October 11, 2010
Tube and pipe producers and fabricators who are familiar with the conventional encircling coils used in eddy current testing might be interested in learning more about this technology. Eddy current can be used on a variety of product shapes and does more than detect cracks. It can detect relative hardness and hardness changes (good for verifying heat treatment) and the presence or absence of physical features such as splines and threads.
Eddy current system users tend to fall into two categories. The first category consists of proactive companies that know about eddy current and invest in a system when installing a new manufacturing line. The advantage is that eddy current testing becomes an integral part of the manufacturing system. The second category is reactive. In many cases, they have failed a major quality audit and need to fix a problem immediately. The panicked manufacturer usually sends parts overnight to an eddy current systems manufacturer with a note that says, “Help!”
Installing an eddy current system when setting up a production line allows all parts to be tested immediately following critical manufacturing processes. For example, for a tubing manufacturer, this means that material can be checked for cracks immediately after the drawing process. This helps in two ways. First, it ensures that if a crack forms, the system detects it before any additional value is added to the material; second, it allows equipment operators to identify the faulty process quickly, which leads to troubleshooting, a remedy, and getting back to producing good parts.
Hypodermic needle manufacturing is a typical use. The eddy current process checks the tubing for cracks before it is cut to length and sharpened. The detection criteria for this application are through-wall cracks down to 0.100 inch (2.5 mm) by 0.004 in. (0.1 mm) in needles as small as 0.011 in. (0.28 mm) in diameter. The eddy current probe used in this application is shown in Figure 1. It uses ceramic inserts to reduce the wear on the coil and small magnets to reduce electromagnetic noise from residual magnetism in the material. A typical flaw response from an eddy current system is shown in Figure 2.
What if you fabricate tube into components and do myriad other manufacturing processes? While eddy current is well-suited to tube and pipe, which pass through an encircling coil during the manufacturing process, eddy current lends itself to other manufacturing processes also.
It can be used for testing components such as wheel bearings and pistons. For crack testing applications, an eddy current coil must pass over the material to be tested. A material handling station rotates the wheel spindle shown in Figure 3 while the robot arm holds a housing with eight eddy current coils near the part. The white ceramic wheels keep the replaceable nylon modules, which encapsulate the coils, a small distance away from the test surface to prevent premature wear.
Eddy current coils can be manufactured to inspect complex shapes. The eddy current probe shown in Figure 4 was designed to inspect the inside bowl of an aluminum piston cylinder head. The four spring-loaded probes modules inspect the upper combustion bowl radius of the piston.
In addition to crack detection, eddy current can be used to detect differences in the structure of a material. This makes the process well-suited to check components for proper heat treatment. Unlike crack testing with eddy current, the heat-treat verification tests on large components are normally run as static tests, in which the part is stationary. The entire component, or just a portion of it, is held briefly in the eddy current field. A test on small components, such as bearings, can be run as a dynamic test, in which the part moves when tested.
The reading is taken and compared to a stored set of values or a known good part. Modern eddy current instruments are capable of evaluating the data for several heat-treatment anomalies, such as no heat treat, short heat, shallow case, and delayed quench. By simultaneously testing at several frequencies, the system can separate bad parts from good. This allows a 100 percent test—it evaluates all of the parts. While not an absolute hardness test like a Rockwell tester, eddy current testing can achieve sorting results on par with Rockwell testing.
Figure 5 shows a set of three hardness coils in a fixture used for testing various lengths and diameters of water pump shafts. These three encircling-style coils are connected to a single eddy current instrument, which monitors all three coils simultaneously. The center tube is made from thin-walled titanium, which acts as a guide for the parts and prevents wear on the eddy current coils. Titanium was selected for its durability as well as its low conductivity so it would not interfere with the eddy current test.
The shafts are metered into the top of the coils and momentarily stopped when the heat-treat test is run. The half-round, black nylon piece on the side of the fixture establishes coil spacing for each of the different pump shaft styles.
Eddy current testing also can check for proper formation of external features such as threads or splines. One simple fixture shown in Figure 6 is a test station probe which verifies that all the ball bearings are installed in this particular bearing race. The copper eddy current coil is visible just outside of the bearing race. This particular fixture is used as a manually run, go/no-go checkpoint, just before final assembly.
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