The reliability of a flange connection system depends not only on the flange body and gasket—the correct selection of fasteners is equally decisive for sealing success. In actual engineering practice, however, fasteners are often treated as “standard parts” and overlooked, and sealing failures arising from this neglect are not uncommon.
Fastener material selection must be systematically matched with the flange and gasket. When one of a pair of flanges is cast iron, bronze, or another cast flange, the fasteners shall be of a material compatible with the lower‑strength flange. The logic behind this requirement is that fastener strength should not exceed the load‑bearing capacity of the weaker connected component; otherwise, the flange body may be damaged during tightening. Similarly, when flanges of different pressure ratings are bolted together, the tightening torque shall comply with the requirements of the lower‑rated flange.
Bolt type selection is closely related to flange type and service conditions. Hexagon head bolts, when subjected to high tensile forces, are prone to fracture at the junction between the shank and the head; therefore, they are not suitable for medium‑ and high‑pressure flanges and are commonly used with slip‑on flanges and non‑metallic gaskets in relatively mild operating conditions. Double‑ended studs can be used for medium‑ and high‑pressure flanges, allowing tightening from both sides, and are commonly paired with welding neck flanges in more demanding service. It is worth noting that full‑threaded studs offer greater load‑bearing capacity due to the absence of cross‑sectional transitions; partially threaded studs have relatively lower load‑bearing capacity.
Thread pitch is an easily overlooked technical detail. The thread pitch of fasteners for flange connections should generally not exceed 3 mm; for fasteners of M30 and above, fine threads may be used. The primary consideration is to facilitate tightening during installation and to avoid excessive load deviation among bolts. When the pitch is too large, the preload corresponding to a given tightening torque becomes more sensitive, making uniform load control across bolts more difficult.
Hardness matching between nut and bolt also merits attention. Generally, the nut material should be slightly softer than the bolt material, with the nut hardness approximately HB30 lower than that of the bolt. The purpose of this hardness differential is to ensure that wear preferentially occurs on the nut—the nut, being the relatively cheaper component, is less costly to replace, and the bolt threads are protected from damage during repeated assembly and disassembly.
Under severe cyclic conditions, bolts or studs for flange connections shall be made of alloy steel. Alloy steel bolts shall be made of high‑grade quality steel, with the letter A added after the material designation, such as 35CrMoA and 25CrMoVA. This requirement addresses the risk of fatigue fracture of ordinary carbon steel bolts under cyclic loading.
Fasteners, though small, are an inseparable part of the flange connection system. Incorporating fasteners into the overall design considerations of the flange connection—rather than treating them as an afterthought accessory—is an important prerequisite for improving sealing system reliability.
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