RFQ Today
Certifications available: EN 10204 3.1 / 3.2 MTRs, NACE MR0175 compliance, ASTM A193 B7 / B8 / B7M grades, hydrogen embrittlement relief, Third-Party Inspection (SGS / BV / DNV / Lloyds), and complete EPC export documentation packages.
Custom Bolt
& Stud
A world-class technical reference for EPC contractors, structural and piping engineers, procurement heads, TPI inspection agencies, and global project buyers specifying custom-length stud bolts, anchor bolts, foundation bolts, reactor stud assemblies, pressure vessel closure bolts, and non-standard fasteners across Oil & Gas, Power Generation, Petrochemical, Offshore, Nuclear, Heavy Civil, and Industrial Construction sectors.
Type Classification
& Design Principles
Custom bolts and studs are engineered fasteners manufactured to non-standard lengths, diameters, thread forms, or material grades that are not available from standard stock catalogues — produced specifically to meet project drawings, equipment vendor specifications, or unique structural and pressure-containment requirements that cannot be satisfied by off-the-shelf fasteners.
1.1 — Technical Definition and Scope
A custom bolt or stud is a threaded fastener that differs from standard catalogue items in one or more of the following dimensions or properties: overall length (non-standard), thread length (partial or full thread, non-standard engagement), thread form (UNC, UNF, metric M-series, ACME, Whitworth, NPT, or customer-specific), head geometry (hex, square, round, countersunk, dome, T-head, eye, J-hook, L-hook, or bespoke), material grade (ASTM A193 B7, B7M, B8M, ASTM A320 L7, ASTM A354 BD, Inconel, Duplex, or exotic alloy), surface treatment (hot-dip galvanised, PTFE/Xylan coated, Molykote, electroless nickel, thermal spray), or certification level (EN 10204 3.1/3.2, NACE MR0175, hydrogen embrittlement relief tested).
In EPC and heavy industrial projects, custom bolts and studs are required for: pressure vessel manway closures (extra-long stud bolts beyond catalogue range), reactor head stud assemblies (large diameter M100–M200 high-strength studs), structural anchor bolt packages (equipment foundations, pipe rack columns, vessel skirts), custom flange bolting (non-standard pressure class or facing combinations), offshore conductor and casing head fasteners (API 6A / 16A class), pipeline isolation joint anchor bolts, wind turbine foundation anchors, and any application where standard fastener dimensions or grades are specified but stock availability is unreliable for the project schedule.
RR Hydraulic manufactures custom bolts and studs under all applicable international standards with full EN 10204 3.1 / 3.2 material traceability and project documentation packages.
1.2 — Custom Fastener Type Classification
Stud Bolt (Fully Threaded / Partial Thread)
A cylindrical bar threaded on both ends (or full length) with no integral head — assembled with two heavy hex nuts. The standard fastener for ASME B16.5 flanged joint bolting per ASME B18.31.2 and ASME B16.5 Table 4. Custom stud bolt lengths are required when standard catalogue lengths (per ASME B16.5 bolting data) are unavailable in the required material grade (A193 B7, B7M, B8M), when the flange assembly includes non-standard spacers or insulation kits, or when extra engagement length is required for specialty flanges. Typically 2A thread class on both ends per ASME B1.1 (UNC) or per ASME B1.13M (metric).
Anchor Bolt (J-Bolt, L-Bolt, Headed)
A fastener embedded in concrete foundations — typically with a hooked lower end (J or L profile) or an anchor plate — to secure structural steel, equipment skids, vessel skirts, pipe rack columns, and machinery bases to civil foundations. Custom anchor bolts are specified on every EPC structural package — the embedment length, projection above the grout, hook geometry, and material grade are determined by the structural engineer for each equipment pad design. ASTM F1554 Grade 36, 55, or 105; ASTM A307 Grade A; or ASTM A193 B7 for high-strength equipment anchors.
Foundation Bolt / Holding-Down Bolt
Large-diameter (M36–M160 or 1½”–6″) bolts for securing rotating machinery (compressors, turbines, pumps) to their foundations — distinct from anchor bolts in that they pass through the equipment baseplate and engage cast-in sleeves or anchor plates in the concrete. Custom length is always required — determined by baseplate thickness plus grout pad depth plus nut engagement. ASTM A193 B7 or ISO 898-1 Grade 10.9 for high-strength holding-down bolts. Dual-nut design (upper and lower nuts with lock nut) is standard for rotating machinery foundations subject to vibration.
Reactor / Pressure Vessel Stud Assembly
Large-diameter (M80–M160 or 3″–6″) high-strength studs for pressure vessel closures — autoclave covers, reactor head flanges, and high-pressure separator manways. Custom manufactured to vessel builder’s drawing with specified thread form, stud length, thread engagement, and material grade per ASME VIII / PD 5500 / EN 13445 design calculation. Typically ASTM A193 B7 (chromium-molybdenum alloy steel, 4140 base) or B16 (high-strength alloy). Hydraulic stud tensioning (not torquing) is specified for large reactor stud assemblies — elongation-controlled tightening method.
