Principles of emerging techniques are presented in the first section of the paper. These techniques include ultrasonic analysis, resonance spectroscopy, acousto-ultrasonics, acoustic pyrometry, power ultrasonics, LASER ultrasonics, and electromagnetic acoustic transducer based ultrasonics (hereafter referred to as EMAT ultrasonics). The second section of the paper describes the applications of both ultrasonic spectroscopy to characterize microstructure in steels and EMAT ultrasonics to study case carburized structure in steel components.

The case carburized structure is very common in engineering components used in rail, aerospace, and automotive industries. The case carburized structure is obtained using carburizing; carburizing is the diffusion of carbon into a component producing a layer with gradient in carbon content [1]. In a case carburized structure, carbon level varies with depth and the case depth is usually defined as the depth to 0.50 percent carbon. The ultrasonic measurements of case depth helps to understand the physical basis of carburizing and is used in closed-loop control of process to achieve the desired case depth in products.

Reference 2 describes a technique developed using EMAT to determine case depth, and the technique is based on the correlation analysis between the group velocity and case depth. The velocity, instead of amplitude or frequency of backscattered wave, was selected based on the correlation analyses between the group velocity and each of four material properties (carbon content, residual stress, hardness, and grain size) in steel specimens [3-5]. Furthermore, the group velocity was selected due to precision in measurements being better than the phase velocity or shear velocity [4].

The group velocity was measured at various frequencies, and the selection of frequency was based on the interrogation depth of interest, as the high frequency waves propagate less deep below the surface than low frequency waves. These results show that the group velocity decreased linearly with case depth, but the linearity depended on frequency of surface wave. The curve between the group velocity and case depth is recommended for use to determine the case depth nondestructively [5].

Another emerging technique is ultrasonic spectroscopy; ultrasonic spectroscopy deals with the study of frequency content of back echo in a specimen [6]. The paper will describe the applications of pulse-echo spectroscopy to characterized microstructure in AISI 9310 VAR, 52100 and CBS 600 steel specimens. The results from these studies show that peak frequency, bandwidth, and attenuation constant and its frequency response depend on the grain size, austenitizing temperature, and volume fraction of phases present in specimens. REFERENCES

1. S. Lampman, "Surface Hardening of Steels", ASM Handbook, Volume 4, Heat Treating, pp 259-462, 1992
2. S. Singh, D. Leeper, and R. Fuquen, "Process For Measuring Case Depth of Case-Carburized Steel", Patent Application, Serial No. 08/519, 989 (Revised), October 1995.
3. S. Singh, P. Shuttleworth, and H. Burrier, "Depth of Hardness in Carburized and Induction Hardened Materials", Industrial Heating, August 1992, pp 8-9.
4. R. D. Mitra, "Case Depth Evaluation of Carburized Specimens using Ultrasonic Methods", MS Thesis, MIT, 1993
5. S. Singh, R. D. Mitra, D. Leeper, and R. Fuquen, "Ultrasonic Evaluation of Case Depth in Case-Carburized Steel Components", To appear in Review of Progress in 22nd QNDE, Plenum Press, Volume 16, 1996
6. S. Singh, D. M. Keener, and R. Fuquen, "Ultrasonic Analysis for Characterization of AISI 9310 VAR-grade Steels", Proceedings of 1993 ASNT Spring Conference, March 29-April 2, Nashville, Tennessee, pp 58-62.