Fasteners for Pressure Service —
ASTM A193, A194
and EN ISO 898

Fastener specification is one of the most under-engineered aspects of process piping and pressure vessel design. The pipe material is carefully selected, the flange class and gasket are correctly specified, the bolt load is calculated — and then the bolts and nuts are specified as "stainless steel" or "high-tensile" without reference to a material standard, a grade, or a temperature limit. The result is either bolts that are over-specified and expensive, or bolts that corrode, stretch, or fail at operating temperature and are the actual cause of the flange joint leakage that gets attributed to the gasket.

This article covers the principal fastener material standards for pressure service — ASTM A193 for bolting, ASTM A194 for nuts, and the EN ISO 898 equivalents — the grades used across the temperature range from cryogenic to high-temperature service, sour service hardness requirements, and the practical decisions involved in matching fastener specification to service conditions.

Why Fastener Specification Matters

A flanged joint is a mechanical assembly whose integrity depends on maintaining adequate bolt load through all operating conditions. That bolt load depends on the fasteners maintaining their mechanical properties — strength, hardness, and ductility — throughout the service life. Three failure mechanisms make fastener material selection critical:

• Relaxation and creep at elevated temperature. Carbon steel loses strength progressively above approximately 300°C. A carbon steel bolt that provides the correct preload at ambient temperature may have relaxed significantly after the first heat-up cycle, losing clamp load and allowing the gasket to unseat. Alloy steels (chromium-molybdenum grades) maintain strength to significantly higher temperatures.
• Corrosion in aggressive environments. Carbon steel bolts in contact with moisture, process fluid leakage, or atmospheric humidity in coastal or industrial environments corrode and reduce in effective cross-section. Corroded bolts cannot be reused — they are often seized, requiring destruction to remove. Specifying a corrosion-resistant material (stainless, hot-dip galvanised, or coated) eliminates this failure mode at modest cost premium.
• Hydrogen-induced and stress corrosion cracking in sour service. In the presence of hydrogen sulphide (H₂S), high-strength steels are susceptible to sulfide stress cracking (SSC) — a form of hydrogen embrittlement that occurs at stresses well below the material's yield strength. NACE MR0175 / ISO 15156 imposes hardness limits on all metallic materials in sour service, including fasteners, to prevent SSC. High-strength fasteners that are perfectly adequate for sweet service can fail catastrophically in even low concentrations of H₂S.

ASTM A193 — Alloy Steel and Stainless Steel Bolting

ASTM A193 is the primary US standard for bolting materials intended for use in pressure vessels, valves, flanges, and fittings for high-temperature or high-pressure service. It covers stud bolts, bolts, and screws in alloy steel and stainless steel. The grade designation combines a letter (material family) and a number (specific alloy and heat treatment).

Grade B7 — The Workhorse

Chrome-molybdenum alloy steel (AISI 4140/4142), quenched and tempered. The default specification for carbon steel and low-alloy piping systems from −45°C to +427°C (−50°F to +800°F). Minimum tensile strength 125 ksi (862 MPa) for diameters up to 2½". The combination of high strength, good toughness, and elevated temperature performance makes B7 the standard choice for Class 150 to Class 2500 flanged joints in normal process service. B7 is not suitable for sour service without meeting the hardness limits of NACE MR0175 — standard B7 may exceed 22 HRC, which is the maximum permitted hardness in sour service.

Grade B7M — Sour Service B7

The same chrome-molybdenum alloy as B7, but controlled to a maximum hardness of 22 HRC (235 HBW) — within the NACE MR0175 limits for sour service. Lower strength than standard B7 as a result of the hardness restriction: minimum tensile strength 105 ksi (724 MPa). Used wherever H₂S may be present in the process fluid or the downstream fluid reaching the bolted joint. The "M" designation indicates the maximum hardness control.

