Measurement of Induction-Hardening Depth
Induction hardening is one of the classical surface-modification methods for steel products such as shafts and gears. The surface is heated using a high-frequency AC current up to the austenitic phase (fcc) and then quenched. Phase transformation (fcc to bct) produces a case of hardened metal and compressive residual stress, both of which enhance tolerance for fatigue, wear, and corrosion during service. There are currently two techniques for case-depth testing. One is direct measurement of hardness on the cross-section, but it is destructive. The other detects the ultrasonic-backscattering signals from the grain-structure change in the steel parts, which is less laborious and nondestructive (Good, 1984; Fujisawa and Nakanishi, 1989). Because the core metal has larger grains, the core-to-case interface produces a backscattering echo, whose arrival time relative to the front-surface echo gives the case thickness in an A-scope display. It, however, requires the parts to be immersed in a water tank for acoustic coupling, making on-line inspection inapplicable. Another shortcoming is that the dominant front echo masks the signal from a shallow interface and this technique fails for a thin surface layer.
KeywordsRayleigh Wave Compressive Residual Stress Thin Surface Layer Case Depth Acoustic Coupling
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