Laser-ultrasonics as an NDT method for non-contact determination of the surface hardening depth

Haderer Wolfgang - Research Center for Non Destructive Testing (RECENDT) GmbH (Austria)

A wide variety of methods exist for surface hardening. Here, we focus on thermally hardened parts which exhibit a change in the surface near microstructure. The extend of this hardened surface layer is the surface hardening depth (SHD). We will discuss an improved NDT methodology based on all-optical laser ultrasound (LUS) and supervised machine learning to determine the SHD in a non-destructive and non-contact way.

LUS uses two lasers to generate and detect ultrasonic waves. A short-pulse excitation laser excites ultrasonic waves at the surface of steel parts. These waves propagate into the sample and are preferentially backscattered at the interface between hardened layer and core region due to the difference in grain size. The backscattered acoustic waves are subsequently detected at the sample surface by a second laser (detection laser). The expansion of the hardened layer is determined via the time-of-flight of the backscatter signal and the known speed of sound of the sample material similar to pulse-echo scheme.

Both the measurement method and the results will be presented using representative samples of typical induction hardened automotive steels. We present two different approaches for data evaluation. First, data evaluation in time domain and second, data evaluation applying a supervised machine learning approach.