RFQ Today
Certifications available: EN 10204 3.1 / 3.2 MTRs, NACE MR0175 compliance, Third-Party Inspection witness, ASME / ASTM / DIN / ISO standards, full export documentation.
Flange Bolts
A world-class technical reference for EPC contractors, piping engineers, pressure vessel engineers, procurement heads, TPI inspection agencies, and global project buyers specifying flange bolts in high-pressure, high-temperature, and critical industrial flanged joint systems.
Sealing Principles
& Load Characteristics
Flange bolts are headed, externally-threaded fasteners used in combination with nuts to clamp flanged pipe joints, pressure vessel nozzles, heat exchangers, and valve bonnets. Distinguished from stud bolts by their integral hexagonal head, flange bolts are used where accessibility permits head-side tightening and where the flange design specifies bolt-through configurations per ASME B16.5, ASME B18.2.1, DIN 933, and ISO 4014.
1.1 — Technical Definition and Functional Role in EPC Systems
A flange bolt is a headed fastener — typically with a hexagonal head conforming to ASME B18.2.1 (inch series) or ISO 4014 / 4017 (metric series) — used to clamp two mating flange faces together. Unlike a stud bolt, which is headless and engages a nut at both ends, a flange bolt bears against the flange face through its head on one side and a single nut on the opposite side.
In EPC project specifications, flange bolts are typically applied in ASME B16.5 Class 150 and 300 flanges where head clearance is available, and in non-process utility piping, water service, fire water, instrument air, and low-pressure cooling water systems. For Classes 600 and above, stud bolts are the standard — however, flange bolts in heavy hex configuration per ASME B18.2.1 are still encountered in ASME-coded equipment and in European DIN-standard flanged systems across all pressure classes.
RR Hydraulic manufactures and exports flange bolts to global EPC projects under all applicable international standards with EN 10204 3.1 / 3.2 material traceability and full project documentation capability.
1.2 — Flange Bolt Classification and Type Descriptions
Hex Head Bolt (Full Thread)
Hexagonal head; thread continuous from under head to tip. Conforms to ASME B18.2.1 (hex cap screw), ISO 4017, or DIN 933. Used in Class 150–300 flanged joints and low-pressure utility piping. Thread class 2A (UNC/UNF) or 6g (metric).
Hex Head Bolt (Partial Thread)
Hexagonal head with unthreaded shank between head and thread runout. Conforms to ASME B18.2.1 (heavy hex bolt), ISO 4014, DIN 931. Shank passes through bolt hole clearance; thread engages nut only. Preferred for flange bolting where shank bears against bolt hole bore.
Heavy Hex Bolt
Larger hex head width-across-flats (WAF) than standard hex. Per ASME B18.2.1 heavy hex series. Provides greater bearing area under head, reducing surface stress on the flange face — required for high-strength alloy steel grades on soft flange face materials.
Flange Head Bolt (Serrated)
Integral washer-face flange on the underside of the hex head, often with serrations to prevent loosening. Eliminates the need for a separate washer. Common in automotive and structural applications but not typically used in pressure piping flange service.
Square Head Bolt
Four-sided head; historically used in flanged steam piping and older ASME equipment designs. Still encountered in maintenance and replacement supply for legacy industrial plant. Less ergonomic than hex for wrench access but dimensionally robust.
T-Head / Special Profile Bolt
Custom head geometry (T, round, eye, or captive) for specific flange or equipment designs where standard hex heads cannot be installed due to head clearance restrictions. Manufactured to OEM drawing; requires engineering approval and full dimensional verification.
1.3 — Flange Bolt vs Stud Bolt: Engineering Selection Logic
| Parameter | Flange Bolt (Headed) | Stud Bolt (Headless) | Engineering Implication |
|---|---|---|---|
| Head configuration | Integral hex / heavy hex head | No head; nut both ends | Bolt: head-side access required; Stud: equal both ends |
| Clamping symmetry | Asymmetric (head bears one side) | Symmetric (nut–nut) | Stud bolt preferred for high-temp cycling joints |
| Torque application | Nut only (or head and nut) | Nut both ends; hydraulic tensioning possible | Stud bolt allows precise tensioner tooling |
| ASME B16.5 Class 150–300 | Bolt or stud; both acceptable | Standard preferred | Flange bolt commonly used in utility lines |
| ASME B16.5 Class 600–2500 | Rarely specified; stud preferred | Standard requirement | High-pressure joints demand stud bolt loading symmetry |
| Maintenance access | Faster removal (head side wrench) | Both ends require nut access | Flange bolt advantageous in tight head-clearance spaces |
| Bolt replacement in-service | Can be replaced without disturbing gasket | Requires both nuts to be free | Flange bolt may be preferred in maintenance-critical joints |
| Thread engagement | One nut; engagement critical | Two nuts; symmetric engagement | Minimum 1× diameter thread engagement in nut mandatory |
| Standard references | ASME B18.2.1, ISO 4014/4017, DIN 931/933 | ASTM A193, ASME B16.5, DIN 976 | Different purchase standard — verify before ordering |
1.4 — Flanged Joint Sealing Mechanics with Flange Bolts
Whether the joint uses flange bolts or stud bolts, the sealing mechanism is identical: bolt preload compresses the gasket to the seating stress required to prevent leakage at operating conditions. The critical difference with flange bolts is that load path asymmetry (head bearing on one flange face, nut on the other) can result in unequal bearing stress distribution if the head-face contact area is insufficient — particularly relevant for high-yield-strength alloy bolts on softer flange materials.
