Welcome to my part of “Technical Talk,” where we will discuss brazing, a very important metal-joining technique that continues to grow in usage each year in many industries around the world. Most people are familiar with welding and soldering, but many are not familiar with where brazing might fit into the overall metals-joining landscape.
Shown here are some images of common items we use around our home or shop that were created by joining pieces together using a brazing process. Figure 1 shows common vice-grip pliers, the gripping surfaces of which are specially hardened steel inserts that are brazed onto standard forged-steel tooling.
Figure 2 shows a sterling-silver knife in which the sterling-silver handle is brazed to a stainless steel blade. Figure 3 shows a bicycle frame in which the tubular structure of the frame is brazed together. Figure 4 shows a pair of Aviator sunglasses in which all the tiny wire connectors are brazed together. Figure 5 shows a diamond-grit cutting blade in which all the diamond bits are brazed securely to the round blade around its periphery. Figure 6 shows one of our U.S. astronauts floating in space. His backpack life-support system contains a brazed heat exchanger to handle the liquids and air supply necessary for his survival in outer space.
As you can see, brazing is a widely used and vital joining process in our daily life. We will explore some of these applications in more detail in future columns, and we will also look more in-depth at some of the many brazing principles that enable such processes to be performed.
Let’s start by taking a look at how brazing, as a joining process, differs from soldering and welding. Take a look at the “thermometer” shown in figure 7. There is a line across the lower portion of that image separating brazing (above the line) from soldering (below the line). That’s right – temperature is pretty much the primary thing that differentiates soldering from brazing. That line across the image shown at 450°C (840°F) is merely an artificial line “pulled out of the air,” so to speak, to help separate the two methods of metals joining.
You will notice that there is a significant space between the melting temperature of zinc (787°F/420°C) and aluminum (1220°F/660°C). So, many years ago, professional engineering organizations agreed to use 800°F as a temperature that would separate brazing (above 800°F)from soldering (below 800°F). Note that only Fahrenheit is being talked about here, since most engineering at that time was focused in the U.S. and many countries of the expansive British realm around the world.
The definition of brazing (Fig. 8) is taken from the first edition of the Brazing Manual published by the American Welding Society (AWS) in 1955. At the time, these engineering groups were only working with the British system. Therefore, they were only specifying temperature via Fahrenheit, with no consideration for metric units. Thus, 800°F was a nice, even number and a very convenient number to use to separate the two joining methods of soldering and brazing.
As we all know, however, the world is much “smaller” now given the speed of transportation (I can travel to the opposite side of the world in less than a day). I can also easily speak to someone on the opposite side of the world right now, instantaneously, via phone and video (like Zoom, WebEx, etc.). It is truly amazing. But if that is the case, then it is important to ask: “What is the predominant measuring system in the world today?” The answer is, of course, the metric system, not the British system of measurement. Therefore, negotiations between the various engineering groups around the world took place, and over the years a new definition of brazing temperatures came about – one based on the metric values.
The temperature that was finally agreed upon to separate soldering from brazing was 450°C, which is a bit of a compromise between the then-British system values and an ideal selection of perhaps 500°C to separate the two processes. Thus, 450°C became the official separation point between soldering and brazing, which is the same as 842°F. The engineering groups agreed to “round down” that value to 840°F as the approximate equivalent to the 450°C, and those newer, correct values are shown in figure 7.
I gave this detailed temperature description because I still receive questions from brazers asking me why 840°F was chosen as the temperature separating soldering from brazing. They ask me, “Is there some kind of metallurgical reaction occurring at 840°F that causes that temperature to be chosen?” And I always answer, “No, the only reason that 840°F is used is because it is an approximate equivalent to the 450°C temperature, which is the actual, official temperature separating those two processes.”
It is important for readers to understand that it is important to always write it as 450°C (840°F) when speaking of this artificial temperature line between the processes, since the 450°C is primary. It should never be written as 840°F (450°C). I remember reading an article someone wrote a few years ago about brazing, and they mistakenly defined brazing as a process occurring at 840°F (449°C), which is incorrect. Always write it as 450°C (840°F) knowing that metric must be primary in this definition.
Unlike this somewhat arbitrary separation of brazing and soldering, welding is always easy to distinguish since, in welding, the base metal is always melted at the joint interface. The base metal is never melted when performing either soldering or brazing. So, on the image in figure 7, there will be no separating line to distinguish welding from the other two processes. We could technically weld zinc at a temperature below the line shown on the chart, and I can weld iron at temperatures near the top of the chart. Therefore, welding is determined by whether or not the base metal is melted, whereas soldering and brazing are differentiated by temperature.
We will look at how brazing works to achieve excellent results in my next column. When done properly, brazed joints should be as strong as the base metals being joined, and any failure/breakage of a brazed assembly should only occur in the base metal – never in the joint itself.