Heat Treatment for DOT 3AL Cylinders

This is in response to your May 8, 2001 letter concerning the heat treatment requirements in 49 CFR 178.46(f) and (j) for cylinders made to the DOT 3AL specification. Your questions are paraphrased and answered as follows:

Q1. What constitutes a heat treatment under §178.46(f)?

A1. Under § 178.46(f), a heat treatment consists of two parts, a solution heat treatment conducted at a high temperature to dissolve the alloying constituents, followed by an aging treatment (artificial or natural) to allow the alloying constituents to precipitate. The aging treatment may involve more than one heating cycle (artificial aging) in order to achieve the desired material properties.

Q2. Are additional aging treatments considered reheat treatment under § 178.46(j)?

A2. No. The aging treatment alone is not considered reheat treatment. The reheat treatment described in § 178.46(j) begins only when the cylinder is subjected to a second solution heat treatment.