Incoloy 925 (UNS N09925) derives its high strength from a two-step heat treatment that precipitates Ni₃(Al, Ti) gamma-prime particles inside an austenitic nickel-iron-chromium matrix. Step 1 is a high-temperature solution anneal at 1900°F (1040°C) followed by rapid quench; this dissolves all carbides and gamma-prime back into solution and locks the matrix in single-phase austenite. Step 2 is a controlled age cycle at 1365°F (740°C) for 8 hours followed by a slow furnace cool to 1150°F (621°C) where a second hold may be applied. The age cycle precipitates nano-scale gamma-prime particles uniformly through the matrix, lifting yield strength from ~50 ksi (annealed) to a guaranteed 110 ksi minimum, and tensile strength to 165 ksi minimum, the values codified in ASTM B805 and required for API 6A wellhead bolting.
Need certified heat-treated Incoloy 925? Email info@torqbolt.com with product type, size and quantity. TorqBolt operates calibrated nitrogen-atmosphere ageing furnaces with continuous chart-recorder traceability. Every order ships with EN 10204 3.1 manufacturer's MTC documenting solution-anneal temperature, age-cycle time-temperature profile, post-cycle hardness and mechanical test results. WhatsApp +91-22-66157017.
Standard Heat-Treatment Cycle (ASTM B805 / SMC HA 46)
| Step | Temperature | Hold Time | Cooling | Atmosphere |
|---|---|---|---|---|
| 1. Solution Anneal | 1900°F (1040°C) | 1 h per 1" (25 mm) of section, min 1 h | Water Quench (WQ) within 60 s of furnace exit | Air or inert gas |
| 2. Age, Step A | 1365°F (740°C) | 8 h | Furnace cool 100°F/h (55°C/h) to 1150°F | Air or inert gas |
| 3. Age, Step B (optional) | 1150°F (621°C) | 8 h | Air Cool | Air |
Total elapsed time for the age cycle is approximately 18 hours including ramp, hold and slow cool. The two-step ageing (740°C followed by slow cool through 621°C) is preferred over a single-temperature age because the second hold completes the gamma-prime precipitation kinetics and gives more reproducible mechanical properties across heat lots.
Solution Anneal, Critical Parameters
Temperature window: 1900±25°F (1040±14°C). Below 1875°F, residual gamma-prime and grain-boundary carbides may not fully dissolve, leading to inconsistent ageing response. Above 1925°F, grain growth accelerates and toughness suffers.
Hold time: 1 hour per 1 inch (25 mm) of section thickness, with a minimum of 1 hour regardless of size. For thick sections (above 4" / 100 mm), interrupted holds with section-temperature surveys are mandatory ASTM B805.
Quench medium: Water quench is preferred. Polymer quench (5–10% PAG) acceptable for thick forgings to limit distortion. Air cool is NOT acceptable for material destined for sour service, the slow cool through 1400–800°F allows intergranular carbide precipitation that creates pits in service.
Quench delay: Material must enter the quench medium within 60 seconds of furnace door open. Section temperature drop during transfer must not exceed 100°F to avoid passing through the gamma-prime nucleation window prematurely.
Age Hardening, Precipitation Kinetics
During the 1365°F (740°C) hold, Ti and Al atoms diffuse to ordered Ni₃(Al, Ti) clusters that grow into coherent gamma-prime particles 5–20 nm in diameter. The precipitates are coherent with the FCC austenite matrix, their lattice parameter mismatch creates the strain field that pins dislocations and gives the alloy its high yield strength.
The slow furnace cool from 740°C to 621°C (100°F/h ramp) is critical: it allows additional gamma-prime precipitates to nucleate at slightly lower temperatures, broadening the size distribution and giving more uniform strengthening. A direct quench from 740°C bypasses this secondary precipitation and gives 5–10 ksi lower yield strength.
Resulting Mechanical Properties
| Property | Annealed Only | Annealed + Aged (ASTM B805 min) | Annealed + Aged (typical) |
|---|---|---|---|
| Tensile Strength | 110 ksi (760 MPa) | 165 ksi (1140 MPa) | 175 ksi (1207 MPa) |
| 0.2% Yield Strength | 50 ksi (345 MPa) | 110 ksi (760 MPa) | 120 ksi (827 MPa) |
| Elongation in 4D | 30% | 18% | 22% |
| Reduction of Area | 40% | 25% | 35% |
| Hardness | 92 HRB max | 35 HRC max | 32–35 HRC |
See full mechanical properties page for elevated-temperature data and Charpy impact envelope. The 35 HRC maximum is the cap codified in NACE MR0175 / ISO 15156-3 Table A.10 for sour-service qualification, over-ageing must be controlled to keep aged hardness below this value.
