It is now completely assumed that strong reliable variations are observed between the ADI heat treatment parameters and the answer to NDE, especially Eddy Currents and Barkhausen Noise measurements. On the other hand, conventional techniques (ultra-sonic propagation velocity and Young's modulus measurements) are too much sensible to graphite structure to provide any reliable information about the matrix structure.
Pulsed eddy current measurements give a global response, which depends on the variations of conductivity and permeability with the microstructure.
Magnetic Barkhausen noise measurements, can be used to characterise microstructure. Each type of micro-structural constituent exhibit typical signature. So, unexpected constituents can be detected (ferrite, pearlite, martensite...) and ADI microstructure can be characterised.
The peak position is especially sensitive to the type of ausferrite which mainly depends on austempering temperature, the peak amplitude is linked to austempering time and is also influenced by the other parameters (Austenitisation temperature and composition).
The prediction of microstructure (upper or lower bainite) and (knowing the composition) of the heat treatment parameters has been done on the industrial test bars. Micrographic study and information obtained from heat-treatment performers confirm these predictions.
Multi-frequency eddy current measurements (with up to ten test frequencies and harmonic analysis) allow optimum sorting of complex ADI structures. The technique enables Austempered Ductile Irons to be ranked in order of hardness. For a given chemical composition, the eddy current response is sensitive to changes in the heat treatment variables. However similar eddy current responses can be given by different combinations of heat treatment variables.
By combining multi-frequency eddy current data with data from other NDT tests which are sensitive to graphite form, it is possible, using algorithms, to predict the retained austenite content, proof and tensile strengths to within ±3 and 10 percent respectively of the measured values. This is considered to be of practical significance to the producers and users of ADI castings. It is not yet possible to predict the elongation to an acceptable degree of accuracy.
Barkhausen Noise measurements and Multi-frequency eddy current testing can identify castings which have not received the correct heat treatment, thereby rapidly indicating that a process problem exists. The need for expensive and slow mechanical testing of cast-on bars or sacrificial of good castings is reduced
Obviously, the main objective of the Project is to design a commercial apparatus able to check ADI parts in a Non Destructive way, i.e. to detect a part that would have undergone a wrong heat treatment and/or a part that would have not undergone any treatment at all. The apparatus must be rapid, easy to use (by no-qualified staff) and able to work in electromagnetic highly polluted environment.
The expected achievements of the project are given here below:
* the implementation of a new technology for ADI control, based on NDE methods,
* the development of ADI in large production volume,
* the creation of new markets by the replacement of forged, case hardened steel,
* the markets extension, where improved performance would be achieved by the replacement of existing ductile iron castings by austempered materials
Barkhausen Noise measurements have been chosen as the most promising for industrial validation. The currently available apparatus proposed by SYSMAT, especially the probe, must be modified to be able to distinguish the different classes of materials.
Main project results
* Magnetic Barkhausen Noise measurement for assessing micro-structural characteristics of ADI materials
* Use of Multi frequency Eddy Current combined with ultrasonic velocity measurement to predict the mechanical properties and structure of ADI
* Pulse Eddy Currents measurements for assessing heat treatment of ADI materials.
Austempered Ductile Iron (ADI) is a special "Heat treated" ductile iron which offers attractive combinations of mechanical and service properties (high strength, ductility, wear resistance and toughness). In spite theses remarkable properties, it exists a key area, which to be tackled: the establishment of quality control procedures which guarantee the integrity of the final product. The application of Non Destructive Evaluation (NDE) methods to guarantee that ADI components have consistent structure and properties should help the concerned SMEs foundries in the development of a new market for mass produced ADI. Therefore in this project, NDE methods will be chosen (Eddy Current, Barkhausen Noise,...) and applied on ADI test samples in order to characterize their structure. Then, the same NDE methods will be applied on real parts, and the results should confirm the selection of an adequate and accurate method to characterize ADI structure by these techniques. The objectives of the project are (1) the implementation of a new technology for ADI control, based on NDE methods, (2) the development of ADI in large production volume, (3) the creation of new markets by the replacement of forged, case hardened steel, and (4) the markets extension, where improved component performance would be achieved by the replacement of existing (ductile) cast iron castings by austempered materials. The interest of the project consist in three points mainly:1. Industrial interest:* Optimisation of the ADI structure and properties. * Development of new NDE methods for ADI control. 2. Economical interest: *Cost reduction (replacement of other materials), and development of new markets*Improvement of ADI product quality. 3. Environmental and social interest:* To increase the workers level (due to the implementation of advanced testing methods.) * To improve the effective quality assurance system in ADI production
Funding SchemeCRS - Cooperative research contracts
B71 4ND West Brownwich
B15 2TT Birmingham
DY9 7AN Stourbridge