Standards
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Incoloy 925 Washers (UNS N09925)
Flat / spring lock / Belleville / structural F436 — load distribution + retention for nickel-alloy bolted joints
Incoloy 925 washers (UNS N09925) are the load-distribution and vibration-retention companion to stud bolts, hex bolts, heavy hex bolts and nuts in API 6A wellhead, Christmas tree, BOP, pressure-vessel and structural bolted joints. Washers serve four distinct functions depending on type: (1) FLAT washers distribute the bolt-clamp load across the joint surface to prevent crushing of the underlying material; (2) STRUCTURAL washers (F436 equivalent) provide thicker, harder bearing surface for high-preload structural bolting; (3) SPRING / LOCK washers provide vibration-loosening retention by storing elastic energy under bolt preload (where allowed by code); (4) BELLEVILLE / DISC washers provide spring-loaded preload for fatigue-critical joints with thermal cycling. TorqBolt manufactures Incoloy 925 washers in M3 to M64 (1/8" to 4") sizes ASME B18.22.1 (flat), ASME B18.21.1 (spring lock), DIN 6796 (Belleville), ASTM F436 equivalent (structural), in the solution-annealed and aged condition (per heat-treatment specification) with hardness controlled within the NACE MR0175 35 HRC sour-service cap.
Need Incoloy 925 washers? Email info@torqbolt.com with bolt size, washer type (flat / spring / Belleville / structural / DTI), quantity, applicable standards (NACE MR0175 stamp). WhatsApp +91-22-66157017.
Washer Variants Manufactured
| Variant | Description | Standard | Function |
|---|---|---|---|
| Flat washer (Type A plain) | Punched / machined flat ring; centered-hole + parallel faces | ASME B18.22.1 / ISO 7089 | Load distribution under nut / bolt head |
| Flat washer (Type B narrow) | Smaller OD than Type A; for tight-clearance joints | ASME B18.22.1 / ISO 7090 | Load distribution where joint OD is restricted |
| Heavy structural washer (F436 equivalent) | Thicker, larger OD, hardness-controlled bearing surface | ASTM F436 (in CRA) / EN 14399-6 | High-preload structural bolting; pretensioned joints |
| Spring lock washer (Type A helical) | Single-coil split helical spring; bears on both nut + joint | ASME B18.21.1 (regular) / DIN 127B | Vibration-loosening prevention (where code allows) |
| Internal tooth lock washer | Internal radial teeth dig into nut bearing surface | ASME B18.21.1 / DIN 6797 | Anti-rotation lock; small-diameter bolting |
| External tooth lock washer | External radial teeth dig into joint bearing surface | ASME B18.21.1 | Anti-rotation lock; visible tooth pattern |
| Belleville (disc) washer | Conical spring washer; provides controlled-spring preload | DIN 6796 / DIN 2093 | Spring-loaded preload; thermal-cycling joints; bolt fatigue mitigation |
| DTI washer (direct tension indicator) | Bumps on bearing face that flatten under controlled load | ASTM F959 / EN 14399-9 | Visual preload verification in pretensioned structural joints |
| Square washer | Square OD with circular bore; for square-head bolting | ASME B18.22.1 | Square-head bolt + nut load distribution; structural timber-bolting |
| Sealing washer (bonded) | Metal washer with bonded elastomer / PTFE sealing element | EN 16983 / customer | Pressure-sealing service; instrument fitting bolting |
Standard Size Range & Dimensions (Flat Type A)
| Bolt Size | Inside Diameter (ID) | Outside Diameter (OD) | Thickness (Type A flat) | Structural F436 Thickness |
|---|---|---|---|---|
| 1/4" / M6 | 9/32" / 6.4 mm | 5/8" / 12.5 mm | 0.065" / 1.6 mm | |
