The discussion on metal treatment is fine,BUT: for a SPRING Fuller????Just take an old tire iron,shape your spring fuller,throw the xxxx thing on the floor or somewhere to cool and put it to work!!! I've never had one fail. I'm not a blacksmith pro like some of you,but I used spring fullers for years when horseshoeing so can speak from experience at least in that area.

i use mild steel, or, flattened to about 4mm and normalised, car coil spring for long spring fullers...no need to harden and temper....for me any way.