Introduction: When H₂S Rewrites the Alloy Selection Rulebook
Incoloy 925 and Inconel 718 are both age-hardenable nickel alloys that serve high-strength oil and gas applications. Both carry approximately 20% chromium. Both carry approximately 3% molybdenum. Both age-harden to the 34–44 HRC range. And yet, in sour gas wells where H₂S partial pressure exceeds 3–5 psi, these two alloys cannot be treated as interchangeable.
The difference comes down to two elements: copper and niobium. Incoloy 925 adds 1.5–3.0% Cu — a deliberate metallurgical design feature that provides NACE MR0175 Level VI sour service pre-qualification without supplementary testing. Inconel 718 omits copper but carries 4.75–5.50% Nb to form the γ″ (Ni₃Nb) strengthening phase, producing 23% higher yield strength — at the cost of NACE qualification uncertainty.
The trade-off is real and has measurable financial consequences: a 10,000-ft sour gas completion string specified in 925 saves approximately $2 million and eliminates 6–8 months of SSC qualification testing versus 718. But that saving only materializes if 925’s lower strength (YS 895 vs 1100 MPa) is sufficient. Selecting the wrong alloy costs far more — either in unnecessary material spend or in premature failure.
This article compares the chemistry, metallurgy, mechanical properties, corrosion resistance, fabrication behavior, and total cost of ownership of Incoloy 925 and Inconel 718 — specifically for downhole tubular, wellhead, and fastener applications where H₂S and NACE compliance drive material decisions.
For a broader discussion of Inconel 718’s temperature limits against other aerospace alloys, see our Inconel 718 vs Waspaloy article. For the Incoloy family evolution, see our Incoloy 800 vs 825 comparison.
1. Chemical Composition: Cu vs Nb — The Metallurgical Trade
The two alloys belong to different sub-families within nickel-base systems. 925 is an Incoloy — a Ni-Fe-Cr system with high iron content for cost control (~28% Fe). 718 is an Inconel — a Ni-Cr-Fe system with lower iron and higher nickel, prioritizing high-temperature stability.
| Element | Incoloy 925 (N09925) | Inconel 718 (N07718) | Key Difference |
|---|---|---|---|
| Ni | 42.0–46.0% | 50.0–55.0% | 925: 8–13% less Ni → lower cost |
| Fe | ≥22.0% (balance) | balance (~17–19%) | 925: ~28% Fe (approaching stainless steel levels) |
| Cr | 19.5–22.5% | 17.0–21.0% | Similar range |
| Mo | 2.5–3.5% | 2.8–3.3% | Virtually identical |
| Cu | 1.5–3.0% | ≤0.30% | 925-specific: H₂S-resistant design |
| Nb | ≤0.50% | 4.75–5.50% | 718-specific: γ″ strengthening |
| Ti | 1.9–2.4% | 0.65–1.15% | 925: 2–3× more Ti → γ′ strengthening |
| Al | 0.10–0.50% | 0.20–0.80% | Similar range |
Why Copper Matters: The H₂S Story
The 1.5–3.0% Cu in 925 is not an accidental impurity — it is a deliberate metallurgical design feature for sour gas environments. Copper deposits as a metallic film on the alloy surface during H₂S exposure, forming a catalytic poison layer that inhibits the hydrogen evolution reaction (HER). Since hydrogen sulfide stress cracking (SSC) requires atomic hydrogen to diffuse into the metal lattice, suppressing HER at the surface directly reduces the driving force for cracking.
This mechanism is fundamentally different from the corrosion resistance provided by Cr and Mo (which form passive oxide films). Copper operates on the electrochemical kinetics of the hydrogen reaction, not on oxide stability. This is why even small copper additions (1.5–3.0%) produce disproportionate improvements in NACE MR0175 compliance — a phenomenon well-documented in API and NACE literature for sour service alloys.
Why Niobium Matters: The Strength Story
718’s 4.75–5.50% Nb is the key to its superior strength. During the standard two-step aging treatment (718°C/8 hr + furnace cool to 621°C/8 hr), niobium precipitates as γ″ (gamma double prime) — Ni₃Nb — a body-centered tetragonal (BCT) phase that produces coherency strains roughly 3× larger than the γ′ (Ni₃AlTi) phase used in 925.
