RFQ Today
Certifications available: EN 10204 3.1 / 3.2 MTRs, NACE MR0175 compliance, Third-Party Inspection witness, ASTM / DIN / ISO / BS standards, and full EPC export documentation packages.
Hex Bolts
A world-class technical reference for EPC contractors, mechanical and structural engineers, procurement heads, TPI inspection agencies, and global project buyers specifying hex bolts in pressure equipment, structural steel, heavy machinery, piping systems, and critical industrial assembly applications across Oil & Gas, Power, Petrochemical, and Infrastructure.
Geometry &
Load Characteristics
Hex bolts are the most universally specified fastener in mechanical and structural engineering — defined by their six-sided (hexagonal) head, externally threaded shank, and nut-and-washer clamping system. From ASME B16.5 Class 150 piping flanges to structural steel moment connections, hex bolts constitute the primary mechanical fastening solution across Oil & Gas, Power, Petrochemical, Infrastructure, and Heavy Machinery EPC project scopes.
1.1 — Technical Definition and Functional Role in EPC Systems
A hex bolt is an externally threaded fastener with a regular hexagonal head, designed to be driven by a wrench on the hex head flats and secured by a nut (and washer) on the threaded end. The hexagonal head provides six engagement faces for torque application, enabling efficient installation with open-end, ring, or socket wrenches, hydraulic torque tools, and tensioning equipment. Unlike stud bolts (which are headless) or carriage bolts (which have anti-rotation necks), hex bolts require wrench access to both head and nut during assembly — a key installation geometry constraint that must be verified in the design phase.
In EPC projects, hex bolts appear in structural steel connections (moment frames, braced frames, base plates), equipment mounting and skid fabrication, heat exchanger and pressure vessel shell connections, valve actuator mounting, machinery base frames, pipe support structures, grating and checkered plate fastening, and general mechanical assembly across all process plant disciplines. Their prevalence in project BOMs makes correct specification — grade, dimensional standard, thread system, coating, and certification level — a high-impact procurement engineering task.
RR Hydraulic manufactures and exports hex bolts to global EPC projects under all applicable international standards with EN 10204 3.1 / 3.2 material traceability and full project documentation.
1.2 — Hex Bolt Classification and Type Descriptions
Hex Cap Screw (Full Thread)
Hexagonal head; thread continuous from under-head to tip. Per ASME B18.2.1 (hex cap screw) and ISO 4017 / DIN 933. Used in tapped-hole (threaded-body) applications where the bolt threads directly into the component. Thread class: 2A (UNC) or 6g (metric).
Hex Bolt (Partial Thread)
Hexagonal head with unthreaded shank (grip) between head and thread start. Per ASME B18.2.1 (heavy hex bolt), ISO 4014, DIN 931. Shank passes through clearance holes; thread engages nut only. The preferred geometry for through-bolted structural and equipment connections.
Heavy Hex Bolt
Larger head width-across-flats (WAF) than standard hex — provides greater bearing area under the head, reducing contact stress on soft flange or structural faces. Per ASME B18.2.1 heavy hex series. Required for high-strength alloy grades (A193 B7) on pressure piping flanges.
High-Strength Structural Bolt (A325 / A490)
Hex bolt in medium-carbon (A325, F3125 Gr.A325) or alloy steel (A490, F3125 Gr.A490) with minimum UTS 830 MPa and 1040 MPa respectively. Used in preloaded (slip-critical) and snug-tight structural connections per AISC 360. Proof-load tested; supplied with matching A563 nut and F436 washer.
Tension Control (TC) Bolt
High-strength hex bolt with an integral spline end that shears off when target preload is achieved during installation — providing a visible, auditable preload indicator. Per ASTM F1852 (A325TC) and F2280 (A490TC). Eliminates torque wrench and calibration requirements; widely specified in structural steelwork erection.
Flange Head Hex Bolt (Hex Flange)
Integral washer-flange on the underside of the hex head — eliminates need for a separate washer. Often serrated to provide vibration loosening resistance. Common in machinery, automotive, and equipment assembly. Not typically used in pressure piping flange service but specified in machinery base frame bolting on EPC projects.
1.3 — Hex Bolt vs Alternative Fastener Types: Engineering Selection Logic
| Parameter | Hex Bolt | Stud Bolt | Socket Head Cap Screw | Carriage Bolt | Engineering Implication |
|---|---|---|---|---|---|
| Head geometry | Hexagonal — open / ring wrench | No head — two nuts | Hex socket — Allen key / bit | Dome — no wrench needed | Hex bolt: versatile wrench access |
| Clamping symmetry | Asymmetric (head / nut) | Symmetric (nut / nut) | Asymmetric | Asymmetric (dome / nut) | Stud bolt preferred for high-temp cycling |
| Load capacity | High — full shank section | High | Very high (alloy) | Moderate | All suitable for structural/mechanical |
| Head access space | Moderate (hex wrench space) | Both ends — nut space | Compact — recessed socket | None required | Socket head where hex clearance limited |
| Pressure piping flanges | Class 150–300 common | Standard Class 300–2500 | Not standard | Not applicable | Stud bolt preferred above Class 600 |
| Structural steel connections | Primary standard (A325/A490) | Rare | Not standard | Timber connections | Hex bolt is the structural standard |
| Tensioning method | Torque wrench / hydraulic / TC | Hydraulic tensioner / torque | Torque wrench | Torque wrench (nut only) | Hex bolt compatible with all tightening methods |
| Standards | ASME B18.2.1, ISO 4014/17, DIN 931/933 | ASTM A193, DIN 976 | ASME B18.3, DIN 912 | ASME B18.5, DIN 603 | Verify standard per project spec |
1.4 — Bolt Preload Behaviour: Embedment, Thermal Cycling, Vibration
Embedment Relaxation
Surface asperities on thread flanks and under-head bearing faces plastically deform within the first 24–72 hours after installation, reducing effective preload by 5–15%. Re-torquing after initial embedment (snug check) is mandated by ASME PCC-1 and AISC 360 for critical joints. Heavy hex head bolts exhibit less embedment than standard hex due to larger bearing area.
