Pulsed eddy currents (PEC) method can be easily applied to ADI materials in a differential mode using a reference specimen. In the case of Austempered Ductile Iron (ADI), the study has been conducted on bars with different heat treatment conditions. The magnitude of the resulting differential signal appears as the most significant parameter, which is clearly sensitive to heat treatment conditions. This parameter is not unique for various combinations of austenitisation temperature, austempering temperature and time. However, for a given composition PEC may be used to sort real parts, provided a reference specimen representative of the required microstructure state is available. The resulting differential signal would then be a good indicator of wrong treatment conditions. Especially, the results show that this parameter is expected to increase markedly for too long austempering times. Previously, Pulsed Eddy Currents had been used mostly for flaw detection. This work shows that the PEC response is sensitive enough to ADI microstructure state to be used as a non-destructive characterisation tool. However, it should be noted that, in contrast to Barkhausen noise, PEC provide global information, i.e. averaged over the various constituents of ADI. So the exploitable result arising from the project consists of a potential application of PEC differential measurements for ADI characterisation. However, considering the appropriate equipment well adapted for a practical application in industrial conditions (especially, a hand held probe is required), complementary studies remain to be carried out.
Magnetic Barkhausen noise measurements for assessing microstructural characteristics of ADI materials
The properties of Magnetic Barkhausen Noise can be measured easily on ADI cast iron. The Barkhausen peak characteristics (height, excitation field position, morphology) appear to be very sensitive to ADI heat treatment conditions. These characteristics can be exploited for the assessment of the microstructural state of ADI resulting from the heat treatment conditions. More precisely, the peak position gives relevant information on the type of bainite (or ausferrite) linked wit the austempering temperature. The peak height can be sued as an indicator of the bainitic reaction progress. Furthermore, the appearance of unexpected peaks allows us to assess the presence of anomalous magnetic constituents. It must be noted that it is the first time that Magnetic Barkhausen Noise techniques are applied to characterise ADI materials (Previously, it was applied to steels evaluation only). The key innovative feature consists in providing distinguishable information from any magnetic constituents of ADI, in contrast with the other electromagnetic techniques, which give overall -and averaged - information of all ADI constituents only. This exploitable result consists in a specific application of this technique applied to ADI materials. It includes suitable equipment for the measurement of the Magnetic Barkhausen Noise response of the material as well as a complete set of reference data acquired on specimens prepared under well-controlled heat treatment conditions.
Use of multi-frequency eddy current testing combined with ultrasonic velocity measurement to predict the mechanical properties and structure of ADI castings
Multi-frequency eddy current testing has been shown to have the following attributes: - It can automatically sort acceptable and unacceptable casting. - It can rank ADI castings in order of hardness. - It can separate castings austenitised at different temperatures or austempered for different times at different temperatures. - It can predict the retained austenite content to within +/-3% if individual algorithms are developed for specific compositions of ductile iron. - By linking multi-frequency eddy current testing with other NDT tests which are sensitive to graphite form (such as resonant frequency or ultrasonic velocity measurement), it is possible to predict proof and tensile strengths of ADI casting to within +/-10% of their actual values. - However, it is not yet possible to predict the % elongation to an acceptable degree of accuracy and the need for routine mechanical tests can only be reduced but not eliminated. - This development is considered to be of practical significance since every casting can be evaluated non-destructively and a reasonable assessment made of its properties. This should allow potential users of ADI castings to have a much-improved confidence in the consistency of engineering components made from this material and hence increase the development of new markets for ADI castings. Laboratory research has shown considerable promise in the use of combined NDT tests to predict the structure and mechanical properties of ADI materials. Although much of this has been carried out on simple test bars, a more limited number of tests have also demonstrated that the technique has reached a stage where it would be appropriate to asses it more extensively under production conditions in a foundry.