1. Why gasket, stud bolt & nut are the most-mis-specified trio in piping

Walk any EPC warehouse during a hydro-test campaign and you will see the same three problems repeating: a spiral-wound gasket with a paper-thin graphite filler paired with a low-grade B7 stud; an A194 grade 8 nut on a stainless B8M stud in a chloride service; or, most commonly, a gasket from one supplier, studs from a trading company, and a nut from yet another source — none of which arrived with a heat-traceable MTC. A flanged joint is a system, and the system is only as strong as its weakest component. The gasket, stud bolt & nut set is the actual mechanical seal; the pipe and the flange are merely the structure that holds them.

In a B2B procurement context the trio is also the easiest place to lose money: a 1,000-joint refinery job can burn through 8,000 studs, 16,000 nuts, and 1,200 gaskets, and a 5% over-spec on stud material alone is enough to wipe out the margin on the entire piping package. This guide is written for procurement engineers, QC inspectors, and project buyers who want a single reference to take from RFQ through goods-in inspection.

2. The three families of gaskets and how to choose between them

A gasket is not a generic commodity. The three families — non-metallic, semi-metallic, and metallic — map to very different pressure and temperature envelopes, and the wrong choice is the single most common cause of a flange leak in service. EZ Steel Industrial supplies gasket, stud bolt & nut packages across all three families, with materials matched to ASME B16.5 / B16.47 / EN 1514 / HG / JB/T flange standards.

2.1 Non-metallic gaskets (soft gaskets)

Soft gaskets — compressed non-asbestos fibre, PTFE, flexible graphite — are the workhorses for low-pressure utility and chemical service. They are cheap, conform well to minor flange-face imperfections, and ｒｅｐｌａｃｅ easily during turnaround. Their envelope is narrow:

• Compressed non-asbestos (CNA / aramid + NBR binder): up to Class 150 and around 200 °C, suitable for water, air, mild chemicals.
• PTFE: best-in-class chemical resistance but prone to cold-flow; do not use above 2.5 MPa or above 150 °C, and never on a serrated face.
• Flexible graphite (with or without metal ｉｎｓｅｒｔ): the highest temperature envelope of the non-metallics, -200 °C to +650 °C in oxidising service, but low mechanical strength, so always reinforce with a metal eyelet on higher classes.

2.2 Semi-metallic gaskets (spiral-wound, kammprofile, jacketed)

The spiral-wound gasket (SWG) is by volume the most-used semi-metallic gasket in refineries, petrochemical plants, and power stations. A 304 or 316L stainless winding strip is spirally wound with a graphite or PTFE filler, and the assembly is finished with an inner ring (to prevent inward buckling), an outer ring (to centre the gasket in the bolt circle), or both. EZ Steel Industrial's gasket, stud bolt & nut line ships SWGs in all four style combinations: basic, with inner ring (IR), with outer ring (OR), and with both rings (CGI). The structure is what gives the SWG its characteristic ability to absorb temperature and pressure cycling and flange-face distortion.

Kammprofile gaskets use a grooved metal core with a soft facing layer; they are used where the resilience of an SWG is not needed but a softer seal is preferred. Metal-jacketed gaskets wrap a non-metallic core in a thin metal skin and are common in heat-exchanger and large-diameter service where handling strength matters.

2.3 Metallic gaskets (ring-joint)

Ring-joint (RTJ) gaskets — octagonal (R) and oval (RX / BX for very high pressure) — are the only choice for ASME Class 600 and above, and for hydrocarbons above 600 °C or in service where any leak is unacceptable. The gasket is solid metal, soft enough to plastically deform into the RTJ groove under bolt load. Pressure ratings on BX profiles exceed 100 MPa in API 6A wellhead service. EZ Steel Industrial stocks RTJ gaskets in soft iron, low-carbon steel, 304 / 316 / 321 / 347 stainless, Monel 400, Inconel 600/625, and Hastelloy B2/C276, in all standard ring numbers (R, RX, BX).