Thermal Cycling
Temperature differential between bolt and connected members causes cyclic preload variation. In high-temperature service (above 250°C), creep of bolt material reduces sustained preload — ASTM A193 B7 or B16 grades with creep resistance must be specified. Retorquing at operating temperature is mandated in ASME B31.1 for hot piping flange joints.
Vibration Loosening (Junker Effect)
Transverse vibration causes progressive nut rotation and loss of preload. Risk zones: compressor and pump base frames, rotating equipment supports, conveyor structures, and pipeline vibrating supports. Mitigation: achieve ≥75% yield preload, specify prevailing-torque nuts (ASTM F594), or apply approved thread-locking compound per project engineering.
Stress Relaxation at Elevated Temperature
Bolt material stress relaxation at sustained high temperatures reduces preload independently of mechanical loosening. Rate depends on material grade, temperature, and initial stress. A193 B7 rated to 454°C; B16 (Cr-Mo-V) rated to 593°C. Above these limits, alternative grades (Alloy 718, Inconel 625) are required.
Hydrogen Embrittlement
High-strength hex bolts (A325, A490, Grade 10.9, 12.9; hardness >34 HRC) are susceptible to hydrogen embrittlement from electroplating processes. Post-plate baking per ASTM F1941 (190°C, min 4 hours) is mandatory for electroplated high-strength hex bolts. Hot dip galvanising, Geomet, or PTFE coatings are preferred alternatives.
Fatigue Failure Modes
Thread root is the primary fatigue crack initiation site (Kt = 3.0–4.5 for standard UNC / metric coarse threads). Fatigue life is maximised by: achieving target preload ≥50% yield (reduces alternating stress amplitude), rolled threads (compressive residual stress), fillet radius at head-shank transition, and avoiding stress concentrations from thread damage or corrosion pitting.
1.5 — Pressure Cycling and Fatigue Life Assessment
In pressure equipment and dynamic machinery applications, hex bolts subject to cyclic tensile loading must be assessed for fatigue using the cumulative damage approach. The Miner’s Rule summation governs remaining bolt life:
nᵢ = Applied cycles at stress amplitude level i
Nᵢ = Cycles to failure at stress amplitude i from bolt S-N curve (grade-specific)
Thread root stress amplitude: Δσ = (ΔF / A_s) × Kt
ΔF = Alternating load amplitude (N); A_s = Stress area (mm²); Kt = Thread stress concentration factor (3.0–4.5)
1.6 — Design Safety Factors by Application Category
| Application Category | Safety Factor (UTS/Working Load) | Target Preload (% Yield) | Governing Code | Key Engineering Note |
|---|---|---|---|---|
| General structural — snug-tight | 3:1 | Snug only (no preload target) | AISC 360 Table J3.1 | Bearing-type connection; hole bearing governs |
| Structural — pretensioned | 3:1 | 70% proof load (AISC) | AISC 360 / EN 1993-1-8 | Turn-of-nut, TC bolt, or direct tension indicator |
| Structural — slip-critical | 4:1 | 70% proof load | AISC 360 / RCSC | Friction interface governs; bolt preload critical |
| ASME B16.5 flanges Cl.150–300 | 3:1–4:1 | 50–65% yield | ASME B31.3 / PCC-1 | Hex bolt or stud bolt; gasket seating check required |
| Equipment mounting / skid | 3:1 | 50–60% yield | OEM / project spec | Dynamic load factor applied; vibration lock-nut |
| High-temperature service (>300°C) | 4:1 | 50–60% yield | ASME B31.1 / BPVC | A193 B7 or B16; retorque at operating temp |
| NACE sour service | 4:1 | ≤50% yield | NACE MR0175 / ISO 15156 | Max 22 HRC; B7M grade; hardness certificate mandatory |
| Cryogenic / LNG (below −29°C) | 4:1–5:1 | 55–65% yield | ASTM A320 / B31.3 | Charpy impact test mandatory; L7 / L7M grade |
| Offshore / marine structural | 4:1 | 60–70% yield | DNV-ST-N001 / AISC | HDG or SS grade; DAF factor applied to design loads |
Submit your BOM, material specification, dimensional standard, grade, and quantity for a fully documented RFQ within 24 hours.
Grades & Standards
Compliance
Hex bolt dimensions are governed by ASME B18.2.1 (inch), ISO 4014 / ISO 4017 (metric), DIN 931 / DIN 933 (metric), and BS 3692 (metric). Grade and mechanical properties are prescribed by ISO 898-1, ASTM A307, A325, A490, A193, and A320. All applicable standards are supported at RR Hydraulic with full certification.
Submit grade, standard, size, coating, and quantity to sales@rrhydraulics.com for a fully certified commercial offer.
