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ASME
B18
Fasteners
A world-class technical reference for EPC engineers, mechanical designers, procurement heads, and TPI inspection agencies specifying fasteners to ASME B18 standards — covering the complete B18 fastener family from hex bolts and socket head cap screws to pins, washers, and specialty fasteners; their dimensional requirements; material grade and strength class selection; the critical ASTM-to-ASME B18 material cross-reference for process piping flange bolting; NACE MR0175 fastener compliance; installation torque engineering; and the full quality and documentation framework for EPC and industrial fastener project supply.
Family: Structure,
Scope & Coverage
ASME B18 is the comprehensive American standard for fasteners — governing the dimensional, mechanical, and quality requirements of bolts, screws, nuts, pins, washers, and specialty fasteners used in pressure equipment, process piping, structural assemblies, and industrial machinery worldwide.
1.1 — ASME B18: Purpose and Engineering Authority
ASME B18 is published by the American Society of Mechanical Engineers (ASME) and defines the complete dimensional and quality requirements for inch-series fasteners used in mechanical engineering, pressure equipment, and industrial construction. Unlike ASTM standards — which govern the material properties of fasteners (chemical composition, mechanical strength, heat treatment) — ASME B18 governs the geometry of fasteners: head dimensions, thread length, shank diameter tolerances, thread form (per ASME B1.1 Unified National thread), drive recess dimensions, and washer bore and OD. The complete fastener specification for any EPC or industrial application therefore requires both an ASME B18 dimensional standard (defining the geometry) and an ASTM material specification (defining the steel grade, strength, and heat treatment) — neither alone is sufficient.
ASME B18 covers the full range of inch-series fastener types used in process piping (flange stud bolts and heavy hex nuts), pressure vessel nozzle connections, structural steel connections, hydraulic and pneumatic manifold cap screws, machine assembly, and precision mechanical components. The B18 family is organised by fastener type: B18.2.x (hex bolts, screws, and nuts), B18.3 (socket products), B18.6 (machine screws), B18.8 (pins), B18.21 (spring washers), and B18.22 (flat washers).
1.2 — The Complete ASME B18 Fastener Family
| Standard | Fastener Type | Size Range | Primary EPC / Industrial Application |
|---|---|---|---|
| B18.2.1 | Square and Hex Bolts and Screws (inch) | ¼” – 4″ | Structural bolting; machinery; general hex bolt dimensions |
| B18.2.2 | Square and Hex Nuts (inch) | ¼” – 4″ | Heavy hex nuts for ASME B16.5 flange bolting (with B18.2.1 stud bolts) |
| B18.2.3.1M | Metric Hex Cap Screws | M5 – M100 | Metric fastener dimensional reference for mixed-unit projects |
| B18.2.4.6M | Metric Heavy Hex Nuts | M5 – M100 | Metric heavy hex nut reference |
| B18.3 | Socket Cap Screws and Set Screws (inch) | #0 – 4″ | Hydraulic manifold cap screws; valve body assembly; precision machinery |
| B18.6.2 | Slotted Head Machine Screws | #0 – ¾” | Instrument panel assembly; electrical enclosure; light machinery |
| B18.6.3 | Machine Screws and Nuts (inch) | #0 – ½” | General light-duty machine screw and nut dimensions |
| B18.6.7M | Metric Machine Screws | M1.6 – M8 | Metric machine screw dimensions for instrument and electronics assembly |
| B18.7 | General Purpose Semi-Tubular Rivets | Various | Sheet metal and thin-wall structural connections |
| B18.8.1 | Clevis Pins and Cotter Pins | ⅛” – 2″ | Linkage pins; hydraulic cylinder clevises; actuator pins |
| B18.8.2 | Taper Pins, Dowel Pins, and Spring Pins | Various | Precision locating; coupling alignment; machine assembly |
| B18.21.1 | Helical Spring Lock Washers | No.2 – 3″ | Vibration-resistant assemblies; machinery; general bolted joints |
| B18.22.1 | Plain (Flat) Washers (inch) | No.6 – 3″ | Load distribution; protection of flange face and bearing surface |
| B18.29.1 | Helical Coil Screw Thread Inserts | #2 – 1½” | Thread repair and reinforcement in aluminium and soft materials |
| B18.31.1 | Continuous-Thread Stud Bolts | ¼” – 4″ | ASME B16.5 flange stud bolts; pressure vessel bolting — critical EPC item |
1.3 — ASME B18 and ASTM: How They Work Together
1.4 — Thread Standards Referenced by ASME B18
ASME B18 inch-series fasteners use threads per ASME B1.1 (Unified National Thread Standard): UNC (Unified National Coarse) for general fasteners; UNF (Unified National Fine) for thin-wall, vibration-sensitive, or precision applications; and UN (constant-pitch series, e.g., 8UN for large-diameter stud bolts) for pressure vessel and flange bolting above 1″ diameter. The thread designation format is: Nominal diameter – TPI Thread-series Thread-class, for example: ¾”-10 UNC-2A (¾” diameter, 10 threads per inch, Unified National Coarse, Class 2A tolerance for external thread). For ASME B16.5 flange bolting: stud bolts above 1″ use the 8UN constant-pitch series (8 threads per inch regardless of diameter) — this provides consistent nut run-up behaviour across all stud bolt diameters in the same flange bolt pattern and is a mandatory thread form for heavy hex nuts in high-pressure flange service.
