Nondestructive determination of hardening depth in induction hardened components by ultrasonic backscattering

Ultrasonic backscatteing technique has been applied to measure hardening depth of the range, 2-10 mm in induction hardened components. The method is based on the sensitivity of backscattering to microstructural variations at the transition from the hardened surface layer to base material. The change in microstructure leads to the backscattered amplitude which was found to be very sharp at the transition region for the investigated samples. Measurements were performed in immersion using 45 degree-shear waves at a frequency of 18 Mhz. Spatial averaging was used for smoothening the backscattered signals. The hardening depth is evaluated from the difference in time-off-flight between the surface reflection and the rising part of the backscattered signal. The measurements and signal evaluation were carried out using a PC-interfaced prototype equipment specially developed for backscattering studies. The results which were compared with metallographic examination as well as hardness measureme nts suggest that the backscattering technique can be applied for process monitoring during induction hardening.