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A “Hardening Penetration Depth Evaluation” technique using ultrasonic wave is developed, instead of destructive inspection. An induction hardened structure possesses extremely fine(∼10μm) grain size, whereas, an non-hardened structure possesses coarse(∼100μm) grain size. This non-destructive technique measures scattering echo, scattered from boundary of hardened structure and non-hardened structure. The advantage of this technique is that through the visualization, one can measure scattering echo around the material and thus acquire quenching pattern. In addition throughout its resolution, one can measure scattering probability distribution of the ultrasonic wave and obtain hardening penetration depth.

The fluorescent magnetic particle inspection is widely used as a visual inspection method for checking cracks generated in hardening and grinding of induction-hardened parts. However, automation of this inspection process has strongly been demanded, due to poor environmental conditions and production line speed. To satisfy such a demand, we have developed a method for picking up images of automotive parts with higher S/N ratio and an original algorithm for image processing which helps measure cracks accurately without being affected by the illuminance and magnetic particle solution concentration. Then we selected the front axle shaft as the object to study practical use and have solved various technical problems in actual use, thereby succeeding in actual application to our production lines.