Variation in mechanical properties of 4340 steel owing to thermal aging and study of corresponding change in structure sensitive magnetic permeability

The eddy current testing has been utilized to study the structure sensitive magnetic properties and electrical conductivity of tempered and roll-aged samples of steel as well as non-ferrous alloys such as aluminum. The major objective is to detect the aging and auto-tempering conditions in the steels and non-ferrous alloys nondestructively via eddy current inspection. Phasec 3D was used for eddy current inspection of the samples of steel and aluminum which had been quenched, normalized, annealed, aged and tempered in a Muffle furnace. The optical microscope was used to study the microstructures with computer software 'Microstructure Characterizer' for the multiphase analysis. The settings of Phasec 3D were optimized to get the maximum signal resolution maintaining readable display. Frequency, gain, phase angle are among the major parameters of Phasec 3D which were used to study the heat treatment conditions which were applied to steel and aluminum samples. The non-ferrous alloys such as aluminum were studied on the basis of their conductivity but all other ferrous alloys were studied for their change in magnetic permeability. It is clear from the results that eddy current testing can not only be used for microstructure characterization of low carbon steels and detection of heat treatment conditions but also for an important phenomena 'auto-tempering' of quench-hardened steels and for the detection of aging of rolled and cold worked steels. (author)