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Which stud bolts are used for sour service?
Short answer
For sour (H₂S) service, exposed carbon and low-alloy steel bolting is usually ASTM A193 B7M studs with A194 2HM nuts, both hardness-limited to resist sulphide stress cracking under NACE MR0175 / ISO 15156. Severe environments call for corrosion-resistant alloy bolting such as Alloy 625, 825 or super duplex.
Key takeaways
In brief
- Exposed vs non-exposed bolting: Bolting that is buried in insulation, or otherwise exposed to leaking sour fluid, is treated as exposed and must meet hardness limits.
- Strength trade-off: B7M has lower strength than B7, so re-check bolt loads and gasket seating when substituting.
- CRA bolting: For severe environments, Alloy 625, 825 or super duplex bolting may be specified.
Exposed vs non-exposed bolting
Bolting that is buried in insulation, or otherwise exposed to leaking sour fluid, is treated as exposed and must meet hardness limits. That's where A193 B7M with 2HM nuts comes in.
Strength trade-off
B7M has lower strength than B7, so re-check bolt loads and gasket seating when substituting.
CRA bolting
For severe environments, Alloy 625, 825 or super duplex bolting may be specified. See our bolting selection guide.
How to decide whether bolting is "exposed"
Bolting is treated as exposed when it is in direct contact with the sour fluid, or could be wetted by a leak and cannot dry out: inside insulation, under flange guards or in buried flanges. Bolting open to the atmosphere, where any leak would evaporate, is often classed as non-exposed and may stay as standard B7 if the project specification allows it. The decision belongs to the project materials engineer, not the supplier, so it should be written into the purchase order.
What the order should say
- Stud grade and nut grade, e.g. A193 B7M / A194 2HM
- "100% hardness tested" and the maximum hardness
- Coating, e.g. PTFE fluoropolymer, and whether it is permitted for the service
- Length per the flange class and gasket
- Certificates showing hardness per piece or per lot as required
Related guides, comparisons and articles
Comparisons
Frequently asked questions
Can I use B7 bolting in sour service?
Only where the bolting is non-exposed and the project allows it. Exposed bolting in H₂S service normally needs hardness-controlled B7M with 2HM nuts or a corrosion-resistant alloy.
Does B7M need different torque values than B7?
Yes. B7M has a lower minimum yield strength, so the bolt load available is lower. Recheck the joint design and gasket seating stress before substituting.
Do coatings affect sour-service suitability?
Coatings such as PTFE improve corrosion and seizure resistance but do not change hardness limits. Some coatings involve processes that can introduce hydrogen, so follow the project specification.
Does Bright Tubes supply the materials discussed in this article?
Can I get technical advice for my application?
Yes. Share your service conditions and specification, and our team can help shortlist suitable grades for your engineer to confirm.