2.1 — ASME B18.2.1 Heavy Hex Bolt Dimensional Table (Inch Series)
The following table provides key dimensional data for heavy hex bolts per ASME B18.2.1 — the standard geometry for pressure piping flange bolting and high-strength structural bolting in ASME-coded systems. Heavy hex dimensions provide greater bearing area and wider wrench engagement than standard hex.
| Nom. Dia. (in) | TPI (UNC) | WAF Standard (in) | WAF Heavy (in) | Head Height Std (in) | Head Height Heavy (in) | Stress Area (in²) | Min Thread Length (in) |
|---|---|---|---|---|---|---|---|
| ¼” | 20 | 0.438 | 0.500 | 0.163 | 0.188 | 0.0318 | 0.750 |
| 5/16″ | 18 | 0.500 | 0.563 | 0.211 | 0.235 | 0.0524 | 0.875 |
| ⅜” | 16 | 0.563 | 0.625 | 0.243 | 0.268 | 0.0775 | 1.000 |
| ½” | 13 | 0.750 | 0.875 | 0.323 | 0.364 | 0.1419 | 1.000 |
| ⅝” | 11 | 0.938 | 1.063 | 0.403 | 0.444 | 0.2260 | 1.250 |
| ¾” | 10 | 1.125 | 1.250 | 0.483 | 0.524 | 0.3340 | 1.375 |
| ⅞” | 9 | 1.313 | 1.438 | 0.563 | 0.604 | 0.4620 | 1.500 |
| 1″ | 8 | 1.500 | 1.625 | 0.627 | 0.700 | 0.6060 | 1.750 |
| 1¼” | 7 | 1.875 | 2.000 | 0.780 | 0.876 | 0.9690 | 2.000 |
| 1½” | 6 | 2.250 | 2.375 | 0.876 | 1.000 | 1.4050 | 2.250 |
| 1¾” | 5 | 2.625 | 2.750 | 1.036 | 1.136 | 1.9000 | 2.500 |
| 2″ | 4.5 | 3.000 | 3.000 | 1.196 | 1.196 | 2.5000 | 2.750 |
| 2½” | 4 | 3.750 | 3.750 | 1.444 | 1.444 | 3.9500 | 3.250 |
| 3″ | 4 | 4.500 | 4.500 | 1.628 | 1.628 | 5.6200 | 3.750 |
2.2 — ISO 4014 / ISO 4017 Metric Hex Bolt Dimensional Table
| Thread | Pitch (mm) | WAF e (mm) | WAC (mm) | Head Height k (mm) | Stress Area As (mm²) | Thread Length ISO 4014 b (mm) | ISO Property Class |
|---|---|---|---|---|---|---|---|
| M6 | 1.00 | 10 | 11.55 | 4.15 | 20.1 | 18 | 4.6 / 8.8 / 10.9 / 12.9 |
| M8 | 1.25 | 13 | 15.01 | 5.45 | 36.6 | 22 | 4.6 / 8.8 / 10.9 / 12.9 |
| M10 | 1.50 | 16 | 18.48 | 6.58 | 58.0 | 26 | 4.6 / 8.8 / 10.9 / 12.9 |
| M12 | 1.75 | 18 | 20.78 | 7.76 | 84.3 | 30 | 4.6 / 8.8 / 10.9 / 12.9 |
| M16 | 2.00 | 24 | 27.71 | 10.29 | 157.0 | 38 | 4.6 / 8.8 / 10.9 / 12.9 |
| M20 | 2.50 | 30 | 34.64 | 12.85 | 245.0 | 46 | 4.6 / 8.8 / 10.9 |
| M24 | 3.00 | 36 | 41.57 | 15.44 | 353.0 | 54 | 4.6 / 8.8 / 10.9 |
| M27 | 3.00 | 41 | 47.34 | 17.21 | 459.0 | 60 | 8.8 / 10.9 |
| M30 | 3.50 | 46 | 53.12 | 19.12 | 561.0 | 66 | 8.8 / 10.9 |
| M36 | 4.00 | 55 | 63.51 | 22.78 | 817.0 | 78 | 8.8 / 10.9 |
| M42 | 4.50 | 65 | 75.06 | 26.97 | 1121.0 | 90 | 8.8 |
| M48 | 5.00 | 75 | 86.60 | 30.65 | 1473.0 | 102 | 8.8 |
| M56 | 5.50 | 85 | 98.15 | 35.72 | 2030.0 | 116 | 8.8 |
| M64 | 6.00 | 95 | 109.70 | 40.35 | 2676.0 | 132 | 8.8 |
2.3 — Applicable Standards and Compliance Framework
ASME B18.2.1
Square, hex, heavy hex bolts and hex cap screws — inch series. The governing dimensional standard for hex bolts in North American and ASME-coded EPC projects. Defines head geometry, WAF, head height, thread class (2A, UNC), and length tolerances for standard and heavy hex variants.
ISO 4014 / ISO 4017
Hex head bolts — metric series, Product Grade A and B. ISO 4014: partial thread (hex bolt). ISO 4017: full thread (hex cap screw). Thread tolerance 6g/6H per ISO 965. Property classes per ISO 898-1. The governing metric standard for European and Asian EPC projects.
DIN 931 / DIN 933
DIN 931: hex head bolts — partial thread. DIN 933: hex head bolts — full thread. Both metric coarse pitch. Historically the dominant European standard; now largely superseded by ISO 4014/4017 but still widely specified in legacy plant, replacement supply, and DIN-coded equipment documentation.
ISO 898-1
Mechanical properties of bolts, screws, and studs — metric series. Defines property classes 4.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9 with UTS, yield/proof, elongation, and hardness requirements. The primary metric strength classification system — equivalent to ASTM A307/A325/A490 for inch-series bolts.
ASTM A307
Low-carbon steel bolts, min UTS 414 MPa (60 ksi). Grade A: structural and general use. Grade B: flanged joints on cast iron flanges per ASME B16.1. The lowest structural grade — used in non-critical connections, secondary structures, and utility piping supports where design loads are modest.
