Nondestructive techniques are required for the in-process characterization of rapidly solidified and surface modified layers to fulfill the role of sensors in emerging intelligent materials processing technologies. In steels, where surface modificationvia directed high energy sources is being investigated for surface hardening, it has been found that a difference exists in the Rayleigh wave velocity of martensite and pearlite. The difference in velocity can be used to characterize the hardness of a surface modified layer on a pearlite substrate. By varying the Rayleigh wave frequency (and thus the depth of wave penetration) and measuring velocity dispersion, it has also been possible to determine nondestructively the depth of modified surface layers on both AISI 1053 and 1044 steels produced by electron beam melting.