NDT is not only applied for quality and production control, but also during design of new products and materials. [1]. This article describes such an ultrasonic application. The investigation concentrates on special workpieces that are used in machines for construction material recycling purposes (breakers). For this application strong wear-resistant workpieces must be developed through the design of material composites and treatment, furthermore, the new design must permit the economical manufacture of these parts. Conventional materials do not include all necessary properties as there are high wear resistance (hardness) and high impact resistance (toughness) [2].. This ultrasonic project is part of a program for material characterization of micro and macro structures, involving many other metallurgical disciplines.

Table of contents

• Abstract
  
• Workpieces Tested
  
• Composite casting
  
• Ultrasonic testing
  
• Results
  
• Conclusion
  
• Literature
  

Workpieces Tested

The investigation was carried out on composite casting plates, based on a steel body (C45 rolled plate) on top casted chromium white cast iron G-X 300 CrMo 15 3, both applied with a thickness of 15mm.


Fig 1: Principles of Composite Casting

This composite will include the advantages of both materials, fulfilling the requirements outlined above. Good bonding between the two materials is an essential factor. Traditionally, methods such as soldering, adhesive bonding, and welding have been applied in order to achieve this. [3]. Composite casting provides an especially economical solution.

Composite casting

Composite casting is the production of metal workpieces, carried out through simultaneous or sequential casting of different melted materials [4]. The example given in the article uses the method of casting chromium white cast iron on a pre-warmed steel body inside an ingot mould. This production was performed at the Institute for production methods at the Otto-von-Guericke-University Magdeburg. [5]. Variation of production conditions, e.g. pre- warming, melting temperature, cooling condition and the application of different kinds of flux is included in the examinations.

Ultrasonic testing

The ultrasonic testing method was applied for determination workpiece conditions, especially bonding quality. The pulse echo technique with straight beam probe and a pitch and catch technique with angle probes for scattering measurements was applied, both with post processing of C-scan and D-scan presentations. Fig 1 and Fig 2 display the principles of both techniques. According to the gate position, echo signals were recorded in the area of bonding. In the case of angle beam testing, the gates were positioned in other depth zones. For the pulse echo technique a high damped 10 MHz transducer was used for achieving best transversal and longitudinal resolution.

Fig 2: Pulse echo technique	Fig 3: Pitch and catch technique for scattering measurements

In the scattering method a miniature angle transducer 45°, 4 MHz. was used for both. To avoid the disadvantages associated with non-straight surfaces, the transducers were coupled through delay lines and the evaluation was released by the interface echo (interface echo start).

The test was executed from the side of the steel body, because high attenuation in chromium cast iron (alpha=450 dB/m for longitudinal waves at 5 MHz) and the surface conditions of the casting would would make a sound transmission from this side impossible. Use of ultrasonics for measurement of crack depth, sound velocity and attenuation are further objects of investigation.

Results

With Ultrasonic testing it is possible to locate disbonding with good local resolution and reproducibility. (Fig 4a) displays a C-scan presentation of a plate and shows the mentioned advantages. Further determinations of defect type are possible through evaluation of echo amplitude. Acoustical impedance was estimated to be 12*10E6kg/m²s by the composition of the flux, thus providing a reflection factor of about 0.6. Hence it could be concluded from this that the defects are flux inclusions with cavities. This statement can be proven by cut workpieces. (Fig 4b) ilustrates that. The position of cavities corresponds as expected with positions of highest amplitude in the C-Scan.

Bild 4a): C-scan reflection	Fig 4b): Picture of a cut at the marked scan result.
Fig 5: D-scan reflection	For an illustration of simultaneously recorded D-scan presentations showing clearly the sections with disbonding via the scan area, please see Fig 5.

The ultrasonic scattering results should provide more information on bonding interface within areas without lack of fusion. The amplification was increased, so that in the area of defects the signals are displayed in a saturation condition. Correlations to the interface process are possible through analysis of these results. Fig. 6 shows an example of low scattering at the marked line which corresponds with an area of melting of the steel body, to be seen in the cross section at figure Fig.6b.

