The short answer: ASTM A193 Grade B8 is Type 304 stainless-steel bolting. Class 1 is carbide solution treated; Class 2 is solution treated and strain hardened for higher strength. Class 2B is a separate, lower-strength strain-hardened class that extends the available B8 diameter range through 3 in and has its own mechanical-property schedule. An order that says only “A193 B8” does not fully define the required strength condition.[1] [2]
For an alloy-steel-versus-stainless selection, read ASTM A193 B7 vs B8. This guide explains the B8 class decision in detail.
What material is ASTM A193 Grade B8?
B8 is the Type 304 austenitic stainless-steel grade within ASTM A193/A193M. It is intended for bolting used with pressure vessels, valves, flanges, fittings and other high-temperature, high-pressure or special-purpose applications covered by the specification.[1]
“304 stainless,” “B8” and “B8 Class 2” are not interchangeable descriptions. Type 304 identifies the stainless alloy family. Grade B8 adds ASTM A193 manufacturing, testing, marking and certification requirements. The class then identifies the processing condition and mechanical-property level.
| Callout | What it establishes | What it does not establish |
|---|---|---|
| Type 304 stainless steel | Alloy family | ASTM A193 compliance or B8 class |
| ASTM A193 Grade B8 | A193 Type 304 bolting grade | Complete class selection when strength matters |
| ASTM A193 Grade B8 Class 1, Class 2 or Class 2B | Grade plus processing/strength condition | Product dimensions, nut grade or service suitability |
B8 Class 1 vs Class 2 vs Class 2B
Class 1 is carbide solution treated. Class 2 receives solution treatment followed by strain hardening, which increases strength. Class 2B is not a continuation of the Class 2 table: it is a separately defined class with a different diameter range and strength schedule.[1] [2]
| Grade, class and nominal diameter | Minimum tensile strength | Minimum yield strength |
|---|---|---|
| B8 Class 1, all diameters covered by the specification | 75 ksi | 30 ksi |
| B8 Class 2, up to and including 3/4 in | 125 ksi | 100 ksi |
| B8 Class 2, over 3/4 through 1 in | 115 ksi | 80 ksi |
| B8 Class 2, over 1 through 1 1/4 in | 105 ksi | 65 ksi |
| B8 Class 2, over 1 1/4 through 1 1/2 in | 100 ksi | 50 ksi |
| B8 Class 2B, up to and including 2 in | 95 ksi | 75 ksi |
| B8 Class 2B, over 2 through 2 1/2 in | 90 ksi | 65 ksi |
| B8 Class 2B, over 2 1/2 through 3 in | 80 ksi | 55 ksi |
The boundary wording matters. A 3/4 in Class 2 stud uses the 125/100 ksi row; a diameter greater than 3/4 in through 1 in uses the 115/80 ksi row. The table gives material-specification minimums, not allowable design stresses at temperature.
How to select the B8 class
Choose the class from the joint calculation and governing specification, not from the word “stainless.”
| Requirement | Starting point | Check before ordering |
|---|---|---|
| Type 304 is accepted and Class 1 strength is adequate | B8 Class 1 | Required preload, design temperature and joint code |
| Type 304 is accepted and higher strength is required through 1 1/2 in | B8 Class 2 | Exact diameter row, manufacturing availability and nut requirement |
| The design explicitly calls for the Class 2B range | B8 Class 2B | Exact diameter row and complete class designation |
Do not “upgrade” Class 1 to Class 2 without checking the design and procurement documents. Class 2 is a different manufacturing condition with different mechanical properties and supply constraints, not simply a higher number. Conversely, a Type 304 test report without the A193 grade, class and applicable tests does not establish B8 Class 2 compliance.
Corrosion resistance does not remove application limits
B8 provides the corrosion behavior of Type 304 stainless steel, but it is not suitable for every corrosive medium. Chloride-bearing or more aggressive environments may require a molybdenum-bearing stainless grade such as the Type 316 route, subject to the project corrosion review. See 304 vs 316 Stainless Steel Fasteners for that alloy comparison.
B8 and ASTM A193 Grade B8M are also different material grades: B8 is Type 304, while B8M is Type 316. The B8M guide also explains where the B8C Type 347 and B8T Type 321 stabilized grades fit. These grades should not be substituted merely because they are all austenitic stainless steels.[5]
Product form, marking and nut pairing
B8 can be ordered as studs, threaded rod or headed bolts. For inch-series A193 bolting, the usual default thread requirements are ASME B1.1 Class 2A, with coarse threads through 1 in and 8-thread series above 1 in unless otherwise specified.[3] State the exact product form, diameter, pitch and length on the RFQ.
Class and certification must agree. The Grade B8 symbols are B8 for Class 1, underlined B8SH for Class 2, and B9 for Class 2B, together with the manufacturer identification and subject to the standard’s small-product and alternate-location provisions. Receiving inspection should match the physical marking to the material test report and the ASTM edition stated on the order.[1] [2] [3]
ASTM A194 Grade 8 nuts are the common companion to B8 bolting. Use the compatibility guide to confirm the class, nut condition and any S1 requirement; it also covers lubrication and galling control.[4]
B8 details to confirm before ordering
Write the complete B8 class and nominal diameter. Check the resulting strength row and identification symbol against the certificate before accepting the lot. Use the assembly ordering examples to add the nut, matching thread, dimensions, service conditions and quantity.
Requesting B8 bolting? Include the full class, diameter, thread, length, product form, nut requirement, quantity and service conditions. HexFastener can check whether the requested configuration and documentation are defined 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 scope, solution-treatment and strain-hardening routes, and purchaser requirements.
- Portland Bolt. ASTM A193 Grades. B8 Class 1 and Class 2 processing, marking and mechanical-property summary.
- Portland Bolt. ASTM A193. Product configurations, default inch-series threads and identification marking.
- Portland Bolt. A193 Nut Compatibility. Common A193/A194 pairings and the optional strain-hardened nut for Class 2 stainless bolting.
- Portland Bolt. ASTM A193 B8 vs B8M. Type 304 and Type 316 grade distinction.