Still continuing from the last post of Piping & Fabrication, but now we will enter the other topics and here we go.
Other Forming Operations. Some additional forming operations which can be performed in a pipe shop are extrusion, swaging, and lapping. Extrusions involve forming outlets in pipe by pulling or pushing a hemispherical or conical die from the inside of the pipe through an opening in the wall. The work may be done hot or cold depending on the characteristics of the material. Ferritic steels, austenitic steels, and nickel alloys are usually formed hot; aluminum and copper are usually formed cold. In order to assure that the outlet will have sufficient reinforcement, it is necessary to increase the wall thickness of the header as a function of the outlet size desired. An increase of 30 percent may be needed for large outlet-toheader ratios.
Swaging involves the size reduction of pipe ends by forging, pressing, or rolling operations. The operation is usually used to produce reductions of one to two pipe sizes. Ferritic steels, austenitic steels, and nickel alloys are usually formed hot. Aluminum and copper are formed cold. In lapped joints, a loose flange is slipped over the end of the pipe which is then heated to forging temperature, upset, and flared at right angles to the pipe axis. After heat treatment and cooling, the lapped section is machined on the face to attain a good gasket surface and on the back for good contact with the flange. The finished thickness of the lapped flange should be equal to or exceed the thickness of the pipe.
Layout, Assembly, and Preparation for Welding. In fabrication shops, piping subassemblies are often assembled on layout tables. Aprojection of the subassembly is laid out on the table in chalk. This establishes the baseline for locating the components and terminal dimensions of the subassembly, and the components are assembled relative to the layout. Prior to fit-up, it is essential that all weld surfaces be properly cleaned of rust, scale, grease, paint, and other foreign substances which might contaminate the weld. If moisture is present, the weld joint should be preheated. For alloy steels the heat-affected zone (HAZ) which results from thermal cutting should be removed by grinding or machining.
Depending on the configuration of the subassembly and root opening required by the welding procedure, some allowance may be required for weld shrinkage in the longitudinal direction. Actual shrinkage is difficult to predict and can vary considerably because of the many variables involved. For most open butt and backing ring joints, one-half the root opening is a reasonable allowance. For joints with other root configurations it may be as little as 1⁄₁₆ in (2.0 mm) for the lighter walls, increasing to as much as ⁵⁄₃₂ in (8 mm) for walls 4 to 5 in (100 to 127 mm) thick. Each weld joint should be carefully aligned within required tolerances using alignment fixtures, spacers, or jigs if necessary. Poor alignment may result in a poor weld. Once alignment is attained, the joint is usually tack-welded to maintain the alignment. The process used for tacking is usually that being used for the root-pass weld. Numbers and size of tacks should be kept to a minimum, but if the subassembly is to be moved elsewhere for weld out, their size must be sufficiently large so as not to crack during the moving operation. Temporary lugs or spacer bars may also be used for this purpose provided they are of a compatible material, the temporary welds are removed, and the surface examined after removal to assure sound metal.
Tack welds made by the shielded metal are welding (SMAW) or gas metal arc welding (GMAW) processes at the root of a weld should be removed or ground smooth since they can become a source of lack of fusion. For gas tungsten arc (GTAW) root welds, tacks usually fuse the adjacent lands to each other or to the insert, and filler metal is often not used. Tack welds are then fused into the weld during the root pass without further preparation. After tacking, the recommended practice is to complete the root pass and one or more weld out passes before starting to complete the weld by other processes to avoid burning through the relatively thin root.
Other Forming Operations. Some additional forming operations which can be performed in a pipe shop are extrusion, swaging, and lapping. Extrusions involve forming outlets in pipe by pulling or pushing a hemispherical or conical die from the inside of the pipe through an opening in the wall. The work may be done hot or cold depending on the characteristics of the material. Ferritic steels, austenitic steels, and nickel alloys are usually formed hot; aluminum and copper are usually formed cold. In order to assure that the outlet will have sufficient reinforcement, it is necessary to increase the wall thickness of the header as a function of the outlet size desired. An increase of 30 percent may be needed for large outlet-toheader ratios.
Swaging involves the size reduction of pipe ends by forging, pressing, or rolling operations. The operation is usually used to produce reductions of one to two pipe sizes. Ferritic steels, austenitic steels, and nickel alloys are usually formed hot. Aluminum and copper are formed cold. In lapped joints, a loose flange is slipped over the end of the pipe which is then heated to forging temperature, upset, and flared at right angles to the pipe axis. After heat treatment and cooling, the lapped section is machined on the face to attain a good gasket surface and on the back for good contact with the flange. The finished thickness of the lapped flange should be equal to or exceed the thickness of the pipe.
Layout, Assembly, and Preparation for Welding. In fabrication shops, piping subassemblies are often assembled on layout tables. Aprojection of the subassembly is laid out on the table in chalk. This establishes the baseline for locating the components and terminal dimensions of the subassembly, and the components are assembled relative to the layout. Prior to fit-up, it is essential that all weld surfaces be properly cleaned of rust, scale, grease, paint, and other foreign substances which might contaminate the weld. If moisture is present, the weld joint should be preheated. For alloy steels the heat-affected zone (HAZ) which results from thermal cutting should be removed by grinding or machining.
Depending on the configuration of the subassembly and root opening required by the welding procedure, some allowance may be required for weld shrinkage in the longitudinal direction. Actual shrinkage is difficult to predict and can vary considerably because of the many variables involved. For most open butt and backing ring joints, one-half the root opening is a reasonable allowance. For joints with other root configurations it may be as little as 1⁄₁₆ in (2.0 mm) for the lighter walls, increasing to as much as ⁵⁄₃₂ in (8 mm) for walls 4 to 5 in (100 to 127 mm) thick. Each weld joint should be carefully aligned within required tolerances using alignment fixtures, spacers, or jigs if necessary. Poor alignment may result in a poor weld. Once alignment is attained, the joint is usually tack-welded to maintain the alignment. The process used for tacking is usually that being used for the root-pass weld. Numbers and size of tacks should be kept to a minimum, but if the subassembly is to be moved elsewhere for weld out, their size must be sufficiently large so as not to crack during the moving operation. Temporary lugs or spacer bars may also be used for this purpose provided they are of a compatible material, the temporary welds are removed, and the surface examined after removal to assure sound metal.
Tack welds made by the shielded metal are welding (SMAW) or gas metal arc welding (GMAW) processes at the root of a weld should be removed or ground smooth since they can become a source of lack of fusion. For gas tungsten arc (GTAW) root welds, tacks usually fuse the adjacent lands to each other or to the insert, and filler metal is often not used. Tack welds are then fused into the weld during the root pass without further preparation. After tacking, the recommended practice is to complete the root pass and one or more weld out passes before starting to complete the weld by other processes to avoid burning through the relatively thin root.
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