Process for detecting defects on a surface by eddy currents and

Details Process for detecting defects on a surface by eddy currents and Process for detecting defects on a surface by eddy currents and

1986-05-20

1989-01-17

Strecker, Gerard R.

Electricity: measuring and testing

Magnetic

With means to create magnetic field to test material

324232, 324233, 324242, G01N 2790

Patent

active

047990104

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for detecting defects on a surface, by eddy currents, and more particularly a process of the type whereby a magnetic field is generated by way of an emitter placed close to the surface of the product to be controlled, and a receiver separate from the emitter is placed also close to the surface of the product in order to collect a signal indicating possible disturbances in the eddy currents circulation on the surface of the product, due to defects in said surface.
The field of application of the invention is that of the detection of defects on the surface of semi-finished metallurgical products, in particular the detection of cracks on semi-finished iron and steel products such as steel slabs, blooms or billets exiting from a continuous casting installation.
Detection of surface defects by eddy currents is presently performed with double-action sensors, namely sensors in which the same winding is emitter and receiver, or as in the case of the process of the aforedescribed type, with sensors having separate functions in which the emitter and receiver windings are separate.
Sensors with separate functions permit a greater detection response, yet this is often very difficult due to a high level of noise. Indeed, in a conventional sensor with separate functions, the receiver winding or windings are placed in the acting zone of the emitter winding. Yet the response of the receiver is affected by numerous factors such as geometry, conductivity and permeability of the product, and the inevitable variation of these factors, even in the absence of actual defects, consideraly lowers the signal
oise ratio.
To improve this ratio, it is known to use differential sensors with two receiver windings before which moves the product to be controlled. Detection is achieved by processing a signal representing the difference between the signals delivered by the two receiver windings. It is then possible with the relative closeness of these two windings to overcome the effect of slow variations in the structure or in the characteristics of the product.
However, even with differential sensors, it may still prove difficult to obtain a reliable detection because of a high noise level. This is so for example, with the detection of cracks on a slab, particularly because of the presence of surface irregularities (wrinkle or waviness formations) which disturb the field without actually constituting defects. Also, and speaking more generally, the detector response is not only affected by variations in the geometry of the product, but also by conductivity and permeability variations due to localized heterogeneities, and also by distance variations between receiver and product.
Therefore, it is the object of the invention to propose a process for detecting surface defects by eddy currents in which an important improvement of the signal
oise ratio is obtained over the previous processes.
This object is reached with a process of the type described hereinabove and in which, according to the invention, the receiver is placed with respect to the emitter, on the one hand, in such a way as to be aligned with the emitter in a direction corresponding to the general orientation of the defects to be detected on the surface of the product, and, on the other hand, so as not to be substantially affected by the eddy currents generated by the emitter when the surface of the examined product is defect-free, so that a significant signal is collected by the receiver only in case of deviation of the eddy currents in its direction due to a discontinuity on the surface of the product.
The position of the receiver in a zone situated outside the zone of direct influence of the emitter presents many advantages.
First of all, anything liable to disturb the eddy currents has no effect on the detection as long as these currents remain confined to the zone of the surfaces facing the emitter. For example, local variations of conductivity or permeability cannot hinder the detection. In the same way, to some extent, the presen

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