ASTM A325 / F3125 Gr.A325
High-strength structural hex bolts, medium-carbon or alloy steel, quenched and tempered. Min UTS 830 MPa (120 ksi) for sizes ≤1″. The primary structural bolt grade per AISC 360 for bearing-type and pretensioned connections. F3125 is the current consolidated ASTM standard replacing A325 and A490.
ASTM A490 / F3125 Gr.A490
High-strength alloy steel structural hex bolts. Min UTS 1040 MPa (150 ksi). Used where A325 capacity is insufficient — heavily loaded moment connections, seismic connections, and high-load base plates. Must not be galvanised (hydrogen embrittlement risk at this strength level).
ASTM A193 / A320
High-temperature (A193) and low-temperature (A320) alloy steel bolting. B7 (4140/4142, Q&T) is the standard grade for pressure piping hex bolts in carbon steel service. L7 (A320) is the cryogenic equivalent with mandatory Charpy impact testing. Used where ISO 898-1 grades are insufficient for temperature or corrosion requirements.
NACE MR0175 / ISO 15156
Hardness limits for carbon and alloy steel in H₂S sour service: max 22 HRC (237 HB). ASTM A193 B7M (controlled hardness Q&T, max 22 HRC) is the standard NACE-compliant grade for hex bolts in sour service equipment. 100% hardness verification on every bolt lot is mandatory for sour service supply.
2.4 — Thread Form and Compatibility Reference
| Thread System | Standard | Size Range | Pitch / TPI | Bolt / Nut Tolerance | Governing Bolt Standard | Regional Application |
|---|---|---|---|---|---|---|
| UNC Coarse | ASME B1.1 | ¼”–4″ | 4–20 TPI | 2A / 2B (std); 3A / 3B (precision) | ASME B18.2.1 / A193 / A307 / A325 | USA, Canada, global ASME EPC |
| UNF Fine | ASME B1.1 | ¼”–1½” | 12–28 TPI | 2A / 2B | Special order / precision machinery | Aerospace, precision equipment |
| Metric Coarse | ISO 261 / DIN 13 | M1.6–M100 | 0.35–6.0 mm | 6g / 6H (std); 5g / 5H (precision) | ISO 4014 / 4017 / DIN 931 / 933 | Europe, Asia, global metric EPC |
| Metric Fine | ISO 261 | M8–M52 | 0.75–3.0 mm | 6g / 6H | Special order — precision mechanical | High-precision machinery, thin-wall assemblies |
| BSW (Whitworth) | BS 84 | ⅛”–4″ | 2.5–28 TPI | Class 1–3 | Legacy BS 1083 / BS 4190 | UK legacy; Commonwealth plant replacement |
| BA (British Association) | BS 57 | 0BA–16BA | Fine | BS 57 | Legacy instrumentation / electrical | Legacy electrical enclosures; not structural |
2.5 — Torque Chart: Hex Bolt Installation
The following torque values target 70% of bolt proof load — the standard pretensioning target per AISC 360 for structural bolts and ASME PCC-1 for pressure equipment bolts. Values are provided for ISO 898-1 property classes 8.8 and 10.9, and ASTM A325 / A490, across the most common hex bolt sizes.
| Size | Thread | Grade | Proof Load (kN) | 70% Proof Preload (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) |
|---|---|---|---|---|---|---|---|---|
| M12 | M12×1.75 | 8.8 | 59.3 | 41.5 | 100 | 75 | 60 | 50 |
| M12 | M12×1.75 | 10.9 | 83.4 | 58.4 | 140 | 105 | 84 | 70 |
| M16 | M16×2.0 | 8.8 | 110.4 | 77.3 | 247 | 185 | 148 | 124 |
| M16 | M16×2.0 | 10.9 | 155.2 | 108.6 | 348 | 261 | 209 | 174 |
| M20 | M20×2.5 | 8.8 | 171.5 | 120.1 | 480 | 360 | 288 | 240 |
| M20 | M20×2.5 | 10.9 | 245.0 | 171.5 | 686 | 515 | 412 | 343 |
| M24 | M24×3.0 | 8.8 | 247.0 | 172.9 | 830 | 622 | 498 | 415 |
| M24 | M24×3.0 | 10.9 | 353.0 | 247.1 | 1186 | 890 | 712 | 593 |
| M30 | M30×3.5 | 8.8 | 392.7 | 274.9 | 1650 | 1237 | 990 | 825 |
| M30 | M30×3.5 | 10.9 | 560.7 | 392.5 | 2355 | 1766 | 1413 | 1177 |
| M36 | M36×4.0 | 8.8 | 572.0 | 400.4 | 2883 | 2162 | 1730 | 1441 |
| M36 | M36×4.0 | 10.9 | 817.0 | 571.9 | 4118 | 3088 | 2471 | 2059 |
| ½” A325 | ½”–13 UNC | A325 | 85.9 | 60.1 | 153 | 115 | 92 | 76 |
| ¾” A325 | ¾”–10 UNC | A325 | 201.4 | 141.0 | 536 | 402 | 321 | 268 |
| 1″ A325 | 1″–8 UNC | A325 | 310.0 | 217.0 | 1105 | 829 | 663 | 552 |
| 1″ A490 | 1″–8 UNC | A490 | 388.0 | 271.6 | 1382 | 1037 | 829 | 691 |
| 1¼” A490 | 1¼”–7 UNC | A490 | 600.0 | 420.0 | 2673 | 2004 | 1604 | 1336 |
2.6 — Preload Calculation Guide
T = Applied torque (Nm)
K = Nut factor: 0.20 (dry), 0.15 (machine oil), 0.12 (MoS₂ / Molykote), 0.10 (PTFE)
d = Nominal diameter (m)
Proof Load (AISC 360 pretension target = 70% proof load):
F_proof = A_s × Sp (proof strength per ISO 898-1 or ASTM grade)
Target: F_b = 0.70 × F_proof (structural) or 0.50–0.65 × F_yield (pressure equipment)
A_s = 353 mm², Sp (10.9) = 830 MPa, Proof Load = 353 × 830 = 292,990 N = 293.0 kN
Target F_b = 0.70 × 293.0 = 205.1 kN
Required torque T = 205,100 × 0.12 × 0.024 = 591 Nm
Table 2.D MoS₂ value at 70% proof = 712 Nm (using full proof load basis) — adjust proportionally to actual target preload.
