with an elongation of about 18%.16
Regarding the influence
of the section size on mechanical properties, tensile strength and elongation until failure decrease as the cast part size increases, while the yield stress remains unchanged. Elon- gation until failure was also affected by the cast part size.17 Tadayon et al.23
studied the mechanical properties of dual
phase ADI microstructures austempered at different tem- peratures, ranging from 662F (350C) to 743F (395C). The authors reported that suitable tensile properties could be achieved by austenitizing at 1562F (850C) and austemper- ing at 743F (395C).
After reviewing the papers referenced above, it can be con- cluded that dual phase ADI can combine interesting me- chanical properties, especially good tensile strength/elonga- tion until failure relationships, when compared to other con- ventional DI matrices. This behavior has been described in a previous work by the authors24
3, where the dual phase ADI region is added to a tensile strength/elongation until failure graph for the different DI types. Several points obtained by Basso et al.16, 17
showing
the effect of austempering temperatures also are included. Impact and Fracture Toughness Properties
So far, few results have been reported in the bibliography about impact and fracture toughness properties of dual phase ADI. Basso et al.16
studied the impact energy absorbed in unnotched
Charpy tests for dual phase ADI with different amounts of ferrite and ausferrite in the microstructure, austempered at 662F (350C). The results did not reveal noticeable variations for the dual phase ADIs studied, showing that impact energy values, in all cases, lie within the range values reported in the literature for fully ferritic DI and ADI (i.e., between 115 and 145 J/cm2
).25 Verdu et al.15 also reported that impact strength
remains unchanged, or is even slightly higher when compared to fully ferritic DI. Moreover, he stated that new microstruc- tures do not embrittle the material.
Valdés et al.21 reported a similar tendency in the impact prop-
erties of dual phase ADI. The best results of impact strength were found when the ductile irons were heat treated in the intercritical region between 1427 - 1526F (800 – 830C) and austempered at 707F (375C). Impact strengths of 130 - 145 J and 100 - 140 J were obtained for unalloyed and alloyed ductile iron, respectively.
The fracture toughness properties of this new type of DI have been characterized by Basso et al.16
for different dual phase ADI matrices. These values were com- pared to fully ferritic DI and ADI austempered at the same temperature, which were used as reference values. Figure 4 summarizes these values. The authors reported that the frac- ture toughness (KIC
who reported KIC and is summarized in Figure values
ferritic samples, which is in agreement with the data accounted for in the literature.25
increases with the ausferrite content in the microstructure. The curve begins with the lower KIC
) for different variants of dual phase ADI values corresponding to fully
in the matrix promotes higher values of KIC
mately 14% and was considered promising since this kind of dual phase ADI combines high strength and elongation values (typical for ADI) with enhanced fracture toughness properties. On the other hand, the authors also reported the relationship (KIC
This result improved the ADI KIC
its highest value for dual phase ADI matrices composed of al- lotriomorphic ferrite and small amounts of ausferrite (less than 25%). This was attributed to the encapsulating effect of the aus- ferrite located around the last-to-freeze zones. These areas have the highest concentrations of small casting defects, such as,
/σYS
to the critical crack size (see Figure 4). This index proves to be useful when measuring the relative toughness of the material, given the fact that it is desirable that a part in service tolerates large flaws without fracture. The relationship (KIC
/σYS )2 yielded )2 , which is a fractomechanical parameter proportional
An increase in the amount of ausferrite , up to the value
found for the fully ausferritic matrix. It is worth pointing out that a peak value corresponding to samples with mainly ausfer- ritic matrices and ferrite acting as second disperse phase was found.16
values by approxi-
Figure 3. Tensile strength vs. elongation until failure for DI with different microstructures.20
International Journal of Metalcasting/Winter 2012
different kinds of dual phase ADI. Comparison with fully ferritic DI and ADI properties.
Figure 4. Review of KIC and (KIC/σYS)2 11
values for
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