Bending steel in ways not typically accustomed is no small feat. In fact, given the idea that all steel has a memory and is highly prone to springing back to its natural flat form, it can seem near impossible at times. But that’s when having a fine-touch to the steel-forming process comes into play.
Let’s focus on one process in particular: Transforming plate steel into a cylinder in order to form such products as the shells for pressure vessels or boilers. Given the critical end use of these products, if the plate-rolling and welding process isn’t done to specification, the result could ultimately prove catastrophic.
So how does it happen? Sure, it takes the use of equipment and technology to ensure the steel doesn't revert back to its original form, but let’s not overlook the fine touch of a highly skilled operator. Together, they provide the right combination of knowledge and automation to produce the best-rolled plate for your product.
It starts with steel. All grades are comprised of different alloying elements, which impact the steel in various ways. This is important when determining how well a plate will roll in the machine; for instance, carbon, nickel and chromium content result in different mechanical properties.
The plate that is shipped from the mill is never perfectly square. In this condition, rolling the plate would result in improper alignment of the edges. Therefore, it is a standard practice to re-square the plate via precision burning prior to rolling. During this process, customer-requested bevels are also incorporated into the plate blank. Bevels are needed to join the seam of the cylinder after it is rolled.
To accommodate the re-squaring and beveling process, the plate is brought in from the mill in either 97” or 121” widths. The extra inch of material provides enough stock to both re-square and bevel. A 97” width will yield a cylinder that has a 96” length. A 121” width will yield a cylinder that has a 120” length.
It is now time to calculate “stretch out,” which accounts for the full amount of plate needed to make a given cylinder dimension. If the requirement is to produce a cylinder that has a 100” inside dimension (ID), the plate must be 121” wide and 314” long.
Once these dimensions are calculated, the material is ready for the plate-burning table. These machines will typically use oxy-fuel and/or plasma technology and employ the use of computer-numerical controlled techniques that ensure that the plate is sized to the correct dimensions for rolling.
Now the plate is ready to be rolled. During this process, the steel plate is fed into the plate roll. This is where craftsmanship meets technology. Repeated dimensional checks by the experienced plate roller confirm that the process will produce a finished product that meets both customer and industry standards for diameter and ovality, which is the distortion of the cross-section from its normal (round) shape.
Remember the alloying elements? This helps during the plate-roll process as well, as this information factors into how best to modify the force of the plate roll. As one example, the nickel and chromium in stainless steels call for more force to roll the cylinder.
Once rolled, the cylinder is transferred to the welding department. There, submerged arc welding (sub-arcs) join the beveled edges using defined processes which are detailed in the ASME code. When finished, the weld is thoroughly inspected for discontinuities using x-ray technology. This helps determine if any minor cracks or defects are present in the weld. Such imperfections in the weld could lead to catastrophic failure under internal pressure. If present, these defects will be ground out before re-welding the product.
Dimensional changes can occur as part of the welding process. The heat that is involved in the welding process can cause a cylinder to go "out of round". When this occurs, re-rolling is required to remove the ovality and to restore the cylinder to customer and industry tolerances. Documenting the diameter measurements helps to ensure that fitting up the heads to cylinders do not require unnecessary labor to grind the cylinder to match head dimensions.
The Finished Piece
Transforming plate steel into a cylinder involves careful attention to detail at every step of the process. This means that having the right combination of highly skilled operators and technology can be vital to success. Therefore, ensure your partner brings both to the table in order to ensure that the steel forming is being done with a fine touch like no other.