How to Order Stud Bolts and Nuts
Stud bolts and nuts are the fasteners that clamp flanges together and compress the gasket to create a pressure-tight seal. An incomplete fastener specification, missing the material grade, length, thread standard, or NACE compliance, leads to wrong deliveries, failed inspections, and potential joint leakage.
This checklist covers the mandatory and optional parameters for ordering flange stud bolts, heavy hex nuts, and washers per ASME B16.5 and ASTM A193/A194.
Core Attributes
Every stud bolt and nut line item must include all of the following attributes.
| # | Attribute | Values / Options | Notes |
|---|---|---|---|
| 1 | Bolt type | Stud bolt (double-ended, threaded both ends or fully threaded) | Flange stud bolts per ASME B16.5 are continuous threaded (fully threaded) studs. Short-thread studs have a reduced unthreaded shank; specify if required by project spec |
| 2 | Nominal diameter | Imperial: 1/2”, 5/8”, 3/4”, 7/8”, 1”, 1-1/8”, 1-1/4”, 1-3/8”, 1-1/2”, up to 4”. Metric: M12 through M100 | Diameter is determined by flange size, class, and flange standard (ASME B16.5 Table 2) |
| 3 | Length | Per ASME B16.5 Table E-1, or specify in mm/inches | Length is a function of flange size, pressure class, and flange type (WN, SO, blind). It must account for: flange thickness + gasket thickness + 2 nut heights + thread protrusion. Always cross-check against Table E-1 |
| 4 | Thread standard | UNC (Unified National Coarse) per ASME B1.1 (standard for ASME flanges), or metric per ISO 261/898 | UNC is the default for ASME B16.5 flanges. Do not assume; always state the thread standard. Metric threads require ISO designation (e.g., M20 x 2.5) |
| 5 | Stud material | ASTM A193 or ASTM A320; see material table below | Always specify the full ASTM standard and grade |
| 6 | Nut material | ASTM A194; see material table below | The nut grade must be paired correctly with the stud grade |
| 7 | Quantity | Number of sets (1 set = 1 stud bolt + 2 heavy hex nuts), or state stud and nut quantities separately | Always clarify whether the quantity is sets, individual studs, or individual nuts. Ambiguity causes shortages or overages |
Stud Material Specifications (ASTM A193 / A320)
| Grade | Base Material | Typical Service | Key Properties |
|---|---|---|---|
| A193 B7 | AISI 4140 / 4142, quenched and tempered | General high-temperature service up to 450 °C | The most common stud bolt grade for process piping |
| A193 B7M | Same as B7, tempered to lower hardness (max 235 HB / 22 HRC) | Sour service per NACE MR0175 | Mandatory for H2S environments; hardness-controlled version of B7 |
| A193 B16 | Cr-Mo-V alloy, quenched and tempered | High-temperature service above 450 °C | Higher tensile strength than B7 at elevated temperatures |
| A193 B8 Class 1 | SS 304, solution annealed (carbide solution treated) | Corrosive environments, cryogenic | Lower strength (annealed condition), maximum corrosion resistance |
| A193 B8 Class 2 | SS 304, strain hardened | Corrosive environments requiring higher strength | Higher strength than Class 1, but reduced corrosion resistance due to cold work |
| A193 B8M Class 1 | SS 316, solution annealed | Corrosive/chemical environments, cryogenic | Molybdenum addition for improved pitting resistance |
| A193 B8M Class 2 | SS 316, strain hardened | Corrosive environments requiring higher strength | Higher strength than Class 1, but reduced corrosion resistance |
| A320 L7 | AISI 4140 / 4142, impact tested | Low-temperature service (down to -101 °C) | Charpy impact tested at the specified design temperature |
| A320 L7M | Same as L7, tempered to max 235 HB / 22 HRC | Low-temperature sour service | Combines low-temperature impact toughness with NACE hardness compliance |
| A320 B8 | SS 304, solution annealed | Cryogenic service (down to -196 °C) | Same base material as A193 B8 Class 1, but impact tested for cryogenic temperatures |
Nut Material Specifications (ASTM A194)
| Grade | Base Material | Paired With (Studs) | Notes |
|---|---|---|---|
| 2H | Carbon steel, quenched and tempered, heavy hex | A193 B7, A320 L7 | The standard heavy hex nut for CS stud bolts |
| 2HM | Carbon steel, tempered to max 235 HB / 22 HRC | A193 B7M, A320 L7M | NACE MR0175 compliant; mandatory for sour service with B7M studs |
| 4 | Alloy steel (Cr-Mo), quenched and tempered | A193 B16 | For high-temperature service above 450 °C |
| 8 | SS 304 | A193 B8 (Class 1 or 2), A320 B8 | Austenitic stainless steel nut for corrosive/cryogenic service |
| 8M | SS 316 | A193 B8M (Class 1 or 2) | Molybdenum-bearing SS nut for chemical service |
Optional and Special Attributes
| Attribute | When Required | Details |
|---|---|---|
| Coating / finish | Corrosive environments, outdoor, subsea | Plain/black (standard, no coating). Hot-dip galvanized (HDG) per ASTM A153 for outdoor CS bolting. PTFE-based coatings (Xylan, fluorocarbon) for anti-galling and corrosion resistance. Zinc flake coatings for subsea applications. Cadmium plating for specific aerospace/military specs |
| NACE MR0175 / ISO 15156 | Sour service (H2S-containing environments) | Mandatory: use A193 B7M studs + A194 2HM nuts for carbon steel. Maximum hardness 22 HRC. Must be verified on the material test report |
