49 CFR §179.400-12

1. (a)Postweld heat treatment of the inner tank is not required.
2. (b)The cylindrical portion of the outer jacket, with the exception of the circumferential closing seams, must be postweld heat treated as prescribed in AAR Specifications for Tank Cars, appendix W (IBR, see § 171.7 of this subchapter). Any item to be welded to this portion of the outer jacket must be attached before postweld heat treatment. Welds securing the following need not be postweld heat treated when it is not practical due to final assembly procedures:
   1. (1)the inner tank support system to the outer jacket,
   2. (2)connections at piping penetrations,
   3. (3)closures for access openings, and
   4. (4)circumferential closing joints of head to shell joints.
3. (c)When cold formed heads are used on the outer jacket they must be heat treated before welding to the jacket shell if postweld heat treatment is not practical due to assembly procedures.