Joint clearances must be tight for effective Ni-brazing
1. Nickel-based brazing filler metals (BFM) can leave a hard, non-ductile eutectic phase in the middle of a brazed joint.
The hard, non-ductile metallurgical phase-structures that form upon solidification of Ni-brazed joints must be carefully controlled, or else they can, and will, result in cracks inside the joint in stressful mechanical or thermal-cycling service.
The famous photomicrograph in Fig. 1 clearly shows how the hard centerline matrix forms in the center of the joint (since solidification moves from the edge of the base-metal/brazed-joint interface to the center of the joint).
The last phases to solidify when brazing with nickel-based brazing filler metals (BFMs) will be those phases that are the lowest-melting, i.e., those phases rich in the temperature-lowering, eutectic-forming, elements (meaning those that are rich in boron, silicon, or phosphorus). Remember, “eutectic” refers to the composition of an alloy that is the lowest melting point portion of the BFM. Thus, eutectic phases will not only be the first composition to start melting during heating of the BFM but also will be the last to solidify during cooling. Thus, during cooling these eutectic-phases will “migrate” towards the center of the joint as the “solidification-front” of the BFM moves from the base-metal/BFM interface toward the center of the joint, and will be forced to solidify right at the center of the joint.
Unfortunately, all of these temperature-lowering, eutectic-forming, elements in nickel-based BFMs are also hardeners, that is, the phase-structures resulting from solidification of these elements have virtually zero ductility! Thus, the last phases to solidify (in the center of the joint) will be hard, and non-ductile. If the joint is thicker than only about 0.004” (0.10mm) max., these hard centerline eutectics can actually form a continuous line down the center of the joint, and cause the joint to become very prone to cracking under any kind of thermal or mechanical stress or strain in service.
Notice, however, the portion of the brazement shown on the far left side of the photo in Fig. 1, which is at a right angle to the larger horizontal joint structure in the photo. Because that joint on the far left side is very thin (less than 0.003”/0.075mm), the amount of eutectic “hardener” available in that joint is minimal, so much so, that there is physically not enough of that temperature-lowering additive present to enable the formation of any kind of continuous centerline eutectic. Therefore, this thin joint would behave in a very ductile fashion in service.
All nickel-brazed joints, therefore, should be designed with this in mind, if they are to perform adequately in service.
2. These centerline-eutectics can become initiators of cracks in components when subjected to thermal or mechanical shock and/or fatigue.
To further illustrate this concept, look at the photo in Fig. 2, which shows a polished cross-section of a round-pin that was nickel-brazed into a much larger diameter round hole. Note how the solid-pin drifted to one side of the hole, causing the joint on the right side of the photo to be thin, with just a few spots of what is called a “non-continuous centerline eutectic”. Those darks spots appearing in the joint on the right side of the photo are not voids, but are actually dark-etching centerline-eutectic phases, surrounded by ductile nickel-chrome solid solution material. By contrast, the left side of the joint in that photo is quite wide, resulting in the formation of a continuous centerline-eutectic structure, which then cracked in service, as clearly shown in that photo.
This can also become a very important issue when brazed-joints are designed such that fillets at the edge of the joint (as shown at the right side of the nickel-brazed joint in Fig. 3) are large enough to exhibit this non-ductile continuous centerline structure right at the point where high-stresses may be concentrated due to service conditions.
Figure 4 is a photo from a nickel-brazed component that actually failed in service because of a crack that progressed right through the center of the joint. The cross-section photo clearly illustrates the problem of centerline-eutectic cracking under thermal/mechanical shock in service.
3. To control the formation of hard centerline eutectics in nickel-brazing, joint-clearances must be kept to a minimum of about “0.000-0.003″ (0.000-0.075 mm) max at brazing temp.” Joint thicknesses larger than this can show continuous centerline eutectic structure, which is highly prone to cracking in service.
Testing by a variety of brazing companies around the world has shown that there is indeed a “threshold” clearance, above which continuous centerline-eutectics tend to form in nickel-brazed joints. This is shown in Fig. 5, in which it can be seen that gap clearances less than about 0.003” (0.075mm) are good, whereas those gaps larger than that are prone to the formation of continuous centerline eutectics, making them very susceptible to cracking in service.
Obviously, large fillets at the ends of nickel-brazed joints will tend to show similar behavior, and, as discussed earlier, can also be a real problem.
Shown in Fig. 6 below is a typical cross-section of the brazed joint between some corrugated heat-exchanger fins and the thin plates to which they are brazed. Note on the right side how a typically designed corrugation requires fairly large fillets to hold it in place since the end of the corrugation is merely rounded.
In the large fillets, you can see the formation of large centerline eutectic phases in the center of the fillet, which, under loading may become crack-initiators. On the left side of the diagram, however, is a properly designed corrugation for brazing, in which the corrugation is flattened so that the length of the actual brazed joint is 3T-to-6T, where T represents the thickness of the thinner of the two members in the joint (the corrugated sheet, or the thin plate to which it is brazed).
Many of today’s high-temperature heat-exchangers (which are often nickel-brazed) exhibit similar issues, which have, unfortunately, not been adequately addressed by many companies that design such brazements. Designers must learn that good brazing depends entirely on the goodness of the metallurgical interaction between the base-metal and BFM between the faying surfaces INSIDE the joint, and should never depend on external fillets.
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 46-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at [email protected], and his website can be visited at http://www.kaybrazing.com/
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