Industrial Fasteners: Stud Bolts, Hex Bolts, Nuts & Washers — Specifications & Selection Guide
A flanged joint is only as strong as the bolts holding it together. The most expensive weld neck flange in the world is worthless if it is fastened with the wrong stud bolt grade, the wrong nut, or the wrong coating. A single bolt failure on a high-pressure flanged joint can shut down a process unit, cause a hydrocarbon release, or injure personnel.
Every pipe order needs flanges. Every flange needs stud bolts and nuts. This guide covers the fastener specifications, grades, mechanical properties, coatings, and selection rules that procurement engineers need to order the correct fasteners for flanged connections in oil and gas, refinery, power plant, and industrial piping systems.
Stud Bolts vs Hex Bolts: What Goes on a Flange?
Stud bolts are threaded rods with a nut on each end. They pass through the bolt holes of both flanges, and the two heavy hex nuts are tightened from each side. Stud bolts are the standard fastener for all industrial flanged connections because the stud stays stationary during tightening — only the nuts rotate. This provides more uniform and accurate clamping force compared to headed bolts, and makes torque control more reliable.
Hex bolts (machine bolts) have a hexagonal head on one end and a nut on the other. They are used in lower-pressure, non-critical applications (typically Class 150 with raised face flanges). Hex bolts are less expensive but provide less consistent clamping force because the bolt head can rotate during tightening.
The rule for industrial piping: Use stud bolts with heavy hex nuts for all ASME B16.5 and B16.47 flanged connections. Hex bolts are only acceptable where explicitly permitted by the project specification, usually limited to Class 150 utility service.
ASTM A193 — The Bolt Standard
ASTM A193 is the standard specification for alloy steel and stainless steel bolting material for pressure vessels, valves, flanges, and fittings for high-temperature or high-pressure service. It is the most referenced bolt specification in industrial piping worldwide.
Key Grades
Grade | Material | Min. Tensile (MPa) | Min. Yield (MPa) | Max. Hardness | Temperature Range | Application |
|---|---|---|---|---|---|---|
B7 | Cr-Mo alloy (AISI 4140/4142), quenched & tempered | 860 (125 ksi) | 720 (105 ksi) | 35 HRC | -40°C to ~450°C | Standard for all general flange bolting — refinery, O&G, petrochemical, power |
B7M | Same as B7, lower hardness | 860 (125 ksi) | 720 (105 ksi) | 235 HBW (22 HRC) | -40°C to ~450°C | Sour service (H2S) per NACE MR0175 — hardness ≤22 HRC mandatory |
B8 | Stainless 304 (AISI 304), solution treated | 515 (75 ksi) | 205 (30 ksi) | 223 HBW | Cryogenic to ~815°C | SS flanges, corrosive environments |
B8M | Stainless 316 (AISI 316), solution treated | 515 (75 ksi) | 205 (30 ksi) | 223 HBW | Cryogenic to ~815°C | Superior corrosion resistance, chemical service |
B8 Class 2 | SS 304, strain hardened | 860 (125 ksi) | 550 (80 ksi) | 321 HBW | Limited range | When higher strength SS bolting is needed |
B16 | Cr-Mo-V alloy, quenched & tempered | 860 (125 ksi) | 690 (100 ksi) | 35 HRC | Up to ~540°C | High-temperature service (power plant, steam) |
A193 B7 is what you need in 85% of all cases. It is the default stud bolt for carbon steel flanged joints across every industry. When a piping specification says "stud bolts" without specifying the grade, it means A193 B7.
B7M is mandatory for sour service. NACE MR0175/ISO 15156 limits the maximum hardness of bolting material to 22 HRC in H2S-containing environments. Standard B7 can exceed this limit (up to 35 HRC), so B7M — which is the same material but tempered to a lower hardness — must be specified.
ASTM A194 — The Nut Standard
ASTM A194 covers carbon and alloy steel nuts for bolts used in high-temperature or high-pressure service. The nut grade must match the stud bolt grade.
Key Grades
Nut Grade | Material | Matches Bolt Grade | Application |
|---|---|---|---|
2H | Carbon steel, quenched & tempered | A193 B7 | Standard for all CS flange bolting |
2HM | Carbon steel, lower hardness (≤22 HRC) | A193 B7M | Sour service (NACE MR0175) |
8 | Stainless 304 | A193 B8 | Stainless steel service |
8M | Stainless 316 | A193 B8M | Corrosive / chemical service |
4 | Alloy steel | A193 B16 | High-temperature service |
7 | Alloy steel | A193 B7 (alternative to 2H) | Higher-temperature applications |
A194 2H is the standard nut for A193 B7 stud bolts. Always heavy hex pattern per ASME B18.2.2. Regular hex nuts are not acceptable for ASME flanged connections.
ASTM A320 — Low-Temperature Bolting
ASTM A320 covers alloy steel and stainless steel bolting for low-temperature service, with mandatory Charpy impact testing.
