Welded roll forming system versus tube mills: What’s the difference?
Welded roll forming system versus tube mills: What’s the difference?
Steel pipe rolling forming process is an elastic-plastic deformation process, the boundary conditions is very complicated and nonlinearity, so it is difficult to get the exact solution by using the analytical method and the traditional implicit static algorithm based on finite element method. The contact collide arithmetic based on explicit dynamic program LS-DYNA provides an effective way for accurate research on the elastic-plastic deformation of the hot-rolled seamless pipe rolling process. This paper takes the SMS MEER rolling mill of Φ 177PQF steel tube rolling units in Ansteel seamless steel pipe rolling mill as the research object, makes a coupled thermo-mechanical finite element method computation on steel pipe rolling process, gets the residual stress and strain change rule in the rolling process. Written the rolling process parametric programs by APDL, analyzed the influence of roller spacing and velocity parameters on residual stress and strain, which provides a reliable theory basis for improving the performance of hot rolling seamless steel pipe and the optimization of rolling technological parameters.
The welding of streel strip into a product, such as round tube or pipe, box frames, and structural members, is a multibillion-dollar industry that had its beginnings more than a century ago. Many items that we use today, like furniture tubing, oil pipe, and fuel lines, are manufactured on welded pipe cold rolling mill.
In the last 20 years OEMs have been asked to apply the principles learned from cold pilger to the much larger roll forming processing market. This has created another technology area today known as welded roll forming.
Companies may move from nonwelded open shapes to welded roll formed shapes for many reasons. Welded roll formed shapes have structural strength and integrity, help eliminate secondary operations in downstream manufacturing, and can offer savings in steel and construction labor. While there are benefits, there are also costs, which typically involve an investment in capital equipment and skilled people who know how to use the technology.
What are the differences between a tube mill and a welded roll form system? Which one should you consider? Can the same parts be run on both systems? Will the part quality be affected?
These questions, asked often within the industry, are plagued with many conditional aspects that cloud the final answer. Intricate details often are required to help machine builders focus on the best option.
In the classic sense, a tube mill is a kind of welded roll form system; however, not every welded roll form system is a tube mill. Tube mills are welded roll form systems that have been fine-tuned to run a specific diameter range, typically at fast speeds (see Figure 1).
Diameter range is mentioned because a tube mill welds a round product. Progressive male and female roller dies (or rolls) shape the incoming strip for welding a specific diameter.
On all welded roll form systems, including tube mills, welding can occur with processes like high-frequency (HF) induction, HF contact, electric resistance, and laser beam welding. For the sake of this discussion, it is assumed that the correct welding process has been determined based on the specific material requirements.
Figure 2 shows the typical forming flower of a tube mill forming the strip into a round product just before welding. From there the round product can stay round or can be sized or reshaped into a square or rectangle.
In a tube mill, the forming machine forms the strip into a round, weldable product. The forming machine has two main parts: breakdown and fin-pass sections. Once welded, the tube can be left round, though it undergoes further forming to size it to a more accurate outside diameter. The sizing section shown in Figure 3 has round roll stands with specialty reshaping stands toward the exit and a double turks-head to finish. Used for straightening, a turks-head has two pairs of rolls, one arranged vertically and the other horizontally.
Defining a welded roll forming system is a little more difficult. Again, in the classical sense, a pipe hot rolling mill is one kind of a welded roll forming system. But if someone refers to a “welded roll forming system,” that person is probably not talking about a tube mill, but other roll forming machines able to form various, often highly complex shapes to within tight tolerances.
Much like forming on the tube mill, with breakdown and fin-pass sections, welded roll forming systems have a similar forming setup, with the fin-pass section occurring in the last few stands before welding (see Figure 4).
When working with nonround shapes, such as a step beam (see Figure 5), the roll forming system tends to form the shape as is before welding. Some in the industry call this near-net-shape forming. Others call this the form square-weld square process. Once the shape is formed and welded, most recommend at least two additional passes to work the welded shape once more to finalize the dimensions.
