Using Eddy current testing to identify casting defects.

With manufacturing standards stressing the need for castings of near perfect quality, nondestructive testing has become widely used throughout the foundry industry. A nondestructive test can be performed over arid over on the same part, or any number of parts, with no lasting effect on quality.

While the most popular methods of nondestructive testing have been X-ray inspection and ultrasonic testing, eddy current testing is an excellent inspection method to compliment existing testing technology. Using more than one nondestructive testing system is the best way to insure shipment of quality components.

For Waupaca Foundry's Marinette, Wisconsin plant, this fact came to life when they were having difficulty detecting carbidic ductile iron castings via ultrasonic testing. Typically, ultrasonic testing is used to measure nodularity in ductile iron. While decreasing nodularity in turn decreases the ultrasonic velocity, the presence of carbides will increase the ultrasonic velocity. With this being the case, it is possible to have a casting with low nodularity as well as carbides pass the ultrasonic test as a good casting. Waupaca needed to find the castings that were "white iron," or heavily carbidic.

The incorrect readings resulted in parts that had a high level of carbides passing through the ultrasonic test. Carbidic castings are very brittle, and can cause serious problems for the end-user. The quick solution to this problem was to add eddy current testing to its inspection line, complementing the existing ultrasonic testing.

By adding eddy current testing to its inspection line, Waupaca-Marinette was able to identify the carbidic castings and scrap them at the foundry.

How it Works

Eddy current testing is based on the principals of electromagnetic induction and is used to identify a wide variety of physical, structural and metallurgical conditions in electrically conductive metals.

It is used to detect seams, laps, cracks, voids and inclusions in ferrous and nonferrous castings. The inspection method also measures or identifies properties such as electrical conductivity, magnetic permeability, grain size, heat treatment condition, hardness and physical dimensions.

Eddy current testing begins by inducing a low-energy circular alternating electrical current through coil windings that are located in a probe or a coil housing into a casting, which in turn induces electrical currents in the pickup coil (Fig. 1).

The alternating current creates an expanding and collapsing magnetic field in a longitudinal direction across the coil windings. The magnetic lines of force created extend into the casting, which in turn induces the flow of eddy currents. The induced eddy currents generate their own magnetic field that interacts with the test coil magnetic field.

When a physical discontinuity (cracks, voids, seams) is present, it alters the interaction between the two magnetic fields. This altered interaction is displayed on the eddy current instrument display, identifying the casting defects. Those castings may then become scrap.

Eddy current testing is sensitive to many properties and characteristics inherent with a material, including alloy content, microstructure, heat treatment, part geometry, surface defects and the distance of the test probe or coil from the casting.

Basically, any discontinuity that appreciably alters the normal flow of eddy currents can be detected by eddy current inspection. These properties, which may not all affect the material properties or part function, may interfere with the inspection by causing instrument signals that mask critical variables or are mistakenly interpreted to be caused by critical variables.

In production of ductile iron castings it is necessary to suppress any variables not of interest and then test for the one variable that is of interest. In the case of Waupaca Foundry, the variable of interest was microstructure. Suppression of variables that are not of interest is achieved by producing consistent castings each production run. Casting chemistry, dimensions, surface finish and minimal casting defects are examples of variables that are needed to be kept as constant as possible for accurate and repeatable test results.

Advantages and Limitations

There are several advantages to eddy curent testting. The inspection method, which can be adapted readily to high-speed automatic scanning equipment, can measure for a number of different variables, provided the physical requirements of the material are compatible with the inspection method. The discontinuities in the castings give an instantaneous response on the monitoring equipment.

Unfortunately, the same aspects that define eddy current testing also offer some limitations. Eddy current testing should only be used to test one variable at a time, and because it is sensitive to so many different properties and characteristics, there is a need to suppress all variables that are not of interest.

