13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Ferritic-pearlitic ductile cast irons: is ΔK a useful parameter? Francesco Iacoviello1,*, Vittorio Di Cocco1, Alessandra Rossi1, Mauro Cavallini2 1 Università di Cassino e del Lazio Meridionale, DICeM, via G. Di Biasio 43, 03043, Cassino (FR), Italy 2 Università di Roma “La Sapienza”, DICMA, via Eudossiana 18, Roma, Italy * Corresponding author: iacoviello@unicas.it Abstract In the last years, the role played by graphite nodules was deeply investigated by means of tensile and fatigue tests, performing scanning electron microscope (SEM) observations of specimens lateral surfaces during the tests (“in situ” tests). According to the experimental results, it is evident that graphite nodules damaging micromechanisms can’t be merely classified as matrix nodule debonding, but depend on different parameters (e.g., loading conditions and matrix microstructure). In this work, the influence of microstructure and loading conditions on fatigue crack propagation resistance in DCIs is discussed. On the basis of experimental results, the applicability of ASTM E399 standard on the characterization of fatigue crack propagation resistance in ferritic-pearlitic DCIs is critically analyzed, mainly focusing the stress intensity factor amplitude role. Keywords Fatigue crack propagation; Ductile cast irons; Damaging micromechanism; Stress intensity factor. 1. Introduction In 1943, during the American Foundry Society (AFS) convention, J. W. Bolton made the following statements: Your indulgence is requested to permit the posing of one question. Will real control of graphite shape be realized in gray iron? Visualize a material, possessing (as-cast) graphite flakes or groupings resembling those of malleable iron instead of elongated flakes [1]. A few weeks later, in the International Nickel Company Research Laboratory, an addition of magnesium (as a copper-magnesium alloy) allowed to obtain nearly perfect spheres of graphite and, consequently, the first ductile cast iron (DCI), also known as nodular cast iron [2]. In 1948, always in the AFS Convention, the production of spherical graphite in iron by the addition of small amounts of cerium was announced by Henton Morrogh of the British Cast Iron Research Association. The first advantage of this production technique is evident: no high temperature/long duration heat treatments are necessary to obtain the desired shape of graphite elements (nodules), with a consequent strong cost reduction and really interesting mechanical properties: DCIs are able to combine the good castability of gray irons and the toughness of steels! In the last decades, different chemical compositions and heat treatments have been optimized in order to control the matrix microstructure and obtain different combinations of mechanical properties. Nowadays, DCIs are mainly used in the form of ductile iron pipes (for transportation of raw and tap water, sewage, slurries and process chemicals), but they are also widely used in safety related components for automotive applications (gears, bushings, suspension, brakes, steering, crankshafts) and in more critical applications as containers for storage and transportation of nuclear wastes. Matrix controls mechanical properties and matrix names are used to designate spheroidal cast iron types [3-5]. Ferritic DCI are characterized by good ductility and a tensile strength that is equivalent to a low carbon steel. Pearlitic DCIs show high strength, good wear resistance and moderate ductility. Ferritic–pearlitic grades properties are intermediate between ferritic and pearlitic ones. Martensitic ductile irons show very high strength, but low levels of toughness and ductility. Bainitic grades are characterized by a high hardness. Austenitic ductile irons show good corrosion resistance, good strength and dimensional stability at high temperature. Austempered grades show a very high wear resistance and fatigue strength. Focusing the graphite nodules, Magnesium is the most common spheroidizing element used in the DCIs production: other elements like Si, Ca and rare earths are commonly added to reduce the
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