Hot Working
Hot working range: 1600–2150°F (870–1175°C). Below 1600°F, deformation work-hardens the matrix and tooling forces become excessive. Above 2150°F, incipient melting at grain boundaries causes cracking. The optimum forging-finish temperature is 1900°F (1040°C) followed by water quench, this in-line solution anneal eliminates a separate annealing cycle.
At temperatures up to 2000°F (1095°C), Incoloy 925 has hot-working characteristics similar to Incoloy 825, both alloys can be processed on the same forging equipment with similar reduction-per-pass schedules.
Cold Working
Cold work above 10–15% reduction must be removed by solution anneal + water quench before age hardening. Reasons:
- Cold work alone can raise hardness above the 35 HRC sour-service cap, even before ageing
- Heavily cold-worked material recrystallises during the 1365°F age hold, giving inconsistent grain size
- Residual cold-work strain reduces ductility and Charpy toughness in the final aged condition
For applications requiring cold-worked + aged condition (some API 6A high-strength bolting), specify the cold-work percentage and post-CW heat treatment at RFQ stage.
Comparison, Heat Treatment vs Adjacent Alloys
| Alloy | Solution Anneal | Age Cycle | Strengthening Mechanism |
|---|---|---|---|
| Incoloy 925 | 1900°F WQ | 1365°F × 8h FC + 1150°F × 8h AC | Gamma-prime Ni₃(Al, Ti) |
| Incoloy 825 | 1725°F WQ | None, solid solution | Solid solution (Mo, Cu) |
| Inconel 718 | 1750°F WQ | 1325°F × 8h FC + 1150°F × 8h AC | Gamma double-prime Ni₃Nb |
| Monel K-500 | 1600°F WQ | 1080°F × 8h FC + 900°F × 8h AC | Gamma-prime Ni₃(Al, Ti) |
Heat-Treatment Process Control
TorqBolt's heat-treatment shop is qualified to API 6A Annex M (heat-treatment requirements) with the following controls:
- Furnace calibration: Pyrometric survey per AMS 2750 every 6 months, ±10°F across qualified work zone
- Charge-thermocouple control: Each load instrumented with at least 2 work-zone thermocouples; recorder traces archived per heat lot
- Quench monitoring: Quench-medium temperature logged at start and end of each batch; quench delay timed and recorded
- Material traceability: Heat number stamped on every piece pre-treatment; barcoded movement through anneal → quench → age sequence
- Post-treatment NDE: 100% hardness check (Rockwell C); sample tensile per ASTM E8; sample Charpy per ASTM E23
Frequently Asked Questions
Why is the age cycle so long (18 hours total)? Gamma-prime precipitation is a diffusion-controlled process. At 1365°F the diffusion rate of Ti and Al in nickel is slow enough that 8 hours is required to grow precipitates to the optimum 5–20 nm size. Shorter cycles give finer, less-effective precipitates; longer cycles cause Ostwald ripening and loss of strength. The 8 + 8 hour cycle with intermediate slow cool is the optimised compromise validated by Special Metals across decades of production.
Can the heat treatment be reversed? Yes. Re-annealing at 1900°F dissolves all gamma-prime back into solution; the material returns to the soft annealed condition with ~50 ksi YS. This re-anneal can be repeated multiple times without degrading the alloy, useful when components require re-machining or if a heat-treatment lot fails QA inspection.
What if the age cycle is interrupted? An interrupted cycle (power failure, furnace fault) is recoverable: the partially-aged material can be re-solution-annealed and the full age cycle restarted. Do not simply restart the age cycle from the interrupted state, the partial precipitation is non-uniform and the resulting properties unreliable.
How does TorqBolt prove the heat treatment was performed? Every order MTC includes: (1) actual furnace chart-recorder trace for both anneal and age cycles, (2) post-anneal hardness reading (sample), (3) post-age hardness reading (100% inspection), (4) age-cycle work-zone thermocouple data. EN 10204 3.2 with third-party witness is available on request, the witness signs the chart recorders and the MTC.
Request a Quote
For Incoloy 925 heat-treated to ASTM B805 / API 6A / NACE MR0175 specifications:
- Email: info@torqbolt.com
- WhatsApp: +91-22-66157017
- Furnace chart recorder traces + sample MTC available on request
Specify product type (stud bolts, nuts, hex bolts, pipe, plate), size range, quantity, applicable standards (ASTM B805, ASTM B983, API 6A, NACE MR0175), required condition (annealed or aged), and any special heat-treatment requirements (third-party witness, chart-recorder archive period, work-zone thermocouple data).
References: Special Metals Corporation: INCOLOY® alloy 925 Technical Bulletin, Tables 6, 7, 12 and Figures 4, 7–12. ASTM B805 Standard Specification for Forgings and Forging Stock for Nuclear and General Use Wrought Precipitation Hardened UNS N09925. API Specification 6A Annex M Heat Treatment Requirements. AMS 2750 Pyrometry. INCOLOY® is a registered trademark of Special Metals Corporation.
