Inspecting welds on complex tube forms

Many manufacturers are familiar with using eddy current systems to inspect their sheet and tubular products. Eddy current testing (ECT) is a nondestructive electromagnetic test that offers a rapid examination to detect surface-breaking flaws or cracks. It can be applied easily to straight sections of both seamless and welded tubing. Tests can be performed online as part of the manufacturing process or offline as part of a supplier or customer quality assurance check.

However, difficulties arise with tubes that are bent or that are welded to other tubes. Can eddy current testing be used to inspect more complex tube forms?

Eddy Current Principles

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 measurement instruments analyze eddy currents, which are affected by flaws and cracks in the material.

Three fundamental material properties can affect the way that the eddy currents travel through the test object:

1. Conductivity or resistivity - These are the material's electrical properties, defined by how well the induced electron flow moves through the test material.

2. Permeability - This property defines how magnetic energy is altered as it tries to move through a test material. Nonferromagnetic metal, such as aluminum, titanium, and copper, do not have any permeability because these alloys do not contain iron, nickel, or cobalt. Some alloys, such as 304L stainless steel and INCONEL® and MONEL® alloys, are only slightly ferromagnetic. While they contain iron, the manufacturing processes that created them alters their molecular structures to lessen their permeability.

Many of the nonaustenitic steels have medium to high permeability ratings. This dramatically changes the way that a low-level AC-generated magnetic field, like that found around an eddy current test coil, can move through them. Many of the nonaustenitic steels have medium to high permeability ratings. In these materials, there is little penetration of the eddy current field, so flaws must be surface-breaking to be detected.

3. Geometry - The quality of any eddy current inspection is affected by how the available energy moves out of the coil and into the test specimen. The more complex the geometry of the test specimen (curves and bends), the more difficult it is to link the coil's energy effectively to the test specimen.

Eddy current testing can very easily detect cracks at or near the surface. As material thickness, conductivity, or permeability increases significantly, eddy currents often cannot move completely through the test specimen, limiting testing to the outer surface in some cases.

Weld Inspection Process

What is a flaw? In most weldments there are acceptable levels for some types of discontinuities in the weld. These normally are classified as either internal inclusions (solid) or porosity (gas bubbles). If a weld has cracks, it almost always is rejected, and the part must be scrapped or repaired. Inspectors decide which types of discontinuities will make a particular product unacceptable to either the manufacturer or the end user.

While straight tubing is quickly inspected by automated methods, complex geometries, such as bends and welds, may require inspection by hand-held probes. These probes are available in a variety of configurations, all intended to give access to tight places, keep lift-off to a minimum, and ensure the maximum response given the expected type of flaw.

Figure 1a and 1b

One of the more common types of eddy current probes is the pancake coil, used to find cracks on the surface of nonferromagnetic materials such as aluminum or stainless steel (see Figure 1a). The use of pancake coils around a weld zone is limited because of the complex information near the weld zone.

Another option is the plus-point, or cross-point, coil. The coil actually is a differential pair of coils that interrogate the same test area. In general, this probe is insensitive to everything except cracks and other material discontinuities, which allows suppression of localized geometry variations (curved surfaces, corners, weld splatter) and material composition variations (filler metals, heat-affected zone) (see Figure 1b). The coil also is relatively insensitive to permeability changes, so it can be used on both ferromagnetic and nonferromagnetic materials.

This coil most often is used in conjunction with lightweight, portable eddy current test systems. Portable testers can be set up fairly easily with a minimum of training. Since the plus-point coil suppresses unwanted noise while responding to cracks, test result interpretation is fairly straightforward.

Dan DeVries is vice president of the manufacturing market segment and Jim Cox is training manager for Zetec Inc., 1370 N.W. Mall St., P.O. Box 140, Issaquah, WA 98027-0140, 425-392-5316, ddevries@zetec.com, www.zetec.com. Zetec Inc. provides complete integrated support for every aspect of eddy current testing used in manufacturing environments, such as inline hardness testing and flaw detection.

MONEL and INCONEL are registered trademarks of the Special Metals group of companies.