Bending steel is defined as deforming steel around a straight axis. We use force to manipulate steel until it reaches a desired angle or shape. The difference between bending versus rolling depends on the radius. For larger radii we call it rolling, and for smaller radii we use bending. In this post, we’ll be focusing on bending plate.
Material Elasticity Material tends to want to bend back to its original shape – this is commonly known as “spring back”. Every piece of material has to be evaluated and taken into consideration when bending. In some cases, the dies used to roll the material will bend the material further than the desired radius, but the over bend will be compensated by the material´s spring back.
Curvature Radius The curvature radius has an important impact on thickness variations. One bend with no inside radius could imply material fractures and we should always avoid this kind of bend. Below are two examples on how stretching from press compression relates to thickness reduction:
If the radius equals the thickness of the steel plate, the stretching of the material due to the tightness of the bend causes the plate thickness to decrease up to 20%.
If the radius equals up to 5 times the thickness, the material does not stretch nearly as much and material thickness decreases less than 5%.
Bending Direction The bend must always be performed perpendicular – or at most 45-degrees – to the grain lamination direction. This will increase bent resistance and reduce the risk of fractures.
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Recently, I worked on a quote that involved rolling cones that were made out of AR400 steel plate. AR400 was something that I was not very familiar with so I took some time to research it and see how it would react during the bending/rolling process. AR400 and AR360 are wear-resistant grades of abrasion resistant steel plate. The “AR” in the material name comes from the “abrasion resistant” terminology. This material is commonly used in situations where the material is constantly being impacted by outside forces. Some functional examples that are typical of the need for abrasion resistant steel would be the bed of a dump truck, the scooping bucket of a backhoe, or an agriculture funnel. Continue reading Understanding “Springback” When Rolling Steel→
Bending thick steel plates to a tight radius is not an easy task. There are many variables to consider such as thickness, surface and edge condition, as well as the material’s chemical composition. Plate bending, also referred to as plate rolling, is a process whereby a force is applied to a plate causing deformation along the bending axis forcing the plate to bend at a desired angle. At that moment of bend, the material is experiencing tension, wherein it expands on the outside surface of the bend, and compresses, or shrinks, on the inside.
Upon the release from the bending force, the residual stress in the material will cause the plate to spring-back. To compensate for the spring-back, plate needs to be bent to a greater angle than the angle required. An experienced operator can calculate the amount of spring-back to be expected based on several factors, helping to speed material construction with fewer attempts.
Typically, plate bending is performed on a press-brake. A press brake contains two tools: the upper tool – known as “the punch” and the lower tool – known as “the die”. The plate to be bent has to be positioned over the die and held in place while the punch is lowered onto the plate and the applied force causes it to bend. The bend angle is controlled by the depth to which the punch forces the plate into to the die. Most common plate bending is called V-bending where the used punch and die tools are V-shaped.
When the punch does not push the material all the way to the bottom of that die cavity, leaving a space underneath, then we have “air bending”, and when the punch pushes the material all the way to the bottom of the die cavity then we have “bottoming”. The bottoming process allows more control over the bent angle do to the lower occurrence of springback. What also needs to be remembered while bending on the press brake, is that the thicker and harder the plate is, it will require a larger minimum bend radius.
One of the most common requests when working up bids for rolled plate segments or cylinders is to include weld preps in your scope of work. Welding preparation or weld prep for short is the process of readying the welding surfaces to a specific joint configuration; predetermined by a welding engineer through a written welding procedure which details the joint design or bevel detail and the method by means of beveling/chamfering.
Including weld preps in your rolled plate and cylinder quotes can be the deciding factor when it comes to the customer placing an order or not, as beveling is a much more involved process than most companies that put weld preps as an afterthought, would like to think. This is especially true when dealing with heavy plate, as the amount of material that needs to be removed during the beveling operation could be quite extensive. Without the proper beveling equipment, weld preps could easily be costing you more than realized in terms of quality and time, which in the end is really dollars flying out of your pocket.
To the welder, the weld prep is an extremely important part of the joint design, as it allows the filler metal a crevice to penetrate into the base metal being welded. The important word there is penetration; any weld that does not penetrate into the base metal, is somewhat like sticking bubble gum on the joint. The weld has no penetration to it and sits atop the base metal; this will likely not hold up to, nor pass, any testing. So a beveled edge on the seam of a rolled cylinder is a must if the joint is designed for complete or partial penetration. There are 4 main types of bevels/chamfers when it comes to rolled plate/cylinders, they are: Y, V, X & K. Y is symbolic for a beveled edge with a land. V is representative of a bevel with a knife edge. X & K are both double bevels where the plates edge at the seam of a rolled cylinders is chamfer cut on both sides; where X signifies both sides of the seam to be double beveled, and K is indicative of only one side of the seam being double beveled an but square to the other side of the seam. Another important aspect of the weld prep to the welder is edge quality. Edge quality and weldability go hand in hand; it is extremely important that whichever method of beveling is put into use, the quality of the cut edge must be clean and free of oxides which may compromise the welding process.
