ASTM A193 Grade B8 is the 304 stainless steel bolting spec for pressure vessels, valves, flanges, and piping in corrosive or elevated-temperature service. It's one of the most common stainless fastener specifications in industrial work — and one of the most frequently ordered without a close read of what you're actually getting.
The issue isn't the alloy. It's the class. B8 comes in Class 1 and Class 2, and those two classes aren't just slightly different — the strength numbers are so far apart that the same spec number can describe bolts that behave completely differently under load. Most ordering problems in B8 trace back to someone specifying the grade but not the class.
What A193 B8 covers
A193 B8 defines austenitic stainless steel bolting — studs, threaded rod, and hex bolts — manufactured from 304 stainless (AISI 304, UNS S30400), for service in pressure equipment and piping operating at elevated temperature or pressure. The material is non-magnetic in the annealed state. Some magnetism can develop after cold working, which is normal and expected in Class 2.
B8 is usable up to approximately 1000°F (538°C) with declining allowable stress above that range. For design temperatures below -20°F (-29°C), use A320 instead — that's the low-temperature companion spec and it has different impact toughness requirements.
Class 1 vs Class 2 — the decision that actually matters
Class 1 is solution-annealed. The material is heated and quenched to put the carbides back into solution, which maximizes corrosion resistance and ductility. This is the softer state — 75 ksi minimum tensile strength, 30 ksi minimum yield, 30% minimum elongation. It's the right choice when the environment is the threat and load is secondary: food and pharmaceutical, cleanroom service, dilute acid contact, elevated temperature where you need the full corrosion resistance of the alloy.
Class 2 is strain-hardened. The material is worked beyond yield to develop strength without heat treatment — the stainless equivalent of cold drawing. This is where B8 gets real mechanical capacity, and this is where most people don't read the spec carefully enough.
Class 2 strength is diameter-dependent, and the drop is significant. At 3/4" and under: 125 ksi minimum tensile, 100 ksi minimum yield. At 7/8" through 1": 115 ksi tensile, 80 ksi yield. At 1-1/8" through 1-1/4": 105 ksi tensile, 65 ksi yield. At 1-3/8" through 1-1/2": 100 ksi tensile, 50 ksi yield.
That's a 50 ksi yield swing between a 3/4" stud and a 1-1/2" stud, both legitimately called "B8 Class 2." Cold working doesn't penetrate the full cross-section of larger diameters the way it does in small ones, so the spec accommodates what heat treatment can reliably achieve. The practical takeaway: always confirm the diameter-specific strength numbers before using Class 2 in a load calculation. "B8 Class 2 = 125 ksi" is only true up to 3/4".
The 304 vs 316 question
Most engineers ask this early in a project. B8 is 304; B8M is 316. The common instinct is that 316 is "more corrosion-resistant" because it's the upgrade. The reason isn't what most people think.
304 carries 18–20% chromium. 316 actually has slightly less — 16–18% chromium. What 316 adds is 2–3% molybdenum, and that molybdenum is what gives it dramatically better resistance to chloride pitting and crevice corrosion. So the rule isn't "more chromium is better." It's that 316's molybdenum is what matters in saltwater, coastal air, pool chemistry, cleaning agents, and chemical process service.
For dry or mildly corrosive service — food equipment in low-chloride environments, general chemical processing away from halide chemistry, elevated temperature without chloride exposure — B8 (304) is the correct specification and costs less. Move to B8M (316) when chlorides are present.
Compatible nuts
B8 bolts pair with ASTM A194 Grade 8 nuts, manufactured from 304 stainless. For Class 1 bolts, Grade 8 Class 1 nuts. For Class 2 bolts, Grade 8 Class 1 or Class 2 nuts. The alloy match matters — don't mix stainless grades between bolt and nut in corrosive service, and don't use carbon steel nuts with stainless bolts in wet environments.
The galling problem that isn't in the spec
One field reality A193 won't tell you: stainless nuts on stainless studs gall. The threads cold-weld to each other under torque, seize, and the assembly is ruined — irreversible and common, especially with faster installation tools and longer thread engagement.
It catches crews accustomed to plated carbon steel. Use a nickel-based anti-seize on any stainless assembly you plan to torque. This applies to B8, B8M, and any other austenitic stainless in the spec.
The L-grade (304L)
304L is a low-carbon variant of 304 — 0.030% carbon maximum versus 0.08% for standard 304. The lower carbon prevents carbide precipitation at grain boundaries during welding, which eliminates sensitization and the intergranular corrosion that follows it in heat-affected zones.
Specify 304L when the fastener will be welded in service or when the application is in a corrosive environment and post-weld corrosion resistance needs to be maintained. Dual-certified 304/304L material meets both grades simultaneously and is standard commercial practice — your cert will show both designations.
How B8 relates to F593
If you work with both pressure piping and general construction, you'll encounter both A193 B8 and ASTM F593 Group 1 — same base alloy (304 stainless), different spec intent. B8 is for pressure vessels, valves, flanges, and piping under ASME code. F593 is for general corrosion-resistant structural and mechanical applications. A drawing calling for stainless bolts on a flange in a code-governed system will say B8; one calling for stainless bolts on an architectural railing or food equipment frame will say F593 Group 1. Know which world your application lives in.
What California Fastener supplies
We stock A193 B8 in both Class 1 and Class 2, with matching A194 Grade 8 nuts, across a full range of diameters and thread pitches in stud bolt, threaded rod, and hex bolt configurations. Mill certs ship with every order, traceable to heat number and confirming chemistry, mechanical properties by diameter, and carbide solution treatment records for Class 1. Pre-kitted bolt-and-nut packages are available for flange assemblies.
Contact California Fastener to discuss A193 B8 specifications, quantities, and lead times.