Also, because the test is built around electromagnetic induction, it only can be used to test conductive alloys. Another disadvantage is that the eddy currents typically have a shallow depth of penetration, which can cause testing problems. Discontinuity detection is limited to surface and near-surface discontinuities.

Penetration Depth 

An important eddy current concept is standard depth of penetration. Standard depth of penetration is defined as the depth at which the eddy current is at 37% of its surface value. The currents start out at a certain surface value and diminish as they travel deeper into the part. The standard depth varies with testing variables as well as material constants. For constant materials, it varies only with testing frequency. By decreasing the testing frequency, the level of penetration is increased. At low frequency (100 Hz), the standard depth of penetration is approximately 9 mm for aluminum, 15 mm for ductile iron and 40 mm for steel.

Another convention of penetration is called effective depth of penetration, which is the depth at which the eddy current is 5% of the surface value, or basically the end of the material's effect on the test. It is also mathematically represented as being three times the standard depth of penetration. For Waupaca, a typical test frequency of 20 kHz achieves an effective depth at 1-3 mm, depending on the matrix structure of the ductile iron that is being tested, Waupaca experienced problems with pearlite skin condition, which aggravated testing results. Because the eddy current test has less penetration in pearlitic iron, the pearlite skin will not allow the eddy currents to penetrate very deeply and most of what it reads is pearlite. These conditions can result in a false "high" reading. While there is no clear solution for this problem, it should be taken into account and monitored.

Testing at Waupaca-Marinette

Implementing eddy current testing at Waupaca required four steps.

1. Creating Standards--Both good! pass and bad/fail standards were created to not only set up the machine, but to verify the test.

2. Correlation Study--After creating the standards, correlation studies were performed to check the measurement capability of the test. Waupaca performed a 50-piece study on castings with various microstructures (Fig.3). A voltmeter was plugged in to the machine, quantifying the values and correlating the microstructure.

After trying to graph pearlite vs. voltage and carbides vs. voltage with no luck, Waupaca combined pearlite and carbides to graph ferrite content of matrix vs. voltage and found a good correlation. The chart shows that in a region of 100%-80% ferrite, the data is linearly flat; from 80%-20% ferrite, the graph slopes steeply; and from 20%-0%, the data flattens out again. This is similar to a ductile/brittle transition curve.

3. Equipment Set-Up--After the machines proved capable in the correlation studies, it was necessary to properly set up the equipment. This included three important factors: consistent probe placement, instantaneous testing and a marking mechanism for scrap castings.

The eddy current testing was implemented in Waupaca's trim press operation, which allowed for consistent probe placement because the parts were fixtured. The eddy current test also is faster then the trim press operation, so it does not slow down the line. Paint markers were added to identify failed castings. Those that pass eddy current testing then must pass ultrasonic testing before gaining final approval.

4. Create Work Instructions--The final step of installation was the creation of work instructions, which includes setting up the machines, standard checks, verification, training procedures, maintenance procedures and obtaining standards.

Factors for Good Results

While using eddy current testing, several factors can influence the testing results. First, testing conditions such as test frequency and the location chosen on the casting to perform the eddy current test must be optimized.

Casting consistency, such as alloy content, dimensional accuracy, surface quality and defect control, also is important to eddy current testing. With so many variables that may effect the eddy current, these casting consistencies must be correct.

Final Results

Overall, eddy current testing has been an effective inspection method for Waupaca Foundry's Marinette plant. By using it to complement existing ultrasonic testing, all areas are covered in the inspection of Waupacas ductile iron castings.

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For More Information

Eddy Current Inspection, ASM Committee, Metals Handbook, 9th Edition, Vol. 17, p.164-194.

Nondestructive Methods for Testing Casting Quality, M. Jacobs, Engineered Casting Solutions, Summer 2002, p.84-85.

About the Authors

Dan Korpi, is the manager of quality and metallurgy at Waupaca Foundry in MarinetteX, Wisconsin. He received a bachelor of science degree in metallurgical engineering from Michigan Technological Univ.