ASTM A320 Grade B8F (303 Free-Machining Stainless)
303 free-machining stainless bolting certified to A320 — sulfur or selenium for fast, clean machining of complex cold-service fasteners, impact exempt above −325 °F.
What A320 B8F is.
ASTM A320 B8F is the free-machining austenitic grade, 303 or 303Se, and it exists only in A320: ASTM A193 has no Grade B8F. Its room-temperature strength table is the same as B8, and A320 adds the low-temperature framework: impact-testing rules, the requirement for matching austenitic A194 Grade 8F nuts, and certification to the specification piping and vessel codes call out for sub-zero design temperatures.
B8F is the only A320 grade that exists specifically for machinability: 303 carries 0.15 percent minimum sulfur, or 303Se 0.15 to 0.35 percent selenium, which breaks chips and lets complex fasteners be turned at production speeds. The price is lower corrosion resistance than 304 and no weldability, so B8F is specified for machined studs, plugs and instrument fasteners rather than pressure-flange bolting. A320 has no A193 equivalent grade for B8F.
Chemical composition.
| Element | Symbol | Min % | Max % |
|---|---|---|---|
| Carbon | C | — | 0.15 |
| Manganese | Mn | — | 2.00 |
| Phosphorus | P | — | 0.20 |
| Sulfur (303) | S | 0.15 | — |
| Sulfur (303Se) | S | — | 0.06 |
| Silicon | Si | — | 1.00 |
| Chromium | Cr | 17.0 | 19.0 |
| Nickel | Ni | 8.0 | 10.0 |
| Selenium (303Se only) | Se | 0.15 | 0.35 |
Values per ASTM A320/A320M-26, Tables 1 and 3 (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 2 in., min (%) | Reduction of area, min (%) | Hardness, max |
|---|---|---|---|---|---|
| Class 1 — all diameters | 75 | 30 | 30 | 50 | 223 HBW / 96 HRB |
| Class 1A (B8FA) — all diameters | 75 | 30 | 30 | 50 | 192 HBW / 90 HRB |
| Class 2 — ¾ and under | 125 | 100 | 12 | 35 | 321 HBW / 35 HRC |
| Class 2 — over ¾ to 1 | 115 | 80 | 15 | 35 | 321 HBW / 35 HRC |
| Class 2 — over 1 to 1¼ | 105 | 65 | 20 | 35 | 321 HBW / 35 HRC |
| Class 2 — over 1¼ to 1½ | 100 | 50 | 28 | 45 | 321 HBW / 35 HRC |
Values per ASTM A320/A320M-26, Tables 1 and 3 (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. Class 2 is limited to 1½ in. Strain-hardened austenitic steel may not show uniform properties through the section, particularly over ¾ in. diameter.
| Grade and class | Impact test exempt above | Below that temperature |
|---|---|---|
| B8, B8C, B8P, B8LN — Class 1 and 1A | −425 °F (−254 °C) | Impact tested to agreed criteria |
| B8M, B8T, B8F, B8MLN — Class 1 and 1A | −325 °F (−198 °C) | Impact tested to agreed criteria |
| All austenitic grades — Class 2 | −325 °F (−198 °C) | Impact tested to agreed criteria |
| Any grade ½ in. and smaller | Exempt at all temperatures | Unless S1 is specified |
Per ASTM A320/A320M-26 (verified against the specification). When impact tests are required for austenitic grades, the test temperature and acceptance criteria are agreed between purchaser and supplier; Supplementary Requirement S1 may be ordered.
Specifications that apply.
How A320 B8F 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
A320 B8F fasteners we make.
Every item is manufactured to your drawing or standard in A320 B8F, imperial or metric, with an EN 10204 3.1 certificate.
What is ASTM A320 B8F?
A320 B8F is free-machining austenitic stainless bolting certified for low-temperature service. The specification allows two chemistries: 303 (UNS S30300), where sulfur forms manganese sulfide stringers that act as chip breakers, and 303Se (UNS S30323), where selenium does the same job with a better surface finish and slightly better corrosion resistance. Both keep the toughness of the 18-8 family at low temperature, with impact exemption above −325 °F. The sulfide inclusions reduce pitting resistance, so B8F is kept away from chloride service. Class 1, Class 1A (B8FA) and Class 2 (B8FSH) are available, with A194 Grade 8F nuts.
Impact testing under A320
- Class 1 and 1A material of this grade is exempt from impact testing above −325 °F. Below that, the test temperature and acceptance values are agreed between purchaser and supplier, or ordered under Supplementary Requirement S1.
- Class 2 (strain hardened) austenitic bolting is exempt above −325 °F.
- Bolting ½ in. and smaller is exempt at all temperatures unless S1 is specified.
- Nuts follow 10.2: matching A194 austenitic grades, with the same exemption temperatures.
Why B8F is an A320-only grade
A193 does not list a free-machining austenitic grade, so a 303 bolt or stud that must carry a bolting-specification certificate is ordered to A320 B8F. The chemistry follows A582, the strength table is the B8 table, and the impact exemption for Class 1 and 1A is −325 °F. Because the sulfur or selenium that makes 303 machinable also lowers its pitting resistance and weldability, B8F is used for machined fittings, plugs and instrument fasteners in mild environments rather than for pressure-flange studs, where B8 or B8M is the right grade.
Testing, certification and marking
- Tensile and hardness tested per lot on the finished product
- Grade symbol B8F (B8FSH for Class 2) and manufacturer's identification on every piece
- EN 10204 3.1 material test report with heat number and chemistry; 3.2 witness available
- Impact test results, or the 6.2.1.1 exemption, stated on the certificate
- Nuts from A194 Grade 8F, impact tested when the service temperature requires it
Recent A320 B8F jobs.
ASTM A320 Grade B8 Class 1 (304 Stainless)
304 stainless bolting certified to the low-temperature specification — the same 75 ksi solution-treated material as A193 B8, with A320 impact rules and matching A194 Grade 8 nuts.
ASTM A320 / ASME SA-320ASTM A320 Grade B8M Class 1 (316 Stainless)
316 stainless bolting certified to the low-temperature specification — chloride resistance for LNG terminals and coastal cold service, impact exempt above −325 °F.
ASTM A320 / ASME SA-320ASTM A320 Grade B8 Class 2 (304 Strain Hardened)
Strain-hardened 304 bolting certified to A320 — 125 ksi tensile in ¾ in. and under, for cryogenic joints that need more bolt strength than annealed stainless.