been extensively proven that the kinetic transformation of ferrite into austenite occurs very quickly-45 minutes is enough to reach the equilibrium. Then, it is possible to ob- tain accurate percentages of phases in the microstructure as a function of the intercritical austenitizing temperature. In- deed this is of special relevance when industrial production is considered, as it does not depend on time (austenitizing time in this case).
Mechanical Properties of Dual Phase ADI Tensile Properties
Due to the technological interest in the new kind of DI, sev- eral studies have been conducted to determine its mechani- cal response, centering mainly on evaluating tensile prop- erties. A review of the main mechanical properties (tensile strength, yield stress, elongation until failure and hardness) obtained by the authors cited above for dual phase ADI are particularized in Table 1. The mechanical properties corre- spond to dual phase ADI structures obtained from heat treat- ments such as those particularized above, starting from fully
ferritic matrices (except those reported by Druschitz et al. 19, 20
and Valdés et al.21 who employed ferritic-pearlitic as-
cast matrices). In all cases, the chemical composition of the DI melts were in the following range: C: 3.2-3.6%; Si: 2.3- 3.0%; Mn: 0.1-0.5%; Mg: 0.03-0.05%; Cu: < 1%; Ni: <1%.
The results reveal that dual phase ADI can offer a wide range of mechanical properties depending on the relative percent- age of microconstituents present in the matrix (Table 1). Aranzabal et al.13
developed dual phase ADI for use in the
production of suspension automotive parts and reported re- markable improvements in mechanical properties. The re- ported results revealed that the yield stress, tensile strength and hardness values were similar to those of fully pearlitic DI, while ductility kept the same level of ferritic DI. Wade et al.14
and Verdu et al.15 studied the mechanical properties of
dual phase ADI microstructures austempered at 707F (375C), with ferrite as the majority phase and encapsulating graphite nodules with ausferrite. These authors focused on improv- ing the mechanical properties of ferritic DI, by surrounding graphite nodules (which can be considered a phase with no resistance) with a high resistant second phase (in this case, ausferrite). They found that yield stress, tensile strength and hardness increased when the ausferrite volume fraction did so in the microstructure. In particular, the presence of 20% ausferrite in the microstructure achieved increments in tensile strength and yielding stress of about 30 percent, compared to fully ferritic DI. Kilicli et al.18
and Sahin et al.22 explored the
mechanical properties of dual phase ADI on a wide range of microstructures composed of different ausferrite volume frac- tions and morphologies austempered at 689F (365C). These microstructures showed, once again, that the yield stress and tensile strength increased when the quantity of ausferrite was
(a)
(b)
(c)
(d)
(e)
Figure 2. Microstructures obtained for samples partially austenitized at different temperatures within the intercritical interval and then austempered at 662F (350C). a: 92% Ferrite-8% Ausferrite; b: 80% Ferrite-20% Ausferrite, c: 50% Ferrite-50% Ausferrite, d: 35% Ferrite-65% Ausferrite, e: 15% Ferrite-85% Ausferrite. (The graphite areas were not considered in the percentage of the reported microconstituents.)
International Journal of Metalcasting/Winter 2012 9
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