Coil joining criteria for tube and pipe mills
A coil end joiner, shear welder, end welder, coil splicer, strip welder, shear and end welder, or butt welder—whatever you call it, it performs the same simple task coil after coil: It quickly shears strip ends, butts them, and provides a smooth ductile weld so that the newly joined coil can pass through a tube mill.
A coil end joiner's obvious benefit is that it maintains the continuity of stock flow through the line, preventing rethreading downtime and the resulting scrap, tooling wear, and lost production.
The spectrum of tube and pipe applications for coil joining presents various challenges and considerations.
Maximum Cycle Time. The coil end joiner must perform its shearing, strip end butting, and welding functions before the accumulated coil is consumed. The minimum strip accumulator storage time determines the maximum coil end joiner cycle time.
To calculate the maximum coil end joining cycle time, add the time needed to weld across the widest strip in the heaviest gauge for that width to the 40 seconds generally required for shearing and fit-up of strip ends, plus strip feed time. Figure 1shows weld speed ranges for various welding methods. The type of welding equipment and coil joiner automation used play large roles in minimizing cycle times.
Duty Cycle. To determine the weld power source needed and physical durability requirements of the equipment, duty cycle must be considered. In terms of welding torches, hydraulics, chill bars and backup bars and the power source, heat is the enemy.
When a power source shuts down during a welding operation, it usually occurs after frequent use near or at its maximum output. This may be caused by exceeding the welding source's duty cycle specified by the manufacturer. Most power sources have a thermostat that shuts down the unit automatically to protect it from permanent damage.
Variable-displacement hydraulic units that shut down automatically between cycles can tackle the heat problem normally before a heat exchanger would be considered necessary for the hydraulic fluid. Using water-cooled welding torches and selecting torches with ratings in excess of the maximum requirements can ensure that heat will not cause interruptions. Water-cooled chill bars or copper clamping pads also are available.
Strip Parameters. Physical and chemical material properties, as well as strip width and gauge ranges, determine the size and features required in a coil end joiner. Most important, the carbon content will determine if weld normalizing is required. Also, shear strength and gauge determine the type of welding equipment employed.
Weld Requirements. Some coil splices are used just as a transport weld and then are discarded as scrap; others, such as submarine cable, can be used in the end product. A full understanding of weld requirements is necessary both to satisfy end-product expectations and to pass the weld through the mill unbroken and without damaging the mill.
Mills equipped with induction-type high-frequency (HF) coil end joiners can make welds that are aligned with or perpendicular to strip edges. However, bias shearing and welding 3, 7, or 15 degrees to the strip edge can produce weld seam ends that will not come together at once at the tube weld apex. This avoids the potential for "arcing out" in transformers on contact-style HF setups.
In addition to presenting staggered end weld edges to the HF contact shoes, bias shearing and welding introduce the end weld seam to roll tooling angularly. Even though end welds should be flat with parent metal thickness, buildup at the weld joint can range from 3 to 10 percent of parent metal thickness, depending on the type of welding used.
|Coil end joiners can be stationary or portable.|
Selecting Equipment and Welding Systems
Gas tungsten arc welding (GTAW) is most frequently employed on tube and pipe mills that run gauges up to 0.150 in. GTAW is at its best in the 0.004- to 0.160-in. range, in which weld seam buildup can be held below 5 percent. Because GTAW is strictly a fusion of parent metal without the introduction of outside chemistry, the weld area itself can be held close to the parent metal's mechanical and electrical properties.
GTAW speeds generally range from 5 inches per minute (IPM) on the heaviest gauges to 20 IPM. Faster weld speeds produce a narrower weld bead, creating the need for more precise tungsten electrode alignment and, in some cases, magnetic gas cups to help arc tracking or arc length control devices.
Many options can be used with GTAW power sources to achieve various welding results. These include pulsed arc for smoother welds, a cleaning cycle for red metals, and up- and downsloping weld currents to prevent suck-back or edge burnout.
Gas metal arc welding (GMAW) is the next most commonly used technique in tube and pipe mills. It operates successfully in 0.055- to 0.500-in. gauges or thicker. Because GMAW wire is driven into the weld seam and becomes a part of the nugget, buildups of 10 percent or so can be expected.
However, GMAW speeds are 25 to 50 IPM. GMAW offers the advantage of quickly incorporating additional chemistry to the weld for strengthening or flattening, as is the case with heavily oxygenated wires. A C-25 carbon dioxide-argon mixture is used with many GMAW wire types and diameters.
Plasma arc welding is a form of GTAW that uses two gases -- a shielder (argon) and a driver or plasma gas, which is a hydrogen-argon mixture. An enlarged tungsten electrode sharpened to a point sits in a retracted position inside the gas cup. It introduces its charge to a restricted gas flow through the nozzle and welds strip at speeds of 35 to 60 IPM.
Fully penetrated, narrow weld beads, or nuggets, are maintained through keyholing. This process involves melting each strip edge to be rejoined or flowing strip edges together behind the passing electrode.
High-frequency starting circuits, which run constantly within the plasma arc torch, are used to start multiple torch heads simultaneously on wide-strip applications for twice the weld width coverage in the same pass.
Mash seam, flash butt, and laser welding can offer superior weld integrity and speed, but may come with a high price tag, in addition to requiring precise settings.
