Induction Hardened Case Depth Measurement Using Ultrasonic Backscattering By Miguel Equihua, InTech NDT, USA

Induction hardening is a critical process in the manufacturing of automotive, agricultural and aeronautical components, such as crankshafts, camshafts, constant velocity joints, axle shafts, etc. The procedure for the evaluation of metallurgical characteristics is carried out in the laboratory and is destructive testing, this means that the component will be unusable, this procedure is time consuming, expensive, and cannot be integrated into the production line. Over time the industry has sought faster and more efficient methods to evaluate metallurgical characteristics. The methods that have had the most development are eddy current testing, magnetic methods and ultrasound. In this article we will address the use of industrial ultrasound, which uses the backscattering technique, which offers a direct determination of the depth of the transition zone between the martensitic phase and the core (ferrite and pearlite). This method is simple and does not require prior calibration to evaluate the components.

Ultrasonic Backscatter technique

Ultrasonic waves propagating in polycrystalline materials (steel) are scattered at material interfaces with changes of density and/or elastic properties. In general, the ultrasonic waves are scattered in all directions, one part also back to the ultrasonic transducer that generated the ultrasonic pulse. The intensity of backscattered ultrasound received by the transducer depends on the ratio of geo- metric size of scattering geometry to the wavelength of ultrasound and on the degree of material property difference at the interface denoted by the term acoustic impedance change.

In the regime where the ultrasonic wavelength is large compared to the size of scattering geometry, higher ultrasound frequencies (or shorter wavelengths) increase the intensity of ultrasonic backscattering. Further, intensity of backscattering increases with the average effective size of the scattering geometry, for example the grain size of the polycrystalline steel. Using the appropriate frequency of about 20 MHz, the microstructure changes between the hardened case (usually fine grain Marten￾site) and the core material (usually quenched and tempered) with coarse-grained microstructure causes a distinct increase of backscattering intensity. This effect can be observed when the ultrasonic pulse crosses the interface and standard time of flight evaluation yields the depth position of the interface that corresponds to the Surface Hardening Depth (SHD), see figure below:

Figure 1: Principle of the Ultrasonic Backscatter Technique (UBT) The best results are achieved under the following conditions:

– Parts are induction hardened;
– parts are forged, not cast;
– the minimum SHD value is higher than ~ 1.2 mm;
– there is a distinct interface between base material and hardened layer; backscattering within the base material is of sufficiently high intensity for ultrasonic frequencies of 20 MHz.


The manual device consists of a four-channel ultrasonic board controlled by a software pack- age for program settings, signal processing, reporting, and general quality assurance (QA) requirements. The components are assembled into an industrial notebook designed for use in rough industrial environments. Our probe systems enable testing of complex shaped components. The wedge of the probe system is adapted to the geometry of the required test position. For SHD values larger than 1.5 mm, rough technical surfaces are available that enable control before machining.

The main source of measurement error is evaluation of surface position: The shape of the surface signal depends on accurate coupling and operator skill. Another error source is setting the marker that provides the time of flight when the pulse reaches the interface. The steeper the signal rises, the lower the error. Thus, a shear wave angle as low as reason- able is used and scanning into the direction of decreasing SHD is recommended. Achievable accuracy of better than ±0.1 mm is possible for standard parts with high quality sur- faces. However, the operator must control the “good” shape of the A-scan during data acquisition. Accuracy based on mi- croindentation hardness profiles compared with the back- scatter method is slightly lower, estimated as ±0.2 mm on average, depending on the material microstructure.

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