Cracking discovered in portions of an atmospheric tower overhead piping system prompted this investigation. Alloy 400 had been used to replace carbon steel that had shown unacceptable metal loss. Intergranular stress corrosion cracking (IGSCC), which was identified as the corrosion-failure mechanism, of Alloy 400 is rare. Furthermore, this material is frequently used with success in many atmospheric overhead line installations. Slow strain rate (SSR) testing was employed to determine the worst environmental condition for stress corrosion cracking (SCC) in a simulated atmospheric distillation overhead environment. A statistically designed experimental matrix was prepared from operational parameters, which included temperature, sulfides, pH and ammonia levels, specified by the refinery. SSR testing results confirmed the susceptibility of Alloy 400 to SCC in the as-welded and stress relieved material condition. SCC susceptibility was observed to vary depending on the time of the year when the water condensate samples were collected. Keywords: SCC, atmospheric overhead refinery environment, failure, welding, Alloy 400, slow strain rate testing, SSR, constant extension rate testing, CERT, sulfide stress cracking