Characterization of laser hardened steels by laser induced ultrasonic surface waves

Ultrasonic surface waves are suitable to characterize surface hardened materials. This is shown at laser hardened turbine blades. The martensitic microstructure within the surface layer of surface hardened steels has a lower surface wave propagation velocity than the annealed or normalized substrate material. Because the propagation velocity depends on the ratio of layer thickness to wavelength d/A, it allows to determine the hardening depth. If the surface wave frequency is high enough, the surface wave propagates mainly within the hardened layer. A correlation of the surface wave velocity to the surface hardness has been found. Because the variation of the surface velocity in hardened steels is small, a high measuring accuracy is necessary to obtain the interesting hardening parameters with sufficient certainty. Therefore, a measuring arrangement has been developed where laser pulses guided by optical fibres to the surface hardened structure generate simultaneously surface wave impul ses at two different positions. The two ultrasonic impulses are received by a piezoelectric transducer. The surface wave velocity is obtained from the time delay between these impulses which is determined by the cross-correlation method. To evaluate simultaneously surface waves with different penetration depth from the same signal acquisition. digital filtering has been used in connection with the crosscorrelation