Effect of ausferrite volume fraction and morphology on tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

Abstract

In the present study, an unalloyed ductile iron containing 3?50%C, 2?63%Si, 0?318%Mn and 0?047%Mg was intercritically austenitised (partially austenitised) in the two phase region (azc) at temperatures of 795 and 815uC for 20 min, and then was quenched into a salt bath held at an austempering temperature of 365uC for various times to obtain various ausferrite volume fractions. Fine and coarse dual matrix structures were obtained from the two different starting conditions. Some specimens were also conventionally austempered from 900uC for comparison. Results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferritezhigh carbon retained or stabilised austenite) was developed. The phase previously described as new ferrite (also called epitaxial ferrite) was observed to form following heat treatment only in specimens with coarse austenite dispersion after austempering from the (azc) temperature range. It was observed that the parent austenite dispersion present at the intercritical austenitising temperature has an effect on the volume fraction of high carbon austenite following austempering. Finer austenite dispersions produced more stabilised high carbon austenite than coarse ones for a given austempering time. The volume fractions of proeutectoid ferrite, new ferrite, ausferrite can be controlled to influence the strength and ductility. Specimens austempered from the (azc) temperature range exhibited much greater ductility than conventionally austempered samples. Within each of the series austempered from the (azc) temperature range, the tensile strength increased and ductility decreased with increasing ausferrite volume fraction and decreasing proeutectoid ferrite volume fraction. However, specimens having a fine ausferritic structure without new ferrite exhibited lower ductility and higher strength than specimens with a coarse ausferritic structure under the same austempering conditions. The specimen with ,60% AFVF (coarse structure) and ,65% AFVF (fine structure) exhibited the best combination of high strength and ductility compared to pearlitic grades, but their ductility was slightly lower than ferritic grades. These materials satisfy the requirements for the strength of quenched and tempered grades and their ductility superior to that of this grade.

Mechanical Characterization of Dual Phase Austempered Ductile Iron

“…[4][5][6][7] An adequate handling of the heat treatment variables allows to control the amount, morphology and distribution of the micro constituents (free ferrite and ausferrite), thereby enabling to attain a wide range of mechanical properties.…”

“…The results have revealed that a strong relationship exists between tensile strength and elongation until failure in the case of some Dual Phase ADI microstructures. [4][5][6][7][8] Along these lines, fracture toughness tests have yielded interesting values when compared to those of fully ferritic or fully ausferritic matrices. 4) In view of the fact that many mechanical components used in the industry need tolerate cyclic loads at elevated stresses; another property to be analyzed is fatigue resistance.…”

“…The number of cycles for run-out was set at 1ϫ10 7 cycles. The sample "endurance limit" was defined as the highest stress level at which non failure occurred after 1ϫ10 7 cycles. Tensile tests were conducted in agreement with ASTM E8M/88 standard, using a universal testing machine (IN-STRON 8501).…”

“…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.…”