Alternating current potential-drop measurement of the depth of case hardening in steel rods

Abstract

The case depth of induction-hardened steel rods has been determined using multi-frequency alternating current potential-drop measurements. Experimental results are analyzed using a model which approximates the variation in the material properties of a hardened rod by assuming that a homogeneous core is surrounded by a homogeneous case-hardened layer of uniform thickness. Experimental measurements on an untreated rod are used to estimate the core conductivity and permeability of similar hardened rods. The implicit assumption is that the material parameters in the core region are unchanged in the hardening process. The case depth is found by parameter fitting to minimize a penalty function representing the overall difference between multi-frequency potential-drop measurements and theoretical predictions. Case-depth values found nondestructively show reasonable agreement with those found using Rockwell hardness measurements on sectioned rods.

Magnetic characterization of surface-hardened steel

“…Considering the difference in electrical resistivity between the induction-hardened layer and that of the core steel, a directcurrent four-point potential drop measurement to evaluate the depth of case hardening was proposed in [4], and a two-point alternating-current potential-drop measurement with axisymmetric applied current in [5]. In both cases the hardened layer was assumed to have uniform depth.…”

“…In [4], the potential drop for various arrangements of the four probe points on the surface of a layered rod was analyzed, by means of finite element calculations, in order to optimize the probe arrangement either axially or circumferentially for maximum sensitivity to case depth. In [5], an analytical model of broadband potential drop due to an axisymmetric current applied by circumferential band contacts at either end of induction-hardened rods was used to interpret potential drop measurements over the frequency range from 1 Hz to 5 kHz. The analytical model assumed a linear relationship between B and H. Obtaining values of core permeability and resistivity from a measurement on an unhardened rod, the layer permeability and resistivity, and the depth of case hardening, were found by a leastsquares-fit of experimental data to theory.…”

“…This method is destructive and time consuming. There have been many efforts to evaluate the hardness and case depth using new methods, which include magnetic characterization [1][2][3], measurement of direct-current potential drop via a four-point probe [4], measurement of alternating-current potential drop via a two-point probe and axisymmetric applied current [5] and others discussed below. In this paper, the differential permeability and saturation magnetization of both induction-hardened and carburized steel rods are studied to assess their relationship to the depth of case hardening.…”

ACPD detection and evaluation of 475 °C embrittlement of aged 2507 super duplex stainless steels

“…For decades, various electromagnetic techniques such as the eddy current technique (ECT) and alternating current potential drop (ACPD) have been used for nondestructive evaluation (NDE) of materials. The ACPD has been used to evaluate microstructural changes by measuring conductivity and permeability of materials [4][5][6]. A novel directional four point ACPD probe was employed for creep monitoring [7] and further investigated by Prajapati et al [8] for creep detection in the vicinity of a weld.…”

Nondestructive characterization of flake graphite cast iron by magnetic adaptive testing

“…Because of this an easy nondestructive check-up of properties of the cast iron is highly desired. Various non-destructive evaluation techniques have been examined so far as an alternative method; alternating current potential drop [2], laser acoustic wave [3], ultrasonic back-scattering [4], eddy currents [5][6][7], photothermal radiometric radiometry [8].…”