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inclusions, porosity, carbides, etc. (see Figure 5). In this case, ausferrite acts as a reinforcing phase of the weakest zones, in- creasing fracture toughness.


Fatigue Properties


The first results about fatigue properties of dual phase ADI reported in the literature corresponded to Verdu et al.15


As


mentioned above, the authors centered on the development of thermal cycles, to obtain dual phase ADIs with graphite nodules surrounded by different amounts of ausferrite as a way to enhance mechanical and fatigue properties. The au- thors sought to favor the fatigue properties of ferritic DI by surrounding graphite nodules with ausferrite, a high resistant second phase, thus providing higher resistance to the crack initiation stage. They found that, microstructures composed of 80% ferrite and 20% ausferrite (austempered at 707F [375C]) yielded values as high as 25% in fatigue life com- pared to those of fully ferritic DI.15


Basso et al.23 also worked in this area using four different dual


phase ADI matrices austempered at 350C (662F), containing different percentages of ferrite and ausferrite. The authors reported the endurance limit as a function of the amount of ausferrite in the dual phase ADI matrix, and it was compared against fully ferritic DI and ADI values (see Figure 6). The fa- tigue tests revealed that an increase in the ausferrite percentage incremented the endurance limits compared to a fully ferritic matrix. A small amount of ausferrite (20% approximately) in the microstructure, in this case located mainly around last- to-freeze zones, improved the endurance limit around 25%. These results confirm the effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze zones and are similar to the values accounted for by Verdu et al.15


Higher amounts of


ausferrite in the dual phase ADI microstructures increase the endurance limit, reaching values corresponding to ADI. Fig- ure 6 summarizes the results of the fatigue tests. In the figure,


The microstructure of this new kind of ductile iron can be composed of different amounts and morphologies of free ferrite and ausferrite, depending on the parameters of the thermal cycle employed. Heat treatments involving a partial austenitizing stage within the intercritical interval followed by austempering led to the production of dual phase ADI structures with controlled amounts of microconstituents.


It was found that dual phase ADI can confer a wide range of mechanical properties based on the relative percentage of microconstituents present in the matrix. As a general rule, when the amount of ausferrite increases, tensile strength, yield stress and fracture toughness increase too, while elon- gation slightly diminishes.


The best combinations of mechanical properties for these new mixed structures were found when using high austem- pering temperatures, around 662F (350C). In particular, dual phase ADI microstructures containing approximately 20% of ausferrite in their microstructures have widely superseded the mechanical and fatigue properties of fully ferritic DI, at the same time, preserving a high ductility. This combination of properties is appreciated in safety parts.


Dual phase ADI offers a wide range of mechanical prop- erties in view of the relative microconstituents percentage present in the matrix and could replace other conventional microstructures (fully ferritic, ferritic-pearlitic, pearlitic, or tempered martensitic) since dual phase ADI provides en- hanced mechanical properties combinations.


A review of the main mechanical properties and methodologies employed to obtain dual phase ADI ductile irons is presented.


values of endurance limits corresponding to fully ferritic DI and ADI reported in the bibliography 25-29


are added. Conclusions


Figure 5. Last-to-freeze zones encapsulated with ausferrite. 12


Figure 6. Endurance limit vs. % of ausferrite in matrix in dual phase ADI. Values corresponding to fully ferritic DI and ADI are added.


International Journal of Metalcasting/Winter 2012


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