Submit fastener type, B18 standard, material grade, size, and quantity for a documented RFQ within 24 hours.
Material Grades
& Strength Selection
The most critical ASME B18 sub-standards for EPC and industrial projects are B18.2.1 (hex bolts), B18.2.2 (heavy hex nuts), B18.3 (socket cap screws), and B18.31.1 (stud bolts) — each paired with the appropriate ASTM material grade for the service condition. All applicable grades are supported at RR Hydraulic.
Submit type, size, material, standard, and quantity to sales@rrhydraulics.com for a certified offer.
2.1 — ASME B18.2.1 — Hex Bolts and Screws
ASME B18.2.1 covers square head bolts, heavy hex bolts, hex cap screws, and hex screws in the inch series from ¼” through 4″ nominal diameter. For EPC structural and machinery bolting the key product is the heavy hex bolt — a hex bolt with a larger across-flats and greater head height than a standard hex bolt, designed for structural steel connections and high-torque applications. Heavy hex bolts per B18.2.1 are the geometry standard; the material for structural steel is ASTM A325 (medium-carbon steel, UTS ≥ 830 MPa, sizes ½”–1½”) or ASTM A490 (alloy steel, UTS ≥ 1035 MPa, sizes ½”–1½”) for high-strength structural bolting per AISC/RCSC specification.
| Nominal Size | Thread (UNC) | Width Across Flats (mm) | Head Height (mm) | Min Thread Length (mm) | ASTM Material Options |
|---|---|---|---|---|---|
| ½” (12.7 mm) | ½”-13 UNC | 22.2 | 11.2 | 38.1 | A307 Gr.B; A325; A490; A193 B7 |
| ⅝” (15.9 mm) | ⅝”-11 UNC | 27.0 | 14.3 | 44.5 | A307 Gr.B; A325; A490; A193 B7 |
| ¾” (19.1 mm) | ¾”-10 UNC | 31.8 | 17.5 | 50.8 | A325; A490; A193 B7 / B7M |
| ⅞” (22.2 mm) | ⅞”-9 UNC | 36.5 | 20.6 | 57.2 | A325; A490; A193 B7 / B7M |
| 1″ (25.4 mm) | 1″-8 UN | 41.3 | 23.8 | 63.5 | A193 B7 / B7M; A490 |
| 1¼” (31.8 mm) | 1¼”-8 UN | 50.8 | 30.2 | 76.2 | A193 B7 / B7M; A490 |
| 1½” (38.1 mm) | 1½”-8 UN | 60.3 | 36.5 | 88.9 | A193 B7 / B7M |
| 2″ (50.8 mm) | 2″-8 UN | 79.4 | 49.2 | 101.6 | A193 B7 / B7M; B16 |
2.2 — ASME B18.31.1 — Continuous-Thread Stud Bolts (Critical EPC Item)
Continuous-thread stud bolts per ASME B18.31.1 — threaded full-length from one end to the other with the same thread form and diameter — are the standard bolting element for ASME B16.5 raised-face and ring-joint flanges in EPC process piping. The stud bolt is used rather than a headed bolt because: (1) studs allow equal engagement depth in both nuts without a head-height variable; (2) studs allow easy replacement from either side of the flange without disturbing the opposing side; and (3) studs provide more predictable bolt load distribution across the flange because both threaded interfaces are identical. A194 heavy hex nuts are used at both ends of the stud bolt.
| Stud Bolt Dia. | Thread Series | Nut (A194 2H / 2HM) | Nut Height (mm) | Flange Class Use | ASTM Material |
|---|---|---|---|---|---|
| ½” | ½”-13 UNC | Heavy hex ½”-13 | 17.5 | ASME 150# (small bore) | A193 B7 / B7M; A193 B8M |
| ⅝” | ⅝”-11 UNC | Heavy hex ⅝”-11 | 22.2 | ASME 150# / 300# | A193 B7 / B7M; A193 B8M |
| ¾” | ¾”-10 UNC | Heavy hex ¾”-10 | 25.4 | ASME 300# / 600# | A193 B7 / B7M; A193 B8M |
| ⅞” | ⅞”-9 UNC | Heavy hex ⅞”-9 | 28.6 | ASME 300# / 600# | A193 B7 / B7M |
| 1″ | 1″-8 UN | Heavy hex 1″-8 | 31.8 | ASME 600# / 900# | A193 B7 / B7M; B16 |
| 1⅛” | 1⅛”-8 UN | Heavy hex 1⅛”-8 | 34.9 | ASME 900# / 1500# | A193 B7 / B7M; B16 |
| 1¼” | 1¼”-8 UN | Heavy hex 1¼”-8 | 41.3 | ASME 1500# / 2500# | A193 B7 / B7M; B16 |
| 1½” | 1½”-8 UN | Heavy hex 1½”-8 | 47.6 | ASME 2500# | A193 B7 / B7M; B16 |
2.3 — ASME B18.2.2 — Heavy Hex Nuts
Heavy hex nuts per ASME B18.2.2 have a larger across-flats (AF) dimension and greater height than standard hex nuts of the same thread — the additional bearing area distributes bolt load more evenly across the flange face and reduces the risk of nut embedment (local yielding under the nut face) in softer flange materials. Heavy hex nuts are mandatory for ASME B16.5 process piping flange bolting — standard hex nuts (smaller AF and height) are not acceptable for flange bolting in coded pressure piping.