When deciding on a cutting method to implement for your weld preps it is important to consider what type of material is selected for the rolled plate or cylinder, whether it be Stainless Steel, Carbon Steel or Aluminum. But more so, the particular alloy of the metal that is specified for the rolled plate/cylinder, as the hardness of the determining alloy can vary greatly from one to another. Alloy hardness can significantly increase your cutting time when it comes to bevel/chamfer cuts.
On thin plates 1/8” to 3/8” a few options are available that can satisfy the bevel detail in the joint design. The most primitive of these options, which is likened to using tools from the “stone age”, is the angle grinder. The angle grinder takes little to no setup but it is extremely primitive and slow going. Depending on the hardness of the material alloy being beveled/chamfered, the angle grinder may even be ineffective. The inability to precisely control the angle grinder makes it extremely difficult to achieve any kind of complicated bevel design. The amount of material removed through the grinding process is minimal compared to the amount of effort it takes which really makes the angle grinder an undesirable method for beveling. Another method for cutting a bevel detail in thin plate is the portable milling head beveler. These machines are equipped with rotary milling heads and are small enough to be operated by hand. They produce a machined quality cut edge and have replaceable carbide cutting inserts. They are somewhat slow going and they do take a considerable amount of force to produce a cut as they are operated by hand but when compared to the angle grinder, the portable beveler is light years ahead in terms of technological advancement.
For heavy plate which can be considered anything over 3/8” up into the range of 2” there is larger, more efficient equipment for achieving a quality weld prep. It should be stated though, that while the machinery that is designed for performing chamfer cuts for bevel details on heavy plate, they are also very capable of performing the same high quality chamfer cut on thin plates all the way down to 3/16”; so this machinery is highly versatile. The peeling/shearing beveler is a common type of beveling machine that can perform simple bevels on heavy plate. Though the hardness of the alloy plays a major role in the success of this machine, as it depends on the material’s ability to be sheared, so tough materials like stainless steel are somewhat difficult to achieve a quality bevel. A negative aspect to this machine is that it has a high propensity for slipping off the plate’s edge and it must be adjusted and watched throughout the beveling process. Also, the edge quality of the cut that the peeling/shearing beveler leaves, looks somewhat like it was nibbled/chewed by a beaver’s teeth, which to some is considered undesirable. Another common method for performing chamfer cuts on heavy plate is a plasma cutting machine. Plasma cutters are automated through the use of a track & buggy system and leave high quality edge cuts and are capable of beveling carbon, stainless and aluminum in thicknesses over 2”. These machines use different combinations of consumables, shielding and cutting gases to give dross-free, oxide-free, highly weldable cut edges. The few drawbacks on this method of beveling are the time to setup the plasma cutter/track system and that it naturally leaves a heat affected zone on the plate’s cut edge that can make the edge of the plate much harder than the rest of the material. Also some alloy’s chemistry can change when enough heat is put into it leaving your edges as a completely different material alloy than the rest of the rolled plate/cylinder. The final option of performing weld prep cuts on rolled plate/cylinders 3/8” – 2” is the rotary mill head double beveler. With the same cutting principle as its little brother the portable beveler, the rotary mill head double beveler performs the bevel through a rotary milling head equipped with replaceable cutting inserts, but uses much more powerful motors to achieve the cuts. This type of machine usually is mounted to a 3D trolley of sorts that allows the beveling machine to “float” in air, adjusting itself to any uneven surfaces, and is fully automatic. The cut edge quality is superb and even high detailed bevel designs could be achieved through the precise control of bevel degree and amount of material being removed by the cutting heads. The great thing about the double beveling machines is that the machine rotates 180deg about the 3D trolley that it is mounted to, this allows bender/rollers to achieve a double bevel on their plate edges without having to flip the plate!
A vast array of equipment is available when it comes to selecting methods of achieving beveled cuts for weld preps. Some may consider weld preps as an afterthought, but those who are looking for a competitive advantage over the competition will give beveling a serious consideration on their next project.
