What is ASTM A193 Grade B8M?
ASTM A193 Grade B8M is a standard material specification for externally threaded fasteners, bolts, stud bolts, screws and threaded rods manufactured from AISI 316 austenitic stainless steel. The “M” suffix distinguishes B8M from the closely related Grade B8 (AISI 304 SS) by the addition of 2–3% molybdenum to the alloy, which significantly improves resistance to chloride pitting and crevice corrosion.
Grade B8M fasteners are produced in two tensile classes: Class 1 (carbide solution treated) and Class 2 (carbide solution treated and strain hardened). Class 1 delivers uniform, consistent strength across all diameters and is used in applications requiring NACE MR0175 sour-service compliance. Class 2 provides substantially higher tensile and yield strength through cold work hardening, making it the preferred choice for high-pressure flanged joints. ASME SA193 Grade B8M is the technically identical specification under the ASME Boiler & Pressure Vessel Code.
Both grades are austenitic stainless steel fasteners under ASTM A193. Grade B8 uses AISI 304; B8M uses AISI 316 with 2–3% molybdenum added. That molybdenum addition is responsible for B8M's markedly superior resistance to chloride pitting, crevice corrosion, and seawater attack making it the standard choice for offshore, marine, chemical process, and coastal environments.
| Base Alloy | AISI 304 SS |
| Molybdenum | None |
| Chloride resistance | Moderate |
| Marine suitability | Limited |
| Nut match | A194 Gr. 8 |
| NACE MR0175 | Cl.1 only |
| Base Alloy | AISI 316 SS |
| Molybdenum | 2.0–3.0% |
| Chloride resistance | Superior |
| Marine suitability | Excellent |
| Nut match | A194 Gr. 8M |
| NACE MR0175 | Cl.1 compliant |
Class 1 vs Class 2 — Key Differences
The tensile class is the single most important selection factor for A193 B8M bolts. The class determines heat treatment method, strength, hardness, and sour-service eligibility.
| Heat treatment | Solution annealing only |
| Tensile strength (all dia.) | 515 MPa (75 ksi) min |
| Yield strength (all dia.) | 205 MPa (30 ksi) min |
| Elongation | 30% min |
| Reduction of area | 50% min |
| Hardness max | 223 HBW / 96 HRB/td> |
| NACE MR0175 | Compliant |
| Head marking | B8M |
| Typical use | Sour service, cryogenic, general corrosive |
| Heat treatment | Solution anneal + cold work |
| Tensile strength (≤3/4") | 760 MPa (110 ksi) min |
| Yield strength (≤3/4") | 665 MPa (95 ksi) min |
| Elongation | 15% min (≤3/4") |
| Reduction of area | 545% min |
| Hardness max | 321 HBW / 35 HRC |
| NACE MR0175 | Not permitted (H₂S sour service) |
| Head marking | B8MC2 or B8M2 |
| Typical use | High-pressure flanges, pressure vessels |
Class 2 strength decreases with increasing bolt diameter because larger cross-sections cannot be uniformly strain hardened. Always specify the required class and diameter range on your purchase order. Class 2 bolts must not be used in H₂S sour-service environments per NACE MR0175 / ISO 15156.
ASTM A193 B8M Bolts Chemical Composition
ASTM A193 B8M is manufactured from AISI 316 austenitic stainless steel. The molybdenum content (2.0–3.0%) is the defining element that differentiates B8M from B8 and provides its enhanced corrosion performance.
| Element | Carbon (C) | Manganese (Mn) | Phosphorus (P) max | Sulfur (S) max | Silicon (Si) | Chromium (Cr) | Nickel (Ni) | Molybdenum (Mo) |
|---|---|---|---|---|---|---|---|---|
| B8M (AISI 316) | 0.08 max | 2.00 max | 0.045 | 0.030 | 1.00 max | 16.00–18.00 | 10.00–14.00 | 2.00–3.00 |
Note: The intentional addition of Bi, Se, Te, and Pb is not permitted per ASTM A193. All heats require mill certification confirming compliance with the above limits.
B8 vs B8M Chemical Comparison
| Property | Grade B8 (AISI 304) | Grade B8M (AISI 316) |
|---|---|---|
| Carbon (C) max | 0.08% | 0.08% |
| Chromium (Cr) | 18.00–20.00% | 16.00–18.00% |
| Nickel (Ni) | 8.00–10.50% | 10.00–14.00% |
| Molybdenum (Mo) | None | 2.00–3.00% |
| Pitting Resistance Index (PRE) | ~19–20 | ~24–26 |
PRE = %Cr + 3.3 × %Mo. Higher PRE indicates better resistance to pitting corrosion in chloride environments.
