Clad, Metal-Coated, and Lined Pipe.
With the continuous of post from Piping & Fabrication, Clad, Metal-Coated and Lined Pipe will explain here. There are instances when it is economically desirable to construct a piping system from relatively inexpensive material but with an interior surface having corrosion- or erosion-resistant properties. Clad pipe may be made by seam welding of clad plate, by weld metal overlay of the inside surface, or by centrifugal casting of a pipe with two metal layers. Lined pipe is made by welding a linear, sometimes as strips, to the inside surface of the pipe. Metal-coated pipe is made by dipping, metal spraying, or plating the entire pipe.
Before choosing construction which requires welding of clad, lined, or metalcoated pipe, such factors as filler metal compatibility, filler metal strength relative to the base metal strength, dilution of base metal into the finished weld, and need for postfabrication heat treatment must be considered. Because it is not possible to cover the great many combinations of base metals and cladding, lining, or metal coatings, some examples of the more common applications will be given. For corrosion services, a carbon steel base material, clad or lined with austenitic stainless steel, is often used. The cladding is usually about 3⁄32 to 5⁄32 in thick. Where the inside of the weld is accessible, the preferred method is to weld the base metal from the outside with carbon steel filler metal, back-gouge the root from the inside, and weld the root from the inside with two or more passes of austenitic filler metal to minimize dilution from the base metal. Where the inside surface is not accessible, a backing strip of the same composition as the cladding, fillet welded to the cladding on the upstream side may be used.
The root weld between the two clad surfaces and the austenitic backing strip is then made with austenitic filler metal. The root weld can also be made with the GTAW process using austenitic filler or preplaced inserts. The carbon steel should be removed for a sufficient distance back to preclude dilution into the root weld. In most instances, the balance of the weld is usually made with austenitic filler metal since it is not good practice to deposit carbon steel or low-alloy steel directly against the stainless steel deposit. See Fig. A6.22b. In some cases, nickel-base alloys are used for cladding where high-temperature corrosion is involved. The joints may be treated much like the austenitic cladding, except that appropriate nickel-base filler metals are used.
Some services require the use of carbon steel pipe nickel plated on the inside surface. Since the plating is relatively thin, different approaches are needed. First, as much fabrication as possible should be done prior to plating. For joints to be welded after plating, the ends to be prepared for welding should be buttered with nickel filler metal and machined to the required contour prior to plating. The root weld is made using the GTAW process with nickel filler metal.
Some occasions require the use of aluminized pipe. Steel pipe is prefabricated and coated with aluminum by immersion in a bath of molten aluminum or by metal spray. Where the inside of the weld will not be accessible for metal spray, one method of joining is to counterbore the ends and use a solid machined backing ring which is fit and welded into one side of the joint prior to coating. After coating, the weld is made using an appropriate base metal process and filler, taking care not to blister the aluminum coating on the underside of the backing ring.
Galvanized steel pipe is often used for external corrosion applications. Since welding of galvanized pipe releases toxic vapors and since the welded area most often cannot be regalvanized, welding of galvanized pipe is not recommended. It is preferable that the assemblies be fabricated with provisions for mechanical joining in the field and then galvanized.
For services involving erosion, carbon steel pipe is often lined with cement or some type of abrasion-resistant material which cannot be welded. In this case the joints are butted together to minimize the gap between the adjacent linings. The weld is then made between the two carbon steel weld bevels, recognizing that full penetration through the carbon steel joint may not be achieved and that additional thickness may be necessary for strength. The gap between the adjacent linings is usually not a problem if only erosion is present.
With the continuous of post from Piping & Fabrication, Clad, Metal-Coated and Lined Pipe will explain here. There are instances when it is economically desirable to construct a piping system from relatively inexpensive material but with an interior surface having corrosion- or erosion-resistant properties. Clad pipe may be made by seam welding of clad plate, by weld metal overlay of the inside surface, or by centrifugal casting of a pipe with two metal layers. Lined pipe is made by welding a linear, sometimes as strips, to the inside surface of the pipe. Metal-coated pipe is made by dipping, metal spraying, or plating the entire pipe.
Before choosing construction which requires welding of clad, lined, or metalcoated pipe, such factors as filler metal compatibility, filler metal strength relative to the base metal strength, dilution of base metal into the finished weld, and need for postfabrication heat treatment must be considered. Because it is not possible to cover the great many combinations of base metals and cladding, lining, or metal coatings, some examples of the more common applications will be given. For corrosion services, a carbon steel base material, clad or lined with austenitic stainless steel, is often used. The cladding is usually about 3⁄32 to 5⁄32 in thick. Where the inside of the weld is accessible, the preferred method is to weld the base metal from the outside with carbon steel filler metal, back-gouge the root from the inside, and weld the root from the inside with two or more passes of austenitic filler metal to minimize dilution from the base metal. Where the inside surface is not accessible, a backing strip of the same composition as the cladding, fillet welded to the cladding on the upstream side may be used.
The root weld between the two clad surfaces and the austenitic backing strip is then made with austenitic filler metal. The root weld can also be made with the GTAW process using austenitic filler or preplaced inserts. The carbon steel should be removed for a sufficient distance back to preclude dilution into the root weld. In most instances, the balance of the weld is usually made with austenitic filler metal since it is not good practice to deposit carbon steel or low-alloy steel directly against the stainless steel deposit. See Fig. A6.22b. In some cases, nickel-base alloys are used for cladding where high-temperature corrosion is involved. The joints may be treated much like the austenitic cladding, except that appropriate nickel-base filler metals are used.
Some services require the use of carbon steel pipe nickel plated on the inside surface. Since the plating is relatively thin, different approaches are needed. First, as much fabrication as possible should be done prior to plating. For joints to be welded after plating, the ends to be prepared for welding should be buttered with nickel filler metal and machined to the required contour prior to plating. The root weld is made using the GTAW process with nickel filler metal.
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| Examples of welding clad |
Some occasions require the use of aluminized pipe. Steel pipe is prefabricated and coated with aluminum by immersion in a bath of molten aluminum or by metal spray. Where the inside of the weld will not be accessible for metal spray, one method of joining is to counterbore the ends and use a solid machined backing ring which is fit and welded into one side of the joint prior to coating. After coating, the weld is made using an appropriate base metal process and filler, taking care not to blister the aluminum coating on the underside of the backing ring.
Galvanized steel pipe is often used for external corrosion applications. Since welding of galvanized pipe releases toxic vapors and since the welded area most often cannot be regalvanized, welding of galvanized pipe is not recommended. It is preferable that the assemblies be fabricated with provisions for mechanical joining in the field and then galvanized.
For services involving erosion, carbon steel pipe is often lined with cement or some type of abrasion-resistant material which cannot be welded. In this case the joints are butted together to minimize the gap between the adjacent linings. The weld is then made between the two carbon steel weld bevels, recognizing that full penetration through the carbon steel joint may not be achieved and that additional thickness may be necessary for strength. The gap between the adjacent linings is usually not a problem if only erosion is present.
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