Eddy-current testing of cast parts with rough surface.

Comparative tests of different types of eddy-current probes are presented. Their results helped to improve techniques of casts eddy-current testing. Techniques of effective testing of cast parts of rolling stock of railway transport are considered.

The acoustic and X-ray methods are widely used for inner defects detection (1-2). These methods have a lot of restrictions. The X-ray method is expensive and requires protection of the staff from the harmful radiation. Readjustment is needed for X-ray method because of different parts thickness equipment. Ultrasonic method requires a good prepared surface. It is complicated for inspection of parts with variable arch radius of surface and not enough effective by surface defects detection. Thereby, the ultrasonic testing of casts is carried out after cleaning to attain a reliable acoustic contact.
The aim of current work is to draw attention to possibility of effective eddy-current method application for surface defects detection on parts with rough surface by example of cast parts testing of railway transport rolling stock.
Eddy current testing of railway transport cast parts is provided by normative documents of Russia and Ukraine [4, 5]. Normative documents provide eddy-current resting of following units:
•    Wheel pairs and box unit;
•    Freight-car trucks, refrigerator trucks and passenger cars;
•    Brake linkage;
•    Automatic coupling device.
Mentioned documents create application of eddy current flaw detectors of Russian (VD-12 NFM, VD-12NF, VD-15NF, VD 113, VD 213[4]) and Ukrainian production (VD 30[5]). Most of these installations are based on a phase variant of the eddy-current method. Compensated absolute eddy-current transducer (ECT), windings of which are located on big (4,5mm) ferrite cores, is used for testing on a rough surface.
But practice showed that by using these installations only testing of units and wheel pairs were fulfilled satisfactorily. The problem of defect detection in units, made by cast method with the help of mentioned installations, could be not solved. The carried out analysis and the preliminary study results in cast parts testing (with elements of surface radius of curvature) allowed to identify the main reasons of methods shortcomings:
•    High noise level, typical for cast items with rough surface;
•    Low-frequency “trend” of ECT signal, conditioned by discontinuity of testing material magnetism, by different radius of curvature and insufficient offset from spacing change and distortion of ECT while scanning the testing surface.
We were offered to use more effective technologies of the eddy-current testing based on selective multidifferential ECT [7] and modern flaw detector VD 3-71[8]. Previous experience of using this eddy-current flaw detector showed its possibility to solve difficult problems of flaw detection in conditions of strong noises. We saw the effectiveness of differential mode for full rejection of low-frequency variations and trends of ECT signal and incensement of signal-noise relations under noises of random character.
Let us consider the use of the eddy-current testing of rough surface parts exemplified with truck solebar of rolling stock. These units are made of steel of 20GL grade with casting in sand method. After normalization the material has a hypopearlitic structure. The roughness of the surface corresponds to RZ320. Detected cast defects are subject to cutting and with further welding repair. A typical testing surface is shown on fig.1. There can be observed jointing (Deepings with rounded edges), which are typical for cast surface and gas roughness, conditioned with the growth of core blows on the form border (a) and convex net form marking as well.

 

 

 

Fig.1. Typical surface of cast parts of bogie frame and bolster.
     Comparative tests of different ECT were carried out. A standard ECT of PN10-   
     TD and ECT of Leotest lot of different diameter (MDF 0701, VDF 0901, VDF 1201)
First two numbers indicate operating platform diameter in millimeters. Windings on ferrite cores about 12, 2 to 3, 5 in diameter are used in these ECT. Standard pieces with RZ 320 surface roughness and defect of crack type are prepared for analysis. Besides, unit fragments with natural defects were selected during manufacture. The analysis showed that the best signal-noise relations in testing of pieces with rough surface made ECT of MDF 1201 type.  Standard ECT of PN10-TD type showed a low sensitivity and was no longer considered. On the ECT operating surface was pasted a ceramic washer about 5 mm thick.
On the fig. 2-4 signals of ECT of MDF 1201 type on 170 kHz operating frequency are presented.  They are observed on the VD 3-71 screen in complex plane (a) and with time base (b). On the fig.2 signals for a crack with 0, 6 mm depth and with 10mm length without differential processing are presented. On the time defectogram (fig. 2b) we can see signals of noises connected with testing surface irregularities.
On the fig.3 signal of the same crack after a differential processing with base equal 10, i.e. 1 and 10 of signal generation is presented.

 

Fig.2 Signals of ECT of MDF 1201 type from a crack without differential processing.
Comparison of signals on fig 2 and 3 shows the effectiveness of differential processing by suppression of smooth signal variations. Defect signal after a differential processing changes the mark by scanning over defect that eases the interpretation of the signals while testing. Besides, “slow” signal changes are suppressed. These signals are caused by convexes of the testing surface.

A defectogram from a void defect about 10mm in diameter after differential processing , which shows the possibility of local void defects detection and confirms the effectiveness of the differential signal processing is presented on the fig.4.

 

Fig. 4. ECT MDF 1201 signal from a void after differential processing.
The analysis allowed to improve the eddy-current technology of cast parts. The testing methodology of bogies frame provides separation of a part to areas: interior and exterior angles of box apertures, flange of upper chord and strengthening rib, box body alighting area, inclined chord, borders of operational opening, angles and borders of spring opening. Center bowl, interior and exterior end collar borders, transmission from exterior center bowl end collar to upper beam chord are thoroughly tested during truck bogie beam inspection. Besides, the bolster beam upper chord, borders of operational openings, side wall areas, bottom chord of bolster beam, inclined planes for wedge and transmissions from restrictive end collar to inclined planes are being tested.
There are some examples of rolling stock cast parts testing. On the fig.5 there are scanning circuits by inclined chord testing (a) and operating openings (b) of the truck solebar.

Fig.5. Example of scanning while truck solebar parts testing.
On the fig.6 there are scanning circuits by center bowl of a bolster. The scanning of center bowl area is recommended to carry out over radial (a) and circuit (b) paths.
On fig. 7 there are scanning circuits by operating openings in upper (a) and bottom (b) bolster.
Fig.7 Example operating bolder openings of scanning.  
Conclusions
1)    The eddy current method with using of selective eddy current multidifferential transducers and VD 3-71 transducers can be effectively applied for flaw detection of cast parts with rough surface.
2)    Further improve of signal-noise relations by testing of rough surfaces can be achieved by means of differential processing of signal while scanning.
3)    Introduced technologies are used for an effective test of railway cars cast parts.