| Thread Size | Heavy Hex AF (mm) | Heavy Hex Height (mm) | Std Hex AF (mm) | Std Hex Height (mm) | AF Increase (%) |
|---|---|---|---|---|---|
| ½”-13 UNC | 22.2 | 17.5 | 19.1 | 14.3 | +16% |
| ⅝”-11 UNC | 28.6 | 22.2 | 23.8 | 17.5 | +20% |
| ¾”-10 UNC | 33.3 | 25.4 | 28.6 | 20.6 | +16% |
| 1″-8 UN | 44.5 | 31.8 | 38.1 | 25.4 | +17% |
| 1¼”-8 UN | 54.0 | 41.3 | 47.6 | 31.8 | +13% |
| 1½”-8 UN | 63.5 | 47.6 | 57.2 | 38.1 | +11% |
2.4 — ASME B18.3 — Socket Head Cap Screws and Set Screws
Socket Head Cap Screws (SHCS)
ASME B18.3 socket head cap screws (SHCS) — cylindrical head with internal hex (Allen) drive socket — are the standard fastener for hydraulic manifold block port closures, cartridge valve retention, valve block face plates, and compact machinery where the flush or countersunk installation of a hex head bolt is not possible. Key dimensional features: head diameter ≈ 1.5× nominal diameter; head height ≈ 1.0× nominal diameter; hex socket depth ≈ 0.75× nominal diameter. Thread range: #0 (M1.6 equivalent) through 4″ (M100 equivalent). Standard material for SHCS: ASTM A574 (alloy steel, UTS ≥ 1240 MPa for sizes ¼”–1½”) — the highest-strength standard fastener material in inch series, providing approximately equivalent load capacity to ISO 12.9 metric cap screws. For SS applications: ASTM F879 (A2 SS) or ASTM F880 (A4 SS 316) SHCS.
Button Head Socket Screws
Low-profile dome head with internal hex socket — smaller head height and reduced head diameter vs standard SHCS, providing a flush or near-flush fastener profile for cover plates, guard panels, and thin-section assemblies where the full SHCS head height would protrude. Shear and tensile strength are lower than SHCS of the same nominal diameter (smaller head-to-shank bearing area) — button head socket screws must not be substituted for SHCS in structural or high-clamping-load applications. ASTM A574 material for alloy steel button heads; ASTM F879 / F880 for stainless. Standard hex key drive sizes per B18.3 Table 7.
Flat Head Socket Screws (Countersunk)
82° or 90° countersunk flat head with internal hex socket — provides a completely flush installation in countersunk holes where protruding fastener heads are unacceptable (fluid system flow-path cover plates; precision instrument panels; cleanroom equipment). The countersunk head geometry per ASME B18.3 requires a matching 82° or 90° countersunk hole in the mating part — verify the countersink angle standard used on the mating component (ASME B18.3 specifies 82° for inch-series; ISO 10642 specifies 90° for metric) to avoid head-height mismatch that leaves the screw proud of the surface or creates a stress concentration at the countersink edge.
Socket Set Screws
Headless set screws (grub screws) per ASME B18.3 — hex socket drive, various point styles (cup point, flat point, dog point, half-dog point, oval point, cone point) — used for shaft-to-hub locking, position locking of components on shafts, and locking of threaded adjustments. Cup point is the standard general-purpose set screw point — the cup digs into the shaft surface and provides good holding power. Dog point (cylindrical pilot extension) is used where the set screw must locate in a pre-drilled hole in the shaft for positive location and easy re-assembly. ASTM A574 alloy steel, case-hardened — hardness 45–53 HRC to enable the cup or dog point to indent into the softer shaft surface without the set screw point deforming first.
2.5 — ASME B18.8 — Pins
Dowel Pins (B18.8.2)
Precision ground cylindrical pins for accurate component location and alignment — used to locate hydraulic manifold block covers, actuator mounting plates, coupling halves, and precision machine assemblies. Standard tolerance: +0.000″ / −0.0002″ OD (Class RC2 to LC2 press fit in steel) per B18.8.2 — this provides an interference fit in the mating hole that holds the pin securely without fasteners. Material: ASTM A108 Grade 1144 (resulfurised free-machining steel), heat treated to HRC 58–62 for maximum dimensional stability and wear resistance. Dowel pins for stainless steel assemblies: AISI 303 or 316 SS (softer, HRC 20–25) — provide location without galling in SS-to-SS or aluminium-to-SS interfaces. Diameter range: 3/32″ through 1″ per B18.8.2.
Taper Pins (B18.8.2)
Conically tapered pins (taper 1:48 on diameter) for permanent or semi-permanent location of coupling flanges, handwheels, and machine components on shafts. The taper pin drives into a matching tapered hole reamed with a standard taper pin reamer — the self-locking taper angle (1:48) holds the pin in the hole under service loads without additional fasteners. Pin sizes per ASME B18.8.2 are designated by number (#0 through #10) with OD at the large end; the small end diameter is the large end diameter minus the taper × pin length. Taper pins are specified by number, length, and material — AISI 1045 carbon steel, AISI 303 stainless, and AISI 316 stainless are standard material options. For precision coupling alignment: use a reamed taper pin over a parallel dowel pin where easy disassembly for shaft re-alignment is required.
