Scope
ASTM A193 / ASME SA-193 Grade B8M is the molybdenum-alloyed 316 austenitic stainless steel bolting designation (UNS S31600) under the ASTM A193/A193M specification for pressure-vessel, valve, flange, and fitting bolting in high-temperature, corrosive, and chloride-bearing service. B8M is supplied in four heat-treatment classes — Class 1 (solution annealed), Class 1A (solution annealed in the finished condition), Class 1D (solution annealed and liquid quenched, for heavier sections), and Class 2 (solution annealed and strain hardened). Class 1 and Class 1D are the NACE MR0175 / ISO 15156-3 qualified condition for sour service; Class 2 is the high-strength variant for non-sour applications. TorqBolt supplies B8M bolts, stud bolts, hex bolts, socket-head cap screws, and threaded rods to ASTM A193/A193M-19 from 1/4 inch through 4 inch and M6 through M100, with EN 10204 3.1 or 3.2 inspection certification, NORSOK M-650 manufacturer qualification, and PED 2014/68/EU compliance.
The 2.00 to 3.00% molybdenum addition is the defining chemistry difference between B8M (316) and B8 (304). Molybdenum raises the pitting-resistance-equivalent number (PREN) to approximately 24, stabilises the passive film against chloride attack, and is the reason B8M is specified for offshore, marine, refining, and petrochemical flange bolting where B8 would not pass corrosion-pitting acceptance. B8M is the ASTM A193 designation for the same chemistry that ASME BPVC calls 316 stainless, AISI calls Type 316, and UNS designates as S31600.
ASTM A193 Grade B8M Classes
The four classes share the same chemistry (UNS S31600 limits: C 0.08 max, Mn 2.00 max, Si 1.00 max, Cr 16.0-18.0, Ni 10.0-14.0, Mo 2.00-3.00, P/S restricted). They differ in heat-treatment route and final mechanical properties. Class 1 and Class 1D are supplied at 75 ksi (515 MPa) minimum tensile and 30 ksi (205 MPa) minimum yield with 96 HRB maximum hardness, within the NACE MR0175 austenitic stainless steel acceptance threshold. Class 1A shares the same tensile floor but is held to 90 HRB maximum because it is solution-annealed in the finished condition. Class 2 is solution annealed and strain hardened, raising minimum tensile to 90-110 ksi (size-dependent) at 35 HRC maximum.
Material identity and cross-references
International Material Equivalents
B8M is the ASTM A193 bolting designation that maps to the same Type 316 / S31600 chemistry used across other international standards: AISI Type 316 (American), UNS S31600 (Unified Numbering), EN 1.4401 / X5CrNiMo17-12-2 (European), SUS 316 (JIS Japanese), GOST 08Kh17N13M2T (Russian), and GB 0Cr17Ni12Mo2 (Chinese). The matched ASTM A194 Grade 8M nut designation covers the same chemistry on the nut side, and ASTM F593 covers commercial 316 SS fasteners outside the A193 pressure-bolting scope.
Sour-service and offshore qualification
NACE MR0175 / ISO 15156-3 Sour-Service Compliance
For sour-service oil-and-gas applications under NACE MR0175 / ISO 15156-3, the solution-annealed B8M classes (Class 1, 1A, 1D) are the qualified condition for UNS S31600 austenitic stainless steel. ISO 15156-3 Table A.2 covers continuous exposure up to 60°C (140°F) at H₂S partial pressure up to 100 kPa without further qualification; supplementary conditions allow extended use per ISO 15156-3 Annex B.
NORSOK M-650 and Offshore Certifications
For North Sea and offshore service, TorqBolt's manufacturing route carries NORSOK M-650 Rev.4 qualification for high-alloyed austenitic stainless steel bolting. API 6A and PED 2014/68/EU Category III supply is covered with EN 10204 Type 3.2 inspection certification.
B8M product types and dimension references
B8M supply covers the full ASTM/ASME fastener inventory paired with the relevant ASME dimensional standards: hex bolts (ASME B18.2.1), heavy-hex structural bolts (ASME B18.2.3.3M, metric), socket-head cap screws (ASME B18.3, inch), socket-head cap screws (ASME B18.3.1M, metric), threaded rods (ASME B18.31.2, inch), and threaded rods (ASME B18.31.1M, metric). For flange-bolt assemblies, B8M studs pair with ASME B16.5 flange dimensions and class-150 through class-2500 pressure ratings.