Mechanical Properties of ASTM A193 B8M Bolts
Mechanical requirements for ASTM A193 B8M vary by class and diameter. Class 2 strength values step down as bolt diameter increases because of limitations in uniform strain hardening of larger cross-sections.
Class 1 — Carbide Solution Treated (All Diameters)
| Diameter | Tensile Str. Min (MPa) | Tensile Str. Min (ksi) | Yield Str. Min (MPa) | Yield Str. Min (ksi) | Elongation 4D Min (%) | Reduction of Area Min (%) | Hardness Max |
|---|---|---|---|---|---|---|---|
| All sizes | 515 | 75 | 205 | 30 | 30 | 50 | 223 HBW / 96 HRB |
Class 2 — Solution Treated & Strain Hardened (Diameter-Dependent)
| Diameter (Inch) | Diameter (Metric) | Tensile Str. Min (ksi) | Tensile Str. Min (MPa) | Yield Str. Min (ksi) | Yield Str. Min (MPa) | Elongation Min (%) | RA Min (%) | Hardness Max |
|---|---|---|---|---|---|---|---|---|
| ¾” and under | M20 and under | 110 | 760 | 95 | 665 | 15 | 45 | 321 HBW / 35 HRC |
| Over ¾” to 1″ | Over M20 to M24 | 100 | 690 | 80 | 550 | 20 | 45 | 321 HBW / 35 HRC |
| Over 1″ to 1¼” | Over M24 to M30 | 95 | 655 | 65 | 450 | 25 | 45 | 321 HBW / 35 HRC |
| Over 1¼” to 1½” | Over M30 to M36 | 90 | 620 | 50 | 345 | 30 | 45 | 321 HBW / 35 HRC |
For Class 2 diameters 1½” and over, core (centre) properties may be lower than indicated. Test reports based on values at ½ radius per ASTM A193.
ASTM A193 B8M Bolts Specification & Dimensions
Jade Alloys supplies ASTM A193 B8M bolts to the following dimensional, thread, and certification standards. Custom lengths and non-standard dimensions are available on request.
| Parameter | Details |
|---|---|
| Standard | ASTM A193 / ASME SA193 Grade B8M |
| Dimensions | ASME B18.2.1 (inch hex bolts), ASME B18.2.3.3M (metric heavy hex bolts), ASME B18.3 (socket head cap screws) |
| Stud bolt dimensions | ASME B16.5 (pipe flanges), ASME B16.47 (large-diameter flanges) |
| Thread standard | ASME B1.1 2A/3A — Unified Inch Threads; ASME B1.13M 6h — Metric Coarse Threads |
| Metric size range | M2 to M160 |
| Inch size range | ¼” to 4″ (standard); larger sizes on request |
| Length range | 3 mm to 3000 mm (custom lengths available) |
| Heat treatment (Class 1) | Carbide solution treatment — heated above 1040°C, quenched in water or rapidly cooled |
| Heat treatment (Class 2) | Carbide solution treatment + strain hardening (cold drawing or cold working) |
| Dual certification | Dual certified as per ASTM & ASME codes |
| Additional compliance | NACE MR0103 / MR0175 / ISO 15156 (Class 1 only) |
| PED | PED 97/23/EC; Merkblatt AD 2000 W2 |
| Marking (Class 1) | Manufacturer’s ID mark + “B8M” |
| Marking (Class 2) | Manufacturer’s ID mark + “B8MC2” or “B8M2” |
| Finish options | Plain (bare), Passivated, Xylan PTFE, Zinc Flake, Hot-Dip Galvanized, Black Oxide, Cadmium Plated |
Types of ASTM A193 B8M Bolts & Fasteners
Jade Alloys manufactures all standard fastener forms in ASTM A193 B8M — Class 1 and Class 2 — to ASME dimensional standards. Custom forms and non-standard configurations are available on request.
Jade Alloys also supplies ASTM A194 Grade 8M heavy hex nuts and ASTM A240 Type 316 washers as complete fastener sets simplifying procurement and ensuring full material traceability.
