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Status quo' of the magneto-inductive testing - Innovation in industrial series testing by digital technology approach Michael MAASS, Jürgen NEHRING, Detlef SIEVERS Contact

Introduction

FOERSTER testing systems have been the worldwide standard for magneto-inductive testing from the very beginning. The term 'MAGNATEST testing' as a worldwide synonym for magneto-inductive testing impressively reflects this.

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'.

Basics of magneto-inductive testing in the industrial use

Magneto-inductive testing stands for low frequency (high penetration depth) eddy current testing. It is generally usable on all electrically conductive materials: e.g. steel, aluminium, brass, copper, etc.. Whereas the high/medium frequency eddy current testing is primarily used for the detection of defects (cracks, pores, shrink holes, exogenous inclusions, etc.) near the surface, the primary use of the low frequency eddy current testing is for the checking of material qualities (sorting of different alloy compositions, determination of surface hardness, hardness depth (HD), case depth (CD), tensile strength, etc.).

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.

Test instruments and accessories for the magneto-inductive testing

Modular testing devices, such as the FOERSTER MAGNATEST ECM (Fig. 1) -ECM= Eddy Current Module - are today increasingly integrated in automatic testing equipment. As sophisticated programmable logic control (PLC) has frequently already been used for the mechanics of the automatic test equipment, it is often practical to also effect the control of the test instrument (e.g. test on / test off) and the management of the test procedure via the PLC.

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).

The new MAGNATEST D

The latest member of the MAGNATEST family is the MAGNATEST D (Fig. 3) with digital test electronics from the signal excitation to the evaluation. A very compact size in 19" design and a very low weight were realised by way of consistent use of the latest digital technology. The well-known features of the MAGNATEST S were also extended: multi-frequency technology and harmonic wave evaluation combined with the multi-parameter analysis form the basis for a simultaneous high test sensitivity and more effective disturbance suppression. In addition to the single-frequency testing and the serial multi-frequency feature, the MAGNATEST D also offers the simultaneous multi-frequency feature as a high-speed version of the multi-frequency testing.

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 11^th harmonic.

At the same time, all known hardware functions and features of modern PC and network technologies and WINDOWS^TM 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 WINDOWS^TM 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.

Automated test systems for industrial component testing

An automatic test system can be divided into 5 components (Fig. 5). In the case of a non-linked test system, when the automatic test equipment is not directly incorporated in the production cycle of the testing line, the components to be tested are separated or isolated (for example, with a vibration feeder) during a first step. The component to be tested is then fed and loaded into the core mechanics of the test system. When encircling-coil sensors are used, the core mechanics are often limited to a lowering of the test coil or a lifting of the component into the test position.

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.

Hardness testing on steering racks

Steering racks are components of the steering gear of vehicles and transfer the steering forces from the steering wheel to the steering trapeze of the front wheel suspension. The characteristics of the steering can be influenced in a targeted manner by variations in the heat treatment and the different tooth designs. The type and requirement dependent hardening of the toothing and the shaft serves for the individual adaptation to the respective operational loads and the wear behaviour.

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:

• automatic setting to the steering rack type with call-up of the associated test settings and type display on the system monitor,
• operator-guided and largely automated setting and calibration when already existing settings are not to be used,
• calibration setting check by way of requesting a defined 'not OK' sample (master),
• check of the respective operating conditions (steering rack in the test station and correctly positioned) via appropriate proximity sensors,
• permanent query of the test coil position by the test system and automatic measurement value triggering after the respective test position has been reached,
• software-aided monitoring of the test sequence with output of respective fault messages.

Hardness testing on strip-hardened cylinder bores of engine blocks

A further area of application of the PC-aided eddy current test stations from FOERSTER is the checking of strip hardenings in cylinder bores. In contrast to the previously common checking of hardness depth at a few selected locations of the component with destructive methods, new to this application is that a line-shaped measurement value recording occurs over the cylinder circumference.

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.

Multi-channel hardness testing on crankshafts

The use of the PC-based simultaneous multi-frequency eddy current instruments from FOERSTER with the test station described below enables a 100% 'in-line' check of different crankshaft types in mixed operation in the series production. In the current version, 4 and 5 cylinder crankshafts can be tested with a cycle time (floor/floor) of 24 seconds, respectively, 28 seconds at a total of 24 measurement positions (Fig. 8).

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.

Eddy current sensing: temperature checking on hot forged parts

Forged blanks are manufactured for the automotive industry with a hot former like, for example, the HATEBUR AMP 70. Up to now, it has been necessary to subject the formed parts to a separate heat treatment (normalisation). The installation of a cooling facility on the press now makes it possible to cool the parts in a controlled way immediately after the forming process. This so-called forge perleatic cooling (BY) achieves a normalisation-equivalent structure condition. A precondition is that the forged blanks leave the press with an austenitic structure condition. This means that the temperature of the formed parts must be above the GOS line of the steel-carbon diagram before the cooling. This is not always guaranteed when failures occur in the process. The structure condition, respectively the temperature of the parts, must therefore be accurately monitored. In the case of the material used, the temperature of the phase change is sufficiently identical to the Curie point, 769°C in the case of Fe-C alloys, so that it is possible to use the magneto-inductive characteristics of the material as test criterion for the structure condition.

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'.

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