| 5/16" / M8 | 11/32" / 8.4 mm | 11/16" / 17 mm | 0.065" / 2 mm | |
| 3/8" / M10 | 13/32" / 10.5 mm | 13/16" / 21 mm | 0.065" / 2.5 mm | |
| 7/16" / M12 | 15/32" / 13 mm | 59/64" / 24 mm | 0.083" / 2.5 mm | |
| 1/2" / M14 | 17/32" / 15 mm | 1-1/16" / 28 mm | 0.083" / 2.5 mm | 0.122" / 3 mm |
| 5/8" / M16 | 11/16" / 17 mm | 1-5/16" / 30 mm | 0.095" / 3 mm | 0.122" / 3 mm |
| 3/4" / M20 | 13/16" / 21 mm | 1-1/2" / 37 mm | 0.134" / 3 mm | 0.136" / 4 mm |
| 7/8" / M22 | 15/16" / 23 mm | 1-3/4" / 39 mm | 0.134" / 3 mm | 0.177" / 4 mm |
| 1" / M24 | 1-1/16" / 25 mm | 2" / 44 mm | 0.134" / 4 mm | 0.177" / 4 mm |
| 1-1/8" / M30 | 1-3/16" / 31 mm | 2-1/4" / 56 mm | 0.134" / 4 mm | 0.177" / 5 mm |
| 1-1/4" / M33 | 1-5/16" / 34 mm | 2-1/2" / 60 mm | 0.165" / 5 mm | 0.177" / 5 mm |
| 1-3/8" / M36 | 1-7/16" / 37 mm | 2-3/4" / 66 mm | 0.165" / 5 mm | 0.177" / 5 mm |
| 1-1/2" / M39 | 1-9/16" / 40 mm | 3" / 72 mm | 0.165" / 6 mm | 0.177" / 6 mm |
| 1-3/4" / M45 | 1-13/16" / 46 mm | 3-1/2" / 85 mm | 0.180" / 6 mm | 0.177" / 6 mm |
| 2" / M48 | 2-1/8" / 50 mm | 4" / 92 mm | 0.180" / 6 mm | 0.177" / 8 mm |
| 2-1/2" / M56 | 2-5/8" / 58 mm | 5" / 105 mm | 0.220" / 8 mm | |
| 3" / M64 | 3-1/8" / 66 mm | 5-1/2" / 115 mm | 0.236" / 8 mm |
Mechanical Property Requirements
| Property | Aged Condition (ASTM B805 min) |
|---|---|
| Tensile Strength | 165 ksi (1140 MPa) min |
| 0.2% Yield Strength | 110 ksi (760 MPa) min |
| Hardness (flat washers) | 32–35 HRC for sour service (NACE cap) |
| Hardness (F436 structural) | 38–45 HRC (per F436); for sour service controlled to 35 HRC max |
| Hardness (Belleville / spring) | 40–50 HRC for spring function (sour-service review required, specify NACE compliance need) |
| Charpy V-notch at -46°C | Not normally required for washers; available on special order |
Spring-function washers (Belleville, lock): Higher hardness gives the spring rate, for sour service, the design must trade spring force vs hardness cap. For most sour applications, flat / structural washers are preferred; spring functions handled by stud-bolt elasticity instead.
Applicable Standards
- ASTM B805, UNS N09925 forging stock (input material)
- ASTM B983, UNS N09925 plate / sheet (alternate input for flat washer blanking)
- ASME B18.22.1, Plain Washers (inch series)
- ASME B18.21.1, Lock Washers (inch series)
- ISO 7089 / 7090, Plain Washers (metric, regular / narrow)
- ISO 7091 / 7093, Plain Washers (metric, large / extra-large series)
- DIN 125 / DIN 9021, German plain washer standards (regular / large)
- DIN 127B, Spring lock washers (German pattern)
- DIN 6796 / DIN 2093, Belleville / disc spring washers
- DIN 6797, Internal / external tooth lock washers
- ASTM F436, High-Strength Hardened Washers (structural; CRA equivalent)
- ASTM F959, DTI (Direct Tension Indicator) washers
- EN 14399-6 / -9, European structural HV system washer + DTI
- API 6A, Wellhead and Tree Equipment
- NACE MR0175 / ISO 15156-3, sour-service Table A.10
- EN 10204 3.1 / 3.2, certification
API 6A PSL Documentation Levels
| PSL | Pressure Class | Documentation |
|---|---|---|
| PSL-1 | 2,000–5,000 psi | Visual + chemistry |
| PSL-2 | 5,000–10,000 psi | + sample mech |
| PSL-3 | 10,000–15,000 psi | + 100% hardness |
| PSL-3G | 10,000–15,000 psi gas | + low-temp Charpy on sample |
| PSL-4 | 15,000–20,000 psi | + TPI witness on heat-treatment + final inspection |
Manufacturing Sequence
- Plate / sheet input: Incoloy 925 plate (per ASTM B983) in annealed condition for flat washers; bar input (per ASTM B805) for machined / Belleville washers.