This explains why 718 achieves a higher ultimate tensile strength (1375 MPa) than 925 (1210 MPa): γ″ is inherently a stronger hardener than γ′, because its BCT crystal structure has a larger lattice mismatch with the FCC nickel matrix, generating more intense strain fields that impede dislocation motion.
However, this strength advantage comes with a temperature penalty. Above ~650°C, γ″ transforms to the stable δ phase (orthorhombic Ni₃Nb), which provides no strengthening and embrittles grain boundaries. This transformation effectively caps 718’s service temperature.
2. Heat Treatment & Precipitation Hardening: Two Routes to Strength
Both alloys achieve their high strength through precipitation hardening (age hardening), but the precipitates — and therefore the optimal aging temperatures — differ.
Incoloy 925: γ′ (Ni₃AlTi) Route
925 uses a single-step solution anneal + two-step age cycle:
| Step | Temperature | Time | Cooling |
|---|---|---|---|
| Solution Anneal | 980–1040 °C | Per section thickness | Water quench or rapid air cool |
| Primary Age | 732 °C (1350 °F) | 8 hours | Furnace cool to 621 °C |
| Secondary Age | 621 °C (1150 °F) | 8 hours | Air cool |
The primary aging at 732°C precipitates fine γ′ (Ni₃AlTi) particles throughout the matrix. The secondary aging at 621°C coarsens these precipitates slightly while relieving residual stresses. The resulting microstructure consists of uniform, coherent γ′ precipitates ~10–30 nm in diameter, distributed in an austenitic (FCC) matrix with occasional TiC carbides at grain boundaries.
Inconel 718: γ″ (Ni₃Nb) Route
718 uses the classic AMS 5662/5663 cycle:
| Step | Temperature | Time | Cooling |
|---|---|---|---|
| Solution Anneal | 940–1010 °C | Per section thickness | Rapid air cool or water quench |
| Primary Age | 718 °C (1325 °F) | 8 hours | Furnace cool @ 55°C/hr to 621 °C |
| Secondary Age | 621 °C (1150 °F) | 8 hours | Air cool |
The critical detail: the controlled furnace cool rate (55°C/hr maximum) between 718°C and 621°C. This slow cooling allows γ″ particles to grow to their optimal size (~20–50 nm diameter, ~5–10 nm thickness as disc-shaped precipitates). Cooling too rapidly produces under-aged material with lower strength; cooling too slowly over-ages the γ″, reducing coherency strains.
Why the different primary aging temperatures? The γ′ phase in 925 nucleates optimally at 732°C; the γ″ phase in 718 nucleates optimally at 718°C. The 14°C difference reflects the different thermodynamic driving forces for the two distinct precipitate phases — not an arbitrary choice.
3. Mechanical Properties: Strength, Ductility & Temperature Limits
Room Temperature Comparison (Aged Condition)
| Property | Incoloy 925 | Inconel 718 | Ratio (925/718) |
|---|---|---|---|
| UTS | 1210 MPa (175 ksi) | 1375 MPa (200 ksi) | 88% |
| YS (0.2% offset) | 895 MPa (130 ksi) | 1100 MPa (160 ksi) | 81% |
| Elongation | 20% | 18% | Slightly more ductile |
| Hardness | 34–40 HRC | 36–44 HRC | — |
| Elastic Modulus | 200 GPa | 205 GPa | Similar |
| Density | 8.05 g/cm³ | 8.19 g/cm³ | Similar |
| Charpy Impact (room temp) | 80–100 J | 40–60 J | ~2× higher toughness |
The key insight: 718’s YS is ~23% higher than 925’s, but 925 shows ~2× higher room-temperature impact toughness. This toughness advantage makes 925 less susceptible to brittle fracture during installation handling and thermal cycling — a practical consideration for downhole tools that experience multiple trip-in/trip-out cycles.