F_b = Bolt preload force (N)
A_bearing = Net bearing area under head = π/4 × (W²_AF − D²_hole) (mm²)
W_AF = Width across flats of hex head (mm)
D_hole = Bolt hole diameter in flange (mm)
W_AF = 31.75 mm (heavy hex ¾”), D_hole = 22.4 mm (Class 150 ¾” flange bolt hole)
A_bearing = π/4 × (31.75² − 22.4²) = π/4 × (1008.1 − 501.8) = π/4 × 506.3 = 397.8 mm²
F_b at 50% yield (A193 B7, Sy = 723 MPa, As = 220.4 mm²) = 220.4 × 723 × 0.50 = 79.7 kN
σ_b = 79,700 / 397.8 = 200 MPa vs A105 Sy = 250 MPa → 0.9×Sy = 225 MPa ✓ Acceptable
1.5 — Bolt Preload Behaviour: Embedment, Thermal Cycling, Vibration Loosening
Embedment Relaxation
Thread flank asperities and head bearing face micro-irregularities plastically deform under initial preload, reducing effective bolt load by 5–15% in the first 24–72 hours. Re-torquing to compensate is specified in ASME PCC-1. Heavy hex head bolts exhibit less embedment than standard hex due to larger bearing area.
Thermal Cycling Derating
Differential thermal expansion between flange bolt and flange body (e.g., carbon steel bolt in stainless steel flange) generates alternating preload variation per thermal cycle. For service above 300°C, specify bolt grade with matching thermal expansion coefficient or apply retorquing procedure.
Vibration Loosening (Junker)
Transverse vibratory loads cause progressive nut rotation and preload loss. Flange bolts on compressor outlet piping, pump discharge flanges, and vibrating equipment require: target preload ≥75% yield, prevailing-torque nuts (ASTM F594), or approved thread-locking compound per project specification.
Gasket Creep Relaxation
Gasket material creeps under sustained compression, reducing its reaction force on the bolt. Spiral wound gaskets exhibit lower creep than CAF (Compressed Asbestos Fibre) or PTFE alternatives. Preload target must include a gasket-specific relaxation allowance — typically 5–20% of initial seating load.
Hydrogen Embrittlement Risk
High-strength alloy steel flange bolts (≥B7 grade, hardness >22 HRC) are susceptible to hydrogen embrittlement in sour service and electroplated conditions. Electroplated coatings (zinc plate) require post-plate baking per ASTM F1941 to expel absorbed hydrogen. Hot dip galvanising does not introduce hydrogen.
Head-Side Bearing Galling
Under nut rotation, galling can occur between the bolt head and flange face — especially for stainless steel bolts against stainless flanges. Prevention: apply anti-seize lubricant (MoS₂ or nickel-based) to head bearing face and thread, and torque from the nut side where possible.
1.6 — Design Safety Factors and Pressure Class Application
| Service Category | Min. Safety Factor (UTS/Operating Load) | Target Preload (% of Proof/Yield) | Code Reference | Engineering Note |
|---|---|---|---|---|
| Class 150 utility piping | 3:1 | 50–60% yield | ASME B31.3 / B16.5 | Flanged water, air, steam service |
| Class 300 process piping | 3:1–4:1 | 55–65% yield | ASME B31.3 / PCC-1 | Standard for most process lines |
| Class 600–900 high-pressure | 4:1 | 60–70% yield | ASME B31.3 / B16.5 | Stud bolt typically preferred above Class 600 |
| Sour / NACE service | 4:1 | ≤50% yield | NACE MR0175 / ISO 15156 | Hardness limit governs grade selection, not strength |
| Cryogenic (LNG) | 4:1–5:1 | 55–65% yield | ASTM A320 / B31.3 | Impact test mandatory; preload target adjusted for CTE |
| High-temperature (>400°C) | 4:1 | 50–60% yield | ASME B31.1 / BPVC | Creep-resistant grade required; retorque at temperature |
| Fire water / utility | 3:1 | 45–55% yield | NFPA / project spec | A307 Grade B flange bolts commonly used |
Submit your flange schedule, material spec, pressure class, and quantity for a documented RFQ within 24 hours.
Pressure Classes
& Standards Compliance
Flange bolt dimensions are governed by ASME B18.2.1, ISO 4014, ISO 4017, DIN 931, and DIN 933 — with specific bolt diameter, thread pitch, length, and head geometry defined for each pressure class and flange size. Material grades are prescribed by ASTM A193, A307, and A320 depending on service conditions. All applicable standards are supported at RR Hydraulic with full certification capability.
