Resistivity measurement by eddy current methods for real-time monitoring of age hardening in heat treatable alloys

Abstract

In this research, the design of an eddy current sensor system that continuously monitors age hardening during aging of heat treatable alloys was modified to allow for operation at temperatures up to 595 deg C. With two eddy current coils in an impedance bridge circuit, eddy currents are generated in a pure aluminum reference sample standard and an age harden able test sample. The difference in the resistivity of the aging test sample relative to the reference sample results in a bridge unbalance voltage, Delta V sub BCA, which is continuously measured by a multimeter. Also, calibration procedures were developed to allow conversion of the values of delta V sub BCA to a resistivity difference, Delta p=Ptest - Pref, between the test and reference samples. These calibration curves were generated by measuring delta V sub BCA at various temperatures for six test sample standards of known resistivity. The resistivity of the aging sample is determined by adding the known resistivity value for pure aluminum to Delta p. Real-time monitoring of an aging alloy's resistivity may allow heat treaters to integrate this monitoring system with a control system to achieve 'intelligent heat treating'.