Eddy-Current Testing of the Fatigue Degradation of a Surface-Hardened Medium-Carbon Structural Steel

Gorkunov, Eduard; Institute of Engineering Science, Ural Branch of the Russian Academy of Sciences; Russian Federation

Savrai, R.A.; Institute of Engineering Science UB of RAS; Russia
Makarov, A.V.; Institute of Engineering Science UB of RAS; Russia
Gorkunov, E.S.; Institute of Engineering Science UB of RAS; Russia
Kogan, L.K.; Miheev Institute of Metal Physics; Russia

ID: ECNDT-0059-2018
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Session: Materials Characterization - ElectroMagnetic Techniques 1
Room: J2
Date: 2018-06-15
Time: 09:40 - 10:00

The paper studies the capabilities of electromagnetic techniques for testing fatigue degradation in hardened steel 50 (0.51% С), subjected to combined strain-heat treatment according to the optimal conditions including nanostructuring frictional treatment followed by tempering at 350 °C, under low-cycle fatigue loading. These optimal conditions increase hardness and wear resistance 2 to 3 times without sacrifice of the level of the mechanical properties (including plasticity) inherent to the steel quenched and tempered at the same temperature.
It has been found that, when the processed steel undergoes “hard” cyclic loading, on the specimen surface there appear cracks increasing in length and number with the number of loading cycles. The study demonstrates that it is possible to test the initial stages of cracking by the eddy-current technique, at frequencies ranging between 24 and 108 kHz, when the calculated depth of the analysed layer (220-120 µm) does not exceed the depth of the layer hardened by frictional treatment (~220 µm). This is based on the first-discovered effect of a decrease in the readings of an eddy-current instrument, which results from the higher resistivity of the thin surface layer during cracking. Therewith, on the initial portion of the dependences of the eddy-current readings on the number of loading cycles there is a minimum, which becomes more pronounced as the excitation frequency of the eddy-current transducer increases and the layer depth correspondingly decreases. Plastic strain accumulated under cyclic loading is shown to be testable by taking the eddy-current readings at a minimum frequency of 2.4 kHz, at a calculated layer depth of 800 µm, when the contribution of the cracking of the hardened surface layer into the eddy-current characteristics is insignificant. The obtained results can be used to predict fatigue degradation in products made of medium-carbon steels with hardened surface layer under cyclic loading.