Patent ID: 7488392
Filing Date: 2009-02-10
Classification: C21D,C22C,C23C

Abstract:
1. A process for treating a stainless steel surface comprising from 20 to 50 weight % of chromium, 25 to 50 weight % of Ni, from 1.0 to 2.5 weight % of Mn less than 1.0 weight % of niobium, less than 1.5 weight % of silicon, less than 3 weight % of titanium and all other trace metals and carbon in an amount less than 0.75 weight % to produce an outer surface at least 80% of which is spinel of the formula Mn x Cr 3−x O 4 wherein x is from 0.5 to 2, having a surface area at least 50% greater than the surface area of underlying chromia on the surface of said stainless steel which process comprises: (i) heating the stainless steel in a reducing atmosphere comprising from 50 to 100 weight % of hydrogen and from 0 to 50 weight % of one or more inert gases at rate of 100° C. to 150° C. per hour to a temperature from 800° C. to 1100° C.; (ii) then subjecting the stainless steel to an oxidizing environment having an oxidizing potential equivalent to a mixture of from 30 to 50 weight % of air and from 70 to 50 weight % of one or more inert gases at a temperature from 800° C. to 1100° C. for a period of time from 5 to 40 hours; and (iii) cooling the resulting stainless steel to room temperature at a rate of less than 200° C. per hour.