Carburization resistance of austenitic stainless steel tubes

A method for enhancing the protection of a cast austenitic stainless steel tube against carburization, the method which comprises:

• (a) shot-peening the inner surface of said tube to deform the surface of said tube to such a degree that upon the subsequent heat treatment of step (b) below, dissolution of the M.sub.23 C.sub.6 carbides in the deformed region is accompanied by recrystallization resulting in a refined micrograin structure, in the deformed surface region, to a uniform minimum depth of at least about 20 microns, and
• (b) heating the shot-peened inner surface of the tube, for an effective amount of time, at a temperature between its recrystallization temperature and its melting temperature, in an atmosphere which is at least oxidizing with respect to chromium,
• wherein the cast austenitic stainless steel tube comprises about;
• 17 to 40 wt.% chromium, 15 to 50 wt.% nickel, and 0.6 to 4 wt.% silicon, based on the total weight of the alloy.