Submit grade, flange class, thread standard, size, and quantity to sales@rrhydraulics.com for a fully certified commercial offer.
2.1 — ASME B18.2.1 Heavy Hex Bolt Dimensional Table (UNC Thread)
The following table provides key dimensional data for heavy hex bolts per ASME B18.2.1 — the standard geometry for pressure piping flange bolt supply in ASME-coded systems. Heavy hex dimensions provide the larger bearing area and wider wrench engagement required for high-torque assembly of alloy steel flange bolts.
| Nom. Dia. (in) | TPI (UNC) | Width Across Flats (in) | Width Across Corners (in) | Head Height (in) | Stress Area (in²) | Proof Load (psi) — A193 B7 | Standard ASTM Grade |
|---|---|---|---|---|---|---|---|
| ½” | 13 | 0.875 | 1.010 | 0.323 | 0.1419 | 85,000 | A307 / A193 B7 |
| ⅝” | 11 | 1.063 | 1.227 | 0.403 | 0.2260 | 85,000 | A193 B7 |
| ¾” | 10 | 1.250 | 1.443 | 0.483 | 0.3340 | 85,000 | A193 B7 |
| ⅞” | 9 | 1.438 | 1.660 | 0.563 | 0.4620 | 85,000 | A193 B7 |
| 1″ | 8 | 1.625 | 1.876 | 0.627 | 0.6060 | 85,000 | A193 B7 |
| 1⅛” | 7 | 1.813 | 2.093 | 0.718 | 0.7630 | 85,000 | A193 B7 |
| 1¼” | 7 | 2.000 | 2.309 | 0.813 | 0.9690 | 85,000 | A193 B7 |
| 1½” | 6 | 2.375 | 2.742 | 0.940 | 1.4050 | 85,000 | A193 B7 |
| 1¾” | 5 | 2.750 | 3.175 | 1.036 | 1.9000 | 85,000 | A193 B7 |
| 2″ | 4.5 | 3.000 | 3.464 | 1.196 | 2.5000 | 85,000 | A193 B7 |
| 2¼” | 4.5 | 3.375 | 3.897 | 1.348 | 3.2500 | 85,000 | A193 B7 |
| 2½” | 4 | 3.750 | 4.330 | 1.444 | 3.9500 | 85,000 | A193 B7 |
2.2 — ISO 4014 / 4017 Metric Hex Bolt Dimensional Table
| Thread | Pitch (mm) | W.A.F. (mm) | W.A.C. (mm) | Head Height k (mm) | Stress Area (mm²) | Min Shank Ø (mm) | ISO Property Class |
|---|---|---|---|---|---|---|---|
| M12 | 1.75 | 18 | 20.78 | 7.76 | 84.3 | 11.57 | 8.8 / 10.9 |
| M16 | 2.00 | 24 | 27.71 | 10.29 | 157.0 | 15.57 | 8.8 / 10.9 |
| M20 | 2.50 | 30 | 34.64 | 12.85 | 245.0 | 19.48 | 8.8 / 10.9 |
| M24 | 3.00 | 36 | 41.57 | 15.44 | 353.0 | 23.48 | 8.8 / 10.9 |
| M27 | 3.00 | 41 | 47.34 | 17.21 | 459.0 | 26.48 | 8.8 / 10.9 |
| M30 | 3.50 | 46 | 53.12 | 19.12 | 561.0 | 29.48 | 8.8 / 10.9 |
| M36 | 4.00 | 55 | 63.51 | 22.78 | 817.0 | 35.38 | 8.8 / 10.9 |
| M42 | 4.50 | 65 | 75.06 | 26.97 | 1121.0 | 41.38 | 8.8 / 10.9 |
| M48 | 5.00 | 75 | 86.60 | 30.65 | 1473.0 | 47.38 | 8.8 / 10.9 |
| M56 | 5.50 | 85 | 98.15 | 35.72 | 2030.0 | 55.26 | 8.8 |
| M64 | 6.00 | 95 | 109.7 | 40.35 | 2676.0 | 63.26 | 8.8 |
2.3 — Applicable Standards and Compliance Framework
ASME B18.2.1
Square, hex, heavy hex bolts and hex cap screws — inch series. Defines head geometry, dimensional tolerances, thread class, and material requirements for flange bolts in ASME-coded piping and pressure equipment. The governing dimensional standard for US EPC projects.
ISO 4014 / ISO 4017
Hex head bolts, product grades A and B — metric series. ISO 4014: partial thread (hex bolt); ISO 4017: full thread (hex screw). Both conform to ISO 898-1 property classes. Used in European and Asian EPC projects with DIN / EN-coded flanges and equipment.
DIN 931 / DIN 933
DIN 931: hex head bolts with partial thread; DIN 933: hex head bolts with full thread — both metric coarse pitch. Legacy German standard widely used in European industrial plant and DIN-coded flanged systems. Dimensionally aligned with ISO 4014 / 4017 but with some tolerance differences.
