Rolling with tube reduction
March 7, 2013
The forge-welding process is one of the most efficient methods of making tube and pipe from 1/2 to 24 in. diameter, but it’s not limited to this range. An ERW mill’s capability can be expanded by adding a cold-stretch-reducing section, allowing it to make smaller diameters.
Manufacturing capabilities have advanced and expanded quite a bit over the past few decades, and tube and pipe mills have kept pace. Once limited to just a few welding processes and one product shape, round, mills these days have become increasingly specialized. They handle a growing number of alloys; use a diversity of welding processes; and turn out tube and pipe in endless combinations of sizes, shapes, and wall thicknesses.
Electric resistance welding (ERW) has been a boon to the tube and pipe industry, providing a fast, efficient way to make much of the tube and pipe used in manufactured products today—tubular components that go into industrial, commercial, military, and consumer products too numerous to list here. Tube and pipe products can be made on a variety of other machines, of course. Piercer mills, extrusion presses, draw benches, pilger mills, and mills that use welding processes such as laser, plasma, and tungsten inert gas (TIG), alone or in combination—each has its niche, the products it makes most efficiently.
However, these machine types aren’t always limited to a conventional niche. In some cases, a little innovation can expand a machine’s capabilities.
Although the forge welding process on an ERW mill doesn’t have specific limits to the diameters it can produce, it’s considered that it makes tube and pipe efficiently in diameters from 1⁄2 to 24 in. Larger diameters are limited by the cost of the tooling, the power needed to pull the strip through the mill, and the wear on the drive stands; smaller diameters are limited by the impeder size. The impeder, the device that directs the welding current toward the weld seam, has to fit inside the tube or pipe. Impeders can be manufactured smaller than ½ in. in diameter, but their effectiveness decreases as the size decreases.
This isn’t to say that a forge welding mill can’t make products smaller than ½ in.; it’s a matter of making small-diameter products efficiently. This type of mill can make a tube 3⁄16 in. in diameter, but without an impeder inside the tube, the current induced by the welding coil doesn’t get directed to the V. The mill operator compensates by applying more power, but it costs more to run the mill, and the excess heat often discolors the tube.
Another way to skin this cat is to form the tube to 0.500 in. dia., then use additional roll stands to reduce its size. A conventional forge welding mill already includes sizing stands to reduce the tube to its final size, normally a 1 percent reduction. Adding a few roll stands and using the cold-stretch-reducing (CSR) process can expand this mill’s capability, enabling reductions of more than 50 percent. This always the mill to make brake and fuel lines for automobiles; coolant lines for refrigerators, freezers, and air conditioners; and other small-diameter products.
Adding the CSR process to a forge welding mill isn’t new, but it is far from gaining industrywide acceptance. One holdup has been changeover times. Years ago changeovers required re-arranging roll passes, and usually adding or subtracting passes. Even though the passes were rafted, or mounted to subplates, a changeover often required several hours.
The emergence of programmable logic controllers (PLCs) enabled a new strategy. It consists of setting up additional passes, usually up to 10. Typically, nine of these are working passes and one is a finishing pass. All working passes remain in place, regardless of the amount of reduction. The operator replaces the finishing pass to suit the finished product and uses the PLC to engage or disengage the appropriate working stands, a strategy that can reduce changeover times by half. Shorter changeover times are possible if the line is set up with several dedicated finishing passes.
With this arrangement, a mill can make a small reduction, such as ½ in. down to 7⁄16 in. (12.5 percent), in two passes. A more severe reduction, ½ in. to 3⁄16 in. (62.5 percent), requires 10 stands.
CSR requires careful setup. Each tooling set rotates at a unique speed, which is programmed in the PLC. This is necessary because the mill must comply with a chief law of physics, the conservation of mass, which states that mass flow in must equal mass flow out. On a tube or pipe mill, this means that as the diameter of the material decreases, the speed of the material must increase. If an upcoming roll pair rotates too fast, the rolls slip or the material tears. If an upcoming roll pair is too slow, the rolls slip or the material buckles, creating a jam and bringing production to a halt.
It also requires an additional proc-essing step. The compressive forces work-harden the material, so if the product is to be fabricated in an end-forming or bending operation, it needs some of its original formability restored. An induction annealing system is needed at the end of the mill.
A final consideration concerns the mill setup parameters (before the CSR section). For conventional tube and pipe production, every product requires specific setup parameters, but they aren’t dead-on; they allow a little bit of latitude. The roll positions and weld power have a tolerance, so if a roll is out of position by a few thousandths of an inch, or if the welding unit is supplying a little more or less heat than specified, the mill is still capable of turning out a product that meets the customer’s specifications. The CSR process shrinks this operating window. The compressive forces and the annealing process put a lot more stress on the tube or pipe, revealing poor welds or flaws in the parent material more so than the conventional forming process does. If the product isn’t formed properly and the welding unit doesn’t achieve full penetration, the weld is more likely to fail.
Despite the drawbacks, adding CSR is a viable strategy for expanding a mill’s capabilities. A mill set up to reduce ½ in. dia. to 3⁄16 in. dia. also can handle conventional tube or pipe in diameters up to 1 in., a minimum-to-maximum diameter ratio of 1-to-5, with a line speed up to 600 feet per minute (FPM). Conventional mills typically utilize a minimum-to-maximum diameter ratio of 1-to-4.
Diameter reduction isn’t the only capability. CSR also can increase or decrease the wall thickness. The wall thickness change, a function of the law of conservation of mass, is determined by the initial D/t ratio of the tube before it reaches the CSR section and the amount of diameter reduction in the CSR section.
Finally, this strategy isn’t just for reducing ½-in.-dia. carbon steel products. It has been used successfully to reduce stainless steel pipe from 1.875 in. to 1.720 in. without changing any of the rolls.