Brazing

From copper.org

http://www.copper.org/applications/plumbing/techcorner/soldering_brazing_explained.html

A major difference between brazed and soldered joints is in the amount of joint overlap or fill necessary to develop full strength of the joint. In a brazed joint, full insertion of the tube to the back of the fitting cup is still highly recommended; however, complete fill of this joint space throughout this entire length is not necessary to achieve full joint strength. According to the American Welding Society (AWS), it is suggested that the brazing filler metal penetrate the capillary space at least three times the thickness of the thinnest component being joined, which is usually the tube. This is known in the industry as the AWS 3-T Rule.

Because of the increased strength of brazing alloys, even this rather small amount of fill penetration will result in a properly fabricated brazed joint stronger than the tube and or fitting themselves. However, unlike a solder joint, where the cap or fillet provides minimal additional strength, a brazed joint should be fabricated so that a well-developed fillet or "cap" of filler metal is provided between the tube and fitting on the face of the fitting. This fillet, or cap as it is often referred to in the trade, permits the stresses developed within the joint (by thermal expansion, pressure or other cyclic reactions such as vibration or thermal fatigue) to be distributed along the face of the fillet. In a brazed joint fabricated without the well-developed concave fillet, all stress would be concentrated at the sharp point of contact between the tube, braze alloy (filler metal), and the fitting, possibly leading to development of a stress fracture in the tube at that point. Creation of the fillet when fabricating the brazed joint greatly minimizes this possibility.

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Besides the strength of the filler metal in the joint, the overall strength of the joint or assembly (tube, fitting and joint) following the joining operation must also be considered when choosing whether to use soldered or brazed joints. As discussed, by definition the temperature that defines the difference between soldering and brazing of copper is approximately 840°F/449°C. This temperature is much more important than just an arbitrary definitional threshold. It is important because 700°F/371°C is the temperature at which copper begins to anneal, or be changed from hard temper (rigid) to annealed temper (soft). With this change in temper comes an inherent loss in strength - hard temper copper is stronger than annealed temper copper. The overall amount of annealing that occurs, and thus strength that is lost, is determined by the temperature and the time the material spends at that temperature. The higher the temperature, the less time it takes to change from hard temper to soft temper.

Since brazing temperatures must exceed the melting point of the brazing alloys, between 1,150°F/621°C and 1,550°F/843°C, the process of making a brazed joint causes the base metals to anneal or soften, resulting in a reduction in the overall strength of the assembly. While a brazed joint is demonstrably stronger than a solder joint, the Rated Internal Working Pressure, that is the 24/7 allowable working pressure of the system, is lower for annealed tube (see Copper Tube Handbook, Tables 3a through 3e).