Brazing metals

All soft solders are alloys composed mainly of lead and tin mixed in proportions that vary from as little as 30% lead (and 70% tin), to as much as 98% lead (and only 2% tin). The brazing metals are mostly alloys of copper or silver their composition, as well as that of other alloys used since antiquity for soldering, are listed in Table 48 (Zhadkevich 2004). 

This is the most common test method employed to qualify the leak characteristics of a new seal material. The test method involves applying the seal between two ceramic discs or between a ceramic and a metal disc, pressurizing the cavity formed by the seal and monitoring the pressure decay as a function of time.22 Alternatively, a metal tube and a ceramic disc can also be used [34], Typically, the cavity is pressurized to about 2 psi and the leak rate is determined by the pressure decay as a function of time. These tests can be done at room temperature or elevated temperatures. Similar test arrangement has also been used to test a plastically deformable brazed metal seal between fuel cell anode material and Haynes 214 washer [35], The cavity is pressurized to measure the rupture strength of the seal material. 

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. 

FLUX (Solder). A material which by its chemical action facilitates the soldering and brazing of metals. Such a flux applied to a metallic surface cleans it and renders it receptive to amaigamalion with the solder or brazing metal. Some fluxes are rosin, for soldering tin muriatic acid, fnr galvanized iron and other zinc surface and borax for brazing,... 

Repair pinholes in glass or poorly welded or brazed metal parts. 

Workers concerned with liquid/solid systems with exceptionally good wettability, such as some braze/metal workpiece systems, find that the drops do not preserve a perfectly circular area of contact because of the sensitivity of the liquid to changes in capillary attraction caused by minor variations in the surface texture of the workpiece. Because of such irregularity, some workers have used the contact area of a small volume of braze as a measure of wettability. Thus Feduska used area measurements to differentiate between the wetting of different stainless steels by a wide range of metals and alloys (Feduska 1959). 

The predicted behaviour of individual liquid metals depends on their specific physical properties, such as those listed in Table 10.1 for Hg and a number of solder and braze metals and alloy solvents. Substitution of these values into... 

Figure 10.3. Some preferred joint configurations for brazed metal components.

Use of the evacuated apparatus (style E) may improve some transport reaction rates. The tantalum portion is attached to a glass joint and stopcock by a brazed metal-to-glass seal. The flat part can be produced by a pair of vise blocks which are driven together by tightening bolts through them while the apparatus is attached to the vacuum line. The evacuated and closed container is then welded from the side at the flat portion and cut off above the weld. The same blocks may be utilized within the dry box instead of a vise. 

The C-C, C-SiC, and Cu-clad-Mo plates were sliced into 2.54 cm x 1.27 cm x 0.25 cm pieces, and ultrasonically cleaned in acetone for 15 min. prior to brazing. Braze powders were mixed with glycerine in a viscous paste with dough-like consistency, and a small (-0.2 g) quantity of this paste was applied to the joint region between the composite and metal. The sandwiched composite/braze/metal structure was placed under -6.0 kPa pressure (2.0 N load) in a vacuum furnace which was heated to the brazing temperature (-20°C above Tl) under a pressure of 10 torr. After a 5 min. soak at the brazing temperature, the assembly was slowly cooled at a controlled rate ( 5°C per min.) to 400°C, followed by furnace cooling. 

An active filler metal approach, using Incusil-ABA braze metal fabricated by GTE-Wesgo was used to braze Zirconia to modular cast iron. This material has the nominal composition ... 

Similar to the ceramic-ceramic joints, the constitutive response of the base zirconia, cast iron, and braze metal were determined in order to predict joint response based on stress-strain properties of the constituent materials. Tensile tests were performed on cast iron and Incusil-ABA, while bend tests were performed on the base zirconia. The temperature of testing ranged from room temperature to 400 C, the upper temperature limit for this type of joint. The strength and toughness of joints were evaluated using unnotched and notched bend bars and disk specimens. 

Geoeetry Base Zr02 For Zr02/CI Joint Cast I ron Braze Metal 2r02/Cast Iron Joint MASZ-67 Interlayer UASZ-80 Inter 1aycr MASZ-50 Interlayer... 

Where permanent types are used, the parts themselves can be joined and narrow gaps filled with weld or brazing metal. Where separation is wider, a strip of metal can be welded or brazed at both ends across the gap. Bonds connecting one vibrating part to another part that may or may not vibrate should be of a flexible material that will not fail under vibration. Corrosion because of the joining of dissimilar metals may cause the electrical resistance across the bond to increase. This is especially noticeable in humid or corrosive atmospheres. The types of metal for the bond and its fastenings must therefore be selected with care. Grounds and bonds should... 

Active metal brazing (AMB) is a process to braze metals, such as Kovar , titanium, copper, and molybdenum, to ceramics. The process does not require a metallization on the ceramic. For this chapter, only copper brazing will be addressed. The AMB process is used by some manufacturers as an alternative to DBC as it provides higher adhesive strength while using a less critical furnace profile. It also does not rely on oxide formation as in the DBC process. 

The condition of the brazed-on wear pads after irradiations in NRX and Halden has been excellent. Pig. 16. The braze metal fillet has a grey appearance and undergoes a little more corrosion than the can, but the actual penetration Is very small. The H concentration in the wear pad is up to 50 greater than in the can, presumably because of the lower temperature. Specimens having brazed wear pads attached have been irradiated In the Dounreay Fast Reactor (DFR) to total neutron doses of 2.5x10 without any noticeable effects on micro-structure. Mechanical tests on similar samples are being carried out. 

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 that appears to be due to the breakdown of the bond between the filler and the braze metal. De-zincification 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. 

Heat is applied to the parts to be joined which melts a manually fed or pre-placed filler braze metal (which has a melting temperature >450 °C) into the joint by capillary action. A flux is usually applied to facilitate wetting of the joint, prevent oxidation, remove oxides and reduce fuming (see 7.12F). 

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