September 17, 2001
An eddy current flaw detection system is suitable for detecting discontinuities in tube and pipe during the production process. Understanding about eddy current system principles and this technology's capabilities and limitations can help tube and pipe producers learn how to use such a system.
Eddy-current flow follows a closed-loop pattern unless interrupted by a crack, pin-hole, or similar discontinuity.
Eddy currents are alternating electrical currents that can be induced to flow in any electrically conducting material, which covers all metals. Eddy current flow follows a closed-loop pattern unless it is interrupted or diverted by a nonconductivity area such as a crack, pin-hole, or similar discontinuity (see Figure 1).
Eddy current testing is the science of detecting flaws while ignoring other influences on the flow pattern created by dimensional variations, stress, chemistry changes, magnetic properties, electrical interference, mechanical movement, vibration, etc.
Collectively, the signals to be ignored are termed as "noise," while the ones which are of interest are called "signals."
The state of the art is to achieve significant improvement in signal-to-noise ratio to obtain a desirable minimum of 3 to 1 for satisfactory in-line operation. Often, signals are totally drowned out by noise at their source, and this must be corrected by various physical, mechanical, and electrical means to optimize the end result.
The depth of penetration of eddy currents on tubes is influenced by test frequency, conductivity, and other variables.
Much can be done with electronic filtering and transducer design, but tackling the interference at the source may still be necessary in many instances, and expert evaluation of the proposed inspection site may be required.
Depth of penetration of eddy currents on tubular products is a complex matter influenced by test frequency, coupling factors, inside diameter (ID) to outside diameter (OD) relationships, and the electromagnetic characteristics of the material (see Figure 2). The lower the test frequency, the greater the penetration but the poorer the sensitivity to defects. One or two kilohertz is the normal practical base frequency.
When testing ferromagnetic material with optimized magnetic saturation of the material, the maximum wall that eddy currents penetrate is about 0.322 inch, but this may be augmented by magnetic effects from flaws on the ID. Tubing and pipe up to 0.500-in. wall has been successfully tested using a combination of eddy current and magnetic flux leakage effects.
On higher conductivity materials such as aluminum, copper, and brass, penetration is much less, ranging only up to 0.080 in. as the practical limit at which sensitivity to flaws is still reasonable.
Eddy current testing is widely used for nondestructive testing in the tube and pipe industry. It is relatively simple to install and operate and can detect a range of defects and discontinuities at varying mill speeds.
Tube and pipe mills have four likely locations for inspection head installation.
Once calibrated, modern drift correction techniques help ensure that systems operate for periods of years with little maintenance or attention, except for size changing.
There is no physical contact between the transducer and the material under test, so wear is not a factor, although damage sometimes results from misalignment or from crashes on the mill.
With seam or spiral welded products, the most vulnerable area is the weld itself. Flare and flattening tests are essential tests on any mill, but 100 percent inspection of the heat-affected zone (HAZ) indicates anomalies or deviation in the process as early as possible. This allows the operator to correct the process and contribute to better overall quality and reduced potential scrap.
Depending on their shape and construction, eddy-current transducers check just a few degrees of the tubular shape or the entire circumference or some amount in between.
Eddy current inspection can be made fully automatic with accurate tracking, marking, and rejection of defective sections. However, the main focus must be on motivating the mill operator to respond and correct early trend signals, since this can significantly improve efficiency.
The first consideration before installing eddy current technology is where to place the inspection head on the mill. Four likely locations are shown in Figure 3.
From a quality aspect, the location should be as late as possible to catch defects that occur or propagate because of heat treatment, cooling, or sizing. From a production aspect, the location should be early, giving the operator the quickest response. In practice, the location may be dictated by availability of space.
Once the location is decided, the next consideration is the type of transducer (see Figure 4):
1. A seam inspection probe that focuses on the weld itself. Generally, this must be located close to the welding platform since a weld twist of only a few degrees can be accommodated. Probes may be air- or water-cooled. One probe covers all tube or pipe sizes.
