It would be unfair to call it a revolution, but it is not unfair to call it a very fast evolution of fabricated steel products.
Abrasion resistant (AR) steels have been around us for some time now, and though people in my industry tend to associate them with mining only, there are many other products that benefit from the features of these alloys.
Not only are these alloys harder and therefore resistant to abrasion, Some are also ductile enough to be fabricated, and so they are being used for structural fabrication.The best example being transportation, where a lighter truck means more profitability per load/transport.
So, what is this “hard stuff” that fabricators are dealing with now? How is it different from standard steel, and why is it important to know and understand the materials before dealing with them?
Abrasion is by definition: “The process of scraping or wearing something away”. In our case, of course: STEEL
Mining industry people know all about abrasion. Imagine how much steel is scraped away during the loading of a truck when tons of mineral rock are thrown in a bucket. Eventually, this scraped material starts compromising the shape and structural holding of the bucket itself. So, the less it’s scraped the longer the bucket is useful, secure and profitable. Basically, there is a need to make the bucket “harder.”
Hardness is the property of a material to resist penetration from another material
AR steels are “designed” with different alloys to increase their hardness, adding Manganese, Nickel, Silicon or Molybdenum among other elements. There are also heat treatments that can help increase hardness, for example by tempering the steel. All done to provide a harder steel that will resist abrasion longer.
What about ductility and malleability? Ductility is the ability of a given material to withstand deformation under traction without cracking, while malleability is the ability to withstand compression. And as we know, in press brake bending we have both tensile strength and compression.
Both ductility and malleability relate directly to a mechanical property always provided by the steel supplier: “ultimate tensile strength” or “UTS.” The UTS is basically the maximum load to traction before cracking.
Many AR steels have good ductility and malleability. We could actually talk about a good “toughness” which would be the ability to absorb energy before cracking.
These are in my experience the most common AR steels currently being used: Hardox 400, 450 or 500 – Armox 370, 440 or 500 or SECURE 400, 450 or 500. Armox and Secure are alloys known as “ballistic,” which means they are even harder and therefore even more difficult to fabricate, but they surely resist abrasion, so we are including them here.
In all these AR steels you will notice how the tensile strength is higher than mild steel. The higher the UTS, the more pressure is needed to bend the part. Here’s a formula and an article about this specific point.
So, let’s focus on bending AR now. We know it’s a harder and tougher material with good ductility. This means that, although it will be difficult to bend, it can still be bent! Here’s what you must know:
- Thickness / V open ratio: There is NO unique rule for all AR steels, but know that V openings need to be at least 10 times a thickness when bending AR steels. Some materials even require a V= 20 x Thickness. The harder the material, the larger the V required. Luckily most suppliers have run tests and provide a chart for each material type. So ask for those charts when buying AR steel.
- Punch radius / material thickness ratio: Again, this is not a unique ratio for all AR steels, BUT the radius of upper tools needs to be very large compared to normal MS bending. Usually, the ratio starts from 5 times and can go up to 12 or 14 times the thickness, depending on the AR type.
- Springback: Every AR steel will be different, and every bending setup (press brake and tooling) will also be different. Indeed, many factors influence the real springback, so it cannot be provided exactly, but only an estimation. In any case, this will be far greater than what MS offers. Springback on AR steels can go as high as 15° to 25° degrees. This means tools must be designed with acute angles to allow deep bends that will then open (visibly) to 90°.
- Grain or roll direction: Bending along the grain or against it forces some press brake parameters to be readjusted while bending MS or SS as well. But unless the part being fabricated needs to be really precise, most of the time the difference remains within tolerance. On AR steel this is not the case. bending along or against the grain becomes critical and not changing the punch radius usually means cracking the material or having wider angles. So before you start cutting, make sure you know how you will bend the part and what direction the grain goes, then choose your tools and program your PB accordingly
WHY is all this required?: As we have said an AR steel is harder but still ductile, which means it can be bent (achieving plastic deformation) but will require a larger V opening in order to distribute the force on a larger surface.
The AR steel fibers need to be bent on larger surface in order not to crack. That’s the reason why AR steel requires larger radius on upper tools as well, If we fail to do so, the risk of cracking such a hard material becomes higher.
Using a larger V opening and a larger radius upper tool increases the springback on any given material we try to bend. Adding this to an already high UTS material results in parts that “hit back” as much as 20° or 25° degrees.
Bending against the grain requires a radius punch somehow smaller, while bending along it urges the need of a larger radius punch in order to meet more fibers and therefore distribute the force among them, otherwise, the pressure focuses only on a few fibers resulting in cracks, usually on the outer part of the steel.
Hardness and toughness meet on AR steels. This forces the use of the exact and correct compromise between pressure and angle on the bending process. Using a V opening far wider than recommended trying to reduce the pressure needed for a part will result on a greater springback that will simply “hit back” too much, sometimes even preventing the material to achieve plastic deformation and surely not achieving 90° angles.
On the other hand using a V that’s smaller than recommended trying to reduce springback will not only increase the pressure needed but most likely will result in cracks on our part.
Press brake and tooling for AR steel: It is no secret in stating a stronger press brake is always welcome when bending AR steel, especially with greater thicknesses and harder alloys. But power is not the main feature: precision and control mean a lot more in order achieve an exact angle with such a difficult material. Adjusting to springback after testing and repeating exact position of the ram on each stroke are vital for this kind of fabrication.
Of course, the tools will need strength to hold the force of bending, but precision is vital as well in order to have correct angles. Also, tooling must not wear out since this could cause cracking on the bends or variations in the resulting radius (usually due to a worn out die).
At Rolleri we’ve had experience with a wide range of AR steels, working alongside fabricators, machine manufacturers and even steel suppliers. We’ve learned our way around all the complications this kind of work brings into a job shop.
As usual, with any bending project, if you learn beforehand and program your steps from first to last, problems and issues will be reduced to a minimum, and you will have the ability to solve them. When working with AR steels this means:
- Know the chemical and mechanical properties of the exact steel you will use. If possible, even before designing the part.
- Before starting to cut any blank, know the PB specifics and tools that will be used. You can learn more about blank calculation on this article
- last, and most importantly: verify with a specialist that the setup is correct and that it will grant long-term results as well. We can help with this at ROLLERI or you can just contact me here on linkedIn.