Heat treatment is an invaluable process that allows manufacturers to optimize the mechanical and physical properties of their metallic components. This helps provide a desired level of performance and life expectancy of a product. The end results are tailored by parameters such as heating method, temperatures, cycle times, atmospheres, quench media and tempering.
Heat-treat results can vary due to variations or mistakes in manufacturing processes. Variations can occur from differing temperature zones in an oven, condensation dripping onto parts, bent induction coils and tooling misalignment.
Failure to quickly identify components with improper heat treatment can lead to an increase in both scrap and warranty costs. Here are five recent recalls related to heat-treatment anomalies.
- In September 2017, there was a recall of Hyundai Santa Fe vehicles due to crankshaft assemblies produced with insufficient heat treatment because of an improperly positioned heat-treatment coil.
- In September 2017, SIG SAUER found that a limited number of rifles were built that may have had an improperly heat-treated hammer that could cause a significant safety hazard.
- In June 2016, Meritor issued a recall for a small number of non-drive front steer axles because they may have not received the correct heat treatment. Improperly heat-treated axles could result in fracture and loss of vehicle control.
- In 2015-2016, Dodge found Ram 1500 pickups built within a three-month period with parts of the rear axle shaft that may have been improperly heat treated, which could cause the axle to overheat, wear and fracture.
- In 2014, GM announced a recall of certain model-year pickup trucks due to an improper heat treatment. This could cause the rear axle to fracture while the vehicle is being driven.
Methods to Ensure Heat-Treatment Quality
There are three traditional ways to check heat-treatment quality:
- Monitor heat-treatment manufacturing processes
- Sample (batch) testing
- Continuous (in-line) testing
Monitoring heat-treatment manufacturing involves monitoring the performance of heating and cooling processes. Variances in these processes can signal a potential heat-treatment process failure.
Sample (batch) testing can be accomplished using various methods. Impact (hardness) testing is most commonly used, as well as tensile testing for some critical parts. Rockwell or Brinell hardness testing and Knoop/Vickers microhardness testing are traditional impact-test standards.
In sample testing, microstructural measurements are often needed to obtain qualitative results. Microstructure analysis can be subject to operator interpretation, and image analyzers are often used to improve result consistency.
Microstructure analysis usually involves cross-sectioning a component using a water-cooled metallurgical cut-off saw or water-jet cut-off. Local grinding burns must be minimized during the cutting process.
The cut area is then etched to visually enhance the case pattern in order to obtain a depth measurement. For case-depth measurements, one commonly used etchant is Nital – a weak solution of nitric acid and alcohol. Figure 1 shows a cut and etched wheel bearing clearly displaying the heat-treatment patterns.
Continuous (in-line) testing involves checking components while on the production line. Integral testing stations are installed downstream of heat-treat and quench processes. Testing systems are integrated with sorting mechanisms to automatically reject out-of-tolerance components. Eddy current technology is one of the primary methods used for continuous in-line heat-treatment validation.
How Eddy Current Heat-Treatment Testing Works
Eddy current testing is based upon electromagnetic induction, which was discovered by Michael Faraday in 1831. It is a standardized testing technique that is widely used in aircraft and nuclear power-plant testing. It has also been used for the last several decades in the automotive, industrial and medical markets.
Eddy currents are used to detect variations in the structure of metallic components. These structural variations can be caused by differing material alloys, component geometry or because components received differing heat treatment.
Unlike the testing processes mentioned in the previous section, eddy current testing is a comparative test, not an absolute test. It doesn’t yield a numeric hardness or case-depth value. It merely indicates that a component under test is different from a group of parts with known good parameters. For in-line production testing, this is adequate to identify components that fail defined quality standards.
Eddy current testing offers the advantage of testing every single part while on the production line. With batch testing, if a problem is discovered the entire manufacturing run or “lot” is then suspect and must be tested, reworked or scrapped.
Eddy Current Production Testing Systems
A typical eddy current production testing system consists of an eddy current instrument, test coils or probes, mechanical fixturing and a material-handling system. A typical eddy current instrument contains eddy current coil drivers, digital signal processing circuitry and computer processing to rapidly identify when material structure differences have been detected. The instruments also have communication interfaces to PLCs, which activate material-handling system sorters to remove noncompliant products.
Newer eddy current systems have easy-to-use touch-screen user interfaces (Fig. 2). Automated setups using multiple frequency selections help to ensure that multiple variations can be identified. Older eddy current systems were often difficult to set up and manage, which resulted in manufacturers not using systems to their full potentials.
