Gasket, Stud Bolt & Nut Buyer's Guide: How to Match the Three Together and Source from a Mill-Direct Supplier

A two-bolt flange joint on a Class 150 line and a 48-bolt Class 1500 heat-exchanger channel cover share one engineering truth: the gasket, stud bolt & nut set is a system, not three line items. A soft non-asbestos sheet that can’t take 300 °C, a B7 bolt that loses strength above 450 °C, a 2H nut that gall on a B16 stud — any one of these mismatches can turn a routine start-up into a hot rework or a near-miss report.

This guide focuses on the three components buyers actually compare on a purchase order: gasket type and facing, stud-bolt material grade, and the matching nut specification. The goal is to give you a defensible selection logic and a sourcing checklist, and to show how ezindustrialtube.com ships those three components as a single coordinated package from the same mill rather than three resold lots.

1. Why the three components must be specified as a system

Every flanged joint is governed by four numbers: design pressure, design temperature, media, and the bolt-up procedure. A change in any one of them changes the gasket selection, the bolt grade, and the nut grade. Buyers who specify each in isolation end up with components that meet the data sheet but fight each other on the flange face.

A coordinated package also matters for documentation. The MTC that arrives with the gasket references a different standard (ASME B16.20, B16.21 or B16.5) than the MTC for the stud bolt (ASTM A193) and the nut (ASTM A194). When all three come from the same mill-direct order, the heat numbers, batch records and country-of-origin statements line up; when they are pulled from three stockists, traceability gaps appear and third-party inspectors flag them at goods-in.

2. The four gasket families buyers actually compare

Although the keyword gasket, stud bolt & nut reads as one phrase, the gasket side alone breaks into four functional families. Picking the wrong family is the most common cause of chronic seeping at the flange face, especially on services that swing between ambient start-up and full operating temperature.

A practical note on RTJ: the gasket hardness must be at least 30 HB softer than the flange groove hardness, otherwise the ring will not seat properly and will score the groove on first torque-up. This is why soft iron (HB ~120) and 304 (HB ~150) are common ring choices while the flange itself is a harder carbon-steel grade.

3. Stud bolt materials — ASTM A193 grade selection

The stud bolt is the structural member of the joint. It must hold preload against internal pressure and thermal expansion, day after day, often at temperature. Carbon & carbon alloy steel stud bolts are made to ASTM A193 / A193M, which covers alloy-steel and stainless-steel bolting for high-temperature and high-pressure service. Five grades do the heavy lifting in the B2B market.

For low-temperature service (down to −101 °C, LNG / ethylene), the grade flips to ASTM A320 L7 / L7M (similar chemistry to B7 / B7M, Charpy impact-tested at −101 °C). For very low temperature down to −196 °C, A320 L43 (304 stainless) or B8 Class 2 are used.

4. Nut materials — ASTM A194 grade pairing

A nut is not just a hex block of steel. Its hardness, alloy and proof load have to be matched to the stud bolt grade, otherwise two failure modes appear: (1) thread galling on torque-up, and (2) thread stripping under thermal cycling when the nut is too soft. ASTM A194 / A194M defines the matching nut grades.

5. Match the set to the service — five real scenarios

Theory tables are useful, but buyers read the following five-scenario matrix more often. It maps the three components together against common service lines.

6. Cross-standard equivalents buyers should keep handy

EPC projects in the Middle East, China and Europe often demand EN or JIS marks alongside ASTM. The same three-component logic applies, but the grade names change. The table below maps the most common equivalents buyers ask about on cross-border RFQs.

Note on the B16 line: EN 10269 does not have a single one-to-one replacement for ASTM A193 B16. Some mills dual-certify to 21CrMoV5-7 (1.7709) under EN 10269 and to A193 B16 under ASTM, and the MTC will carry both marks. Always request the dual-cert on the MTC if your project crosses standards.

7. Dimensions, threads and what to put on the RFQ

The three components have to match in three dimensions: stud length (so the nut fully engages and exposes 1–3 threads), stud thread size and pitch (UNC for imperial, metric coarse or fine for EN/JIS), and the gasket thickness that determines the compressed gasket height. Here is the RFQ line set that captures all of them.

