The short answer: ASTM A193 Grade B8M is Type 316 stainless-steel bolting. Class 1 is carbide solution treated; Class 2 adds strain hardening for higher strength. B8M is commonly selected when a project requires the molybdenum-bearing 316 material route, but it is not interchangeable with B8 Type 304 and it is not universally immune to chloride corrosion.[1] [2] [3]
B8M belongs to the austenitic stainless-steel grades in ASTM A193. Read the ASTM A193 Grade B8 guide for the related Type 304 grade, or return to the ASTM A193 and A194 overview when the alloy-steel-versus-stainless decision is still open.
What material is ASTM A193 Grade B8M?
B8M is the A193 grade based on Type 316 austenitic stainless steel. Compared with B8 Type 304, B8M contains molybdenum and follows the 316 chemistry route. That generally improves resistance to localized corrosion in many chloride-bearing environments, but actual suitability still depends on chloride level, temperature, crevices, stress, cleaning chemicals and the governing corrosion specification.[1] [2] [3]
In procurement, B8M identifies the molybdenum-bearing Type 316 grade; do not interpret the suffix by analogy with B7M, where it identifies modified mechanical and hardness requirements.
| Grade | Stainless material | Main distinction | Common A194 nut |
|---|---|---|---|
| ASTM A193 B8 | Type 304 | General 304 stainless route | Grade 8 |
| ASTM A193 B8M | Type 316 | Molybdenum-bearing 316 route | Grade 8M |
B8M Class 1 vs Class 2
Class 1 is carbide solution treated. Class 2 receives solution treatment followed by strain hardening, which raises the minimum tensile and yield strength. The order must state the class because “A193 B8M” alone does not identify the intended mechanical-property row.[1] [3]
| ASTM A193 B8M class and nominal diameter | Minimum tensile strength | Minimum yield strength |
|---|---|---|
| Class 1, all diameters covered by the specification | 75 ksi | 30 ksi |
| Class 2, up to and including 3/4 in | 110 ksi | 95 ksi |
| Class 2, over 3/4 through 1 in | 100 ksi | 80 ksi |
| Class 2, over 1 through 1 1/4 in | 95 ksi | 65 ksi |
| Class 2, over 1 1/4 through 1 1/2 in | 90 ksi | 50 ksi |
A 3/4 in B8M Class 2 stud uses the 110/95 ksi row; a diameter greater than 3/4 in through 1 in uses 100/80 ksi. These are specification minimums, not allowable design stresses at temperature.
B8M Class 2 values are also not copied from B8 Class 2. For example, through 3/4 in, B8 Class 2 has a 125 ksi minimum tensile strength while B8M Class 2 has 110 ksi. The required alloy, class and diameter must all be included in the joint calculation and purchase order.
B8C and B8T: where the stabilized grades fit
B8C and B8T are separate A193 grades, not classes of B8M. They belong in the same austenitic stainless family but use stabilized alloys intended for service where the project specifically selects resistance to sensitization during elevated-temperature exposure.[1]
| ASTM A193 grade | Stainless type | Alloy distinction | Common A194 nut |
|---|---|---|---|
| B8M | Type 316 | Molybdenum-bearing stainless | Grade 8M |
| B8C | Type 347 | Niobium/columbium-stabilized stainless | Grade 8C |
| B8T | Type 321 | Titanium-stabilized stainless | Grade 8T |
Do not substitute B8C or B8T for B8M merely because all three are austenitic stainless steels. B8M addresses the Type 316 material route; B8C and B8T address different stabilized chemistries. Each grade can have its own class, processing, marking and mechanical requirements, so copy the complete designation from the governing design rather than reducing the order to “stainless A193.”
Corrosion limits
Type 316 often performs better than Type 304 against pitting and crevice corrosion, but “marine grade” is not a design calculation. Concentration, temperature, oxygen, deposits and cleaning cycles can change performance. 304 vs 316 Stainless Steel Fasteners provides the broader alloy comparison; final selection belongs to the project corrosion review.
Marking, threads and product form
B8M Class 1 uses the B8M grade symbol. B8M Class 2 uses the underlined B8MSH symbol, together with manufacturer identification and subject to the standard’s small-product and alternate-location provisions.[1] [3] Match the physical marking to the material test report and the edition stated on the order.
B8M may be supplied as studs, threaded rod or headed fasteners. For inch-series A193 bolting, threads are ASME B1.1 Class 2A; UNC applies through 1 in and 8UN above 1 in unless otherwise specified.[2] State the exact product form, diameter, pitch and length.
Which nut goes with B8M?
ASTM A194 Grade 8M nuts provide the usual Type 316 companion to B8M bolting. The compatibility guide covers Class 2/S1 selection, B8C/8C and B8T/8T pairings, and assembly lubrication.[4] [5]
B8M details to confirm before ordering
State B8M and the complete class, then confirm the diameter-dependent strength and Type 316 chemistry. B8C and B8T require their own grade callouts. Use the assembly ordering examples to add the nut, matching thread, dimensions, service conditions and quantity.
Requesting B8M bolting? Include the exact class, dimensions, matching A194 8M nut requirement, service conditions and documentation requirements. HexFastener can review whether the callout is complete before quoting.
References
- ASTM International. ASTM A193/A193M-26 — Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications. Current grade chemistry, processing, mechanical properties and marking requirements.
- ASTM International. ASTM A962/A962M-26 — Common Requirements for Bolting Intended for Use at Any Temperature from Cryogenic to the Creep Range. Common thread, testing, certification and marking requirements invoked by A193.
- Portland Bolt. ASTM A193 Grades. B8 and B8M material, class, strength and marking summary.
- ASTM International. ASTM A194/A194M-26 — Standard Specification for Carbon Steel, Alloy Steel, and Stainless Steel Nuts for Bolts for High Pressure or High Temperature Service, or Both. Companion nut specification and supplementary requirements.
- Portland Bolt. ASTM A193 Hardware. Common B8M/8M pairing and strain-hardened nut option for Class 2 stainless bolting.