ASTM A193 Grade B8C Class 1 (347 Stainless)
347 stainless bolting stabilised with niobium against sensitisation — for hot hydrogen, refinery and welded stainless service between 800 and 1500 °F.
What A193 B8C Class 1 is.
ASTM A193 Grade B8C is bolting made from Type 347, an 18-8 stainless with niobium (columbium) added to tie up carbon. Ordinary 304 can sensitise, forming chromium carbides at the grain boundaries, when it spends time between roughly 800 and 1500 °F; stabilised 347 does not, so it keeps its intergranular corrosion resistance through prolonged hot service and welding.
B8C is the stainless bolting grade for refinery hydrogen units, hot process piping in 347 and 321, and high-temperature exhaust and flue systems. Class 1 is solution treated; Class 2 is strain hardened for strength in small sizes. We machine it from certified 347 bar and pair it with ASTM A194 Grade 8C nuts.
Chemical composition.
| Element | Symbol | Min % | Max % |
|---|---|---|---|
| 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 |
| Niobium (columbium) | Nb | 10 × C | 1.10 |
Values per ASTM A193/A193M-24, Tables 1 and 2 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team.
Mechanical properties.
| Nominal diameter (in.) | Tensile strength, min (ksi) | Yield strength 0.2% offset, min (ksi) | Elongation in 4D, min (%) | Reduction of area, min (%) | Hardness, max |
|---|---|---|---|---|---|
| Class 1 — all diameters | 75 | 30 | 30 | 50 | 223 HBW / 96 HRB |
| Class 1A (B8CA) — all diameters | 75 | 30 | 30 | 50 | 192 HBW / 90 HRB |
Values per ASTM A193/A193M-24, Tables 1 and 2 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team. Class 1 sizes ¾ in. and smaller may be 241 HBW / 100 HRB. Strain-hardened Class 2 properties are on the B8C Class 2 page.
Specifications that apply.
How A193 B8C Class 1 ships from Houston.
- Full-thread studs and stud bolts
- Double-end and tap-end studs
- Hex and heavy hex bolts
- Socket head cap screws
- Threaded rod (cut and rolled thread)
- Machined components from certified bar
A193 B8C Class 1 fasteners we make.
Every item is manufactured to your drawing or standard in A193 B8C Class 1, imperial or metric, with an EN 10204 3.1 certificate.
What is ASTM A193 B8C?
B8C is A193's Type 347 grade. Niobium has a stronger affinity for carbon than chromium does, so in 347 the carbon is locked into niobium carbides and the chromium stays in solution where it protects the steel. That makes 347 immune to the sensitisation that can leave 304 vulnerable to intergranular attack after welding or long exposure between 800 and 1500 °F. Mechanically it behaves like B8: Class 1 is soft and ductile, Class 2 is cold worked to up to 125 ksi in small diameters.
B8C or B8T?
Both are stabilised 18-8 grades and are interchangeable in most bolting. 347 (niobium) is slightly stronger at high temperature and is the common choice in refinery and power piping specified as 347; 321 (titanium) is more common where the pressure equipment itself is 321. Match the bolting to the flange material unless the specification says otherwise.
Nuts and galling
Use ASTM A194 Grade 8C nuts with B8C studs. As with every austenitic stainless joint, threads must be lubricated or coated to prevent galling; anti-seize, PTFE coating or silver-plated nuts are all available.
Testing, certification and marking
- Tensile and hardness tested per heat-treat lot on the finished product
- Grade symbol B8C and manufacturer's identification on every piece
- EN 10204 3.1 material test report with heat number and chemistry; 3.2 witness available
- Intergranular corrosion testing (ASTM A262) and PMI arranged on request
Recent A193 B8C Class 1 jobs.
ASTM A193 Grade B8C Class 2 (347 Strain Hardened)
Niobium-stabilized 347 bolting cold worked to 125 ksi tensile in ¾ in. and under — strength for moderate-temperature joints that must not sensitize.
ASTM A194 / ASME SA-194ASTM A194 Grade 8C (347 Stainless)
347 stainless heavy hex nut for B8C studs — stabilised against sensitisation for hot hydrogen, refinery and welded stainless service.
ASTM A193 / ASME SA-193ASTM A193 Grade B8T Class 1 (321 Stainless)
321 stainless bolting stabilised with titanium — the match for 321 piping and vessels in hot, cyclic and welded service.