Eddy Current Heat Treat Verification: Case Study
Modern eddy current test systems overcome the limitations of destructive testing and provide the ability to test 100% of production parts in-line for proper heat-treat process. These systems are fast, environmentally friendly, meet lean manufacturing requirements and integrate well with induction heating processes.
Metallurgists and heat-treat professionals have traditionally used a small number of parts for destructive testing to verify proper heat-treat processes. In today’s competitive manufacturing environment, sample destructive testing alone is not sufficient to ensure zero-defect policies.
Eddy Current Heat-Treat Verification
To Rockwell hardness test every bearing made at KBI would be cost prohibitive. Also, with mechanical testing it is technically impossible to verify the case depth and placement without destroying each part. Modern eddy current test systems overcome these limitations and provide the ability to test 100% of production parts in-line for proper heat-treat process. These systems are fast, environmentally friendly, meet lean manufacturing requirements and integrate well with induction heating processes.
In eddy current testing, an electromagnetic field is induced into a metallic object by way of an electrical coil. The eddy currents generated will differ depending on the microstructure of the metal. The eddy current test system compares a “known-good” test condition to each part under test and then rejects parts that are outside established tolerances. Multiple coils can be integrated into a single eddy current probe, allowing multiple areas to be tested simultaneously. Figure 1 shows a cut-away of a KBI spindle identifying the different areas under test.
Eddy Current Probe Design
A properly designed eddy current probe must be capable of detecting all metallurgical defects while not rejecting “good” parts that have acceptable manufacturing geometric variations. To create the proper eddy current test criteria, KBI utilizes parts made with known metallurgical defects such as misplaced case, shallow case, short quench and ground-out condition. One hundred good parts with normal variation in geometry are also created with minimum and maximum acceptable heat-treat tolerances. The signatures of these test parts are stored in the eddy current instrument and establish the acceptance-limit criteria. This allows the eddy current system to pass parts with acceptable tolerances and only reject out-of-tolerance components. Because of this advanced testing setup, KBI has a low or negligible false reject rate with a very high uptime.
KBI attributes much of their success with eddy current testing to the 15-year relationship they have had with Criterion NDT probe design and application engineering personnel. Criterion NDT probe designers worked closely with KBI to take into account the areas to be tested, the different conditions to be tested for and how the test will be run on the production line. Figure 2 shows a Criterion NDT production spindle probe. The coil seen on the bottom of the probe is used to verify the proper location of the flange bearing-race induction hardening pattern.
The spindle-probe in Figure 2 has a 300 series stainless steel body with nylon housing components to precisely locate the coils and prevent premature wear. This ensures that the probe is robust enough to work in a factory environment 24/7 while maintaining maximum sensitivity. Small metal parts like ball and roller bearings, which have a uniform heat-treat application, can be dynamically passed through an encircling-type coil and a sorting chute, allowing multiple parts per second to be inspected.
Eddy Current Instrument Design
Modern eddy current instruments, such as the Criterion/Zetec InSite, are capable of simultaneously testing up to eight coils at eight different frequencies for 64 channels of data. Microprocessor-controlled architectures have Ethernet connectivity and support industrial I/O, which interfaces to production material-handling systems. The InSite has many unique features, including Frequency Plot and Drive Voltage Plot, that greatly simplify new test development. It also has the ability to adjust the alarm gate by shape as well as size, which greatly reduces the false reject rate. The InSite eddy current instrument platform is capable of both heat-treat process testing and crack/flaw detection, which allows KBI to reduce spare parts’ inventory and training time.
Eddy Current System Design
KBI uses induction heat treating to properly harden their bearings because superior mechanical properties are easily achieved by allowing surface areas to have a hard case and soft inner core. The eddy current test systems are installed directly in the manufacturing production line downstream of the induction heating stations, allowing 100% of the parts to be tested at production-line rates. Figure 3 shows an eddy current testing system using a Zetec eddy current tester installed in a KBI production line.
Eddy current testing of the spindle is run as a static test. The material-handling station positions the spindle under the eddy current probe, which is then lowered onto the spindle (Fig. 4). A PLC on the material-handling station signals the instrument to start the test once the probe is in place. If an out-of-tolerance condition occurs, the Zetec eddy current instrument automatically identifies the failure and signals the material-handling system to reject the part.
Heat-Treat Process and Eddy Current System Verification
To validate the eddy current process, one part per production line is cut and nital etched every eight hours. The case depth is checked with a 6-inch ruler, and the surface hardness is tested with a Rockwell hardness tester. This destructive test takes about half an hour. Once per shift, the gauge QC inspector will verify the eddy current system by “nulling” with a known-good eddy current master and testing the system with six reject test parts with various out-of-tolerance conditions. Once per week, one part per line is sent to the metallurgy lab to perform a microstructure test verifying the heat-treat processes.
Many companies have been hesitant to replace traditional destructive-test methodologies with automated eddy current testing in their manufacturing process. For KBI, the partnership with Criterion NDT and the investment made in eddy current technology has paid off in warranty, testing and scrap cost savings. It has also enhanced the quality reputation of KBI as a world-class automotive wheel-bearing supplier. IH
For more information: Bablu Ratnaparkhi is a Sr. Metallurgist at KBI Bearings and can be contacted at email@example.com. For more information on KBI, visit their website at www.kbibearing.com/website. Joe Jessop is the president of Criterion NDT and can be contacted at firstname.lastname@example.org. For more information on Criterion NDT visit their website at www.criterionndt.com. Dan DeVries is a senior marketing consultant with Wild Horse Strategies and can be contacted at email@example.com. For more information on Wild Horse Strategies visit www.wildhorsestrategies.com.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media search at www.industrialheating.com: eddy current, electromagnetic field, crack detection, induction heating, Rockwell hardness, microstructure
SIDEBAR: Eddy Current Verification of Heat-Treat Processes: Pros and Cons
- Allows 100% testing of components – Modern eddy current instruments have industrial I/O and are made to work with material-handling stations. Because it is a clean non-destructive test, all tested parts can be used.
- Fast – Testing a component like a spindle takes between one and six seconds including material-handling time. Small parts can be tested at 100 parts per minute or more.
- Test multiple areas on a single part – Modern eddy current instruments can test up to eight areas simultaneously.
- Test for multiple anomalies – Because eddy current tests at the microstructure level, it identifies parts that have material structure defects as well as heat-treat process defects. A properly designed eddy current test can identify variations including missing heat treat, shallow case, delayed quench, short quench, air cooled and misplaced case.
- Reduce scrap costs – By having a testing system in-line, process issues can be immediately identified and addressed.
- Reduce warranty costs – With 100% testing, you only ship good components. An automated eddy current system removes the subjectivity of human error.
- Eddy current is a test relative to a known-good condition. Periodic mechanical sample testing still needs to be done but at a greatly reduced sampling rate.
- It cannot give you an absolute reading like a Rockwell hardness test.
- A set of known-good and bad “master samples” must be made.
- A gauge or quality technician should “null” the instrument on a good part once per shift. This is superior to an “auto-null” procedure.
- A high initial purchase price of the eddy current instrument, probe and material-handling station. This is quickly offset by the savings in warranty, scrap and destructive-testing costs.