May 30, 2001
This article is aimed not at tube producers, but at fabricators of tubing, to provide an overview of the process.
This article is aimed not at tube producers, but at fabricators of tubing, to provide an overview of the process.
In the simplest terms, a welded tube is made by taking a piece of steel strip, rolling it into a cylinder, and then heating the edges and forging them together to make a tube.
Four different welding processes are typically used for making tube. Since there is no filler metal in these welds, the welds are at least as strong as the parent metals.
High Frequency (HF) Welding. Today, most mild steel and aluminum tube is welded with HF welders, which are also suitable for 400 series stainless and even 300 series if the tubing is for decorative use.
Nearly 95 percent of the HF welders sold are solid-state, using transistors to generate the alternating frequency current. Research has been undertaken to weld products less than 1/4 inch in diameter and to extend the ability of the welder to produce a pressure-quality weld on 300 stainless steel at high line speeds.
Compared to other welding techniques, high frequency is somewhat forgiving of less-than-perfect edge registration.
Electric Resistance Welding (ERW). Low-frequency welding, accomplished through a rotating copper electrode, is used most often for mild steel tube from .250 to .375 inch in diameter.
This process is most suitable for the smallest diameters, as the impeders needed for HF welding cannot fit into the tiny tubes. As the welding transformer rotates, the copper electrodes come into contact with the tube edges, heating them so that they can be welded.
Gas Tungsten Arc Welding (GTAW). GTAW is used for pressure-quality welds on 300 series stainless steel at slower welding speeds. It produces a good weld with little bead.
A welding torch with a tungsten tip, usually employing argon or an argon/oxygen mixture, heats the edges of the strip until the metal begins to melt or puddle, and the next pair of rolls squeezes the strip edges together. Here, edge registration is critical.
Laser Welding. The cost of laser welding is high relative to that of GTAW, although it is faster. It is used primarily for exotic metals and 300 series stainless steel, though some manufacturers use it on 400 series stainless.
Laser welding produces the smallest heat-affected zone (HAZ) and a bead even smaller than that produced by GTAW. Since there is almost no change in the grain structure of the parent metal, the finished piece has greater flexibility than a GTAW part. However, with this technique, edge registration is more critical than with the others.
It is often stated that form follows function. This is no truer than in the specification and design of a tube mill. As mentioned previously, mills can now be almost completely customized.
The client first determines what product or range of products will be made. The client also determines the preferred rate of production, quality level, and any particular characteristics such as end condition, length tolerance, position of the seam on shapes, etc.
Generally, the material chosen dictates the type of welder used. The required output of the welder is determined by the diameter, wall, and required speed of the mill. If the client has not chosen a speed, the mill builder can determine that based on annual production requirements, estimated uptime, number of shifts, and expected changeover frequency. With this information, the mill builder can also calculate the horsepower requirements.
The number and size of the stands and the diameter of the roll shafts is determined by the diameter of the tube, its wall thickness, and the material being formed. The ratio of thickness to diameter, the surface finish, and the shape of the product also have a direct bearing on the number of stands required.
Other factors, such as the need to preserve existing tooling or specific placement of the seam, may also affect the design.
Finally, after the number of stands and the required horsepower are finalized, the number and placement of motors can be determined.
Most tube mill buyers want a mill that is large and sturdy, but they must not oversize the mill for the task at hand. The mill must be large enough to do the job, the shafts heavy enough to withstand bending, and the castings large enough to house the shafts and bearings, but the mill must be small enough to minimize the distance between the points where the roll contacts the material.
The diameter of the shafts and their effect on the throat diameter of the tooling must also be considered. A larger shaft diameter requires a larger throat, which means more costly tooling, but also a greater number of possible regrinds.
A tube mill line consists of a number of different pieces of equipment, but the machinery is interdependent. Changing the capacity of any one piece affects the operation of the others.
The major factors to consider in a high-frequency tube making line are:
The physical size of the tube and the welding and forming characteristics of the material are critical in determining production speed. Large, heavy-wall tubes require more heat and drive power to run at a given speed than do smaller, lighter wall tubes.
Weld power and drive power should be matched. Sufficient horsepower should be available to support the ability of the welder to weld the tube.
The type of cutoff and its accelerator may affect production speeds. If the operator has sufficient power to run product at maximum speed but wishes to cut short lengths off the mill, the ability of the press to cycle quickly and accurately will be the determining factor in line speed.
Often, when a mill is upgraded, the entry end of the line is overlooked. While the mill capacity may be 600 or 700 feet per minute, this can be limited by the capacity of the accumulator or the speed of the end welding equipment.
A mild steel welded tube line typically consists of the following equipment.