Flange Stud Bolt (Custom Length)
Standard ASME B16.5 or B16.47 stud bolts manufactured in custom lengths for non-standard applications: extra length for insulation flange kits (cathodic protection isolation flanges add 50–100 mm per bolt); extra length for cryogenic cold box flange assemblies with thick insulation pads; or non-standard pressure class combinations not covered by ASME B16.5 bolt data tables. Custom flange stud bolts in ASTM A193 B7 (standard process) or B7M (sour NACE service) with ASTM A194 2H (standard) or 2HM (NACE) heavy hex nuts per ASME B16.5 material group.
Structural Bolt (Non-Standard Length / Grade)
High-strength structural bolts per ASTM A325, A490, or ISO 898-1 Grade 8.8 / 10.9 manufactured in non-standard lengths for structural connections where standard stock lengths are not available for the required grip length. Custom structural bolts are required on complex structural connections (splice plates, moment connections, crane runway brackets) where the standard ASME B18.2.1 length range does not include the required grip plus nut engagement length. Supplied with matching hardened washers and ASTM A563 or ISO 898-2 nuts per structural connection specification.
1.3 — Custom vs Standard Fastener: Engineering Selection Drivers
| Condition | Standard Stock | Custom Required | Engineering Driver |
|---|---|---|---|
| Length within catalogue range | Yes — stock A193 B7 | No | Use standard stock for cost and lead time |
| Non-standard length (outside catalogue) | No | Yes | Custom cut and thread to drawing |
| Insulation flange kit extra length | No | Yes | +50–100 mm to standard ASME B16.5 bolt length |
| Material: A193 B7 (standard) | Yes — most sizes | Conditional | Availability for large diameter (>2″) often requires custom |
| Material: A193 B7M (NACE sour) | Limited | Often yes | B7M heat treatment specific; not all sizes stocked |
| Material: A193 B8M (SS 316) | Limited | Often yes | Non-standard lengths; large diameter |
| Material: Duplex / Inconel / Titanium | No | Always yes | Exotic alloy — always custom manufacture |
| Reactor stud: M100+ / 4″+ | No | Always yes | Large diameter reactor studs are always custom forgings |
| Anchor bolt: J/L hook geometry | Limited | Often yes | Custom hook length and bend geometry per structural drawing |
| Special coating (PTFE, Xylan, HDG) | Limited | Often yes | Coating specification per project corrosion protection schedule |
| NACE + EN 10204 3.2 + TPI | No | Always yes | Certified supply with TPI always requires custom order |
1.4 — Bolted Joint Design and Preload Principles
Bolt Preload and Joint Integrity
The bolt clamps two flanges together with a controlled preload (bolt tension) that must exceed the combined hydrostatic end force plus the minimum gasket seating force under all operating conditions. Preload is achieved by torquing the nut to a calculated torque value (torque method) or by measuring bolt elongation (hydraulic tensioning, elongation method). For stud bolts on ASME B16.5 flanged joints, the torque method using calibrated torque wrenches is standard for NPS ≤ 4″ ASME B16.5 joints; hydraulic tensioning is used for large diameter (NPS 6″+) and high-pressure Class 900+ stud bolt assemblies.
Torque-Tension Relationship
Applied torque T converts to bolt tension F_bolt: T = K × d × F_bolt, where K is the nut factor (≈ 0.15 for lubricated threads; ≈ 0.20 for dry threads), d is the nominal bolt diameter, and F_bolt is the resulting bolt tension. Uncertainty in K (±30% for typical assembly conditions) means torque control alone provides bolt tension scatter of ±30%. For critical joints (Class 900+, high-pressure vessel closures), elongation-controlled tensioning with a dial micrometer or hydraulic tensioner eliminates this scatter — bolt elongation is directly proportional to bolt tension with ±5% accuracy.
Thread Engagement and Nut Stripping
Minimum nut engagement length to prevent thread stripping: L_engage = 1.0 × d for equal-strength bolt and nut material. For A193 B7 bolts with A194 2H heavy hex nuts (both alloy steel): engagement = 1.0d is adequate. For A193 B8/B8M (SS 316) bolts with SS nuts: engagement = 1.5d required due to the lower yield strength of SS. Custom stud bolt length must include: grip length (compressed joint thickness) + nut engagement at both ends. Under-length studs with insufficient nut engagement are a catastrophic failure mode — verify engagement per VDI 2230 bolt design calculation for all custom stud bolt applications.
Hydrogen Embrittlement (H₂E)
High-strength bolts and studs (above 1000 MPa tensile strength — approximately ASTM A193 B7 at 1035 MPa minimum) are susceptible to hydrogen embrittlement (H₂E) when electroplated — the acid pickling and electroplating process introduces atomic hydrogen into the steel lattice. H₂E causes delayed fracture under sustained load at stresses below the yield strength. Relief heat treatment (embrittlement relief bake) at 190–220°C for 3+ hours after electroplating is mandatory per ASTM F1941 / ISO 15330 for electroplated bolts above 1000 MPa. H₂E is not a risk for hot-dip galvanised bolts (galvanising is not an acid-based electroplating process).