Grade | Material | Impact Tested To | Application |
|---|---|---|---|
L7 | Cr-Mo alloy (same base as B7) | -101°C | LNG, cryogenic piping, cold-climate pipelines |
L7M | Same as L7, lower hardness | -101°C | Low-temp sour service (NACE + cryogenic) |
L43 | Ni-Cr-Mo alloy | -101°C | Severe low-temperature service |
Matching nuts for A320: A194 Grade 7 or Grade 4.
When to use A320 instead of A193: Whenever the design temperature drops below -29°C (-20°F). This typically includes LNG facilities, cryogenic storage, cold-climate pipelines (arctic/subarctic), and air separation plants.
Fastener-to-Flange Matching Guide
This is the table your procurement team needs. Match the bolt and nut specification to the service condition:
Service Condition | Stud Bolt | Nut | Notes |
|---|---|---|---|
Standard CS piping (ambient to ~450°C) | A193 B7 | A194 2H | Default for 85%+ of industrial flange bolting |
High-temperature (up to ~540°C) | A193 B16 | A194 4 or 7 | Power plant steam piping, high-temp refinery |
Low-temperature (down to -101°C) | A320 L7 | A194 7 | LNG, cryogenic, cold-climate |
Sour service (H2S, NACE MR0175) | A193 B7M | A194 2HM | Hardness ≤22 HRC, mandatory for H2S environments |
Sour + low-temperature | A320 L7M | A194 7M | NACE-compliant, impact tested |
Stainless steel flanges (304) | A193 B8 | A194 8 | Corrosive, food-grade, pharmaceutical |
Stainless steel flanges (316) | A193 B8M | A194 8M | Chemical plants, offshore, severe corrosion |
Subsea / offshore (duplex flanges) | A193 B7 + special coating | A194 2H + special coating | Verify with project specification |
Thread Standards
All industrial stud bolts for ASME flanged connections use UNC (Unified National Coarse) thread per ANSI/ASME B1.1.
For diameters 1" and above, the thread pitch is 8 TPI (threads per inch) — also designated 8UN. This is universal across all ASME B16.5 and B16.47 flange bolting.
Thread class: 2A for external threads (stud bolt), 2B for internal threads (nut).
Thread engagement: The stud bolt must extend through the nut by at least one full thread beyond the nut face after tightening. If the stud is too short, there is insufficient thread engagement and the joint is unsafe. If too long, the excess thread wastes material and can interfere with insulation.
Stud Bolt Length Calculation
The correct stud bolt length for a flanged joint is calculated from the flange dimensions, gasket thickness, and nut dimensions. The formula is:
Stud bolt length = flange thickness (×2) + gasket thickness (compressed) + nut height (×2) + allowance for thread protrusion
For standard ASME B16.5 raised face flanges with spiral wound gaskets, stud bolt lengths are tabulated in reference charts available from flange manufacturers and in ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly).
When ordering from Kasko Makine, simply specify the flange size, class, and face type — we calculate the correct stud bolt length and supply the matching set.
Coatings and Surface Treatments
Bare alloy steel fasteners corrode rapidly in outdoor and marine environments. Coatings protect against corrosion and can also provide lubrication for more consistent torque.
Coating | Standard | Application |
|---|---|---|
Plain (black oxide) | — | Indoor service, painted piping, short-term storage |
Hot-dip galvanized (HDG) | ASTM A153 / F2329 | Outdoor structural, utility piping, water treatment |
Xylan (PTFE fluoropolymer) | — | Offshore, subsea, marine — excellent corrosion resistance + lubricity |
Zinc nickel | — | Offshore, high-corrosion environments |
Cadmium plating | QQ-P-416 | Aerospace, military (restricted in some regions) |
Dacromet / Geomet | — | Automotive, industrial, high-corrosion |
PTFE coated | — | Chemical service, anti-galling for SS bolts |
For most onshore oil and gas projects in Africa and the Middle East: plain (black) finish or hot-dip galvanized, depending on the environmental conditions. For offshore projects: Xylan or zinc-nickel coated per the project specification. For stainless steel bolts: anti-galling compound or PTFE coating is recommended to prevent thread seizure during tightening.
Important note on HDG: Hot-dip galvanizing adds thickness to the threads. Galvanized stud bolts must be supplied with oversize (overtapped) nuts to accommodate the zinc coating thickness. Always order galvanized studs and nuts as a matched set.
Washers
Washers distribute the clamping load over a larger area and protect the flange face from damage during bolt tightening.
ASTM F436 — Hardened steel washers are the standard for structural and industrial bolting. Used under both nuts on a stud bolt assembly.
Practice note: Many piping specifications do not require washers for ASME B16.5 flanged connections because the heavy hex nut face provides sufficient bearing area. However, some project specifications and all structural bolting applications require washers. Always check your project specification.