Uncoiler. The uncoiler pays strip out to the line. The typical uncoiler has two arms so that a second coil can be readied while the first is being run.
In lines for larger tube, the uncoiler may be equipped with power rotation of the head and powered coil keepers. In higher-speed lines, the extras may also include powered expansion of the arms and water-cooled brakes.
Shear/End Welder. The end welder is key to producing continuous tube. Its function is to shear the trailing edge of the first coil and the leading edge of the next coil, then weld the two together so that the strip can be fed continuously through the mill.
The end welder is available as a portable or stationary model in various levels of automation, depending on line speed. In lines for heavier-wall tube, the shear/end welder may be preceded by a strip flattener to flatten the ends sufficiently for welding.
Strip Accumulator. Strip must be stored so that the mill will have an ample supply while the end weld is being completed. The most commonly used ways to store strip are the horizontal and vertical accumulators.
Each type can accept strip at a rate two to three times the speed of the mill, creating a storage bank of strip in the form of a large coil, which the mill then depletes during the end welding process.
Welder. The types of welders have been already been discussed. Regardless of type, each one heats the strip edge to the temperature required for forging.
Drive System. The drive electrical system consists of the drive (or controls) and the motors. Typically, there is one motor for each section of the mill.
The lead motor is in the fin section, and the controls are designed so that the overall mill speed is set by the fin motor. However, the other two motors can be trimmed appropriately by the operator.
The breakdown motor runs slightly slower than the fin motor, and the sizing motor is slightly faster. In this way, the tube is pulled rather than pushed through the mill. The operator also uses the speed controls to minimize or eliminate any scuffing of the tube caused when the rolls run at a speed different from that of the tube itself.
Forming/Welding Mill. The forming and welding section is discussed in greater detail later in this article. It consists of vertical and horizontal passes that form the strip into an open tube. The weld area forges the strip into a closed tube and then removes the bead created during the weld.
Sizing Mill. This section is also discussed in greater detail later. Once the tube is formed and cooled, the sizing section brings it to its final diameter. Also, if the tube is to be reshaped, that is accomplished here.
Cutoff System. A typical cutoff system used today is the double-cut flying shear. The press receives a signal from a length encoder mounted at the end of the mill. The encoder signals the die set to accelerate to the speed of the tube. The die set can be powered by an alternating current (AC) motor, direct current (DC) motor, or hydraulic power system.
When it reaches line speed, the die set clamps on the tube, holding it so that it can be cut. The press receives a signal, the ram descends, and the horizontal blade makes a broach cut across the top of the tube. The vertical blade then descends, completing the cut.
Runout Table. The runout table transfers the tube from the mill to the packaging system. A typical runout table consists of undriven bronze vee rollers on which the tube decelerates from the press. Bronze is used to prevent marks on the tube.
Usually, the tabletop is pneumatically actuated to dump the tube. In the past, tube would typically be dumped onto an inclined table and then allowed to fall into an accumulation rack for packaging or transfer to another area of the plant. In most large tube making operations today, however, tube is fed into an automated packaging system.
Exit Tube Handling System. The automated exit tube handling system, or bundler, accepts tube from the runout table or from an intermediate source, such as a conveyor, and then forms the tubes into a bundle for strapping.
The tube can then be strapped manually or automatically. After strapping, the completed bundle is transferred to exit/storage conveyors until it is removed.
Testing. The most common type of on-line testing equipment is the eddy current tester, which induces a current into the entire tube or the weld zone. It is used to detect weld flaws that can damage the integrity of the tube. Pressure tubes may be hydrostatically tested (filled with fluid and pressurized for a given time). Ultrasonic testing may also be used.
Detailed Description of Mill Elements
The tube mill itself consists of four major areas:
The forming section is further divided into three areas:
Entry Guide. As its name implies, the entry guide ensures that the strip enters the mill properly. It usually contains conveyor or pinch rollers set at the tube bottom line height, as well as two pairs of guide rolls to ensure that the strip is straight upon entering the forming section.
If the strip is not straight, the tube will be formed incorrectly, resulting in poor weld quality as well as tube that wants to bend or bow.
The entry guide may also contain scarfing tools or other edge preparation equipment to trim the strip to precisely the right width or to remove aluminum or galvanized coatings for a better quality weld.
Breakdown Passes. The breakdown section typically consists of three or four driven vertical stands. They break the edge of the strip and form it into an initial U shape. The side roll, or idler passes in between the driven stands, holds the shape in place between the working stands.
The number of stands is determined by both the material being run and its relative thickness-to-diameter ratio. Very heavy-wall tubing requires extra stands so that less work is done in each stand, while very thin-wall tubing sometimes requires extra idler passes and vertical passes to ensure better control.