Step brazing defined: The brazing of successive joints on a given part with filler metals of successively lower brazing temperatures so as to accomplish the joining without disturbing the joints brazed previously. A similar result can be achieved at a single brazing temperature if the remelt temperature of prior joints is increased by metallurgical interaction. (AWS Brazing Handbook)

Step brazing is used as an alternative to a single braze run only when the entire assembly cannot be brazed with one alloy or when brazing a second joint could cause remelting in the first joint.

Lucas-Milhaupt experts recommend step brazing if  the preferred practice of brazing the entire assembly in one run is not possible. However, in some cases it is necessary to braze one component prior to another, and the best way to accomplish this is by step brazing. The key is to avoid overheating and remelting the original brazed joint.

Exposing a braze joint to excessive temperatures can result in:

• Loss of high-vapor-pressure elements

• Dissolution and erosion of the base materials (depending on the braze alloy and base materials)

• Joint failure, increased scrap rates and decreased efficiency

Applications and Alloys
In furnace brazing, the entire assembly reaches brazing temperature at the exact same time, so it is possible to braze all joints in one run. Several examples of when step brazing may be a benefit are:

• The part being braze is also being heat treated, but you are adding components later that require lower temperatures (so as not to affect the previous heat treat)

• Lower-melting-temperature parts need to be inserted into an assembly, but some components must be brazed first (the initial braze alloy may have a higher melting temperature than some parts in the assembly)

• The complexity of the components. Some components inside of assemblies may need to be fixtured in place during the brazing process, but the fixtures cannot be sealed into the assembly.

Open-air brazing using torch or induction heating may have multiple joints brazed with the same alloy consecutively; this is not considered step brazing. Take care also when brazing with the same alloy; the heat used to braze one joint could transfer to and remelt a previous joint. If this separation is not possible, then use a lower-melting-temperature alloy for true step brazing or braze both joints at the same time.

When an application requires step brazing, we recommend using a braze alloy or solder that has a liquidus temperature as far below the solidus temperature of the previously used alloy as possible. Step brazing can be performed in a furnace-brazing application with alloys such as Lucas-Milhaupt's Handy Hi-Temp® 820 (BNi-2) in combination with a Lucas-Milhaupt Silvaloy® 721 (BVAg-8). In open-air, torch- or induction-brazing applications, we recommend using alloys including Silvaloy 450 (BAg-5) and Silvaloy 560 (BAg-7).

For successful step brazing, it is critical that the original braze joint not reflow. Our Nickel Filler Metals, like Handy Hi-Temp® 820 (BNi-2), initially braze at one temperature but do not reflow until reaching a higher temperature. This is due to diffusion of elements (including boron and silicon) from the braze alloy into the base materials at brazing temperature. The braze alloy-which now has less boron and silicon in its chemistry-will have a higher melting temperature once solidified. This allows you to form the second braze joint in a step-brazing process at the same initial temperature as the first joint or lower.

Pitfalls and Prevention
The inadvertent reflow of a previously brazed joint can cause the joint to fail-due to either hermeticity or lack of strength. The braze alloy from the previous joint could flow from one area to another, opening a pinhole leak that was previously sealed. When open-air brazing two joints that are close to one another, be careful to limit the heat to the joint you are currently brazing. Excessive heating of the joint could conduct to previously brazed joints in close proximity.

CONCLUSION:
Step brazing is used as an alternative to a single braze run only when an entire assembly cannot be brazed with one alloy or when brazing a second joint could cause remelting in the first joint.  

Lucas-Milhaupt offers alloys that help avoid remelting of the original braze joint in step brazing and result in quality braze joints for the finished product. Questions? Contact us for further assistance.

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