| Hardness testing | NACE service, project specification | Max 22 HRC for CS per NACE MR0175. Report hardness values (Brinell or Rockwell) on the MTR. Hardness must be tested per ASTM E18 or ASTM E10 |
| Washers | When specified by project or design | Hardened washers per ASTM F436 (structural) or ASME B18.22.1. Specify material (CS, SS), size, and quantity. Typically used under the nut to protect the flange face and distribute load |
| Certificates | Almost always required | EN 10204 Type 3.1 (MTR) minimum. Must include: chemical composition, mechanical properties (tensile, yield, elongation, reduction of area), hardness values, heat treatment condition, and heat/lot number |
| Length tolerance | Per ASME B16.5, ASME B18.2.1, or project spec | Standard tolerance per ASME B18.31.2 for continuous threaded studs. Tighter tolerances require explicit statement in the purchase order |
| Anti-galling treatment | Stainless steel studs and nuts | SS-to-SS contact is prone to galling (cold welding) during tightening. Specify anti-seize compound application, nitrided nuts, or different hardness between stud and nut (e.g., B8 Class 2 stud with Grade 8 Class 1 nut) |
Stud Bolt Length Selection
Stud bolt length must account for the complete joint assembly. The required length is:
Stud length = Flange thickness (x2 for pair) + Gasket thickness + Nut height (x2) + Thread protrusion (x2)
ASME B16.5 Table E-1 provides pre-calculated stud bolt lengths for each flange size, class, and type. Always reference this table as the primary source. When ordering for non-standard joints (e.g., equipment nozzle flanges with different thicknesses), calculate the length manually and round up to the nearest standard increment.
Example Line Items
Correct
100 sets - Stud bolt + 2 heavy hex nuts, 3/4” x 4-1/4” (108 mm), UNC, ASTM A193 B7 studs + ASTM A194 2H nuts, plain finish, MTR EN 10204 3.1
This line item defines: quantity and unit (sets), bolt type (stud bolt + 2 nuts), diameter, length, thread standard, stud material, nut material, finish, and certification.
Correct (NACE Service)
48 sets - Stud bolt + 2 heavy hex nuts, 7/8” x 5-1/2” (140 mm), UNC, ASTM A193 B7M studs + ASTM A194 2HM nuts, plain finish, NACE MR0175, max hardness 22 HRC, MTR EN 10204 3.1
Correct (Stainless Steel)
32 sets - Stud bolt + 2 heavy hex nuts, 3/4” x 4-3/4” (121 mm), UNC, ASTM A193 B8M Class 1 studs + ASTM A194 8M nuts, PTFE coated, MTR EN 10204 3.1
Incorrect
100 - Bolts 3/4 inch
This is missing the bolt type (stud bolt vs hex bolt), length, thread standard, material specification (A193 grade), nut specification (A194 grade), quantity unit (sets vs pieces), finish, and certification. The supplier cannot determine what to manufacture or supply.
Common Pitfalls
| Pitfall | Why It Matters |
|---|---|
| Ordering pieces instead of sets | ”100 stud bolts” without specifying nuts means you receive 100 studs and zero nuts. Always order in sets (1 stud + 2 nuts) or explicitly state the stud and nut quantities separately (e.g., “100 stud bolts + 200 heavy hex nuts”) |
| Wrong stud bolt length | The length must account for both flange thicknesses, the gasket, two nut heights, and thread protrusion on both ends. A stud that is too short will not achieve full nut engagement; a stud that is too long wastes material and may interfere with insulation. Always verify against ASME B16.5 Table E-1 |
| Using A193 B7 instead of B7M for sour service | Standard B7 studs are quenched and tempered to high hardness (up to 35 HRC). NACE MR0175 limits hardness to 22 HRC for carbon steel in sour (H2S) service. B7M is the same alloy tempered to a lower hardness (max 235 HB / 22 HRC). Using B7 in sour service will fail NACE compliance and risk sulfide stress cracking |
| Confusing Class 1 vs Class 2 for stainless steel studs | A193 B8/B8M Class 1 is solution annealed (lower strength, maximum corrosion resistance). Class 2 is strain hardened (higher tensile and yield strength, but reduced corrosion resistance due to cold work). Specifying the wrong class results in either inadequate strength or accelerated corrosion. Always state the class explicitly |
| Not specifying anti-galling measures for SS bolting | Austenitic stainless steel studs and nuts are highly susceptible to galling (seizure) during tightening. Without anti-seize compound, coatings, or differential hardness, the fasteners can cold-weld during torquing and become impossible to remove. Specify PTFE coating or anti-seize application in the purchase order |
| Missing nut specification | Ordering “A193 B7 stud bolts with nuts” without specifying the nut grade (A194 2H) leaves the nut material to the supplier’s discretion. Always pair the stud grade with the correct nut grade explicitly |
| HDG bolting without re-tapping nuts | Hot-dip galvanizing adds thickness to the stud threads. Nuts for HDG studs must be tapped oversize (after galvanizing the stud, or the nut is supplied oversized per ASTM A563 DH). Failure to account for this results in nuts that will not thread onto the studs |
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