Bending high-strength steel

Bending HSS requires high-quality tooling made from steel that can withstand the high amount of force that is generated when bending these materials.

Since the early 1980s, sheet metal fabricators have, for the most part, been following a pretty basic set of rules when it comes to air bending mild steel. To determine the appropriate die opening for bending materials of 0.500 in. (12.7 mm) thick and less, they simply multiplied the material thickness by 8. To calculate the resulting inside radius, they multiplied the width of the die opening by 16 per cent or divided it by 6, whichever they preferred. And one punch with a 0.039-in. (1.0-mm) tip radius could bend virtually anything from 20- to 11-ga. mild steel.

Those that were bending plate materials pretty much followed the same basic set of rules. The exceptions included the need to use punches with larger tip radii and dies with a die opening of a minimum of 10 times the material thickness when the material thickness exceeded 0.500 in. (12.7 mm). While all of this is still largely true for bending mild steel, the ever-increasing use of high-strength steels (HSSs) is changing all of that.

High-strength Steels

So, what are some of these new HSSs and what purposes do they serve? Some of the most commonly used HSSs that come to mind are:

Domex: An advanced HSS that reduces the weight of heavy vehicles, trailers, cranes, and containers.

Hardox: A material that increases the useful life of such products as excavator buckets, truck beds, and a variety of wear parts.

Weldox: A construction-grade sheet steel that reduces the weight and increases the lifting capacity in applications such as mobile cranes, truck cranes, and trailers.

Greencoat: A sustainable colour-coated steel option for roofs, facades, and rainwater systems.

Docol: An advanced lightweight HSS for the automotive industry, which is available in a large number of standard grades.

Armox: An armored steel for military vehicles, limousines for corporate VIPs, safe rooms, and the like.

All of these HSSs are produced by SSAB of Sweden. While there are other suppliers of HSSs, such as U.S. Steel, those listed above are the most well-known among North American sheet metal fabricators.

A heavy duty punch holder, large radius punch, and a heavy duty 60-degree die is needed to bend this thick HSS.

Formability

So, what are the unique requirements when it comes to forming HSSs on a press brake? Unfortunately, there isn’t enough space in this article to cover all the rules that apply. If you are considering taking on a job that will require you to form one or more of these materials, you should first consult with the manufacturer or the technical sta at your local steel service centre and your press brake tooling supplier before ordering the first piece of material. In doing so, you will want to ask the following questions:

1. What is the tensile strength?
2. What is the yield strength?
3. What is the minimum bend radius?
4. What is the recommended punch radius?
5. What die opening is recommended for the material thickness that I am planning to use?
6. What is the minimum flange length requirement based on the specific V opening required?
7. What effect if any does grain direction have on forming this material? For example, according to the website www.steelforge.com, "Most HSLAs have directionally sensitive properties. Formability and impact strength vary significantly depending on whether the steel is bent parallel or perpendicular to the grain. Bends parallel to the grain are more likely to crack around the outside of the tension-bearing surface of the bend. This is more pronounced in thick sheets. HSLA steels treated for sulfide shape control reduce this directionally sensitive characteristic."
8. What special considerations do I need to take into account?

While all of these points are very important, No. 8 is something that you should pay very close attention to. Many of the special considerations that come with forming these materials are not common to mild steel, and if they are ignored, they could potentially cause you a lot of problems. Examples of some of the special considerations provided by various manufacturers follow:

• Always bend the material to the largest radius possible. This will help to reduce material fatigue and the subsequent cracking that may occur on the outside of the bend radius.
• Grind away all surface scratches and all other surface defects as they may function as initial cracks.
• Some HSSs require that all bends be created in a single stroke. These materials must not be allowed to spring back during the stroke. It is very important for you to verify whether this is indeed the case with the material that you will be forming if your press brake has automatic springback compensation. Also, the yield strength of the material has a tremendous influence on the amount of springback that will occur during bending. The higher the yield strength is, the greater the amount of springback will be. Be sure to check the yield strength of the material that you will be forming with your steel supplier.
• Die shoulder radii should be hardened and ground to a very smooth surface finish. This will help to reduce drag during forming, which in turn will help to reduce the amount of force (tonnage) required to bend the material.
• Dies that use hardened rollers on the shoulders significantly reduce bending force requirements.
• Lubrication of the material is often recommended. This helps to reduce drag as the material flows over the die shoulder radii during bending, thus reducing the amount of bending force that will be required. It also helps to reduce tooling wear and damage to the surface of the material.
• Preheating the material is often recommended to reduce the bending force required to bend the material and can reduce cracking.
• Because of the high yield strength that is common to these materials, V openings of 12 to 15 times the material thickness and even larger are often required to reduce the bending force requirements and the possibility of the material cracking along the outside bend radii.
• Because of the large V openings required, bends with small flange lengths most often cannot be formed. This should be taken into consideration during part design.