ASTM A193
Alloy steel and stainless steel bolting for high-temperature service. Grade B7 (alloy steel Q&T) is the dominant grade for pressure piping flange bolts. B8/B8M covers SS 304/316. Full material properties, heat treatment, and proof load requirements defined per grade and diameter range.
ASTM A307
Low-carbon steel bolts and studs for general-purpose service. Grade A: for structural and general use; Grade B: for flanged joints on cast iron flanges (fire hydrants, valves, pipe flanges per ASME B16.1 Class 125/250). Not suitable for pressure-retaining flanges above Class 150.
ASTM A320
Alloy steel and SS bolting for low-temperature service. Grade L7 (4140 Q&T, impact tested to −101°C) for cryogenic flanged joints. Charpy impact testing per ASTM A320 mandatory. Required for all flanged piping systems below −29°C operating temperature.
ASTM A394
Steel transmission tower bolts — zinc-coated. Galvanised hex head bolts for structural bolting in outdoor and utility structures. Not a pressure-piping standard; encountered in plant structural steelwork and cooling tower framework bolting on EPC project BOMs.
ISO 898-1
Mechanical properties of fasteners — bolts, screws, and studs; metric series. Defines property classes 4.6 through 12.9 with UTS, yield, elongation, and hardness limits. The primary metric equivalent to ASTM A193 for strength classification of metric flange bolts.
NACE MR0175 / ISO 15156
Materials for H₂S sour environments. Carbon and alloy steel flange bolts in sour service: maximum hardness 22 HRC (237 HB). A193 B7M (controlled hardness variant) is the standard NACE-compliant alloy steel grade. Hardness verification on 100% of bolts is mandatory for sour service supply.
2.4 — Thread Form and Compatibility Reference
| Thread System | Standard | Size Range | Pitch / TPI | Tolerance Class (Bolt/Nut) | Application | Interchangeability Note |
|---|---|---|---|---|---|---|
| UNC Coarse | ASME B1.1 | ¼”–4″ | 4–20 TPI | 2A / 2B (std); 3A/3B (precision) | ASME B18.2.1 flange bolts; B16.5 flanges | Standard for US-coded EPC projects |
| UNF Fine | ASME B1.1 | ¼”–1½” | 12–28 TPI | 2A / 2B | Precision machinery, small bore flanges | Not interchangeable with UNC |
| Metric Coarse (ISO) | ISO 261 / DIN 13 | M6–M100 | 1.0–6.0 mm | 6g / 6H (std); 5g/5H (precision) | ISO 4014/4017; DIN 931/933 bolts | Default for European / Asian EPC |
| Metric Fine | ISO 261 | M8–M52 | 0.75–3.0 mm | 6g / 6H | High-pressure, thin-wall flanges | Must specify; not interchangeable with coarse |
| BSW (Whitworth) | BS 84 | ⅛”–4″ | 2.5–28 TPI | Class 1–3 | UK legacy BS 1083 / BS 4190 bolts | Not interchangeable with UNC |
| BA (British Association) | BS 57 | 0BA–16BA | Very fine | BS 57 | Legacy instrumentation; rare in flanges | Legacy only; verify before supply |
2.5 — Torque Chart: Flange Bolt Installation
Torque values below are for A193 B7 heavy hex flange bolts with A194 2H heavy hex nuts. Values represent target torque to achieve 50% of bolt proof/yield load — the standard initial preload target per ASME PCC-1 Appendix A. Torquing sequence must follow a cross-bolt (star) pattern in minimum three passes: 30%, 70%, 100% of target torque.
| Bolt Ø (in) | TPI (UNC) | Yield Load @ 100% (kN) | Target Preload @ 50% Yield (kN) | Dry Torque Nm (K=0.20) | Oiled Torque Nm (K=0.15) | MoS₂ Torque Nm (K=0.12) | PTFE Torque Nm (K=0.10) |
|---|---|---|---|---|---|---|---|
| ½” | 13 | 102.7 | 51.4 | 130 | 98 | 78 | 65 |
| ⅝” | 11 | 163.5 | 81.8 | 260 | 195 | 156 | 130 |
| ¾” | 10 | 241.6 | 120.8 | 458 | 344 | 275 | 229 |
| ⅞” | 9 | 334.2 | 167.1 | 742 | 557 | 445 | 371 |
| 1″ | 8 | 438.4 | 219.2 | 1114 | 836 | 668 | 557 |
| 1⅛” | 7 | 552.0 | 276.0 | 1579 | 1184 | 948 | 790 |
| 1¼” | 7 | 701.2 | 350.6 | 2236 | 1677 | 1342 | 1118 |
| 1½” | 6 | 1016.6 | 508.3 | 3876 | 2907 | 2326 | 1938 |
| 1¾” | 5 | 1374.9 | 687.5 | 6094 | 4570 | 3657 | 3047 |
| 2″ | 4.5 | 1809.6 | 904.8 | 9194 | 6896 | 5516 | 4597 |
2.6 — Preload Calculation Guide
T = Applied torque (Nm)
K = Nut factor / torque coefficient:
K = 0.20 (dry, uncoated threads)
K = 0.15 (machine oil / light lubricant)
K = 0.12 (MoS₂ paste / Molykote)
K = 0.10 (PTFE coat / Teflon thread tape)
d = Nominal bolt diameter (m)
d = 0.01905 m, K = 0.12, A_s = 215 mm², S_y = 723 MPa
Yield Load = 215 × 723 = 155,445 N = 155.4 kN
Target preload @ 50% yield = 77.7 kN
Required torque T = F_b × K × d = 77,700 × 0.12 × 0.01905 = 177.6 Nm
Compare: Table 2.D MoS₂ torque for ¾” = 275 Nm @ 50% yield (using stress area 334 mm²) — verify A_s per actual bolt lot MTC.