Stress Relaxation at Elevated Temperature
At elevated service temperatures (above ~200°C for carbon steel; above ~300°C for Cr-Mo alloy), bolt preload decreases over time through stress relaxation — the bolt elongation remains constant but the stress reduces as the material creeps at the thread root. For high-temperature flanged joints (Class 600+ steam, hot oil, reformer piping), the bolt material and initial preload must be selected to maintain adequate residual bolt tension at the design temperature after 100,000+ hours of service. ASTM A193 B7 (Cr-Mo alloy) is specified precisely for its superior relaxation resistance compared to plain carbon steel bolts in elevated-temperature service.
Galvanic Corrosion in Fastener Assemblies
Dissimilar metals in contact in the presence of an electrolyte create a galvanic cell — the more active (anodic) metal corrodes preferentially. In fastener assemblies: carbon steel bolts in SS flanges (SS is cathodic — CS bolt corrodes); titanium bolts in CS flanges (CS is anodic — CS flange corrodes locally); zinc-plated bolts in aluminium structures (zinc is anodic — zinc corrodes). For SS, Duplex, or exotic alloy flanges: always specify the same alloy grade or a noble metal for the stud bolt material, not plain carbon steel. Match the fastener alloy to the flange alloy or step up to a more noble alloy — never step down.
1.5 — Stud Bolt Length Calculation
t_flange = Flange thickness (mm) — per ASME B16.5 table for NPS and class (one flange; × 2 for pair)
t_gasket = Compressed gasket thickness (mm) — 3.2 mm for spiral wound; 1.5 mm for ring gasket
t_washer = Hardened washer thickness (mm) — per ASME B18.22.1 or project spec (often zero for SW gasket flanges)
H_nut = Heavy hex nut height (mm) — per ASME B18.2.2 heavy hex; one at each end (× 2 total)
L_extra = Additional length (mm) — 2 threads protruding beyond nut face (minimum per ASME B16.5) + insulation kit allowance (if applicable)
Standard protrusion rule: ASME B16.5 requires minimum 2 thread pitches visible beyond the nut face after tightening.
For custom insulation flange kit bolts: add kit insert thickness (typically 25–50 mm per side) to L_stud above.
Flange thickness (Class 600, 6″): 57.2 mm each; gasket compressed: 3.2 mm (SW gasket)
Nut height (1″ UNC heavy hex A194 2H): 27.8 mm; insulation kit insert: 25 mm per side
L_stud = 2×57.2 + 3.2 + 2×27.8 + 2×25 + 6 (protrusion) = 207 mm → round up to 210 mm custom length
Standard ASME B16.5 bolt for 6″ Cl.600 = 165 mm — 45 mm shorter, not suitable with insulation kit.
Submit your drawing, dimensional spec, material grade, coating, and quantity for a documented RFQ within 24 hours.
Dimensional Reference
& Compliance Framework
Custom bolt and stud dimensions — diameter, length, thread form, thread class, head geometry, and under-head geometry — are governed by ASME B18.2.1 (hex bolts), ASME B18.31 (studs), ASME B1.1 (UNC/UNF threads), ISO 261 / ISO 965 (metric), and the project-specific drawing. All applicable standards are supported at RR Hydraulic.
Submit diameter, length, thread, material, coating, quantity to sales@rrhydraulics.com for a certified offer.
2.1 — ASTM A193 Stud Bolt Mechanical Properties Reference
| Grade | Base Material | UTS (MPa) | Yield 0.2% (MPa) | Elongation (%) | Hardness Max | Diameter Range | NACE | Application |
|---|---|---|---|---|---|---|---|---|
| B7 | Cr-Mo alloy (4140/4142) | 1035 | 862 | 16 | 35 HRC | ≤ 2½” | No | Standard EPC process flanges; ASME B16.5 |
| B7 | Cr-Mo alloy | 1000 | 896 | 16 | 35 HRC | 2½”–4″ | No | Large diameter stud bolts |
| B7M | Cr-Mo alloy (modified HT) | 862 | 724 | 18 | 26 HRC max | ≤ 2½” | Yes | NACE sour service flanges; H₂S environment |
| B8 | SS 304 (A2) | 517 | 207 | 30 | 223 HB | All sizes | Yes | Non-magnetic; moderate temp; food/pharma |
| B8M | SS 316 (A4) | 517 | 207 | 30 | 223 HB | All sizes | Yes | Marine; offshore; chloride; cryogenic |
| B8M Cl.2 | SS 316 strain-hardened | 758 | 551 | 20 | 35 HRC | ≤ ¾” | Cond. | Higher strength SS stud; small diameter |
| B16 | Cr-Mo-V alloy | 1000 | 862 | 18 | 35 HRC | All sizes | No | High-temp elevated service; power generation |
| B6 | 13Cr (410 SS) | 862 | 724 | 15 | 26 HRC | All sizes | Cond. | Moderate sour; valve stem bolting |
2.2 — ASTM A320 Low-Temperature Stud Bolt Grades
| Grade | Base Material | UTS (MPa) | Yield (MPa) | Test Temp (°C) | Min CVN Energy (J) | NACE | Application |
|---|---|---|---|---|---|---|---|
| L7 | Cr-Mo alloy (4140) | 862 | 724 | −73 | 20 | Cond. | LNG; cryogenic CS piping bolting; −46°C service |
| L7M | Cr-Mo alloy (modified) | 862 | 724 | −73 | 20 | Yes | NACE + low-temp combined service |
| L43 | Ni-Cr-Mo alloy (4340) | 862 | 724 | −73 | 20 | No | Very low-temp; LNG cryogenic bolting |
| B8/B8M | SS 304 / SS 316 | 517 | 207 | Down to −196°C | No CVN reqd | Yes | Austenitic SS: inherently impact-resistant at cryogenic |