Elevated Temperature Properties
Both alloys are limited to approximately 650°C (1200°F) for continuous service, but performance within that range differs:
| Temperature | 925 UTS | 718 UTS | 925 YS | 718 YS |
|---|---|---|---|---|
| 25 °C | 1210 MPa | 1375 MPa | 895 MPa | 1100 MPa |
| 200 °C | 1100 MPa | 1275 MPa | 820 MPa | 1020 MPa |
| 400 °C | 1030 MPa | 1200 MPa | 780 MPa | 960 MPa |
| 540 °C | 950 MPa | 1120 MPa | 740 MPa | 900 MPa |
| 650 °C | 790 MPa | 900 MPa | 650 MPa | 750 MPa |
| 760 °C | ~500 MPa (not recommended) | ~550 MPa (δ phase formation) | — | — |
Both alloys lose ~35% of room-temperature strength by 650°C, but the absolute values show 718 maintaining a consistent ~100–150 MPa advantage across the entire temperature range. For applications above 650°C, neither alloy is suitable — consider Waspaloy (to 730°C) or Nimonic 105 (to 900°C) instead (see our Inconel 718 vs Waspaloy comparison).
Cryogenic Properties
718 is preferred for cryogenic service (LNG, liquid oxygen, liquid hydrogen) because of its well-documented toughness retention down to -253°C. The γ″ phase remains stable at cryogenic temperatures, and 718’s FCC matrix does not undergo a ductile-to-brittle transition. 925 has less published cryogenic data, making 718 the conservative choice for services below -50°C.
4. Corrosion Resistance: The H₂S Differentiator
General Corrosion
In oxidizing acids (HNO₃, aerated H₂SO₄), both alloys perform similarly — their ~20% Cr content provides comparable passive film stability. In reducing acids, Mo (~3% in both) provides the primary protection, yielding broadly equivalent performance.
However, the critical difference emerges in sour environments containing H₂S.
Hydrogen Sulfide (H₂S) & NACE MR0175/ISO 15156
This is where 925’s copper addition produces a decisive advantage. Under NACE MR0175/ISO 15156, the maximum acceptable hardness for sour service depends on the alloy’s susceptibility to SSC:
| Alloy | NACE MR0175 Level | Max HRC for Sour Service | H₂S Partial Pressure Limit |
|---|---|---|---|
| Incoloy 925 | Level VI (highest) | 40 HRC (aged) | No limit (up to material YS) |
| Inconel 718 | Level IV–V | 35–40 HRC | Requires qualification testing |
The practical implication: 925 can be specified for the most aggressive sour gas wells (H₂S > 10 psi partial pressure, chloride > 100,000 ppm) without supplementary qualification. 718 can serve in sour environments but may require project-specific testing per NACE TM0177 Method A (tensile) or Method C (C-ring), adding time and cost to material qualification.
This difference is not marginal — it is the primary reason operators specify 925 for high-H₂S deep gas completions (e.g., Middle East Khuff formation, Sichuan Basin, China, North American Rockies sour gas trends). When H₂S partial pressure exceeds 3–5 psi, the decision often shifts decisively toward 925.
Chloride Stress Corrosion Cracking (Cl-SCC)
Both alloys resist Cl-SCC in NaCl and CaCl₂ brines — a fundamental advantage of nickel-base alloys over austenitic stainless steels. Neither alloy suffers Cl-SCC in neutral chloride environments up to their yield strength at temperatures to 200°C.
Pitting Resistance
| Alloy | Cr | Mo | N | PREN (= %Cr + 3.3×%Mo + 16×%N) |
|---|---|---|---|---|
| Incoloy 925 | ~21% | ~3.0% | — | ~31 |
| Inconel 718 | ~19% | ~3.0% | — | ~29 |
The PREN values are comparable and relatively modest — neither alloy is designed for pitting-dominated services. For aggressive chloride pitting environments, consider super austenitics (254SMO, PREN 46) or nickel-chromium-molybdenum alloys (C-276, PREN 69; 686, PREN 76).
5. Weldability & Fabrication
Welding
Incoloy 925: Weldable with matching filler metal (ERNiCrMo-16 / AWS A5.14 ERNiCrMo-16). Requires post-weld solution anneal + full re-age to restore strength in the weld zone. As-welded strength is roughly 60–70% of aged strength, as the γ′ precipitates dissolve in the heat-affected zone (HAZ).
Inconel 718: Excellent weldability — one of the most weldable precipitation-hardening nickel alloys. Gas tungsten arc welding (GTAW) with ERNiFeCr-2 (AMS 5832) filler. The sluggish γ″ precipitation kinetics mean 718 resists strain-age cracking in the HAZ far better than γ′-hardened alloys. Post-weld aging (no re-solution required for many applications) restores most strength.
