How is ASTM A193 Grade B8M Class 2 strength achieved?

Class 2 B8M is first carbide solution treated, then strain hardened to develop properties. Per ASTM A193 Section 6.2.5, the bolting material is heated from ambient and held at a temperature at which chromium carbide goes into solution, cooled rapidly to prevent precipitation, then cold worked to raise tensile and yield. Per Table 2, strain hardening lifts minimum tensile from 75 ksi (Class 1) to 110 ksi at 3/4 inch and below, and yield from 30 ksi to 95 ksi. Maximum hardness rises to 321 HBW (35 HRC). All Class 2 strain-hardened bolting is tested full-size after strain hardening per Section 9.1.1.

How do Class 2 tensile properties vary with bolt diameter?

Per ASTM A193 Table 2, Class 2 B8M tensile and yield step down as diameter increases. At 3/4 inch and under the minimum tensile is 110 ksi and yield is 95 ksi. From over 3/4 inch to 1 inch the minimums drop, and continue to drop through 1-1/4 inch and 1-1/2 inch. Above 1-1/2 inch, strain-hardened Class 2 B8M is not listed and the engineer must move to Class 2B (B8M2) or Class 2C (B8M3) for the larger diameters. This taper exists because strain hardening cannot be driven uniformly through thick cross-sections.

ASTM A193 / SA-193 B8M Class 2 Stud Bolts

Name: ASTM A193 Grade B8M Class 2 Stud Bolts
Brand: TorqBolt
SKU: B8M-Class-2
Availability: InStock

ASTM A193 / ASME SA-193 Grade B8M Class 2 is a carbide-solution-treated and strain-hardened 316 austenitic stainless steel bolting material (UNS S31600) for high-temperature and corrosive service. Class 2 raises the minimum tensile strength to 110 ksi (760 MPa) for sizes 3/4 inch and under (M20 and under), decreasing to 90 ksi (620 MPa) for sizes through 1-1/2 inch (M30 to M36). Maximum hardness is held at 321 HBW or 35 HRC across the diameter range. The molybdenum addition of 2.00 to 3.00% sets B8M apart from B8 by improving resistance to pitting and crevice corrosion in chloride-bearing service. TorqBolt supplies B8M Class 2 stud bolts, hex bolts, and threaded rods to ASTM A193/A193M-19 with EN 10204 3.1 or 3.2 inspection certification.

Strain hardening follows the carbide-solution treatment in the standard A193 production sequence. The solution treatment dissolves chromium carbides above 1900°F (1040°C) and quenches the bar in water to retain the austenitic structure. The strain-hardening step that follows develops the mechanical-property values listed in the tables below. Class 2 finished bars are not annealed after strain hardening; the cold-worked structure carries the elevated tensile and yield strength through to the finished fastener.

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	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 Class 2	M20 & under	strain hardened	760	655	15	45	321 HBW or 35 HRC
Over M20 to M24	690	550	20	45	321 HBW or 35 HRC
Over M24 to M30	655	450	25	45	321 HBW or 35 HRC
Over M30 to M36	620	345	30	45	321 HBW or 35 HRC

For sour-service applications under NACE MR0175 / ISO 15156-3, the Class 1 (solution-annealed) variant of B8M is the qualified condition — its maximum hardness of 96 HRB (approximately 22 HRC) sits within the NACE acceptance threshold for austenitic stainless steels. Class 2 at 321 HBW (35 HRC) exceeds the general MR0175 hardness limit for austenitic SS and is therefore selected only where the application does not invoke NACE compliance, or where the operator's project specification explicitly permits strain-hardened austenitic SS in sour service through case-by-case qualification.

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 2 chosen over Class 1?

Class 2 strain-hardened B8M is chosen when the joint requires higher preload than Class 1 can sustain, typically on large-diameter flanges where the 515 MPa (75 ksi) tensile of Class 1 produces inadequate gasket stress at design torque. Class 2 raises minimum tensile to 620 to 760 MPa (90 to 110 ksi) depending on diameter, at the cost of reduced elongation and sour-service eligibility. Class 2 hardness reaches 321 HBW (35 HRC), above the ISO 15156-3 Table A.2 limit for austenitic stainless in sour service, so the strain-hardened condition is selected only where NACE MR0175 compliance is not required.

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 (NACE-qualified solution-annealed), Class 1A (annealed in finished condition), Class 1D (heavier sections, liquid-quenched). See also: B8M vs B8 comparison (316 vs 304). Compliance context: the ASME SA-193 B8M specification page mirrors ASTM A193 within the BPVC framework, and the UNS S31600 datasheet anchors the underlying Type 316 chemistry that Class 2 strain-hardens.

Related Specifications

Surface Treatments

Xylan 1070 / 1014 / 1400 / 1424

Xylar 2 / 101

Geomet 321 / 500 / 720

SermaGard 1105 / 1280

Dacromet 320 / 500

TakeCoat 1000 Ceramic

Delta Tone/Seal

Magnigard

MolyKote

Sherardized

Zinc-Cobalt (ASTM B840)

Zinc-Nickel (ASTM B841)

Certifications

ISO 9001 - 2015 Certified
PED 2014/68/EC
API 6A ; API 20E; API 20F
NACE MR0175/ISO 15156-2
NORSOK M-650
DFAR
MERKBLATT AD 2000 W2/W7/W10

ASTM A193 Grade B8M Class 2 Stud Bolts

Scope