May 30, 2001
Preparing the edge of a metal strip properly before it enters an electric resistance welding tube mill makes a huge difference in the quality of the final product. Make sure your prep methods match your quality requirements.
Achieving a quality weld requires starting with good edges.
In manufacturing electric-resistance- welded (ERW) pipe and tube, this is critical. Perfect edges are almost impossible to obtain consistently by rotary side trimming single-width coils or side trimming and slitting master coils.
In rotary side trimming of single-width, hot-rolled coils and slitting and side trimming of a master coil into multiple strands, it is critical to remove material equally from each side of a coil, leaving a skelp strand of desired width for forming, welding, and sizing.
There are three fundamental but critical processes that must be controlled in producing ERW pipe and tube over which a mill operator has a margin of control--skelp edge preparation, skelp forming, and ERW.
This article looks at three aspects of skelp edge preparation:
1. The need for the cleanest, smoothest, most solid edges possible from the rotary shear unit so the best-quality ERW seam can be made
2. The basic mechanics of rotary shearing
3. Quality standards that a sheared edge must attain so that a mill operator can produce the best weld seam possibleWhile the focus of this article is not high-frequency welding, examining the characteristics of the current flow in this welding process is necessary.
The depth of the heat sink (penetration) into the edges before the close of the welding "V" for a 450,000 cycles per second (cps) welder is approximately 1/32 inch. Shear stringers, rough face, or any irregularities produced on the surface of separation that approach 1/32 inch may produce welding defects. High-frequency current also will jump readily to surfaces with sharp edges and points.
Although edge irregularities pass through finned pass rolls, they can pull away from the face when they are heated because they are very hot. These irregularities can oxidize on both sides and generate a weld defect, normally called a black spot. The movement of these surface irregularities is caused by the heating and magnetic forces in the welding "V."
When a formed can exits the last finned pass stand, it passes under an encircling coil or sliding contacts. The edges will be heated equally if they are parallel to each other in the vertical and horizontal planes. If the bottom edges are closer to each other than the top edges--and this is usually the case--the edges nearest to each other will become the hottest.
If clean, smooth, solid edges are presented to the welder, the only maladies or defects in the weld seam result from a too-hot or too-cold heating condition.
If the weld power setting at a constant welding speed is so high that a hot welding condition occurs, then defects such as popouts and penetrators will result. If the weld power setting at a constant welding speed is so low that a cold welding condition occurs, then defects such as pasty weld seams, cold weld seams, or no weld at all result.
The shearing action between two offset circular blades.
The primary considerations for using rotary shearing to remove or separate material are:
1. Force and power requirement.
2. Tooling performance.
3. Effects of the process on the physical properties of the material.
4. Condition of the surface of separation.
An operator has no control over the shape of the skelp. Skelp shape, good or bad, reflects the material's cross-sectional contour.
Problems that cause bad shape include edge wave, end hook, camber, center buckle, or oil canning. Skelp can be acceptable to the ERW pipe and tube manufacturing process even if it has some of these shape problems.
When skelp is processed through a rotary shearing unit, material is forced between pairs of circular blades set in an offset or staggered position and mounted on parallel shafts. The blades overlap each other, causing a shearing action that separates the material (see Figure 1).
The blades only partially cut the material from the top and bottom. The remainder is fractured or broken. It is this fractured surface that produces the undesirable edge condition in the ERW pipe and tube making process.
Before examining the sheared edges of the skelp, a manufacturer should think about the goals it wants to achieve as it prepares the edges for welding.
(A) shows a trimmed and slit strand of skelp that is theoretically perfect. (B) is a theoretical cross section of a hot-rolled coil. (C) shows the shape of individual strands when a coil is slit.
The cross section in Figure 2Ashows a trimmed and slit strand of skelp that is theoretically perfect. Its edges are clean, solid, burr-free, and perfectly rectangular.
If such an edge could be produced, it would greatly minimize the welding problems encountered in the pipe and tube manufacturing process. This is impossible, however, because of the material with which manufacturers start. Figure 2Billustrates the theoretical cross section of a hot-rolled coil.Figure 2Cshows the shape of individual strands when a coil is slit.
