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What is the difference between B7 and B7M stud bolts?
Short answer
B7M is a hardness-limited version of B7, tempered to a maximum of 235 HBW with every piece hardness-tested, so it resists sulphide stress cracking in sour (H₂S) service under NACE MR0175. The trade-off is lower strength, so joint loads must be checked when substituting B7M for B7.
B7 vs B7M at a glance
| Property | B7 | B7M |
|---|---|---|
| UNS | — | — |
| EN | — | — |
| Family | — | — |
| C (%) | 0.37 – 0.49 | 0.37 – 0.49 |
| Cr (%) | 0.75 – 1.20 | 0.75 – 1.20 |
| Mo (%) | 0.15 – 0.25 | 0.15 – 0.25 |
| Tensile strength | — | — |
| Yield strength | — | — |
| PREN | — | — |
| Density | — | — |
| Magnetic | — | — |
| Typical use | Process piping flanges (ASME B16.5 / B16.47) | Sour gas and oil processing |
Which should you choose?
There's no universal "winner": the right choice depends on service conditions, codes and cost. Share your conditions with us for a recommendation.
Matching nuts
Marking
B7M studs carry the "B7M" marking and nuts "2HM". Check marking against the certificate on receipt, because the two look the same.
Products in both grades
Related guides, comparisons and articles
Frequently asked questions
Is B7M required for all sour service?
For exposed bolting in sour environments, yes, or a corrosion-resistant alloy. Non-exposed bolting may remain B7 if the project allows.
Do B7 and B7M have the same dimensions?
Yes. Dimensions follow ASME B18.2.1 / B18.31.2; only strength and hardness differ.
What is the main difference between B7 and B7M?
B7M is hardness-limited (235 HBW max, every piece tested) for sour service, and has lower strength than B7. B7 is the general-purpose high-strength grade.
When should I choose B7?
General flange bolting.
When should I choose B7M?
Sour (H₂S) service per NACE MR0175.
Can B7 replace B7M?
Only when the design, code and specification allow it. Strength, corrosion resistance and temperature limits differ, so substitution must be approved by the design authority.