Eddy current coils consist of copper magnet wire that is wound into a desired shape. This coil is energized by the eddy current instrument at multiple frequencies and generates eddy current flow within the component under test. Figure 3 depicts a gear passing through an eddy current coil.
Often, a second eddy current coil housing with a known “good” component is also connected to the eddy current instrument. This reference part helps to increase test sensitivity of the eddy current system.
Eddy current probes can be configured with one or more windings that surround the part under test. Some smaller parts can be quickly passed through a coil for high-speed sorting. Complex components can be evaluated with a multi-coil probe (Fig. 4), providing results within fractions of a second.
Fixturing and Material Handling
Eddy current coils must also consistently align with every component under test. Stainless steel guides and rings are often used to provide positive locating on the part as well as prevent damage to the coils within the housing.
For small, simple components, a sorting mechanism (Fig. 5) is often an adequate solution. For larger or more complex parts, a dedicated material-handling system or robotic system is often implemented.
Figure 6 shows a SCARA-type robot made by Epson and a demonstration test station that has been set up for testing differential cam/side gears. The robot is programmed to pick up a part from the tray on the left and place it into the eddy current coil closest to the robot. The second eddy current coil holds a “reference master” that is used as part of the differential comparison test.
Valve Case Study
An engine valve manufacturer’s process involved hardening the tips of the valve to ensure durability over the life of the automotive engine. Before using eddy current, valve-tip hardness testing was performed using statistical batch-testing methods. This involved gathering samples of the recently hardened parts and destructively examining them. Test methods included Rockwell testing as well as visual inspection (cut, polish and etch).
Several months after installing an eddy current test system, 300 valves in a row were suddenly rejected. The production line was stopped, and the valve tips were examined with a Rockwell test and found to be “soft.” Upon cutting, polishing and chemically etching, the parts visually showed an incorrectly located heat treatment. The cause was found to be a mechanical misalignment on the induction hardening machine. A guide rail had loosened, resulting in a heat-treatment zone intended for the upper valve shaft and tip to be located further down the shaft than desired.
Eddy current testing is a good way to ensure that all components on a production line are validated for proper heat treatment. This technology also helps save time and money by reducing both scrap and warranty costs associated with improperly heat-treated components.
What makes eddy current technology unique?
- Testing is fast. Typical testing times are in the millisecond range, which makes it perfect for in-line testing. For small parts like pins or balls, testing can be integrated with a feeder or sorter mechanism that can test several parts per second. For larger parts, you are only limited by time for part handling.
- Testing is repeatable. Eddy current tests are consistent and don’t require an operator to make a human judgment. Consistent mechanical positioning is required to ensure accurate and consistent results.
- Testing is easily integrated into production lines. Modern eddy current instruments are made to integrate right into production lines. They communicate with material-handling PLCs to work in conjunction with robotics and sorting devices. Instruments can be programmed to stop assembly lines when consecutive heat-treat anomalies are discovered.
- Testing is clean. There is no need to apply couplants for testing. It is not necessary to wash parts clear of oils or cutting fluids prior to testing.
For more information: Contact Dan DeVries, director of marketing at Criterion NDT, 3702 W. Valley Hwy, Auburn, Wash.; tel: 425-891-5163; e-mail: firstname.lastname@example.org; web: www.criterionndt.com. USA-based Criterion NDT specializes in providing engineered eddy current solutions for hardness verification and flaw detection.
- NHTSA Part 573 Safety Recall Report - https://static.nhtsa.gov/odi/rcl/2017/RCLRPT-17V578-1441.PDF
- SIG SAUER - https://www.sigsauer.com/press-releases/sig-sauer-safety-warning-recall-notice/
- North Carolina Consumers Council https://www.ncconsumer.org/news-articles/meritor-issues-recall-of-trucks-due-to-improper-heat-treatment-of-axles.html
- Dodge News, Photos and Reviews http://www.dodge-motors.com/news/fiat-chrysler-recalls-141000-ram-cherokee-models-for-potential-axle-fractures-fires/
- NHTSA https://static.nhtsa.gov/odi/rcl/2014/RCRIT-14V819-4255.pdf
- Steel Heat Treatment: Equipment and Process Design by George E Totten
- Practical Induction Heat Treating by Richard E. Haimbaugh