primarily from nitrogen (0.15-0.25%) and molybdenum.
Q8: Is Alloy 31 weldable?
Yes, with ERNiCrMo-13 (Alloy 59) or ERNiCrMo-3 (Alloy 625) filler. Over-alloyed filler ensures weld metal PREN >= base metal. Post-weld anneal recommended for most aggressive service.
Q9: What heat treatment does Alloy 31 require?
Solution annealing at 1150-1180C with mandatory water quenching. Critical to avoid sigma phase during cooling.
Q10: What is the price of Alloy 31?
$35-60/kg EXW for bars. Positioned between 254SMO ($20-35) and C-276 ($50-80). Contact sales@hangboalloy.com.
Q11: Does Shanghai Hangbo ship internationally?
Yes -- FOB/CIF/DAP to 40+ countries. Stock 2-4 weeks; mill order 10-14 weeks.
Q12: Is Alloy 31 non-magnetic?
Yes, magnetic permeability <=1.005 (fully austenitic, non-magnetic).
Q13: What product forms?
Round bars (6-300 mm), seamless tubes (OD 6-219 mm), plates (3-50 mm), forgings, welded pipes.
Q14: What is the carbon content and why so low?
C <=0.015% -- ultra-low carbon prevents chromium carbide precipitation (sensitization), ensuring the 27% Cr remains fully in solid solution for corrosion resistance.
Q15: Why copper?
1.0-1.4% Cu provides resistance to sulfuric acid in the mid-concentration range by raising the alloy's electrochemical potential into the passive region where the hydrogen evolution cathodic reaction would otherwise drive active corrosion.
Contact Shanghai Hangbo Alloy Group
| Channel | Details |
|---|---|
| Company | Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier |
| Website | www.nickel-alloy.com |
| Email (Sales) | sales@hangboalloy.com |
| Email (Technical) | hangbo@nickel-alloy.com |
| WhatsApp (Lisa) | +86 13611656360 |
| Address | Room 1508, No. 288 Shiyi Road, Baoshan District, Shanghai 200940, China |
Shanghai Hangbo Alloy Group - Complete Product Range
| Family | Available Grades |
|---|---|
| Pure Nickel | NI200 (N02200), NI201 (N02201), N4, N5, N6 |
| Monel (Ni-Cu) | Monel 400, Monel 401, Monel 404, Monel K-500, Monel R-405 |
| Inconel (Ni-Cr) | Inconel 600, 601, 617, 625, 690, 718, X-750, 725 |
| Incoloy (Fe-Ni-Cr) | Incoloy 800, 800H, 800HT, 825, 925, 926, A-286, 25-6HN |
| Hastelloy (Ni-Mo-Cr) | Hastelloy C-276, C-22, C-2000, C-4, G-30, G-35, X, B-2, B-3, N |
| Nimonic (Ni-Cr-Co) | Nimonic 75, 80A, 90, 95, 105, 115, 263 |
| Haynes (High-Temp) | Haynes 25, 188, 230, 242, 282, 556 |
| Precision Alloys | 1J22 (Hiperco 50), 1J46, 1J50, 1J79, 1J85, 3J21, 4J32, 4J36 (Invar), 4J45 |
| High-Strength Alloys | 18Ni250 (Maraging 250), 18Ni300 (Maraging 300), AerMet 100, PH13-8Mo |
| Specialty Alloys | Alloy 20, Alloy 31, Alloy 59, 254SMO, RA330, Inconel 686, 602CA, 713C, K403 |
Product Forms
| Product Form | Typical Size Range | Applicable Standards |
|---|---|---|
| Round Bars & Rods | Diameter 6 - 500 mm | ASTM / ASME / AMS / GB |
| Seamless Tubes & Pipes | OD 6 - 610 mm, WT 0.5 - 30 mm | ASTM / ASME / EN 10216-5 |
| Plates & Sheets | Thickness 0.1 - 100 mm | ASTM / ASME / EN 10095 |
| Forgings | Discs, Rings, Blocks, Tube Sheets | ASTM B564 / A182 / AMS |
| Welded Pipes | OD up to 1219 mm | ASTM B619 / B622 / B626 |
| Fittings & Flanges | Elbows, Tees, Reducers, Weld Neck, Slip-On | ASTM B366 / A403 / A182 |
| Wire & Welding Wire | Diameter 0.05 - 10 mm, spooled or coiled | AWS A5.14 / AMS |
| Strip & Coil | Thickness 0.02 - 4.0 mm, slit to width | ASTM / GB/T 14985 |
Third-Party Inspection: SGS, Bureau Veritas, TUV Rheinland, DNV, Lloyd's Register available on request.
Contact: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com
