In high-temperature flange and pressure-equipment service, strength is only one part of the bolting requirement. Some projects also place a strict limit on hardness. ASTM A193 Grade B6X is the controlled-hardness version of the 410-type stainless B6 family: it uses UNS S41000 chemistry, with minimum tensile and yield requirements of 90 ksi and 70 ksi, while limiting hardness to 26 HRC maximum. This separates B6X from standard Grade B6, which uses the same 13% chromium chemistry but is specified at higher minimum strength.
Unifit Fastener manufactures ASTM A193 B6X bolts, stud bolts and threaded bolting in India for refinery, petrochemical, valve, pressure-equipment and high-temperature process applications where the project specification calls for B6X. For the wider grade family, see ASTM A193 grades and specifications. For the broader product range, see our industrial fasteners page.
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Technical Data
Why ASTM A193 Grade B6X Is Different from B6
B6 and B6X share the same 410 / UNS S41000 chemistry, so the distinction is not created by adding a different alloying element. It comes from the mechanical-property and hardness requirements applied to the finished bolting. B6X lowers the minimum strength requirement and introduces a 26 HRC maximum hardness limit. That controlled-hardness requirement is the reason B6X should have its own page and its own procurement intent.
For engineers and buyers, the practical question is therefore not “Is B6X better than B6?” It is “Which condition does the project specification require?” B6 is the higher-strength option in this 13% chromium family; B6X is the lower-hardness condition. Do not substitute one for the other without engineering and purchaser approval.
For full B6 technical information, use the dedicated ASTM A193 Grade B6 fasteners page rather than duplicating it here.
ASTM A193 B6X Product Forms
The grade defines the bolting material and mechanical requirements; the product geometry, dimensions, thread and matching hardware still have to be specified separately. Keep this page focused on the industrial forms most relevant to A193 service.
For geometry-led information across multiple materials, use the industrial bolts and stud bolts pages.
ASTM A193 B6X Chemical Composition
ASTM A193 groups B6 and B6X under 410 / UNS S41000 chemistry. The chemistry below is the grade-level composition used for B6X procurement. Confirm it against the ASTM edition named in the purchase specification.
The 11.5-13.5% chromium range gives B6X its 410-type stainless chemistry, but this should not be presented as equivalent to 304/316 corrosion performance. The service environment still has to be checked against the actual corrosion mechanism and project requirement.
Heat Treatment and Mechanical Properties of Grade B6X
B6X belongs to the ferritic-steel group in ASTM A193 and is heat treated to develop the required mechanical properties while staying within the maximum hardness limit. Public A193/A193M-24 data shows a minimum tempering temperature of 1100°F (593°C) for B6X in the listed size range. The final heat-treatment route and test results should be verified against the edition specified on the order.
Important hardness note
The 26 HRC maximum is a core B6X requirement. Do not state that every B6X fastener is 100% hardness tested unless Unifit’s actual inspection plan or project specification requires and documents 100% testing. Publish only the test frequency Unifit can verify.
B6X Bolting with ASTM A194 Grade 6 Nuts
ASTM A193 controls the B6X bolting material. The matching nut is specified separately under ASTM A194. For the B6 family, ASTM A194 Grade 6 heavy hex nuts are a common matching specification. The complete bolted joint still needs the correct nut dimensions, thread class, coating or lubricant and project tightening procedure.
Related Unifit page: ASTM A194 Grade 6 hex nut.
Where B6X Is Specified - and Where It Needs More Engineering Review
B6X should be described through the actual bolting requirement, not through a broad industry list. Typical project contexts include high-temperature or high-pressure valves, flanges, fittings and pressure equipment where a 410-type stainless grade with controlled hardness is specified.
- Refinery and petrochemical flange assemblies - where the line class or project bolting specification explicitly calls for A193 B6X.
- Valves and pressure equipment - headed bolts or studs requiring the B6X strength/hardness condition.
- High-temperature process service - where 13% chromium bolting is required and the design calls for B6X rather than standard B6.
- Selected H2S / sour-service requirements - the 26 HRC cap can be relevant to hardness-controlled service, but NACE MR0175 / ISO 15156 suitability must be confirmed for the actual environment and project specification.
Do not over-position B6X
Do not call B6X a universal sour-service, seawater, marine or low-temperature fastener. ASTM A193 is not the low-temperature bolting family, and 410-type stainless does not provide the chloride resistance of 316-based or higher-alloy stainless grades. Material selection remains project-specific.
For another controlled-hardness A193 option in chromium-molybdenum alloy steel, see ASTM A193 Grade B7M fasteners. The material family and service logic are different, so the two pages should not share the same primary keyword set.
Marking, Inspection and Traceability
For critical B6X bolting, procurement confidence comes from traceability and verified test data rather than generic quality claims. The purchase order should identify the required marking, inspection and documentation before production or dispatch.
- Grade and manufacturer identification - use the marking required by the applicable ASTM/A962 requirements and project drawing.
- Heat / lot traceability - connect finished bolting to the source material and test records.
- Chemistry and mechanical results - verify B6X composition, tensile/yield, elongation, reduction of area and hardness.
- Dimensional / thread inspection - diameter, length, thread pitch/TPI, class/fit and head/stud geometry against the order standard.
- Inspection certificate / MTC - where ordered, provide EN 10204 3.1 documentation or the agreed material certificate. Treat EN 10204 3.1 as an inspection document, not a company certification.
- PMI or third-party inspection - include only where the buyer/project specifies it and Unifit has accepted the scope.
What is ASTM A193 Grade B6X?
ASTM A193 Grade B6X is a 410 / UNS S41000 stainless bolting grade for high-temperature, high-pressure and special-purpose service. Its key distinguishing requirement is a maximum hardness of 26 HRC, with minimum tensile and yield strengths of 90 ksi and 70 ksi in the listed size range.
What is the difference between ASTM A193 B6 and B6X?
B6 and B6X share the same 410 / UNS S41000 chemistry, but their mechanical requirements differ. B6 has higher minimum tensile and yield strength, while B6X has lower minimum strength and a 26 HRC maximum hardness limit. The correct grade should follow the project specification; they should not be substituted without approval.
Is ASTM A193 B6X the same as 410 stainless steel?
B6X uses 410-type chemistry identified as UNS S41000 under ASTM A193, but the B6X designation also defines bolting-specific heat-treatment and mechanical-property requirements. A generic 410 stainless component is therefore not automatically ASTM A193 B6X bolting.
Which nuts are commonly used with ASTM A193 B6X studs?
ASTM A194 Grade 6 heavy hex nuts are a common matching specification for the B6 family. Confirm the nut grade, dimensions, finish, marking and project temperature requirements as part of the complete bolted joint.
Is B6X automatically NACE MR0175 / ISO 15156 compliant?
No. The B6X 26 HRC limit can be relevant to hardness-controlled sour-service requirements, but material suitability still depends on the actual H2S environment, heat treatment, stress, temperature, project requirements and the applicable NACE / ISO 15156 rules. Do not treat the grade name alone as proof of compliance.
Is B6X suitable for seawater or marine service?
Do not assume this from the stainless-steel designation. B6X is based on 410-type stainless chemistry, which does not provide the chloride pitting resistance of 316-based or higher-alloy grades. The corrosion environment and project material specification should determine the final choice.
What information should I send for a B6X quotation?
Send the B6X grade, product form, diameter, length, thread, dimensional standard or drawing, matching nut/washer requirement, finish or lubricant, service condition, quantity, delivery location and any MTC, PMI, hardness, third-party inspection or marking requirement.