In refinery heaters, pressure equipment and other assemblies that remain hot for long periods, austenitic stainless steel can become vulnerable to intergranular corrosion if chromium carbides form at the grain boundaries. ASTM A193 Grade B8C uses Type 347 / UNS S34700 stainless steel stabilized with niobium (historically called columbium) and tantalum, helping keep chromium available for corrosion resistance after elevated-temperature exposure.
Unifit Fastener manufactures ASTM A193 Grade B8C bolts, stud bolts and threaded bolting in India for valves, flanges, pressure vessels, process piping and high-temperature equipment where B8C is specified. B8C is available in Class 1 (carbide solution treated) and Class 2 (carbide solution treated and strain hardened). For the wider ASTM grade family, see ASTM A193 grades and specifications. For generic Type 347/347H products across standards, see Stainless Steel 347 / 347H Fasteners.
Request a Quote | Send ASTM A193 B8C SpecificationB8C is based on Type 347 rather than standard Type 304. Its defining addition is niobium/columbium with tantalum. These stabilizing elements combine preferentially with carbon, reducing the amount of carbon available to form chromium carbides during elevated-temperature exposure.
That stabilization is useful where the bolting may remain in the sensitization temperature range for extended periods or see repeated thermal cycles. It does not make B8C universally superior to every stainless grade. For severe chloride pitting service, a molybdenum-bearing grade such as B8M or a higher alloy may still be more appropriate.
ASTM A193 B8C should also be kept separate from generic 347H material terminology. B8C is tied to the Type 347 / UNS S34700 A193 grade requirements; 347H is a higher-carbon material variant with its own generic material intent.
| Parameter | Details |
|---|---|
| Grade | ASTM A193 Grade B8C |
| Material family | Type 347 / UNS S34700 stabilized austenitic stainless (niobium/columbium + tantalum) |
| Classes | Class 1 and Class 2 |
| Typical product forms | Stud bolts, heavy hex / hex bolts, threaded studs / rods, special / drawing-based bolting |
| Common uses | Valves, flanges, pressure vessels, process piping and high-temperature equipment |
| Matching nuts | ASTM A194 Grade 8C |
| Documentation | EN 10204 3.1 MTC, PMI, hardness or third-party inspection |
The chemistry remains Type 347 in both classes. The difference is the mechanical condition. Class 1 is carbide solution treated. Class 2 receives additional strain hardening to increase tensile and yield strength, with the required values changing as diameter increases.
| Class | Diameter | Tensile min | Yield min | Elong. min | RA min | Hardness max | Marking |
|---|---|---|---|---|---|---|---|
| Class 1 | All diameters | 75 ksi / 515 MPa | 30 ksi / 205 MPa | 30% | 50% | 223 HBW / 96 HRB max | B8C |
| Class 2 | 3/4 in. and under | 125 ksi / 860 MPa | 100 ksi / 690 MPa | 12% | 35% | 321 HBW / 35 HRC max | B8CSH |
| Class 2 | Over 3/4 to 1 in. | 115 ksi / 795 MPa | 80 ksi / 550 MPa | 15% | 35% | 321 HBW / 35 HRC max | B8CSH |
| Class 2 | Over 1 to 1-1/4 in. | 105 ksi / 725 MPa | 65 ksi / 450 MPa | 20% | 35% | 321 HBW / 35 HRC max | B8CSH |
| Class 2 | Over 1-1/4 to 1-1/2 in. | 100 ksi / 690 MPa | 50 ksi / 345 MPa | 28% | 45% | 321 HBW / 35 HRC max | B8CSH |
Class 2 is not simply a higher grade. It is a different strength condition. The higher-strength requirement is most pronounced at smaller diameters, where strain hardening can be developed more effectively through the section.
The B8C chemistry below is specific to the ASTM A193 Type 347 grade and includes the stabilization requirement that is missing from the current Unifit table. Verify final values against the ASTM A193/A193M edition named in the project specification.
| 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) | 17.0 | 19.0 |
| Nickel (Ni) | 9.0 | 12.0 |
| Columbium/Niobium + Tantalum (Cb/Nb + Ta) | 10 x C | 1.10 |
ASTM traditionally uses the term columbium (Cb); niobium (Nb) is the modern name for the same element. The Cb/Nb + Ta requirement is central to B8C's stabilized Type 347 identity.