2.3 — Anchor Bolt and Foundation Bolt Material Reference
| Standard | Grade | UTS (MPa) | Yield (MPa) | Weldable | Application |
|---|---|---|---|---|---|
| ASTM F1554 | Grade 36 | 400–550 | 248 min | Yes | General structural anchor bolts; pipe rack; buildings |
| ASTM F1554 | Grade 55 | 517–655 | 379 min | Yes (Supplement S1) | Medium-load anchors; equipment skids; vessels |
| ASTM F1554 | Grade 105 | 862 min | 724 min | No | High-load anchors; compressors; turbines; heavy lift |
| ASTM A307 | Grade A | 414–827 | — | Yes | General purpose; non-critical structural connections |
| ASTM A193 | B7 | 1035 min | 862 min | No | High-strength machinery anchor; rotating equip bases |
| ISO 898-1 | Grade 8.8 | 800 min | 640 min | No | Metric structural bolting per European codes |
| ISO 898-1 | Grade 10.9 | 1040 min | 940 min | No | High-strength metric structural; wind turbine anchors |
| ASTM A354 | BD | 1034 min | 896 min | No | High-strength hex bolt alternative to A490 |
2.4 — Applicable Standards and Compliance Framework
ASTM A193
Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications. The primary standard for EPC process piping and pressure vessel stud bolts. Grade B7 (Cr-Mo alloy, standard service), B7M (modified heat treatment, NACE sour service, ≤26 HRC), B8 (SS 304), B8M (SS 316), and B16 (Cr-Mo-V, elevated temperature) cover the full range of EPC flanged joint bolting requirements. Custom lengths, diameters, and thread forms are manufactured per A193 grade specifications with EN 10204 3.1 certification.
ASTM A194
Carbon and Alloy Steel Nuts for Bolts for High Pressure or High Temperature Service, or Both. Grade 2H (heavy hex nut for B7 stud bolts — standard service), 2HM (NACE-compliant for B7M sour service bolts — ≤22 HRC), 8M (SS 316 for B8M stud bolts). The nut grade must match the stud bolt grade for NACE compliance — mixing B7M studs with non-2HM nuts creates a NACE non-conformance. Custom nut heights or bearing face geometries are manufactured per ASME B18.2.2 heavy hex dimensions.
ASME B18.2.1 / B18.31
ASME B18.2.1: Square, Hex, Heavy Hex, and Askew Head Bolts and Hex, Heavy Hex, Hex Flange, Lobed Head, and Lag Screws (Inch Series). B18.31: Studs and Continuous Studs. Governing dimensional standards for head geometry, body tolerances, thread runout, and under-head radius. Custom bolts and studs are dimensionally specified per B18.2.1 or B18.31 as appropriate — all dimensions except the non-standard length or diameter are manufactured to the applicable ASME B18 table value for the nominal size.
ASME B1.1 / ISO 261
ASME B1.1: Unified Inch Screw Threads (UNC/UNF). Defines thread form, pitch, class of fit (1A/2A/3A external; 1B/2B/3B internal), and tolerance for all inch-series fasteners. ISO 261 / ISO 965: Metric ISO General Purpose Threads — M-series thread form, pitch, tolerance class (6g external; 6H internal). Custom bolts and studs are threaded per the specified standard — 2A/2B for standard process piping (ASME B16.5 flanged joints); 3A/3B for close-tolerance pressure vessel stud assemblies. Thread form must be specified explicitly on the purchase order.
NACE MR0175 / ISO 15156
For stud bolts and anchor bolts in H₂S sour environments: ASTM A193 B7M (≤26 HRC — mandatory, not just recommended); ASTM A194 2HM nuts (≤22 HRC). Plain B7 (35 HRC) is NOT acceptable in sour service. Custom stud bolt lengths in B7M must be heat treated (quench + temper) to the B7M hardness limit — not simply cut from B7 bar stock. Individual hardness testing per lot with EN 10204 3.1 certification is mandatory. NACE non-compliance on pressure vessel stud assemblies in sour service is a safety-critical issue.
ASTM F1554
Standard Specification for Anchor Bolts, Steel, 36, 55, and 105-ksi Yield Strength. Covers all grades of weldable (36 and 55 Grade) and high-strength (105 Grade) anchor bolts for structural foundations. Custom configurations per ASTM F1554: J-bolt (hooked end), L-bolt, headed bolt, and plate-anchor bolt — all manufactured to the dimensions shown on the project civil/structural drawing. Supplement S1 certifies weldability of Grade 55 anchor bolts for applications where the anchor bolt is field-welded to a baseplate during installation.
VDI 2230
Systematic Calculation of High Duty Bolted Joints — German engineering standard that is the global reference method for pressure vessel and flanged joint bolted joint design calculation. VDI 2230 defines the method for calculating required bolt preload, minimum bolt diameter for the joint, fatigue load at the thread root under cyclic loading, and required tightening method. For critical custom stud bolt assemblies (reactor heads, high-pressure vessels, engine mounts), VDI 2230 calculation is required to verify the stud bolt design before manufacture.
