August 16, 2001
Eddy current testing offers several features that makers of welded tube may find to their liking—in particular, high throughput speeds and sensitive flaw detection.
To use this method, a manufacturer must have, as a minimum, eddy current test electronics, test coils and cabling, and some form of output device such as a strip chart recorder, paint marker, or alarm. More advanced systems include material handling and provide sophisticated flaw evaluation, network interfacing, and data archiving.
Monitor Production. Tube makers basically have two reasons to conduct eddy current tests of their tubing. The first is monitoring the production line for mistakes in methodology. A properly implemented eddy current test can aid in process control and usually results in reduced scrap costs. By correcting mistakes early in the manufacturing process, a shop can minimize waste and maximize profits.
Eddy current testing can be conducted at locations very near the welding point, thereby providing almost instantaneous information on weld quality. If the mill performs a bad weld, the welding process can be adjusted quickly, before large amounts of scrap are produced.
Many different parameters influence the quality of a welded tube. Some of these are related to the quality of the strip that makes the tube. Strip thickness and width variations, edge defects, and surface conditions all can affect the final quality of the finished tube. Other variables introduced by the process itself determine the final tube quality. If not properly controlled, processing parameters such as the welding current and contact pressure result in such weld defects as porosity, lack of fusion, or weak welds. Poor weld mill rolls can create periodic defects in the tube that are unacceptable to the end user.
Many factors affect the final quality of tubing, and they often occur randomly. They have a complex effect on quality, making it extremely difficult — if not impossible — for weld line operators to detect the resulting defects. Online eddy current testing is an effective tool for detecting these defects.
Meet Customer Expectations. A second reason for testing is to fulfill industrial and customer test specifications and standards. Many such specifications exist that detail testing procedures and define flaw rejection criteria for nearly all types of tubing. Acceptable quality levels can be very stringent, thus necessitating test equipment that is rugged, reliable, and highly sensitive. An eddy current testing capability will allow a tube producer to compete in markets that demand higher-quality tubing.
At the heart of any eddy current test system is the test coil or sensor. This coil consists of field and measurement windings that are used to produce an alternating magnetic field, create eddy currents in the tube, and sense changes in those eddy currents that are caused by localized defects in the tube.
The field winding of a test coil is energized by an alternating current. When this current flows, an alternating magnetic field (the primary magnetic field) surrounds the field winding. As an electrically conductive tube moves through or near the test coil, the primary magnetic field from the field winding penetrates the tube. As a result, eddy currents flow in a circumferential direction within the tube. This is known as electromagnetic induction. The eddy currents induced to flow in the tube create their own magnetic field (known as the secondary magnetic field), which flows in the opposite direction to the generating primary magnetic field. It is the interaction between the primary and secondary magnetic fields that provides valuable information on the condition of the tube.
Defects in the tube wall will cause a disruption of the eddy current flow because the eddy currents must flow around the defect. This disruption of the eddy current flow causes variations of the secondary magnetic field. Measurement windings in the test coil sense the changes of the secondary magnetic field as the tube passes through the coil system. The electrical phase and amplitude of these voltage variations are evaluated and provide an indication of the size and type of a defect.
A differential measurement winding provides a comparison of neighboring sections of the tube as it passes through the test coil and is essential for detecting small defects such as holes or cracks. An absolute measurement winding provides real-time indications of open welds or disrupted weld conditions. Test coils containing both differential and absolute windings are available to provide a simultaneous differential and absolute test of the tubing.
In the test object, the primary magnetic field created by the test coil is superimposed on the secondary magnetic field that is produced by the eddy currents to form the resulting field. The resulting field in a tested tube is determined by the following variables:
Encircling Coils. Encircling coils are used to test the full circumference of round or shaped tubing. They usually are not installed directly into the welding line because of space limitations and because the tube must be threaded through an encircling coil during start-up.
Weld line size changes can be difficult because they require that the tube be cut so that the encircling coil can be removed. However, encircling coil tests often are performed as a final inspection method using offline mechanical handling systems, especially because the entire tube circumference is tested during the process. Therefore, the encircling eddy current test sometimes is used as an alternative to leak testing. Encircling test coils work well for smaller-diameter tubing, but sensitivity decreases as tube diameter increases. Testing with this type of coil generally becomes unreliable when tube diameters are larger than 170 millimeters.
Segment Test Coils. Used for in-line testing of the weld seam zone without threading the product through encircling guides, segment test coils can be installed in tight spaces and are easily lifted off the welding line for quick size changes or offline calibration checks.
Segment coil transmitters usually are installed directly into a tube manufacturing line to provide rapid feedback of weld quality at all welding speeds. They are especially helpful if the weld migrates out of range at the test point because they scan a defined section of the tube perimeter.
The total number of segment coils necessary for a given product diameter range depends on the degree to which the weld zone migrates at the test point. The segment coil size must match the tube diameter, but it is possible to cover a larger diameter range without changing the segment coil size. Migration of the weld seam position from the 12 o'clock position up to 45 degrees in each direction is tolerable in most cases. Segment coils cover a certain angular range of the tube circumference, typically the weld seam area. They are held at a constant height above the tube by height-adjustable mechanics and can be used with permanent or electromagnetic DC magnetizing holders.
Weld Seam Probes. A simple, low-cost tube testing method for all tube diameters, weld seam probes are used to monitor the welding process for early detection of common process failures. They are highly sensitive to weld seam defects such as inclusions, cold shuts, edge defects, and poor scarfing that result from welding-related process irregularities.
Water-cooled probes can test tubing just after it exits the weld box and can detect problems before they flow downstream in the production process. Weld seam probes are the simplest testing instruments and take the least amount of space — just one probe is needed for all tube dimensions. The probes can be lifted off easily when starting the line, thereby minimizing risk of damage to the tube. They are suitable for ferritic, austenitic, and nonferrous materials.
Special precautions must be taken when testing tubes that are made from magnetic grades of steel. The magnetic permeability of these materials usually has an adverse effect on the signal-to-noise ratio of the eddy current test.
The signal-to-noise ratio can be improved with additional DC field magnetization using segmented, all-round permanent, or electromagnetic magnets that surround the test coil. Special DC magnetization test coil holders are available to magnetize the tube at the point of test. Such DC field magnetization simultaneously reduces the tube permeability and increases the penetration depth of the eddy currents.
When testing austenitic tubes, permeability variations also may occur. These variations can be caused by pressure from the weld line rolls when shaping the tubing, by delta ferrite concentration in the weld zone, or by the presence of strain-induced martensite. DC magnetization during eddy current testing helps to eliminate the permeability variations and thereby improve the quality of the test. Depending on the intended end purpose of the tubes, they may have to be demagnetized after testing to reduce the residual DC field in the tube to an acceptable level. Special demagnetization equipment is available for this.
The eddy current method for testing can be conducted at high throughput speeds without actual physical contact with the material. High testing speeds help to ensure that testing always keeps up with production. Eddy current tests are reliable even at extreme temperatures. It is a clean method that does not contaminate tube products with other media such as oil or water. Eddy current testing systems are easily automated and can be readily integrated into your existing production processes.