November AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 9 5 8 Surface hardening is commonly used to manufacture parts with high tensile properties and fatigue strength to re- sist surface cracking and abrasion. In addition, surface hard- ening depth (SHD) must be controlled in accordance with various international specifications [1] . Although evaluation of SHD can be done by cutting samples, this approach is time consuming, expensive, and does not lend itself to integration into a production line. These concerns led to the demand for a fast, nondestructive method to test hardened parts and to enable quick optimization of the induction hardening pro- cess when treating different parts and/or changing process parameters. Methods developed to accomplish this include eddy current testing, magnetic methods [2] , and ultrasound. Electromagnetic methods take advantage of material char- acteristics that correlate to physical values like conductivity and permeability. However, various part specifics interfere with the correlation, which requires a set of calibration sam- ples. Further, electromagnetic methods have limited pene- tration depth. By comparison, ultrasound backscattering offers a direct determination of the interface between two materials with different microstructures, including grain size, which is discussed here. This type of measurement is simple and does not require correlation of any kind nor cali- bration samples. PRINCIPLES OF ULTRASOUND BACKSCATTERING The ultrasonic method uses the physical phenomenon of Rayleigh scattering. Scattering in the Rayleigh regime oc- curs when randomly distributed scattering objects are much smaller than the wavelength. Backscattered power increas- es with the fourth power of the incident wave frequency. In ultrasonic testing, it is applied for microstructure character- ization and effective grain size assessment, for example [3] . In the Rayleigh regime, backscattering power depends on the effective size of the scattering object by the third power. In solidmaterials, shear waves can be used with the advantage of a higher scattering coefficient due to the acoustic crystal anisotropy of iron, but with the practical disadvantage of an- gle beam scanning. Grains in polycrystalline metals are densely packed. The fundamental equation is simplified assuming an effec- tive average grain size, the absence of a multiscattering con- tribution, averaged interference, and averaged refraction indices at the grain boundaries [4] . The Rayleigh scattering coefficient σ λ is estimated as: σ λ ∼ ƒ 4 ⋅ A e 3 ⋅ R M This simplified relation shows the role of material fac- tors in backscattering intensity. Frequency, ƒ , is a scaling factor; an increase in frequency increases the resolution of material characteristics by a power of four. Effective grain size, A e 3 , implies the contribution of various scattering sourc- es and the effect of multiscattering, and R M indicates the averaged impedance contrast at the interface of scatter- ing objects. Assuming an interface between the fine-grained mar- tensitic case and the coarse-grained core material, an in- crease of measured backscattering intensity provides in- formation about the depth of the interface by time-of-flight evaluation, knowing the angle of incidence and the sound velocity. The method senses the transition to the unaffect- ed core material, thus indicating the thickness of the total hardening depth (THD). However, it is not possible to mea- sure case hardening depth (CHD) or nitriding hardness depth (NHD) because there is no well-defined interface between case and core by means of effective grain size. TECHNICAL CAPABILITY Measured A-scans provide information about the part surface position by specular reflection of the sound pulse at the interface probe system to the part surface. The width of the surface peak is due to beam spread and from the rough- ness at both the wedge and part surface. At a frequency of 20 MHz, the fine, needle-shaped martensitic case should not scatter, but most of the coarse-grained core material does (Fig. 1). The wedge of the probe system is made of plastic material with low attenuation at high ultrasonic frequencies and does not scatter. The angle of incidence is above the first critical angle, but as low as reasonable for a steep increase of SURFACE HARDNESS DEPTH MEASUREMENT USING ULTRASOUND BACKSCATTERING Ultrasound backscattering enables direct determination of the interface between two materials with different microstructures. Mike Bogaerts, CNH Industrial, Antwerp, Belgium Michael Kroening, TPU, Tomsk, Russia Paul Kroening and Tobias Mueller, Q NET Engineering, Saarbruecken, Germany 8 *Member of ASM International