Clevis Pins (B18.8.1)
Shouldered pins with a through-hole for a cotter pin or snap ring retention — used in hydraulic cylinder clevis-end connections, toggle linkages, actuator mounting clevises, and any pivoting or articulating mechanical connection requiring a pin that can be easily removed for maintenance. Clevis pins per B18.8.1 are specified by nominal diameter (⅛” through 2″), grip length, and head style (flat head or round head). Material: AISI 1018 carbon steel (general purpose) or AISI 304/316 stainless (corrosive environments). Head diameter ≈ 1.75× body diameter; grip length = distance between head bearing face and cotter pin hole centre. Always specify cotter pin or snap ring size together with clevis pin size to ensure the retention element fits the hole in the pin end.
Spring Pins / Roll Pins (B18.8.2)
Slotted or spirally wound cylindrical pins manufactured undersize and sprung into an interference-fit hole — the spring pin compresses radially during installation and expands against the hole wall to create a press-fit without a precision reamed hole. Spring pins (slotted type) per B18.8.2: OD 1/16″ through 1″; material carbon steel or SS 302/304; hardness 42–48 HRC for carbon steel slotted spring pins. Used in: hinge pins; linkage assemblies; coupling retention; and applications where easy removal and reassembly is required and where the precision location of a dowel pin is not critical. Spirally wound roll pins provide better radial load distribution and are less susceptible to fatigue cracking under cyclic shear loads than slotted spring pins — specify roll pins (ASME B18.8.2 Type B) for high-cycle applications.
2.6 — ASME B18.21 / B18.22 — Washers
B18.21.1 — Helical Spring Lock Washers
Split ring spring-steel washers that bite into the bolt head / nut bearing surface and the mating part surface when the nut is tightened — the cut ends of the split ring act as friction elements resisting rotation (loosening) under vibration. Standard series (B18.21.1) dimensions: OD ≈ 1.75× nominal bolt diameter; ID ≈ 1.02× nominal bolt diameter (slip fit); thickness ≈ 25% of nominal diameter. Material: carbon steel, hardened spring steel (HRC 40–48); stainless steel 302 or 316 for corrosive service. Engineering limitation: spring lock washers provide only modest vibration resistance — for high-vibration applications (machinery, engines, vehicles) specify Nord-Lock wedge locking washers, prevailing torque nuts, or thread-locking compound (Loctite) as more reliable anti-loosening solutions. Spring lock washers are not acceptable in ASME pressure vessel bolted joint design per ASME VIII Appendix 2.
B18.22.1 — Plain (Flat) Washers
Plain flat washers per B18.22.1 serve three functions in bolted joints: (1) load distribution — spreading the bolt head or nut bearing load over a larger area, reducing bearing stress in soft materials (aluminium, plastics, timber) below the material’s compressive yield stress; (2) surface protection — preventing the rotating nut or bolt head from scoring the mating surface finish during tightening; and (3) cover for oversized holes — standard clearance holes for B18.2.1 hex bolts are approximately bolt diameter + 1/16″, which may not provide adequate bearing area for the nut in thin sheet metal; a washer bridges the clearance gap. Size designations per B18.22.1 use “Narrow”, “Regular”, and “Wide” series for each nominal bolt size — “Regular” is the standard EPC and structural specification. Plain washers are hardened (ASTM F436 for structural applications) or unhardened (plain steel, SS, or brass for general applications).
F436 Hardened Washers (Structural Bolting)
ASTM F436 hardened flat washers (HRC 38–45) are mandatory in structural steel bolting per RCSC (Research Council on Structural Connections) specification when using A325 or A490 high-strength structural bolts per ASME B18.2.1 — the hardened washer prevents the soft structural steel from yielding under the nut or bolt head bearing face during high-torque pretensioning. An F436 washer must be used under the turned element (the element being rotated during tightening — nut for standard bolting; bolt head for reverse-torque bolting) when installing A325 or A490 structural bolts. For slotted holes in structural steel: two F436 washers (one each side of the connection) are required per RCSC specification to prevent hole deformation during high-torque bolt tensioning.
Fender Washers and Dock Washers
Extra-large OD flat washers providing maximum bearing area for oversized holes, soft materials, and specific structural applications. Fender washers (B18.22.1 Wide series): OD ≈ 3–4× nominal bolt diameter; used for thin sheet metal panels, plastic covers, and wood applications where the standard washer OD would punch through the material under bolt load. Dock washers (heavy-duty fender washers): OD up to 100 mm on ½” nominal bolt — used for marine deck hardware, dock cleats, and heavy structural connections in timber framing. Both types available in zinc-plated carbon steel, hot-dip galvanised (ASTM A153) for marine and outdoor service, and SS 316 for direct seawater exposure. Specify OD, ID, and thickness explicitly when ordering large OD washers to avoid non-standard proportions.
Material Grades,
Torque & NACE Compliance
Fastener material grade selection determines the load capacity, temperature rating, corrosion resistance, and NACE sour service compliance of any bolted joint. The ASTM A193 / A194 pairing for process piping flange bolting is the most critical fastener specification in EPC projects. RR Hydraulic supplies all standard and specialty fastener grades with full certification.
