Side-by-side comparison of Incoloy 925 (UNS N09925) and Inconel 718 (UNS N07718). Both are Special Metals age-hardenable nickel alloys, both qualified for sour service under NACE MR0175 / ISO 15156-3, both used for high-strength oilfield bolting. The difference is strengthening mechanism (Incoloy 925 uses gamma-prime; Inconel 718 uses gamma double-prime), iron content (925 has 22% min Fe; 718 has 17% Fe), and the resulting strength envelope (925: 110 ksi YS / 165 ksi UTS minimum; 718: 150 ksi YS / 180 ksi UTS minimum). Inconel 718 wins on raw strength but costs roughly 2.5–3× per kg; Incoloy 925 wins on cost-per-strength when the application fits within its envelope. This page covers the metallurgy, the sour-service envelopes, the temperature limits, and the selection logic.
Need to compare a specific application? Email info@torqbolt.com with the bolt size, design load, service H₂S / temperature, and we'll size both alloys with full MTC. WhatsApp +91-22-66157017.
Chemistry Side-by-Side
| Element | Incoloy 925 (UNS N09925) | Inconel 718 (UNS N07718) |
|---|---|---|
| Ni | 42–46% | 50–55% |
| Cr | 19.5–22.5% | 17–21% |
| Fe | 22% min (bal) | 17% (bal) |
| Mo | 2.5–3.5% | 2.8–3.3% |
| Cu | 1.5–3.0% | 0.30 max |
| Nb | 0.5 max (residual) | 4.75–5.50% (primary) |
| Ti | 1.9–2.4% | 0.65–1.15% |
| Al | 0.1–0.5% | 0.20–0.80% |
| C | 0.03 max | 0.08 max |
The signature difference is niobium: Inconel 718 has 4.75–5.50% Nb (the primary strengthener), while Incoloy 925 has only residual Nb. The other signature is copper: Incoloy 925 has 1.5–3.0% Cu (giving extended reducing-acid resistance), while Inconel 718 has essentially none.
Strengthening Mechanism, The Real Difference
| Property | Incoloy 925 | Inconel 718 |
|---|---|---|
| Precipitation phase | gamma-prime, Ni₃(Al, Ti) | gamma double-prime, Ni₃Nb |
| Lattice / structure | Coherent FCC ordered L1₂ | Coherent BCT ordered DO₂₂ |
| Particle size | 5–20 nm | 20–30 nm |
| Volume fraction | ~10–15% | ~15–20% |
| Age temperature | 1365°F (740°C) × 8h + 1150°F × 8h | 1325°F (718°C) × 8h + 1150°F × 8h |
| Stable up to | ~1100°F (593°C) | ~1200°F (650°C) for short term, over-ages above |
| Long-term over-age temp | ~750°F (400°C) | ~1100°F (593°C), gamma double-prime transforms to delta phase |
Gamma double-prime (718) gives more strength per unit volume but is meta-stable: above ~1200°F it transforms into the brittle delta phase Ni₃Nb, embrittling the alloy. Gamma-prime (925) is thermodynamically stable and tolerates longer service exposure at moderate temperature without embrittlement. For deep, hot wells (above 1200°F bottom-hole), 925 is more reliable than 718 even though 718 starts stronger.
Mechanical Properties, Aged Condition
| Property | Incoloy 925 (ASTM B805 min) | Inconel 718 (AMS 5662 min) |
|---|---|---|
| Tensile Strength | 165 ksi (1140 MPa) | 180 ksi (1241 MPa) |
| 0.2% Yield Strength | 110 ksi (760 MPa) | 150 ksi (1034 MPa) |
| Elongation | 18% min | 12% min |
| Reduction of Area | 25% min | 15% min |
| Hardness max (sour service) | 35 HRC (NACE Table A.10) | 40 HRC (NACE Table A.13) |
| Charpy at -50°C, J min | 40 J typical | 35 J typical |
Inconel 718 has a strength advantage of ~36% YS and ~9% UTS over Incoloy 925, but pays for it in ductility (12% vs 18% elongation, 15% vs 25% RA). For fatigue-loaded applications the higher ductility of 925 is sometimes preferable despite the strength shortfall. See mechanical properties page for full Charpy / elevated-temperature data.
Sour-Service Envelope
| Sour-Service Limit | Incoloy 925 (Table A.10) | Inconel 718 (Table A.13) |
|---|---|---|
| H₂S Partial Pressure max | 1000 psi (6.9 MPa), effectively unrestricted | 15 psi (0.1 MPa), restricted |
| Temperature max | 400°F (204°C) | 350°F (177°C) |
| Hardness cap | 35 HRC | 40 HRC |
| Cl⁻ Concentration | Saturation | Saturation |
This is the most-cited difference between the two alloys. Inconel 718 is restricted to 15 psi maximum H₂S in NACE MR0175, while Incoloy 925 is qualified to 1000 psi. For high-H₂S sour wells (deep gas, sour-service production tubing), Incoloy 925 is qualified where 718 is not. Inconel 725 (UNS N07725) is a related Special Metals alloy that combines 718-class strength with higher H₂S tolerance, the alternative for ultra-high-strength sour service.
