In chemical-processing lines, refinery equipment and pressure-service flange joints, Type 304 stainless bolting may not provide enough resistance when chlorides or aggressive process media are part of the service. ASTM A193 Grade B8M Class 1 uses Type 316 austenitic stainless steel, with molybdenum added to improve pitting and crevice-corrosion resistance compared with Grade B8.
Unifit Fastener manufactures ASTM A193 B8M Class 1 bolts, stud bolts and threaded bolting in India. Class 1 is the carbide-solution-treated condition of B8M and provides the standard baseline mechanical properties for this grade. For the full B8M family, including the higher-strength strain-hardened condition, see ASTM A193 Grade B8M fasteners. For the wider grade family, see ASTM A193 grades and specifications.
Request a Quote | Send B8M Class 1 SpecificationB8M identifies the ASTM A193 austenitic stainless-steel bolting grade based on Type 316 / UNS S31600 chemistry. Class 1 identifies the heat-treatment condition. The material is carbide solution treated so chromium-rich carbides are dissolved and the austenitic structure is restored before the bolting is supplied in the Class 1 condition.
Class 1 should not be confused with Class 2. B8M Class 2 receives additional strain hardening to obtain higher mechanical strength. If the purchase order only says 'B8M' without the class, the class should be clarified before production because the strength, marking and processing requirements are different.
Where strain-hardened B8M is required, use the dedicated ASTM A193 B8M Class 2 fasteners page.
The B8M chemistry is based on Type 316 stainless steel. The limits below are the standard B8M values commonly used for engineering reference; the final purchase should follow the ASTM edition specified on the project.
| Element | B8M requirement, % |
|---|---|
| Carbon (C) | 0.08 max |
| Manganese (Mn) | 2.00 max |
| Phosphorus (P) | 0.045 max |
| Sulfur (S) | 0.030 max |
| Silicon (Si) | 1.00 max |
| Chromium (Cr) | 16.0-18.0 |
| Nickel (Ni) | 10.0-14.0 |
| Molybdenum (Mo) | 2.00-3.00 |
For generic 316/316L fastener material information across standards, see stainless steel 316/316L fasteners. This page should remain focused on ASTM A193 B8M Class 1.
Class 1 is the baseline-strength condition of B8M. Unlike Class 2, its minimum tensile and yield values do not increase through strain hardening.
| Size | Tensile, min | Yield, min | Elongation, min | Reduction of area, min | Hardness, max |
|---|---|---|---|---|---|
| All diameters | 75 ksi / 515 MPa | 30 ksi / 205 MPa | 30% | 50% | 223 HBW or 96 HRB max |
These values are useful for grade verification, but design allowable stresses, temperature limits and joint loading must still come from the governing design code and project specification.
ASTM A193 controls the bolting material and mechanical requirements; it does not replace the dimensional standard, thread specification or matching-nut requirement. For flange and pressure-equipment work, the RFQ should therefore state both the B8M Class 1 grade and the required product geometry.
| Product form | Typical use | What procurement should specify |
|---|---|---|
| Stud bolts | Flanges, valves and pressure equipment | Diameter, overall length, thread series/pitch, full/double-end/tap-end form, nut quantity |
| Heavy hex / hex bolts | Pressure equipment and bolted assemblies where headed bolts are specified | Diameter, length, dimensional standard, thread, head form |
| Threaded studs / rods | Custom-length threaded bolting and maintenance requirements | Diameter, length, thread, end configuration, cut/rolled thread if specified |
| Special / drawing-based bolting | Project-specific dimensions or geometry | Latest drawing revision, tolerances, grade/class, finish and inspection requirements |
Related product pages: industrial bolts | stud bolts
| Grade / class | Material | Condition | Key distinction | Content owner |
|---|---|---|---|---|
| B8M Class 1 | Type 316 / UNS S31600 | Carbide solution treated | 75 ksi tensile / 30 ksi yield baseline | This page |
| B8M Class 2 | Type 316 / UNS S31600 | Solution treated + strain hardened | Higher, size-dependent strength | Dedicated Class 2 page |
| B8 Class 1 | Type 304 / UNS S30400 | Carbide solution treated | 75 ksi tensile / 30 ksi yield baseline | Dedicated B8 Class 1 page |
| B8MLCuN | 6Mo / UNS S31254 family | A193 grade-specific condition | Higher-alloy chloride resistance; class depends on order | Dedicated B8MLCuN page |
Compare with ASTM A193 B8 Class 1 fasteners when Type 304 is being considered.
