Induction hardening depth and hardness measurement by laser ultrasonics for automotive industry

Induction hardening is an important heat treatment process for improving the surface hardness and wear resistance in critical components such as engine crank shafts. In this paper, we demonstrate the use of laser ultrasonics (LUS) for non-destructive measurement of the hardness penetration depth and surface hardness of several induction hardened crank shaft samples. The samples were removed from an automotive production process, and one goal of the paper is to study how process quality control could be improved by LUS measurements and what challenges remain. We performed measurements on multiple surfaces of the crank shaft bearings, including the bearing surface, the sidewall, and the radius between the two. We find that grinding the surface under study with sandpaper yields the best measurement results. In total, 25 samples were studied. The hardness penetration depth was measured on 17 samples and at 50 positions and compared with destructive testing, resulting in a coefficient of determination ( R 2 ) of 97%. Measurements were performed for a full revolution on all crucial regions of the crank pin, including the bearing surface, the corner radius at 45°, as well as the 90° side wall. The surface hardness was measured by comparing surface wave velocities on 8 heat treated samples at 22 positions. The coefficient of determination ( R 2 ) between laser ultrasonic measurements and destructive testing was 87 %. The ability to quickly and nondestructively measure the hardness penetration depth and surface hardness represents a step forward in the quality control of induction hardened automotive components.