·Table of Contents
·Materials Characterization and testing
Status quo' of the magneto-inductive testing - Innovation in industrial series testing by digital technology approachMichael MAASS, Jürgen NEHRING, Detlef SIEVERS
The new digital MAGNATEST D is used here as an example to describe the extent to which the increasing use of the computer and data processing technologies have worldwide determined and significantly expanded the potential use of magneto-inductive testing. This applies to both the continuous shift of the use from the laboratory area to the 'on-line' manufacturing process and the different application requests which are currently wide-ranging from the simple yes/no test result and the multi-frequency homogeneity evaluations,up to the determination of the physico-technological characteristic values.
Moreover, features that are today taken for granted, such as intuitive user interfaces, defined test result documentation and network connection can also be integrated into the measuring technology at a low cost due to the development of PC hardware and software.
The aim of this contribution is to describe the currently available spectrum of the magneto-inductive testing and its system solutions by way of industrial application examples both in the well-known 'classical' fields of application (material sorting, hardness checks) and in the 'non-classical' fields of application such as 'eddy current sensing'.
The eddy current sensors are predominantly constructed with coils, usually one transmitter coil combined with one or more receiver coils. In general, there are two different sensor versions: encircling-coil sensors and probe sensors.
The probe sensors permit a high local measuring point resolution. The encircling-coil sensors induce eddy currents in a larger component volume and enable a powerful eddy current induction which can, for example, be used for harmonic wave excitation in ferrite materials.
|Fig 1: FOERSTER MAGNATEST ECM||Fig 2: FOERSTER MAGNATEST S|
More sophisticated excitation and evaluation functions continue to be necessary for more complex testing requirements where there is a need for more than a simple yes/no or good/bad result, for example, in the case of a multi-dimensional regression for the determination of an accurate hardness or case depth value.
The reference instrument for the magneto-inductive testing is the FOERSTER MAGNATEST S (Fig. 2) which has multi-frequency excitation, multi-parameter evaluation and, as an option, also offers the multi-dimensional linear regression for absolute value determination (e.g. for hardness and case depth).
|Fig 3: FOERSTER MAGNATEST D|
The MAGNATEST D features improved the effective harmonic wave evaluation for the testing of ferrite materials: powerful signal excitation and digital signal evaluation permit a sensitive harmonic wave analysis up to the 11th harmonic.
At the same time, all known hardware functions and features of modern PC and network technologies and WINDOWSTM software features (MAGNAWIN), such as TFT colour monitor, standardised interface (printer, etc.), internal/external data storage (hard disk/ floppy disk, ZIP, etc.), have been implemented in the new digital MAGNATEST D. A further standard feature, the universal network capability, makes it possible to connect the test instrument to QA systems and, in particular, also the remote on-line customer support.
The data compatibility with the WINDOWSTM standard permits the user to use well-known software functions like, for example, the temporary saving of measuring results on the " clip board" for a subsequent printing of investigation reports or measuring records.
|Fig 4: Icon-aided user-interface of the MAGNATEST D|
Simple menu-driven user interface and test parameter management combined with the icon-assisted function selection of modern software permit an intuitive operability (Fig. 4). Help wizards support the user during set-up operations. The PC-based technology of the MAGNATEST D offers a suitable basis for the user-friendly visualisation of test results and, especially, also for complex test situations. Data documentation as per ISO 9000 and integrated system self-checks for the integration in automatic test sequences are part of the standard scope of supply.
|Fig 5: Components of an automatic test station|
Much more complex and thus more expensive is the scanning of complexly shaped
component geometries with probe sensors for achieving a higher local resolution. The component is removed from the core mechanics after the actual test process and then
sorted according to the test result. In most cases, the sorting is limited to a
good/bad separation or a sorting according to defect-free and defective. Additional measurement information such as long-term tendencies of the test results or the
absolute measurement values of a hardness or case depth testing can be subsequently read from the result memory of the automatic test station and, for example, fed into a statistical process check.
Described below as examples are 4 FOERSTER eddy current automatic test systems for the magneto-inductive testing, which display the actual status of modern magneto-inductive test-stations.
