Artem Komarov clarified that many manufacturers are familiar with the use of eddy current systems to inspect sheet and tubular products. Eddy current testing (ECT) is a non-destructive electromagnetic testing that offers a quick inspection to detect surface defects or cracks. It can be easily applied to straight sections of both seamless and welded pipes. Tests can be performed online as part of the manufacturing process, or offline as part of a supplier or purchaser quality check.
However, difficulties arise with pipes that are bent or welded to other pipes. Can eddy current testing be used to test more complex pipe shapes?
ECT uses electromagnetic induction, a process in which an alternating current is passed through a test coil, which creates a primary electromagnetic field. When an electrically conductive test object (any piece of metal) is placed in or near this primary field, another electrical current (eddy current) is induced in the test object. Eddy current measuring instruments analyze eddy currents that are affected by defects and cracks in the material.
Three main material properties can affect how eddy currents pass through the test object:
1. Conductivity or resistivity. These are the electrical properties of a material, determined by how well the induced electron flow passes through the material under test.
2. Permeability. This property determines how the magnetic energy changes as it tries to pass through the material under test. Non-ferromagnetic metals such as aluminium, titanium and copper are not permeable because these alloys do not contain iron, nickel or cobalt. Some alloys, such as 304L stainless steel are only slightly ferromagnetic. Although they contain iron, the manufacturing processes that created them alter their molecular structure to reduce their permeability.
Many of the non-austenitic steels have medium or high permeability. This dramatically changes the way a low-level AC-generated magnetic field passes through them, similar to the field around an eddy current test coil. Many of the non-austenitic steels have medium or high permeability. In these materials, the field of eddy currents penetrates insignificantly, therefore, in order to detect defects, it is necessary that they destroy the surface.
3. Geometry. The quality of any eddy current test is affected by how the available energy moves from the coil to the test piece. The more complex the geometry of the test piece (curves and bends), the more difficult it is to effectively couple the energy of the coil to the test piece.
Eddy current testing can very easily detect cracks at or near a surface. Because the material thickness, conductivity or permeability is greatly increased, eddy currents often cannot pass completely through the test piece, limiting the test to the outer surface in some cases.
Weld Inspection Process
What is a disadvantage? For most welds, there are acceptable levels for some types of discontinuities in the weld. They are usually classified as either internal inclusions (solid) or porous (gas bubbles). If a weld is cracked, it is almost always rejected and the part must be scrapped or repaired. Inspectors decide what types of defects will make a particular product unacceptable to either the manufacturer or the end user.
While straight pipes are quickly inspected using automated methods, complex geometries such as bends and welds may need to be inspected with manual probes. These transducers are available in a variety of configurations and all are designed to provide access to hard-to-reach areas, minimize pull-off, and provide maximum response for the type of defect expected.
One of the most common types of eddy current transducers is the pancake coil, used to find cracks in the surface of non-ferromagnetic materials such as aluminum or stainless steel. The use of flat coils around the weld zone is limited due to complex information near the weld zone.
Another option is a coil with a positive point or crossover point. The coil is actually a differential pair of coils that interrogate the same test area. In general, this sensor is insensitive to everything except cracks and other material defects, which allows you to suppress local changes in geometry (curved surfaces, corners, welding spatter) and changes in material composition (filler metal, heat affected zone). The coil is also relatively insensitive to changes in permeability, so it can be used with both ferromagnetic and non-ferromagnetic materials, said Artem Komarov.