Issue 13

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 13 (2010) 3-16; DOI: 10.3221/IGF-ESIS.13.01 4 After more than fifty years, ductile iron should be considered as a family of materials offering a wide range of properties depending on the chemical composition and heat treatment and the consequent microstructure modifications. Matrix microstructure importance is emphasized by the use of matrix names to commonly designate the different types of ductile irons (Fig. 1): - Ferritic DCI: this DCI is characterized by a good ductility and impact resistance; ultimate tensile and yield strength are equivalent to a low carbon steel. - Pearlitic DCI: a pearlitic DCI is characterized by high strenght, good wear resistance and reduced ductility and impact resistance. - Ferritic-pearlitic DCIs: these are the most common DCI; properties are intermediate between ferritic and pearlitic grades (Fig. 2), and good machinability is obtained with low production costs. - Austenitic DCI: this DCI shows a high corrosion and oxidation resistance, with good strength and dimensional stability at high temperature. - Martensitic DCI: these DCI are obtained controlling both the chemical composition (to prevent pearlite formation) and the heat treatment (quench and temper): very high strength and wear resistance are obtained, but with lower values of ductility and toughness. - Bainitic DCI: this DCI is obtained controlling chemical composition and/or heat treatment: the result is a hard and wear resistant material. - Austempered DCI (also ADI): ADI are obtained after an austempering heat treatment, with very high tensile strength values (twice than a pearlitic DCI), high elongation and toughness. Figure 1 : DCI microstructures (different magnifications). From left to right: ferritic, ferritic-pearlitic, pearlitic, martensitic, bainitic, tempered, austempered (UTS = 1050 MPa), austempered ( UTS = 1600 MPa), austenitic [1]. Figure 2 : Elastic and yielding behavior for steel, gray iron and ferritic and pearlitic DCIs [1]. Focusing fatigue crack propagation resistance, references results show an evident influence of matrix microstructure, graphite elements morphology, size and volume fraction and chemical composition [3-10]. The aim of this work is the analysis of microstructure influence on fatigue crack propagation micromechanisms, considering different loading conditions (applied  K and R). Five different DCI were analyzed [11-21]: four DCI were characterized by a ferritic pearlitic matrix (different ferrite and pearlite volume fractions); the fifth investigated DCI was an austempered one.