Heat Treatment
& Manufacturing Process
Flange bolt material selection is governed by service temperature, pressure class, fluid composition, corrosion environment, and NACE / low-temperature requirements. RR Hydraulic manufactures flange bolts in all standard and high-alloy grades — from A307 carbon steel to Super Duplex and Inconel — with full heat treatment documentation and EN 10204 traceability.
3.1 — Material Grade Overview and Mechanical Properties
| Grade / Spec | Material Base | UTS (MPa) | Yield (MPa) | Elong. (%) | Temp Range (°C) | HRC Max | Primary Service Application |
|---|---|---|---|---|---|---|---|
| A307 Gr.B | Low-carbon steel | 414–724 | — | 18 | −29 to +260 | — | Cast iron flanges, fire water, Class 125/250 |
| A193 B7 | 4140/4142 Cr-Mo Q&T | 860–1000 | 723 | 16 | −45 to +454 | 35 (26 NACE) | Standard alloy steel for all pressure classes |
| A193 B7M | 4140/4142 Cr-Mo Q&T (controlled) | 690–862 | 552 | 18 | −45 to +454 | 22 (NACE) | Sour service, H₂S environments, NACE-mandatory |
| A320 L7 | 4140 Cr-Mo Q&T + impact tested | 860 | 723 | 16 | −101 to +343 | 35 | LNG, cryogenic piping, low-temperature |
| A193 B8 Cl.1 | SS 304 solution annealed | 515 | 205 | 30 | −254 to +816 | Max 192 HB | Corrosive, high-temperature, food/pharma |
| A193 B8M Cl.2 | SS 316 strain-hardened | 690 | 414 | 12 | −254 to +816 | 321 HB max | Chloride, marine, offshore low-temp |
| Duplex 2205 | UNS S31803 | 620 | 450 | 25 | −50 to +315 | 28 HRC | Offshore, subsea, sour + chloride service |
| Super Duplex S32750 | UNS S32750 | 750 | 550 | 20 | −50 to +260 | 32 HRC | Seawater, aggressive chloride, subsea |
| Inconel 625 | UNS N06625 Ni-Cr-Mo | 827 | 414 | 30 | −196 to +980 | — | Extreme corrosion, high-temp, sour + Cl |
| A193 B16 | Cr-Mo-V alloy steel Q&T | 1000 | 827 | 16 | −30 to +593 | 35 | High-temp steam turbine, boiler flanges |
3.2 — Yield Strength and Mechanical Properties by Heat Treatment
| Grade | Heat Treatment | UTS (MPa) | 0.2% Yield (MPa) | Elongation (%) | Reduction of Area (%) | Hardness (HB) | Charpy (J) @ Test Temp |
|---|---|---|---|---|---|---|---|
| A193 B7 (≤2½” dia) | Q&T — 860°C/620°C | 860–1000 | 723 | 16 | 50 | 248–302 | Not req. (ambient) |
| A193 B7M (≤2½” dia) | Q&T (max hardness control) | 690–862 | 552 | 18 | 50 | 200–235 | Not req. |
| A320 L7 | Q&T + Charpy impact test | 860 min | 723 | 16 | 50 | 248–302 | 20 J min @ −101°C |
| A193 B8 Cl.1 | Solution annealed 1040°C | 515 | 205 | 30 | 50 | Max 192 | N/A (austenitic) |
| A193 B8M Cl.2 | Solution Ann. + strain hardened | 690 | 414 | 12 | 35 | 260–321 | N/A |
| A307 Gr.B | Normalised (or as-rolled) | 414–724 | Min. not spec. | 18 | — | — | Not req. |
| Duplex 2205 | Solution annealed 1020–1100°C | 620 | 450 | 25 | 45 | Max 293 | N/A |
| Super Duplex S32750 | Solution annealed 1025–1125°C | 750 | 550 | 20 | 40 | Max 310 | N/A |
| A193 B16 | Q&T — 940°C/620°C | 1000 | 827 | 16 | 50 | 248–321 | Not req. |
3.3 — Corrosion Resistance by Material vs Service Media
| Material | H₂S Sour* | CO₂ / Wet Gas | Cl⁻ / Seawater | HCl (dilute) | H₂SO₄ (dilute) | Caustic / Alkali | High Temp Oxidation |
|---|---|---|---|---|---|---|---|
| A307 Gr.B / A193 B7 (coated) | Conditional* | Fair | Poor | Poor | Poor | Good | Good to 454°C (B7) |
| A193 B7M (NACE-compliant) | Good* | Fair | Poor | Poor | Poor | Good | Good to 454°C |
| A193 B8 (SS 304) | Fair | Good | Poor (SCC) | Fair | Fair | Very Good | Very Good to 816°C |
| A193 B8M (SS 316) | Good | Very Good | Fair | Good | Good | Very Good | Very Good to 816°C |
| Duplex 2205 | Very Good | Excellent | Very Good | Very Good | Very Good | Very Good | Limited above 315°C |
| Super Duplex S32750 | Excellent | Excellent | Excellent (PREN>42) | Excellent | Excellent | Excellent | Limited above 260°C |
| Inconel 625 | Excellent | Excellent | Excellent | Excellent | Excellent | Excellent | Excellent to 980°C |
* Sour service: Carbon and alloy steel must comply with NACE MR0175 / ISO 15156 hardness limit ≤22 HRC. Standard B7 (up to 35 HRC) is NOT acceptable without B7M controlled hardness designation.
