Application of Austempered Ductile Iron in Hydraulic Components with Dynamic Loads

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Austempered ductile iron (ADI) and isothermed ductile iron (IDI) are heat treated ductile irons (GJS). The heat treatment of ADI consist of the steps austenitising, quenching and isothermal austempering. During the austenitisation the austenite enriches with carbon. As the austenite partially transforms into ferrite during the isothermal austempering, the carbon enrichment of the remaining austenite increases making it stable at room temperature. IDI on the other side is intercritically austenitised followed by a controlled cooling. The cooling rate is selected in a way that pearlite forms. The microstructure of IDI therefore consist of proeutectoid ferrite and pearlite.Compared to the base material (GJS) the tensile strength or elongation of ADI can be increased by factor two or more. IDI shows mechanical properties between those of ADI and ductile iron. These materials are therefore interesting for components with hydraulic loads such as valve control clocks. As increasing hydraulic pressures along with constant design spaces are required for these components, the increase of fatigue properties is limited with standard ductile irons.The principle aim of the research project therefore is to increase the fatigue properties of components, which are exposed to dynamic oil pressures by the application of ADI. As a possible cost-effective alternative IDI will also be tested for this application.On the basis of extensive preliminary work, the heat treatment of the used materials will be characterised by neutron diffraction. The combination of preliminary work and new data allows a sound alloy decision with respect to the requirements of the demonstrator. An appropriate heat treatment strategy will be developed using static and dynamic tests. On the basis of these results ADI and IDI samples will be produced, which will be tested under hydrodynamic loads.The samples of the numerous dynamic tests will be used for the analysis of transformation of austenite to martensite under dynamic loads. The fractured samples will be analysed using metallography and diffraction in order to investigate the supposed correlation of transformation and crack propagation.