The Nature of the Tensile Fracture in Austempered Ductile Iron with Dual Matrix Microstructure

Abstract

The tensile fracture characteristics of austempered ductile irons with dual matrix structures and different ausferrite volume fractions have been studied for an unalloyed ductile cast iron containing (in wt.%) 3.50 C, 2.63 Si, 0.318 Mn, and 0.047 Mg. Specimens were intercritically austenitized (partially austenitized) in two phase region (a + c) at various temperatures for 20 min and then quenched into a salt bath held at austempering temperature of 365°C for various times and then air cooled to room temperature to obtain various ausferrite volume fractions. Conventionally austempered specimens with fully ausferritic matrix and unalloyed as-cast specimens having fully ferritic structures were also tested for comparison. In dual matrix structures, results showed that the volume fraction of proeutectoid ferrite, new (epitaxial) ferrite, and ausferrite [bainitic ferrite + high-carbon austenite (stabilized or transformed austenite)] can be controlled to influence the strength and ductility. Generally, microvoids nucleation is initiated at the interface between the graphite nodules and the surrounding ferritic structure and at the grain boundary junctions in the fully ferritic microstructure. Debonding of the graphite nodules from the surrounding matrix structure was evident. The continuity of the ausferritic structure along the intercellular boundaries plays an important role in determining the fracture behavior of austempered ductile iron with different ausferrite volume fractions. The different fracture mechanisms correspond to the different levels of ausferrite volume fractions. With increasing continuity of the ausferritic structure, fracture pattern changed from ductile to moderate ductile nature. On the other hand, in the conventionally austempered samples with a fully ausferritic structure, the fracture mode was a mixture of quasi-cleavage and a dimple pattern. Microvoid coalescence was the dominant form of fracture in all structures.

Effect of Microstructure on Fatigue Strength of Intercritically Austenitized and Austempered Ductile Irons with Dual Matrix Structures

“…The austempering time 120 min were selected because of the previous studies showed that the optimum tensile and ductility combinations of ADI with DMS obtained this austempering time. 4,5,9,10) It may be interpreted as a useful range or process window for present ADI to obtain ideal ausferritic structure.…”

“…This result is in good agreement with the existing literature. [4][5][6][7][8][9][10][11][12][13][14][15] Austempering from different ICATs produced a DMS consisting of ausferrite + proeutectoid ferrite at the specimens with different AFVFs restricted to eutectic cell boundaries and an isolated or continuous network of ausferritic structure along the eutectic cell boundary formed depending on AFVF (Figs. 8 to 10).…”

“…In the newly developed ductile cast iron with dual matrix structure, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] the structure consists of proeutectoid ferrite, and ausferrite (bainitic ferrite and high carbon austenite (it is also called retained, stable or transformed austenite) or martensite. Therefore it is also called Dual Matrix Structure (DMS).…”

“…Therefore it is also called Dual Matrix Structure (DMS). [1][2][3][4][5][6][7][8][9][10][11] Austempered ductile iron (ADI) with DMS has been found to be validated for suspension parts of the automotive, owing to its greater ductility than the conventionally heat treated austempered ductile iron. This material showed tensile and proof stress, together with hardness, similar to pearlitic grades, and ductility at the same level as ferritic grades.…”

“…Authors had examined the phase transformations, microstructure, mechanical properties (tensile and hardness) and wear behavior of ADI with DMS comprehensively. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Fatigue properties of ADI with DMS matrix was studied by Verdu et al 41) reported prior to 2005. They produce DMS matrix in ADI using short austenization time (incomplete austenization) following by austempering in their research.…”