ASTM A193 controls the B8C bolting material and class. Product geometry, dimensions, thread and the compatible nut specification still need to be stated on the RFQ.
| Product form | Typical use | What to specify |
|---|---|---|
| Stud bolts | Flanges, valves, pressure vessels and process equipment | Diameter, overall length, thread series/pitch, B8C class and nut quantity |
| Heavy hex / hex bolts | Pressure equipment and assemblies requiring a headed bolt | Diameter, length, head/dimensional standard, thread and B8C class |
| Threaded studs / rods | Custom-length threaded bolting | Diameter, length, thread, end configuration and class |
| Special / drawing-based bolting | Project-specific geometry or tolerances | Latest drawing, B8C class, dimensions, finish and inspection plan |
For generic geometry, use industrial bolts and stud bolts.
| RFQ field | What to send |
|---|---|
| Grade / class | ASTM A193 Grade B8C Class 1 or Class 2; include the required ASTM edition when project-controlled |
| Product form | Stud bolt, heavy hex/hex bolt, threaded stud/rod or drawing-based component |
| Diameter / length | Metric or inch; Class 2 strength is diameter-dependent |
| Thread | UNC, UNF, 8UN or metric; pitch/TPI and class/fit |
| Dimensional standard | ASME/ANSI, ISO, DIN or project drawing |
| Matching components | ASTM A194 Grade 8C nut requirement and washers where specified |
| Finish / lubricant | Self-colour/passivated or project-approved condition; state lubricant if tightening is controlled |
| Quantity / delivery | Quantity, destination and required schedule |
| Inspection / documents | MTC, PMI, mechanical/hardness testing, TPI, marking and project documentation |
Identifying different fastener types allows buyers to pick the exact products tailored to their unique industrial application needs.
ASTM A194 Grade 8C is the corresponding Type 347 stainless nut grade commonly used with A193 B8C bolting when the joint specification calls for that pairing. For Class 2 bolting, any required strain-hardened nut condition or supplementary requirement should be stated under the applicable A194 purchase specification rather than assumed from the bolt class.
B8C's main material advantage is stabilization against sensitization-related intergranular corrosion after elevated-temperature exposure. It should not be positioned as a universal chloride or seawater grade. Type 347 does not have the molybdenum content of Type 316/B8M, so chloride pitting conditions may point to B8M or a higher-alloy material.
Like other austenitic stainless fasteners, B8C threads can gall during tightening. Thread finish, cleanliness, lubrication and the tightening procedure should be controlled. Do not publish one universal B8C torque chart without defining the thread size, lubricant/coating, nut factor and target preload because these variables materially change torque-tension behaviour.
B8C is used where the equipment or piping specification calls for stabilized Type 347 ASTM A193 bolting. Typical applications include:
The application list is not a substitute for engineering selection. Operating temperature, pressure, process chemistry, chloride exposure, required strength, gasket/joint design and the governing code determine the final grade.
| Grade | Material | Key alloy feature | Selection emphasis | Owner |
|---|---|---|---|---|
B8C |
Type 347 / UNS S34700 |
Niobium/columbium + tantalum stabilized |
Elevated-temperature service where stabilized 347 is specified |
THIS PAGE |
B8T |
Type 321 / UNS S32100 |
Titanium stabilized |
Alternative stabilized austenitic grade; different stabilizing element |
|
B8 |
Type 304 / UNS S30400 |
Unstabilized 18-8 stainless |
General A193 austenitic stainless service |
|
B8M |
Type 316 / UNS S31600 |
Molybdenum bearing |
Improved chloride pitting resistance vs 304/347; not a stabilized 347 grade |
Related grade pages: ASTM A193 B8T fasteners | ASTM A193 Grade B8 fasteners | ASTM A193 Grade B8M fasteners.
B8C receiving inspection should confirm the grade, class and traceability. For Class 2, B8CSH marking is a key indicator of the strain-hardened condition.
ASTM A193 Grade B8C is Type 347 / UNS S34700 austenitic stainless-steel bolting stabilized with niobium/columbium and tantalum. It is used in high-temperature, high-pressure and other special-purpose A193 applications where the project specifies stabilized Type 347 bolting.
Both classes use the same B8C Type 347 chemistry. Class 1 is carbide solution treated and has a 75 ksi minimum tensile and 30 ksi minimum yield baseline. Class 2 receives additional strain hardening, is marked B8CSH and has higher diameter-dependent mechanical requirements.
B8CSH is the ASTM A193 marking used for Grade B8C Class 2 strain-hardened bolting. The SH identifies the strain-hardened condition and should be checked against the order documentation during receiving inspection.
B8C uses Type 347 / UNS S34700 chemistry, but ASTM A193 also controls the bolting class, mechanical properties, testing and marking. A generic 347 fastener is therefore not automatically ASTM A193 B8C compliant.
No. B8C is tied to the Type 347 / UNS S34700 ASTM A193 grade requirements. Stainless Steel 347H is a higher-carbon material variant with separate generic material intent. Do not use 347H as an automatic synonym for B8C.
Both are stabilized austenitic A193 grades used for elevated-temperature service. B8C is based on Type 347 and uses niobium/columbium plus tantalum stabilization. B8T is based on Type 321 and uses titanium stabilization. The project specification determines which grade is required.
ASTM A194 Grade 8C is the corresponding Type 347 stainless nut grade commonly paired with A193 B8C bolting when specified. Nut dimensions, thread fit and any supplementary requirements should be confirmed as part of the complete joint.
Send the B8C class, product form, diameter, length, thread, dimensional standard or drawing, matching nut/washer requirement, finish or lubricant, quantity, delivery location and any MTC, PMI, mechanical testing, third-party inspection or marking requirement.