Fig 6a): C-scan scattering

Fig 6b): Picture of a cut at the marked scan result.

The D-Scan presentation also shows scattering from a deeper area, obviously from the cast iron. After cutting the workpiece at this section a correlation proved the existence of microshrinkage in this area. Please see Fig.7.

Fig 7a): D-scan scattering

Fig 7b): Picture of microshrinkage at the marked scan result.

Another application of the ultrasonic method involved crack depth measurement. After cutting the workpieces cracks on the surfaces were visible, and had to be measured. A Rayleigh-wave technique was applied and due to time of flight measurement the crack depth could be determined. Multiple measurements with transversal waves showed similar results of a few millimeters of crack depth.

To achieve specific mechanical properties, it is important to determine Elastic modulus of the chromium cast iron. By known equations this figure can be calculated through measurement of longitudinal and transversal sound velocity, as well as density. Through attenuation and coupling changes it is possible that echo signals are distorted and this could cause changes of trigger points with further disadvantages in time of flight measurement accuracy. However, a pre - calculation of possible errors showed a result of just +/- 4 GPa, so it met the necessary standard of accuracy. In general, the results show ultrasonics to be a good tool for materials characterisation. It should also be mentioned that the workpiece preparation demands high efforts, hence an improvement is necessary. In addition to the results for material characterization, we also gain feedback for production processes. Correlations of ultrasonic results to production parameters helps to optimize production.

Conclusion

In general the Ultrasonic method provides advanced information on composite castings in an early stage. The results lead directly to the material characterization and to the developments for production process. It is possible to optimize other investigations and properties of interest can be determined fast and non-destructively.

This task was part of a DFG and LSA program "Material characterization", included in the SFB 385 "Baustoffrecycling". The investigation was carried out in the Institut for materials technology and material testing at the Otto-von-Guericke-University Magdeburg.

LITERATURE

1. Kröning, M. Neue Aufgaben für die ZfP: DGZfP-Jahrestagung 1990, Trier, S. 8-38
2. Motz, J. M.; Christianus, D. Schlagbeanspruchte Werkstoffe für die Aufbereitung von Altautos und von Bauschutt: VDI-Berichte 773: Werkstoffe für die Umwelttechnik; VDI-Verlag Düsseldorf 1990 S. 1-31
3. Norman, T. E.; Röhrig, K. Verschleißfeste martensitische Chromgußeisen: Aufbereitungstechnik (1970),6, S. 356-364
4. Ißleib, A.; Friedel, A.; Lubojanski, I. Verbundgießen für Verschleißteile - Stand und Zukunftsaussichten: Gießerei-Praxis (1995), 7/8, S. 146-150
5. Lubojanski, I.; Friedel, A.; Ißleib, A.; Ambos, E. Grundlagen zum Verbundgießen plattenförmiger Bauteile Vorträge zum Kolloquium des Sonderforschungsbereiches 385 "Baustoffrecycling" der Otto-von-Guericke-Universität Magdeburg am 08.02.1996 / Preprint Nr. 3, S.50-55

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Authors

Dr. - Ing. Jürgen Pohl , Juergen.Pohl@Masch-Bau.Uni-Magdeburg.DE	Dipl.Ing. Dirk Wiedemann, Dirk.Wiedemann@Masch-Bau.Uni-Magdeburg.DE	Prof. Dr. - Ing. habil. Karl-Otto Prietzel, Karl-Otto.Prietzel@Masch-Bau.Uni-Magdeburg.DE
The authors are researchers at the Institute for materials technology and material testing at the Otto-von-Guericke-University Magdeburg. - Institut Home Page in German :). PF 4120 - 39016 Magdeburg

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Rolf Diederichs 1.July 1996, info@ndt.net /DB:Article /AU:Pohl_J /AU:Wiedemann_D_ /AU:Prietzel_K_O /CN:DE /CT:UT /CT:composite /CT:steel /ED:1996-08