Special alloy fasteners for Offshore & Marine.
Corrosion-resistant flange sets, high-strength pump studs and valve fasteners for seawater service. Order the mating parts together.
Which fasteners suit your marine exposure?
BYBOLT supplies Inconel 625 stud & nut sets to resist corrosion at seawater-exposed flanges, Super Duplex 2507 stud bolts for loaded pump covers, and Monel K-500 double-end studs for high-strength valve connections. Monel 400 hex bolts suit flowing-seawater pipework. Get the fasteners and mating parts for each equipment tag in one quotation.
Fasteners for marine equipment.
Seawater pumps
A seawater-pump cover transfers stud tension through the nut threads and bearing face. 2507 heavy hex nuts provide super duplex corrosion resistance at both locations. Their larger hex is useful for repeated overhaul work, provided the casing layout leaves clearance for the socket beside each adjacent nut.
Super Duplex 2507 Heavy Hex NutsRelated material guides
Connection requirements
- Stud tension and nut proof load
- Socket clearance on the bolt circle
- Washer and spotface dimensions
Shipboard cooling lines
Monel 400 hex bolts suit small shipboard cooling-line flanges where nickel-copper bolting meets the water exposure and closure load. Complete bolt removal is useful when a pipe section is lifted out for maintenance. The flange layout needs tool access at both the head and nut, with length allowed for the full assembled stack.
Monel 400 Hex BoltsRelated material guides
Connection requirements
- Flange stack and bolt projection
- Head and nut access beside pipework
- Cooling-water chemistry and shutdown periods
Deck winches & topside equipment
On topside equipment brackets, washers bridge clearance holes and spread bolt-head pressure onto the support. Inconel 625 is useful where salt spray and chemical washdown reach the underside of the washer. Slot width and edge distance set the bearing footprint, rather than the bolt diameter alone.
Inconel 625 Flat WashersConnection requirements
- Hole or slot width
- Support material and coating
- Washdown exposure and washer footprint
Marine valve bonnets
On a marine valve bonnet, these bolts clamp a removable cover to the valve body. K-500 is useful when the joint needs a stronger nickel-copper fastener than Monel 400, particularly where the available bolt diameter is already fixed by the bonnet. Exposed heads remain accessible for servicing the stem and packing.
Monel K-500 Hex BoltsRelated material guides
Connection requirements
- Bonnet joint thickness, gasket type, and required preload
- Bolt diameter, grip length, and nut/washer stack-up at installed position
- Chloride exposure, age-hardened condition, and heat-number certification
Subsea & ROV connections
2507 heavy hex nuts can serve subsea stud connections whose drawing and installation tool require that profile. They provide strength and chloride resistance at the loaded threads. For cathodically protected assemblies, the qualification also addresses hydrogen-induced stress cracking under the actual load and protection conditions.
Super Duplex 2507 Heavy Hex NutsRelated material guides
Connection requirements
- Installation tool and nut envelope
- Cathodic-protection conditions
- Stud pairing and nut acceptance requirements
Plate & shell heat-exchanger covers
On a seawater heat-exchanger cover with closely spaced studs, regular K-500 nuts offer an aged nickel-copper component without a heavy hex nut's footprint. They suit a design that needs the specified nut strength while leaving enough tool clearance for repeated cover removal.
Monel K-500 Hex NutsConnection requirements
- Mating bolt diameter, thread form, and thread class
- Nut height, width across flats, and bearing-face condition required
- Chloride exposure, age-hardened condition, and heat-number certification
Immersion, load, and splash exposure.
2507 bolts combine strength with chloride resistance at seawater connections; 625 studs offer strong pitting and crevice-corrosion resistance. Monel 400 suits flowing seawater, but sheltered threads and stagnant shutdown water need a separate corrosion assessment.
What to specify
- Chloride level
- Stagnation duration
- Temperature
Age-hardened K-500 studs add strength to loaded marine connections. For cathodically protected equipment, select the final alloy condition against the protection potential and tensile load; changing to 718 alone does not remove hydrogen-cracking risk.
What to specify
- CP potential
- Applied load level
- Hardness level
316L can serve accessible, rinsed deck attachments, while 625 and titanium offer options for more persistent marine exposure. At aluminium fittings, electrical isolation and drainage protect the less noble component; a more corrosion-resistant bolt alone does not prevent galvanic attack.
