July 17, 2009
Eddy current testing does more than detect product defects. When used with a proper monitoring system on a mill staffed by highly trained operators, it can help to optimize the mill's efficiency. The first part of this two-part article covers eddy current system principles.
Editor's Note: This article, the second of a two-part series, is adapted from Richard Fisher's paper presented at Metal Matters, March 14-16, 2007, Lake Buena Vista, Fla. © 2007 by the Fabricators & Manufacturers Association Intl. (FMA).
Understanding eddy current principles is just the first phase in implementing an eddy current system. The next steps involve selecting the equipment, determining the test location, learning how to read the system's output, and using it to maximize the mill's efficiency.
A sector coil is a good choice for inspecting welded tube or pipe. Its half-moon shape concentrates the eddy current field in the weld zone, improving sensitivity, especially on larger diameters (see Figure 1). Also, a single coil can work on several tube or pipe sizes, as long as the weld is held at the top dead center position.
The drawback is that sector coils or probes focus on the weld and do not inspect the body of the tube, which is a requirement for many grades of American Petroleum Institute-certified tube and pipe. They also are ineffective if severe twist rotates the weld out of the inspection zone.
On the other hand, encirclingcoils are not affected by twist and cover the entire tube circumference. They also yield good sensitivity to weld defects when applied to diameters less than 2 in. The main reasons encircling coils are not used more often on weld mills are their initial cost and their susceptibility to damage.
To ensure adequate sensitivity, encircling coils must be matched carefully to each tube size. Proper inspection relies on a suitable fill factor, which is the ratio of the workpiece diameter to the coil diameter. Several sets of coils and guide bushings are required to cover the entire production size range. The fillfactor results in a tight clearance between the workpiece and the coil, which subjects the coil to damage from weld splits or bad end welds. Tube pileups at the coil can become a common, and expensive, occurrence.
As a rule, online inspection has several advantages over offline testing:
Although offline testing can create a bottleneck in the production process, it may be the only option for producing seamless tube or if no space is available on the mill for the required equipment. In the case of seamless tube, the system may be inline, but will still require a drive mechanism to propel the tubes through the inspection coil and a subsequent sorting mechanism to segregate rejected tubes.
Another drawback involves the tube or pipe ends. The leading and trailing ends generate an enormous signal at the test coil. These end effect signals normally are suppressed by a system that involves proximity sensors and timing schemes to inhibit testing during these short intervals. The net result, however, is that several inches near the end of each piece remain untested.
Four specific areas on a typical tube mill are common inspection head locations (see Figure 2). Each of these locations has its advantages and disadvantages.
1. After the Welder. Positioning the sensor immediately after the welder assists with early defect detection, which can be quite useful. For example, weld splits can be especially problematic in stainless steel tube or pipe mills that inject hydrogen gas up the bore for inline bright annealing. A weld split may allow the gas to escape, resulting in a hydrogen explosion when the tube enters the annealing furnace. An eddy current probe can immediately detect the split and automatically shut down the mill before the split reaches the annealer.
Probes also may be installed on induction weld lines. However, the potential for damage is a major concern. Coils or probes at this location must be adequately cooled by water or air; also, loose scarf can destroy the probe.
2. In the Cooling Trough. Many sector coils are waterproof, and the eddy current's magnetic field usually is unaffected by water or mill coolant.
On smaller mills, the cooling trough often is the only location available for the inspection head. However, the tube or pipe is usually unsupported at this point, and care must be taken to ensure that its centerline is held steady. Support rolls may be required to reduce tube movement and shock vibration, which often occur when the downstream cutoff activates.
3. After the Cooling Trough. This is perhaps the most common location for test rigs. This area usually has adequate space for the inspection head assembly; the tube has good support from the sizing rolls on the downstream side; the weld is typically still at top dead center; and the tube has been adequately cooled. An additional support roll may be needed at the output of the cooling trough for stability, but usually few (if any) additional modifications are necessary.
4. In the Sizing Section. The sizing process places additional stress on the tube or pipe, which tends to open up certain types of defects, such as cold welds, that may be difficult or impossible to detect further upstream. The sizing rolls also provide excellent stability of the tube passline and additional protection for encircling coils. This location commonly is used for full-body inspection on mills that use gas tungsten arc welding (GTAW).
A common difficulty in testing carbon steel tube or pipe is that the mill usually doesn't have enough space between the roll stands to install the magnetizing assembly. Such an assembly is usually much larger than the simple coil holders designed for nonferrous materials.
Before an eddy current tester is put into use, the system must be calibrated to a baseline, or reference standard. This is a matter of putting a known flaw in a length of otherwise good material and running it through the coil. Although filed or machined notches may be used as reference flaws, the easiest method is to drill a hole through the weld. The most common reference standard for tubular sections is a 0.031-in. (0.8-mm) drilled hole, which satisfies the requirements of ASTM A-450 (ferrous) and E-426 (nonferrous).
After the eddy current system is up and running and the operators have been trained, the system can start detecting flaws and providing feedback for optimizing the mill's efficiency and troubleshooting production problems.
Optimizing Welding. Cold welds in carbon steel are difficult to detect because of their continuous and uniform nature. Many are caused by too much heat rather than too little. Excess heat and pressure squeeze out most of the molten material, leaving behind two cold edges that are joined weakly.
Excess heat causes the internal bead to thicken and become globular. The globules create eddy current disturbances, which in turn cause a significant increase in the background signal, or noise. The rise in noise is visible on the display and can be used to trigger a warning device. The operator can use visual cues from the display to reduce the heat to the optimal level—hot enough for a good weld but not so hot as to create a boiling condition, which can cause cold welds and porosity.
GTAW mill operators can also take advantage of background noise to minimize gas usage. Many GTAW lines inject helium up the bore to minimize spatter on the ID. The operator reduces the flow rate until the spatter causes the noise to increase, then increases the helium flow gradually until the signal returns to normal. Some tube and pipe producers have used this technique to reduce helium consumption up to 30 percent, reducing gas costs considerably.
Spotting Mill Setup and Strip Problems. Many mill problems can be diagnosed quickly by merely observing the display. For example, a broken fin roll nicks the strip once each revolution and often produces pinholes. These dings show up as a regular, continuously recurring disturbance in the signal.
Other similar cyclical signals include pickup on a roller, a common occurrence on aluminum mills, or strip edge damage caused by improperly shipping or handling of the coil. Observing the distance between signals makes it easy to deduce the circumference related to the flaw, which points to either a damaged mill roll or the much larger strip roll.
Some signals are not cyclic. Irregular signals require further investigation to determine their source. These include total loss of ID or OD scarf, impeder failure, and scarfing chatter.
Any eddy current system can indicate the presence of a problem of some sort; it cannot tell exactly what the problem is. Operator training and involvement are critical in initiating corrective actions to complete the process control loop.
An eddy current testing system can detect many defects, but it has some limitations. As mentioned previously, cold welds, open seams, and other continuously uniform faults are difficult to detect by eddy current methods. In addition, a product that has an irregular surface condition or composition usually produces a high background noise level.
Hot-rolled black steel and some galvanized, or zinc-coated, material that has been remetalized may present problems. Their normally high background noise levels tend to mask smaller defects. In such a case, reducing the system sensitivity reduces false signals. The downside is that reducing the sensitivity reduces valid signals too. Nevertheless, even with reduced gain, an eddy current inspection system generally reveals about 85 percent of the most common defects.
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