In chemical-processing lines, pressure equipment and chloride-bearing service, stainless bolting may need more corrosion resistance than Type 304 can provide. ASTM A193 Grade B8M is the Type 316 stainless-steel bolting grade in the A193 family; its molybdenum content gives it better resistance to pitting and crevice corrosion than Grade B8/Type 304, subject to the actual environment and project specification.
Unifit Fastener manufactures ASTM A193 Grade B8M bolts, stud bolts and threaded bolting in India for pressure vessels, valves, flanges, fittings and process piping. B8M can be ordered in Class 1 or Class 2, so the class, dimensions, thread, matching nut, surface condition and inspection documentation should be stated on the purchase requirement. For the wider grade family, see ASTM A193 grades and specifications. For the broader product range, see our industrial fasteners page.
Request a Quote | Send B8M SpecificationsASTM A193 B8M is an austenitic stainless-steel bolting grade based on Type 316 chemistry. Compared with B8/Type 304, the addition of molybdenum improves resistance to localized corrosion in many chloride-bearing and chemical environments. That difference is why B8M is commonly considered for process plants, water-treatment equipment and other pressure-service joints where 304-type bolting may not provide enough corrosion resistance.
B8M is still a specification-driven material choice. “Stainless steel” does not mean immunity to chloride attack, and B8M should not automatically be described as suitable for every seawater, offshore or acid service. Chloride concentration, temperature, crevices, process chemistry, stress, cleaning regime and the governing code all influence the final selection.
If the requirement is generic Type 316/316L fasteners rather than ASTM A193 B8M bolting, use the Stainless Steel 316/316L fasteners page instead.
| Parameter | Details |
|---|---|
| Grade | ASTM A193 Grade B8M |
| Material family | Type 316 austenitic stainless |
| Classes | Class 1 and Class 2 |
| Typical product forms | Stud bolts / threaded studs, heavy hex / hex bolts, drawing-based special bolting |
| Common uses | Pressure vessels, valves, flanges, fittings and process piping |
| Matching nuts | ASTM A194 Grade 8M heavy hex nuts |
| Documentation | MTC, PMI, mechanical test, TPI, marking, ITP or project documentation |
The B8M grade has two common strength conditions. Class 1 is carbide solution treated. Class 2 is solution treated and then strain hardened, which raises the strength but makes the requirements size-dependent. Under A193 practice, B8M is generally treated as Class 1 unless Class 2 is specifically ordered.
| Class | Condition | Parent-page strength summary | Marking | Selection logic |
|---|---|---|---|---|
| B8M Class 1 | Carbide solution treated | 75 ksi min tensile; 30 ksi min yield; 223 HBW / 96 HRB max | B8M | General B8M condition where Class 1 properties meet the design |
| B8M Class 2 | Carbide solution treated + strain hardened | 110 ksi min tensile / 95 ksi min yield up to 3/4 in.; values reduce by size | B8MSH / Class 2 marking per applicable edition | Higher-strength B8M when Class 2 is specifically required |
Detailed class pages: ASTM A193 B8M Class 1 fasteners | ASTM A193 B8M Class 2 fasteners.
The B8M chemistry below reflects the Type 316 composition limits used in the current Unifit page and common ASTM A193 reference tables. Verify the final values against the ASTM A193/A193M edition specified for the order.
| Element | Minimum % | Maximum % |
|---|---|---|
| Carbon (C) | - | 0.08 |
| Manganese (Mn) | - | 2.00 |
| Phosphorus (P) | - | 0.045 |
| Sulfur (S) | - | 0.030 |
| Silicon (Si) | - | 1.00 |
| Chromium (Cr) | 16.00 | 18.00 |
| Nickel (Ni) | 10.00 | 14.00 |
| Molybdenum (Mo) | 2.00 | 3.00 |
The molybdenum content is the most useful buyer-facing distinction from B8/Type 304. It improves resistance to pitting and crevice corrosion, but the final corrosion suitability still depends on the complete service environment.
Keep the B8M product range focused on pressure-service bolting rather than a long catalogue of unrelated screw types. Typical project forms include:
| Product form | Typical use | What to specify |
|---|---|---|
| Stud bolts / threaded studs | Flanged piping, valves, pressure vessels and process equipment | Diameter, length, thread, Class 1/2, nut grade, flange standard/drawing |
| Heavy hex / hex bolts | Pressure-equipment or machinery joints where a headed bolt is specified | Diameter, length, head/dimensional standard, class, finish/lubricant |
| Drawing-based special bolting | Non-standard project requirements | Latest drawing, class, material, dimensions, tolerance, finish and inspection plan |
General product-form information: industrial bolts | stud bolts.
