Development of Ni-free austenitic and duplex ductile irons

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Cast iron is commonly known as a metal matrix composite containing graphite particles distributed as an embedded constitute within the matrix. Due to its relatively low production cost, excellent thermal conductivity and good tribological performance, it is widely used in applications where the component is exposed to wear and heat. The foremost reason for good tribological properties is attributed to the presence of graphite as a solid phase self-lubricating agent in the matrix. Additional improvement of the tribological properties of cast iron is being achieved by obtaining an austenitic matrix in it. The austenitic cast iron is a series of materials known as Ni-Resist cast irons, which typically have a high nickel content of 18 to 36%. The primary aim of this proposal is to introduce a new alloying-concept for producing ductile irons with austenitic as well as with duplex (austenitic-ferritic) structure without alloying with Ni. The concept of stabilizing the austenite in this proposal relies principally on saturating it with the maximum possible carbon content by applying a high temperature austenitization, which is followed by quenching to room temperature (RT). Additionally, an adequate amount of Mn is to be added for further suppressing of Ms to be below RT and hence obtaining austenite matrix at RT. Another variant, in which production of ductile iron with duplex matrix structure is also intended. In this variant the Si level is increased to shift the “ferrite + austenite” intercritical region to high temperature and hence a saturated “intercritical austenite” with high carbon level is obtainable. By quenching, a novel matrix of austenite and ferrite can be produced. In this proposal the composition-temperature ranges within the predicted Si-based phase diagrams that are necessary to obtain the duplex structure with sufficiently stabilized austenite (Duplex Window) are defined. Upon mechanical loading of the duplex matrix, the metastable austenite fraction progressively transforms into martensite, thereby increasing the work hardening rate at higher strain levels (TRIP Effect). This expectedly superior strain hardening characteristics would be beneficial for both mechanical and tribological behavior of the duplex ductile iron. In the framework of the proposed project, the alloy compositions and processing parameters for economic alloying of the austenitic and duplex ductile irons are to be defined. Additionally, the effects of Mn and Si contents and the processing routes on the microstructural development and the mechanical behavior of the developed materials are to be investigated.