ISO 898-1 / ISO 898-2
Mechanical properties of fasteners made of carbon steel and alloy steel. Part 1: Bolts, screws and studs. Part 2: Nuts with specified proof load values. ISO 898-1 Grade 8.8 and 10.9 are the metric equivalents of ASTM A325 and A490/A354 BD. Custom metric bolts and studs for European EPC projects, offshore modules, and wind turbine structures are manufactured per ISO 898-1 with ISO 898-2 matching nut grades. EN 10204 3.1 MTCs per ISO 898-1 property class are the standard certification for all metric custom bolts in EPC projects.
Surface Treatment
& Manufacturing Process
Custom bolt and stud material is selected to match the service environment — pressure, temperature, corrosion, and NACE requirements — of the connected flanged joint or structural assembly. RR Hydraulic manufactures custom fasteners in all standard and exotic grades with full EN 10204 3.1 / 3.2 traceability and post-manufacture coating per project corrosion protection schedules.
3.1 — Material Grade and Corrosion Resistance Reference
| Material / Grade | H₂S Sour | Seawater / Cl⁻ | High Temp >350°C | Cryogenic <−46°C | Acids | Typical Application |
|---|---|---|---|---|---|---|
| A193 B7 (4140 Cr-Mo) | No (too hard) | Poor | Good to 450°C | Not suitable | Poor | Standard EPC process flanges — non-sour |
| A193 B7M (Cr-Mo, modified HT) | Yes (≤26 HRC) | Poor | Good to 400°C | Not suitable | Poor | NACE sour service flanges; H₂S pipework |
| A320 L7 (4140 low-temp) | Cond. | Poor | Limited | Good to −73°C | Poor | LNG utility; cryogenic piping flanges |
| A193 B8M (SS 316) | Good | Fair | Good to 400°C | Excellent | Good | Marine; offshore; chemical; cryogenic |
| Duplex (UNS S31803) | Very Good | Very Good | Limited >300°C | Good to −50°C | Very Good | Offshore seawater; sour+Cl⁻ combined |
| Super Duplex (S32750) | Excellent | Excellent | Limited >260°C | Good to −50°C | Excellent | Seawater injection; extreme chloride |
| Inconel 718 / 625 | Excellent | Excellent | Excellent to 700°C+ | Excellent | Excellent | Reactor studs; extreme corrosion + temp |
| Titanium Grade 5 (Ti-6Al-4V) | Excellent | Excellent | Good to 300°C | Good | Excellent | Offshore lightweight; subsea; titanium flanges |
| A193 B16 (Cr-Mo-V) | Poor | Poor | Excellent to 540°C | Not suitable | Poor | Power boilers; superheated steam; HRSG |
| F1554 Grade 55 (carbon steel) | No | Poor (HDG) | Limited | Not suitable | Poor | Structural anchor bolts; equipment bases |
3.2 — Surface Treatment and Coating Options
Hot-Dip Galvanising (HDG)
Zinc coating applied by immersion in molten zinc at ~450°C. Coating thickness 45–85 µm per ISO 1461. Provides excellent atmospheric and weathering corrosion protection for structural anchor bolts and pipe support bolts. Threads must be tapped oversize after galvanising to maintain thread engagement. HDG is NOT an electroplating process — no hydrogen embrittlement risk. Recommended for all above-ground structural bolt packages in marine or atmospheric corrosion environments. Not suitable for buried or immersed service without additional protection.
PTFE / Xylan Coating
PTFE (polytetrafluoroethylene) or Xylan fluoropolymer coating applied as a thin film (15–25 µm) to the bolt body and threads. Provides: dry-film lubrication (reduces friction coefficient at thread-nut interface, reduces torque-tension scatter to ±15% vs ±30% for dry threads), chemical resistance (acids, alkalis, solvents), and corrosion protection equivalent to HDG for mild atmospheric environments. Mandatory for SS stud bolts to prevent thread galling. Xylan coating is colour-coded by grade — blue (B7), green (B8M), yellow (B7M NACE) — for on-site grade identification.
Electroless Nickel Plating (ENP)
Chemical nickel deposition without electrical current — uniform coating on all surfaces including threads, recesses, and blind holes. Coating thickness 12–75 µm per ASTM B733. Provides excellent corrosion resistance (saltwater, acid, alkali), wear resistance, and dimensional accuracy (no rack marks or edge buildup as in electroplating). H₂E relief bake at 190°C for 3+ hours required after ENP on fasteners above 1000 MPa tensile strength per ASTM B733 Supplementary Requirement S1. Used for offshore subsea bolts and subsea wellhead stud assemblies requiring corrosion protection without increased thread diameter.
Molykote / MoS₂ Dry Film Lubricant
Molybdenum disulphide (MoS₂) dry film lubricant applied to bolt threads and bearing surfaces. Reduces friction coefficient to 0.04–0.08 (vs 0.15–0.20 for clean dry threads) — provides the most accurate and repeatable torque-tension relationship available for threaded fasteners. Mandatory for large-diameter reactor stud assemblies (where even small torque-tension variability causes unacceptable preload scatter), for high-temperature flanged joints (prevents thread seizure at operating temperature), and for all stud bolt assemblies specified to be installed and removed multiple times during the plant life.
