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A time-of-flight Bragg-edge neutron transmission imaging was used to investigate the microstructure and strain distributions in a gear hardened by a newly developed two-step induction-heating method: precursor (Sample 1) and final product (Sample 2). The edge-position and edge-broadening were determined and mapped with high spatial resolution, which enabled us to confirm the two-dimensional distributions of the microstructure and residual strain. A deep hardened layer was made for Sample 1 in which martensite was formed on the entire teeth and the outer peripheral portion of the gear body. Sample 2 was subjected to double induction-hardening, where a tempered martensite was formed as the thermal refined microstructure between a fine-grained martensite at the tooth surface and a ferrite-pearlite microstructure at the core. The relationship between edge-broadening and the Vickers hardness described by a linear equation was employed to derive the elastic residual strain. The residual strain map for Sample 2 revealed that a steep compressive strain was introduced into the fine-grained martensite at the tooth surface by the super rapid induction-heating and quenching process. The reversal of tension was speculated to occur below 2 mm from the tooth tip, and the strain was almost zero in the core region. [ABSTRACT FROM AUTHOR]

The inspection of the surface-hardened depth of the mechanical part in a large-sized heavy industrial machine is important in the intensity or the guarantee of quality of these parts. When the size of its hardened steel parts is large, the destructive testing using the Vickers hardness tester is made difficult. Therefore, the non-destructive inspection method is needed for the evaluation of the hardened depth. The electromagnetic characteristic inside the hardened domain is lower than the non-hardening domain. Then, the non-destructive inspection of the hardened depth is possible by detecting the difference of these electromagnetic characteristics. In this paper, the high-sensitivity inspection method for detecting the surface-hardened depth is investigated by a 3-D nonlinear electromagnetic finite-element method using the evolution strategy method. [ABSTRACT FROM AUTHOR]