Surface deoxidation mechanisms of stainless steels in vacuum brazing processes

摘要

Stainless steels are used in a wide variety of applications due to their excellent corrosion resistance. Yet, in a vacuum brazing process, wetting the workpiece surface with molten braze is a crucial procedural step - which is inhibited by the surface oxide layer, responsible for corrosion protection. Removal of this oxide layer thus becomes of outmost importance. Procedural knowledge is vast and its implementation is evident from the successfully carried out fluxless brazing processes. But the exact thermophysical and chemical processes leading to a wettable surface remain unknown. We compare two stainless steels, AISI 304 and 446, the first one being known to be easily joined by vacuum brazing, the latter on the other hand being much more demanding. We study the effect of oxygen partial pressure in the ranges between ultrahigh vacuum (UHV) and high vacuum (HV) on the composition of both surface and bulk. While AISI 304 shows no detectable dependency on oxygen partial pressure within the observed range, AISI 446 shows an onset oxygen partial pressure above which its surface manifests coverage with aluminium oxide. In conclusion, a deoxidation mechanism is proposed.