ASTM A320

Covers — alloy-steel and stainless-steel bolting for low-temperature service with mandatory impact testing: grades L7, L7M, L43, L1, and austenitic B8/B8M Class 1/2, with chemistry, mechanical properties, heat treatment, and Charpy requirements.

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

• Ambient/high-temperature bolting → ASTM A193 (B7).
• Matching nuts → ASTM A194 (impact-tested grades for cold service).
• General low-strength bolting → ASTM A307.

A320 = A193-type strength plus mandatory low-temperature impact toughness.

A320 sets minimum requirements for low-temperature bolting:

• Chemistry — L7 (Cr-Mo, like B7), L43 (Ni-Cr-Mo), or austenitic B8/B8M.
• Mechanical Properties — tensile/yield plus mandatory Charpy impact energy at the specified low temperature.
• Heat Treatment — quench-and-temper (L-grades) or solution-anneal/strain-harden (B8).
• Testing & Marking — impact and tension testing, with low-temperature grade marking.

ASTM A320 is the low-temperature bolting standard. Grade L7 shares B7's chromium-molybdenum chemistry but adds mandatory Charpy impact testing to guarantee toughness in cold service; L43 and austenitic B8 grades extend to lower temperatures. It pairs with impact-tested A194 nuts.

The defining difference from A193 is the impact requirement — the whole point is to avoid brittle fracture of the bolting when a joint sees sub-ambient temperatures. Swapping A320 L7 in for A193 B7 on cold-service lines is a routine material-selection correction.

Engineering Note: Confirm the impact test temperature matches the joint's minimum design metal temperature — an L7 stud tested at the wrong temperature does not qualify the cold-service joint.