Heat Treatment
& Manufacturing Process
Hex bolt material selection spans the widest range of any fastener type — from low-carbon A307 for general structural use, through high-strength A490 for slip-critical structural connections, to A193 B7 for pressure piping and exotic alloys for extreme environments. RR Hydraulic manufactures hex bolts in all standard and high-performance grades with full heat treatment documentation and EN 10204 traceability.
3.1 — Material Grade Overview and Mechanical Properties
| Grade / Class | Spec | Base Material | UTS (MPa) | Yield / Proof (MPa) | Elong. (%) | Temp Range (°C) | HRC Max | Primary Use Case |
|---|---|---|---|---|---|---|---|---|
| Class 4.6 | ISO 898-1 | Low-carbon steel | 400 | 240 | 22 | −20 to +300 | — | Non-critical general structural |
| Class 8.8 | ISO 898-1 | Medium-carbon alloy Q&T | 800 | 640 | 12 | −20 to +300 | 34 | Standard structural, equipment mounting |
| Class 10.9 | ISO 898-1 | Alloy steel Q&T | 1040 | 940 | 9 | −20 to +300 | 39 | High-strength structural, heavy machinery |
| Class 12.9 | ISO 898-1 | Alloy steel Q&T | 1220 | 1100 | 8 | −20 to +300 | 44 | Very high-strength; not for HDG; socket head typical |
| A307 Gr.A/B | ASTM A307 | Low-carbon steel | 414–724 | — | 18 | −29 to +260 | — | General construction, secondary structure |
| A325 (≤1″ dia) | ASTM A325 / F3125 | Med-carbon steel Q&T | 830 | 635 | 14 | −29 to +300 | 33 | Structural steel connections, AISC 360 |
| A490 (≤1½” dia) | ASTM A490 / F3125 | Alloy steel Q&T | 1040 | 940 | 14 | −29 to +300 | 38 | High-load structural, seismic connections |
| A193 B7 | ASTM A193 | 4140/4142 Cr-Mo Q&T | 860–1000 | 723 | 16 | −45 to +454 | 35 (26 NACE) | Pressure piping flanges, process equipment |
| A193 B7M | ASTM A193 | 4140/4142 Q&T controlled | 690–862 | 552 | 18 | −45 to +454 | 22 | NACE sour service, H₂S environments |
| A320 L7 | ASTM A320 | 4140 Q&T + impact tested | 860 | 723 | 16 | −101 to +343 | 35 | Cryogenic, LNG, low-temperature piping |
| A193 B8M Cl.2 | ASTM A193 | SS 316 strain-hardened | 690 | 414 | 12 | −196 to +816 | — | Corrosive, high-temp, SS flanges |
| Duplex 2205 | UNS S31803 | Duplex SS | 620 | 450 | 25 | −50 to +315 | 28 | Offshore, subsea, sour + chloride |
| Super Duplex S32750 | UNS S32750 | Super Duplex SS | 750 | 550 | 20 | −50 to +260 | 32 | Seawater, aggressive chloride, subsea |
| Inconel 625 | UNS N06625 | Ni-Cr-Mo alloy | 827 | 414 | 30 | −196 to +980 | — | Extreme temp, sour + Cl⁻, cryogenic |
3.2 — Yield Strength and Mechanical Properties by Heat Treatment
| Grade / Spec | Heat Treatment | UTS (MPa) | Yield / Proof (MPa) | Elongation (%) | Reduction of Area (%) | Hardness (HB) | Charpy Impact (J / Temp) |
|---|---|---|---|---|---|---|---|
| ISO 4.6 | As-rolled / normalised | 400 | 240 | 22 | — | Max 250 | Not req. |
| ISO 8.8 (d ≤ 16 mm) | Q&T or cold-work | 800 | 640 | 12 | 52 | 245–320 | Not req. |
| ISO 8.8 (d > 16 mm) | Q&T 840°C / 550°C | 800 | 660 | 12 | 52 | 245–320 | Not req. |
| ISO 10.9 | Q&T 880°C / 500°C | 1040 | 940 | 9 | 48 | 300–380 | Not req. |
| ISO 12.9 | Q&T 880°C / 480°C | 1220 | 1100 | 8 | 44 | 365–435 | Not req. |
| A325 (≤ 1″) | Q&T — medium carbon | 830 | 635 | 14 | 40 | 235–308 | Not req. |
| A490 | Q&T — alloy steel | 1040 | 940 | 14 | 40 | 300–360 | Not req. |
| A193 B7 (≤ 2½”) | Q&T 860°C / 620°C | 860–1000 | 723 | 16 | 50 | 248–302 | 54 J (not req.) |
| A193 B7M | Q&T (hardness controlled) | 690–862 | 552 | 18 | 50 | 200–235 | Not req. |
| A320 L7 | Q&T + Charpy impact tested | 860 | 723 | 16 | 50 | 248–302 | 20 J min @ −101°C |
| Duplex 2205 | Solution annealed 1020–1100°C | 620 | 450 | 25 | 45 | Max 293 | N/A (austenitic-ferritic) |
| Super Duplex S32750 | Solution annealed 1025–1125°C | 750 | 550 | 20 | 40 | Max 310 | N/A |
3.3 — Corrosion Resistance by Material vs Service Media
| Material | H₂S Sour* | CO₂ / Wet Gas | Cl⁻ / Seawater | HCl (dilute) | Caustic / Alkali | High-Temp Oxidation | Marine Atmosphere |
|---|---|---|---|---|---|---|---|
| ISO 8.8 / A307 (HDG) | Conditional* | Fair | Poor (bare) / Fair (HDG) | Poor | Good | Good to 300°C | Fair (HDG) |
| A193 B7 (plain) | Conditional* | Fair | Poor | Poor | Good | Good to 454°C | Poor |
| A193 B7M (NACE) | Good* | Fair | Poor | Poor | Good | Good to 454°C | Poor |
| A193 B8 (SS 304) | Fair | Good | Poor (SCC risk) | Fair | Very Good | Very Good to 816°C | Good |
| A193 B8M (SS 316) | Good | Very Good | Fair | Good | Very Good | Very Good to 816°C | Very Good |
