Brazing is a metal-joining process. Metal joining is necessary when parts are too complex to be cost-effectively manufactured from a single piece of material. How does brazing differ from soldering or welding? How does brazing make good things? We shall soon know.
Brazing is a technique that utilizes applied heat and a brazing filler metal (BFM). The BFMs melt at temperatures above 450°C (840°F). The melted BFM flows by capillary action into the gap between the metals to be joined, creating a metallurgical bond between them at the molecular level. The standard for braze joint strength is greater than either of the metals being joined. For this reason, when a brazed part is stressed, it will fail in the base metal before it fails at the joint.
Brazing is similar to soldering except that joints are stronger and the filler metal melts at a higher temperature. The temperature of 450°C has become the cut-off temperature above which the process is defined as brazing and below which it is soldering. If you have been “around a while,” you may have heard terms like “silver soldering” or “hard soldering” incorrectly used to describe the process of brazing. Welding is typically performed at temperatures higher than those involved in brazing by melting the base metal and adding a filler material.
There are 11 basic brazing processes, and we don’t have the space to talk about all of them here. Torch brazing involves heating the metal with a torch. BFM may be preplaced in the joint or fed into it. Variations of this process are induction, resistance, dip and infrared brazing.
Depending on the type of base metal to be joined, different BFMs will be used. For nickel braze, the top process temperature ranges from 1700-2200°F (925-1200°C) depending on the braze alloy used. Silver braze is usually performed at temperatures of 1500-1650°F (815-900°C). The lower temperature of brazing versus welding means that distortion is less of a problem with brazing. Joint speed is typically faster for brazing, and less fuel gas is consumed.
Furnace brazing is used for high-volume production situations. It is also utilized when the parts are machined or formed to their final dimensions. When the furnace atmosphere is controlled – typically hydrogen or vacuum – a protective flux is unnecessary. Vacuum brazing creates superior braze joints that are extremely clean and flux-free with high integrity and strength. Because the cleanliness can be described as “sterile” after vacuum brazing, this process is used for medical equipment, food equipment and analytical instruments to name a few. Medical equipment, such as dental tools, must have very clean braze results (no pits or dirt) because these create a place for bacteria to live and grow.
Braze joints for the food industry must meet FDA regulations. Examples of food equipment utilizing the vacuum-brazing process are the manifolds and spouts seen in the photo (above). The 304 or 316 stainless steel manifolds can range from 4 to 30 inches long and are typically about an inch in diameter. Spouts can be a single port like those pictured, or they can have up to four ports. Spouts are 1 to 12 inches long and about ¼ inch in diameter.
Can you guess what the manifolds and spouts are used for? They are used to fill cupcakes and pies for TastyKake, Hostess, Mrs. Smith and others. Filling – creams and jellys – pass through the manifolds and feeder tubes and finally out through the spouts to fill cakes and pies with a tasty treat. It’s important that the brazing of these parts is clean and defect-free to prevent the collection of bacteria, which would contaminate the product.
Now you know the difference between brazing and soldering, and you are also up to speed on how brazing makes good things. Think about it next time you bite into a cream-filled TastyKake treat.IH