How to Order Fasteners
A complete fastener order for flanged connections requires:
1. Stud bolt specification and grade: ASTM A193 B7, A193 B7M, A320 L7, etc.
2. Nut specification and grade: ASTM A194 2H, A194 2HM, A194 7, etc.
3. Diameter: In inches (3/4", 7/8", 1", 1-1/8", 1-1/4", etc.) — determined by the flange class and size per ASME B16.5 bolt chart
4. Length: In inches or mm — determined by flange thickness, gasket, and nut height
5. Thread: UNC / 8UN per ANSI B1.1 (standard for ASME flange bolting)
6. Coating: Plain, HDG, Xylan, zinc-nickel, etc.
7. Quantity: Number of stud bolts + nuts (each stud bolt requires 2 nuts)
8. Washers (if required): ASTM F436, quantity = 2 per stud bolt
Example order line:
200 sets — Stud Bolt ASTM A193 B7, 3/4" dia × 4-1/2" long, UNC thread, with 2 pcs Heavy Hex Nut ASTM A194 2H, plain finish. Per ASME B18.2.1 / B18.2.2.
Common procurement mistakes:
Ordering B7 stud bolts for sour service (must be B7M for NACE compliance)
Specifying regular hex nuts instead of heavy hex nuts (ASME flanges require heavy hex)
Ordering galvanized studs with standard nuts (must use overtapped nuts for HDG)
Wrong stud bolt length — too short = unsafe joint, too long = wasted material and insulation interference
Forgetting that each stud bolt needs 2 nuts (total nut quantity = stud quantity × 2)
Using A193 B7 for temperatures below -29°C (must switch to A320 L7)
Supply from Kasko Makine
Kasko Makine supplies the complete range of industrial fasteners for flanged piping connections:
Stud bolts: ASTM A193 B7, B7M, B8, B8M, B16. ASTM A320 L7, L7M, L43. Diameters 1/2" through 4". Lengths per ASME B16.5 and B16.47 bolt charts.
Nuts: ASTM A194 2H, 2HM, 4, 7, 8, 8M. Heavy hex per ASME B18.2.2.
Washers: ASTM F436 hardened steel washers.
Coatings: Plain, hot-dip galvanized (ASTM F2329), Xylan/PTFE, zinc-nickel.
We supply fasteners as part of complete piping packages — pipe + flanges + fittings + stud bolts + nuts + gaskets, all from one source, documented and shipped together to your project site. One purchase order covers everything. No mismatched specifications, no separate shipments to coordinate.
All fasteners supplied with EN 10204 Type 3.1 material test certificates showing chemical composition, mechanical properties, and hardness values. Third-party inspection available on request.
FAQ SCHEMA
Q: What is the standard stud bolt for industrial flange connections? A: ASTM A193 Grade B7 stud bolts with ASTM A194 Grade 2H heavy hex nuts is the standard combination for carbon steel flanged connections in oil and gas, refinery, petrochemical, and power plant piping. This combination covers approximately 85% of all industrial flange bolting requirements.
Q: What is the difference between ASTM A193 B7 and B7M stud bolts? A: Both are made from the same chromium-molybdenum alloy steel (AISI 4140/4142) with the same minimum tensile and yield strength. The difference is maximum hardness: B7 allows up to 35 HRC, while B7M is tempered to a maximum of 22 HRC. B7M is mandatory for sour service (H2S environments) per NACE MR0175/ISO 15156, which limits fastener hardness to prevent sulfide stress cracking.
Q: What fasteners are used for low-temperature (cryogenic) piping? A: ASTM A320 Grade L7 stud bolts with ASTM A194 Grade 7 nuts. A320 L7 is impact tested to -101°C, making it suitable for LNG, cryogenic storage, and cold-climate pipeline applications. For sour service at low temperatures, A320 L7M with A194 7M is specified.
Q: Why are stud bolts used instead of hex bolts on flanges? A: Stud bolts provide more uniform and accurate clamping force because the stud stays stationary while only the nuts rotate during tightening. This allows better torque control and more consistent joint sealing pressure. Hex bolts can rotate during tightening, leading to inconsistent clamping. Stud bolts are standard for all ASME B16.5 and B16.47 flanged connections.
Q: What coating should I use for stud bolts in offshore environments? A: Xylan (PTFE fluoropolymer) or zinc-nickel coatings provide excellent corrosion resistance for offshore and marine environments. Hot-dip galvanizing (ASTM F2329) is used for general outdoor service. For stainless steel bolting, anti-galling compound or PTFE coating prevents thread seizure. Always verify the coating requirement with your project specification.
Q: How do I determine the correct stud bolt length for a flanged joint? A: Stud bolt length is calculated from: (flange thickness × 2) + compressed gasket thickness + (nut height × 2) + thread protrusion allowance. For standard ASME B16.5 flange connections, bolt lengths are available in published bolt charts. When ordering from a supplier, specify the flange size, class, and face type — the supplier calculates the matching bolt length.
Request fastener pricing — send your flange summary, bolt chart, or piping MTO to info@kaskomakine.com or WhatsApp at +90 (537) 521 1399. We respond within 24 hours and deliver across Africa, the Middle East, Central Asia, and beyond.