- Blanking (flat washers): Punched or laser-cut from plate to OD + ID dimension. For thicker structural washers, machine-turned from bar stock.
- Forming (Belleville / spring): Cold-forming on a press die to give the conical / helical profile. Belleville washers require precision die set per DIN 6796 / 2093 height-vs-load curve.
- Internal / external teeth (lock washers): Stamped / coined teeth on punched flat blank.
- Solution anneal + age cycle: Per heat-treatment specification. For Belleville / spring washers, the age cycle hardens the alloy for spring function while keeping flat-washer hardness within sour-service NACE 35 HRC cap.
- Final inspection: ID + OD + thickness dimensional check per applicable standard; flatness check (flat washers); spring-rate test (Belleville per DIN 6796); 100% Rockwell hardness; sample tensile.
- Marking: Heat number + grade + standard reference on packaging (washers too small for individual stamping).
Companion Components
- Stud bolts + heavy hex nuts, primary mating fasteners
- Hex bolts + hex nuts, standard fastener mating
- Heavy hex bolts, structural bolting for F436 washer service
- Cotter pins for slotted nut + flat washer locking arrangement
- Thread compound on bolt, sulphur-free, copper-free anti-seize
Application Examples
- API 6A wellhead bolting (flat washer under nut bearing face): Standard load-distribution practice; one washer per nut
- Compact-flange bolting (F436 structural washer): Higher preload requires hardness-controlled washer to prevent embedment
- Pretensioned structural-steel joints (DTI washer): Visual verification of preload reaching design value
- Subsea connector bolting (F436 + flat washer stack): Subsea pressure containment with verified preload
- Bellow seal bolt joints (Belleville stack): Maintains preload through thermal cycling
- Pump baseplate alignment (flat washer + shim): Sour-service rotating-machinery foundation
- Heat exchanger flange bolting (flat washer under nut): Pressure-vessel + sour-service exchanger bonnet retention
- Vibration-prone instrument mounting (spring lock or DIN 127B): Where vibration-loosening is the design risk
- Slotted-nut pin retention (flat washer + cotter pin): Wheel hubs, axle ends, drilled-shank-bolt locking
Standard Coatings & Finishes
- Bare (passivated): Per ASTM A380 / A967, standard sour-service finish
- PTFE coating: Xylan 1424 / 1070, reduces friction at the washer-nut bearing interface; matches PTFE-coated bolt finish
- Phosphate + oil: MIL-DTL-16232 + light oil for storage protection
- Cad plate / hot-dip galvanise: NOT applied for nickel-alloy CRA service
Frequently Asked Questions
Do I need a washer under both the bolt head and the nut? Yes for high-preload joints, both bearing surfaces benefit from load distribution. Standard practice in API 6A wellhead bolting is one washer per nut bearing face plus an additional washer under any bolt head that contacts the joint surface (stud bolts have nuts both sides, so 2 washers per stud).
What's the difference between Type A and Type B flat washers? Both ASME B18.22.1. Type A is the "regular" flat washer (larger OD); Type B is the "narrow" flat washer (smaller OD, fits in tight clearance). Most general fastening uses Type A; Type B for compact assemblies where joint clearance is tight.
