thread177-297351Fire Box Quality means suitable for use in a boiler firebox. Retains pressure and is exposed to flame. Such as a steam locomotive. Check this site out. trainweb.org/tusp/r_links.html1Just to add to the above after a little digging around of my files; Firebox Quality steel plate had tighter control over chemical composition to assure specified minimum mechanical properties prior to the development of SA 515 steel plate. SA-285 Grade C is not the same as fire box quality SA 285 Grade C. If anything, SA 515 plate would be an equivalent material substitution.As an old steel maker, SA-285 Grade C, fire box quality, was fully deoxidized - Silicon killed.1I agree with metengr. It is unlikely you will find any FBQ and if you do the paper trail may not be there. Have hand forged a couple of rear flue sheets in some restorations and used 515,but thats another story. Do your calcs. and check with your AIA.Additionally, the 1950 ASME Code information on SA 285-C FBQ material states 55k min. tensile, 1375psi allow. stress upto 650oF, 13k upto 700oF & 11900 upto 750oF. Information from the 1951 drawings state this material shall have "shear test per ASME Code case #896 required (1948 ruling)". Finally, I would like to especially thank metengr, deanc & stanweld for their insight and shedding light on this issue. I want you to know that this small clarification has helped me and it has, once again, helped influenced my supervisors opinion of my ASME Code abilities. I am appreciative of these replies and for Eng-Tips.