The short answer
- ASTM A193 B7: general high-temperature and high-pressure bolting. Chromium-molybdenum steel, quenched and tempered.
- ASTM A193 B7M: the same steel, tempered higher and hardness-limited so it resists sulphide stress cracking in sour (H₂S) service, for example under NACE MR0175 / ISO 15156. It is weaker than B7.
- ASTM A320 L7: similar steel, impact tested at low temperature for cold service.
Mechanical properties (diameters up to 2½ in)
| Grade | Min. tensile | Min. yield | Hardness | Special test | Matching nut |
|---|---|---|---|---|---|
| A193 B7 | 125 ksi (860 MPa) | 105 ksi (720 MPa) | max. 35 HRC (321 HB) | – | A194 2H |
| A193 B7M | 100 ksi (690 MPa) | 80 ksi (550 MPa) | max. 235 HB (99 HRB), every stud tested | Hardness on 100% of studs | A194 2HM |
| A320 L7 | 125 ksi (860 MPa) | 105 ksi (720 MPa) | – | Charpy impact at −150 °F (−101 °C) | A194 4 or 7, often with S3 impact test |
For B7, the strength steps down for larger sizes: over 2½ in up to 4 in the minimums are 115 ksi tensile and 95 ksi yield, and over 4 in up to 7 in they are 100 ksi and 75 ksi. B7M keeps 100 ksi and 80 ksi up to 4 in. Always check the current edition of the standard for your size.
Nuts: 2H and 2HM
B7 studs are paired with A194 grade 2H heavy hex nuts (hardness 248 to 327 HB). For sour service, B7M studs need 2HM nuts, which are hardness-limited (159 to 237 HB) in the same way. Mixing a B7M stud with a standard 2H nut defeats the point of buying B7M.
How to tell them apart
Studs are marked on one end with the grade (B7, B7M, L7) and the maker's identification; nuts carry 2H, 2HM, 4 or 7. B7M studs are usually also identified by colour or an extra mark as the purchaser specifies. Ask for an EN 10204 3.1 certificate that shows chemistry, heat treatment, tensile results and, for B7M, the hardness of every stud.
Coatings
Plain (black) B7 is common for indoor plant. For corrosion protection, PTFE (fluoropolymer) coated studs and nuts are widely used offshore and in chemical plants, and hot-dip galvanised studs need oversize-tapped nuts. For hot-dip galvanising and electroplating of high-strength studs, ask about the risk of hydrogen embrittlement.
Before you order
- Flange standard and class (for example ASME B16.5 class 300), which sets stud diameter, length and quantity
- Service: sour, low temperature, or high temperature
- Grade of stud and nut, coating, and certificate type
- Any end-marking, colour code or third-party inspection your client requires
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.