Vacuum Levels Influence on the Properties of Various High Temperature Brazing Alloys

Paper in proceeding, 2015

The importance of a good vacuum for brazing Chromium containing alloys on stainless steel is well known. The vacuum level required is depending on the Cr content in the brazing alloy, higher content requires a higher vacuum level. Also the alloying composition of the steel in the substrate from the point of view of elements with high oxygen affinity as e.g. chromium, plays a significant role as well. However, due to the higher Cr-content in the brazing alloy, the dominating factor for what vacuum quality is required is the composition of the brazing alloy. This paper deals with thermodynamic and experimental investigation of the vacuum level required to achieve a good brazing behavior of different alloys. The vacuum levels required, or being more specific, oxygen partial pressures needed, are not at all enough to reduce the Chromium oxide even at the lower chromium activities present in low Chromium containing compositions at the typically applied brazing temperatures. However, brazing powder is covered by heterogeneous oxide, formed by the thin oxide layer of the main matrix element and oxide islands, rich in stronger oxide forming elements. Hence, the oxides controlling the behavior of the powder during process is strongly determined by the removal of this thermodynamically unstable surface oxide layer of base matrix element. Importance of vacuum level/quality is therefore expressed in terms of its effect on oxide transformation and distribution during brazing process that will determine final properties of the component.