I made a rubbish bar which was from about 8mm mild steel, it worked but didn't do much damage but I think that was due to the motor. I hardened the tip using hardening mig welding which actually a seemed to work quite well, it didn't blunt too much in its one fight. It could work as a cheap solution but wouldn't be the best- you could always use something already made of tool steel like a lathe tool or centre punch.