Tooling Selection

Simply put, bending HSS requires high-quality tooling. For example, the tip radius on the punch and the shoulder radii on the dies must be hardened and must remain intact. And the deeper the hardening zone is, the better, because any breach of the hardened area will lead to a rapid deterioration of the working surfaces, causing accuracy to decrease and a subsequent increase in tooling costs. Wear on the die shoulder radii will also increase drag on the material as it flows over the shoulder radii, resulting in a significant increase in the amount of force required to bend the material. The tooling must also be made of a high-quality steel that can withstand the high amount of force that is generated when bending these materials.

Other considerations include making sure that the tooling you plan to use has an included angle on both the punch and the die that will provide you with adequate capacity to overbend the material as necessary to compensate for springback. As the yield strength is often very high in HSSs, in addition to the fact that they normally require larger inside bend radii, the amount of springback that will occur during bending will almost certainly be much higher than you experience when bending materials such as mild steel, aluminum, and so forth. If so, your existing tooling may not have the ability to compensate for it.

One of the greatest contributors to the cost of bending thick materials in general is setup time. This is due in part to the difficulty of handling the large, heavy tooling that is required. The punches normally have large tip radii and often comprise a combination of a high-load-capacity punch holder and a large-radius punch insert.

As noted previously, the dies normally have large V openings and often weigh well in excess of 100 lbs. (45 kg), requiring two or more operators and possibly a forklift to load and unload them. The use of short, modular tooling segments that match properly when installed will make tooling changes must faster, safer, and easier, and will eliminate the need for additional personnel or forklifts. Punches and dies with steel roller mechanisms also make locating them in the correct position much easier.

Of course, reducing the number of tool changes or eliminating them altogether is even better. This is where an adjustable die can be beneficial, especially one with quickchange capability.

As always, safety must be a very high priority. The rated capacity of the punch and die should always be checked against the amount of force that will be applied during the bending process. Most press brake tooling is rated on a basis of tons per foot, tons per meter, or both. Of course, you should never exceed a tool’s rated capacity as it puts both the operator and the tooling at serious risk.

It is also important to remember that a tool with a rated capacity of, say, 12 tons per foot does not mean that you can apply a total of 12 tons of force over any length, regardless of how long or how short the part length or the tooling is. It means that a tool with a rated capacity of 12 tons per foot is capable of handling a maximum of 1 ton per inch of bending force. Subsequently, if you were to apply 12 tons of bending force over 6 in. of a length of tooling that is rated at 12 tons per foot, you would be using the tooling at twice its rated capacity. In like manner, a tool with a rated capacity of 24 tons per foot is rated at 2 tons per inch.

Finally, it is always a good idea to use punches with Safety-Clicks or a safety tang on them, especially with large, heavy punches. This will help to reduce the risk of operator injury and damage to the tooling during loading/unloading.

One of the challenges with bending HSS is that cracking can occurs along the outside bend radius when bending this material with a die that has too small of a V opening.

Clamping and Crowning Systems

As mentioned previously, handling large, heavy tooling is one of the greatest contributors to the cost of bending thick materials. It can also be dangerous as the potential for injury grows as the tooling increases in weight. Besides using high-quality, short, modular tooling segments, one of the best ways to reduce setup time is to install a hydraulic or pneumatic clamping system on the ram of the machine. These clamping systems enable you to clamp the punches without the use of cumbersome wrenches and levers. Some have the ability to clamp, seat, centre, and align the tooling with the push of a single button.

The higher amount of bending force required to bend HSS and thick materials in general will increase machine deflection and the need to compensate for it when bending parts that require good accuracy and parts that are typically 8 ft. in length or longer. Using the old method of shimming dies is rarely acceptable as it is extremely time-consuming and can result in damage to the die holder as the force is partially concentrated over the shims as opposed to being dissipated over the full load-bearing surface of the lower die holder. This will create the need to have the lower die holder remachined periodically, thus adding cost to the overall operation.

The best solution for attacking the problem of machine deflection is to install a crowning system on the bed of the machine. Today’s crowning systems come with a host of features that are designed to allow you to tailor them to your specific needs and budget.

They include opposing waves for virtually infinite accuracy, localized adjustments that allow you to compensate for local wear on the ram and bed of the machine and the tooling, drive motors that are interfaced with the control, lowercost manual hand cranks for the adjustment of the crown, hydraulic die clamping for rapid die changes, and manual clamping in the form of multiple set screws that allow for the use of sectionalized tooling and to provide an inexpensive method of securing long planer made dies.

Get Started

Today’s HSSs offer countless opportunities to improve product quality by reducing weight while increasing product strength, durability, and product life cycles. However, they can present challenges as the traditional rules of forming do not always apply. Fortunately, the manufacturers and many of the steel service centres have plenty of data available to assist you, and most of it can be found on their websites. Combining their technical knowledge with that of your press brake machine and tooling suppliers should more than enable you to use these exciting materials to produce products that meet or exceed your expectations and those of your most demanding customers.

David Bishop is business development manager, west, for Wila USA, 7380 Coca Cola Dr., Hanover, Md. 21076, 443-459-5496, www.wilausa.com.

SSAB, www.ssab.com