Transition joints

Radiographic. Radiography is an exceUent nondestmctive test (NDT) method for evaluating the bond of Al—steel electrical and Al—Al—steel stmctural transition joints. It provides the capabiHty of precisely and accurately defining all nonbond and flat-bond areas of the Al—steel interface, regardless of size or location (see Surface and interface analysis). 

The clad plate is x-rayed perpendicular from the steel side and the film contacts the aluminum. Radiography reveals the wavy interface of explosion-welded, aluminum-clad steel as uniformly spaced, light and dark lines with a frequency of one to three lines per centimeter. The waves characterize a strong and ductile transition joint and represent the acceptable condition. The clad is interpreted to be nonbonded when the x-ray shows complete loss of the wavy interface (see X-ray technology). 

Vessel heads can be made from explosion-bonded clads, either by conventional cold- or by hot-forming techniques. The latter involves thermal exposure and is equivalent in effect to a heat treatment. The backing metal properties, bond continuity, and bond strength are guaranteed to the same specifications as the composite from which the head is formed. AppHcations such as chemical-process vessels and transition joints represent approximately 90% of the industrial use of explosion cladding. 

Transition Joints. Use of explosion-clad transition joints avoids the limitations involved in joining two incompatible materials by bolting or riveting. Many transition joints can be cut from a single large-area flat-plate clad and deflvered to limit the temperature at the bond interface so as to avoid undesirable diffusion. Conventional welding practices may be used for both similar metal welds. 

Usually, copper surfaces are mated when joints must be periodically discoimected because copper offers low resistance and good wear. Junctions between copper and aluminum bus bars are improved by using a copper—aluminum transition joint that is welded to the aluminum member. Deterioration of aluminum shunt connections by arcing is eliminated when a transition joint is welded to both the primary bar and the shunting bar. 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Explosion-welded constmction has equivalent or better properties than the more compHcated riveted systems. Peripheral benefits include weight savings and perfect electrical grounding. In addition to lower initial installation costs, the welded system requires tittle or no maintenance and, therefore minimizes life-cycle costs. Applications of stmctural transition joints include aluminum superstmctures that are welded to decks of naval vessels and commercial ships as illustrated in Figure 11. 

Explosion-bonded metals are produced by several manufacturers in the United States, Europe, and Japan. The chemical industry is the principal consumer of explosion-bonded metals which are used in the constmction of clad reaction vessels and heat-exchanger tube sheets for corrosion-resistant service. The primary market segments for explosion-bonded metals are for corrosion-resistant pressure vessels, tube sheets for heat exchangers, electrical transition joints, and stmctural transition joints. Total world markets for explosion-clad metals are estimated to fluctuate between 30 x 10 to 60 x 10 annually. 

Transition joints are used to join dissimilar metals where flanged, screwed, or threaded connections are not practical. They are used when fusion welding of two dissimilar metals forms interfaces that are deficient in mechanical strength and the ability to keep the system leak-tight. Transition joints consist of a bimetallic composite, a stainless steel, and a particular kind of aluminum bonded together by some proprietary process. Some of the types in use throughout the cryogenic industry are friction- or inertia-welded bond, roll-bonded joint, explosion-bonded joint, and braze-bonded joint. 

In-service inspection and monitoring is based on the requirements of ASME section XI, Div 3. For the main vessel, in addition to ASME requirements of continuous monitoring, ultrasonic examination is planned to be carried out through the main vessel - safety vessel interspace (300 mm nominal gap). A periscope is provided for visual examination of reactor internals. Eddy current inspection is under development for the SG tubes. SG tube size and expansion bend design takes into account this inspection requirement. Ultrasonic examination is planned for the dissimilar joints of the roof slab - main vessel and SG transition joint. The subject of ISI for other reactor components important to safety is under study. For the safety related reactor assembly components, which are non-inspectable, an additional factor of safety in design is envisaged. 

Channel tubes (88 mm o.d. and 4 mm thick) are of welded design and contain fuel assemblies which are cooled by boiling light water. The upper and lower parts of the channel are made of stainless steel and the central part, located in the active zone, is made from a zirconium/2% niobium alloy. The central part is joined to the upper and lower parts by vacuum diffusion-welded stainless steel/zirconium transition joints. The channel tube is attached to the upper duct by a welded joint, and to the lower one by a compensator unit, which is necessary to compensate for the difference in thermal expansion of the channels and ducts without destroying the leak-tightness of the reactor cavity. This type of joint makes it possible to replace a channel during reactor shutdown. 

American Society for Testing and Materials (ASTM), ASTM D3138-04, Standard specification for solvent cement for transition joints between acrylonitrile-butadiene-styrene (ABS) and poly(vinylchloride) (PVC) nonpressure piping components. Annual Book of ASTM Standards, Vol. 08.04. 

Experience shows that erosion or damage often starts at joints and transitions. Therefore, important aspects of revetment constructions, which require special attention are the joints and the transitions joints onto the same material and onto other revetment materials, and transitions onto other structures or revetment parts. A general design guideline is that transitions should be avoided as much as possible, especially in the area with maximum wave attack. If they are inevitable, the discontinuities introduced should be minimized. This holds for differences in elastic and plastic behavior and in the permeability or the sand tightness. Proper design and execution are essential in order to obtain satisfactory joints and transitions. 

Process gas from the outlet of the reformer tubes passes through the transition joint outside the furnace to the refractory lined outlet collector. This outlet system is used when the outlet temperature is high (above ca. 900 C). In other applications, an alternative outlet system with hairpins and hot collector is used (Kawai et al., 1984) and (Dybkjaer et al., 1989). 

Assemblies between steel and aluminium can be simplified by using transition joints that are weldable on both sides (Figure B.3.4) [18]. Their use has become widespread in shipbuilding for welding joints between steel and aluminium (such as aluminium superstructures on a steel deck) and could be used also in the transportation sector [19]. 

Fig. 8-10. Zircaloy-2 type-347 stainless steel transition joint for HRE-2 pressure vessel.

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