Braze joint design

All brazed joint designs shall be suitable for the intended critical service of hydrogen applications. There are two basic types lap and butt. 

In general, the rate of dezincification increases as the zinc content rises, and great care needs to be exercised in making brazed joints with copper/zinc brazing alloys, particularly if they are to be exposed to sea-water. Under these conditions, a properly designed capillary joint may last for some time, but it is preferable to use corrosion-resistant jointing alloys such as silver solders (e.g. BS 1845, Type AGJ or /4G5) . 

Brazed joints are limited to tubular lap or butt-lap joints and shall meet the more stringent requirements of this Code or the engineering design. See Nonmandatory Appendix A, para. A-3.3 for AWS C3.3, Recommended Practices for the Design, Manufacture, and Examination of Critical Brazed Components. 

Requirements for vessels fabricated by forging in Part UF include unique design requirements with particular concern for stress risers, fabrication, heat treatment, repair of defects, and inspection. Vessels fabricated by brazing are covered in Part UB. Brazed vessels cannot be used in leth service, for unfired steam boilers, or for direct firing. Permitted brazing processes as well as testing of brazed joints for strength are covered. Fabrication and inspection rules are also included. 

The same capillary phenomena affect brazing practice for joining both ceramic and metal components, but the relative importance of the phenomena differs, and this makes it convenient to discuss their effects in a different sequence. Further, most joining of ceramics is to metals and the different thermal expansion and mechanical characteristics of these two families of materials, as exemplified in Table 10.4, have a profound effect on joint design that is not related to capillarity. 

Typical Joint designs using brazing lap and scarf in thin Joints with large contact areas or a combination of lap and fillet. Fillets can help to distribute stresses at the Joint. Butt Joints are possible but can cause stress concentrators in bending. 

Seal, permanent (vacuum technology) A seal that is designed so as not to be easily disassembled. Example A weld or braze joint. 

Design and strength of brazed joints, M. H. Sloboda, Welding and Metal Fabrication, 1961, 6, 291-296. 

Recuperator designs in the past have incorporated plate overlays which are brazed to the sides of the recuperator and welded to the top/bottom plates and the manifold structures to minimize the number of exposed brazed joints to space. One example of this design approach is provided in Reference 9- 49. NRPCT recommends that a similar all-welded external structure (vs exposed brazed joints) be incorporated to minimize the likelihood of a recuperator missionending leak to space. 

Branch Welds These welds eliminate the purchase of tees and require no more weld metal than tees (Fig. 10-127). If the branch approaches the size of the run, careful end preparation of the branch pipe is required and the run pipe is weakened by the branch weld. See subsection Pressure Design of Metallic Components Wall Thickness for rules for reinforcement. Reinforcing pads and fittings are commercially available. Use of the fittings facilitates visual inspection of the branch weld. See subsection Welding, Brazing, or Soldering for rules for welded joints. 

Brazements included in a piping system that is subjected to a temperature 1,000°F (538°C) and greater shall require tests in addition to those of ASME BPV Code Section IX. These tests shall be considered a part of the qualification procedure for such design temperatures. Two tension tests on production type joints are required, one at the design temperature and one at 1.05T (where T is the design temperature in degrees Fahrenheit). Neither of these production-type joints shall fail in the braze metal. 

Rapid fluid flow cannot be achieved with active metal brazes because of the need to form solid wettable reaction product layers for their liquid fronts to advance. Equations (10.1) to (10.2) relating liquid flow rates to the opposed effects of surface energy imbalances and of viscous drag are not relevant. Actual penetration rates are so slow, usually of the order of 1 pm.s, that the usual practice is to place the active metal braze alloy within the joints rather than expecting it to fill them, and, as explained already, gap width is not the dominant consideration when designing ceramic-metal joints. 

---