What to specify
- Splash exposure frequency
- Galvanic potential difference
- Chloride level
Compare marine
fastener alloys.
| Material grade | Mechanical Duty / Focus | Environment | Relative cost | Popular fastener forms |
|---|---|---|---|---|
| Inconel 625UNS N06625Crevice resistance |
| Immersed flanges & salt-exposed deck fixings | $$$ |  Stud & nut sets |
| Super Duplex 2507UNS S32750High strength |
| Seawater-pump & exchanger covers | $$ |  Fully threaded studs |
| Monel K-500UNS N05500Age hardened |
| Loaded marine valve bonnets | $$$ |  Double-end studs |
| Monel 400UNS N04400Flowing seawater |
| Flowing-seawater cooling lines | $$$ |  Hex bolts |
Budget guide: $$ moderate · $$$ higher · $$$$ premium. Relative raw-material bands; dimensions, quantity, processing and inspection determine the finished-fastener quote.
Material & cost references
- Special Metals · Alloy 625
- Alleima · SAF 2507 bar
- Special Metals · Alloy K-500
- Special Metals · Marine engineering
- Published round-bar price comparison · Monel supplier price guide
Cost bands group overlapping supplier price ranges for initial budgeting; they are not live BYBOLT prices.
Marine bolts, studs & matching nuts.
Inconel 625 stud & nut sets
Resist pitting and crevice corrosion at seawater flanges, with mating nuts supplied for a complete connection.
Super Duplex 2507 stud bolts
Combine high strength and chloride resistance at seawater-pump and heat-exchanger covers.
Monel K-500 double-end studs
Carry high bonnet loads in marine valves, with separately sized threads for the valve body and mating nut.
Monel 400 hex bolts
Resist corrosion in flowing-seawater cooling lines while keeping the bolt head accessible for maintenance.
Marine pump & flange kits.
Bring the mating parts for each seawater connection into one order, ready to identify by pump, valve or flange tag.
Marine fastener traceability & inspection.
EN 10204 3.1 MTC, heat traceability and mechanical test results with each shipment. Add the tests your equipment specification requires.
Material & condition
Match heat identity, chemistry and the specified delivery or heat-treatment condition to the order.
Alloy 625 · sample material certificateMechanical testing
Tensile properties, Rockwell hardness, impact or metallurgical testing to the specified method and acceptance criteria.
Alloy 718 · sample laboratory reportDimensions & threads
Drawing dimensions, thread gauging and part identification included in the inspection scope.
Alloy 400 · sample inspection certificatePMI & third-party testing
PMI and testing by CNAS-accredited laboratories can be included. Specify sampling, test methods and any witness points.
Sample reports show previous test records. Your order documents identify the actual supplied material and batch.
Before you order marine fasteners.
Have a drawing or a demanding project specification? Our sales team can work through the material, manufacturing and inspection requirements with you.
Get a QuoteIs 316L always enough for seawater?
No. Warm, stagnant or creviced seawater can require a different alloy. Specify the exposure before replacing a 625, Monel or duplex component with stainless steel.
Can K-500 be used under cathodic protection?
That requires a review of the material condition, load and protection system. Send the project limits and required qualification; an alloy name is not approval for every subsea joint.
Can the nuts use a different alloy from the studs?
Yes, if the connection specification permits it. Define nut strength, thread fit, corrosion compatibility and the assembly lubricant as one system.
Which inspection records can be included?
Each shipment includes an EN 10204 3.1 MTC, heat traceability and mechanical test results. Add PMI, hardness, impact, metallography, dimensional records or third-party tests to the enquiry with the method, sampling and acceptance criteria.
Guides for marine fastener selection.
Resolve the questions behind your marine bolting specification.
Nickel alloy fasteners in seawater: what to check before choosing a grade
Identify exposure, sheltered crevices, connected metals and the load your joint must retain.
Read the guide02SUBSEA SPECIFICATIONCathodic protection: what must the marine bolting enquiry include?
Send the protection conditions with the bolting callout so material condition, strength and inspection can be reviewed together.
Start with the connection, not just the alloy name
A submerged valve bonnet and a splash-zone support can need different bolting even on the same project. Mark the fastener location, wetted surfaces, mating metals and whether it is electrically connected to the cathodic-protection system. Include seawater temperature, design load and the specified service life.
Put the protection envelope in the RFQ
Attach the project cathodic-protection specification. Give the specified potential range and reference electrode, together with operating and commissioning conditions. The potential value without its reference electrode is incomplete. Identify any coating, insulation or electrical-continuity requirements at the joint.
