In a business known for quality management, automotive component suppliers have few clear guidelines when it comes to nondestructive testing (NDT). While most Tier 1 suppliers – companies that provide parts or systems directly to OEMs – inspect heat-treated materials and finished components as part of their internal quality controls, specific requirements in supplier agreements can be vague, and there are virtually no industry standards when it comes to NDT.
“The contract may state that the supplier has to subject the component to nondestructive testing but not provide specifics about testing frequency or what techniques are acceptable,” said Bablu Ratnaparkhi, chief executive officer at Eddy Current Solutions in Sheffield, Ohio. “So you have big variations in knowledge and capabilities among suppliers. Some have sophisticated and well-established inspection systems while others you only hear from because they’re responding to a problem.”
Ratnaparkhi gets calls from both camps.
He spent more than 30 years in research and development at General Motors and Delphi, specializing in eddy current inspection systems for heat-treated materials and components. He wanted to retire but was coaxed into consulting after old colleagues continued to phone for advice.
“If you supply a defective part that goes into an axle or transmission and there is a recall, every other supplier associated with that axle or transmission is going to be affected,” Ratnaparkhi said. “Taking everything into account, the cost of that defective part can easily exceed the cost of an inspection system that would have caught it.”
NDT in Automotive Applications
NDT techniques range from simple manual and visual tests to sophisticated technologies that can detect surface and subsurface flaws, porosity, variations in metallurgical structure and other discontinuities without having to destroy, damage or waste assets.
At a Tier 1 supplier, nondestructive testing of steel happens at two points in the production process. The first is to verify whether the manufacturer has been supplied with the proper grade of steel, and the second is after heat treatment.
“Whether you’re using furnace heat treating or induction hardening on a part, you want to be certain that the microstructure is good and there are no cracks or other manufacturing defects,” Ratnaparkhi said. “By identifying variations due to case depth, hardness, incorrect heat treatment, coil locations and improper quenching, you determine which parts to scrap and work on processes that might prevent those defects in the future.”
Additionally, an NDT program can safeguard against rejecting good parts. So-called “false rejects” occur when the inspection process and quality standards are imprecise or inconsistent.
“Your inspection process can be very good at sorting defective parts,” Ratnaparkhi said, “But if you’re also rejecting parts that have a slight variation but are serviceable, you’re canceling out the benefits.”
Why Eddy Current?
Eddy current testing is a technique for detecting surface and subsurface defects and metallurgical qualities in conductive materials. It is used to inspect for surface cracks, hardness, inclusions, grinding burns, case depth and other material defects, as well as the quality of welds and seams.
A technician or machine places a probe or coil to a metal surface, and the probe generates an electromagnetic field that induces electrons to flow into it. Any discontinuities in the metal will distort the flow of electrons. This is captured and analyzed by an eddy current instrument, such as the Zetec InSite.
Eddy current array (ECA) takes eddy current a step further. ECA probes have multiple coils in one assembly. They are positioned at longitudinal, transverse or off-axis orientations and fire at coordinated times. ECA instruments and probes can capture more inspection data in a single pass. It is a speedier, more-thorough approach.
In high-volume automotive component manufacturing, eddy current inspection systems can be automated and incorporated into the production line, typically right after heat treatment.
A machine picks up the part, moves it into the test position, brings the coil down on the part or lifts the part onto the coil, and sends the signal to the instrument. There is no variation from one inspection to the next. Once the test is complete, there is a determination of “accept” or “reject.” Then the part goes down the appropriate chute.
Unlike other NDT techniques, in-line eddy current inspections are fast, clean and keep the line moving. They also have the benefit of consistency.
“You are ensuring every part is produced to the same standard, regardless of which machine did the heat treating or which operator is running the equipment or whether you’re in Ohio or someplace overseas,” Ratnaparkhi said. “Eddy current can improve manufacturing efficiency while reducing scrap.”
With in-line eddy current testing, probes can be customized to the shape of a component – imagine a probe that nestles into the teeth of a spline shaft, for example – allowing the eddy current coils to stay close and nominally perpendicular to the material.
For lower production volumes, the latest generation of handheld eddy current instruments – like Zetec’s MIZ-21C – have the processing power, software and battery life to perform inspections virtually anywhere, with C-Scan capability that provides real-time visual feedback to the technician and production crew.
Probes like Zetec’s Surf-X array probe with flexible surfaces allow the eddy current coils to stay close and nominally perpendicular to the material, even when that surface is rough or the geometry is complex.
“Portable eddy current instruments are easy for an operator to handle, and they generate a valuable digital record of inspection that can be stored, analyzed and compared against a history of results,” said Jesse Herrin, eddy current product manager at Zetec, a leading developer of nondestructive testing instruments, probes and software. “They’re ideal for low-production runs or whenever you need to stop and take a closer look at a part or piece without having to destroy it or take extra steps to prepare the surface for testing.”
There are other nondestructive techniques for testing heat-treated parts, but each involves a set of compromises.
Magnetic testing (MT) uses magnetic fields to locate surface and near-surface discontinuities in ferromagnetic material. Fine ferromagnetic particles are applied to the material and are drawn into any surface-breaking defects. The process is slow, disruptive to manufacturing, messy and depends on an individual operator’s ability to see and interpret indications of defects.
Ultrasonic testing (UT) uses high-frequency sound energy to indicate flaws both on and beneath the surface. Ultrasonic waves enter the material at precise intervals and at a set angle. When a wave encounters a defect, some of that energy is reflected back and generates an echo. The time it takes for that energy to reflect back to the probe is calculated and analyzed by the test instrument and presented instantaneously as a graphic on a screen. The inspector can validate completed welds and provide feedback to the crew as they go about their work.
UT is highly effective at detecting surface and subsurface flaws, but it cannot detect low hardness or poor microstructure as effectively as eddy current.
The Right Balance
With the right combination of instruments and probes, eddy current can detect a wide range of defects, which increases probability of detection and product quality.
Whether the inspection is incorporated into the production line or takes place at a bench top with a handheld instrument and probe, eddy current technology continues to evolve in ways that produce inspections that are quick to set up and complete. Given the full range of techniques available, eddy current can strike the right balance of inspection speed, detection capability and cost effectiveness – without having to destroy the piece you are testing.
For more information: Contact Zetec Inc. 8226 Bracken Pl. S.E., Suite 100, Snoqualmie, WA 98065 USA; tel: 425-974-2700; e-mail: firstname.lastname@example.org; web: www.Zetec.com.