The ratio of cut to fracture depth is governed by the chemistry and hardness of the skelp and the lateral clearances between the top and bottom blades. This lateral clearance is primarily determined by the skelp thickness and the quality of the sheared edge. The goal is to produce as much cut surface as possible without completely sacrificing blade life.
A microscopic examination of a sheared edge would show that the edge is developed by a combination of rollover, cut depth, fracture, and burr formation.
The final edge condition will be determined primarily by the lateral clearance of the blades. Whether the blades are sharp or well-used also contributes to the final condition of the edges.
As the plastic deformation increases, edge rollover is initiated as blade penetration continues. The edge rollover is plastic flow caused by a combination of compressive force, the force couple created by the shearing pressure, and lateral clearance between the blades. (see Figure 3).
These figures illustrate the effects of edge rollover caused by compressive force, the force created by shearing action, and lateral clearance between blades.
As blade penetration increases, the burnished land is formed. This surface is relatively flat, smooth, and solid, and it is usually a small portion of the skelp thickness. As the skelp is pulled through the blades, penetration increases until the ultimate strength of the skelp is exceeded. Then, fracturing occurs.
The fracture surface appears granular and is a large portion of the skelp thickness in depth.
Next comes the burr. It is nearly always present and is the result of a compressive displacement formed by the radius (dull portion) of the blades.
It would be inappropriate to discuss a quality standard for sheared edge in manufacturing ERW tube and pipe without looking at what defines overall skelp quality.
ERW pipe and tube mill operators realize that the coil slitting and side trimming operation may introduce quality problems that the incoming coils did not have. Slitting and side trimming do not affect such properties as chemistry or hardness of the skelp, but they can affect surface appearance, width dimension, flatness, camber, and edge wave. These may affect the final quality and acceptability of the pipe or tube being made.
This figure shows edge surfaces produced by different blade clearances.
If the skelp is not contained transversely with full-width support rolls and top hold-down rolls on the entry and exit sides of the slitter/side trimmer unit, it will bow up near the center, and the trimmed width will be greater than expected. This is especially true of skelp for large pipe.
The important skelp quality characteristics for manufacturing acceptable ERW pipe and tube are:
1. Surface appearance.
2. Physical shape.
3. Dimensional accuracy.
4. Physical strength consistency.
5. Edge surface finish.
The most common process control element that seems to have the greatest effect on the condition of the edge surface finish is blade clearance. Figure 4shows edge surfaces produced by different blade clearances.
Blade clearance is always calculated as a percentage of skelp thickness. Figure 4 illustrates two types of edges and lists data for five types of edges.
Data from a table such as this, which the mill operator should collect over time, is a good starting point to develop statistical process control (SPC). Several devices are available to measure surface finish electronically in microinches.
The simplest method for gauging the condition of a sheared edge is to visually compare the sheared edge with an acceptable standard. This standard may be a machined surface made to represent a good sheared edge.
Other pipe mill manufacturing elements that influence the quality of skelp for ERW pipe and tube manufacturing include equipment, tooling, maintenance, and operating procedure.
Although some edge irregularities are rolled into the face of the skelp and may cause some welding problems, others are so robust that they resist being rolled into the face and smoothed out.
At least two different types of equipment are available to improve the surface of separation of the skelp after rotary side trimming or slitting. The older of these two is the edge planer, at times called a skiver. The other type is the edge miller.
The edge miller can be used in conjunction with an existing side trimming unit or may be used exclusively to prepare the mill edge of coils. Normally, an edge miller can remove 10 to 15 millimeters per side at typical production rates. At reduced production rates, the edge miller can remove 1-1/2 inches (37.5 millimeters) per side.
In a highly competitive marketplace that demands products of the highest quality and reliability, pipe and tube manufacturers must execute every process to the fullest. Nothing less will satisfy the customer.
Tube and pipe producers must develop a comparative method that will alert them to the type of sheared edge moving into the tube mill. From this, they can develop SPC charts en route to producing precision skelp edges. These charts may even indicate that existing equipment is insufficient to produce edges that lead to better welding.