Hi everyone,
I work for SSAB (the manufacturer of Hardox) as the technical development manager for the UK market. Having seen your interest in the use of Hardox in fighting robots and bending related questions, I just wanted to share some pointers in regard to grade selection and fabrication. My experience of robots fighting is pretty much watching Robot Wars many years ago...so my assumptions may be very incorrect.
Basics of what gives Hardox strength -
Hardox gets its strength from quenching i.e. cooling the steel after rolling from around 900 degress C to room temperature in a matter of seconds, depending on the grade. This has the effect of "locking in" the Carbon atoms in the atomic lattice structure. This gives it a very high strength and hardness level. A useful rule of thumb is the Tensile Strength / 3 gives the hardness level in HBW. It then undergoes some tempering (in a large oven) at fairly low temperatures (in comparison to conventional practices), hence, there is a heating limit of 250 degrees C on Hardox. Heating it above this temperature will essentally enable the "locked in" carbon atoms to migrate and the martensitic structure is gradually lost as the temperature increases. So when fabricating Hardox, i.e. welding, thermal cutting, machining, many of our recommendations revolve around minimising the temperature. For example;
Welding - we recommend minimising the heat input (product of volts and amps) to minimise the extent of the Heat Affected Zone and also improve toughness in the joint - something I assume would be important in a fighting robot which could be subjected to high dynamic loads. SSAB WeldCalc can help with this, its a free online software which visually plots your welding settings and tells you if they are optimal. If you want access to this please send me an email - bottom of post.
Thermal cutting - tips such as underwatter cutting with an oxy torch (or even feeding water with a water hose) whilst cutting reduced the HAZ by about 30%, which means that the softened zone from cutting is 30% less giving you a longer life in that edge. Maybe if you were making a flipping arm that flips the opponent and scrapes the floor, this preserved hardness would be beneficial
Machining - the use of plenty of coolant whilst machining is very beneficial to both the thread strength and the drill bit. Usually about 10% coolant mix is sufficient, but I would consider 20% mix for grades Hardox 550 and above.
Selecting the grade
Skin of robot - I would suggest either Hardox 450 or, preferably, Hardox 500. As has already been suggested, bending is always much better than welding, since a weld is essentially a point defect, so from a durability perspective a bend is always better.
Some comments on bending Hardox -
Bending high-strength steel is different to mild steel, as has already been highlighted. The reason is that the high-strength steel follows the punch much more closely than mild steel, since it has a higher capacity for local straining. This means that the relationship between punch radius is critical since it strongly dictates the strains on the outer radius, which, if high enough, leads to “fissures”, which can eventually lead to cracks and failure. On the other hand, bending performance and weldability is a strong indicator of quality, so when it comes to Wear Plate, Hardox is very user friendly. Another thing to consider when bending Hardox is springback i.e. you ned to overbend it by a certain amount of degrees to get the desired angle. A good rule of thumb for this is about 1 degree per 100MPa of yield strength, i.e. expect to overbend by about 15 degrees. This can vary from equipment type, so a quick test run is always recommended. Also, when it comes to required bending force, it can often be less in comparison to conventional steel since you can use a thinner material (or upgraded) and the bending force is proportial to the thickness squared. See links at end of post.

A final note on material selection. Generally speaking, the smaller your part is e.g. a damaging spike or saw-shaped ram, the higher in grade you can use. So you could consider a Hardox 600 in certain small parts, which would give you very high hardness and possibly a competitive edge. However, as mentioned, ensure to consider the processing effects, abrasive water jet is by far the best for small critical parts since the bulk hardness is preserved, or even a laser cut is also fantastic. Of course, it needs to be within budget, so apologies if I’m suggesting practices that are beyond scope.

Please drop me an email if you would like more information related to high strength steels.

Bending force calculator - BendCalc
http://www.ssab.com/en/Products--Ser...App-functions/

Upgrading rule of thumb - how much weight can be saved going from conventional steel to HSS
http://www.ssab.com/en/Products--Ser...d-films/Tools/


Good luck.
Email – aaron.middleton@ssab.com
Regards,
Aaron