Sorry to hear about the disk fracture - D2 is a wear resistant steel with really poor impact resistance, although I never heard about the 6x comparison to S7 before. Case hardening would only have helped to promote fracturing by raising the surface hardness and welding it up will likely just delay the inevitable. The best options are to get a softer disk in some type of Hardox or to go all out on an S7 disk that will last longer than wear steel but will cost a fortune. The S7 beater in Mr Mangle cost around $700 USD back in 2007 and its WAY smaller than Binky's disk