3.1 — ASTM Fastener Material Grades for EPC Applications
| ASTM Grade | Material | UTS min (MPa) | Yield min (MPa) | Hardness | Max Temp (°C) | NACE MR0175 | Primary Application |
|---|---|---|---|---|---|---|---|
| A307 Gr.B | Low carbon steel | 415 | — | — | 300 | No | Low-pressure piping; equipment; non-structural |
| A325 Type 1 | Medium carbon steel Q+T | 830 | 635 | — | 300 | No | Structural steel connections per RCSC/AISC |
| A490 Type 1 | Alloy steel Q+T | 1035 | 895 | — | 300 | No | High-strength structural bolting per RCSC/AISC |
| A193 B7 | 4140/4142 Cr-Mo Q+T | 860 | 725 | ≤ 35 HRC (if B7M) | 450 | B7M only (≤ 35 HRC) | ASME B16.5 flange stud bolts; pressure vessel bolting |
| A193 B7M | 4140/4142 Cr-Mo Q+T modified | 725 | 585 | ≤ 35 HRC | 450 | Yes — ISO 15156-2 | NACE sour service flange bolting; offshore; H₂S service |
| A193 B16 | Cr-Mo-V alloy Q+T | 860 | 725 | — | 540 | No | High-temperature steam; power plant; above 450°C |
| A193 B8M Cl.1 | SS 316 strain-hardened | 515 | 205 | — | 815 | Yes (annealed) | SS flange bolting; cryogenic; corrosive service; high-temp |
| A193 B8M Cl.2 | SS 316 strain-hardened | 760 | 450 | — | 425 | No (strain-hardened) | Higher-strength SS stud bolts; lower temp application |
| A574 | Alloy steel (SHCS) | 1240 | 1100 | 39–45 HRC | 260 | No | Socket head cap screws; hydraulic manifolds; machinery |
| A307 Gr.A | Low carbon steel | 415 | — | — | 300 | No | General machinery; non-pressure bolting |
3.2 — ASTM A194 Nut Grades for Flange Bolting
| A194 Grade | Material | Proof Load (MPa) | Hardness | NACE Compliant | Paired With | Application |
|---|---|---|---|---|---|---|
| 2H | Medium carbon steel Q+T | 830 | C24–C36 | No | A193 B7 | Standard CS flange bolting; non-sour service |
| 2HM | Medium carbon steel (modified) | 690 | ≤ C35 (≤ 327 HB) | Yes | A193 B7M | NACE sour service; offshore; H₂S piping |
| 8M | SS 316 (Cl.1 annealed) | 205 | — | Yes (Cl.1) | A193 B8M Cl.1 | SS flange bolting; cryogenic; corrosive service |
| 4 | Alloy steel (high-temp) | 830 | C26–C36 | No | A193 B16 | High-temperature steam; power boiler |
| 7 | Cr-Mo alloy steel | 725 | C28–C34 | No | A193 B7 (high-temp) | Elevated-temperature pressure equipment; alloy piping |
| 7M | Cr-Mo alloy steel (modified) | 585 | ≤ C35 | Yes | A193 B7M (alloy) | High-temp NACE service; sour gas plant bolting |
3.3 — NACE MR0175 Fastener Compliance
3.4 — Bolt Load and Torque Engineering
K = Nut factor (dimensionless) — accounts for thread friction and nut-face friction
K = 0.20 (as-received, no lubricant — typical for clean dry threads)
K = 0.15 (light oil lubricant — machine oil or anti-seize compound)
K = 0.12 (PTFE-coated / Molykote lubricated stud bolt — often used on SS bolting)
K = 0.17 (zinc-plated thread — slightly higher than plain steel)
F_b = Target bolt preload (N) — typically 60–80% of bolt proof load for flange bolting
d = Nominal bolt diameter (m)
σ_y = Bolt yield (proof) stress (Pa) — per ASTM grade (B7: 725 MPa; B7M: 585 MPa)
A_stress = Bolt stress area (m²) = π/4 × [(d_p + d_r)/2]² where d_p = pitch dia, d_r = root dia
Engineering Note: The nut factor K is the largest source of torque-preload uncertainty — variation in K from 0.12 to 0.20 for nominally identical bolts causes a 40% variation in achieved preload at the same applied torque. Use consistent lubrication condition (and specify it on the bolt torque procedure) to minimise K variation across a flange bolt pattern.
σ_y (B7M) = 585 MPa; A_stress (1″-8 UN) = 507 mm²; d = 25.4 mm; K = 0.15 (anti-seize)
F_b = 585 × 507 × 0.65 = 192,800 N (192.8 kN)
T = 0.15 × 192,800 × 0.0254 = 735 N·m (542 ft·lbf)
Cross-check: at K = 0.20 (dry): T = 980 N·m — 33% higher torque for same preload. Use consistent lube.
