Brazing Q &mp A 200510019
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Brazing Q & A Q. There is a requirement on the heat exchangers we are making out of 347 base metal brazed with BNi-2 brazing filler metal that allows a maximum of five cracks in the brazing filler metal, when checking the micrograph taken from the brazement. We are within the acceptable crack limit, with five or fewer cracks that are only in the brazed fillets. The customer is finding many more cracks and they are in the fillet area. What is the source of these cracks and why would the customer be finding so many more and even some in the joint area. The customer believes that we are closing the cracks by smearing the base metal over the crack. Is this possible?A. Over years of investigation, carefully prepared metallurgical specimens confirmed that brazements coming out of the brazing furnace have essentially no cracks. Where then do the observed cracks come from? The source was quite evident in an experience we had several years ago. We had brazed a small, sturdy 304L assembly with BNi-2 in our laboratory. Half of the parts were sent to the customer for testing, and a sample brazement from the remaining lot was sectioned, mounted, polished, and etched and photographs taken of the specimen in our laboratory. When the customer subsequently visited our office to discuss the brazing program, the metallurgist laid out his photomicrographs of eight parts and pointed out that every part was badly cracked. We then laid out our photomicrographs and it was apparent that no cracks were evident in any of these brazements. With these divergent results an extended discussion ensued. We found that the customer had cut the brazements with a large resin-bonded cutoff wheel with a minimum of cooling water. Our brazements were cut using a metallurgical Isomet saw, Model 2000, using a 0.032-in.-thick, 6-in.-diameter diamond wheel rotating at 2500 rpm and flooded with coolant. The force on the specimen was only 300 grams. The conclusion was that the heat and force generated by the large cutoff wheel had caused the cracking in the brazement. The metallurgical cutoff wheel, with slower speed, more coolant, and less force prevented the cracking. Recently, this was again demonstrated when a tube-and-shell heat exchanger was sectioned and prepared by two different methods by an outside laboratory. The first method used a 9-in.-diameter resin-bonded wheel running at 2000 rpm in a regular cutoff machine. This method resulted in 15 to 40% cracking. The second method used a metallurgical cutoff machine with a 5-in.-diameter, 0.032-in.-thick diamond wheel running at 300 rpm and running in water at the bottom of the wheel. The specimen was mounted on top with only about a 300-gram load. This second method exhibited only 2% cracking. Another laboratory investigating a similar brazement and looking at 15 joints only found 1% cracking and one specimen showed no cracks. While this problem is seldom found in brazements containing brazing filler metals of copper, silver, or gold, it has become obvious that more care must be taken when preparing for microexamination of specimens where nickel brazing filler metals have been used. 
Fig.1 — 304 stainless steel (at bottom) brazed with BNi-2 to magnetic material 49%Co-49%Fe-2%V (at top) showing a partly diffusion brazed joint. The white layer on either side of the joint is soft but the center phase is approximately 50–59 HRC. The preparation for micro specimens is important because most brazing shops only hold the brazement for a short time at the brazing temperature and produce large fillets. This results in a hardness of 50–59 HRC — Fig. 1. When the brazements are held for a longer time at the brazing temperature and smaller fillets are employed, the hardness is decreased. When the brazed joint is fully diffused the hardness can drop to 70 HRB to 25 HRC, depending on the base metal — Fig 2. With the lower brazed joint hardness, the problem of brazing cracking in the joint filler metal goes away as a result of the lower hardness and greater ductility. The fillets are usually too large to fully diffusion braze and remain around 50–60 HRC. Thus they can crack if highly stressed. 
Fig. 2 — 304 stainless steel (at bottom) brazed with BNi-2 to magnetic material 49%Co-49%Fe-2%V (at top) showing a fully diffused brazed joint. The white unetched brazed joint in this specimen had a hardness of approximately 70 HRB. Another example was observed in the early days of nickel brazing. A small jet engine nozzle was made as an assembly of an inner and outer shroud with punched holes for blades to be brazed between them. It was requested that the assembly be brazed with BNi-5. After brazing the joints were without imperfections. The brazement was mounted on a milling machine and with a milling saw cut it into three sections. On inspection it was observed that the vibration of the saw had shattered each brazed joint on either side of the cut. The subsequent parts were diffusion brazed and this solved the cracking problem. The causes of cracking during metallographic specimen preparation included the following: bending, heavy loading, heat generation, vibration, and other mechanical stressing. Most brazements are produced and function throughout their service life having only a short time at the brazing temperature. In most applications, this is satisfactory. It is only in special cases where complete diffusion is not required that special care must be taken in specimen preparation. When high-strength joints are required, or where aid is desired in preventing cracking during metallurgical preparation, diffusion brazing and small fillets should be employed. R. L. PEASLEE is Vice President Emeritus, Wall Colmonoy Corp., Madison Heights, Mich. Readers may send questions to Mr. Peaslee c/o Welding Journal, 550 NW LeJeune Rd., Miami, FL 33126 or e-mail to bobpeaslee@wallcolmonoy.com. |