Contribution of dual phase (α, G, B) heat treatments to fatigue properties of SG cast irons

Abstract

Ferritic spheroidal graphite (SG) cast irons are used to make parts that require high ductility and fracture toughness and good high-cycle fatigue resistance. Ferritising anneals have an all-ferritic matrix favouring high fracture toughness and ductility but this does not improve fatigue strength, and even lowers it in some cases. The purpose of the present study was therefore to optimise these heat treatments for the best compromise between fracture toughness and fatigue life. To attain this, the fatigue behaviour of cast irons was studied in the as cast and heat-treated conditions. A comparison of the lives and mechanisms of ferritisation of these metallurgical conditions made it possible to evaluate the influence of various parameters (characteristics of the initiation sites, properties of the matrix). From the present study, a method is proposed to optimise the heat treatments to produce ferrite in association with a stronger phase in the zones where microfissures are initiated. A study of heat treatments favouring the formation of a matrix made up of ferrite and bainite yielded encouraging results. The presence of bainite near the graphite spheroids and zones of porosity can increase resistance to crack initiation. This leads to a substantial improvement of fatigue life of the SG cast iron without at the same time embrittling the material.IJC/578

Mechanical Characterization of Dual Phase Austempered Ductile Iron

“…The D1 samples containing a small amount (20 %) of ausferrite located in the last to freeze zones exhibit a noticeable increase if compared to fully ferritic samples (F). These results are consistent with the values accounted for by Verdu et al, 9) who reported an increase in the endurance limit of around Table 2. 25 % for Dual Phase ADI with small percentages of ausferrite located around the graphite nodules, if compared to those in the fully ferritic DI.…”

“…Verdu et al 9) centered on the development of thermal cycles whose aim was to encapsulate graphite nodules with different amounts of ausferrite as a way to enhance mechanical and fatigue properties. These authors sought to favor the mechanical properties of ferritic DI by surrounding graphite nodules with a high resistant second phase, thus providing higher resistance to the crack initiation stage.…”

“…9) Considering that Dual Phase ADI parts manufacturers and users need to be aware of the quantitative values of the different mechanical properties to be used as design data, the main objective of this work is to provide a complete characterization of Dual Phase ADI containing different percentages of ferrite and ausferrite. The studies include the evaluation of fatigue behavior along with that of other mechanical properties (tensile strength, yield stress, elongation and fracture toughness).…”

“…8, in which the tensile strength-deformation until failure ratio is shown for DI with different microstructures, considering our results as well as those reported by other authors. [4][5][6][7][8][9] …”

High Silicon Ductile Iron: Possible Uses in the Production of Parts with ^|^ldquo;Dual Phase ADI^|^rdquo; Microstructure

“…This improvement is explained by the novelty of this DI microstructure, which is composed of different amounts of free (or allotriomorphic) ferrite and ausferrite (ADI regular structure). [1][2][3][4][5][6][7][8][9][10][11] Special thermal cycles have been developed by different researchers in order to obtain Dual Phase ADI. One of them has been advanced and used by Kilicli et al 7,8) and Basso et al [4][5][6] It consists in subjecting fully ferritic DI parts to an incomplete austenitization stage, at different temperatures within the intercritical interval of the Fe-C-Si diagram (see Fig.…”