3.5 — Flange Bolt Tightening Sequence
- Step 1 — Snug tight (hand + ¼ turn): Install all stud bolts and run nuts down by hand; snug tight with wrench to eliminate gasket surface irregularities and bring flange faces parallel. Confirm faces are parallel and gasket is centred before proceeding
- Step 2 — Cross-bolting at 30% target torque: Tighten all bolts in a star (cross) pattern — not sequential circular — to 30% of the final target torque. Star pattern ensures even gasket seating load distribution; sequential circular tightening creates uneven gasket seating that cannot be corrected in later passes
- Step 3 — Cross-bolting at 70% target torque: Second pass in star pattern to 70% of final target torque. The gasket compresses further; flange faces approach final parallel position
- Step 4 — Full target torque: Third pass in star pattern to 100% of final target torque. Do not exceed the torque specification for the stud bolt diameter and grade — excessive torque yields the bolt thread flanks or gasket
- Step 5 — Circular verification pass: Final circular pass (clockwise around the flange) to verify all bolts hold the target torque without movement. Any bolt that continues to rotate at the target torque requires investigation (cross-threaded, insufficient thread engagement, or yielded bolt)
- PTFE-envelope gaskets: Require a fourth cross-bolt pass at 100% + a re-torque after 24 hours at operating temperature — PTFE creeps under sustained compressive load, reducing bolt preload (gasket relaxation). All PTFE gasket flange joints in process piping should be re-torqued after the first hot pressurisation to restore the target bolt load
- Spiral-wound gaskets (SWG): Do not re-torque spiral-wound gaskets after compression — once the inner ring contacts the flange faces (visible by the “clunk” of full SWG compression), no further tightening is permitted. Excessive torque on a fully-seated SWG crushes the wound metal rings and permanently damages the gasket sealing element
Industry Applications
& Export Documentation
RR Hydraulic maintains full traceability on all ASME B18 fastener supply — dimensional gauging, hardness testing (NACE compliance), PMI, mechanical testing, and complete EN 10204 3.1 / 3.2 export documentation are standard on all project-grade fastener supply.
4.1 — Inspection & QC Protocol
4.2 — Material and Certification Cross-Reference
| Service Condition | Stud Bolt Grade | Nut Grade | B18 Dimension Std | NACE | Key Requirement |
|---|---|---|---|---|---|
| CS piping, non-sour, to 427°C | A193 B7 | A194 2H | B18.31.1 / B18.2.2 | No | Standard EPC flange bolting; most common |
| CS or alloy piping, sour H₂S service | A193 B7M | A194 2HM | B18.31.1 / B18.2.2 | Yes | ≤ 35 HRC per piece; individual hardness cert |
| SS 316L / Duplex piping, corrosive | A193 B8M Cl.1 | A194 8M | B18.31.1 / B18.2.2 | Yes (Cl.1) | SS bolting for SS flanges; NACE compliant Cl.1 |
| High-temp steam, 427–540°C | A193 B16 | A194 4 or 7 | B18.31.1 / B18.2.2 | No | Cr-Mo-V; above B7 temperature limit |
| Structural steel bolting | A325 / A490 hex bolt | A563 Gr.C / DH | B18.2.1 / B18.2.2 | No | AISC/RCSC specification; F436 hardened washers |
| General machinery (non-pressure) | A307 Gr.B hex bolt | A563 Gr.A | B18.2.1 / B18.2.2 | No | Low-cost; non-critical bolting |
| Hydraulic manifold cap screws | A574 SHCS | N/A (threaded hole) | B18.3 | No | 1240 MPa UTS; high-clamping hydraulic assembly |
| Cryogenic SS piping (−196°C) | A193 B8M Cl.1 | A194 8M | B18.31.1 / B18.2.2 | Yes (Cl.1) | Austenitic SS retains toughness at cryogenic temp |
4.3 — Common Fastener Specification Errors in EPC Procurement
- Specifying A193 B7 instead of B7M for sour service: The single most costly fastener error on EPC offshore and sour gas projects — standard B7 (no hardness maximum) will routinely be supplied at HRC 37–42, which is a NACE non-conformance. Every sour service piping class bolting MR line item must explicitly state “A193 B7M” and “A194 2HM“
- Using standard hex nuts instead of heavy hex nuts on flanges: ASME B16.5 and ASME VIII require heavy hex nuts per ASME B18.2.2 for all flange bolting — standard hex nuts have insufficient bearing area for the high bolt loads in pressure flange joints and will embed into the flange face or nut seating under torque
- Omitting the ASME B18 dimensional standard from the procurement document: Specifying only “½” A193 B7 stud bolt” without citing B18.31.1 is incomplete — without the dimensional standard, the thread form, overall length tolerance, and thread length cannot be verified as compliant
- Specifying A193 B8M Class 2 (strain-hardened) for NACE sour service: Only A193 B8M Class 1 (annealed, ≤ 22 HRC equivalent in austenitic SS) is NACE MR0175 compliant — Class 2 (strain-hardened, higher strength) is not NACE compliant for H₂S service. Always specify “B8M Class 1” explicitly for NACE SS bolting
- Using spring lock washers in ASME pressure vessel bolted joints: ASME VIII Appendix 2 design rules for flanged joints do not include spring lock washer capability in the bolt load calculation — spring lock washers are not acceptable in ASME pressure vessel nozzle and flange connections. Use hardened flat washers (ASTM F436) or specify bolts with sufficient thread engagement to prevent loosening
- Incorrect thread form for large diameter stud bolts: Stud bolts above 1″ nominal diameter in ASME B16.5 flange service must use the 8UN constant-pitch thread series — not UNC (which has fewer threads per inch at larger diameters, reducing thread engagement length in the nut). Specifying “1¼” UNC stud bolt” (7 TPI) instead of “1¼”-8 UN stud bolt” gives different thread engagement depth in the 2H / 2HM nut and is not the correct ASME B16.5 bolting specification
- Mixing B7 and B7M stud bolts in the same flange bolt pattern: A flange bolted with a mix of B7 (high hardness) and B7M (lower hardness) stud bolts will have unequal stiffness across the bolt pattern — the harder B7 bolts carry more load than the softer B7M bolts at the same elongation, creating non-uniform gasket stress. All stud bolts in a single flange joint must be the same ASTM grade
4.4 — Applications by Industry
ASME B16.5 Flange Bolting — EPC Process Piping
The dominant ASME B18 fastener application in EPC process piping — every flanged joint from ASME Class 150# through Class 2500# uses continuous-thread stud bolts per ASME B18.31.1 with heavy hex nuts per ASME B18.2.2. A typical large onshore Gas Processing EPC project may have 50,000–200,000 individual stud bolts across all piping classes and sizes. RR Hydraulic supplies A193 B7 / B7M stud bolts and A194 2H / 2HM heavy hex nuts in all sizes from ½” through 4″ with EN 10204 3.1 MTCs, dimensional certificates per B18.31.1 / B18.2.2, hardness per piece for NACE lots, and NACE compliance statements — the complete documentation package for EPC project bolting MR closeout.