| Duplex 2205 | Very Good | Excellent | Very Good | Very Good | Very Good | Limited >315°C | Excellent |
| Super Duplex S32750 | Excellent | Excellent | Excellent (PREN>42) | Excellent | Excellent | Limited >260°C | Excellent |
| Inconel 625 | Excellent | Excellent | Excellent | Excellent | Excellent | Excellent to 980°C | Excellent |
* Sour service: hardness ≤22 HRC mandatory per NACE MR0175. Standard A193 B7 (up to 35 HRC) is NOT acceptable for sour service without B7M controlled-hardness designation.
3.4 — Manufacturing Process
3.4.1 — Cold Heading and Hot Forging of Hex Head
Hex bolt heads are formed by cold heading (for diameters typically up to M24 / 1″) or hot forging (for larger diameters). Cold heading produces a continuous, unbroken grain flow around the head-shank fillet — the highest-stress zone in a tensile-loaded bolt. This grain continuity maximises fatigue resistance at the critical head-shank transition. The hexagonal head geometry is formed in a multi-stage die set, producing dimensional consistency compliant with ASME B18.2.1 and ISO 4014 / 4017 tolerances.
3.4.2 — Thread Rolling vs Thread Cutting
- Thread rolling (cold-forming): Compressive residual stress at thread root; continuous grain flow through thread form — superior fatigue life and surface finish. Standard for ISO 8.8, 10.9, A325, A490, and A193 B7 hex bolts. Surface finish Ra ≤ 1.6 µm at thread root achievable.
- Thread cutting (machining): Interrupts grain flow; tensile residual stress possible at thread root — lower fatigue life than rolled threads. Necessary for large diameters (M48 and above), hard alloys, and exotic grades (duplex, SS, Inconel) where rolling is impractical. Specify rolled threads explicitly when fatigue performance is critical.
- Thread class: 2A (UNC, ASME B1.1) and 6g (metric, ISO 965) are standard. Class 3A / 5g on request for precision-loaded, high-preload joints or hydraulic tensioner applications where dimensional scatter must be minimised.
3.4.3 — CNC Machining Tolerances
- Head width-across-flats (WAF): per ASME B18.2.1 Table 2 (standard) or Table 5 (heavy); per ISO 4014 / 4017 Table 1 for metric
- Head height: ±0.38 mm (sizes ½”–1″); ±0.5 mm (sizes above 1″) per ASME B18.2.1
- Thread pitch diameter: per ASME B1.1 Class 2A or ISO 965 Class 6g, with verified Go/No-Go gauging on 100% of production lots
- Length tolerance: ±1 pitch for lengths up to 150 mm; ±2 pitches above 150 mm per DIN / ISO; ±1/32″ for inch series per ASME B18.2.1
- Head-to-shank squareness: max 1° from perpendicular (ASME B18.2.1)
- Shank bow: max 0.5 mm per 100 mm of shank length (measured on surface plate)
- Surface finish: shank Ra ≤ 3.2 µm; thread form Ra ≤ 1.6 µm; head bearing face Ra ≤ 1.6 µm
3.5 — Surface Finish Options
| Finish | Specification | Thickness (µm) | Salt Spray (h) | Max Service Temp (°C) | Effect on K Factor | H₂E Risk (High-Strength) | Typical Application |
|---|---|---|---|---|---|---|---|
| Plain / Bare | Per ASTM A193 / ISO 898-1 | — | <24 | 454 (B7) | K=0.20 | None | Immediate assembly; lubricant applied |
| Black Phosphate | MIL-DTL-16232 | 5–15 | 24–72 | 150 | K=0.15–0.17 | Low | Short-term storage; standard CS bolts |
| Zinc Electroplate | ASTM B633 Type II/III | 5–25 | 96–500 | 120 | K=0.18–0.20 | HIGH for Grade 10.9 / A490 | General EPC, indoor; bake-out mandatory for 10.9+ |
| Hot Dip Galvanise (HDG) | ASTM A153 / ISO 1461 | 45–85 | 2000+ | 200 | K=0.20–0.22 | None (no hydrogen) | Outdoor structural, marine atmosphere, A307/A325 |
| Geomet / Dacromet | ISO 10683 | 8–12 | 1000–1500 | 300 | K=0.13–0.16 | None | Offshore structural, treated timber adjacent |
| PTFE / Teflon | Proprietary | 20–50 | 1000+ | 260 | K=0.10–0.12 | None | Precision torque control; SS anti-galling |
| Molykote / MoS₂ | Applied at assembly | — | — | 450 | K=0.11–0.13 | N/A | High-temp pressure equipment assembly |
| Nickel Anti-Seize | Applied at assembly | — | — | 1200 | K=0.13–0.16 | N/A | SS and duplex hex bolts; prevents galling |
| Passivation (SS / Duplex) | ASTM A380 / A967 | Passive layer | 500–2000+ | Per alloy | K=0.20 | None | All SS 304/316, duplex, super duplex hex bolts |
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from raw material heat to final packed shipment on all hex bolt orders. Dimensional inspection reports, EN 10204 3.1 / 3.2 MTRs, NACE compliance statements, NDT reports, coating certificates, and complete EPC export documentation packages are standard on all project-grade supply.
