Carburization resistance of austenitic stainless steel tubes

In the thermal cracking of hydrocarbons in contact with cast austenitic stainless steel pyrolysis tubes to form products including C.sub.2 -C.sub.4 olefins and diolefins, wherein the composition of said tubes comprises about 17 to 40 wt. % chromium, 15 to 50 wt. % nickel, and 0.6 to 4 wt. % silicon, the improvement which comprises employing tubes which have been treated by a method comprising:

• (a) cold-working the inner surface of said tubes to deform the surfaces 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 regions will result while maintaining a refined recrystallized micrograin structure, in the deformed surface regions, to a uniform minimum depth of at least about 20 microns;
• and
• (b) heating the cold-worked inner surfaces of said tubes, for an effective amount of time, at a temperature between their recrystallization temperature and their melting temperature, in an atmosphere which is at least oxidizing with respect to chromium.