Clamping Stations. Open-throat weld clamps hold the workpiece in portable coil end joiners; two-post clamping pads hold the workpiece in stationary coil end joiners. On fully automatic coil end joiners -- those that achieve strip end fit-up automatically -- the weld clamp station and the shear can be mounted on an indexing platen.
The shear-weld platen indexes the shear out of position after end cropping operations, simultaneously shifting the weld clamping station into rest beneath the sheared coil ends being held in outboard combination strip clamp and side guide units.
As the strip ends are contained in their outboard side guiding, before clamping the fit-up at the weld station, one end can be matched perfectly with the other if there is misalignment caused by camber. Camber compensation or even skew-adjust capability can be built into one of the side guide units on fully automatic models.
Skew or camber compensation devices can be operated by hand crank or hydraulics. Copper weld clamp pads help control heat in the heat-affected zone (HAZ) adjacent the weld puddle. Adjustable clamps can allow wider spacing on heavier gauges and close spacing to help sink heat and minimize warpage on lighter material.
Shears. Manufacturers employ a wide variety of shear types to clean-cut coil ends in preparation for the fit-up. Crop or guillotine shears with two- or four-post die set shears with spring-loaded pressure pads are most common in stationary models; however, open-throat roller or scissor-type shears also are used.
The gauge range for each application determines upper and lower shear blade clearances, rake angles, and tonnage capacity. The shear force needed initially to cut the heaviest gauge determines the tonnage required. Thus, strip width at the maximum gauge does not affect the shear force needed.
Shear blades can be designed with four-way reversibility to extend blade life and to delay the need for redressing.
Open-throat shears are used in portable applications so the coil joiner can be rolled away from the strip after completing the weld.
Incorporating a two-post die set shear, with the front post moving with the shear blade downward to engage a receiver bushing, closes the shear while it does its work.
These open fronts can be designed to handle strips up to 0.250 in. thick by 24 in. wide. The front pin then retracts from the receiver bushing during the return stroke of the blade to open the shear for strip removal at the end of the shear cycle.
Today's gas metal arc and gas tungsten arc welds
are extremely flat, so the need to use
equipment to return welds to parent metal thickness is rare.
are extremely flat, so the need to use
equipment to return welds to parent metal thickness is rare.
Normalizing Ovens. Processing steel strip high in carbon content (1040 and higher) generally requires changing the GMAW wire or in the case of GTAW, normalizing the weld by heating it to 1,100 degrees F (593 C) prior to processing. Press lines more commonly run higher-carbon steels than do tube mills, but in either case, normalizing is necessary to return the weld zone close to parent hardness and remedy the brittle grain boundary typical with high-carbon-alloy GTAW. Normalizing stations usually are quartz lamps but also can be induction or gas-fired.
GTAW Accessories. The same range of electronics available to help control GTAW arcs on stainless tube mills sometimes are employed for special coil joining applications. Arc length controllers can adjust electrode standoff automatically by measuring open circuit voltage throughout the weld, which then controls a stepper motor adjustment on the GTAW torch holder.
Magnetic gas cups also can be used to keep the arc tracking properly on the fit-up. These devices can be useful on wider strip, for which electrode erosion and misalignment are potential problems.
Skiving or Planishing. Today's gas metal arc and gas tungsten arc welds are extremely flat, so the need to use equipment to return welds to parent metal thickness is rare.
In some GTAW applications it may be desirable to planish the weld seam. Normally, this is done by rolling a crowned D-2 roll across the seam under hydraulic pressure in a postwelding operation.
Gas metal arc welds can be skived to parent thickness with an automated milling cutter, which is fairly common on rolling mills in which an 80 to 90 percent thickness reduction makes it desirable to overfill with GMAW wire and then skive the excess buildup.
Tab Setting. Mills that produce coaxial cable, submarine cables, and other products requiring full edge-to-edge welds commonly employ tab setting -- fitting parent metal tabs to each edge of the fit-up where the weld arc begins and finishes. This allows operators to snap tabs off after the cycle for a full edge-to-edge weld seam.
A recent patent eliminates the problems that have been associated with tab setting, including difficulty in setting tabs while the strip ends are clamped.
This automatic parent tab-setting device consists of a slide rear and front tab-holding backup bar and a pneumatic cylinder-loaded wedge mounted underneath. The backup bar can be relaxed by moving the wedge to allow tab fit-ups while the weld clamps attach strip ends firmly to the table. Next each tab simultaneously slides into position on either end of the fit-up. Finally the cylinder-loaded wedge is actuated to return the backup bar to its home position, thus clamping the tabs in place.
Clamp Leveler. Another device recently patented for use on coil joiners is the clamp leveler. A clamp leveler is used when a portable or rear line side guide machine processes both narrow and wide stock.
A portable coil end joiner with a 10-in. width capacity tends to overclamp the front edge of a 1-in. strip fit-up made at the rear of the clamps. Even heat sink across the entire weld length is critical to prevent weld edge burnout or suck-back. Overclamping the front edge of the fit-up can be eliminated by using a clamp leveler. A clamp leveler provides an offsetting workpiece at the front of the weld clamps.
Gauge pin extension is adjusted by a thumb wheel for each new thickness. Then a pneumatic cylinder-loaded wedge automatically extends the pins up through the front of the weld table for each cycle.
Taking these criteria into consideration will help you select and specify the right coil end joining equipment for your application.