Hydraulic Manifold Assembly (A574 SHCS)
ASME B18.3 socket head cap screws in ASTM A574 (UTS ≥ 1240 MPa; HRC 39–45) are the standard fastener for: hydraulic manifold block subplate cover plates; cartridge valve bodies threaded into aluminium or steel manifold blocks; valve block-to-subplate connections; and all compact hydraulic assembly joints where protruding hex bolt heads are unacceptable and high clamping loads are required in confined space. The A574 SHCS at 1240 MPa UTS provides approximately 2× the clamping load of a standard A307 hex bolt at the same nominal size — enabling shorter-length, higher-load fastener designs in compact hydraulic manifold blocks. For aluminium manifold blocks: specify A574 SHCS with thread torque per ISO 4762 (metric) or B18.3 torque tables to avoid over-torqueing and stripping the aluminium tapped thread.
Offshore Sour Service Bolting (B7M / 2HM)
NACE-compliant A193 B7M stud bolts and A194 2HM heavy hex nuts for all flanged connections in sour oil and gas production piping on offshore platforms, FPSOs, and subsea trees. The combined H₂S + Cl⁻ + seawater atmosphere environment on offshore topsides creates the most aggressive fastener service conditions in the process industry — sulphide stress cracking (SSC) of non-compliant hard alloy steel bolts and hydrogen-induced cracking (HIC) in the heat-affected thread roots are both documented failure mechanisms. EN 10204 3.2 material certification (TPI countersign by DNV, Lloyds, or BV) with individual piece hardness testing and NACE compliance statement is the minimum acceptable quality documentation for offshore NACE B7M / 2HM fastener project supply.
Cryogenic and High-Temperature Bolting
A193 B8M Class 1 (annealed SS 316) stud bolts and A194 8M nuts for cryogenic LNG and liquid nitrogen piping flange connections — austenitic SS 316 retains full ductility and toughness at −196°C (LNG temperature) and at −253°C (liquid hydrogen), while alloy steel B7 bolts become brittle below approximately −100°C. For steam power plant bolting above 427°C where B7 alloy steel reaches its temperature limit: A193 B16 (Cr-Mo-V alloy, max 540°C) or A193 B7 with Grade 4 / Grade 7 nuts for the 427–540°C range. Above 540°C: high-alloy or austenitic SS bolting grades (A193 B8 series) are required — confirm the specific grade with the pressure vessel design engineer per the applicable ASME VIII allowable stress tables for the operating temperature.
Structural Steel Bolting (A325 / A490)
ASTM A325 and A490 high-strength structural bolts in B18.2.1 heavy hex bolt geometry with A563 nuts and F436 hardened washers — the standard structural bolted connection system for EPC steel structure, pipe rack, vessel support frame, and module frame structural steel connections per AISC Steel Construction Manual and RCSC Specification for Structural Joints. A325 (UTS ≥ 830 MPa) covers most structural connections; A490 (UTS ≥ 1035 MPa) for high-load or reduced-size connections. Installation methods: snug-tight; pre-tensioned (turn-of-nut or torque); or slip-critical (pre-tensioned to prevent slip at the faying surface under service load) — each method requires a different inspection protocol per the RCSC specification and the project structural steel standard.
Pin Applications (B18.8) — Hydraulics and Machinery
ASME B18.8 pins cover the full range of precision and semi-precision locating and articulating pin applications in hydraulic and mechanical systems: B18.8.1 clevis pins for hydraulic cylinder rod-end and barrel-end clevis connections (pivot bearing surface requires minimum hardness of RC 25–30 on carbon steel clevis pins to resist galling in the bronze or PTFE bush bearing); B18.8.2 dowel pins for hydraulic manifold block cover plate location (maintains port alignment and O-ring groove concentricity between reassembly cycles); B18.8.2 taper pins for coupling-to-shaft location in hydraulic power unit drive couplings (semi-permanent location with easy removal for shaft re-alignment). All B18.8 pins supplied with dimensional certificates confirming OD tolerance class (RC, LC, or LN fit series per ASME B4.1) and material hardness for the intended bearing application.
