Justification for replacing BAu-4 precious metal braze filler metal with Pal 30 Ni-based braze filler metal for dissimilar stainless-steel joints

摘要

A variety of stainless-steels materials are utilized in the manufacture of fuel nozzle components of some industrial gas turbine engines used for power generation. There are often dissimilar metal joints brazed with precious metal Au-based filler metals, which although allow the braze joint to be easily X-Rayed for defects, are indeed very expensive. BAu-4 is one such braze filler metal of a nominal 82% Au - 18% Ni eutectic composition that is often utilized in production. With the increasingly high price of Au, there has been a demand for the usage of a less expensive braze filler metal. In this study, three Au-free braze filler metals were evaluated with the objective of replacing the gold-based BAu-4 braze namely: 1) Pal 30 (contains low% of Pd, no Au); 2) Pal 36 (contains low% of Pd, no Au); 3) BNi-2 (contains no Pd or Au). The preceding braze filler metal candidates and BAu-4 were brazed to the following dissimilar stainless steel (SS) material combinations: a) 304 SS to 410 SS; b) 304 SS to 17-4PH SS; c) 410 SS to 17-4PH SS. Tensile tests were carried out at 20℃ and 540℃ on all combinations listed above. In addition, low cycle fatigue tests were also carried out at 450℃. The resultant data showed that the BAu-4 brazed joints had the best properties at 20℃, but the Pal 30 braze joint had the best tensile and fatigue strength at elevated temperature, making it a viable candidate for cost reduction, i.e., to replace the expensive BAu-4 braze filler metal. It is evident that there is a mechanical strength advantage to replacing BAu-4 with Pal 30 braze filler metal; however, the replacement comes with a potential loss in corrosion resistance if inappropriate braze parameters are utilized. Plans to mitigate this undesirable phenomenon will be discussed.