Fractographic analysis of austempered ductile iron

Abstract

A B S T R A C T This investigation involves a systematic study of the fracture surfaces of two grades of austempered ductile iron (ADI) broken under quasi-static, dynamic and cyclic loading conditions. The study used electron microscopy, optical microscopy and image postprocessing. The results show that the predominating fracture mechanism in ADI upon impact loading changes from quasi-cleavage to ductile (with little areas of cleavage facets) as the testing temperature increases. Noticeably, even at the lower temperatures tested, the fracture surface of ADI shows clear signs of ductile fracture mechanisms. In particular, graphite nodule cavities suffer marked plastic deformation. Fracture after bending tests at room temperature was characterized by a mix of quasi-cleavage facets, deformation of the contour of nodular cavities and microvoid coalescence. In the case of fatigue fracture at room temperature, the fracture surfaces show a flat appearance which has notorious differences with those reported for other loading conditions, but the typical fatigue striations were not found. The particular features identified on the fatigue fracture surfaces can be used to identify fatigue failures. It was also shown that the determination of the direction of main crack propagation by using the experimental methodology proposed earlier by the authors is applicable to ADI fractured by impact and quasi-static loads.The results provide information potentially useful to fractographic analyses of ADI, particularly in samples that fail in service under unknown conditions.Keywords austempered ductile iron; bending; fatigue; fractography; fracture surface; impact.N O M E N C L A T U R E L' = Projected length of the fracture profile Lt = Fracture profile length Rs = Superficial roughness Ry = Peak to valley ratio from the fracture profile Y = Length of the nodular contour in parallel axis to the main crack propagation direction X = Length of the nodular contour in perpendicular axis to the main crack propagation direction Β = Angle between the major axis of the equivalent ellipse and the main crack propagation direction I N T R O D U C T I O NThe austempered ductile iron (ADI) is a member of the ductile iron (DI) family which is produced by means of an austempering heat treatment. The different grades are produced primarily by varying the austempering temperature and duration of the heat treatment cycle. 1 Nearly twice as strong as pearlitic DI, ADI still retains high elongation and toughness. This combination provides a material with superior tensile strength, wear resistance and fatigue strength. Its usual applications include gears, crankshafts, connecting rods, agricultural and construction ground engaging parts such as plow points, digger teeth and track shoes, shafts; cams; mining and grinding equipment such as balls and hammers; mill liners and rollers; spring hanger brackets and railroad wear applications. Because of its outstanding mechanical properties, ADI represents an excellent choice of replacement for

Microstrain measurements and damage analysis during tensile loading of intercritical austempered ductile iron

“…This observation agree with the classical ductile damage mechanisms for ferrite 6,8,13 . On the other hand, for tregion II, the crack propagates earlier than in region I, following the ausferrite‐ferrite interface (as is pointed out by white arrows), probably by a quasi‐cleavage mechanisms commonly found in ausferrite 6,26,27 . As a result, it is expected that when the crack propagates through a ferritic area, a ductile damage involving void nucleation and necking of the contour of nodular cavities will be observed.…”

Microstructural damage evaluation of ferritic-ausferritic spheroidal graphite cast iron

“…The resulting fracture surface, Fig. 8a, shows a combination between the well-known ductile damage mechanism of Ferritic SGI [13,15,17] and quasi-cleavage fracture mechanism for ADI [16,17] resulting from quasi-static loading conditions. These observations support that there is a competition between the plastic deformation at the internodular zones and the crack propagation along the ausferrite during fracture.…”

Micromechanical impact of solidification regions in ductile iron revealed via a 3D strain partitioning analysis method

“…However, compelling micromechanical evidence unveiling the mechanisms by which the FTS and LTS regions can impact the deformation and fracture process of DI has not been reported so far. To the authors' best knowledge, the only studies on this topic were conducted by Fernandino et al by means of 2D micromechanical simulations [6] and in-situ surface observations supported by digital image correlation [7]. These authors concluded that, during tensile loading of DI, the subdivision between FTS and LTS regions does not play a role at strains below 1 %, whereas it becomes important towards the end of the tensile test in determining the fracture path.…”