What's an F436 washer? ASTM F436 is the structural-bolting hardened washer specification, thicker (typically 0.177" / 4 mm vs 0.083" for Type A flat) and harder (38–45 HRC) than a plain flat washer. Used in pretensioned structural-steel joints with A325 / A490 / F3125 bolts to prevent the washer from embedding into the joint material under high preload. The Incoloy 925 equivalent provides the same dimensional and hardness performance in CRA material; for sour service the hardness is controlled to 35 HRC max per NACE MR0175.
Can I use lock washers in sour service? Generally not recommended, the higher hardness required for spring function (40–50 HRC) exceeds the NACE 35 HRC sour-service cap. For sour-service vibration retention, alternatives include: jam-nut configuration (full nut + half-height jam nut), Belleville stack (hardness still high but isolated from the bolt by intervening flat washer), or thread-locking compound (Loctite 263 / 277 in nickel-alloy-compatible variants). Lock washers acceptable only for non-sour topside applications.
What's a DTI washer used for? ASTM F959 / EN 14399-9 Direct Tension Indicator washers have raised bumps on one bearing face that flatten progressively as preload develops in the bolt. The remaining gap (measured by feeler gauge) gives a direct visual indication of bolt preload. Used in pretensioned structural-steel joints where the preload must be verified rather than calculated from torque (torque is unreliable due to friction variability).
What's the typical lead time for Incoloy 925 washers? Standard flat washers (M6 to M30 / 1/4" to 1-1/8"): 4–6 weeks. Larger sizes / structural F436 / Belleville: 6–8 weeks. Custom DTI / square / sealing washers: 8–10 weeks. Belleville washers require precision die setup, which adds 2–3 weeks for new size on first order.
Documentation Package
- EN 10204 3.1 manufacturer's MTC (3.2 with TPI on request)
- Heat-lot chemistry vs UNS N09925 limits
- Sample tensile + hardness per heat / per heat-treatment lot
- 100% hardness inspection on structural (F436) and Belleville washers
- Heat-treatment furnace chart-recorder traces
- Dimensional check (ID, OD, thickness, flatness) per applicable standard
- Spring-rate test (Belleville per DIN 6796 height-vs-load curve)
- NACE MR0175 Table A.10 reference (sour-service orders)
- Heat-traceable packaging (no individual washer marking due to size)
Related Products
- Stud bolts, primary mating fastener
- Hex bolts, integral-head mating fastener
- Hex nuts + heavy hex nuts, mating nuts
- Threaded rods, for cut-to-length bolting with washers
- Full Incoloy 925 datasheet
Request a Quote
For Incoloy 925 washers with full MTC documentation:
- Email: info@torqbolt.com
- Phone: +91-22-66157017
- WhatsApp: +91-22-66157017
Specify washer type (flat Type A / B / structural F436 / Belleville / spring lock / DTI / sealing), bolt size or specific ID-OD-thickness dimensions, quantity, applicable standards (API 6A PSL level, NACE MR0175 stamp, NORSOK MDS A30, ASTM F436 / F959), required MTC level (3.1 / 3.2 with TPI witness) and any coating / finish requirement.
References: ASTM B805; ASTM B983; ASME B18.22.1 (plain washers inch); ASME B18.21.1 (lock washers inch); ISO 7089 / 7090 / 7091 / 7093 (metric flat washers); DIN 125 / 127B / 9021 / 6796 / 2093 / 6797 (German washer standards); ASTM F436 (structural hardened washer); ASTM F959 (DTI washer); EN 14399-6 / -9 (European HV system washer + DTI); API Specification 6A Annex M; NACE MR0175 / ISO 15156-3 Table A.10; NORSOK M-630 / M-650; EN 10204; ASTM A380 / A967. Special Metals Corporation INCOLOY® alloy 925 Technical Bulletin. INCOLOY® is a registered trademark of Special Metals Corporation.
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