3.4 — Manufacturing Process
3.4.1 — Hot Forging of Hex Head
Flange bolt heads are formed by cold heading (for smaller diameters, typically up to 1½”) or hot forging (for larger diameters). Cold heading produces a continuous grain flow around the head-shank fillet — the highest-stress zone — providing superior fatigue resistance compared to machined head profiles. The head-shank fillet radius must conform to ASME B18.2.1 to avoid stress concentration that precipitates fatigue cracking in cyclic-load applications.
3.4.2 — Thread Rolling vs Thread Cutting
- Thread rolling (cold-forming): Produces compressive residual stress at thread root; grain flow follows thread form — superior fatigue life. Preferred for high-cycle and vibration-prone applications. Rolled threads are the industry standard for alloy steel grades.
- Thread cutting (machining): Interrupts grain flow at thread root; residual tensile stress at root possible. Acceptable for low-cycle, low-vibration applications. Mandatory for some material grades (hard alloys, SS, duplex) where rolling is impractical.
- Thread class: 2A tolerance class (ASME B1.1) is standard for pressure piping flange bolts. Class 3A (tighter) may be specified for precision-loaded joints or hydraulic tensioning applications.
3.4.3 — CNC Machining Tolerances
- Thread pitch diameter: per ASME B1.1 Class 2A (UNC) or ISO 6g (metric)
- Head height tolerance: ±0.38 mm for sizes ½”–1″; ±0.5 mm for sizes above 1″ (ASME B18.2.1)
- Width across flats: +0.00 / −0.40 mm (standard hex); tighter for heavy hex precision grade
- Head-to-shank perpendicularity: max 1° from bolt axis (per ASME B18.2.1)
- Thread run-out to shank: chamfer at 15°–30°; min 1 full thread depth clearance
- Surface finish — shank: Ra ≤ 3.2 µm; thread form: Ra ≤ 1.6 µm; bearing face: Ra ≤ 1.6 µm
- Length tolerance (grip length): ±1 pitch for bolts up to 150 mm; ±2 pitches above 150 mm
3.5 — Surface Finish Options
| Finish | Specification | Coating Thickness (µm) | Salt Spray (h) | Max Service Temp (°C) | Effect on K Factor | Nut Oversize Required? | Typical Application |
|---|---|---|---|---|---|---|---|
| Plain / Bare | ASTM A193 standard | — | <24 | 454 | K=0.20 (dry) | No | Immediate assembly; assembly lube applied |
| Black Phosphate | MIL-DTL-16232 | 5–15 | 24–72 | 150 | K=0.15–0.17 | No | Short-term storage; standard CS bolts |
| Zinc Electroplate | ASTM B633 Type II/III | 5–25 | 96–500 | 120 | K=0.18–0.20 | No (thin coat) | General EPC, indoor; hydrogen bake-out required for A193 B7 |
| Hot Dip Galvanize (HDG) | ASTM A153 / ISO 1461 | 45–85 | 2000+ | 200 | K=0.20–0.22 | Yes — oversize nut mandatory | Outdoor, marine, water service, A307 Grade B bolts |
| PTFE / Teflon | Proprietary | 20–50 | 1000+ | 260 | K=0.10–0.12 | No | Precision torque control; stainless anti-galling |
| Molykote / MoS₂ | Applied at assembly | — | — | 450 | K=0.11–0.13 | No | High-temperature flange assembly standard lubricant |
| Nickel Anti-Seize | Applied at assembly | — | — | 1200 | K=0.13–0.16 | No | SS and duplex bolts — prevents galling on assembly |
| Geomet / Dacromet | ISO 10683 | 8–12 | 1000–1500 | 300 | K=0.13–0.15 | No | Offshore hardware; no hydrogen risk; thin coat |
| Passivation (SS / Duplex) | ASTM A380 / A967 | Passive layer only | 500–2000+ | Per alloy | K=0.20 | No | All SS, duplex, super duplex grade bolts |
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from raw material heat to final packed shipment on all flange bolt orders. Dimensional inspection reports, EN 10204 3.1 / 3.2 MTRs, NACE compliance statements, NDT reports, and complete EPC export documentation packages are provided as standard on project-grade supply.
