UNS N06625 Hex Bolt | Alloy 625 Bolts & Nuts
UNS N06625 hex bolts are high-strength, corrosion-resistant nickel-chromium-molybdenum-niobium fasteners used in offshore, seawater, chemical-processing and other demanding environments. Alloy 625 combines high nickel and chromium contents with molybdenum and niobium to provide strong resistance to pitting, crevice corrosion, chloride-induced attack and a wide range of corrosive media.
We manufacture and supply UNS N06625 hex bolts, stud bolts, fully threaded studs, hex nuts and custom fasteners in metric and inch sizes, with project-specific thread forms, PMI, dimensional inspection and EN 10204 3.1 certification.
UNS N06625 Fastener Quick Specification
| Item | Specification |
|---|---|
| Material | UNS N06625 |
| Common Designation | Alloy 625 |
| EN / W.Nr. | 2.4856 |
| Material Family | Nickel-Chromium-Molybdenum-Niobium Alloy |
| Material Standard | ASTM B446 for rod/bar material where applicable |
| Hex Bolt Standard | ASME B18.2.1 / ISO 4014 / ISO 4017 as specified |
| Hex Nut Standard | ASME B18.2.2 / applicable metric standard |
| Stud Standard | ASME B18.31.2 when specified |
| Thread Standard | ASME B1.1 / metric thread standard |
| Products | Hex Bolt / Stud Bolt / Fully Threaded Stud / Hex Nut |
| Certification | EN 10204 3.1 / 3.2 when specified |
What Is UNS N06625?
UNS N06625 is a nickel-chromium-molybdenum-niobium alloy developed to combine high mechanical strength with excellent corrosion resistance.
Its principal alloying system is:
Ni + Cr + Mo + Nb
This combination provides:
- High strength
- Pitting resistance
- Crevice-corrosion resistance
- Chloride resistance
- Good seawater performance
- Oxidation resistance
- Resistance to many chemical-process environments
The alloy derives much of its strength from solid-solution strengthening by molybdenum and niobium in the nickel-chromium matrix.
UNS N06625 Chemical Composition
| Element | UNS N06625 |
|---|---|
| Nickel, Ni | 58.0% min |
| Chromium, Cr | 20.0–23.0% |
| Molybdenum, Mo | 8.0–10.0% |
| Niobium + Tantalum, Nb + Ta | 3.15–4.15% |
| Iron, Fe | 5.0% max |
| Carbon, C | 0.10% max |
| Manganese, Mn | 0.50% max |
| Silicon, Si | 0.50% max |
| Phosphorus, P | 0.015% max |
| Sulfur, S | 0.015% max |
| Cobalt, Co | 1.0% max |
| Aluminum, Al | 0.40% max |
| Titanium, Ti | 0.40% max |
| Copper, Cu | 0.50% max |
The high molybdenum and niobium contents distinguish N06625 from many conventional nickel-chromium alloys and contribute significantly to its strength and corrosion resistance.
Actual heat chemistry is reported on the Material Test Certificate.
ASTM B446 Material Conditions
ASTM B446 covers UNS N06625 rod and bar in multiple heat-treated conditions.
The ordered grade and heat-treatment condition should be clearly specified because the mechanical requirements and service positioning differ.
Grade 1
Grade 1 is supplied in the annealed condition.
It is commonly used for standard corrosion-resistant applications and normal service conditions.
Grade 2
Grade 2 is supplied in the solution-annealed condition.
It is generally selected for elevated-temperature service where creep and rupture performance become important.
The correct material condition should be selected according to the operating temperature, joint design and project specification.
Mechanical Properties
Mechanical properties of UNS N06625 depend on the ASTM B446 grade, bar size and final processing condition.
For finished bolts and nuts, the required mechanical performance should not be assumed solely from the raw bar data.
Important acceptance properties can include:
- Tensile strength
- Yield strength
- Elongation
- Reduction of area
- Hardness
Actual test values are reported on the MTC.
Final fastener mechanical requirements shall follow:
Material Condition + Finished Fastener Requirement + Project Bolting Specification
Hex Bolt, Nut and Stud Standards
The material standard and dimensional standard perform different functions.
Hex Bolts
Inch-series hex bolts can be manufactured to ASME B18.2.1 when specified.
Metric hex bolts can be supplied to:
- ISO 4014
- ISO 4017
- Other project-specified dimensional standards
Hex Nuts
Inch-series hex and heavy hex nuts can be supplied to ASME B18.2.2 when specified.
Metric nuts can be supplied to the applicable metric nut standard.
Stud Bolts
Inch-series studs can be manufactured to ASME B18.31.2, including:
- Continuous-thread studs
- Flange bolting studs
- Double-end studs
Threads
Unified inch threads can be specified to ASME B1.1, including:
- UNC
- UNF
- 8UN
Metric coarse and fine threads can also be supplied when specified.
Corrosion Resistance of UNS N06625 Fasteners
UNS N06625 fasteners are widely selected where both high strength and corrosion resistance are required.
Pitting Resistance
The high molybdenum content provides strong resistance to localized pitting corrosion.
Crevice Corrosion Resistance
N06625 performs well in many severe crevice-corrosion environments, particularly compared with conventional stainless steels.