Zinc-Nickel Electroplate
Electroplated zinc-nickel alloy coating (12–15% Ni) providing superior corrosion protection vs standard zinc electroplate — 1000+ hours salt spray resistance per ISO 9227 vs 200–400 hours for plain zinc electroplate. Maintains hydrogen embrittlement risk for high-strength fasteners — H₂E relief bake mandatory. Used for automotive and aerospace fasteners; also specified on some offshore and marine structural bolts where a thin, uniform, high-performance coating is required without the dimensional increase of HDG. Less common in EPC process piping than PTFE/Xylan or HDG.
Thermal Spray (Arc Spray / Flame Spray Zinc or Aluminium)
Wire-arc spray or flame spray of zinc or aluminium onto the bolt body — provides thick (75–150 µm) sacrificial metallic coating with excellent adhesion and no H₂E risk (non-electroplating process). Used on large structural anchor bolts and foundation bolts for marine splash zone exposure, where HDG galvanising cannot achieve the required coating thickness, or where the size of the bolt exceeds the galvanising bath capability. Thread areas are masked before spraying and the threads are chased after spraying. Commonly specified for offshore jacket leg anchor bolts and marine structure foundation fasteners.
3.3 — Manufacturing Process
3.3.1 — Bar Stock Selection and Cut-to-Length
Custom bolts and studs begin with certified bar stock of the specified ASTM grade — A193 B7 / B7M Cr-Mo alloy bar per ASTM A193 or ISO 898-1 alloy steel bar. The bar is cut to the required blank length per the stud bolt or bolt length drawing, allowing for thread run-out, head height (where applicable), and machining allowance. For large-diameter stud bolts (above 2″ / M52), billets are individually forged from the bar stock to refine the grain structure before thread cutting.
3.3.2 — Heat Treatment and Hardness Verification
- A193 B7: Quench and temper (Q+T) to achieve UTS ≥ 1035 MPa and ≤ 35 HRC — typically 880°C austenitise, oil quench, 600°C temper; hardness verified on each length
- A193 B7M: Q+T to ≤ 26 HRC — higher temper temperature (650–700°C) reduces hardness to NACE limit; every piece hardness-tested; documented on individual bar certificate
- A193 B8M / B8: Solution annealed ≥1040°C; no additional HT required; Class 2 (strain-hardened) additional cold working after annealing
- A320 L7: Q+T + Charpy impact test at −73°C; individual lot CVN certificate
- A193 B16: Q+T to specific temper temperature per B16 grade requirements; elevated-temperature properties verified by stress rupture test on batch samples
3.3.3 — Thread Cutting and Inspection
- Thread rolling (preferred): Cold thread rolling after heat treatment — rolled threads have higher fatigue strength than cut threads due to compressive residual stress at thread root; mandatory for high-cycle and pressure vessel stud bolt applications
- Thread cutting: CNC thread turning for large-diameter (M80+) studs where rolling tooling is unavailable; thread form and pitch verified by thread ring gauge (Go/No-Go) on 100% of custom studs
- Thread gauge: 2A Go/No-Go ring gauge for all UNC/UNF threads per ASME B1.1; 6g Go/No-Go ring gauge for all metric threads per ISO 965; gauge calibration certificates maintained with QMS records
- Thread length verification: Full-thread and partial-thread lengths verified by calliper measurement against the drawing dimension; both ends of stud bolts verified independently
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from raw bar stock to final coated and packed shipment on all custom bolt and stud orders. Dimensional inspection, thread gauging, hardness (100% for NACE B7M), mechanical testing, EN 10204 3.1 / 3.2 MTRs, and complete EPC export documentation packages are standard on all project custom fastener supply.
4.1 — Inspection & QC Protocol
4.2 — EN 10204 Material Test Certificate Requirements
| Certificate | Content | Signatory | EPC Requirement | When Mandatory |
|---|---|---|---|---|
| 2.1 / 2.2 | Declaration / non-specific | Manufacturer | Not acceptable for pressure service or structural | Not acceptable for ASME B16.5 or structural EPC fasteners |
| 3.1 | Bar heat-traceable mech + chem | Manufacturer’s authorised QC | Minimum for all EPC custom fasteners | All custom bolts and studs for pressure and structural service |
| 3.2 | 3.1 + TPI countersign | Manufacturer + SGS / BV / DNV / Lloyds | NACE B7M; cryogenic L7; reactor studs; offshore | Sour service B7M; A320 L7; pressure vessel reactors; offshore |
4.3 — Applications by Industry
Insulation Flange Kits — Extra-Length Stud Bolts
The highest-volume custom bolt application in EPC: A193 B7 (non-sour) or B7M (NACE sour) stud bolts in custom lengths to accommodate cathodic protection isolation flange kits. Standard ASME B16.5 stud bolt lengths are 30–100 mm too short when an insulation kit (full-face isolating gasket, isolating sleeves, and isolating washers) is installed between the flanges. Every isolation flange connection in a pipeline system requires custom-length stud bolts — typically 1–3 sizes longer than the standard bolting table for the NPS and pressure class. RR Hydraulic supplies matched sets of custom stud bolts + A194 2H/2HM heavy hex nuts per isolation flange kit drawing.
