- February 10, 2015
Techniques for roll forming thin-walled tubing from high strength steels
The Problem: Reducing tube weight while maintaining structural integrity
The Solution: New materials and improvements to the roll forming process
Ever wonder about the legs on a folding chair? How about the exhaust pipe on your car? Tubing is everywhere, but most of us don’t give it a second thought. The technology used to make this important product is quite amazing, however, requiring equipment longer than a football field and more expensive than a mansion on Vancouver Island.
Someone who knows all about tube rolling is Brian Kopack, senior sales engineer at Formtek Inc., Warrensville Heights, OH. The Formtek Group represents a number of metal forming equipment manufacturers, including Cooper Weymouth, Tishken, Hill Engineering, Dahlstrom, Rowe Machinery and the Yoder brand of tube and pipe mills.
Roll forming is a broad topic. It’s used to manufacture everything from bicycle rims and auto bumpers, rain gutters to aircraft fuselage. In fact, almost any shape—angles, channels, S, T, U, V, W and Z-shapes—can be roll formed, and done so quickly and accurately. Tube manufacturing is a small part of the roll forming process (see sidebar page 83).
Kopack explains that tube mills are just another type of roll forming machine, but one with a critical difference: these monstrous systems weld the rolled form shut partway through the manufacturing process, then trim and smooth the weld bead to create a strong, precise and mechanically stable product.
Almost any aluminum, steel or stainless steel, is a candidate for tube making, but as Kopack points out, high strength steel is being called for with increasing frequency. Automakers in particular have their eye on weight reduction, and this means maintaining or in some cases increasing the structural integrity of their products, and doing so with less material.
“The term high strength is relative,” Kopack says. “The generic, low carbon steel tubing that’s been around since the 50s is typically 40,000 to 50,000 psi yield strength material. By comparison, we have ultra high strength, low
alloy (HSLA) steels today that offer yield strengths of 150,000 to 180,000 psi. Those materials form much differently than do commodity grade materials, and may require additional forming passes, different tooling and even modifications to the machine configuration.”
As material becomes stronger, manufacturers can reduce tubing wall thickness with no loss of product strength. This provides for a lighter weight product, and allows manufacturers to cut costs. For example, the ratio of tubing diameter to wall thickness runs between 10:1 up to 50:1 in the “plain Jane” materials mentioned in the chair leg and exhaust pipe examples cited earlier, whereas high performance tubing like that used in a modern automobile could easily be 80:1 or higher.
Those two factors—tougher materials coupled with thinner walls—can give tubing manufacturers a stomach ache. “Many of these HSLA materials are so strong and stiff they simply don’t like to be formed,” explains Kopack. “This is especially true with some of the newer ultra high strength steels—forming those is like trying to bend glass.”
One of the best ways to soothe that stomach ache is to take a kinder, gentler approach when bending gnarly materials, something tube formers refer to as “slowing down.” This doesn’t mean the machine processing speed is reduced, but rather the rate at which the material is transformed: instead of forcing the flat stock into a round shape in four or five steps, ten or more may be used.
Another approach is application of heat. Like glass, metal typically becomes more pliable under high temperatures, and tubing makers can use this phenomenon to their advantage. Yet Kopack says this tactic has largely fallen out of favour in the roll forming community. “There was a time where tubers would heat the steel and hammer it into shape, but doing so is somewhat cost prohibitive. Not only does it take a lot of energy, but machinery doesn’t work well when it’s red hot. Standard bearings don’t last because the grease burns off, and hydraulics and pneumatics become problematic in high heat. There are ways to deal with it, but it drives the cost of the machine sky high—instead of two million dollars for a roll mill, it might cost six million or more.”
Instead, roll formers have turned to steel providers for a better solution. Dual-phased steel is one of these. Kopack explains that dual phase steel is formulated in a way that makes it fairly pliable at room temperatures, but quite hard after heat treating. This makes for a straight forward roll forming process, one where the steel can be formed using conventional techniques and equipment, then hardened after final fabrication.
Tubing manufacturer Lock Joint Tube LLC, South Bend, IN, is one company seeing greater use of this and other specialty metals. Plant engineer Dave Mercurio says dual-phase steel may start out at 40,000 psi yield strength, and triple to 120,000 or more after subsequent processing such as hydroforming, heat treating, or quench and tempering. He agrees high strength steel use is on the rise. Aside from the automotive market, tough, thin-walled tubing is increasingly popular with ATV, snowmobile and motorcycle manufacturers. This puts a strain on roll forming equipment designed to make tubing from less stern stuff.
“There are a couple things you need to consider when rolling high strength materials,” says Mercurio. “For starters, the higher forces required to bend the steel means shafts and support mechanisms deflect more, and bearings wear prematurely. The end result is that you have to adjust your mill capacity accordingly. For example, a 75 mm (3 in.) tube mill may only be able to roll 60 mm diameter (2-1/2 in.) of high strength material without running into mechanical problems.”
It also means adjusting the rate at which the material is formed. As Kopack pointed out earlier, additional forming passes are required as material strength goes up. This means greater tooling expense because more passes means more rollers. Different tooling designs may also need to be utilized to control the forming rate of high strength materials—to ensure the strip is properly prepared for welding, and material is worked uniformly for the required properties in the final tube.
Mercurio says Lock Joint Tube must purchase a complete set of rolling tools for every tubing size, an investment of $20,000 or more for D2 tool steel, and twice that for carbide. “The tools are pretty dedicated. Depending on the finish requirements, material strength and the application, we probably need a set of tools for every 1.5 mm (1/16 in.) size increment.”
Surprisingly, roll forming high strength steel doesn’t automatically mean carbide tools. “It depends on the volume. If we’re tooling up for million foot rollings, spending the additional money for carbide makes sense. Otherwise, steel tools work just fine.”
Far more important than the tooling material itself is how tools and equipment are maintained. Proper lubrication and regular replacement of worn parts is imperative with any piece of roll forming equipment, but especially so when making high strength tubing. Tube mills are expensive and complex pieces of machinery. Lock Joint Tube’s two Yoder mills were installed 15 years ago, and aside from the downtime required for upgrades to the accumulators and the addition of high frequency welders, the mills have been running continuously ever since.
“With the updated accumulators, we don’t have to stop when we thread up a coil of material. Some of our mills have also received flying cold saw cutoffs, which has reduced our secondary end-trimming operations by 80 per cent,” Mercurio says. “These are nice, solid mills. With proper care and regular maintenance, I expect they’ll run for another 20 or 30 years.”
That’s good news, because it doesn’t look like things are slowing down for the high strength steel tubing market anytime soon. The call for lighter, more fuel efficient cars is on the rise, and as steel manufacturers dream up stronger, more workable steels, their use will become even more widespread. It appears that roll forming equipment and tubing manufacturers alike have their work cut out for them. SMT