ASTM A193 Grade Comparison
ASTM A193 covers a wide range of bolt grades for different temperature, pressure, and corrosion service conditions. The table below positions B8M among the most commonly specified grades.
| Grade | Material | Min. Tensile (ksi) | Max Temp | Key Application | Nut Grade |
|---|---|---|---|---|---|
| B7 | Cr-Mo alloy steel | 125 | 450°C (840°F) | Pressure vessels, general flanging | A194 2H |
| B8M | AISI 316 SS | 75 (Cl.1) / 110 (Cl.2) | 800°C / −196°C cryo | Corrosive, marine, cryogenic, chemical | A194 8M |
| B8 | AISI 304 SS | 75 (Cl.1) / 110 (Cl.2) | 800°C / −196°C cryo | General corrosion, food, pharmaceutical | A194 8 |
| B16 | Cr-Mo-V alloy steel | 125 | 593°C (1100°F) | Power generation, steam turbines | A194 16 |
| B8T | AISI 321 SS (Ti-stabilised) | 75 (Cl.1) | 800°C | High-temp service, sensitisation-prone | A194 8T |
Stud Bolt & Hex Bolt Dimension Chart
Stud bolt dimensions per ASME B16.5 flange classes. Hex bolt dimensions per ASME B18.2.1. All dimensions in inches unless stated.
ASTM A193 B8M Stud Bolt — ASME B16.5 Class 150 & 300 Dimensions
| Nominal Pipe Size (NPS) | Flange Class 150 — Stud Dia. × Length | No. of Studs | Flange Class 300 — Stud Dia. × Length | No. of Studs |
|---|---|---|---|---|
| ½” | ½” × 2¾” | 4 | ½” × 3″ | 4 |
| ¾” | ½” × 3″ | 4 | ⅝” × 3¼” | 4 |
| 1″ | ½” × 3¼” | 4 | ⅝” × 3½” | 4 |
| 1½” | ½” × 3½” | 4 | ¾” × 4″ | 4 |
| 2″ | ⅝” × 3¾” | 4 | ¾” × 4½” | 8 |
| 3″ | ¾” × 4¼” | 4 | ¾” × 5″ | 8 |
| 4″ | ¾” × 4½” | 8 | ¾” × 5¼” | 8 |
| 6″ | ¾” × 5″ | 8 | 1″ × 6¼” | 12 |
| 8″ | ¾” × 5¼” | 8 | 1⅛” × 7″ | 12 |
| 10″ | ⅞” × 5¾” | 12 | 1¼” × 7¾” | 16 |
| 12″ | ⅞” × 6″ | 12 | 1¼” × 8¼” | 16 |
Stud lengths include two heavy hex nuts per ASME B16.5. Dimensions subject to verification against current revision of ASME B16.5. Contact us for Class 600, 900, 1500, and 2500 dimensions.
ASTM A193 B8M Heavy Hex Bolt — Size Chart (ASME B18.2.1)
| Nominal Size | Width Across Flats (in) | Head Height (in) | Thread Pitch (UNC) | Min. Thread Engagement |
|---|---|---|---|---|
| ½” | ⅞” | 19/64″ | 13 TPI | ½” |
| ⅝” | 1 1/16″ | 25/64″ | 11 TPI | ⅝” |
| ¾” | 1¼” | 15/32″ | 10 TPI | ¾” |
| ⅞” | 1 7/16″ | 35/64″ | 9 TPI | ⅞” |
| 1″ | 1⅝” | 39/64″ | 8 TPI | 1″ |
| 1¼” | 2″ | 25/32″ | 7 TPI | 1¼” |
| 1½” | 2⅜” | 15/16″ | 6 TPI | 1½” |
Torque Chart for ASTM A193 B8M Stud Bolts
The recommended tightening torque values below apply to ASTM A193 B8M stud bolts with ASTM A194 Grade 8M heavy hex nuts, lightly oiled (µ = 0.15 lubricated condition). Dry or special coating conditions require separate torque calculations.
Torque values are guidelines only. Final torque must be specified by the project engineer based on flange rating, gasket type, bolt load requirements, and lubricant. ASME PCC-1 provides detailed guidance on bolted flange joint assembly.
B8M Class 2 Stud Bolts — Lubricated (µ = 0.15)
| Bolt Diameter | Metric Size | Tensile Stress Area (mm²) | Proof Load (kN) | Torque — Lubricated (Nm) | Torque — Lubricated (ft·lb) |
|---|---|---|---|---|---|
| ½” (Class 2) | M12 | 84.3 | 43 | 37 | 27 |
| ⅝” (Class 2) | M16 | 157 | 80 | 82 | 60 |
| ¾” (Class 2) | M20 | 245 | 125 | 155 | 114 |
| ⅞” (Class 2) | M22 | 303 | 152 | 218 | 161 |
| 1″ (Class 2) | M24 | 353 | 177 | 293 | 216 |
| 1¼” (Class 2) | M30 | 561 | 254 | 524 | 386 |
| 1½” (Class 2) | M36 | 817 | 346 | 856 | 631 |
For Class 1 bolts, torque values are lower due to reduced yield strength (205 MPa). Contact our technical team for Class 1 torque tables. Values are approximate and for reference only.
