Brazed joints

When stronger joints are required, brazing may be used. The filler alloys employed generally melt at much higher temperatures (600-1200 °C), but the effectiveness of the joining process still depends 

Furthermore, brittle diffusion products may be produced at the filler-base-metal interface as a result of the reaction of a component of the filler alloy with a base-metal component, e.g. phosphorusbearing fillers used for steel in which the phosphorus diffuses into the steel. 

There is a whole range of silver-nickel and palladium-based braze fillers of high oxidation and corrosion resistance that have been developed for joining the nickel-rich alloys however, the presence of sulfur, lead, or phosphorus in the base-metal surface or in the filler can be harmful, since quite small amounts can lead to interface embrittlement. 

In the case of the Monels, the corrosion resistance of the joint is generally less than that of the parent metal and the design must be such that as little as possible of the joint is exposed to the corrosive media. 

in an engineering structure, the aluminum-bronzes are used for their corrosion resistance, the selection of braze filler becomes important and although the copper-zinc brazing alloys are widely used, the corrosion resistance of the joint will be that of the equivalent brass rather than that of the bronze. 

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Ak ammonia piping should be standard (Schedule 40) or extra heavy (Schedule 80) steel having welded or screwed joints, respectively. Galvanized piping or brazed joints should never be used. Ammonia accepted for pipeline transportation must meet the fokowing specifications NH, 99.5 vol % min dissolved inerts, 0.16% max, ok, 5 ppm max and water, in the form of steam condensate, 0.2% min, or distiked water, 0.5% max. 

Silver Brazed Joints These are similar to soldered joints except that a temperature of about 600°C (1100°F) is required. A 15 percent silver, 80 percent copper, 5 percent phosphorus solder is used for copper and copper alloys, while 45 percent silver, 15 percent copper, 16 percent zinc, 24 percent cadmium solders are used for copper, copper alloys, carbon steel, and alloy steel. Silver-brazed joints are used for temperatures up to 200°C (400°F). Cast-bronze fittings and valves with preinserted rings of 15 percent silver, 80 percent copper, 5 percent phosphorus brazing alloy are available. 

Silver-brazed joints are used when temperature or the combination of temperature and pressure is beyond the range of soldered joints. They are also more reliable in the event of plant fires and are more resistant to vibration. If they are used for fluids that are flammable, toxic, or damaging to human tissue, appropriate safeguarding is required by the code. There are OSHA regulations governing the use of silver brazing alloys containing cadmium and other toxic materials. 

Maximum recommended pressure-temperature ratings for brazed joints made with copper tubing and copper or copper-alloy fittings and using representative commercial brazing alloys... 

Along with high mechanical properties, copper improves its conductivity in the range of lower temperatures (at -160°C 400, -190°C 450, and -252°C 1600 kcal/m °C hr). It softens in the temperature range of 200 to 220°C as shown in Figure 3.5. The casting properties of copper are rather fair, but copper can be readily stretched, flattened, rolled, welded and brazed. For chemical plant work, welded or brazed joints have become almost universal. 

More than 30 years have passed since the U.S. nuclear submarine Thresher sank, with the loss of 129 lives, and the reasons may have been forgotten. The immediate cause was a leak of seawater from a silver-brazed joint in the engine room. This, it is believed, short-circuited electrical equipment, causing a shutdown of the reactor. As a result, the submarine was unable to empty its ballast tanks and rise to the surface. 

Advice on the reduction of bimetallic corrosion at welded and brazed joints can be found in Reference. The use of replaceable wastage pieces to take up the bimetallic corrosion in various systems is proposed in References... 

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) . 

Brazing Satisfactory brazed joints in molybdenum have been made using oxyacetylene torch and furnace brazing techniques. 

An interesting example of judicious choice of braze filler is to be found in the selection of silver alloys for the brazing of stainless steels to be subsequently used in a tap-water environment . Although the brazed joint may appear to be quite satisfactory, after a relatively short exposure period failure of the joint occurs by a mechanism which appears to be due to the break-down of the bond between the filler and the base metal. Dezincifica-tion is a prominent feature of the phenomenon and zinc-free braze alloys based on the Ag-Cu system with the addition of nickel and tin have been found to inhibit this form of attack. A similar result is obtained by electroplating 0-007 mm of nickel over the joint area prior to brazing with a more conventional Ag-Cu-Zn-Cd alloy. 

Brazing Joint Detail. The type of brazing joint detail shall be limited to the type specified/qualified per BPS/PQR, which shall be representative of the production brazement. For details, see Fig. GR-3.8.1. 

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

All couplings (fittings) for newly constructed and repair brazements shall be manufactured with internal stops. The stops shall control the internal gap between the pipe component ends. For requirements of brazed joint pressure fittings, see ASME B16.50. 

Brazed joints that have been found to be defective may be rebrazed, where feasible, after thorough cleaning, by employing the same brazing procedure used for the original brazement. 

The brazed joints shall be properly cleaned, along with the areas of the inside and outside surfaces for a minimum distance of 25 mm (1 in.). 

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. 

All brazed joints shall have complete penetration, whether brazed from one side or from both sides. 

Brazing fluxes shall be applied to remove oxides and contaminants from base materials to ensure good-quality brazed joints. They remove only surface oxides and tarnish other contaminants (oil, grease, lubricants, and protective coating) must be removed either mechanically or chemically before brazing. 

Using too little flux, however, can lead to premature flux exhaustion and inadequate coverage, producing unsound or unsightly brazed joints. 

Large Diameter Steel Ranges NPS 26 Through NPS 60 Metric/lnch Standard Wrought Copper and Copper Alloy Braze-Joint Pressure Rttings... 

The procedures recommended represent the best current practice and are necessary to the control of brazed joint quality. These practices are applicable to all products and brazing processes. Whenever any or some of these practices are omitted when producing critical components, the omission should be the result of a rational decision, not the result of a lack of knowledge of the best practice. 

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. 

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