• Flange standard & class: ASME B16.5 Class 150 / 300 / 600 / 900 / 1500, or B16.47 Series A or B, or EN 1092-1 PN10 / PN16 / PN40, or JIS B2220 5K / 10K / 20K
• Service fluid & concentration: hydrocarbon, steam, seawater, sour H2S (NACE?), cryogenic, oxygen-clean
• Design temperature (min / normal / max): °C or °F, both normal and upset
• Design pressure: barg / psig, both normal and upset, plus any water-hammer surge
• Bolt quantity per joint, stud size, thread pitch, length: e.g. 16 × M30 × 180 mm, full thread or partial
• Gasket style & facing: SWG with inner/outer ring, RTJ ring number, soft sheet, graphite
• Stud grade & nut grade: A193 B7 + A194 2H, A193 B16 + A194 4, etc.
• Documentation: EN 10204 3.1 MTC, PMI report, hardness report, NACE MR0175 (if sour), 100% UT / MT (if high-temp alloy)
• Surface treatment & packaging: plain oil, PTFE / Xylan, zinc-flake (e.g. Geomet), anti-rust VCI, plywood crate or carton
• Quantity & delivery: per joint or per kilogram, target date, port of discharge or job site

If your PO links to pipe fittings and industrial valves on the same drawing, the gaskets, studs and nuts almost always come from the same supplier. The supplier that ships all four (fittings, flanges, gasket, stud bolt & nut, valves) tends to produce MTCs that line up heat number by heat number, which simplifies the inspector's job at goods-in.

8. The five most expensive procurement traps

Trap 1 — Specifying the stud without the nut grade

A B16 stud with a 2H nut will pass a 1.0× load check at ambient but will fail the relaxation test at 540 °C. The nut will lose preload in 20–30 thermal cycles and the joint will weep. Always write “A193 B16 with A194 4” as one line, never just the stud.

Trap 2 — Soft-iron RTJ on a stainless groove

If the flange is stainless (e.g. 304 / 316 to ASME B16.5) and the ring is soft iron, the ring will smear metal onto the groove. After two torque-ups the groove is no longer round, and the joint will not pass a helium leak test. Match ring hardness to flange hardness: 304 groove needs 304 or softer ring, never soft iron.

Trap 3 — B7 stud in chloride service

B7 in seawater, brine or even routine coastal-air ambient will pit and eventually fail by chloride stress corrosion cracking. Switch to B8M (316 stainless) for any chloride-bearing service, even at ambient. The price premium is roughly 3–5× the B7 price, but the failure cost is a full plant shutdown.

Trap 4 — Reusing a soft gasket

Compressed non-asbestos and graphite gaskets are single-use. Once compressed they take a permanent set and will not recover. RTJ rings can sometimes be re-used if the ring is inspected and the groove is undamaged, but most plants adopt a single-use policy to keep records clean.

Trap 5 — Skipping the cross-standard note on the MTC

A stud marked B7 but lacking the “supplementary requirement S1” for Charpy impact is not the same material as one with S1. Bidders sometimes ｄｒｏｐ S1 to win on price. For any service that swings to sub-zero start-up, the impact test is non-negotiable.

9. Mill-direct versus stockist — what the three-component PO looks like

A stockist reshuffles three brands into one box and writes one invoice. A mill-direct supplier cuts stud, machines nut and cuts gasket from the same heat of raw stock, then issues three MTCs from the same quality system. The second is the only model that survives a third-party audit cleanly.

10. Documentation — the MTC, PMI, NACE and impact test package

For any pressure-containing joint, the MTC has to be EN 10204 type 3.1 at minimum. For critical service, ask for 3.2 (with independent third-party signature). The MTC must reference the standard, the grade, the heat number, the lot number and the chemical / mechanical results. The supplementary documents that often come with the MTC are:

• PMI (positive material identification) report — verifies alloy against the MTC chemistry
• Hardness map — per ASTM A193 / A194 batch requirements (typically 3 samples per heat)
• Impact test report — Charpy V-notch at the agreed test temperature
• NACE MR0175 / ISO 15156 compliance statement — mandatory for any sour service
• UT / MT reports — for high-temp alloy studs (B16) above M30 size, mandatory under most power-plant specs
• Surface treatment certificate — if zinc-flake, PTFE or Xylan coated

11. Frequently asked questions

12. Sourcing the three-component set from EZ Steel Industrial

EZ Steel Industrial ships gasket, stud bolt & nut sets as a coordinated package from the same mill. The set is cut from the same heat of carbon, alloy or stainless bar stock, the gaskets are produced to ASME B16.20 / B16.21, and every stud / nut pair ships with an EN 10204 3.1 MTC, PMI report and (when required) NACE MR0175 compliance statement.

Buyers who want to consolidate the four families on a single PO can pair the gasket set with pipe flanges (carbon, stainless, alloy, steel flanges and copper nickel flanges), pipe fittings (BW, SW, threaded), and industrial valves. Cross-references into the larger piping system are also available: petrochemical facilities, power plants & aerospace, and marine & ship-building all reference the same three-component logic above.

Common request patterns the EZ Steel team handles weekly: Class 150 / 300 / 600 SWG sets for refinery, Class 1500 RTJ sets for high-pressure steam headers, full B7M + 2HM + soft-iron RTJ packages for sour upstream service, and B8M + 8M + PTFE sets for marine seawater systems. A single RFQ can be returned as a coordinated quotation across all four categories (gasket, stud, nut, and mating flanges / fittings), with the MTC numbers pre-aligned to the heat codes the inspector will see at goods-in.