Connect final condition to acceptance
Age-hardened K-500 and high-strength duplex bolting need a review of hydrogen-related cracking under cathodic protection. Specify final heat treatment, strength, hardness and any project qualification testing as one package. Where the purchase order invokes API 20F with IOGP S-725, send the applicable edition and supplementary requirements so the proposed manufacturing and test route can be quoted against them.
Build one traceable supply package
List studs, nuts and washers separately with grades, quantities and drawing revisions. Request links from the material heat number to each finished heat-treatment batch and test record. BYBOLT can work from your project specification to prepare the component list, processing route and inspection scope together.
References
- IOGP · S-725 subsea bolting, March 2021
- DNV · Duplex steel under cathodic protection
- Gangloff et al. · Hydrogen-assisted cracking of K-500 (abstract)
03ALLOY COMPARISON625 vs 2507 for seawater flange bolting
Compare corrosion exposure and the required joint strength first, then price the same stud, nut and inspection package in both grades.
Where each grade earns its place
Alloy 625 brings strong resistance to seawater pitting and crevice corrosion. Super Duplex 2507 combines chloride-corrosion resistance with high strength and a lower raw-material budget band. The useful comparison is the complete flange connection: the wetted surfaces, crevices under nuts and washers, mating flange material and the load that the assembly must retain.
Define seawater at the actual joint
Give the operating and shutdown temperatures, water movement, chlorination and likely deposits. A freely washed external bolt and a threaded crevice holding warm, stagnant seawater are different selection cases. Add cathodic-protection conditions for submerged assemblies. These details let the material review address the connection rather than a general marine label.
Compare strength on the same basis
Alleima lists a minimum 0.2% proof strength of 550 MPa for solution-annealed and quenched SAF 2507 bar at 20°C. This is a bar reference; the purchase specification must establish the required finished-stud and nut properties. Alloy 625 gains strength through solid-solution strengthening and does not need precipitation aging for that mechanism. Ask for the proposed delivery condition and finished-fastener test requirements in both quotations.
Send an equivalent package for pricing
Use the same diameter, overall length, thread series and class, nut pattern, washer requirements and quantity. Include coating or lubricant, inspection and packing by flange tag. Compare delivered cost alongside acceptance records. BYBOLT can quote both routes against one bolting list so the commercial comparison stays tied to the same connection requirements.
References
Get a Quote04VALVE BOLTINGMonel 400 vs K-500 for marine valve connections
Choose between 400 and K-500 around the required load and final condition, then specify the valve connection as a complete assembly.
Strength is the central distinction
Both are nickel-copper alloys. K-500 uses aluminum and titanium additions with aging to achieve greater strength and hardness than 400. For a loaded bonnet stud, that distinction makes the heat-treatment condition a purchasing requirement. For Monel 400, state the required delivery condition and properties rather than assuming every form has the same strength.
Separate flowing and sheltered seawater
Monel 400 is used for its resistance to flowing seawater. Include shutdowns, stagnant pockets and deposits in the enquiry because the local exposure can differ from the main flow. For K-500 in cathodically protected service, supply the protection conditions and the project material requirements alongside the final strength and hardness.
Specify both ends of a bonnet stud
For double-end studs, give overall length, grip length and the thread length, pitch and fit at each end. Identify the end entering the tapped body and the nut end. Include body material, engagement depth, nut grade and condition, surface finish and the installation specification. This helps the supplier check fit and material pairing as well as the stud itself.
Ask for finished-condition records
Make the raw-material heat traceable through the finished heat-treatment batches and inspection records. Where ASTM F468 or F467 is specified, identify the applicable fastener type, unit system and edition; metric requirements should be stated explicitly. BYBOLT can quote the studs, mating nuts and requested tests together against your valve drawing.
References
- Special Metals · Alloy K-500
- Special Metals · Marine engineering
- BYBOLT · K-500 fastener specification
Have a
bolting list?
Upload your bolting list, BOM or flange schedule. We review every line — material, standard, size and documents — and quote the full package.
- 01One list, all mating parts
Studs, nuts and drawing-specified washers quoted together.
- 02Built to your requirements
Custom dimensions, heat treatment and project-specific inspection.
- 03Traceable through delivery
Material heat and finished heat-treatment batch records linked to your parts.