Cost Comparison
Raw-material cost is dominated by nickel content and minor-element cost (niobium is expensive). Approximate relative pricing for finished bolting:
- Incoloy 925 (UNS N09925): 1.0× baseline (e.g., $35–50 per kg for stud-bolt material)
- Inconel 725 (UNS N07725): ~1.6×
- Inconel 718 (UNS N07718): ~2.5–3.0×, reflects higher Ni + 5% Nb content + lower production volume
- Monel K-500 (UNS N05500): ~1.8×, reflects ~63% Ni content
- Incoloy 825 (UNS N08825): ~0.65×, lower-strength solid-solution alternative
For sour-service wellhead bolting, the cost ratio means a 4" diameter stud can cost $400 in Incoloy 925 vs $1100–1200 in Inconel 718. On a 16-stud wellhead this is a $11,000–13,000 budget difference per assembly, significant when 925 meets the design strength requirement.
When to Pick Incoloy 925 vs Inconel 718
| Scenario | Recommended Alloy | Reason |
|---|---|---|
| Sour wellhead / Christmas tree bolting, design YS ≤110 ksi | Incoloy 925 | Meets envelope at lower cost; qualified to 1000 psi H₂S |
| Ultra-high-pressure deep well, design YS >110 ksi | Inconel 725 (preferred) or Inconel 718 (if H₂S <15 psi) | 925 cannot meet strength; 718 has H₂S restriction |
| Aerospace turbine disk / ring | Inconel 718 | Industry standard; long heritage; fatigue + creep proven |
| Subsea Christmas tree, BOP bolting (sour, high strength) | Inconel 725 | Combines 718-class strength with 925-class sour resistance |
| Long-term service above 750°F | Incoloy 925 or specialty alloy | 718 over-ages; 925 stable to 1100°F |
| MWD / LWD downhole tool housing (low magnetic permeability) | Either, 925 (μr ~1.005) or 718 (μr ~1.001) | Both essentially non-magnetic; pick on strength + cost |
Frequently Asked Questions
Is Inconel 718 always the right choice for high-strength bolting? No. The 15 psi H₂S restriction in NACE MR0175 Table A.13 disqualifies Inconel 718 from many sour-service applications. For wellhead bolting where the H₂S partial pressure exceeds 15 psi (i.e., most sour gas wells), 718 is NOT qualified and Incoloy 925 or Inconel 725 must be used.
Why does Inconel 718 cost 3× Incoloy 925? Three drivers: (1) higher nickel content (50–55% vs 42–46%) at $9–12/kg; (2) ~5% niobium content at $35–45/kg; (3) lower production volume because aerospace consumes most 718 capacity, reducing oilfield-grade availability and pricing. The lead-time for Incoloy 925 is also typically shorter.
Can Incoloy 925 and Inconel 718 be welded together? Yes, with INCONEL Filler Metal 625 for solid-solution joints or INCO-WELD 725NDUR for matching-strength welds. The dissimilar joint should be solution-annealed + aged after welding to recover full strength on both sides. Heat-input control is critical to avoid HAZ over-ageing on the 718 side. See the welding page for filler-selection logic.
Which alloy has better Charpy impact at -101°C (cryogenic / API 6A PSL-3G)? Both are qualified, but Incoloy 925 typically delivers 30–50 J at -101°C vs 25–40 J for Inconel 718. The higher elongation of 925 in the aged condition translates to higher impact toughness at low temperatures, another reason 925 is favoured for cryogenic-rated PSL-3G wellhead bolting.
Compare Incoloy 925 to Other Alloys
- Inconel vs Incoloy, What's the difference?
- Incoloy 925 vs Incoloy 825, age-hardenable vs solid solution
- Incoloy 925 vs Incoloy 926, oilfield CRA vs super-austenitic
- Incoloy 925 vs Monel K-500, nickel-Fe-Cr vs nickel-Cu
- Incoloy 925 vs Duplex 2205, nickel CRA vs duplex stainless
Request a Quote
For Incoloy 925 with full MTC, or for Inconel 718 / 725 alternatives in the same enquiry:
- Email: info@torqbolt.com
- WhatsApp: +91-22-66157017
- Datasheet PDF + sample MTC available on request
Specify product type (stud bolts, nuts, hex bolts, pipe, plate), size range, quantity, applicable standards (API 6A, NACE MR0175, ISO 15156, ASTM B805 / B637), required condition, and service H₂S partial pressure / design temperature for alloy-fit verification.
References: ASTM B805 (UNS N09925); ASTM B637 / AMS 5662 (UNS N07718). Special Metals Corporation: INCOLOY® alloy 925 + INCONEL® alloy 718 Technical Bulletins. NACE MR0175 / ISO 15156-3 Tables A.10 and A.13. INCOLOY® and INCONEL® are registered trademarks of Special Metals Corporation.
