What is ASTM A193 Grade B8MA and how does it differ from Class 1?

B8MA is the Class 1A designation of Grade B8M (the suffix A denotes Class 1A per ASTM A193 Section 4.1). Per Section 6.2.4, Class 1A finished bolting components are carbide solution treated AFTER all rolling, forging, heading, and threading operations are complete. Class 1 by contrast permits solution treatment of the bar stock before machining. Both classes share the same minimum tensile (75 ksi) and yield (30 ksi) per Table 2, but Class 1A carries a lower maximum hardness of 192 HBW (90 HRB) versus 223 HBW (96 HRBC) for Class 1.

When is carbide solution treatment in the finished condition critical?

Class 1A is selected when the bolting will see severe corrosion service and any cold work introduced by threading or heading would compromise corrosion resistance. Per ASTM A193 Section 6.2.4, treating in the finished condition removes residual cold work from the threads and ensures a fully austenitic, carbide-free microstructure at every machined surface. Class 1A is the preferred B8M variant for chloride pitting service, polythionic acid stress cracking environments, and any application where corrosion resistance ranks higher than mechanical preload.

Why is the hardness limit lower for Class 1A (192 HBW) than Class 1 (223 HBW)?

The lower 192 HBW (90 HRB) hardness ceiling reflects the absence of any residual cold work. Class 1 bar may be threaded after solution annealing, leaving locally cold-worked thread roots that raise surface hardness up to 223 HBW (96 HRBC). Class 1A solution treats the finished bolt after threading, restoring a fully annealed structure across every surface. The 192 HBW limit also satisfies NACE MR0175 / ISO 15156-3 hardness requirements for sour-service austenitic stainless bolting.

ASTM A193 / SA-193 B8M Class 1A Bolts

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

Scope

ASTM A193 / ASME SA-193 Grade B8M Class 1A is the 316 austenitic stainless steel bolting variant (UNS S31600, sometimes designated B8MA) carbide-solution treated in the finished condition: the solution anneal is performed after all machining and cold work is complete. The post-finish anneal erases residual cold work introduced by thread rolling and machining, so Class 1A is supplied in the softest of the four B8M class designations, with maximum hardness held at 192 HBW or 90 HRB (compared with 96 HRB for Class 1 and 1D, and 35 HRC for Class 2). Mechanical properties otherwise mirror Class 1 at 75 ksi (515 MPa) minimum tensile and 30 ksi (205 MPa) minimum yield. TorqBolt supplies B8M Class 1A bolts and stud bolts to ASTM A193/A193M-19 with EN 10204 3.1 or 3.2 inspection certification.

Class 1A is the procurement choice when maximum chloride-pitting and intergranular-corrosion resistance is paramount and component geometry can withstand the slight dimensional movement that the final solution-anneal step may introduce. The post-finish heat treatment also relieves residual stress in the thread roots, which can be a fatigue benefit on flange bolts cycling between assembly torque and operating pressure.

Properties

Chemical composition

Sour-service acceptance

Grade	Description and UNS Designation	Classes	C	Mn	P	S	Si	Cr	Ni	Mo
ASTM A193 Grade B8M	316, S31600	1A	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

Class 1A sits within the NACE MR0175 / ISO 15156-3 austenitic stainless steel acceptance threshold by virtue of its solution-annealed structure and 90 HRB maximum hardness, which is softer than the Class 1 96 HRB ceiling. ISO 15156-3 Table A.2 environmental limits for UNS S31600 cover continuous exposure up to 60°C (140°F) at H₂S partial pressure up to 100 kPa (15 psi) without further qualification. Class 1A's additional post-finish anneal makes it the preferred selection for sour-service positions where residual cold work from finishing operations would otherwise need to be specifically excluded.

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 (standard solution-annealed), Class 1D (heavier sections), Class 2 (strain-hardened, higher tensile). See also: ASTM A193/A193M-19 cornerstone. Selection aids: the Class 1 vs Class 2 disambiguation explains when solution-treatment in the finished condition is mandatory, and the UNS S31600 datasheet lists the underlying Type 316 chemistry behind B8MA.

ASTM A193 Grade B8M Class 1A Stud Bolts

Scope

Properties

Chemical composition

Sour-service acceptance

Related references

Related Specifications

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