Scope
ASTM A193 Grade B8M and Grade B8 are the two principal austenitic stainless steel bolting designations under the standard. The single chemistry difference between the two grades is molybdenum: B8M carries 2.00 to 3.00% Mo as a required alloying element, B8 carries none. That one alloying line is the primary selection driver between the two grades and shifts the pitting-resistance-equivalent number (PREN) from approximately 18 for B8 to approximately 24 for B8M.
Chemistry side-by-side
Both grades share the same carbon ceiling (0.08% max), manganese (2.00% max), silicon (1.00% max), and the same phosphorus/sulphur restrictions. B8 sits at Cr 18.0-20.0 / Ni 8.0-11.0 (UNS S30400, AISI 304). B8M sits at Cr 16.0-18.0 / Ni 10.0-14.0 / Mo 2.00-3.00 (UNS S31600, AISI 316). The slightly lower chromium range in B8M is partially offset for general corrosion resistance by the higher nickel content; the molybdenum is what shifts pitting and crevice performance in chloride-bearing service.
When to specify B8M over B8
The selection rule used by procurement engineers and repeated across pressure-vessel and piping codes: chloride-bearing service or operating temperature above 60°C in saline atmosphere specifies B8M; clean-environment dry indoor service is acceptable to B8. Specific triggers for B8M selection:
- Marine atmosphere and offshore platform exposure (chloride aerosol)
- Seawater service flanges and pump bolting
- Refining and petrochemical heat-exchanger bonnets in salt-water cooling loops
- Chemical-plant pump and valve flanges handling chloride solutions
- Sulphuric, hydrochloric, and phosphoric acid handling equipment
- Sour-service oil-and-gas positions under NACE MR0175 / ISO 15156-3 where Mo is preferred for chloride margin
- Pulp-and-paper digester bolting
- Pharma and food-grade equipment where local chloride exposure is possible during cleaning
Cost and lead-time delta
B8M typically commands a 10-25% material-cost premium over B8 at the mill bar level, driven by molybdenum price exposure and the lower mill throughput of 316 relative to 304. Lead times are comparable for stock diameters; oversize or large-quantity B8M Class 2 orders may sit 2-4 weeks behind B8 Class 2 because the strain-hardening step is run in dedicated batches per heat to maintain mechanical-property uniformity.
Class compatibility
The same heat-treatment classes apply to both grades: Class 1 (solution annealed), Class 1A, Class 1D, and Class 2 (strain hardened). A B8M Class 2 substitution for a B8 Class 2 specification is upward in corrosion performance and is acceptable on most reviews; the reverse substitution (B8 in place of B8M) is not acceptable without engineering deviation because the chloride-pitting margin is lost. Companion nut designations follow: A194 Grade 8 for B8, A194 Grade 8M for B8M (matched chemistry on both sides of the flange).
Related references
Detailed pages on this site: B8M overview, UNS S31600 material data, B8M Class 1 vs Class 2 selection, ASTM A193/A193M-19 spec overview, ASME SA-193 designation (Mid-East). For the equivalent 304-chemistry B8 grade, astma193b8.com covers the same depth (separate TorqBolt micro-site).
