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Typical ductile cast iron microstructure, showing nodular graphite in ferrite and pearlite matrix

Typical ductile cast iron microstructure, showing nodular graphite in ferrite and pearlite matrix

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The article presents the outcome of a comprehensive program of tensile, compression and fracture toughness experiments, addressing thick-walled ductile cast iron inserts used for the production of three nuclear waste canisters. The resulting data are required as input to the assessment of the failure probability of the canisters. Moreover, these da...

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... is a cast ferrous material in which the free graphite is present in a spherical form. Figure 2 shows a typical microstructure with the graphite nodules in a ferrite and pearlite matrix (Ref 3). Pearlite increases the materialÕs strength but, at the same time, lowers its ductility. ...
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... already stated, the canister inserts are mainly designed for compressive loading conditions. For this reason a limited series of compression tests was also carried out. In Fig. 12 the compressive true stress-true strain curves obtained for the three canister inserts are compared with three selected tensile true stress-true strain plots, corresponding to the same sampling orientation (top longitudinal). For the compression tests the curves could always be developed up to high strain values, as no fracture events ...
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... logarithmic fit was obtained when fracture surfaces only containing oxide films were considered-see Fig. 19. Beyond all it is clear that the presence of the oxide films, even with relatively small dimensions, systematically resulted in low elongation after fracture values, whatever the properties of the other variables were. This is made clear by Fig. 20, which includes all the I24, I25, and I26 oxide film measurements. When really small oxide film dimensions are considered (D eff < 2 mm), the elongation measurements show an increasing trend although the relation- ship becomes more scattered as the other affecting parameters start to influence A more significantly. An indication of the ...
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... small oxide film dimensions are considered (D eff < 2 mm), the elongation measurements show an increasing trend although the relation- ship becomes more scattered as the other affecting parameters start to influence A more significantly. An indication of the impact of one of these parameters-the areas with densely distributed graphite-is given by Fig. 21, from which oxide film containing fracture surfaces have been excluded. It is clear that the measured defective area must exceed relatively large values (~20%) before a detrimental effect can be observed with respect to the materialÕs ...
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... microstructural factors, it was found that pearlite content lowered the elongation after fracture measurements, whereas nodularity had the opposite effect. These trends are confirmed to a large extent by Fig. 22 and 23, in which only specimens including very small oxide films are considered (D eff < 1 mm). Again it is difficult to come to overall conclu- sions quantifying all factors together but at least some interesting individual observations can be made, such ...
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... the small strain formulation was used in the computation. Figure 24 gives a schematic of the model geometry and the boundary conditions. The finite element mesh was very refined at the crack tip to account for the large local stress and strain concentra- tions. ...
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... tensile properties were modeled using a Ramberg-Osgood deformation plasticity model-i.e., the same as presented in Section 4.2.-with the parameters fitted to the tensile data given for each insert respectively. J-integral values were determined for increasing applied displacements u z (corre- sponding strain e 0 = 2u z /L) for models with varying defect diameter D. Figure 25 gives the calculated J)e 0 relation for a test specimen diameter Ø = 14 mm and for three distinct D values, this both for insert I24 and insert I26. The effect of the difference in tensile behavior between I24 and I26 is clearly visible, certainly for smaller defect diameters. ...
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... statistically average elongation after fracture data should be lower for large specimens. This size effect was not seen however in the laboratory tests, as mentioned in Section 4.1. This could partly be explained by the higher J-values obtained for the smaller specimens (Ø 9.5 mm), given a constant defect diameter. This finding is obvious from Fig. 27, which considers two defect diameters (2 and 4 ...

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Citations

... This microstructure feature adds impact resistance to ductile iron cast iron. [11][12][13][14][15]. Perlite is a product of ferrite with a carbide structure called cementite. ...
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... In fact, graphite degeneracy and chucky graphite can be also found in conventional DIs with silicon content lower than 5%wt., when very slow cooling rates occur in heavy section castings [18][19][20][21][22]. It has been reported that in conventional DIs chunky graphite decreases ductility and ultimate tensile strength [20,21], though the most detrimental effect of chunky graphite on mechanical properties is the fatigue resistance decrease [23][24][25][26][27]. However, besides chunky graphite, also other microstructure parameters like nodule count, size and roundness of the graphite nodules can affect the mechanical properties [8,[28][29][30][31][32]. ...
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... Longer solidification times cause an increased risk of formation of microstructural defects, with detrimental effect on the mechanical properties [7]. The most common defects that can be found in castings are non-metallic inclusions, shrinkage porosities, and undesired segregation or graphite particles that deviate from the spheroidal shape. ...
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... By increasing the thickness of the components, the solidification times will increase with the increased risk of finding coarse grains and anomalous structures. 2 In particular, the nodularity and the nodule count will decrease, while the dimensions of graphite particles will increase. Furthermore, the greater the casting dimensions, the greater the probability of finding degenerated graphite (exploded, chunky, or spiky), large Nomenclature: ...
... Compared with Deguchi's model (2) where only σ UTS was considered as representative of the material properties, the new equation takes into account the actual mechanical properties in terms of both the ultimate strength σ UTS and yield strength σ y 0.2% combined with the maximum value of the square root of the area of the initiating defect. The reason behind the choice of considering both the ultimate tensile strength and the yield strength in the fatigue assessment is to include in the fatigue strength evaluation the influence of the microstructural degeneration existing in the material. ...
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... Casting large volume parts of weight on the order of tens of tons is particularly critical as the control of the final microstructure is rather difficult [1][2][3]. In fact the resulting long solidification time introduces into the material intrinsic defects, such as solidification cavities or poor graphite nodularity, dross and degenerated (chunky and spiky) graphite, which can be tolerated up to a certain level. It was found that the increase in nonnodular graphite content decreases the mechanical strength; indeed, the ultimate tensile strength and mostly the elongation to fracture are severely lowered, despite hardness and yield strength remain nearly unaffected. ...
... Best-fit parameters of the crack propagation law expressed by Equation(2). ...
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... For alloys with silicon content lower than 3 wt%, it has been reported that chunky graphite decreases elongation at rupture and ultimate tensile strength without affecting yield strength [25,32,33]. However, the most problematic effect for engineering applications is certainly that chunky graphite does also decrease fatigue resistance [25,[34][35][36][37][38][39]. ...
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... This effect has been previously studied in [71]. However, from 150 to 500°C, the ductility of the material is reduced due to an oxidation phenomena, which was analyzed in the studies of [72][73][74]. On the other hand, Fig. 7b shows that for the HCS material, the lower fracture strain is presented at 150°C. ...
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