Influence of atmosphere during vacuum heat treatment of stainless steels AISI 304 and 446

Abstract Specimens of austenitic AISI 304 and ferritic 446 (contains 1.7 wt.% Al) underwent heat treatments up to 1200 °C in ultrahigh vacuum (UHV) in an analysis chamber with a base pressure of 10 -10 mbar and high vacuum (HV) in an industrial vacuum brazing furnace with a base pressure of 10 -6 mbar. The effect of the two oxygen partial pressure ranges on surface and bulk composition was determined by XPS, AES and EDX, imaging was provided by SEM. For AISI 304, we observe similar behavior for both atmospheres and the removal of the surface oxide layer. For AISI 446, we observe a strong dependency on the atmosphere. Heat treatment in high vacuum yielded a thick aluminum oxide layer on the surface that inhibited wetting with copper silver eutectic. Heat treatments in ultrahigh vacuum did not lead to aluminum oxide formation but rather carbide formation of foreign elements such as titanium was observed on the surface.