# Monel K-500 Bolts

*UNS N05500 · 2.4375 · Alloy K500*

**Category:** Nickel-Copper Alloy

Monel K-500 (UNS N05500, 2.4375) is an age-hardenable nickel-copper bolting grade with a 300°C service limit, supplied to ASTM B865 and QQ-N-286 at 1100 MPa tensile and 790 MPa yield in the age-hardened condition. Ni₃(Al,Ti) gamma-prime precipitation gives it the corrosion resistance of Monel 400 at roughly three times the yield strength, which makes monel k500 bolts and monel k500 fasteners the reference choice for seawater and chloride service.

In flowing seawater it is essentially the reference material: resistant to pitting, crevice attack, cavitation and biofouling-related corrosion, and non-magnetic for sensitive instrumentation.

Age-hardened UNS N05500 bolts are susceptible to hydrogen embrittlement when overaged or cathodically overprotected. We control the aging cycle of all monel k500 fasteners and hardness to 35 HRC max for subsea fasteners in line with API 6A CRA practice.

## Standards

- ASTM B865
- QQ-N-286
- UNS N05500
- 2.4375

## Chemical composition (wt%)

| Element | Min | Max |
|---|---|---|
| Ni | 63.0 | - |
| Cu | 27.0 | 33.0 |
| Al | 2.30 | 3.15 |
| Ti | 0.35 | 0.85 |
| Fe | - | 2.00 |
| Mn | - | 1.50 |
| Si | - | 0.50 |
| C | - | 0.18 |
| S | - | 0.010 |

## Mechanical properties

| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|
| Solution Treated | 690 | 310 | 35 | ≤ 85 HRB |
| Age Hardened | 1100 | 790 | 20 | 27-35 HRC |
| ASTM B865 Bar | 965 min | 690 min | 20 min | 24-35 HRC |

## Applications

- **Marine**: Monel k500 bolts for seawater pump and valve bolting, propeller shafts and fasteners for hull-mounted equipment.
- **Oil & Gas**: Downhole tools, subsea instrumentation housings and API 6A CRA fastening with controlled hardness.
- **Pumps & Shafts**: Impeller and shaft fasteners resisting cavitation and high-velocity seawater erosion.
- **Instrumentation**: Non-magnetic fasteners for sonar, navigation and magnetic survey equipment.

## FAQ

### Why are monel k500 bolts the best choice for seawater?

The nickel-copper matrix of monel k500 fasteners forms a stable protective film in chloride environments and, unlike stainless steels, is not susceptible to chloride stress corrosion cracking. UNS N05500 bolts also resist cavitation and high-velocity erosion far better than 316 or duplex stainless.

### What is the maximum service temperature of monel k500 bolts?

About 300°C (572°F). Beyond that the gamma-prime in monel k500 fasteners over-ages and strength drops rapidly, and the alloy loses its advantage over cheaper materials in dry high-temperature service.

### How do you control hydrogen embrittlement risk?

We limit final hardness to 35 HRC, avoid overaging, and can supply to NACE MR0175 / ISO 15156 with hardness testing on every lot. Cathodic protection potentials should stay above −0.90 V (Ag/AgCl).