Determination of the Case Depth by Ultrasonic Backscatter of Case and Induction Hardened Steel With a Soft Hardness Gradient

Abstract

Heat treatment of steel is a common industrial procedure to alter the microstructure of a component locally. Often, only the surface of a component is heat-treated to a certain thickness such that it benefits from the high durability provided by a hardened microstructure, while the core microstructure remains ductile. The nondestructive determination of this thickness, the case-hardened depth, can be performed by evaluating the slight differences of the ultrasonic backscatter from the grains of changing size in different layers. However, current methods require a small transition zone from the hardened to the core microstructure for accurate results. In this work, a different approach for the determination of the case-hardened depth of components with a large transition zone is described. This approach utilizes ultrasonic frequencies about 20 MHz in contact technique. The ultrasound is introduced in an oblique setup with a wedge and is mode converted to the transverse mode. The statistical evaluation of the backscattering shows promising results for the determination of the case-hardened depth of hardened components with a smooth decrease of the hardness over the depth.