Grade B8 and B8M — Austenitic Stainless Bolting

B8 covers 304 stainless steel bolting; B8M covers 316 stainless. Class 1 (solution annealed, not strain-hardened) has minimum tensile strength of 75 ksi (517 MPa) — significantly lower than B7. Class 2 (strain-hardened) achieves 125 ksi (862 MPa) but only up to approximately 3/4" diameter before strength starts to fall with increasing section. Used for stainless flanged joints to avoid galvanic corrosion and for cryogenic service (austenitic stainless has no low-temperature toughness concern). Not suitable above approximately 315°C (600°F) for the strain-hardened Class 2 grades — sensitisation and carbide precipitation degrade properties at higher temperatures. Use 316L (B8ML) where sensitisation is a concern.

Grade L7 — Low-Temperature Service

AISI 4140, quenched and tempered, with impact tested and certified to −100°C (−150°F). Mechanically identical to B7 but with the additional low-temperature toughness qualification. Required for cryogenic and low-temperature process piping where the flange and bolting will operate below the impact test exemption temperature for standard B7.

Grade B16 — High Temperature

Chrome-molybdenum-vanadium alloy steel, heat treated. Used for high-temperature service up to 540°C (1000°F) where B7 strength has dropped unacceptably. The vanadium addition maintains elevated temperature strength better than the standard Cr-Mo grades. Typically specified for steam lines, fired heaters, and other high-temperature process applications above 427°C.

ASTM A194 — Nuts

ASTM A194 covers nuts for high-pressure and high-temperature service. The grade must be compatible with the bolt grade — mixing incompatible grades causes galling, differential thermal expansion issues, or strength mismatches that produce non-uniform load distribution.

The standard pairing for carbon steel and low-alloy process piping is A193 B7 stud bolts with A194 2H heavy hex nuts. This is what "B7/2H" means in a pipe specification. The combination has matched strength and thermal expansion, and 2H nuts are manufactured to the same dimensional standards as the B7 bolt thread.

EN ISO 898 — The European Standard

European fastener practice for general structural and pressure service uses EN ISO 898 Parts 1 and 2, which define property classes rather than grade designations. The property class is stamped on the fastener head or end face and identifies both the tensile strength and the yield-to-tensile ratio:

For pressure vessel and piping flanged joints in European practice, the pressure equipment standards (EN 13445, EN 1591-1) and piping codes specify the bolt material requirements. EN 13445-2 Annex B lists approved bolt and nut materials for pressure service, referencing EN ISO 3506 (stainless fasteners) and EN ISO 898 for carbon and alloy steel.

The approximate European equivalents to the common ASTM grades used in process piping:

Sour Service — NACE MR0175 / ISO 15156

NACE MR0175 (internationally adopted as ISO 15156) defines the materials requirements for equipment exposed to H₂S-containing environments in oil and gas production. The standard applies where the partial pressure of H₂S in the gas phase exceeds 0.0003 MPa (0.05 psia) — a concentration easily reached in many refinery, gas processing, and upstream production environments. For fasteners exposed to the process fluid or to the wet H₂S environment outside the pipe (external sour service — rare but possible in H₂S-rich atmospheres):

• Carbon and low-alloy steel fasteners — maximum hardness 22 HRC (250 HBW, 237 HV10). This limits the strength of carbon steel bolting significantly below the standard B7 strength — B7M is the sour-service grade that satisfies this limit.
• Austenitic stainless steel fasteners — generally acceptable in cold-worked condition up to the hardness limits stated in NACE MR0175 Annex D. Cold-worked 316L is commonly specified for sour flanged joints.
• High-alloy materials — Alloy 625, Alloy 718, duplex stainless grades — covered by Part 3 of ISO 15156 with specific composition and hardness requirements for each.

The hardness requirement applies to every bolt and nut in sour service — not just the bulk material but the thread roots and any heat-affected zones from manufacturing. Material certificates for sour service fasteners must include hardness test results on the actual components, not just the parent material.

