# Inconel 625 Bolts

*UNS N06625 · 2.4856 · GH3625*

**Category:** Nickel-Chromium-Molybdenum Alloy

Inconel 625 (UNS N06625, GH3625, 2.4856) is a solid-solution nickel-chromium-molybdenum bolting grade rated to 650°C and supplied to ASTM B446, ASTM B564 and AMS 5666 at 930 MPa tensile and 517 MPa yield in the annealed condition. A PREN above 50 sits behind the corrosion side, so inconel 625 bolts and inconel 625 fasteners deliver a combination of heat and corrosion resistance no stainless steel offers.

Strength comes from solid-solution hardening by molybdenum and niobium rather than aging, so properties are stable, welding is straightforward, and the alloy is effectively immune to chloride stress corrosion cracking.

It is the pragmatic choice when a joint sees both heat and an aggressive medium: UNS N06625 bolts are specified for exhaust and flue systems, offshore risers, seawater heat exchangers and chemical reactor bolting.

## Standards

- ASTM B446
- ASTM B564
- AMS 5666
- UNS N06625
- 2.4856

## Chemical composition (wt%)

| Element | Min | Max |
|---|---|---|
| Ni | 58.0 | - |
| Cr | 20.0 | 23.0 |
| Mo | 8.0 | 10.0 |
| Nb + Ta | 3.15 | 4.15 |
| Fe | - | 5.00 |
| Co | - | 1.00 |
| Ti | - | 0.40 |
| Al | - | 0.40 |
| Mn | - | 0.50 |
| Si | - | 0.50 |
| C | - | 0.10 |
| S | - | 0.015 |
| P | - | 0.015 |

## Mechanical properties

| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|
| AMS 5666 Annealed | 930 | 517 | 30 | ≤ 30 HRC |
| ASTM B446 Grade 1 | 827 | 414 | 30 | ≤ 25 HRC |
| ASTM B446 Grade 2 | 690 | 276 | 30 | ≤ 22 HRC |

## Applications

- **Marine & Offshore**: Inconel 625 bolts for riser flange bolting, seawater heat exchanger fasteners and splash zone hardware.
- **Chemical Processing**: Reactor, column and piping bolting in mixed acid and chloride environments up to 650°C.
- **Nuclear**: Reactor core components, control rod hardware and steam plant fasteners.
- **Aerospace**: Exhaust ducting, bellows hardware and engine thrust reverser fasteners to AMS 5666.

## FAQ

### Inconel 625 bolts or Inconel 718: what is the difference?

Inconel 625 fasteners are solid-solution: 930 MPa tensile, outstanding corrosion resistance, no aging required. Inconel 718 is precipitation hardened: 1275 MPa tensile but less corrosion margin and it must be aged. Choose inconel 625 bolts for corrosion, 718 for load.

### Can inconel 625 bolts and inconel 625 fasteners be used in seawater?

Yes. With its 8-10% molybdenum the PREN exceeds 50, so UNS N06625 bolts resist pitting and crevice corrosion in seawater and are immune to chloride SCC. Monel K-500 is preferred only where higher strength in flowing seawater is needed.

### Does Inconel 625 need heat treatment after forming?

Cold worked fasteners are annealed at 1090-1200°C with rapid quench to restore ductility and corrosion resistance. Grade 1 (annealed) is standard for bolting, and Grade 2 (solution annealed) is used above 600°C for creep service.