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 metallurgical 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 brazements. The microstructure discovered in this work is remarkably similar to that of conventional precipitation-hardened, 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 hardening; Hardening; Heat treatment; Reference lists. Welding in the World, Vol. 50, n° ½, 2006 Doc. IIW-1713-05 (ex-doc. I-1147-04) recommended for publication by Commission I “Brazing, soldering, thermal cutting, and flame processes”.
Advanced brazed thin-walled structures and honeycombs have been increasingly used in modern manufacturing. Compact plate-plate and plate-fin heat exchangers, exhaust gas catalytic converters for cars and trucks, fuel converters for fuel cells, seals for aircraft turbines are only a few examples where these brazed assemblies play a critical role in achieving superior energy savings and environmental protection. Other advantages include very low weight combined with high strength per unit weight, compactness, a one-step economical fabrication cycle and relatively low production costs.
In most, if not in all their applications, the base metals (BM)s of the aforementioned brazed structures are made of stainless steels, superalloys, and powder metallurgy foils having thicknesses in order of a few mils (50-200 μm). In spite of such minute thickness, these structures withstand a high temperature/oxidation environment for a very long time without noticeable deterioration. The filler metals (FM)s used are Ni/Co-based alloys that are covered by AWS A.5 specifications as the BNi- and BCo- classifications. These classifications can be applied in forms of powders, powder-based pastes or tapes, or Metglas® amorphous brazing foils (designated MBF). The latter have been used predominantly due to their minute 25-50 μm (1.0-2.0 mil) thickness, resulting in a much lower BM erosion when compared to the powder forms. The low erosion rate is an extremely critical factor for thin-walled parts. Inherently flexible amorphous foils are also convenient to handle when automatically assembling many precise parts in one unit prior to brazing operations. Earlier, the optimization of brazing processing of plate/plate and plate/fin stainless steel parts was reported and some data on their microstructure and properties were presented [1-4]. The combinations of BM/FM studied previously were as follows: 316L/MBF-51/316L [1, 3], 430/MBF-20/430 [2, 3], 436/MBF-20/436 , and PM2000/MBF-50, - 51/PM2000 [2, 3].
This work describes results of detailed microstructural studies of joints manufactured of 304LN and Fecralloy materials as BMs and MBF-51 and MBF-50 as FMs, correspondingly. Table 1 presents their complete chemical compositions (wt. %). Such combinations have a wide range of industrial applications but have never been analysed before. The joints selected for analysis were taken from some industrial heat exchanger (HE) and a metal catalytic support (MCS). In addition, the first example presents a series of joints with BM parts having different thickness; whereas all the other processing parameters were the same.