3. Stud-bolt material families (ASTM A193 / A194) and pressure-class matching

A stud bolt is a high-tensile fastener that lives in some of the most punishing environments on a plant — high temperature, pressure cycling, and a wide range of corrosive media. The two standards that govern almost every stud-and-nut pair in hydrocarbon service are ASTM A193 (alloy-steel and stainless-steel bolting for high-temperature service) and ASTM A194 (carbon and alloy steel nuts for high-pressure and high-temperature service). Choosing a stud without pairing the right nut is one of the most common procurement mistakes EZ Steel Industrial sees in gasket, stud bolt & nut RFQs.

3.1 ASTM A193 grade families

ASTM A193 grade selection is driven first by temperature and then by corrosion. The dominant grades in petrochemical service are:

• B7 — chromium-molybdenum alloy steel, quenched and tempered. The default grade for high-temperature hydrocarbon service, -20 °C to +538 °C. Equivalent to 42CrMo / 4140 / 4142. Always paired with A194 grade 2H nut.
• B7M — same chemistry as B7 but with restricted hardness (≤ 235 HB) and a 100% magnetic-particle test, for sour service per NACE MR0175. Paired with A194 grade 2HM nut.
• B16 — chromium-molybdenum-vanadium, higher creep resistance than B7, rated to +595 °C, for high-temperature steam and refinery furnace service. Paired with A194 grade 7 or 7L nut.
• B8 — 304 stainless, solution-annealed, Class 1 (carbide solution) or Class 2 (strain-hardened). For general corrosion service. Paired with A194 grade 8 nut.
• B8M — 316 stainless, for chloride and marine environments. Paired with A194 grade 8M nut. Avoid in concentrated sulphuric acid; consider B8C (347 stainless) or B8T (321 stainless) instead.
• L7 / L7M — B7 / B7M chemistry but with Charpy impact testing at -46 °C, for low-temperature service (LNG, ethylene, ammonia). Paired with A194 grade 4 or 7 nut.

3.2 ASTM A194 nut grade mapping

A nut must be harder than the stud it is mated to, so that the threads do not gall and the load is reliably distributed. The pairing is not optional; it is the way the joint is designed. EZ Steel Industrial pre-matches every stud in the gasket, stud bolt & nut catalogue to the corresponding A194 nut, and every shipment is packed in matched sets so the two never get separated on site.

4. ASME B16.5 / B16.47 quantity, size, and length tables — the most-missed data

The single biggest cause of a mis-sized gasket, stud bolt & nut order is a buyer who specifies "M20 x 100 stud" without referring to the ASME B16.5 (NPS 1/2 to 24) or B16.47 (NPS 26 to 60) table for the exact quantity and size of studs required for the pressure class, the flange size, and the facing. The data is the same across manufacturers, but only because it is in the standard — guessing it produces a stud shortfall of 20% on a typical Class 300 RF job.

4.1 Why length matters more than diameter

A stud-bolt length is the distance from the first to the last thread, plus the chamfer. The required length is governed by: the flange raised-face or RTJ-joint thickness, the gasket thickness (typically 4.5 mm for a Class 150 SWG, 6.5 mm for a Class 300 SWG, and rising with class), twice the washer thickness if used, and a thread engagement of at least one stud-diameter into the nut. EZ Steel Industrial's gasket, stud bolt & nut technical pack provides a length calculator that takes NPS, class, facing, and gasket style and outputs the recommended stud length, nut height, and washer thickness.

4.2 Thread engagement and nut height

The minimum thread engagement is the stud diameter (1 × d), the recommended engagement is 1.5 × d, and a hard-and-fast rule for ASME B16.5 is that the stud must protrude beyond the nut face by at least one thread but no more than three threads. The nut itself is typically the heavy-hex series ASME B18.2.2, height = 0.875 × d for the standard series. EZ Steel Industrial's studs and nuts are machined to ASME B18.2.1 and B18.2.2 tolerances and shipped in matched sets so the engagement rule is satisfied by default.

5. Flange face finish, RTJ ring numbers, and torque / bolt-up sequence

A gasket, stud bolt & nut set that is correctly specified can still leak if the flange face finish is wrong, the RTJ ring number does not match the groove, or the bolt-up sequence applies uneven load. The mechanical-engineering side of the trio is just as important as the materials side.