Chloride Resistance
The high nickel and molybdenum content provides strong resistance to chloride-induced attack and chloride stress-corrosion cracking.
Seawater Resistance
UNS N06625 is widely used in marine, offshore and seawater systems because of its strong resistance to localized corrosion.
Actual suitability still depends on:
- Temperature
- Chlorination
- Crevice geometry
- Flow conditions
- Contaminants
- Service chemistry
Oxidation Resistance
Chromium contributes to oxidation resistance and makes the alloy useful across a broad range of corrosive and elevated-temperature environments.
Available N06625 Fastener Types
| Product | Description |
|---|---|
| Hex Bolt | Hexagonal-head fastener |
| Stud Bolt | Flange bolting stud used with nuts |
| Fully Threaded Stud | Threaded over full length |
| Double-End Stud | Threaded at both ends |
| Hex Nut | Matching corrosion-resistant nut |
| Custom Bolt | Manufactured to drawing or project specification |
Matching Nuts
Matching UNS N06625 hex nuts can be supplied when specified.
Nut selection should consider:
- Stud or bolt strength
- Nut proof strength
- Corrosion environment
- Galvanic compatibility
- Flange material
- Temperature
- Thread system
- Project requirements
The complete bolt/stud + nut assembly should be considered as one bolting system rather than selecting each component independently.
Washers
Washers are not automatically required for every N06625 bolting assembly.
When required, washer material should be selected according to:
- Joint design
- Mechanical requirements
- Corrosion environment
- Contact materials
- Project specification
Matching corrosion-resistant alloy washers can be supplied when specified.
Typical Size & Thread Range
Typical commercial supply can include:
| Item | Typical Range |
|---|---|
| Metric Diameter | M6–M64 or project-specific |
| Inch Diameter | 1/4″–2″ or project-specific |
| Length | Custom length |
| Metric Threads | Coarse / Fine |
| Inch Threads | UNC / UNF / 8UN |
| Thread Class | As specified |
Actual availability depends on diameter, thread form, material condition, length, quantity and inspection requirements.
Surface Condition
Typical surface conditions can include:
- Plain / as-machined
- Bright machined
- Polished
- Cleaned
- Project-specific surface finish
Additional coatings should only be applied when compatible with the service environment and bolting design.
Alloy 625 vs Alloy 825 Fasteners
Alloy 625 and Alloy 825 are both corrosion-resistant nickel alloys, but they have different compositions and service strengths.
| Item | N06625 / Alloy 625 | N08825 / Alloy 825 |
|---|---|---|
| Base System | Ni-Cr-Mo-Nb | Ni-Fe-Cr-Mo-Cu |
| Ni | 58% min | 38–46% |
| Mo | 8–10% | 2.5–3.5% |
| Nb + Ta | 3.15–4.15% | No major addition |
| Cu | Low | 1.5–3.0% |
| Strength | Higher | Moderate |
| Main Positioning | Severe chloride / offshore / high-strength service | Acid / chloride-SCC service |
Neither material is universally superior.
Selection should be based on corrosion environment, required strength, operating temperature and project requirements.
Applications
UNS N06625 hex bolts and fasteners are commonly used in:
- Offshore piping
- Subsea equipment
- Seawater systems
- Marine equipment
- Chemical-processing plants
- High-pressure connectors
- Valves
- Pumps
- Heat exchangers
- Pressure equipment
- Corrosion-resistant flange joints
- High-strength nickel-alloy assemblies
The alloy is particularly useful where both corrosion resistance and high mechanical strength are required.
Inspection & Certification
UNS N06625 fasteners can be supplied with:
- EN 10204 Type 3.1 MTC
- EN 10204 Type 3.2 when specified
- Heat chemical analysis
- Mechanical test results
- Hardness testing when specified
- PMI
- Dimensional inspection
- Thread inspection
- Visual examination
- Surface inspection
- Raw-material UT when specified
- Third-party inspection
- Full heat-number traceability
Thread dimensions can be inspected using the applicable gauges and project acceptance criteria.
Frequently Asked Questions
What is a UNS N06625 hex bolt?
A UNS N06625 hex bolt is a high-strength corrosion-resistant nickel-chromium-molybdenum-niobium fastener used in offshore, marine, chemical and severe corrosion applications.
What standard is used for Alloy 625 hex bolts?
ASTM B446 can be used as the material basis for UNS N06625 rod and bar, while bolt dimensions are specified separately.
ASME B18.2.1 may be used for inch-series hex bolts, ASME B18.2.2 for nuts and ASME B18.31.2 for studs.
What is the difference between Grade 1 and Grade 2 ASTM B446 N06625?
Grade 1 is annealed, while Grade 2 is solution annealed.
Grade selection depends on the intended temperature range, mechanical requirements and service conditions.
Is UNS N06625 suitable for seawater fasteners?
UNS N06625 is widely used for seawater and offshore fasteners because of its strong resistance to pitting, crevice corrosion and chloride-induced attack.
Final suitability depends on temperature, chlorination, crevice design and service chemistry.
What is the difference between Alloy 625 and Alloy 825 fasteners?
Alloy 625 contains substantially more molybdenum and niobium and generally provides higher strength and stronger localized corrosion resistance, while Alloy 825 contains copper and is particularly useful in several acid-processing and chloride-SCC environments.