Practical winner: 718 for complex welded fabrications. The γ″ sluggish kinetics that limit 718’s temperature capability are actually an advantage during welding — the HAZ does not crack during cooling because γ″ precipitates slowly enough that residual stresses relax before hardening.
Machining
Both alloys are challenging to machine in the aged condition (34–44 HRC). Best practice:
- Machine in the solution-annealed condition (~20–25 HRC), then age-harden
- Use carbide tooling (C-2 grade for roughing, C-3 for finishing)
- Low cutting speeds: 15–25 m/min for turning, 8–12 m/min for drilling
- Copious coolant (sulfo-chlorinated oil recommended for 718)
925 is marginally easier to machine than 718 at equivalent hardness, due to its lower Nb content (Nb forms hard MC carbides that accelerate tool wear) and higher Fe content (which reduces work-hardening rate). Expect 15–20% longer tool life on 925 compared to 718 at equivalent hardness.
Hot Working
| Parameter | Incoloy 925 | Inconel 718 |
|---|---|---|
| Forging range | 980–1150 °C | 940–1120 °C |
| Reduction ratio | ≥ 3:1 recommended | ≥ 3:1 recommended |
| Final forge temp | ≥ 950 °C | ≥ 940 °C |
| Cooling after forge | Air cool or water quench | Air cool |
6. Industry Standards & Specifications
Incoloy 925 (UNS N09925)
| Standard | Product Form | Title |
|---|---|---|
| ASTM B805 | Bar, rod, wire | Ni-Fe-Cr-Mo-Cu alloy (UNS N09925) |
| AMS 5962 | Bar, forgings | Ni-42Fe-22Cr-3Mo-2Cu-2Ti age-hardenable |
| NACE MR0175 | All forms | Level VI sour service |
| API 6A CRA | Wellhead & tree | PSL 3G / PSL 4 |
| ASTM B637 | Bar, forgings (general) | Precipitation-hardening Ni alloys |
Inconel 718 (UNS N07718)
| Standard | Product Form | Title |
|---|---|---|
| AMS 5662 | Bar, forgings, rings | Ni-19Cr-3Mo-5Nb-1Ti (solution + aged) |
| AMS 5663 | Bar, forgings (higher solution temp) | As AMS 5662 with 1021–1052°C solution |
| AMS 5596 | Sheet, strip, plate | Ni-19Cr-3Mo-5Nb-1Ti |
| AMS 5962 | Wire (welding) | ERNiFeCr-2 filler |
| ASTM B637 | Bar, forgings | Precipitation-hardening Ni alloys |
| API 6A CRA | Wellhead & tree | PSL 3G / PSL 4 |
7. Cost Comparison & Total Cost of Ownership (TCO)
Raw Material Cost Drivers
| Cost Factor | Incoloy 925 | Inconel 718 | Impact |
|---|---|---|---|
| Ni content | 42–46% | 50–55% | Ni is the dominant cost driver (~$20–30/kg) |
| Nb content | ≤ 0.5% | 4.75–5.5% | Nb (ferroniobium) is ~$35–45/kg |
| Cu content | 1.5–3.0% | — | Cu is ~$8–10/kg (minor impact) |
| Fe content | ~28% | ~18% | Fe is the cheapest element (< $1/kg) |
The net effect: Incoloy 925 typically costs 15–25% less than Inconel 718 per kilogram of mill product, driven primarily by the lower nickel content (6–13% less) and dramatically lower niobium content (4.5–5.0% less).
TCO Analysis: 10,000-ft Sour Gas Completion String
Consider a 10,000-ft (3,048 m) deep sour gas completion requiring 4-1/2″ OD × 0.350″ wall production tubing:
| Item | Incoloy 925 | Inconel 718 |
|---|---|---|
| Weight per foot | 15.5 lb/ft | 15.8 lb/ft |
| Total tubing weight | 155,000 lb (70.3 t) | 158,000 lb (71.7 t) |
| Material cost per lb | ~$35–40 | ~$45–50 |
| Total material cost | ~$5.8M | ~$7.5M |
| NACE qualification | Pre-qualified | +$150K testing |
| Total installed cost | ~$6.5M | ~$8.5M |
The ~2Msavingswith925(approximately242Msavingswith925(approximately24300,000/day.
