When manufacturing high quality press brake tools, selecting the correct materials and shape are only the first steps in the process. ROLLERI then improves the durability of our tools by hardening them. But hardening is just a small part of the process when we talk about resistance to compression and buckling. It helps, but it is not as important as choosing the right material. If we were to harden our tool completely or “too much,” it would no longer be malleable. The material would become too rigid and crack under pressure.
Then, why do we harden press brake tooling at all?
We said earlier that our tools get “scratched” by the sheet metal we are bending.
Our tools have a disadvantage against the sheet metal: The tools stay there stroke after stroke, so eventually that “scratches” start to build up. They eventually change the shape of the tools by “eating” up some material on each bend.
Now, when we talk about precision bending we assume a CNC press brake is being used, high quality sheet metal, and of course, precision tooling! But what good would a tool be if the original shape has changed?
Just as an example, for each 0.5mm (.019”) that the beam goes down (or up, depending on your PB) the bend you are making closes by 2°. So if our tools were to lose only 0.5mm on some parts, your bend would result 2° wider on that point.
That is why precision tooling must be hardened tooling. So it will keep its shape even after 1 million strokes of your press brake, giving you the same result over and over.
But, what is hardening and how do we apply it on tools?
When we talk about hardening, we are talking about heating. Actually we also call it “heat treatment.” In reality the heat applied to a tool (or any other steel element) is only a part of the so called “treatment.”
By heating the molecules of steel we excite them, forcing these molecules to change structure. A good way to picture this would be to imagine a dance floor where all the atoms are dancing a slow waltz, then at a certain point someone changes music and all of a sudden the atoms are dancing to the rhythm of a hard metal rock band. That is what heat does to molecules.
Heat excites the elements of mass and in our steel-made tool this means carbon atoms tend to go to the outer part of the molecule, giving the molecule a structure called “martensite.”
Martensite is a stage of the molecule that increases volume and hardness without reaching the state of “cementite” which is even harder but less malleable.
So, martensite is all we look for when manufacturing a press brake tool. But there is a problem: martensite shows up in a structure when we apply heat on our tool. But under normal temperature and circumstances most of the molecules will return to an “austenite” phase, which is less hard, and which we do not want. Because, as we already said, our tool will be scratched over and over many times by the sheet-metal we will bend, and we do not want our tool to lose any of its shape, otherwise we would be obtaining different results on our bending process.
Here comes the real hardening part of the process. In order to force the molecules to remain in their martensite structure we must bring down the temperature of the part as fast as we can. That indeed is what is known as hardening. We force the molecules to freeze at the hard metal melody they were dancing (to go back to our metaphor) so that they will remain hard and so our tool will not change its shape even after 1 million hits of your press brake.
We, at ROLLERI, use induction hardening. This method implies that an electromagnetic field is created around the part of our tool that requires hardening.
This way we can localize hardening, making sure that the rest of the tool remains malleable.
Also this method allows a deepness in hardening of 3mm to 6mm (0.11” to 0.24”) and a hardness of 52.60 HrC (basically more than double the normal hardness of steel)
Induction hardening allows a fast heating of the part so we can then rapidly bring the temperature of the part down to normal temperatures. The entire process requires less than half a minute on each tool.
Of course every heat treatment brings along a certain deformation and stress of the material. That is why our tools go through a normalization process after hardening. Normalization consists of bringing the tool to a higher temperature, not as high as the temperature used for hardening though, and then bringing the temperature down slowly in order to allow stress relief on the organization of the molecules. This of course does not change the martensite structure we have reached before. This process is slow and costly, but brings the best result in precision tooling.