| RFQ field | What to send |
|---|---|
| Grade / class | ASTM A193 Grade B8M Class 1 |
| Product form | Stud bolt, heavy hex/hex bolt, threaded stud/rod or drawing-based component |
| Diameter and length | Metric or inch; include any unthreaded shank requirement |
| Thread | UNC, UNF, 8UN or metric; pitch/TPI and class/fit |
| Dimensional standard | ASME/ANSI, ISO, DIN or project drawing as applicable |
| Matching components | ASTM A194 Grade 8M nut requirement and washers where specified |
| Finish / lubrication | Self-colour/passivated or project-approved finish; state assembly lubricant if controlled |
| Quantity / delivery | Project quantity, destination and required schedule |
| Inspection / documents | MTC, PMI, hardness, TPI, marking and any project-specific documentation |
ASTM A193 controls the bolting material and mechanical requirements; it does not replace the dimensional standard, thread specification or matching-nut requirement. For flange and pressure-equipment work, the RFQ should therefore state both the B8M Class 1 grade and the required product geometry.
B8M Class 1 bolting is commonly paired with ASTM A194 Grade 8M stainless-steel nuts in pressure-service assemblies. The final nut class/condition, dimensions, thread fit and any surface treatment should be specified as part of the complete joint rather than assumed from the bolt grade alone.
Unifit reference: ASTM A194 Grade 8M hex nuts. If the project explicitly calls for the Class 1 nut condition, also review ASTM A194 Grade 8M Class 1 hex nuts.
The molybdenum in B8M gives Type 316 better resistance to chloride pitting and crevice corrosion than the Type 304 chemistry used for B8. That does not make B8M Class 1 a universal solution for every seawater, subsea or highly concentrated chloride service. Temperature, chloride level, crevice geometry, process chemistry and the project corrosion assessment still matter.
Austenitic stainless threads can also gall during tightening, especially when similar stainless surfaces are assembled dry. Thread finish, cleanliness, lubrication and the specified tightening method should therefore be controlled. A universal torque chart should not be copied onto this page because the torque required for a target preload changes with lubrication, coating, thread condition and nut factor.
For more severe chloride service where 6Mo / UNS S31254 is specified, see ASTM A193 B8MLCuN fasteners.
B8M Class 1 is used when the project calls for corrosion-resistant Type 316 bolting in the Class 1 solution-treated condition. Common applications include:
The industry name alone should not decide the grade. The final selection should account for temperature, pressure, chloride concentration, process media, required strength, gasket/joint design and the governing code.
For project bolting, the inspection scope should be agreed before order release. Class 1 material identity, heat/batch traceability and mechanical-property verification should be tied to the purchase specification and applicable ASTM edition.
ASTM A193 B8M Class 1 is Type 316 / UNS S31600 austenitic stainless-steel bolting supplied in the carbide-solution-treated Class 1 condition. It is used for pressure vessels, valves, flanges, fittings and other A193 applications where the project requires B8M chemistry and the baseline Class 1 mechanical properties.
Both use B8M Type 316 chemistry. Class 1 is carbide solution treated. Class 2 receives additional strain hardening to obtain higher, size-dependent strength. Because the processing, properties and marking differ, the class should be stated clearly on the purchase order.
For the Class 1 baseline, the commonly referenced minimums are 75 ksi tensile strength, 30 ksi yield strength, 30% elongation and 50% reduction of area, with a maximum hardness of 223 HBW or 96 HRB. Verify the final requirements against the ASTM edition named in the order.
B8M Class 1 uses Type 316 / UNS S31600 chemistry, but the ASTM A193 designation also controls the bolting specification and Class 1 heat-treatment/mechanical condition. A generic 316 fastener is not automatically an ASTM A193 B8M Class 1 fastener.
ASTM A194 Grade 8M nuts are a common matching specification. The exact nut class or condition, dimensions, thread and finish should be defined in the joint or project specification.
Type 316 provides better chloride pitting and crevice-corrosion resistance than Type 304, but B8M Class 1 should not be treated as universally suitable for all seawater or subsea conditions. Chloride concentration, temperature, crevice conditions and the project corrosion assessment determine whether B8M or a higher-alloy grade is appropriate.
Send the grade/class, product form, diameter, length, thread, dimensional standard or drawing, matching nut/washer requirement, finish or lubrication requirement, quantity, delivery location and any MTC, PMI, hardness, third-party inspection or marking requirement.