This invention relates to electromagnetic thickness gauges and more particularly to such gauges which are used to measure the thickness of a weakly ferromagnetic layer deposited on a ferromagnetic base material.
In certain manufacturing processes, a layer of one material is deposited on a different base material. To control these processes and inspect the product, a simple non-destructive method for determining the thickness of the material layer is desirable. If the cladding in the inspected region is found to be less than a specified thickness, then additional material can be added to the layer. For non-magnetic cladding materials such as Inconel 600 and 304 stainless steel on a ferromagnetic base material, the clad thickness can be determined using known techniques and equipment such as low frequency eddy current probes or magnetic force probes.
In the manufacture of certain components for use in the nuclear industry, a layer of austenitic stainless steel is deposited on a steel base. The non-magnetic austenitic stainless steel matrix contains a certain amount of .delta.-ferrite which is added to prevent cracking of the layer. The .delta.-ferrite is a magnetic constituent which has a major influence on characteristics of the austenitic stainless steel layer such as intensity of magnetization and permeability. Therefore the presence of .delta.-ferrite in an austenitic stainless steel layer adversely affects the performance of standard eddy current probes or magnetic force probes.
Since eddy current probe signals are affected by the clad material permeability as well as the clad thickness, a direct clad thickness measurement can only be made if the effect of the clad material permeability is eliminated or at least reduced so that its influence is minimal. For an austenitic stainless steel layer having a .delta.-ferrite constituent, minimizing the permeability effects can be accomplished by magnetically saturating the .delta.-ferrite. In this invention, saturation of the .delta.-ferrite is accomplished through the use of an essentially constant magnetic bias in the probe.
A probe for an eddy current thickness gauge for measuring the thickness of a weakly ferromagnetic layer on a ferromagnetic base in accordance with the present invention includes a measuring coil having an electrical characteristic which varies as a function of the magnetic conductivity of the layer and base, and means for producing an essentially constant magnetic field of sufficient strength to substantially saturate the ferromagnetic constituent of the layer being measured. A gauge in which this probe would be used further includes a means for energizing the coil with an alternating signal and means for measuring changes in the electrical characteristic of the measuring coil when the measuring coil is energized. The use of such a gauge is in accordance with a method for measuring the thickness of a weakly ferromagnetic layer on a ferromagnetic base wherein the method includes the steps of: placing a measuring coil adjacent the weakly ferromagnetic layer; applying a constant magnetic field to the weakly ferromagnetic layer to substantially saturate the magnetic constituents of the layer; applying an alternating signal to the measuring coil; and measuring an electrical characteristic of th measuring coil wherein the electrical characteristic varies in response to the thickness of the weakly ferromagnetic layer.