Emissivity-adjusted based depth profiling in eddy current pulsed thermography

The measurement of hardened case depth is important for evaluating the case treated materials, particularly in the nondestructive means. The conventional method for the task has to cut the samples to evaluate the hardness by crossing surface of it. Here we introduce a new nondestructive testing and evaluation (NDT&E) method to accomplish the task by using eddy current pulsed thermography (ECPT), in comparison with another newly introduced method, magnetic Barkhausen noise. The samples used in this paper were four case nitrided gear steel samples. They have different treating times with the case depth less than 1mm, which are with different surface emissivity values. We investigated the behaviors of thermal intersection points under different lift-off of the excitation coil to the sample under test in ECPT after the emissivity adjustment. It shows that there are two kinds of intersection points. The first one is thermal intersection points of different samples are as further to the maximum heating transient as increasing the lift-off of excitation coil. The second kind is the thermal intersection point appeared in the transient thermal response of one sample with different lift-off of excitation coil, and the positions of the point is shifting close to the maximum heating transient as the case depth of the sample increasing. This paper shows, the depth-profiles of case hardened materials can be retrieved by evaluating the behaviors of lift-off point of thermal intersection (LOTI) in eddy current pulsed thermography.