Scope
ASTM A193 B8M Class 1 and Class 2 are the two most commonly specified heat-treatment conditions for the 316 stainless steel bolting grade (UNS S31600). They share the same chemistry but differ in the post-anneal processing route, which shifts the mechanical-property envelope and the sour-service qualification status. The selection between Class 1 and Class 2 is the most common engineering decision on B8M procurement.
At a glance
Class 1 — carbide solution treated only. Lower strength (75 ksi minimum tensile, 30 ksi yield), maximum hardness 223 HBW or 96 HRB. Within the NACE MR0175 / ISO 15156-3 austenitic stainless steel hardness threshold, so qualified for sour-service oil-and-gas applications.
Class 2 — carbide solution treated and strain hardened (cold-worked after anneal). Higher strength (90-110 ksi minimum tensile size-dependent, 50-95 ksi yield), maximum hardness 321 HBW or 35 HRC. Exceeds the general NACE MR0175 austenitic SS hardness ceiling, so NOT acceptable for general sour service without project-specific qualification.
Decision matrix
Select B8M Class 1 when:
- The bolted joint operates in NACE MR0175 sour-service oil-and-gas conditions (any H₂S exposure on the carbon-steel side of the flange)
- The joint is offshore and project specification calls for the qualified sour-service condition
- Maximum chloride pitting resistance is the governing design driver (the solution-annealed condition has slightly better intergranular corrosion resistance than the strain-hardened condition)
- Design tensile demand is at or below the 75 ksi floor (typical for flange bolting at moderate class ratings)
Select B8M Class 2 when:
- Design tensile demand sits between 75 and 110 ksi and a 316 stainless body is required for corrosion reasons (Class 1 cannot deliver this strength; the higher-strength alternative would otherwise be a B7 carbon-alloy steel that fails chloride pitting)
- The service is high-pressure but explicitly non-sour (no NACE invocation)
- Valve stems, high-pressure pump shafts, and similar fastener positions where the higher yield is the constraint
- Project specification explicitly permits strain-hardened austenitic stainless in sour service through case-by-case qualification (rare; usually carries supplementary hardness and microstructure testing)
What about Class 1A and Class 1D
Class 1A is Class 1 with the solution anneal performed after machining is complete (the A suffix in B8MA). It is the softest variant (90 HRB max) and is the choice when residual cold work from finishing must be eliminated for pharma, food, and sour-service bolts where cold-work exclusion is mandated, and high-cycle fatigue positions where thread-root stress relief matters.
Class 1D is Class 1 with a supplementary liquid-quench step after the solution anneal, sized to maintain quench rate through heavier section thicknesses (typically ≥3/4 inch / M20 diameter). It is the Class 1 variant called out separately on standard tables to emphasise the heavy-section heat-treatment route.
Companion nut grade tracks the bolt class
The matched-condition rule: a Class 1 bolt pairs with an ASTM A194 Grade 8M (solution-annealed) nut; a Class 2 bolt pairs with a Grade 8M nut that has been similarly strain-hardened; a Class 1A bolt pairs with a Grade 8MA nut. Mixing the hardness conditions across the joint (e.g. Class 2 bolt with annealed Grade 8M nut) is permitted but is usually deprecated by project bolting standards because the differential galling response can complicate torque control.
