Method and apparatus for measuring hardened surface layer

A method of measuring depth of a hardened surface layer of a to-be-inspected piece, comprising:

• a calibration curve creation step in which pairs of samples of a material of which the to-be-inspected piece is made are prepared such that carburization depth varies gradually from pair to pair, one of two samples in each pair is subjected to a destructive test to measure hardened surface layer depth, the other of the two samples in each pair is subjected to a non-destructive test performed with a transmitter probe and a receiver probe placed on a surface of interest of the sample, wherein at least propagation time of a first peak of a first wave of waveform of a signal outputted by the receiver probe receiving a surface wave transmitted from the transmitter probe to propagate along the surface of the sample is measured, and a calibration curve is created on the basis of hardened surface layer depths and propagation times thus obtained;
• a waveform measurement step in which the to-be-inspected piece is subjected to the non-destructive test performed with the transmitter probe and the receiver probe placed on a surface of interest of the to-be-inspected piece to obtain waveform of a signal outputted by the receiver probe receiving a surface wave transmitted from the transmitter probe to propagate along the surface of the to-be-inspected piece;
• a propagation time obtaining step in which at least propagation time of a first peak of a first wave of the waveform of the signal obtained in the waveform measurement step is obtained;
• and
• a hardened layer depth estimation step in which the hardened surface layer depth of the to-be-inspected piece is calculated using the calibration curve created in the calibration curve creation step, on the basis of the propagation time obtained with the to-be-inspected sample.