What distinguishes ASTM A193 Grade B8M Class 1D from Class 1?

Class 1D applies only to bar and wire that is cooled rapidly immediately following hot working, while the temperature is still above 1750 F (955 C), so that grain-boundary carbides remain in solution. Per ASTM A193 Section 6.2.3, Class 1D also covers finished bolting machined directly from Class 1D bar without any subsequent hot or cold working. Class 1 by contrast is solution annealed in a separate furnace cycle. Mechanical properties per Table 2 are identical to Class 1 (75 ksi tensile, 30 ksi yield, 223 HBW maximum). The difference is the production route.

Why is Class 1D restricted to service temperatures below 850 F (455 C)?

Per ASTM A193 Section 6.2.3, Class 1D bar relies on rapid cooling from the rolling temperature to keep chromium carbides in solid solution. Sustained service above approximately 455 C (850 F) puts the material in the carbide precipitation range, where Cr23C6 can form at grain boundaries and sensitize the structure to intergranular corrosion. The spec restriction is a defence against in-service sensitization; it does not affect short-term excursions during commissioning or shutdown.

When is Class 1D specified instead of standard Class 1 solution-treated B8M?

Class 1D is specified when the mill route produces bar that is cooled rapidly from the rolling temperature without a separate solution-anneal furnace cycle, which can reduce cost on continuous rolling lines. It is suitable for bolting in service below 455 C with no sour-service NACE MR0175 requirement. For chloride or sour service, or operating temperatures above 455 C, specify Class 1 (solution annealed) or Class 1A (solution annealed in finished condition).

ASTM A193 / SA-193 B8M Class 1D Stud Bolts

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

Scope

ASTM A193 / ASME SA-193 Grade B8M Class 1D is the carbide-solution-treated and liquid-quenched 316 austenitic stainless steel bolting material (UNS S31600) supplied for heavier-section bolts where a supplementary quench step is required after solution annealing. Class 1D delivers the same chemistry as Class 1 with the same 75 ksi (515 MPa) minimum tensile strength, 30 ksi (205 MPa) minimum yield, and 223 HBW or 96 HRB maximum hardness, within the NACE MR0175 austenitic stainless steel acceptance threshold for sour service. The 2.00 to 3.00% molybdenum addition places B8M at PREN approximately 24 for chloride pitting resistance. TorqBolt supplies B8M Class 1D stud bolts, hex bolts, and threaded rods to ASTM A193/A193M-19 with EN 10204 3.1 or 3.2 inspection certification.

The Class 1D heat-treatment route adds an explicit liquid-quench step (water or polymer) after the solution-anneal soak, sized to ensure the quench rate through the section thickness retains the austenitic structure without intergranular carbide precipitation. The supplementary quench is the reason Class 1D is called out separately on the standard's tables despite sharing the property floor with Class 1.

Properties

Chemical composition

Molybdenum at 2.00 to 3.00% is the defining chemistry difference between B8M (316) and B8 (304). It raises PREN to roughly 24, stabilises the passive film against chloride attack, and is the reason B8M is specified for marine, offshore, and refining flange-bolting where B8 would not pass corrosion-pitting acceptance.

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	1D	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 Class 1D	All diameters	carbide solution treated	515	205	30	50	223 HBW or 96 HRB^C

Class 1D shares the Class 1 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. Class 1D's solution-annealed structure and 96 HRB maximum hardness fall within the austenitic SS acceptance threshold; the liquid-quench step preserves the sour-service margin through heavier-section bolts where slower air-quench rates could allow grain-boundary carbide precipitation.

Related references

Spec hub: ASTM A193 / SA-193 B8M overview (covers NORSOK M-650 qualification, EN 10204 inspection certification, A194 8M companion nuts, F436 washers, B16.5 flange pairing, and the full applications register). Sibling classes: Class 1 (lighter-section solution-annealed), Class 1A (annealed in finished condition), Class 2 (strain-hardened, higher tensile). See also: UNS S31600 material data hub. Spec context: refer to the ASTM A193/A193M specification overview for the Class 1D rapid-cool heat-treatment provisions, and the B8M vs B8 comparison for the Type 304 versus Type 316 chemistry differences that shape sour-service selection.

ASTM A193 Grade B8M Class 1D Stud Bolts

Scope

Properties

Chemical composition

Mechanical properties

Sour-service acceptance

Related references

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