Mating Nuts, Washers & Compatible Hardware
Using the correct mating hardware ensures chemical compatibility, prevents galvanic corrosion, and meets the full code requirements of ASTM A193 / ASME SA193.
| Component | Specification | Material | Notes |
|---|---|---|---|
| Heavy Hex Nut | ASTM A194 Grade 8M | AISI 316 SS | Standard mating nut — Class 1 & 2. Available from Jade Alloys. |
| Washer | ASTM A240 Type 316 | AISI 316 SS | Flat and spring washer; prevents galling and distributes load |
| NACE-grade nut | ASTM A194 Grade 8MA | AISI 316 SS annealed | For Class 1 in sour service per NACE MR0175 — max 22 HRC |
Never use ASTM A194 Grade 8 (304 SS) nuts with A193 B8M bolts in marine or chemical service. The 304 nut will corrode preferentially in chloride environments. Always match 316 SS nuts (Grade 8M) to 316 SS bolts (B8M).
International Equivalents
ASTM A193 Grade B8M corresponds to the following international designations. This is important for global projects, dual-stamped supply, and replacement sourcing.
Note: Equivalency is approximate. Always verify chemical composition limits, mechanical property requirements, and heat treatment conditions before substituting across standards. Request dual-certified material for cross-standard projects.
Industries & Applications of ASTM A193 B8M Bolts
The combination of AISI 316 corrosion resistance, austenitic ductility, and wide temperature range (−196°C to 800°C) makes ASTM A193 B8M bolts the first-choice fastener across demanding industries.
Why ASTM A193 B8M for Sour Service and Offshore?
In offshore and sour-gas environments, hydrogen sulfide (H₂S) causes sulfide stress cracking (SSC) in high-hardness steels. ASTM A193 B8M Class 1, with its hardness capped at 22 HRC in the NACE MR0175 interpretation, resists SSC while providing the chloride corrosion resistance demanded in seawater-exposed joints. For subsea wellhead connections, manifold flanges, and topside piping, B8M Class 1 remains the industry default where both H₂S resistance and chloride corrosion resistance are simultaneously required.
NACE MR0175 & Sour-Service Compliance
Understanding NACE / ISO 15156 compliance is critical when specifying B8M bolts for oil and gas projects with H₂S present.
| Class | Hardness (max) | NACE MR0175 Eligible | ISO 15156 Reference | Notes |
|---|---|---|---|---|
| B8M Class 1 | 22 HRC (NACE) / 96 HRB (ASTM) | ✔ Yes — with hardness verification | ISO 15156-3, Table A.3 | Solution-treated condition; hardness must be confirmed on MTC |
| B8M Class 2 | 35 HRC max | ✘ Not permitted | — | Strain hardening raises hardness above NACE MR0175 limits |
When ordering B8M Class 1 for NACE service, always specify on your purchase order: "NACE MR0175 / ISO 15156 compliance required — maximum hardness 22 HRC." Request individual Brinell hardness test reports, not just grade certification, as part of the MTC documentation package.
✓ NACE MR0175 (Class 1)
✓ NACE MR0103 (Class 1)
✓ ISO 15156-3
✓ PED 97/23/EC
✓ AD 2000 W2
✓ ISO 9001:2015
✓ Dual ASTM / ASME
Frequently Asked Questions - ASTM A193 Grade B8M Bolts
What is ASTM A193 Grade B8M?
ASTM A193 Grade B8M is a material specification for externally threaded fasteners — bolts, studs, and threaded rods — manufactured from AISI 316 austenitic stainless steel. It is divided into Class 1 (carbide solution treated) and Class 2 (carbide solution treated and strain hardened). B8M is widely used in chemical processing, oil & gas, and marine applications due to its molybdenum-enhanced corrosion resistance.
What is the difference between A193 B8M Class 1 and Class 2?
Class 1 fasteners are carbide solution treated only, resulting in a minimum tensile strength of 515 MPa (75 ksi) for all diameters, with hardness not exceeding 96 HRB. Class 2 fasteners undergo additional strain hardening after solution treatment, raising tensile strength to 760 MPa (110 ksi) minimum for diameters up to ¾". Class 2 strength reduces as diameter increases. Class 1 is NACE MR0175 eligible; Class 2 is not permitted in sour-service H₂S environments.
