What materials are used for gasket, stud bolt & nut in different service conditions?

Every flanged joint in a process plant is really a three-part system: the pipe flanges on either side, a gasket, stud bolt & nut combination that holds them together, and the contact faces in between. Choose the wrong gasket and it creeps out of the seat at start-up. Pick a stud that cannot hold preload at operating temperature and the joint slowly loosens into a leak. And as straightforward as fasteners look, the material that works at ambient temperature can fail completely in a steam line or a chloride-rich marine service. This guide walks through which materials are used for gaskets, stud bolts and nuts across the most common service conditions, and how to match them before you place an order.

What a service condition actually changes

Material selection for the three parts is driven by four working parameters: operating temperature, design pressure, the media being carried, and the surrounding environment. Each one pulls the choice in a different direction. High temperature reduces bolt yield strength and drives creep, so an alloy steel (or in very hot oxidizing service, a stainless) is called for. Low temperature makes carbon steel brittle, so impact-tested grades take over. A corrosive or sour medium demands either a corrosion-resistant grade or a controlled-hardness alloy. Pressure largely controls the type of gasket rather than the bolt grade, while marine and offshore service that sees chlorides pushes you toward 316 stainless even when the pressure is modest.

Gasket materials by service condition

Gaskets fall into three broad families, and the operating envelope of each family maps onto distinctly different pressure and temperature windows.

Non-metallic (soft) gaskets for low-pressure utility service

For water, air, cooling circuits and mild chemical lines at Class 150 and moderate temperatures, a soft gasket is usually the most economical call. Compressed non-asbestos fibre handles roughly up to 200 °C, PTFE provides outstanding chemical resistance but should stay below about 150 °C and low clamping pressures, and flexible graphite carries the widest temperature range of the three, from about -200 °C to +650 °C in oxidizing service. Flexible graphite has poor mechanical strength on its own, so it needs a thin metal ｉｎｓｅｒｔ or eyelet when used at higher pressure classes.

Semi-metallic gaskets for high-temperature, high-pressure and cycling duty

Where the duty is serious, the spiral-wound gasket is the workhorse. A metallic winding strip — commonly 304 or 316L stainless, with Monel or Inconel for more aggressive duties — is wound spirally with a graphite or PTFE filler, and is finished with an inner ring, an outer ring, or both. That construction lets the gasket absorb thermal and pressure cycling without losing its seal. Kammprofile and metal-jacketed designs cover the cases where a softer, more resilient seal is preferred in heat-exchanger and large-diameter service.

Metallic (ring-joint) gaskets for high-pressure hydrocarbon and critical service

Above about ASME Class 600, and whenever any leak is unacceptable, ring-joint (RTJ) gaskets become the only realistic choice. These are solid metal rings — octagonal R or oval/energised RX and BX profiles — that plastically deform into the flange groove under bolt load. They are supplied in soft iron and low-carbon steel for normal service, and in 304/316/321/347 stainless, Monel 400, Inconel 600/625 and Hastelloy grades where corrosion or temperature demands it. The ring number must match the flange groove exactly, or the joint simply will not seal.

Stud bolt materials by service condition

Almost every stud-and-nut pair in hydrocarbon and power service is governed by two ASTM specifications: ASTM A193 covers alloy and stainless bolting for high-temperature service, and ASTM A194 covers the matching nuts. The dominant A193 grades and where they belong are listed below.

General high-temperature service: ASTM A193 B7

B7 is a quenched-and-tempered chromium-molybdenum alloy steel (42CrMo/4140 family). It is the default grade for the bulk of refinery, steam and general hydrocarbon flanging from about -20 °C to +538 °C, and it is nearly always paired with an ASTM A194 grade 2H nut. If you are not sure what to specify, B7 plus 2H is the safe starting point for non-corrosive, non-cryogenic service.

Sour and hydrogen-sulphide service: ASTM A193 B7M

Where the medium contains hydrogen sulphide (sour oil and gas, amine systems, sour water), B7 is too hard and is at risk of sulphide stress cracking. The answer is B7M — the same chemistry as B7 but with restricted hardness and mandatory magnetic-particle testing to NACE sour-service requirements. It is matched with an A194 grade 2HM nut.

Very high temperature steam and furnace duty: ASTM A193 B16

When the stud must hold preload above the range of B7 — in steam boilers, furnace headers and superheater service — the chromium-molybdenum-vanadium grade B16 takes over, rated roughly up to 595 °C with better creep resistance. It is paired with an A194 grade 7 or 7L nut.

Corrosive, chloride and marine service: stainless grades B8 and B8M

For chemical, food, offshore and chloride-bearing service, stainless studs are the right call. B8 is 304 stainless, used for general corrosion service; B8M is 316 stainless and is the better choice where chlorides and seawater are involved. Both are matched with A194 grade 8 and 8M nuts respectively. Avoid plain 304 in concentrated acids, where stabilised grades such as B8C (347) or B8T (321) are safer.

Low-temperature and cryogenic service: ASTM A193 L7

For LNG, ethylene, ammonia and other low-temperature lines, normality carbon-steel bolts are brittle. Grade L7 is the B7 chemistry but with Charpy impact testing down to about -46 °C, and it is paired with an A194 grade 4 or 7 nut. Where a reduced hardness is also needed for sour low-temperature duty, L7M fills that gap.

Nut materials: match the stud, never guess

A nut should be harder than the stud it mates with so the threads do not gall and the load spreads reliably. The A194 nut grades therefore map directly onto the A193 stud grades, and the two should always be specified and supplied together. For the common pairs, the A194 2H nut goes with B7, 2HM with B7M, 7 with B16, 8 with B8, and 8M with B8M. Ordering fasteners as matched sets removes the single most common field mistake — a stainless nut landed on a carbon stud, or vice versa.

A working summary of material selections

Final practical checks before you order

Once the material family is right, three details decide whether the joint actually holds. First, confirm the flange face finish suits the gasket — around 125 to 250 micro-inches Ra for raised-face joints, with the smoother end of that range preferred for spiral-wound gaskets. Second, verify the stud length against the flange thickness, gasket seating thickness and thread engagement, using the ASME B16.5 or B16.47 data tables rather than a guess. Third, ask for a heat-traceable mill test certificate and, for stainless and alloy lots, specify positive material identification so the delivered grade matches the paperwork.

Sourcing gasket, stud bolt & nut sets together, from one supplier that can pair the material to your exact flange and service condition, removes most of the mismatch failures before they reach the field. Where your system also includes the flanges themselves — from standard steel flanges and pipe flanges to copper-nickel variants — EZ Steel Industrial can package the flanged joint as a single, traceable order rather than a collection of separate parts.