4.1 — Inspection & NDT Protocol
4.2 — EN 10204 Material Test Certificate Requirements
| Certificate | Content | Signatory | EPC Requirement | When Mandatory |
|---|---|---|---|---|
| 2.1 | Conformity declaration only — no test data | Manufacturer | Non-critical utility / secondary structure | General commercial procurement only |
| 2.2 | Non-specific test results; not heat-traceable | Manufacturer | Low-criticality supply | Rarely accepted in EPC O&G or structural projects |
| 3.1 | Heat-traceable mech + chem; specific to production lot | Manufacturer’s authorised QC Inspector | Minimum for all pressure, structural, and NACE supply | All A193 B7, A325, A490, SS/duplex, cryogenic |
| 3.2 | 3.1 content + countersigned by independent TPI | Manufacturer + SGS / BV / DNV / Lloyds | Critical structural, NACE, offshore, cryogenic | All offshore, sour service, Class 900+ pressure joints |
4.3 — Assembly and Pressure Test Requirements
Hex bolts in assembled pressure piping and equipment joints are verified through system hydrostatic or pneumatic testing. Key test parameters per ASME B31.3:
- Hydrostatic test pressure: 1.5 × MAWP at test temperature — ASME B31.3 Clause 345.4
- Pneumatic test pressure: 1.1 × MAWP — ASME B31.3 Clause 345.5
- Hold duration: Minimum 10 minutes at test pressure before inspection
- Acceptance: Zero visible leaks at any bolted joint, gasketed connection, or threaded interface
- Structural bolt verification: Pretensioned and slip-critical structural hex bolt joints are verified by bolt inspection (calibrated torque wrench check) per AISC 360 and RCSC Specification — not by hydrostatic testing
- Direct Tension Indicator (DTI) verification: Where DTI washers are specified, gap compliance measured with feeler gauge at minimum 50% of bolt locations per batch after installation
- Post-hydrostatic retorque: Required for high-temperature service flanges before commissioning at operating temperature per ASME PCC-1
4.4 — Applications by Industry
Oil & Gas — Upstream / Wellhead
Hex bolts in A193 B7 / B7M for wellhead equipment, manifold connections, and surface facility structural steel. NACE MR0175 compliance mandatory for all bolting in H₂S-containing service. EN 10204 3.2 with TPI witness required on critical equipment. API 6A / 20E compliance on wellhead structural bolting.
Refinery & Petrochemical
Hex bolts across all structural steel categories: pipe rack frames, equipment support structures, heat exchanger supports, vessel skirts, fired heater structures, and secondary containment walls. A193 B7 for high-temperature process equipment mounting. A307 / ISO 8.8 for secondary structural. HDG coating standard for outdoor structural supply.
Structural Steel — EPC
ASTM A325 (F3125 Gr.A325) is the standard structural bolt for all EPC building and process structure connections — moment frames, braced frames, column base plates, beam-column connections. Pretensioned or slip-critical classification per structural engineer specification. Turn-of-nut or TC bolt tightening methods standard on EPC projects.
LNG Terminals
Cryogenic hex bolts in A320 L7 for pipe support structures and equipment frames at −165°C service temperature. Charpy impact testing at −196°C on special project specifications. SS A193 B8M or Inconel 625 hex bolts for cryogenic flanged connections. Anti-galling nickel anti-seize lubricant mandatory for SS-to-SS head bearing surfaces.
Power Generation
Steam turbine and boiler structural hex bolts in A193 B16 (Cr-Mo-V, rated to 593°C) or A193 B7 (to 454°C). Generator and transformer base frame bolting in ISO 8.8 or A325 with anti-vibration Nyloc nuts. Cooling tower structural bolting in A307 HDG or SS A4-70 in chemical-exposure zones of the cooling water circuit.
Offshore Platforms & FPSOs
Topside structural bolting in A325 HDG or Duplex 2205 for primary deck and module support frames in marine atmosphere. Equipment module lifting trunnion hex bolts verified by DNV or Lloyds TPI with EN 10204 3.2 MTR. HISC assessment required for super duplex hex bolts in cathodic protection zones per DNV RP-F112.