Reactor Head Stud Assemblies
Large-diameter (M80–M200 or 3″–8″) ASTM A193 B7 or B16 Cr-Mo alloy stud bolts for pressure vessel reactor head closures — high-pressure hydrotreater, hydrocracker, and ammonia synthesis converter closures. Custom length per vessel fabricator’s drawing. Thread form: UNC Class 3A (external) for tightest tolerance fit in the hydraulic tensioner. Surface treatment: Molykote MoS₂ dry film lubrication on threads and spherical nut-bearing surfaces. Supplied complete as a stud assembly: stud + spherical heavy hex nut + hardened washer + hydraulic tensioner engagement thread extension. EN 10204 3.2 with TPI mandatory.
Equipment Foundation Anchor Bolt Packages
ASTM F1554 Grade 36, 55, or 105 J-bolts, L-bolts, or headed anchor bolts for pump, compressor, turbine, vessel, and column foundations — custom length per structural engineer’s civil drawing. Supplied as complete sets per equipment: anchor bolts + levelling nuts + top nuts + lock nuts + washers + template (where specified). HDG coating per project corrosion schedule for above-ground service; epoxy or fusion-bonded epoxy (FBE) coating for embedded service. Pre-installation inspection report confirming anchor bolt set dimensions before concrete pour is standard for critical rotating machinery foundations.
Offshore Subsea and Wellhead Fasteners
Duplex UNS S31803, Super Duplex S32750, or Inconel 625 stud bolts and hex bolts for offshore subsea wellhead connections (API 6A / 16A class), subsea tree bolting, and riser connector bolting. All with EN 10204 3.2 and DNV or Lloyds TPI; PMI on 100% of pieces. Electroless nickel plating (ENP) for subsea service — provides corrosion protection without dimensional impact on the precision thread form required for subsea connector engagement. H₂E relief bake mandatory after ENP for Duplex or high-strength alloy custom studs above 1000 MPa.
LNG and Cryogenic Flange Bolting
ASTM A320 L7 (Cr-Mo, Charpy tested at −73°C) or A193 B8M (SS 316, inherently cryogenic) stud bolts for LNG process and utility flanged connections at temperatures to −165°C. Custom lengths for cryogenic cold box flanges with thick insulation pads. A320 L7: individual Charpy test certificate per lot (not lot average). A193 B8M: solution-annealed, no Charpy test required. PTFE Xylan coating on A320 L7 studs for cryogenic service — prevents thread seizure at low temperature where thread lubrication stiffens. EN 10204 3.2 with TPI for all LNG cryogenic stud bolt supply.
Wind Turbine and Renewable Energy Foundation Anchors
ISO 898-1 Grade 10.9 or ASTM F1554 Grade 105 custom anchor bolt cages (typically 60–120 bolts per turbine foundation, M36–M72 diameter, 2–4 m long) for wind turbine tower foundations. Thermal spray zinc or HDG coating for long-term atmospheric protection. Bespoke anchor cage templates and installation spacers are manufactured alongside the anchor bolts as a complete foundation package. Full lot mechanical testing per ISO 898-1 or ASTM F1554, including proof load and minimum breaking load. EN 10204 3.1 with individual batch traceability from steel bar heat number.
4.4 — Export Packaging Specification
- Custom stud bolts bundled per size (diameter + length) and grade in polybag sets with individual count labels — matched sets for ASME B16.5 flanges (per-joint bolt packs) are the preferred EPC supply format: each polybag contains the exact stud bolt count for one flanged joint, eliminating site counting errors
- Coated bolts individually wrapped or separated in the bundle to prevent coating-to-coating abrasion damage during transit — PTFE/Xylan coated studs are particularly susceptible to surface scratch from metal contact; scratched Xylan coating requires re-coating before installation
- Individual identification: each stud bolt or bolt heat/lot clearly labelled — ASTM grade, diameter, length, thread, coating type, lot number, and PO item number; for NACE B7M lots: each piece individually marked (vibro-engraving or permanent marker on shank) with “B7M” for on-site grade identification separate from the label
- Anchor bolt sets: individual bolts tagged per drawing number and bolt position (e.g., “Foundation Tag F-1001, Bolt Position 1 of 16”) — reduces site installation errors on complex anchor bolt arrangements with multiple sizes
- Moisture protection: VCI poly wrap for all carbon steel and alloy steel bolts; SS bolts in VCI-lined sealed polybags separated from CS hardware
- Stainless and Duplex hardware: stainless steel polybags segregated from carbon steel packaging — iron contamination from CS bolts depositing onto SS stud bolt surfaces causes rust staining and pitting during marine transit
- ISPM-15 heat-treated timber crates for all international export; heavy anchor bolt packages (M72+) individually crated with steel banding
- Documentation package: EN 10204 3.1/3.2 MTC, mechanical test certificates, individual hardness report (B7M — all pieces), Charpy certificate (A320 L7), H₂E relief bake certificate (electroplated), thread gauge report (100%), PMI report (exotic alloys), coating inspection report, and FAI report in waterproof document pocket
