What heat treatment defines ASTM A193 Grade B8M Class 1?

Class 1 B8M bolting is carbide solution treated. Per ASTM A193/A193M Section 6.2, the bar or wire is heated from ambient and held at a temperature at which the chromium carbide goes into solution, then cooled at a rate sufficient to prevent carbide precipitation. The resulting mechanical properties per Table 2 are 75 ksi (515 MPa) minimum tensile, 30 ksi (205 MPa) minimum yield, 30 percent minimum elongation, and a hardness limit of 223 HBW (96 HRBC) maximum. Heat treatment may occur on the bar stock before machining the finished component.

Does the tensile strength of B8M Class 1 vary with bolt diameter?

No. ASTM A193 Table 2 lists Class 1 B8M tensile and yield as 75 ksi and 30 ksi minimum for all diameters. The same values apply whether the bolt is 1/4 inch or 4 inches in diameter, because solution annealing produces uniform properties through the full cross-section. This is the key contrast with Class 2 B8M, whose tensile drops in steps as diameter increases from 3/4 inch up through 1-1/2 inch (per Table 2 footnotes).

ASTM A193 / SA-193 B8M Class 1 Stud Bolts

Name: ASTM A193 Grade B8M Class 1 Stud Bolts
Brand: TorqBolt
SKU: B8M-Class-1
Availability: InStock

Scope

ASTM A193 / ASME SA-193 Grade B8M Class 1 is the carbide-solution-treated 316 austenitic stainless steel bolting material (UNS S31600) at the standard's baseline heat-treatment condition. Class 1 delivers 75 ksi (515 MPa) minimum tensile strength and 30 ksi (205 MPa) minimum yield, with maximum hardness held at 223 HBW or 96 HRB across the diameter range, within the NACE MR0175 austenitic stainless steel limit for sour-service acceptance. The molybdenum addition of 2.00 to 3.00% improves chloride pitting and crevice resistance over B8 (304 chemistry) and places B8M in the PREN approximately 24 range. TorqBolt supplies B8M Class 1 stud bolts, hex bolts, and threaded rods to ASTM A193/A193M-19 with EN 10204 3.1 or 3.2 inspection certification.

The carbide-solution treatment sets the baseline production sequence: heating in the range 1900-2100°F (1040-1150°C) to dissolve chromium-rich carbides, followed by rapid water quench to retain austenite in supersaturated solid solution. Class 1 finished bars are not cold-worked after solution annealing, so the mechanical values listed in the table below represent the annealed-condition floor for B8M.

Properties

Chemical composition

Mechanical properties

Sour-service acceptance

Grade	Description and UNS Designation	Classes	C	Mn	P	S	Si	Cr	Ni	Mo
ASTM A193 Grade B8M	316, S31600	1, 1A, 1D, 2	0.08	2.00	0.045	0.030	1.00	16.0-18.0	10.0-14.0	2.00-3.00
Product Analysis Variation^B	0.01	0.04	0.010	0.005	0.05	0.20	0.15	0.10

Grade Class , mm	Diameter	Heat Treatment^B	Tensile Strength, min,MPa	Yield Strength, min, 0.2 % offset, MPa	Elongation in 4 D,min %	Reduction of Area, min %	Hardness, max
ASTM A193M Grade B8M Classes 1 and 1D	All diameters	carbide solution treated	515	205	30	50	223 HBW or 96 HRB^C

Class 1 is the NACE MR0175 / ISO 15156-3 qualified condition for UNS S31600 austenitic stainless steel in H₂S-bearing oil-and-gas service. ISO 15156-3 Table A.2 environmental limits cover continuous exposure up to 60°C (140°F) at H₂S partial pressure up to 100 kPa (15 psi) without further qualification; elevated temperature and partial pressure are permitted under supplementary chloride and pH conditions per ISO 15156-3 Annex B. Class 1's solution-annealed structure and maximum 96 HRB hardness fall within the austenitic SS acceptance threshold for sour service, where strain-hardened Class 2 at 35 HRC exceeds the general MR0175 limit.

Frequently Asked Questions

What nuts and washers pair with ASTM A193 Grade B8M bolts?

ASTM A194 Grade 8M heavy hex nuts are the standard companion nut for B8M stud bolts and hex bolts. Grade 8M is manufactured from the same Type 316 stainless steel (UNS S31600), ensuring galvanic compatibility and matching pitting resistance. ASTM F436 Type 3 hardened washers are the matched washer for B8M assemblies that require under-head or under-nut bearing surfaces — Type 3 indicates austenitic stainless. TorqBolt supplies matched B8M bolt + 8M nut + F436 Type 3 washer sets under a single EN 10204 3.2 certificate covering all components by heat number.

When is ASTM A193 B8M Class 1 specified over Class 2?

Class 1 solution-annealed B8M is specified when sour-service compliance under NACE MR0175 / ISO 15156-3 is required, or when the joint operates at elevated service temperature where strain-hardened material risks softening. Class 1's 223 HBW (96 HRB, approximately 22 HRC) hardness ceiling sits within the ISO 15156-3 Table A.2 acceptance for austenitic stainless in H₂S environments. The trade-off is lower mechanical strength: 515 MPa (75 ksi) tensile versus 620 to 760 MPa (90 to 110 ksi) for Class 2. Class 1 is therefore the default for offshore, subsea, and refinery sour-gas service under NORSOK M-650 qualification.

ASTM A193 Grade B8M Class 1 Stud Bolts

Scope

Properties

Chemical composition

Mechanical properties

Sour-service acceptance

Frequently Asked Questions

What nuts and washers pair with ASTM A193 Grade B8M bolts?

When is ASTM A193 B8M Class 1 specified over Class 2?

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