Steel plate rolls–whether they have two or three bottom rolls–all have a top roll. The top roll can be sized to roll plate into cylinders or cylinder segments to radii close to the diameter of the top roll. However, relatively small top rolls can deflect in the center under the pressure of curving steel plate. Alternately, larger-diameter top rolls deflect less but limit the machine to rolling only larger diameters.
Benders and rollers, those who specialize in curving steel plate among other steel products, are asked to roll a variety of plate widths, lengths and thicknesses. Some of the possibilities will cause deflection of the top roll in the rolling process. The result will be the plate cylinder with a barrel form and ends that are not parallel.
With plate rolls that have a long top roll with a small diameter, you will see more deflection. Imagine standing on the middle of a 6 x 4 wood beam supported at two ends. As the length on the wood beam gets longer, the deflection where you are standing would sag downwards more. The same concept is applied to steel plate rolling.
To compensate for this deflection in a plate roll, the top roll is “crowned.” A “crown” is the barrel shape of the top roll that is needed to obtain a uniform distribution of the pressure required for the rolling of steel plate. A plate roll has supports at each end and the top roll deflects in the middle when the plate is in motion. This affects the parallelism on the edges of the rolled part.
The process of crowning is achieved through grinding or cutting on large, modern CNC lathes. A program takes the deflection formula or tabulated values to produce the correct shape to equalize the pinch pressure.
Top rolls are recommended to be crowned for the material that is run the majority of the time. If, for example, you are always running ½” thick, 3ft wide plate to make 30in diameter cylinders, you can have your top rolled crowned to those specifications. But then if you run 2in plate with the same width through the machine to the same radius, you will see a barrel-shaped end product. If you roll a 1/8” thick plate through that same plate roll with the crowned top roll you will see an hour-glass shape end product.
Benders and rollers do not run one-size-thickness, width and diameter plate only, so they cannot crown their top roll to be a one-size-fits-all. Plate rolling companies often have a number of different plate rolls that work their way up the plate thickness, width and diameter scale.
If you have a plate that you need to roll using a plate roll that has the wrong crown amount, you may be able to use a shim to artificially add thickness to correct for crowning. The most common shim is a cardboard, but you can also use metal and wood shims. To correct a barrel effect, the machine operator can place a shim on top of the plate at the center; to correct an hour-glass effect, the machine operator can place the shim in each corner on top of the plate when rolling.
Depending on your requirements, then, you should consider crowning your plate roll to produce quality steel cylinders and cylinder segments.
The construction of steel tanks for the storage of hazardous materials must be done carefully. The tanks’ seams are often considered to be a potential point of failure. For structural reasons, therefore, tank construction with as few seams as possible is desired, and, when possible, single seam construction is preferred. But this single-seam requirement can be a daunting task for the people involved in their production because handling and transporting the materials to make these tanks requires a certain level of experience, expertise and special equipment.
The design diameter of these tanks is somewhat restricted by maximum shippable lengths of the materials used in their construction.
If a plate processor who slits and levels the steel can also roll the material into a tank, it can eliminate the transportation of the raw material. If the flat blanks need to be shipped to a company to roll the steel, the maximum shippable length is 70ft loaded on a flatbed “stretch” trailer. Consequently, it is unusual to see single-seam tanks in diameters over 22ft.
Furthermore, coils come in a maximum width of 97.5in which further restricts the design dimensions of the tank. And after slitting, the maximum tank height will become slightly less than 97.5in. Most plate processing companies who level coil can do so at a maximum thickness of ½in, but when referring to these thin-wall tanks, we will only consider steel in 3/16in – ¼in thick.
After materials are slit from coil and ready for shipment, one needs to load and unload the flatbed trailer that will be shipping the steel. In order to handle these very long, thin, cumbersome plates, special, below-the-hook handling equipment is necessary to complete the lifts, e.g. spreader beams, plate hooks, magnets, or vacuum suction cups.
To the inexperienced, this handling may seem like no big deal, but the extreme flexibility of the plates makes them extremely hard to handle even at lengths of only 20ft.
Another worry when loading and unloading the materials for these tanks is not to damage the plates. Plastic deformation or stretching of the material can occur and result in a kink in the plate.
And then rolling of these plates requires a well-thought-out process as shipping anything over 14-15ft in diameter can become expensive and difficult.
So how do you roll steel plate for tanks with diameters up to 22ft when you can only ship rolled plate cylinders up to 14-15ft in diameter? In order to successfully complete a rolling job like this, the ends of the plates must overlap one another until the rolled plate’s diameter is within shippable dimensions. At this point the overlapping plates are tack welded into place.