4.5 — Complete EPC Project Documentation Package
| # | Document | Standard / Format | Mandatory / Conditional | Notes |
|---|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | Mandatory — all custom fasteners | Bar/billet heat-traceable; one MTC per heat lot |
| 02 | Chemical Composition Report | Bar stock certified lab analysis | Mandatory | Per ASTM A193 / A320 / F1554 / ISO 898-1 |
| 03 | Mechanical Properties Report | UTS, yield, elongation, reduction of area | Mandatory | Tensile test on representative lot sample |
| 04 | Hardness Test Report | ASTM E18 Rockwell C / ASTM E10 Brinell | Mandatory — all NACE grades (100% B7M) | A193 B7M ≤26 HRC per piece; B7 ≤35 HRC per lot |
| 05 | Charpy Impact Test Report | ASTM A370 / EN 10045 | Mandatory — A320 L7/L7M; F1554 Gr.55 (arctic) | Test temp; individual + average J-values per lot |
| 06 | Dimensional Inspection Report | Per ASME B18.2.1 / B18.31 / drawing | Mandatory | All critical dims; length ±1.5 mm; thread length verified |
| 07 | Thread Gauge Report | ASME B1.1 / ISO 965 Go/No-Go gauges | Mandatory — 100% of all custom fasteners | 2A (UNC) or 6g (metric); calibrated gauge cert |
| 08 | Heat Treatment Certificate | Furnace chart + HT procedure | Mandatory — all grades | Q+T records; A193 B7M specific temper cycle |
| 09 | Hydrogen Embrittlement Relief Certificate | ASTM F1941 / ISO 15330 bake records | Mandatory — all electroplated >1000 MPa | 190–220°C × ≥3 hours; within 4 hours of plating |
| 10 | Coating Inspection Report | Per coating standard (ISO 1461/ASTM B733) | Mandatory — all coated fasteners | Coating thickness per specified standard; adhesion test |
| 11 | PMI Report (XRF) | Per lot — exotic alloys and SS | Mandatory — Duplex, Inconel, SS, Ti | B8M vs B8; Duplex S31803 vs S32750 differentiation |
| 12 | NACE Compliance Statement | Hardness + HT declaration | Mandatory — B7M and B7M sour service | Individual piece hardness ref; ≤26 HRC per piece |
| 13 | First Article Inspection (FAI) Report | Project-specific format | Mandatory — new project line items | All parameters; released before batch production |
| 14 | TPI Witness Certificate | SGS / BV / DNV / Lloyds | Conditional — EN 10204 3.2 orders | Co-witness at manufacturer; hardness + dimensional |
| 15 | ISO 9001:2015 Certificate | Third-party QMS certification | Mandatory — EPC projects | Scope covers custom fastener manufacture |
| 16 | Country of Origin + Packing List | Chamber of Commerce / item-level | Mandatory | HS tariff code; matched sets per joint where applicable |
| 17 | Commercial Invoice + Bill of Lading | Per INCOTERMS 2020 | Mandatory | Freight forwarder issued; HS tariff code |
4.6 — ISO and Quality System Compliance
ISO 9001:2015
Quality Management System covering bar stock procurement, cut-to-length process control, heat treatment process qualification (Q+T cycle for B7 and B7M with documented furnace records), thread rolling and cutting process qualification, thread gauging protocol (100% Go/No-Go), hardness testing (100% for B7M), H₂E relief bake procedure, coating thickness inspection, and full material traceability from bar heat to finished fastener. Mandatory for all EPC, pressure vessel, and structural custom fastener project procurement.
ASME B16.5 / ASME VIII
ASME B16.5 governs stud bolt material (Group 1, 2, or 3 per material table) and nominal bolt lengths for standard flanged joints — custom lengths are used when the standard table length is insufficient. ASME VIII Division 1 Appendix 2 governs the bolted flange joint design including bolt area calculation and bolt material allowable stress at design temperature — the design calculation must confirm that the custom stud bolt area and grade provide adequate bolt load for the specific flange and gasket design at all operating conditions.
API 6A / 16A
API 6A (Wellhead and Tree Equipment) and API 16A (Drilling Riser Connectors) specify stud bolt materials, thread forms, and mechanical requirements for wellhead connector bolting. API 6A stud bolts are typically ASTM A193 B7 with UNC Class 3A threads and specific heat treatment requirements that may differ from standard ASME B16.5 bolting practice. Custom stud bolts for API 6A wellhead flanges are manufactured to the specific API class dimensions and verified by the API-licensed manufacturer’s quality system. Xylan or MoS₂ coating standard for API 6A stud bolt assemblies.
PED 2014/68/EU / EN 1515
Pressure Equipment Directive — European regulatory framework. EN 1515-1 defines bolting materials for flanged joints in CE-marked pressure equipment — B7 equivalent is EN ISO 898-1 Grade 10.9; B8M equivalent is EN ISO 3506-1 Grade A4-70. Custom stud bolts for European PED-compliant pressure equipment must be manufactured and certified per EN 1515-1 material requirements with EN 10204 3.1 documentation. RR Hydraulic provides PED-compliant custom fastener documentation with Declaration of Conformity for European EPC project supply on request.
Submit your drawing, dimensional specification, material grade, coating, and quantity to RR Hydraulic for a complete, certified commercial offer.