Case Study - Case Depth VerificationScope A transmission shaft manufacturer had a customer with three broken transmission shafts due to unacceptable case depth. Application The transmission shafts are manufactured from two different carbon steel alloys, 1045 and 5140. It was required to sort for acceptable case depth, 3.0 mm to 7.0 mm, within each material as well as sort one material from the other. Initially only acceptable and low case depth samples were provided. High case depth samples were later provided and a case depth separation of 0.1 mm was required. The Challenge Case depth inspections are typically the most difficult eddy current applications. This application was especially challenging since case depths of over 7.0 mm must be inspected while maintaining resolution of 0.1 mm. Three test coil/probe options were available: 1. Self-reference spot probe 2. Self-reference encircling coil 3. External-reference encircling coil Testing with the MIZ-27CT® Instrument and an internal reference hardness “spot probe” (for ease of in-line integration) at several frequencies revealed no separation between the good and the low case depth samples from either material. There was also no separation between the two alloys using the spot probe. The next inspection attempt was with the MIZ-27 CT Instrument (since their sister company is currently inspecting with some MIZ-27 CT’s) and a 29mm “Internal Reference” Hardness Encircling Coil. Testing at several frequencies also gave no separation between the good and the low case depth samples from either material. Little separation between the two alloys was observed. The Solution/Setup The best results were achieved testing with the MIZ-27CT Instrument and two 25 mm InSite HT “External Reference” Hardness Encircling Probes with the center of the probe coils positioned around the small diameter of the shaft near the splines. One of the samples was used as the reference sample and placed within the external reference coil. To build the part buffer and create the alarm boxes for each of the four different tests, a data point at five different orientations from each good sample was used.
Test Results Excellent test results when testing the transmission shafts, including the new high case depth samples. Material 1045 Test: Very good separation when testing the Material 1045 samples. Material 5140 Test: Good results from the reject samples when testing the 5140 samples. Material Sort: All of the 1045 samples from all of the 5140 samples and all of the new high case depth samples were separated.
High Case Depth Test: Great and reliable separation when testing the new, high case depth parts. 10 kHz provided the best results. Each sample was inspected 4 times at orientations of approximately 0, 90, 180 and 270 degrees. Please see the attached “High Case Depth Sort” results. Labels “64” = 6.4 mm, “72” = 7.2 mm, “73” = 7.3 mm and “74” = 7.4 mm. There was great separation between the responses to the 6.4 mm, 7.2 mm, 7.3 mm and 7.4 mm case depth samples. Implementation The customer purchased an InSite HT-4 Instrument and eight External Reference Hardness Coils (four coils for reference and four for testing) for the in-line, simultaneous inspection at four positions along the transmission shaft. This required four External Reference Cables and two InSite Y-Adapter cables. Benefits External reference testing provides the best sensitivity for case depth, alloy and hardness sorting. In many cases, a test can be reliably performed using an internal reference coil. However, if a sort is difficult or impossible to perform with an internal reference coil, especially on case depth applications, it will most likely become a successful test when performed using external reference testing. Other Uses The external reference test would provide similar exceptional results when inspecting any bar-shaped product for proper case depth, alloy or hardness. |
