Hello Mr. Massabie,

The "Y" coefficient is just to address the increased ductility of piping materials as the temperature increases (and we only use it in our "hoop stress" based pipe wall thickness calculation). It has been with us for a long time (pre 1955) but I think you would have to go back to some of the very early ASME collected papers publications to see how it was developed. I don't have anything in the way of a reference here at the office. I shall have a look around.

Regards, John.