CBN( CUBIC BORON NITRIDE ), hardness is just second to diamond, but thermal stability is better than. CBN doesn't react with iron family elements. Its hardness and chemical properties make CBN be the most desirable super hard abrasive for grinding ferrous and other aeronautic material.  

    CBN application:

    Alloy Steels( HRC>50 )

    Cast Iron, Hardened Tool Steel( HRC>50)

    Super Alloys( HRC>40 )

    Stainless Steels( HRC>50 )

Here is a video from Youtube, it is a really good video about CBN wheels, let's enjoy it: