46 CFR 54.25-7: -7 Requirement for postweld heat treatment (modifies UCS-56).

(a) Postweld heat treatment is required for all carbon and low alloy steel Class I, I-L, and II-L vessels regardless of thickness. (Refer to table 2 to § 54.01-5 for applicable requirements.)

(b) Cargo tanks which are fabricated of carbon or low alloy steel as Class II pressure vessels, designed for pressures exceeding 100 pounds per square inch gage and used in the storage or transportation of liquefied compressed gases must be postweld heat treated regardless of thickness.