ASTM A194

Covers — carbon, alloy, and stainless steel nuts for high-pressure/high-temperature bolting: Grade 2H, 2HM, 4, 7, 7M, 8, 8M, etc., with chemistry, hardness, and proof-load/testing.

Does NOT cover (exclusions) — the following are governed elsewhere:

• The bolts/studs themselves → ASTM A193 (high-temp) / A320 (low-temp).
• General low-strength nuts → ASTM A563.
• Nut dimensions (inch) → ASME B18.2.2; metric nut dimensions → ISO 4032.

Match the nut grade to the stud grade (2H↔B7, 8↔B8) — the pairing is defined by the referencing flange code.

A194 sets minimum requirements per nut grade:

• Chemistry — carbon/alloy (2H, 4, 7) or stainless (8/8M) composition.
• Hardness/Strength — grade hardness range ensuring the nut is stronger than the mated stud so threads strip in the stud, not the nut.
• Heat Treatment — quench-and-temper (2H/7) or solution-anneal (8/8M).
• Marking — grade identification on the nut face.

ASTM A194 is the companion nut standard to A193 and A320 bolting. Grade 2H heavy-hex nuts are the default match for B7 studs; Grade 8/8M match stainless B8/B8M studs, and Grades 4/7 cover alloy and low-temperature pairings. The grades are set so the nut is stronger than the mated stud.

Correct pairing matters: an undersized or mismatched nut can gall, strip, or fail to develop the required bolt load. The stud/nut pairing is dictated by the referencing flange code, and dimensions come from B18.2.2 (inch) or ISO 4032 (metric).

Engineering Note: Never substitute a lower-grade nut to save cost — the nut must out-strength the stud, or a thread-stripping failure can occur at bolt-up or in service.