Proper Joint Design
Don’t brush this one off. This, in my experience, is the most important, and it is too often overlooked by designers. It is often assumed that if a joint can be welded, then that same joint design could probably be brazed. That’s totally wrong. Good joint design for brazing is very different from a good welding design. As shown by the criteria in Figure 1, a good brazing designer will be looking inside the joint where the BFM is supposed to fill the inside surfaces (faying surfaces) rather than looking on the outside (fillets).
Good brazing design does not look for external fillets but instead seeks to optimize the gap clearance and total amount of joint overlap of the faying surfaces.
Good brazing design provides for smooth, contoured shoulders at the edge of a braze joint rather than sharp corners. Sharp corners, being points of high stress concentration, are often the place where cracks are initiated in service. And since those sharp corners are often right at the end of a brazed joint, any cracks seen in the sharp corners are often immediately blamed on the braze.
Look at Fig. 2. Notice that the joint design in Fig. 2A resulted in a crack during service due to high stress concentrations in the sharp corner, which then progressed through the base metal due to the bending stresses the joint faced in service.
The cross-sectional drawing shows that the brazed joint itself was fine, did not fail and had nothing to do with the field failure. But, because the crack was observed to start at the external exposed edge of the braze joint, the brazement was incorrectly blamed for the failure. Instead, the designer of the joint should have been blamed for that service failure, because he/she did not properly contour the joint edge so as to spread the service-stresses (Fig. 2b).
For good brazing results, the surface roughness of the HCR alloys being brazed should be in the as-rolled, as-drawn or as-machined condition, typically in the range of 16-64 RMS (root mean square) surface roughness. This is fine for brazing. The “peaks and valleys” of that surface roughness can actually help the wetting (spreading and alloying) by the BFM on the HCR-alloy surface and help to maintain a reasonable gap-clearance between the faying surfaces inside the joint, thus allowing capillary flow of the BFM through the joint, even with so-called “metal-to-metal” contact of the parts being brazed (Fig. 3).
Cleanliness of Joint Surfaces
This, too, is critical to the success of the braze. I have heard too many people say something like, “Oh, don’t worry about cleaning. The furnace will take care of that.”
BFMs do NOT like to bond to or flow over oils, dirt or oxides. Each of these contaminants will prevent the BFM from not only flowing into the joint, but also from diffusing into and alloying with the HCR metal. If diffusion/alloying is prevented or degraded, then the joint strength will be marginal at best and may fail in service. A “catch-22” scenario arises in which some people then say, “See, I told you brazing isn’t really good for this part. You should have welded it.”
If designers and production personnel will take these first two important brazing criteria seriously, great brazing results can (and will) be achieved.