Edge ductility is a common and well-known issue among automotive component manufacturers. Under some circumstances, when a high strength steel is cut mechanically, the edge of the material at the cut can become compromised. Deformations and flaws like micro cracks may occur, which no longer possess the same properties as the steel around it. The big problem comes when it is time to form. As the steel is bent or stretched, brittle edges and micro cracks can grow, resulting in an unusable component. So, what can you do?
One remedy for this is to choose a steel grade with lower strength properties. However, while you may obtain better edge ductility, factors like weight, strength and safety characteristics may be negatively affected.
There is another way, one that can still allow for major upgrades to high strength steel without limitations in the design.
Note that the principle for pre-piercing is not just for holes but works for any geometry.
Known in theory, but not widely used among manufacturers of automotive components is the practice of pre-piercing.
The process works like this: After the first cut or punch in a high strength steel, a second cut or punch is performed that removes the compromised area of the initial cut. A good rule of thumb for the second cut is to remove half of the thickness of the steel. In effect, the technique shaves off a very small additional amount, which can easily be accounted for in the component design. That’s it. Simple and easy.
The second cutting process provides a different mechanical situation and makes it possible to obtain a much smoother surface without any hardening taking place along the edge. The benefit is improved formability and edge ductility. When comparing hole expansion ratios as a method to measure edge ductility, you can actually double the properties by pre-piercing.
Pre-piercing makes sense in connection with high strength steels. Often, the design of the component decides what kind of steel can be used, so the practice can make a big difference and allows for upgrading to higher strength steels. It can also be employed if a manufacturer is simply having trouble with edge ductility for pre-existing components like control arms, A pillars, door beams and chassis parts, to name a few.
There are a few things to keep in mind when planning to implement pre-piercing. While there is no need for additional investments, the practice must be planned into a production process in advance, or there will be problems. For example, the pre-piercing step may require another empty station on a stamping line.
In addition, it is worth noting that while pre-piercing can be used effectively for most high strength steels, it does not work with the same affect when used on dual phase steels. However, very good results have been achieved with steels that have martensitic or complex phase microstructures.
Simple and effective, pre-piercing is a technique that should not be overlooked when considering a material upgrade or developing a new component. The simple act of performing two cuts can mean the difference between a standard component and one that is lighter and stronger, with the potential for a competitive advantage.