I suggest that you refer first to Crystal Acoustics by Musgraves.  I believe this monograph includes the necessary elastic moduli for single crystal iron.  You will need to specify the crack plane and the direction of crack propagation on that plane.  To see how to do the calculations you could look at the work of  David Barnett of Stanford University who has made many contributions to the calculation of surface wave speeds in anisotropic materials. 

Rod Clifton