A number of companies I’ve visited who conduct vacuum brazing operations have asked for assistance in understanding how to properly use a torch-brazing (flame braze) process to repair some of the assemblies that did not fully braze during their vacuum brazing operations. The components were such that they did not want to send the entire assembly back through the vacuum brazing furnace but merely needed to fix a small portion of the assembly where it did not fully braze.
So let’s take a brief look at torch-brazing to see what it is, and how it can be used by brazing shops today to meet some of their production-repair needs.
Figure 1 shows a typical example of a torch-brazing setup, in which a small region of a metal assembly is being heated for application of some brazing filler metal (BFM) so that the entire assembly does not need to be heated.
Some of the reasons why people use torch-brazing processes are:
In addition to the use of torch-brazing as a repair technique on parts that were previously vacuum brazed, there are three other possible reasons why a brazing-shop might wish to understand, and utilize, torch brazing:
1. Perhaps they do not want (or need) to heat the entire component to brazing temperature, but only desire to braze one small location on that component. Because furnace brazing would heat the entire component, torch brazing can be an excellent option to heat only that part that needs to be heated for brazing, or for a braze-repair, thus potentially saving a lot of money compared to the time and cost of heating the entire part in a furnace.
2. Perhaps they cannot heat certain portions of a component because high heat would damage those other portions, etc.
3. Perhaps the part is just too big to fit into a furnace, and methods must be found in which only a local area will be effectively heated for brazing.
A few months ago, I discussed how induction-brazing is also a possible option to be used in vacuum-brazing shops for the same three reasons listed above.
How can torch brazing (flame brazing) fit in with furnace brazing?
Torch brazing uses the intensely hot flame from a torch tip to locally heat up a small portion of a metal assembly, bringing it up to brazing temperature in a matter of from just a few seconds to a few minutes maximum depending on the size and mass of the part being heated. A typical brazing torch might look like the one in Fig. 2, and generally consists of a metal mixer-body into which two different gases are fed. These gases are typically oxygen and a combustible fuel gas (such as acetylene, propane, etc.) These two gases enter the torch body through separate gas lines/hoses, are then blended before exiting out of the curved tip of the torch body where the mixture is ignited and burns, as shown in Fig. 1. The gas flow rates are controlled by the two knobs on the mixer-body.
The torch tip itself may have a single hole in it (like the one in Fig. 1 and Fig. 2), or perhaps several holes in its tip, as shown for each of the two torch tips shown in Fig. 3. The dual-tipped torch shown in Fig. 3 uses multi-flame (“rosebud”) tips which allows much more rapid heating of a region of a component since the part can now be heated from two sides at the same time.
Flux is needed.
When torch brazing out in open air, it is important to prevent oxidation of the surfaces of the part being heated. For this purpose, a flux is applied to the surface prior to heating. Fluxes are excellent oxygen absorbers, and thus will keep the brazing surfaces clean and oxide free until the brazing filler metal (BFM) melts and flows into the braze joint.
Pure copper is the only metal that can be heated in air without flux, but specialized phos-copper BFMs must be used to compensate for the lack of physically applied paste flux. All other metals will require the generous application of a flux to enable the parts to be heated in open air and locally brazed. This flux must be either a paste applied by hand prior to brazing, or, as an alternative, there is a commercial product known as GasFlux® that can be piped through the torch along with the fuel gas so that a gaseous flux-curtain can envelop and protect the part without the need for physically applying a paste flux.
Many brazers and welders talk about adjusting their torch-flame so that it is oxidizing, or neutral, or carburizing. This is important to know about, and is shown in Fig. 4.
An oxidizing flame is, as the name implies, oxygen-rich, and will thus heavily oxidize the surface of the part being heated. A carburizing flame is one that is fuel-rich since most fuel-gases are hydrocarbon gases.
When any metal is heated, it tends to react with oxygen in the air to form oxides on its surface, and the rate of this oxidation increases as the temperature rises. Unfortunately, BFMs cannot effectively bond to oxides, and thus, oxygen must be kept away from the surfaces to be brazed!
By using a fuel-rich gas-flame, the torch brazer will be throwing excess carbon at the surface of the part being heated. This carbon will instantly react with the surface oxidation that is forming as the part is being heated by the flame, and the excess carbon reacts with the surface oxygen to form gaseous CO and CO2, thus keeping the surface of the metal bright and clean. Thus, a carburizing flame is also known as a “reducing flame” since it continuously reduces the surface oxidation that is trying to form.
Please be aware that “neutral“ is really just a line that separates oxidizing from carburizing. There really is no neutral-region to which a torch can be adjusted. Invariably the flame setting will drift, and you don’t want it drifting into the oxidizing region. Therefore, set the torch to slightly carburizing (reducing) and you should then have a good flame that will heat the surface effectively, and keep it clean as well.
Any good vacuum brazing shop will encounter components from time to time that:
These are just a few of the reasons that may cause your vacuum-brazing personnel to consider the viability of adding in some torch-brazing capability to your brazing shop, so that you can do more of a “turn-key” job for your customer, and perhaps be able to more cost-effectively do some needed braze-repairs on some of your vacuum-brazed parts.
So, if the need arose, could your shop do this, or would you be forced to send the parts out to other brazing companies who could perform such torch brazing for your customer?
Dan Kay – Tel: 860-651-5595: – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for more than 40-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. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
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