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
Highlights
Ductile cast irons (DCIs) are characterized by a good castability and good mechanical properties: this peculiar behaviour is obtained only by means of the chemical composition control [1] and not by means of long and expensive heat treatments
The increase of the macroscopic deformation implies the activation of different damaging micromechanisms with cracks that initiate and propagate in the graphite nodules (Fig. 4a), with the debonding at the interface ferritic shield – graphite nodules that is more and more evident (Fig. 4a and b) and with the presence of slip bands in the ferritic shield that, preferably, emanates corresponding to the graphite nodules equators (Fig. 5; different magnifications)
The second micromechanism has been already observed in ferritic Ductile Cast Irons (DCIs) and it has been called “onion-like” mechanisms: in ferritic DCIs it seems to be connected to the presence of a mechanical properties gradient inside the graphite nodules, with an outer shield that is characterized by higher nanohardness and wear resistance values with respect to an inner nucleus [12]
Summary
Ductile cast irons (DCIs) are characterized by a good castability and good mechanical properties (tensile strength, toughness, wear resistance): this peculiar behaviour is obtained only by means of the chemical composition control [1] (using Mg, Ca, or Ce) and not by means of long and expensive heat treatments (as in malleable irons). Chemical compositions and many different heat treatments have been optimized in order to obtain different metal matrix microstructure, obtaining different combination of mechanical properties as yield strength, ductility, toughness, wear and fatigue resistance. Ferritic, perlitic, ferritic-perlitic, martensitic, austenitic, bainitic and austempered ductile irons offer a wide range of mechanical properties at a lower cost than the older malleable iron.
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