2. A sector inspection coil covering the whole HAZ and allowing weld wander up to 1-1/2 in. on either side of center or up to ±90 degrees with sizes up to 2 in. in diameter. The number of coils needed to cover a specified size range depends on the degree of seam wander at the proposed site.
3. Full body testing, either with encircling coils or with multiple sector coils. This test is usually done after sizing and stretch reduction (if applicable) but before the turkshead, since vibration and pass-line variations can create problems. Individual coils and bushings are usually needed for each tube or pipe diameter.
Full body inspection may be mandated by specifications. However, it may sometimes be impractical because of the presence of loose ID scarf within the tube or because of space limitations.
Generally, the in-line use of encircling coils is limited to tubes with less than 2 in. OD. For larger sizes, the area of search may be extended too much to reliably detect the 0.031-in. drilled hole standard prescribed by ASTM-A450. Use of multisector coils is recommended for full body testing of larger sizes.
Most defects occur in the HAZ, but strip defects such as pin holes and laminations may be a problem, particularly with hot rolled material, and these may occur anywhere around the wall.
Seam probes or sector coils are usually better suited for the mill environment, except on small-diameter tube mills (less than 1/2-in. OD) or in stretch reduction. In the latter case, the weld seam may twist beyond the scan of a sector coil.
Two distinct detection methods are available:
1. Differential sensing, whereby one section is compared with the section immediately preceding it to provide high detection sensitivity to short and intermittent defects. It signals the start and finish of the open seam. If the defect is continuous and does not modulate along its length, it could be missed.
2. Absolute sensing, with which the section under review is compared with a pattern stored in memory. The sensitivity of this method can be limited because of continuously changing signals caused by unrelieved stress and roll marks in the material. Absolute sensing provides a continuous signal for the full length of open seams.
Separate detection of open seam can be used to advantage to stop the mill in installations in which subsequent annealing or plating operations could be dangerously affected by the escape of mill coolant through the split.
Combined systems with both differential and absolute facilities are available.
By integrating the noise level over a period of time and alerting the operator to marginal changes, subtle variations in weld integrity can be sensed and hopefully corrected before deteriorating to a rejectable defect level.
No method of nondestructive testing can find every conceivable defect associated with tube quality, but eddy current testing offers a broad spectrum of response. This spectrum cannot be limited only to defects which fail mechanical tests, however.
Typical defects likely to be found by eddy current inspection include open seams, pin holes, cross cracks, splits, porosity, lack of fusion, total loss of ID or OD scarf, scarfing chatter, impede loss or inefficiency, lamination within the inspection zone, butt welds, hook cracks, and edge damage.
Defects that are likely to be missed include undercut or overcut on the scarf, absolute lack of penetration, certain pasty welds, weld embrittlement, signals below the trigger level, and defects caused by subsequent processing.
ISO 9000 dictates that test equipment be checked and calibrated on a regular basis by the equipment manufacturer or an appointed representative and that this inspection be certified.
The tube and pipe maker must also record and document all material being inspected. Information may be collected by a datalogger or through a personal computer.
A typical record of in-line eddy current testing shows relative positions and categorizes flaws with respect to the start of each coil and includes a summary of overall quality and potential yield. Software may be customized to meet individual requirements.
Digitally controlled eddy current equipment is now emerging with total interaction and control through an industrial PC. Control settings and test parameters are automatically downloaded upon entry of the material designation codes.
Eddy-current inspection has become the most widely used nondestructive test tool after pure visual inspection by the mill operator. It is estimated that more than 50 percent of tube and pipe mills are equipped with some form of eddy current tester.
Its primary role has become more than that of a production tool to give early warning to the operator of changes in weld condition that can hopefully be corrected before any significant scrap occurs. Payback can be dramatic through these efficiencies, as long as the operator is motivated to respond correctly.
Unfortunately, the system can only warn that something is changing or has changed. It cannot be specific or define the necessary corrective action.