Temperature Selection Guide

Galvanic Compatibility — Bolt to Flange

As covered in the corrosion article, dissimilar metals in contact in an electrolyte form a galvanic cell. The area ratio of cathode to anode determines attack severity. For flanged joints:

• Carbon steel bolts in carbon steel flanges — no galvanic issue, same potential. Coat or galvanise the bolts to resist atmospheric corrosion.
• Stainless steel bolts in carbon steel flanges — stainless is noble relative to carbon steel. The small cathodic stainless bolts driving attack on the large anodic carbon steel flange faces is a relatively benign configuration. More concerning is carbon steel bolts in stainless flanges — small anode (bolt), large cathode (flange) — which concentrates attack on the bolts. Use stainless bolts in stainless flanges.
• B7 alloy steel bolts in stainless flanges — a common mismatch in mixed-material systems. The alloy steel bolt is anodic to the stainless flange, and the large cathodic flange area drives aggressive attack on the bolt threads in any wet environment. Always specify stainless bolts (B8M) for stainless flanges.

Coating and Surface Treatment

For carbon steel fasteners (B7/2H) in outdoor or marine environments, surface coating significantly extends service life:

• Hot-dip galvanising — zinc coating applied by immersion. Provides sacrificial cathodic protection. Oversize the bolt hole clearances for galvanised nuts (the zinc coating adds approximately 0.1–0.15mm per surface). Not suitable above approximately 200°C — the zinc diffuses into the base metal and embrittles it.
• Geomet/Dacromet — zinc-aluminium flake coating applied by dipping and curing. Better high-temperature performance than hot-dip galvanising (suitable to approximately 300°C), excellent chemical resistance, no hydrogen embrittlement risk (important for high-strength fasteners). Standard in offshore and marine process applications.
• PTFE or fluoropolymer coating — chemical resistance in aggressive atmospheric environments, particularly where chloride stress corrosion is a concern for the base material. Also reduces installation torque and galling risk on stainless fasteners.
• Plain (black) carbon steel — no protective coating. Acceptable in dry indoor service; corrodes rapidly in any outdoor, humid, or splash-zone environment. Never specify plain carbon steel fasteners for outdoor process plant without additional corrosion protection.

Stud Bolt vs Bolt — Form Selection

For flanged joint service in process piping and pressure vessels, stud bolts (fully threaded bar with two heavy hex nuts) are strongly preferred over through-bolts (hex head bolt with one nut) for the following reasons:

• Stud bolts allow equal extension (stretch) on both ends during make-up, distributing the bolt load more evenly through the thread engagement
• If a bolt is seized and must be cut, a stud bolt can be extracted from one side by unscrewing rather than requiring drilling out
• Stud bolts allow precise torque measurement at both ends
• ASME B16.5, B16.47, and ASME VIII all specify stud bolts as the standard fastener for flanged pressure service connections

Hex head bolts may be used for lower-pressure and non-critical connections (instrument connections, small bore fittings, cover plates) but stud bolts with heavy hex nuts should be specified for all Class 150 and above flanged process piping connections.

Summary

The standard fastener specification for the majority of process piping and pressure vessel flanged joints is A193 B7 stud bolts with A194 2H heavy hex nuts — adequate from −29°C to +427°C in clean service. Where temperature exceeds 427°C, upgrade to B16. Where temperature is below −46°C, use L7/4. Where the service is sour (H₂S present), use B7M/2HM to meet NACE MR0175 hardness limits. Where the flanges are stainless steel, use B8M/8M to eliminate galvanic attack. Coat all outdoor carbon steel bolting with Geomet or hot-dip galvanising.

The wrong fastener is one of the most common causes of flange joint failure in process plant — not because the bolts break, but because they corrode, relax, or contribute to galvanic attack on the flange face in a way that is only identified when the joint is opened during a maintenance shutdown, often years after commissioning. Specifying correctly takes five minutes and costs almost nothing extra.