4.1 — Inspection & NDT Protocol
4.2 — EN 10204 Material Test Certificate Requirements
| Certificate Type | Content | Signatory | Typical EPC Requirement | When Mandatory |
|---|---|---|---|---|
| 2.1 | Declaration of conformity only — no test data | Manufacturer | Non-critical utility service only | General procurement; rarely accepted in O&G EPC |
| 2.2 | Non-specific test results — not heat-traceable | Manufacturer | Low-pressure, non-coded service | Not acceptable for pressure-retaining supply |
| 3.1 | Heat-traceable mechanical + chemical test results; specific to production lot | Manufacturer’s authorised QC Inspector | Minimum standard for all pressure piping supply | All Class 150 and above flange bolt orders |
| 3.2 | 3.1 content + countersigned by independent TPI inspector (SGS / BV / DNV / Lloyds) | Manufacturer + TPI | Critical joints, NACE service, cryogenic, offshore | Class 900+, sour service, LNG, all TPI-mandated projects |
4.3 — Pressure Test Requirements for Assembled Flange Joints
Flange bolts in assembled pressure piping joints are subject to hydrostatic or pneumatic pressure testing of the complete system per applicable piping code. Test parameters for ASME B31.3 Process Piping:
- Hydrostatic test pressure: 1.5 × MAWP (Maximum Allowable Working Pressure) at test temperature — ASME B31.3 Clause 345.4
- Pneumatic test pressure: 1.1 × MAWP where hydrostatic is not practical — ASME B31.3 Clause 345.5
- Initial service leak test: System pressurised to operating pressure; flanged joints inspected for leaks using soapy water, UV tracer, or snoop method
- Hold duration: Minimum 10 minutes at test pressure before joint inspection
- Acceptance: Zero visible leaks at any flanged joint; no torque-induced deformation of flange face
- Post-test bolt retorque: Required if embedment relaxation expected; mandatory in ASME PCC-1 for high-temperature service joints before commissioning at operating temperature
- NACE sour service: Flange joints must be fully dried before sour gas introduction; wet H₂S exposure after hydrostatic testing requires drying procedure and retorquing protocol
4.4 — Applications by Industry
Oil & Gas — Upstream
Wellhead, manifold, and Christmas tree flanged connections. High-pressure Class 600–2500 joints predominantly use stud bolts; flange bolts applied in utility and instrument connections. Sour service NACE compliance mandatory for all bolting in H₂S-containing fluid service. EN 10204 3.2 with TPI witness standard on major projects.
Refinery & Petrochemical
ASME B31.3 process piping across all pressure classes. A193 B7 for standard carbon steel service; B7M for sour and NACE-required service; B8M for chloride-containing process streams. High-temperature catalytic cracking and hydrocracker service requires A193 B16 (Cr-Mo-V) above 454°C with creep assessment.
LNG Terminals
Cryogenic piping in A320 L7 or L7M with Charpy impact testing at −101°C or lower. Stainless steel A193 B8M or Inconel 625 flange bolts used for SS 304L / 316L cryogenic piping flanges. Anti-galling treatment mandatory for SS-to-SS flange contact — nickel anti-seize or PTFE coating specified.
Power Generation
Steam turbine casing, HP/IP/LP casing flange bolting in A193 B16 (to +593°C) or B7 (to +454°C). Retorquing at operating temperature per OEM specification mandatory on turbine casings. Boiler pressure parts per ASME B31.1; flange bolt MTC heat traceability to ASME Section II material specification required.
Offshore & Subsea
Topside piping in SS 316 / Duplex 2205 for marine corrosion resistance. HISC (Hydrogen Induced Stress Cracking) risk assessment mandatory for super duplex flange bolts in cathodic protection zones. Subsea tree and manifold bolting in Inconel 625 or Super Duplex S32760 with DNVGL / Lloyds certification.
Fire Water & Utility Systems
A307 Grade B flange bolts are the standard specification for cast iron flanged fittings per ASME B16.1 Class 125 and 250, and AWWA C207 steel pipe flanges in fire water and cooling water service. Hot dip galvanised coating mandatory for outdoor service; oversize A194 Grade 2H nuts required with HDG coating.
