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Why does Brazing require Temperatures above 450C (840F)? Brazing, when performed correctly, is a joining process that produces a permanent bond between two or more materials by heating them to a temperature above 450C (840F), but lower than the melting-temperature of any of the materials being joined, and a permanent, metallurgical bond between these materials is produced when capillary action draws a molten brazing filler metal (BFM) through the clean, closely fitted faying surfaces of the joint.

The filler metal is not supposed to become fully liquid (i.e., have a "liquidus") until the brazing temperature reaches at least 450C (840F). If the liquidus of the filler metal is below 450C (840F) then that filler metal would commonly be called a "soldering alloy". People often wonder about the temperatures used to differentiate brazing from soldering. Why 450C (840F)? Is there some significance to these "exact" numbers? By Dan Kay

Over the years I've helped many brazing shops resolve common brazing problems (such as leakers, non-wetting surfaces, etc.). In evaluating these situations, it is not uncommon to discover that sub-components (such as brackets, or fittings, etc.) from outside suppliers can actually be the trouble-makers!

Often the brazing shop is not aware of how some of their suppliers are making the sub-components that will be subsequently brazed. Then when there is a problem brazing some of the assemblies containing these sub-components, the brazing shop may try to solve the problem by trying to find out what is wrong with their own in-house brazing operations, getting very frustrated when an in-house cause for the problem can't be found. Many suppliers are not aware that their own manufacturing processes can have a negative impact on brazing. Unless you have talked with them extensively about how certain processes will, or will not, hurt brazing, they will continue to do what works best for them in supplying a nice looking product for you in as cost-effective a manner for themselves as possible. By Dan Kay

Even with built-in "holds" when heating low-carbon steel parts up to brazing temp, some heat treaters are getting a high percentage of the tubular brazements "pulling apart" somewhere during the cycle, i.e. the smaller-diameter tubing pulls away from the larger-diameter tubing, even snapping the welded clips off one of the tubes so that they are not brazed together along their length. What's happening, and how can they "fix" this problem? By Dan Kay

A common occurrence (unfortunately) in the brazing world is the need to join two parts together by brazing in which the brazing gap is too large, i.e., in the range of 0.010-inches (0.25 mm) or larger. Ideal brazing clearances should be in the area of 0.000-inches to 0.005-inches (0.00mm to 0.125mm) maximum for most brazing filler metals (BFMs).

Brazing depends on capillary action to draw the liquid BFM into the brazing joint, and tight clearances are needed for best brazing to occur. If the BFM is pre-placed in the brazing joint prior to assembly of the parts then capillary action is not a major factor since the BFM will melt in-situ and join the two members without the need for flowing any distance through tight capillary spaces. But, when the BFM is applied externally, or if a preform ring of BFM is placed in a groove internally, and the BFM must melt and flow through the joint by capillary action, then clearnace become a critically important factor.

Many people use silver-based brazing filler metal (BFM) when torch-brazing (i.e., flame-brazing) a variety of base metals such as copper, copper-alloys, and many types of steel (including stainless steels). Unless you are brazing pure copper to pure copper, a brazing flux should always be used. Pure copper to pure copper can be brazed using a phos-copper BFM, in which the phosphorus content of the BFM acts as a flux on the copper, and no additional paste flux is needed.

However, for all other metals a paste flux will need to be applied to the surfaces of the parts to be brazed in order to protect the components (especially the inside of the braze joint itself) from oxidation during the brazing process, since all metals tend to react with oxygen when heated, to form surface oxidation. BFM does not like to bond to, or flow over, oils, dirt or oxides on the surfaces to be brazing, and so, steps must be taken to insure that the surface is very clean prior to brazing, and then protected from oxidation during that high-temperature brazing process itself.

For lap joints in brazing, we usually use the "3T-to-6T Rule," where "T" is the thickness of the thinner of the two members being joined.

Here's how this was developed: Many overlap brazing tests were made in labs around the world using different base metals, different filler metals and different amounts of joint overlap. These tests have shown that sheet-metal brazing using overlaps less than 2T failed in the brazed joint itself when subjected to tensile tests. As the overlap increased to more than about 2.3T or so, the failures invariably occurred in the base metal away from the joint and not in the brazed joint itself. So, the brazing community at that time decided to use (and publish) the 3T-overlap as a minimum overlap to ensure that failures would always occur in the base metal and not in the brazed joint.

Most people realize that metals expand when they are heated and contract when they are cooled. This fact has been thoroughly explored over the years, and data tables have been published showing how fast (and how much) each metal expands as temperature increases.

Unfortunately, some people forget to use this data properly in a brazing situation. They set up a brazing operation with a desired "brazing clearance" at room temperature for the parts being brazed together and don't think about the fact that as the metals are heated, the clearance can, and most likely will, change dramatically. In some cases, brazing filler metal (BFM) flow is even shut off into the joint completely if the gap closes up tightly upon heating.