However: if the well conditions demand the higher strength of 718 (e.g., deeper setting depths, higher collapse pressures), the cost equation shifts. The 23% higher yield strength of 718 may enable thinner-wall tubing, partially offsetting the higher per-pound cost. This is a site-specific engineering optimization.
8. Application Selection Matrix
| Application | Preferred Alloy | Reason |
|---|---|---|
| Sour gas production tubing (H₂S > 3 psi) | Incoloy 925 | NACE Level VI, Cu for H₂S resistance |
| High-pressure gas injection wellhead | Incoloy 925 | Cost + H₂S margin |
| Downhole safety valve (SCSSV) springs | Incoloy 925 | High toughness, H₂S compatibility |
| Subsurface hangers and packers | Incoloy 925 | Cost + NACE compliance |
| Gas turbine disks (aero-derivative) | Inconel 718 | Higher strength, proven aerospace history |
| Aircraft engine fasteners (bolts, studs) | Inconel 718 | AMS 5662, high-temperature strength |
| Cryogenic valve stems (LNG, -162°C) | Inconel 718 | Proven cryogenic toughness |
| High-strength bolting (>1100 MPa YS) | Inconel 718 | Maximum strength capacity |
| Subsea wellhead connectors | Inconel 718 | Strength at large section sizes |
| Non-sour sweet gas completions | Inconel 718 | Strength advantage without H₂S penalty |
Decision Flowchart Logic
Is H₂S partial pressure > 3 psi (NACE sour)?
├── YES → Does the application require >1200 MPa UTS?
│ ├── YES → Inconel 718 + NACE qualification testing
│ └── NO → INCOLOY 925 (pre-qualified, cost-effective)
└── NO → Is the application cost-sensitive?
├── YES → Incoloy 925 (15-25% cheaper, sufficient strength)
└── NO → Does the application require >1200 MPa UTS?
├── YES → Inconel 718
└── NO → Either alloy (evaluate on other factors: temperature, cryogenic needs, weldability)
9. Notable Case Studies
Case 1: Sichuan Basin High-H₂S Gas Well (China)
Sinopec and PetroChina have deployed Incoloy 925 production tubing in Sichuan Basin gas wells where H₂S content exceeds 5% (50,000 ppm) and CO₂ exceeds 8%. Bottom-hole temperatures of 150–180°C and chloride concentrations exceeding 120,000 ppm rule out duplex stainless steels (Cl-SCC risk) and super 13Cr (SSC risk at high H₂S). 925 is specified over 718 primarily for NACE Level VI pre-qualification — eliminating the 6–8 months of project-specific SSC testing that 718 would require.
Case 2: North Sea HPHT Tieback
A North Sea high-pressure/high-temperature (HPHT) tieback at 180°C used Inconel 718 for the subsea wellhead connector body (requiring 827 MPa minimum YS in sections >250 mm thick), but specified Incoloy 925 for the production tubing and downhole accessories. This hybrid approach optimized cost while maintaining technical performance: 718 where strength dictated, 925 where H₂S margin and cost dominated.
Case 3: Gas Turbine Fastener Replacement
An aero-derivative gas turbine operator in the Middle East investigated replacing Inconel 718 compressor disk bolts with Incoloy 925 to reduce cost. The substitution was rejected because:
- 718 bolts are stress-relieved and aged to AMS 5662 (minimum 1100 MPa YS), while 925 maxes out at ~895 MPa
- The joint design relied on 718’s higher preload capacity
- Rewriting the engine Type Certificate for a new bolt material would cost more than the material savings
This case illustrates that cost savings alone do not justify material substitution — the entire system design (joint preload, fatigue margins, certification) must be revalidated.
10. FAQ: High-Frequency Search Questions
Q1: Can Incoloy 925 replace Inconel 718 in all oil and gas applications?
No. Incoloy 925 replaces 718 specifically where H₂S resistance (NACE MR0175) is critical and 925’s lower strength is acceptable (YS 895 vs 1100 MPa). For non-sour, high-strength applications — deep HPHT completions, high-pressure wellhead connectors, subsea bolting — 718’s 23% higher yield strength is essential. The cost savings of 925 (~15–25%) are only realized when the strength margin is sufficient. Each application requires a fit-for-purpose engineering evaluation.
Q2: Why does 925 cost less than 718 despite using more expensive elements like copper?