4.5 — Export Packaging Specification
- Individual flange bolts wrapped in VCI (Volatile Corrosion Inhibitor) poly film — prevents atmospheric corrosion during ocean freight, port storage, and construction-site pre-installation storage up to 24 months
- Thread protection: plastic caps on threaded ends; head protection sleeve on hex head where coating integrity is critical
- Bundle identification: each bundle tagged with heat number, material grade, ASTM standard, size, thread, length, quantity, surface finish, and purchase order reference — full traceability maintained from manufacturer to site installation
- Segregation by heat: different heat numbers in separate bundles/polybags within the same box — prevents heat mix-up during goods receipt inspection
- Outer crating: ISPM-15 heat-treated wooden crates for all international export; fumigation certificate available on request
- Crate markings: Project PO number, item tag, material grade, surface finish, gross weight, net weight, crate dimensions (L×W×H mm), country of origin, handling symbols per ASTM D5276, stack height limit
- Documentation enclosed: packing list inside lid + waterproof external pocket; cross-referenced to MTC certificate serial numbers and inspection release note
4.6 — Complete EPC Project Documentation Package
| # | Document | Standard / Format | Mandatory / Conditional | Notes |
|---|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | Mandatory — all pressure service | Heat-specific; traceable to production lot |
| 02 | Chemical Composition Report | Heat/lot certified lab analysis | Mandatory (within MTC) | All major and minor alloying elements reported |
| 03 | Mechanical Properties Report | UTS, yield, elongation, RA, hardness | Mandatory (within MTC) | Per ASTM A193 / A320 / ISO 898-1 |
| 04 | Hardness Test Report | ASTM E10 / E18 (Brinell + Rockwell) | Mandatory — NACE / sour service | Max 22 HRC for B7M; reported on every lot |
| 05 | Charpy Impact Test Report | ASTM A370 / EN 10045 | Mandatory — A320 / cryogenic supply | Test temp and J-values per grade and service |
| 06 | Dimensional Inspection Report | Per ASME B18.2.1 / ISO 4014 | Mandatory | Thread gauge, head geometry, length, chamfer |
| 07 | Thread Gauge Report | Go/No-Go per ASME B1.1 / ISO 286 | Mandatory | 100% gauged; results per lot |
| 08 | First Article Inspection (FAI) Report | Project-specific format | Mandatory — new item / first batch | Released before batch production |
| 09 | MPI / LPI Inspection Report | ASTM E709 / E165; EN ISO 9934 / 3452 | Conditional — NACE, critical joints | Head-shank fillet primary inspection zone |
| 10 | PMI Report (XRF) | Per lot; SS / duplex / exotic grades | Mandatory — non-CS grades | 100% of stainless, duplex, exotic bolts |
| 11 | UT Report | ASTM A388 / EN 10308 | Conditional — >1½” diameter | Volumetric bar stock inspection before machining |
| 12 | TPI Witness Certificate | SGS / BV / DNV / Lloyds countersigned | Mandatory — EN 10204 3.2 orders | Co-witness at manufacturer works |
| 13 | NACE MR0175 Compliance Statement | Hardness + heat treatment confirmation | Mandatory — sour service supply | References specific heat number and hardness readings |
| 14 | Hydrogen Bake-Out Certificate | ASTM F1941 (electroplated only) | Conditional — electroplated A193 B7 | 190°C minimum 4 hours; dated and signed |
| 15 | ISO 9001:2015 Certificate | Third-party QMS certification | Mandatory — EPC projects | Current certificate; scope covers bolting manufacture |
| 16 | Country of Origin Certificate | Chamber of Commerce issued | Mandatory — all export | Required for customs and import duty classification |
| 17 | Packing List | Item-level detail per shipment | Mandatory | Cross-references MTC serial numbers |
| 18 | Commercial Invoice | Per INCOTERMS 2020 | Mandatory | Includes HS code for customs clearance |
| 19 | Bill of Lading / Air Waybill | Per freight mode | Mandatory | Issued by freight forwarder |
| 20 | MSDS / Safety Data Sheet | For coated / treated surfaces | Conditional — HDG, PTFE, Geomet, MoS₂ | Required for hazardous material shipping classification |
4.7 — ISO and Quality System Compliance
ISO 9001:2015
Quality Management System covering the full manufacturing cycle: supplier qualification for raw material, in-process dimensional and NDT inspection, heat treatment verification, final inspection release, and traceability documentation. Mandatory for EPC and O&G project procurement qualification.
ISO 10474
Steel and steel products — inspection documents. The source framework for EN 10204 certificate type definitions. Some legacy EPC project specifications reference ISO 10474 types 3.1.B or 3.1.C — these map to EN 10204 3.1 and 3.2 respectively.
ASME PCC-1
Guidelines for Pressure Boundary Bolted Flange Joint Assembly. Covers tightening method selection, K factor determination, cross-bolt sequencing, documentation requirements (Flange Joint Assembly Record), and retorquing protocols. The operative engineering standard for all ASME-coded flanged joint assembly.
ISO 4413
Safety requirements for hydraulic fluid power systems. Relevant where flange bolts are specified for hydraulic system flanged connections and manifold bolting in HPUs and hydraulic power packs. Bolting torque and preload documentation requirements consistent with ISO 4413 safety management approach.
Submit your BOM, flange class schedule, or material specification to RR Hydraulic for a complete, certified commercial offer.