Tube mills tend to have alternating driven forming pass and idle side pass patterns. These patterns tend to help alleviate springback from the previous forming pass.
This specialty pass progression has led tube mill builders to design systems with a given diameter range to have between five and nine driven forming passes, with their sister idle side passes. The actual pass count often is a function of the ratio of the workpiece diameter to the material thickness, material yield strength, as well as the minimum to maximum range for both material OD and thickness.
Welded roll forming mills tend to have mostly driven passes, though using idle side passes is becoming more common. Many factors come into play when determining the most robust pass count for stable welding. Pass counts depend not only on material wall thickness and yield strength, but also on the overall complexity of the shape to be formed. The pass count also depends on material movement during the process, as well as the experience of the roll tooling designer.
Which Is Best?
For round shapes, a tube mill probably is the best choice. But what if you only want to make squares or rectangles? What if you want to have prepunched holes?
To determine the best machine, consider the strengths and weaknesses of each. Tube mills specialize in simple shapes. Besides round shapes, they’re capable of producing squares, rectangles, and ellipses.
If you’re looking to form a few complicated shapes, the tube mill may not be the best choice. Forming some shapes on the steel pipe production line simply isn’t possible, but a welded roll forming mill can form highly complicated shapes quite readily.
Also consider the outside corner radii of those shapes. In some cases, a tube mill can form those radii down to 2 times the material thickness, but this isn’t typical. Usually a tube mill can form radii down to between 2.5 to 3 times the material thickness.
Welded roll forming mills have a different tooling setup that allows for tight, accurate radii. Specifically, it has to do with how the forming rolls engage the material on the corner. On a welded roll forming mill, both the female and the male roll engage the corner, making forming very exact. Typically, radii can be formed down to less than 2 times the material thickness.
Also consider the consistency of the corner radius, as measured from corner to corner on the roll formed product. Shapes formed on a tube mill may have discrepancies between opposite and/or adjacent corners. This occurs because of one-sided bending, with just one roll (not both male and female) fully engaged with the material.
Male and female roll engagement in welded roll form systems has another benefit: It helps produce consistent flat sides or features, with no crowning or other reshaping. Because tooling on a tube mill gives the workpiece no inside support, flat sides or features tend to reshape during the process and emerge from the tube mill with a crown.
Also take into account any pre-punching operations that a job might benefit from or require. Punching typically is not recommended on tube mills. This is because of inconsistencies with so-called round-to-shape material movement. It can be very difficult to “lock” the hole location in a round shape and then reshape it into a square, hoping the hole ends up in the correct position. Moreover, punched holes may warp during the reshaping phase.
Pre-punching in welded roll forming is usually better because it allows you to accurately locate hole features. The punched holes also tend to hold their shape during forming, if those holes are put in the right location. If the hole is near a corner, however, the hole shape still can change.
Finally, consider processing speed. This hinges mainly on pre-punching operations. Since pre-punching is not normally performed on tube mills, these mills have been designed to run up to 750 feet per minute (FPM), depending on the product OD and wall thickness. If a pre-punching job is running on a welded roll forming mill, processing speeds typically are 100 FPM or less, though they can be improved with special tooling like a rotary pierce tool, which looks like a roll with punches sticking out of it. These punch or pierce the material as the roll rotates. (Rotary piercing does have limits and may not work in some applications.)
If pre-punching is not required, then a welded roll forming system can process at the same speeds as a tube mill. To achieve and maintain these speeds, however, a welded roll form system needs some tube mill specialty equipment added, such as an accumulator and high-performance cutoff.
So again, which is best? If you have only simple shapes to form, a cold drawing machine may be the way to go; if you have complex shapes with multiple radii, holes, and other features, a welded roll forming system is the way to go.
But what if you have a simple shape with tight tolerances? What if those radii need to be consistent from corner to corner? What if you need sides to be precisely flat, with no chance of crowning? In these cases, a welded roll forming system may be more suitable for the job, unless product specifications can be changed.
As with anything else, choosing a roll forming system involves considering product requirements and weighing those against your current and future production needs.