Further to Mr. David Diehl's reply, to my knowledge the first mention of the 6,000 psi criteria came from a joint ASME paper by Rossheim-Markl in 1940. Title of the paper is called, "The Significance of, and Suggested Limits for, the Stress in Pipe Lines Due to the Combined Effects of Pressure and Expansion." Both authors once worked at M.W. Kellogg Company, and that was the company I believe first made it into its Piping Mechanical Design Manual. Due to the free migration of stress analysts in the last few decades, other engineering companies began to apply it as well. COADE heard of it in like manner, as Mr. Diehl says.