ASME B16.5 Says: Stud Bolts Only. Here’s Why Hex Bolts Fail in Flanges.

ASME B16.5 allows only stud bolts in pressure-retaining flange joints, a rule that surprises teams who reach for hex head bolts out of habit. The reason is engineering rather than tradition: a stud bolt carries pure tension, tightens evenly from both ends and holds the gasket stress that keeps the joint sealed through thermal cycles. This comparison covers both fasteners, the matching flange gaskets and heavy hex nuts, and where each one belongs.

Why the Code Accepts Stud Bolts Only

ASME B16.5 (Pipe Flanges and Flanged Fittings) and ASME B31.3 (Process Piping) call for stud bolts on flange connections that retain pressure. Four reasons explain the rule.

• Uniform gasket compression: two nuts squeeze the joint symmetrically, so the gasket sees even stress across the sealing face.
• No head fillet: without a forged head there is no transition radius to concentrate stress and start a fatigue crack.
• Repeatable preload: alternating torque from each end lets you control bolt load bolt by bolt, following the sequence in ASME PCC-1.
• Easy maintenance: both ends stay accessible, so the joint can be broken and rebuilt without holding a bolt head.

Symmetric loading also keeps pipe flanges tight when temperature and pressure swing, which is exactly where hex bolts start to lose grip and the gasket stops sealing.

The common exception is ASME B16.1 Class 125 and 250 cast iron flanges, where ASTM A307 Grade B hex head bolts may be permitted. Those are low-pressure, non-critical joints.

Design, Manufacturing and Material Matching

A stud bolt is a headless cylindrical body with threads at both ends. It comes as a full-thread stud, threaded along the whole length, or a double-end stud, with a plain shank between two threaded ends. Both types need two heavy hex nuts, one at each end.

• Full-thread stud: used where grip length varies, since every part of the body can take a nut.
• Double-end stud: the plain shank keeps a larger tensile area and helps alignment in the bolt hole.
• Heavy hex nut (ASTM A194 Gr. 2H, 8 or 8M): larger across-flats and greater height than a standard nut, so the bearing face suits the load.

A hex head bolt has a forged hexagonal head at one end and takes a single nut at the other. Tightened, the head carries an eccentric load and the fillet between head and shank concentrates stress, so fatigue risk rises under cyclic or impact loading.

Manufacturing follows the same logic. Stud bolts are simple to make: small and medium sizes, typically up to M24, are cold headed or machined from bar, while larger diameters have threads cut or rolled directly on round bar. With no head-forming step the process is shorter, material utilisation is higher and cost per tonne is lower.

• Cold forming suits sizes up to about M24: the material deforms in dies at room temperature, with high output and material utilisation above 90%.
• Hot forging is used above M24: heating the bar to red heat prevents cracking and keeps a continuous grain flow from head into shank.
• Forming rather than cutting lets the grain follow the bolt contour, which improves strength and fatigue resistance.

Table: standard stud-to-nut grade pairing used in petrochemical practice. Always match the nut grade to the stud grade.

Torque Control, Identification and Storage

Torque can be applied to both nuts of a stud, so preload is measured and controlled from each end. With a hex bolt the head must be held while the nut turns, and that introduces torsion into the shank, reducing the accuracy of the final preload.

• Tighten in a cross pattern over several passes and finish at the value from ASME PCC-1 for the gasket and bolt size.
• Grade marks sit on one end of a stud and on the top of a hex bolt head, which is the quickest field check.
• Store studs as sets with two nuts so batch traceability and material pairing survive to the site.
• For hot service to 550 °C (1022 °F) specify ASTM A193 Grade B7, B8 or B16, which resist stress relaxation and creep at temperature.

FAQ

Can I use a hex head bolt instead of a stud bolt in an ASME B16.5 flange joint?

No. ASME B16.5 and ASME B31.3 call for stud bolts on pressure-retaining flange connections, because only a stud gives even gasket compression and avoids the head fillet stress concentration.

Why do stud bolts perform better in hot service?

A stud has no head fillet, so the main stress concentration point disappears. Paired with ASTM A193 Grade B7 or B8, it holds preload against creep and stress relaxation up to about 550 °C (1022 °F).

Which nut grade pairs with a B7 stud bolt?

Use heavy hex nuts in ASTM A194 Gr. 2H for B7, Gr. 8 for B8 and Gr. 8M for B8M. Standard hex nuts are too thin and can strip under the required preload.

What is the fastest way to tell the two fasteners apart on site?

A stud is headless, with threads at both ends and two nuts. A hex bolt has a forged head on one side and one nut on the other.

Which grades suit sour service under NACE MR0175?

Specify ASTM A193 Gr. B7M stud bolts with matching ASTM A194 Gr. 2HM heavy hex nuts to resist sulfide stress cracking.