4.5 — Complete EPC Documentation Package for ASME B18 Fastener Supply
| # | Document | Standard / Format | Mandatory / Conditional | Notes |
|---|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | Mandatory — all EPC fastener supply | Heat-traceable; ASTM grade cited; per heat lot |
| 02 | Chemical Composition Report | Per ASTM A193 / A194 composition limits | Mandatory | All elements; Cr+Mo for B7/B7M; C+Cr+Ni+Mo for B8M |
| 03 | Mechanical Properties Report | ASTM F606 — proof load, UTS, yield, elongation | Mandatory | Per heat; B7M ≥ 725 MPa UTS; ≥ 585 MPa yield |
| 04 | Hardness Test Report — Individual Pieces | ASTM E18 Rockwell C | Mandatory — NACE B7M / 2HM / B8M Cl.1 lots | ≤ 35 HRC per piece (B7M/2HM); individual records |
| 05 | NACE Compliance Statement | NACE MR0175 / ISO 15156-2 declaration | Mandatory — all sour service lots | Grade, alloy, condition, max hardness, ISO 15156 ref |
| 06 | Dimensional Inspection Report | ASME B18.2.1 / B18.2.2 / B18.3 / B18.31.1 | Mandatory | Thread gauge pass; AF; head height; length; shank OD |
| 07 | PMI Report (XRF) | 100% — B7M and B8M lots | Mandatory — alloy and SS fastener lots | Cr-Mo confirmation (B7M); 316 vs 304 (B8M) |
| 08 | Surface Finish Certificate | ASTM B633 (zinc plating); ASTM A153 (HDG) | Conditional — coated fasteners | Plating thickness; adhesion test; passivation cert (SS) |
| 09 | First Article Inspection (FAI) Report | Project-specific format | Mandatory — new project fastener configs | All parameters; before batch production |
| 10 | TPI Witness Certificate | DNV / Lloyds / BV / SGS | Conditional — EN 10204 3.2; offshore; nuclear | Witness individual hardness + mechanical + PMI |
| 11 | ISO 9001:2015 Certificate | Third-party QMS certification | Mandatory — EPC projects | Scope covers fastener manufacture per ASTM and B18 |
| 12 | Piping Class Cross-Reference | Project piping class bolting schedule | Mandatory — EPC piping projects | Maps each fastener lot to the piping class(es) it serves |
| 13 | Country of Origin + Packing List | Chamber of Commerce / item-level | Mandatory | HS tariff code; ASTM grade and B18 std per line item |
| 14 | Commercial Invoice + Bill of Lading | Per INCOTERMS 2020 | Mandatory | Freight forwarder issued |
4.6 — ISO and Quality System Compliance
ISO 9001:2015
Quality Management System governing raw material procurement and heat traceability, cold-heading or machining process controls, heat treatment qualification (quench and temper cycle for B7/B7M — temperature, time, quench medium, and furnace calibration per AMS 2750 pyrometry standard), thread rolling or cutting process controls, hardness test procedure (individual piece Rockwell C for NACE lots — calibration, test frequency, and rejection criteria), dimensional gauging procedure (thread gauge calibration schedule; AF and head height measurement method), PMI procedure, surface treatment process control (plating bath chemistry; thickness measurement), and full heat-to-fastener traceability. Mandatory for all EPC, offshore, and NACE-critical ASME B18 fastener procurement.
ASME B1.1 (Unified Thread Standard)
The governing thread standard referenced by all ASME B18 inch-series fasteners — ASME B1.1 Unified National Thread Standard defines the thread form (60° included angle, flat root and crest), thread class (Class 1A/1B loose fit; Class 2A/2B standard fit; Class 3A/3B precision fit), and pitch-diameter tolerances for UNC, UNF, UNEF, and UN constant-pitch series threads. All ASME B18 stud bolts and hex bolts use Class 2A/2B threads as standard — the tolerance class that provides a reliable fit with Class 2A bolts into Class 2B threaded holes or Class 2B nuts across the full temperature and coating range of industrial service. Thread gauging compliance with ASME B1.1 is the primary dimensional acceptance criterion for ASME B18 fastener supply.
ASME VIII / ASME B31.3 (Pressure Design Codes)
ASME Section VIII Division 1 Appendix 2 (Rules for Bolted Flange Connections with Ring-Type Gaskets) specifies the allowable bolt stress values and bolt load calculation method for pressure vessel and piping flange bolting — the design code that makes ASTM A193 B7 / B7M the mandatory material for ASME-coded flange bolting at elevated temperature. ASME B31.3 (Process Piping) Table A-2 lists the allowable stresses for A193 B7 and B7M at temperatures up to 450°C and B16 up to 540°C — these values are the basis for the flange bolt load calculations that determine the minimum required stud bolt diameter and number for each ASME pressure class. Specifying non-ASTM-listed fastener materials on ASME B31.3 coded piping flanges requires an engineering assessment and concession per B31.3 Paragraph 300.2 and 302.3.2.
NACE MR0175 / ISO 15156
The governing standard for fastener material selection in H₂S sour service — ISO 15156-2 Table A.2 lists the requirements for carbon and low-alloy steel fasteners (B7M: 4140/4142 alloy, Q+T, ≤ 35 HRC per piece) and Section 9 covers alloy steel and SS fasteners. The NACE non-conformance for bolting is one of the most frequently cited deficiencies on offshore EPC project quality audits — the hardness limit (35 HRC) and the “M” suffix designation (B7M, 2HM) are the two mandatory compliance markers that procurement, inspection, and receiving teams must verify on every sour service fastener lot. Any fastener lot without individual piece hardness records below 35 HRC cannot be accepted as NACE-compliant regardless of the grade marking, ASTM test reports, or supplier declaration.
Submit fastener type, B18 standard, material grade, size, finish, and quantity to RR Hydraulic for a complete, certified commercial offer.