5.1 Flange face finish (RF and FF)

For raised-face (RF) and flat-face (FF) flanges, the standard stock finish is 125 to 250 micro-inches Ra (3.2 to 6.3 µm). Spiral-wound gaskets require a slightly smoother finish at the higher end of that range (around 125 to 200 µin Ra); soft gaskets can tolerate 250 µin Ra. Serrated-spiral or "chequered" finishes are still seen on older installations but are not recommended for new SWG joints because the serration can damage the winding strip.

5.2 RTJ ring number and groove compatibility

Ring-joint gaskets come in three profiles: R (oval / octagonal, the original API 6A and ASME B16.5 profile, pressures up to Class 2500), RX (a one-piece energised version with a higher pressure rating, used in BOP stacks), and BX (a square-section energised profile, pressures above 100 MPa, used in API 6A wellhead and Christmas-tree equipment). The ring number must match the groove exactly — a Type R ring in a Type BX groove, or vice versa, will not seal and can damage the flange. EZ Steel Industrial's gasket, stud bolt & nut line ships RTJ sets with the corresponding ring number printed on the gasket and on the MTC, and cross-references the flange's groove identifier before shipment.

5.3 Bolt-up sequence and target torque

The classic four-pass bolt-up sequence for an even number of studs is: pass 1, snug to 30% of target torque in a star pattern; pass 2, 60%; pass 3, 100%; pass 4, a final 100% clockwise round to relax residual unevenness. For 12-stud and higher flanges, use a five-pass sequence. The target torque itself is calculated from the desired bolt stress (typically 50% of the stud yield for a Class 150 SWG, rising to 70% for an RTJ), the stud nominal area, the K-factor of the stud-and-nut pair (0.16 to 0.20 for B7 / 2H), and the friction coefficient assumed for the coating (0.10 for PTFE, 0.15 for Dacromet, 0.20 for black oxide, 0.30 for hot-dip galvanised). EZ Steel Industrial supplies a one-page torque table on request for any B16.5 / B16.47 combination.

6. MTC packages, PMI, and the 7-step RFQ checklist

A gasket, stud bolt & nut order that arrives without a mill test certificate is, in a regulated service, the same as a gasket that arrives without a material. The MTC is the audit trail; it is what the inspector checks at goods-in and what the operator pulls up five years later during a root-cause investigation. EN 10204 defines four MTC types — 2.1 (declaration of compliance), 2.2 (test report), 3.1 (mill-issued independent test), 3.2 (third-party witnessed test). For hydrocarbon and pressure-vessel service the minimum is 3.1; for nuclear, sour service above NACE MR0175 level III, and certain offshore applications the standard is 3.2.

6.1 What the 3.1 MTC for a stud-bolt must contain

• Heat number, batch number, and country of origin
• Chemical composition: C, Mn, P, S, Si, Cr, Mo, V (plus Ni for stainless B8 / B8M)
• Mechanical properties: tensile strength, yield strength, elongation, reduction of area, hardness (HB / HRC)
• Charpy impact at the specified temperature (for L7 / L7M, mandatory; for B7, optional but often required)
• Heat-treatment condition: solution-annealed, quenched and tempered, stress-relieved
• NDT report: 100% MPI for B7M, optional for B7; UT for sizes above M100
• Dimensional check: stud length, thread pitch, thread run-out, chamfer

6.2 Positive Material Identification (PMI)

PMI is the on-site XRF check that confirms the actual material of a fastener matches the MTC. It is a 30-second non-destructive test and should be performed on a sample of at least 10% of every batch in a gasket, stud bolt & nut shipment — and on 100% of any batch where stainless and carbon-steel fasteners are co-shipped. EZ Steel Industrial's goods-in process uses a handheld XRF on every lot; the result is recorded on the packing list and can be tied to the MTC heat number.