4.5 — Export Packaging Specification
- Individual hex bolts wrapped in VCI (Volatile Corrosion Inhibitor) poly film for all plain and coated grades — prevents atmospheric oxidation during ocean freight and extended site storage up to 24 months
- Thread end protection: plastic thread protector caps on all bolt ends; hex head protection sleeve for coated grades where coating integrity during transit is critical (HDG, epoxy-coated)
- Bundle tagging: each bundle labelled with heat number, material grade/property class, standard (ASME B18.2.1 / ISO 4014 / DIN 931), nominal size, length, surface finish, quantity, and PO reference — full traceability maintained through shipping chain
- Segregation by heat and grade: different heat numbers and grades in separate compartments / polybags within the same carton — prevents mix-up during goods receipt inspection at project site
- HDG hex bolts: packed with matching oversize nuts (A563 / ISO 898-6 HDG) and structural washers (F436 / ISO HDG) in the same bundle where specified by the purchase order
- A325 / A490 structural bolt sets: hex bolt + A563 nut + F436 washer packed as a matched set per AISC / RCSC requirements — lot-traced and verified as a complete assembly
- Outer crating: ISPM-15 heat-treated pine wooden crates for all international export shipments — crate dimensions optimised to standard 20′ / 40′ container ISO modular pallet grid
- Crate markings: Project PO, item tag, grade, surface finish, gross weight, net weight, dimensions (L×W×H mm), country of origin, “FRAGILE — HANDLE WITH CARE” and stack height symbols per ISO 780 / ASTM D5276
- Documentation: packing list enclosed inside crate lid + external waterproof envelope; cross-referenced to MTC certificate numbers, lot 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 structural and pressure supply | Heat-specific; traceable to production lot |
| 02 | Chemical Composition Report | Heat / lot certified lab analysis | Mandatory (within MTC) | All major and trace alloying elements |
| 03 | Mechanical Properties Report | UTS, yield/proof, elongation, RA, hardness | Mandatory (within MTC) | Per ISO 898-1 / ASTM A193 / A325 / A490 |
| 04 | Hardness Test Report | ASTM E10 / E18 (Brinell + Rockwell) | Mandatory — NACE / sour service; A193 B7M | Max 22 HRC; all lot results reported |
| 05 | Charpy Impact Test Report | ASTM A370 / EN 10045 | Mandatory — A320 L7 / cryogenic supply | Test temp + J-values per grade and service |
| 06 | Dimensional Inspection Report | Per ASME B18.2.1 / ISO 4014 / 4017 | Mandatory | Thread gauge, WAF, head height, length, chamfer |
| 07 | Thread Gauge Report | Go/No-Go per ASME B1.1 / ISO 965 | Mandatory | 100% gauged; results per lot |
| 08 | First Article Inspection (FAI) Report | Project-specific format | Mandatory — new items / first production batch | Released before batch production |
| 09 | MPI / LPI Report | ASTM E709 / E165; EN ISO 9934 / 3452 | Conditional — NACE, A193 B7, A490, offshore | Head-shank fillet primary inspection zone |
| 10 | PMI Report (XRF) | Per lot — SS / duplex / exotic grades | Mandatory — non-CS grades | 100% of SS 304/316/duplex/Inconel hex bolts |
| 11 | UT Report | ASTM A388 / EN 10308 | Conditional — >M48 / >1¾” diameter | Bar stock volumetric inspection pre-machining |
| 12 | Coating / Surface Finish Certificate | ASTM A153 / ISO 1461 / ISO 10683 / ASTM B633 | Mandatory — all coated supply | HDG thickness + salt spray hours |
| 13 | Hydrogen Bake-Out Certificate | ASTM F1941 | Conditional — electroplated Grade 10.9 / A490 | 190°C × 4h min; dated, signed, batch-specific |
| 14 | NACE MR0175 Compliance Statement | Hardness + HT confirmation | Mandatory — sour service supply | References specific heat and hardness readings |
| 15 | TPI Witness Certificate | SGS / BV / DNV / Lloyds countersigned | Mandatory — EN 10204 3.2 orders | Co-witness at manufacturer works |
| 16 | Structural Bolt Assembly Certification | AISC / RCSC — matched bolt-nut-washer set | Conditional — A325 / A490 structural sets | Lot-traced assembly; pretension verification |
| 17 | ISO 9001:2015 Certificate | Third-party QMS certification | Mandatory — EPC projects | Current scope covers hex bolt manufacture |
| 18 | Country of Origin Certificate | Chamber of Commerce issued | Mandatory — all export | Required for customs and duty classification |
| 19 | Packing List | Item-level detail per shipment | Mandatory | Cross-references MTC and lot numbers |
| 20 | Commercial Invoice | Per INCOTERMS 2020 | Mandatory | Includes HS tariff code |
| 21 | Bill of Lading / Air Waybill | Per freight mode | Mandatory | Issued by freight forwarder |
| 22 | MSDS / Safety Data Sheet | For coated / treated surfaces | Conditional — HDG, PTFE, MoS₂, Geomet | Required for hazardous shipping classification |
4.7 — ISO and Quality System Compliance
ISO 9001:2015
Quality Management System covering the full manufacturing cycle: raw material supplier qualification, heat treatment procedure qualification, in-process dimensional and NDT inspection, coating QC, final inspection release, and traceability documentation. Mandatory for all EPC, O&G, and structural project procurement qualification.
ISO 10474
Steel and steel products — inspection documents. Source framework for EN 10204 certificate type definitions. Legacy EPC specifications referencing ISO 10474 types 3.1.B and 3.1.C map to EN 10204 3.1 and 3.2 respectively. Current operative standard is EN 10204.
ASME PCC-1
Guidelines for Pressure Boundary Bolted Flange Joint Assembly. Defines tightening method selection, K factor documentation, cross-bolt sequencing, target preload specification, Flange Joint Assembly Record (FJAR) format, and retorquing requirements. The operative assembly engineering standard for all ASME-coded flanged joints using hex bolts or stud bolts.
ISO 4413
Safety requirements for hydraulic fluid power systems. Relevant where hex bolts are specified for hydraulic power unit (HPU) base frame mounting, hydraulic manifold connections, and hydraulic system structural attachments. Bolting specification on hydraulic equipment must be consistent with ISO 4413 safety engineering requirements and OEM design documentation.
Submit your BOM, grade schedule, coating specification, and quantity to RR Hydraulic for a complete, certified commercial offer.