Damaging micromechanisms characterization in a ferritic-pearlitic ductile cast iron

Abstract

ABSTRACT. The analysis of the damaging micromechanisms in Ductile Cast Irons is often focused on ferritic matrix. Up to ten years ago, for this grades of DCIs, the main damaging micromechanism was identified with the graphite elements -ferritic matrix debonding. More recent experimental results showed the presence of an internal gradient of mechanical properties in the graphite elements and the importance of other damaging micromechanisms, with a negligible importance of the graphite elements -ferritic matrix debonding mechanism. In this work, damaging micromechanisms development in a ferritic -pearlitic DCI have been investigated by means of tensile tests performed on mini-tensile specimens and observing the specimens lateral surfaces by means of a scanning electro microscope (SEM) during the tests ("in-situ" tests). Experimental results have been compared with the damaging micromechanisms observed in fully ferritic and fully pearlitic DCIs.

Characterisation of the damaging micromechanisms in a pearlitic ductile cast iron and damage assessment by acoustic emission testing

“…The matrix‐nodule debonding was exhibited by a very large number of the investigated nodules (85%); this occurred together with both the other mechanisms in a large number of cases (77% of such cases). It is recalled that this mechanism was seldom observed in ferritic DCIs . Onion‐like mechanism was observed in a large number of nodules (70%), whereas disgregation occurred in approximately half of the nodules (54%).…”

“…The elementary damaging micromechanisms identified in ferritic DCIs were also found in pearlitic DCI, even though with different patterns. Such differences might be caused by the different interaction between the matrix and the nodules because of the different matrix microstructures of ferritic and pearlitic DCIs . The onset of the damage within the nodules (or at their interface with the matrix) began below the elastic limit, ie, corresponding to applied stress ranging from 450 to 600 MPa.…”

“…The classical matrix‐nodule debonding behaviour was rarely observed in ferritic DCIs, and multiple damaging micromechanisms were found to be more significant . Figure shows the (A) undeformed and (B) deformed conditions of a graphite nodule embedded within a ferritic DCI subjected to tension loading . The recently identified onion‐like and disgregation mechanisms are highlighted in Figure B.…”

“…4,10 Figure 1 shows the (A) undeformed and (B) deformed conditions of a graphite nodule embedded within a ferritic DCI subjected to tension loading. 4,11 The recently identified onion-like and disgregation mechanisms are highlighted in Figure 1B. The onion-like mechanism defines the initiation and propagation of cracks between the nodule core and the external crown according to the fibrous shape of the onions.…”

Graphite nodules and fatigue crack propagation micromechanisms in a ferritic ductile cast iron

“…Considering the approach that will be followed in this work, some considerations on the graphite nodules nucleation and growth are necessary. Magnesium is the most common spheroidizing element used in the DCI production: other elements such as Si, Ca and rare earths are commonly added to reduce the reaction violence and to promote graphite nodulizing, controlling the effect of impurities on nodules morphology and the matrix microstructure . Different graphite element nucleation theories are proposed in literature, mainly on the basis of ‘heterogeneous mechanisms’ (e.g.…”