7. Mill-direct vs trading-company sourcing for gasket, stud bolt & nut packages

A flange joint is a system, and a system is what a mill-direct supplier does best. EZ Steel Industrial, founded in 1994 with three production locations in Cangzhou, Yangzhou, and Lishui and more than 500 employees, manufactures the gasket, stud bolt & nut set as a matched package rather than a collection of separately-sourced parts. The benefit shows up in five concrete ways:

• One heat number, one MTC. Every stud in a batch traces back to a single heat, every nut to its own heat, and every SWG to its filler-batch and winding-strip heat. The MTC set is shipped stapled to the goods-in copy of the packing list.
• Pre-matched grade pairing. A193 B7 studs are packed with A194 2H nuts, B7M with 2HM, B8M with 8M. The buyer never has to second-guess the pairing, and the site crew never has to open ten crates to find a missing nut.
• Length and quantity per ASME B16.5 / B16.47. For any NPS × class combination, the EZ team can quote the exact stud quantity, length, and thread specification without an engineering sub-consultant.
• Coating and packaging options. Hot-dip galvanised to ASTM F2329, PTFE coated for chloride service, black oxide for B7 / 2H, Dacromet for offshore. Vacuum-packed for stainless; oiled for carbon. EZ Steel Industrial's mill-direct gasket, stud bolt & nut shipments include the option of project-specific labelling, heat-number tracking, and pre-kitted joint boxes.
• Combined package with adjacent product lines. The same mill-direct producer that supplies gasket, stud bolt & nut sets also supplies pipe fittings, pipe flanges, and industrial valves. A single RFQ covers the entire joint, including the BW fittings and SW fittings on the run, the steel flanges at the tie-in, and the industrial valves on the line. One packing list, one MTC bundle, one delivery.

7.1 Trading-company risk in gasket, stud bolt & nut packages

The trading-company model is built on margin, not on traceability. A trading company that wins a gasket, stud bolt & nut RFQ will typically source studs from a small mill, nuts from a different small mill, and gaskets from a third party, then re-pack and re-label. The three parts may not share a heat, may not share a quality system, and almost certainly will not share a project-specific packaging. When the inspector opens the crate, the MTC set is a paper-trail of three different companies with three different quality manuals. The cost saving of 8% is real; the audit-trail risk is also real.

8. Frequently asked questions

Q1. Can I use a stainless B8M stud with a B7 stud grade on the same flange?

No. A stud-and-nut set is designed as a thermal-expansion-matched pair. Mixing grades leads to uneven load as the joint heats up. Specify the same grade on every stud-and-nut position in the bolt circle. EZ Steel Industrial's gasket, stud bolt & nut team rejects mixed-grade orders at quotation, not at goods-in.

Q2. What is the difference between an R, an RX, and a BX ring-joint gasket?

R is the standard ASME B16.5 / API 6A profile, used up to Class 2500. RX is a one-piece energised profile that fits an R-groove flange but cannot be re-used; it is standard in BOP stacks. BX is a square-section energised profile, used in API 6A wellhead above 100 MPa. The ring number must match the groove; the gasket and the groove are matched at the foundry and at goods-in.

Q3. Should I order coated or uncoated B7 studs?

Coating selection is driven by environment, not preference. Black oxide (the default) is fine for dry, indoor hydrocarbon service. Hot-dip galvanising to ASTM F2329 is for outdoor atmospheric exposure. PTFE coating is for chloride-bearing insulation and for ease of torque-up. Dacromet is the standard for offshore and marine. A mill-direct gasket, stud bolt & nut supplier can offer all four on a single batch.

Q4. Is it acceptable to re-use a spiral-wound gasket after a torque-down-and-release cycle?

No. An SWG is designed to be loaded once. The spring-back after release is reduced, the metal strips have taken a permanent set, and the second bolt-up will not reach the design stress. Always fit a new SWG; keep the old one in the spares bin for non-critical work only. EZ Steel Industrial's gasket, stud bolt & nut package is priced to make this the obvious default, not a budget question.

Q5. Do I really need an MTC for a utility-service joint, or is 2.2 sufficient?

For general utility, EN 10204 2.2 is acceptable. For anything inside the battery limit of a process unit — and certainly for hydrocarbon, steam, or pressure-vessel service — 3.1 is the minimum. For nuclear and selected offshore, 3.2 with third-party witness is required. The MTC type is part of the gasket, stud bolt & nut RFQ, not a footnote.