Copper is not the cost driver. The savings come from: (1) lower nickel content (42–46% vs 50–55%), which saves 2–4/kgatcurrentnickelprices;(2)dramaticallylowerniobiumcontent(≤0.52–4/kgatcurrentnickelprices;(2)dramaticallylowerniobiumcontent(≤0.51.5–2.0/kg in ferro-niobium costs. These two factors contribute 80–90% of the 15–25% price difference. Copper, at ~8–10/kgandonly1.5–3.08–10/kgandonly1.5–3.00.30/kg to the alloy cost.
Q3: Is Incoloy 925 weldable? Does post-weld heat treatment require a full solution anneal?
Yes, 925 is weldable, but unlike 718, it generally requires full post-weld solution annealing (980–1040°C) followed by complete re-aging to restore strength in the weld zone. The γ′ precipitates dissolve in the HAZ, and unlike 718’s sluggish γ″, they do not re-precipitate during a simple post-weld age cycle alone. For field weld repairs, 925 is less forgiving than 718 — this is one reason 718 dominates welded fabrications (e.g., aerospace casings, manifold blocks).
Q4: Can I use Inconel 718 in NACE MR0175 sour service without qualification testing?
It depends on the H₂S severity. NACE MR0175/ISO 15156 lists 718 as acceptable for sour service at hardness ≤40 HRC in the solution-annealed + aged condition. However, for H₂S partial pressures exceeding 1–3 psi (depending on the end-user specification), operators commonly require supplementary SSC testing per NACE TM0177 Method A (smooth tensile) or Method D (DCB). 925 eliminates this uncertainty — it is pre-qualified to NACE Level VI for unrestricted sour service at ≤40 HRC. The ~$150K in testing costs and 6–8 months of schedule for 718 NACE qualification often tip the decision toward 925 for sour gas projects.
Q5: Between Incoloy 925 and Inconel 718, which is better for high-cycle fatigue applications?
718 has a better-characterized fatigue database, particularly in the aerospace sector where S-N curves, Goodman diagrams, and fracture mechanics data (da/dN vs ΔK) are well-published. 925’s fatigue performance is broadly comparable — the slightly lower UTS reduces the fatigue limit marginally — but the published data set is smaller. For safety-critical fatigue applications (rotating components, cyclic pressure service), 718’s extensive fatigue qualification heritage is a significant risk-reduction advantage. For static or low-cycle applications, 925 performs adequately.
Key Takeaways
- Strength vs Cost: Inconel 718 delivers 23% higher yield strength (1100 vs 895 MPa) — essential for deep HPHT wells and aerospace fasteners. Incoloy 925 saves 15–25% on material cost, adequate for most downhole strength requirements.
- H₂S is the Decider: 925’s 1.5–3.0% Cu addition is a deliberate H₂S-resistant design feature. NACE MR0175 Level VI pre-qualification saves 6–8 months and ~$150K in testing versus 718’s qualification requirements for sour service.
- Different Precipitates, Different Aging: 925 hardens via γ′ (Ni₃AlTi), aged at 732°C. 718 hardens via γ″ (Ni₃Nb), aged at 718°C. The 14°C aging temperature difference reflects different thermodynamic driving forces — not an arbitrary choice.
- Weldability Favors 718: 718’s sluggish γ″ kinetics prevent HAZ strain-age cracking, making it one of the most weldable precipitation-hardening nickel alloys. 925 requires full post-weld re-solution + re-age, adding fabrication cost.
- Temperature Limit is Comparable: Both alloys cap at ~650°C. Above this, consider Waspaloy (730°C) or Nimonic 105 (900°C).
- Toughness Trade-off: 925 shows ~2× higher room-temperature Charpy impact energy (80–100 vs 40–60 J), an advantage for downhole tools subject to handling and thermal cycling cycles.
- Hybrid Specification is Common: Many operators specify 718 for wellhead connectors and 925 for production tubing — optimizing strength where needed, cost and H₂S margin elsewhere. This is not a “one alloy fits all” decision.
For material selection support, mill certification, or project-specific sour service engineering, contact J&A Alloy’s technical team. We supply Incoloy 925 and Inconel 718 in bar, forging, tubular, and machined component forms to ASTM B805/B637, AMS 5662/5962, and NACE MR0175 requirements.