The extreme flexibility of these plates, which once caused a headache on the material handling side, now plays a significant role in how they are erected. Once in the field the tack welds are broken, and the rolled plate is opened up through the assistance of a crane to their designed diameter.
If the plates were not so flexible, adjusting their diameter this way would not be possible. Also, if the entire length of the plate was rolled to the shippable diameter, when the rolled plates are opened in the field at time of erection, the ends of the plate would not meet as designed at the tank’s seam. Rather, they would curve in further towards the tanks ID, making the tank a sort of heart-shaped geometry (If you try to visualize this, the seam would look somewhat like the number 3.).
In order to avoid this happening, each end of the plate, the portion that gets overlapped on one another, gets rolled to the tank’s true design diameter, and the rest of the plate is rolled to the shippable diameter. The rolled plate is not truly round but somewhat egg shaped/multi-radial. The rolled plate cylinders are then braced properly, loaded onto the trailer just as carefully as the straight long plates were unloaded, paired with an escort and shipped via wide load to their destination.
Bollards are usually a post or other metal or concrete structure that helps define a physical space–think of the posts put up at key buildings after 9/11.
But bollards date back to ancient Rome where they served as road markers. Recently we had an opportunity to make some rather large bollards–or I should say–bollard segments. Some 180 degreees. Some 90 degress. You can see them packed tightly together in the picture below to save shop floor space.
All of the bollard segments are 4ft tall, all are rolled of 3/8 thick steel plate, and all have an inside radius of 36in. But, as I said before, some have a 180 degree of arc and some have 90 degrees of arc.
Furthermore, the 180 degree sections have 16in inward flanges at the end of the arc. The 90 degree sections also have flanges, but they are configured somewhat differently. Please see the sketch below.
In order to make these parts, both a plate roll and a press brake were used: the plate roll to curve the large radii, the press brake to form the flanges.
It is not that unusual to incorporate two different means and machines to curve metal. Especially if the work is being done by a company that specializes in bending metal into various shapes including curves. They strive to find the best methods, machines and machine operators to achieve the best curving of metal. [maxbutton id=”1″]
The underground mining/boring industry has always employed some of the most creative, hands-on workers. One entrepreneur in Missouri used a rolled and welded plate cylinder as the base ring for a 36″ diameter drill bit for underground directional boring. Welding hardened steel cutting teeth to the ring made the product very cost effective. The 3/4″ x 12″ plate provided plenty of stiffness and the Grade 50 steel added extra strength to the equipment.
Another example of creativity in the underground mining industry involves underground metal chutes.
A mine required materials to create a chute that would carry ore from one level down to a rail car on a lower level. Although the chute needed to be long, there were limitations on how large a tube could be brought down into the mine on the elevator. Previous solutions proved impractical particularly in regards to cost.
Working with the customer, Chicago Metal Rolled Products developed several designs before settling on the best. The successful design involved curving 1/4″ thick plate into a 5 ft diameter cylinder 2 ft long and then welding it on the longitudinal seam. Next was to fit two angle flanges that had been rolled and welded to fit on the outside of the top and bottom of the cylinder. The angle rings were then submerged-arc welded to each end of the cylinders. These angle rings become mating flanges pre-punched with slotted bolt holes for ease of connection in the close confines of the mine. The new design provided significant cost savings for the mine operator. Again, a tribute to those who work in the underground mining industry.
Designers, detailers, and fabricators commonly debate the correct formula for a blank length of bent plates. A trial run is always good practice when forming on a press brake because metals stretch so differently on different setups.
This confusion does not need to carry over into the rolling of plate, since the radius is larger than 3 times the thickness.
The K-factor of a bend is the ratio of the neutral axis to the thickness, so when K = 0.50, the neutral axis is in the exact center of the thickness: half the thickness compresses and half of it stretches. On very tight bends, however, the K value changes where the material stretches more than it compresses.
Most of today’s 3D CAD software such as SolidWorks or AutoCAD Inventor will “unfold” a 3D model. The AutoCAD default K value is 0.44, which is the standard for steel with bend radii of 1 to 3 times the thickness.
For rolled items, however, you should set your software to K = 0.50.
Perhaps this debate still exists because the K = 0.44 is close enough when rolling large radii.
Looking at an example of 1/2″ plate to 60″ inside diameter, there is only 3/64″ difference over a 90deg segment. The K-value is not as important on large radii, but if you are a stickler for accuracy use K = 0.50 when rolling.