Abstract
Vastly different microstructures are formed in 304LN austenitic and Fecralloy® ferritic stainless steel joints brazed with Ni-15Cr-1.4B-7.25Si (MBF-51) and Ni-19Cr-1.5B-7.3Si (MBF-50), filler metals, respectively. These joints were cut from an industrial heat exchanger and a metallic catalyst support that were subjected to a short optimal brazing cycle in a vacuum furnace. A detailed description is given of the composition and morphology of phases evolved in these brazements, as a result of complex metallurgical reactions between the base and filler metals. A new metal- lurgical reaction was discovered between Fe-20Cr-5Al Fecralloy base metal (BM) having b.c.c. crystal lattice, and the Ni and B from MBF-50 (Ni-19Cr-1.5B-7.25Si) brazing filler metal (FM). This reaction resulted in the precipitation of fine, regularly distributed Nix (Al)y particles in the base metal matrix phase, thus strengthening Fecralloy braze- ments. The microstructure discovered in this work is remarkably similar to that of conventional precipitation-hard- ened, heat resistant alloys. Therefore, these joints can withstand years of service in the brutal environment observed in automotive exhaust pipes. IIW-Thesaurus keywords: Austenitic stainless steels; Stainless steels; Steels; Ferritic stainless steels; Microstructure; Brazed joints; Amorphous metals; Parent material; Brazing fillers; Filler materials; Gap; Joint preparation; Age hard- ening; Hardening; Heat treatment; Reference lists.
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