Flash butt welding of pipe
Basics of the process
This article describes the flash-butt welding process as used to weld pipes externally and internally. The authors describe the process, how it's used, how its technology is employed, advances in the process, materials that can be welded, and equipment used.
Flash butt welding (FBW) is one of several resistance welding processes. At the E.O. Paton Electric Welding Institute, Kiev, Ukraine, this process was improved, and as a result, new processes were developed and patented along with corresponding equipment. Resistance processes are used for joining pipes, tubes, rails, and rolled stocks.
In FBW, pipes to be welded are moved together while electric contacts are formed on the pipe ends. These contacts are heated, overheated, and burst out, causing pipe-end heating.
FBW can be used for welding pipeline, railroad tracks, and boilers in the construction industry and in fabrication shops. Portable FBW equipment, which moves over the pipe or rail to be welded, is used in field conditions. Fabrication in shops and pipe mills involves a fixed welding station, where the pipe is moved to the welder.
Benefits of flash butt welding include:
- High weld quality because of solid fusion. Lack of a molten pool eliminates many conventional defects.
- Automated remote control system, so manual welding skills are not necessary.
- No consumables or shielding gases are used. This eliminates a source of contamination and alloys.
- Cycle time is one to three minutes.
- Pre- and postheating are not necessary, so productivity and quality are not affected by weather, temperature, or delays caused by other factors.
- Joint-end preparation is not required. A rough cut with a 1/4-inch gap between joints is acceptable.
Flash butt welding uses a flash to provide enough heat to soften the ends of the pipes to be welded to near-molten temperatures. As the pipes are moved together, small areas come into contact, and through resistance heating, become overheated. These high-temperature areas burst out as tiny bits of molten steel.
The flash is generated by applying electric force to the ends of both pipes. Once pipe ends are hot enough, the joints are forced together to form a weld. The process is computer-controlled.
To achieve uniform heating across the pipe circumference, it's essential to maintain a continuous flash without interruption or pipe-end sticking.
Continuous flashing is especially important for quality control of large-diameter pipe. By keeping it continuous from the very start of the process, more uniform heating occurs across the entire cross-sectional area of misaligned pipe joints.
The E.O. Paton Electric Welding Institute has focused on welding commercial steel, from low-carbon to low-alloy, and corrosion-resistant steels.
Low-carbon Steel. In FBW of low-carbon and low-alloy steels, the upsetting pressure and power consumption remain relatively constant. Other parameters vary with pipe diameter and steel composition. Low-alloy steels require higher final flashing feed speeds than low-carbon steels.
Corrosion-resistant Steel. These steels are used in the power and chemical industries; in oil and gas systems; and in pipelines carrying corrosive materials, particularly those high in hydrogen sulfide and carbon dioxide.
Austenitic steels are common in power generation and chemical plants; ferritic-austenitic (duplex) steels are used for oil and gas pipelines. In conventional arc welding of these steels, it's difficult to develop high corrosion resistance in the welds while maintaining weld strength at the level of the base metal.
The program for welding low-alloy steels is similar to that for low-carbon steels. Variations exist in the total process time, the feed speed through the flashing and upsetting phases, and the length of the preset upset stroke. The basic welding parameters for common corrosion-resistant steels are characterized by higher flash and upset speeds and by a higher upsetting distance, or stroke.
FBW machines are available to join 2- to 56-in.-dia. pipe. Two variations of the equipment are external machines that are lowered over the outside of the pipe to weld it and internal machines that make the weld from inside the pipe. The external equipment can handle diameters from 2 to 20 in., while larger pipe can be welded from the inside.
External Machines. These look like a pair of tongs connected by three rods. Transformers are integrated with the welding equipment, allowing it to be handled by one side-boom tractor. The welding circuit is designed for low impedance and uniform current supply to the ends of the pipes to be joined.
Internal Machines. These self-propelled machines travel inside the pipe to the weld area. Clamping shoes located around the circumference press on the joint ends. A ring-transformer built into the alignment system provides uniform current flow around the entire circumference.
The self-contained drive moves the welding machine at approximately 1.5 feet per second (FPS). The equipment has integral hydraulic and electric controls. Cables with special connectors supply power from an external source.
A built-in shearing device removes weld flash while the metal is soft as the unit moves to the next joint.
Field Welding External Machines. Side-boom tractors carry these welding machines along the strung pipe. The tractor also carries the power source. The welding head is lowered over the joints to be welded, and the clamps are engaged. This holds the pipe and aligns the pipe ends. Pipe ends are cleaned before the welding commences. When the weld is complete, a flash remover is inserted into the weld area on a stinger rod, and the internal flash is removed while the metal is still soft. A welding gang for this machine consists of six people.
Field Welding Internal Machines. For these larger machines, two side-boom tractors support the pipe during welding. The machine moves from weld to weld inside the pipe. Eleven people make up this welding gang.
Professor Sergei Kuchuk-Yatsenko is deputy general director and Dr. Boris Kazymov is senior of the scientific staff at E.O. Paton Electric Welding Institute, 11 Bozhenko St., Kyiv, 252650, Ukraine, phone 380-44-227-7446, fax 380-44-227-6329, e-mail email@example.com firstname.lastname@example.org. The E.O. Paton Electric Welding Institute performs fundamental and applied research and develops technologies; materials; equipment; control systems; quality assurance methods; and devices for advanced welding and joining technologies, welded structures, strength, and fatigue life.
Sergei Kuchuk-Yatsenko, Flash Butt Welding (Kiev: Naukova Dumka, 1992), pp. 20-60.
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