The steering rack test system checks the hardness penetration depth (HD) of inductive hardened steering racks. A FOERSTER PC-based eddy current test station with encircling test coil is used for the testing (Fig. 6). The test sequence starts with the steering racks being grabbed between prongs and aligned. The test coil is then positioned via a driven rail at 5 locations (type-dependent) where the HD is checked non-destructively and independently of the other locations. The setting for the steering rack type to be tested, the positioning of the test coil and the change of test parameters occur automatically via the SPC of the automatic test equipment.
|Fig 6: Automatic test station for steering racks|
An initial calibration with sample parts with known HD graduations is necessary because the hardness depth determination occurs not only as an attributive result but also in millimetres (+/- tolerance). These samples were prepared and subjected to metallographic checking by the customer. An HD determination accuracy of approx. +/- 0.15 mm can be achieved with the system.
The measures taken to secure the actual test sequence include the following:
The measuring task is solved by the use of a simultaneously multi-frequency working eddy current system in order to ensure an area-wide sensing of the respective testing area with many testing frequencies in one operation with simultaneous continuous rotation of the test sensor.
The primary aim set by the customer was a significant reduction in the very expensive destructive determination of the hardening depth. This was achieved by incorporating a non-destructive testing method into the production process as an in-process checking instrument.For the checking of the inductively achieved hardness depth, the component to be tested is first ejected from the running process and positioned in a separate test station linked to the production process flow.
A complete testing system (Fig. 7) that can at any time be successively expanded to suit the expected requirement increases (high flexibility in the adaptation to changed workpiece geometries, adaptation of the number of test tracks to the changed hardness structures, increase in the throughput capacity of the test system, etc.) was realised by way of use of a handling system that was specially optimised, particularly in respect of the sensor guidance requirements.
|Fig 7: Automatic test station for cylinder bores||Fig 8: Automatic test station for crankshafts|
Dependent on the test rules of the customer, several test tracks in the hardened area are recorded per cylinder after the test head has been positioned and has reached a constant rotational speed. In addition to the checking of the hardness penetration depth, the targeted analysis also permits a non-destructive check of the presence of the required number of hardness strips.
The test sequence is complemented by test task matching algorithms for taking into account batch effects, fixed sequences for calibration on type change and the checking of the entire system at defined time intervals.
The direct transfer of all relevant test and component data to the customer's quality assurance network after the completion of each individual test enables the customer to quickly react to any failure in the hardening process. The shortened intervention times reduce the reject rate and the associated costs. The capacities in the destructive testing become free and can be used for other purposes.
All main and pin journals and also the oil seals are checked for HD homogeneity at two selected circumference points (for checking of the concentricity of the hardness profile). Screened special probe sensors are used for the sensor technology. These are 'lowered' onto the selected test points by the test mechanics in a defined way and with high reproducibility. The calibration of the measuring station occurs automatically with 'OK' series samples whereby optimised test parameters and calibration fields are determined for the respective test points. After the system set-up and before the start of the series testing, the test system is checked for the required selectivity with so-called master samples (defined 'not OK' parts) in a 'master check cycle'. The use of modern PC technology in the test system ensures not only an optimum operator guidance but also a comprehensive visualisation and documentation of the determined test results.
In order to guarantee a homogeneous hardness distribution of the forged part after the controlled cooling it must be ensured that all forged parts that have already partially dropped below the forging temperature are sorted out prior to the further heat treatment with absolute test reliability.
The solution approach with the magneto-inductive test technology uses the physical effect that even a small local temperature drop below the Curie temperature results in a significant magnetic behaviour of the forged component. It is therefore possible to sensitively detect even the smallest zone of the forged part below the curie temperature by use of an encircling test coil through which the hot part to be tested falls, rolls or slides. With the aid of the device-internal test piece recognition (the automatic triggering is standard for FOERSTER MAGNATEST systems), it is possible to effect a precise evaluation and reliable sorting of even only partially (localised) cooled-down parts independent of the 'run-through speed' of the test piece and independent of the number of the test pieces which run through the test coil simultaneously. A component size which permits the testing of the very different part sizes/weights (Fig. 9) was realised by way of a test coil sensitivity optimisation. A robust test coil design with integrated water cooling also ensures a process-near integration at the run-out point of the forging press.
|Fig 9: Eddy current sensing for temperature checking on forged parts|
A maximum measuring sensitivity with simultaneous suppression of disturbing influences from the forging process was achieved by the selection of an optimised test frequency for driving the coil. The entire forged part spectrum at the customer, GKN-WALTERSCHEID, can be covered with the aid of an adaptable evaluation window and a single calibration setting.
Furthermore, the visualisation of the measuring results on the monitor of the test system offers a high comfort in respect of the setting and the periodic function monitoring of the measuring system with defined 'nonconforming sample parts'.
|© AIPnD , created by NDT.net|||Home| |Top||