Article
A flanged joint is held together by its bolts. If they are the wrong grade, or stretched beyond their limit, the best flange and gasket will leak. In process piping the usual fastener is a fully threaded stud bolt with a heavy hex nut at each end.
ASTM A193: the stud
ASTM A193 covers alloy and stainless steel bolting for high-temperature or high-pressure service. The most common grade is:
- B7: chromium-molybdenum alloy steel (AISI 4140 or 4142 type), quenched and tempered. Minimum tensile strength 125 ksi (860 MPa) and yield strength 105 ksi (720 MPa) for diameters up to 2.5 inches.
- B7M: the same steel, tempered to a lower maximum hardness of 235 HB. Lower strength, but suitable for sour service where hydrogen sulphide could cause harder steel to crack. B7M is the usual choice where NACE MR0175 / ISO 15156 applies.
- B8 and B8M: stainless steel (304 and 316 type), for corrosive conditions and cryogenic service. B8 Class 2 and B8M Class 2 are strain hardened for higher strength.
- B16: Cr-Mo-V steel for higher temperatures than B7.
ASTM A194: the nut
Nuts are specified separately under ASTM A194, and they must be matched to the stud:
- 2H: heavy hex carbon steel nut, the usual partner for B7.
- 2HM: the reduced-hardness version, used with B7M in sour service.
- 8 and 8M: stainless nuts for B8 and B8M.
Coatings
Studs are supplied black (plain, oiled), hot dip galvanised, zinc plated, or with a fluoropolymer coating such as PTFE for corrosion protection offshore and in coastal plants. A coating changes the friction when the bolt is tightened, so the torque value must suit the coating. Galvanised studs need nuts with oversized threads to allow for the zinc.
Size and length
The number of bolts, their diameter and the length of stud for each flange size, class and facing are given in ASME B16.5. Stud length depends on the facing and gasket: a ring type joint needs longer studs than a raised face. Lengths are measured end to end on fully threaded studs, not counting any chamfered point.
What to put on the enquiry
The stud grade and nut grade, the diameter and thread (inch UNC/8UN, or metric), the length, the coating, and the quantity. Or give the flange size, class and facing, and we will work out the bolting set for you.
See A193 B7 stud bolts and the fasteners range.
Frequently asked questions
What is the difference between B7 and B7M?
B7M has a lower maximum hardness for sour service where sulphide stress cracking is a concern.
Which nut is used with B7 studs?
ASTM A194 Grade 2H heavy hex nuts are the standard pairing.
What bolting is used for stainless flanges?
ASTM A193 B8 or B8M studs with A194 Grade 8 or 8M nuts are common.
Key takeaways
- A193 B7 with A194 2H nuts is the standard flange bolting.
- Use B7M / 2HM for sour service.
- Stud length depends on flange size, class, facing and gasket.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.