A complete RFQ helps prevent the wrong class, nut pairing or assembly condition from being quoted.
| RFQ field | What to provide |
|---|---|
| Grade / class | ASTM A193 B8M Class 1 or Class 2 |
| Product form | Stud bolt, hex/heavy-hex bolt, threaded stud or drawing-based item |
| Size | Diameter and overall length; include unthreaded section where relevant |
| Thread | UNC / UNF / 8UN / metric, pitch or TPI, class/fit |
| Dimensional basis | ASME / ISO / DIN / project drawing / flange specification as applicable |
| Matching components | ASTM A194 nut grade, washer requirement and quantity per assembly |
| Surface / lubricant | Plain/passivated or approved coating/lubricant; state any friction requirement |
| Service environment | Temperature, chlorides, process media and any corrosion restriction relevant to selection |
| Quantity / delivery | Quantity, destination and required delivery date |
| Inspection / documents | MTC, PMI, mechanical test, TPI, marking, ITP or project documentation |
Identifying different fastener types allows buyers to pick the exact products tailored to their unique industrial application needs.
ASTM A193 covers the B8M bolting material; the matching nut is specified separately. ASTM A194 Grade 8M heavy hex nuts are a common pairing with A193 B8M Class 1 and Class 2 bolting. For Class 2 bolts, a strain-hardened nut is not automatically required by the B8M designation alone; if a strain-hardened nut is needed, it should be called out in the nut specification / supplementary requirement or project document.
Related nut page: ASTM A194 Grade 8M hex nuts.
B8M is often selected because Type 316 gives better chloride resistance than Type 304, but that does not remove the need for a corrosion review. Warm chloride solutions, stagnant crevices and severe seawater exposure can still attack 316-type stainless steel. Where the corrosion assessment calls for substantially higher pitting resistance, a higher-alloy grade such as B8MLCuN may be the better specification route.
Austenitic stainless-steel threads also need attention during assembly. B8M bolt-and-nut pairs can gall if thread condition, surface finish, lubrication and installation speed are not controlled. These factors also change the relationship between applied torque and achieved preload. For this reason, a generic torque number should not be copied from a chart without defining the lubricant/coating, nut condition, target preload, thread size and calculation basis.
Assembly note
For critical flange joints, use the project-approved tightening procedure. If the customer asks for a torque value, confirm the thread, class, nut grade, lubricant/coating and target preload before issuing it.
B8M is used where the design calls for Type 316 austenitic stainless bolting under the ASTM A193 pressure/high-temperature specification. Common project environments include:
The three grades should not compete for the same search intent. Use the service environment as the routing logic:
| Grade | Material family | Key chemistry difference | Selection direction | Owner |
|---|---|---|---|---|
|
B8 |
Type 304 austenitic stainless |
No molybdenum |
General corrosion-resistant A193 stainless bolting where Type 304 is specified |
https://unifitfastener.com/astm-a193-grade-b8-fasteners.html |
|
B8M |
Type 316 austenitic stainless |
2-3% Mo |
Improved pitting/crevice-corrosion resistance over B8 in many chloride-bearing environments |
THIS PAGE |
|
B8MLCuN |
UNS S31254 / 6Mo-style high-alloy stainless |
Higher Mo + alloying additions |
More severe chloride/process environments where the project specifies B8MLCuN |
https://unifitfastener.com/astm-a193-b8mlcun-fasteners.html |
Compare the exact grade pages: ASTM A193 Grade B8 | ASTM A193 B8MLCuN.
For project bolting, the order should define the class, marking and documentation scope before production or dispatch. B8M Class 1 is marked B8M; Class 2 is differentiated by the applicable Class 2 / SH marking requirements. The page should show a real Unifit marking photograph once the team has a verified sample.
ASTM A193 Grade B8M is an austenitic stainless-steel bolting grade based on Type 316 chemistry. Its molybdenum content differentiates it from B8/Type 304 and improves resistance to pitting and crevice corrosion in many chloride-bearing environments.
Do not treat the terms as automatically interchangeable. B8M is an ASTM A193 bolting grade with Type 316 chemistry and class/mechanical requirements. If a project specifically requires 316L or UNS S31603 chemistry, that requirement should be stated and verified separately.
Class 1 is carbide solution treated. Class 2 is solution treated and strain hardened, giving higher strength with size-dependent mechanical requirements. If Class 2 is required, it should be stated explicitly on the order.
B8 is based on Type 304 stainless steel, while B8M is based on Type 316 and contains molybdenum. The molybdenum improves localized corrosion resistance, so B8M is often considered where chloride exposure is more demanding than a Type 304 service.
ASTM A194 Grade 8M heavy hex nuts are a common matching specification for A193 B8M Class 1 and Class 2 bolting. The final nut condition, dimensions, lubricant/coating and project requirements should be checked as part of the complete joint.
Not automatically. Type 316 B8M has better chloride resistance than Type 304 B8, but severe seawater, elevated temperature or crevice conditions may require a higher-alloy material. The final choice should follow the corrosion assessment and project specification.
Torque depends on thread size, target preload, nut condition and especially friction from the lubricant, coating and surface finish. A generic value can create the wrong preload. Use the project-approved tightening basis for critical joints.
Send the B8M class, product form, diameter, length, thread, dimensional standard or drawing, matching nut/washer requirement, surface or lubricant requirement, service environment, quantity, delivery location and required test/documentation scope.