What is the difference between ASTM A193 B8 and B8M?
Both B8 and B8M are austenitic stainless steel grades. B8 uses AISI 304; B8M uses AISI 316. The key difference is molybdenum content: B8M contains 2.0–3.0% Mo, giving it a Pitting Resistance Equivalent (PRE) approximately 6–7 points higher than B8. This makes B8M substantially more resistant to chloride pitting, crevice corrosion in seawater, and general corrosion in acidic chloride environments. B8M is the preferred choice for marine, offshore, chemical, and coastal applications.
What nuts should be used with ASTM A193 B8M bolts?
ASTM A194 Grade 8M heavy hex nuts are the specified mating nut for A193 B8M bolts. Grade 8M nuts are manufactured from AISI 316 stainless steel, ensuring material compatibility and preventing galvanic corrosion. ASTM A240 Type 316 flat washers are also recommended. Never use A194 Grade 8 (304 SS) nuts with B8M bolts in chloride-containing environments.
Is ASTM A193 B8M NACE MR0175 compliant?
Class 1 B8M bolts, supplied in the carbide solution-treated condition with hardness not exceeding 22 HRC, are listed as acceptable in NACE MR0175 / ISO 15156-3 for sour-service (H₂S) environments. Class 2 bolts, with hardness up to 35 HRC from strain hardening, are not permitted under NACE MR0175. Always confirm compliance by reviewing the individual hardness test results on the Material Test Certificate (MTC), not merely the grade designation.
What is the temperature range for A193 B8M bolts?
ASTM A193 B8M bolts are suitable for service temperatures from −196°C (−320°F) in cryogenic applications — such as LNG and cold-box equipment — up to approximately 800°C (1472°F) for short-term elevated temperature service. Unlike ferritic and martensitic steels, austenitic 316 SS does not undergo a ductile-to-brittle transition at sub-zero temperatures, making B8M the standard choice for cryogenic piping and LNG terminal flanges.
How are ASTM A193 B8M bolts marked?
Class 1 bolts are marked "B8M" on the bolt head or end face along with the manufacturer's identification symbol. Class 2 bolts are marked "B8MC2" or "B8M2" to clearly distinguish the strain-hardened condition from Class 1. Always verify that markings match the class specified on your purchase order, and request a Material Test Certificate confirming heat number, grade, class, chemical composition, and mechanical test results.
What is the difference between ASTM A193 and ASME SA193?
ASTM A193 is published by ASTM International and covers high-strength bolting for high-temperature and high-pressure service. ASME SA193 is the identical specification adopted by the ASME Boiler & Pressure Vessel Code (BPVC). The technical requirements — chemical composition, mechanical properties, heat treatment — are identical. The ASME prefix indicates the material is being supplied and certified under ASME BPVC requirements, which may include additional documentation or witnessing obligations.
Why Choose Jade Alloys?
Jade Alloys is a Mumbai-based manufacturer and exporter with decades of experience supplying precision fasteners to critical industries across 40+ countries. Here’s what sets us apart for ASTM A193 B8M bolt supply.
- ISO 9001:2015 Certified
Every batch manufactured under a certified quality management system with full traceability from raw material heat to finished product. - Full MTC & Third-Party Testing
We provide EN 10204 3.1 and 3.2 Mill Test Certificates. Third-party inspection by TPI agencies available on request. - NACE & Sour-Service Supply
Experienced in NACE MR0175 procurement requirements, including hardness verification and documentation packages for Class 1 B8M supply. - Export to 40+ Countries
Supplying projects in the Middle East, Europe, Southeast Asia, USA, and Africa. Familiar with all major port requirements, fumigation certificates, and export documentation. - Class 1 & Class 2 In Stock
Standard sizes of B8M Class 1 stud bolts and heavy hex bolts maintained in stock for rapid dispatch. Custom sizes manufactured to order. - Technical Pre-Sales Support
Our technical team assists with grade selection between B8 and B8M, Class 1 vs Class 2 selection, NACE compliance queries, and torque specification.
Export Destinations
Jade Alloys exports ASTM A193 B8M bolts to the following countries and regions. Contact us for country-specific documentation, Incoterms, and lead times.
Additional destinations available. All exports supplied with full commercial documentation including Certificate of Origin, Packing List, and Test Certificates.
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Explore other stainless steel and high-alloy fastener products from Jade Alloys for complete bolting solutions.
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