An unalloyed ductile iron containing 3.42 C wt.%, 2.63 Si wt.% and 0.318 Mn wt.% was intercritically austenitized in two-phase region (α + γ) at various temperatures of 795 °C, and 815 °C for 20 min and then quenched into salt bath held at austempering temperature of 365 °C for various times to obtain austempered ductile iron (ADI) with different ausferrite volume fractions distributed in soft proeutectoid ferrite matrix. Some samples were also conventionally austempered for comparison reason. For austempered samples, the tensile strength and hardness decreased with increasing the austempering time, whereas the elongation increased considerably. In addition, the abrasive wear behavior of as cast and ADI's with dual matrix structure was studied under different loads without lubrication. It was found that the wear resistance of ADI with dual matrix structure was superior to that of as cast iron. Among the austempered samples, the lowest weight loss was observed for conventionally austempered samples with its wholly ausferritic structure throughout the specimen while the highest weight loss was obtained for the samples having the lowest ausferrite volume fractions in those samples with dual matrix structures. Moreover, wear resistance increased with increasing ausferrite volume fraction and decreasing austempering time in all tested samples.

Wear behavior of austempered ductile irons with dual matrix structures

In the present study, an unalloyed ductile iron containing Fe–3·50C–2·63Si–0·318Mn–0.047Mg (wt-%) were intercritically austenitised (partially austenitised) in two phase region α+γ at various temperatures of 795, 805, 815 and 830°C for 20 min and then quenched into salt bath held at austempering temperature of 365°C for various times to obtain different ausferrite volume fractions (AFVFs). Results showed that dual matrix structure containing proeutectoid ferrite, new ferrite (also called epitaxial ferrite) and ausferrite (bainitic ferrite+high carbon austenite, which is retained or stabilised austenite) has been developed. Within each of the austempered series in α+γ temperature range, new ferrite volume fraction increased with increasing intercritical austenitising temperature (ICAT). Although, transforming percentage of new ferrite from parent austenite present at ICAT increased with decreasing ICAT. Some specimens were also conventionally austempered from 900°C for comparison. The new ferrite ...

Tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

In the present work, the transformation characteristics of ductile iron austempered from intercritical austenitization temperature ranges were investigated. For this purpose, an unalloyed ductile cast iron containing 3.50 wt% C, 2.63 wt% Si and 0.318 wt% Mn were intercritically austenitized (partially austenitized) at various temperatures and then rapidly transformed to a salt bath held at the 365 °C for austempering for various times to produce dual matrix structure with different ausferrite volume fractions in ferrite matrix. A microstructure map was created to illustrate the transformation of products quantitatively as a function of austempering time for a particular intercritical and austempering heat treatment temperature and time. It was demonstrated that the total volume fraction of transformed phases was approximately constant for all austempering times after rapidly transforming samples from a particular intercritical temperature to austempering temperature. It was found out that the new ferrite (It is also called epitaxial ferrite) introduced into the intercritically austenitized structure during austempering and its content was dependent on the intercritical austenitizing temperature and austempering time.

Transformation characteristics of ductile iron austempered from intercritical austenitizing temperature ranges

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 (α + γ) at temperatures of 795 and 815°C for 20 min, and then was quenched into a salt bath held at an austempering temperature of 365°C 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 900°C for comparison. Results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high 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 (α + γ) temperature range. It was observed that the parent austenite dispersion present at th...

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

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 (α + γ) 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.

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

The present study investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0–7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon, namely 2.02 wt-% and 3.31 wt-%, and austempered at 360°C. The volume fraction of retained austenite and austenite carbon content results were then correlated with microstructural changes and impact toughness results. It is shown that the austenite stability is of great significance with respect to impact toughness, and that ferrite, when present in acicular form, can increase the mechanical stability of the austenite. It is observed that decreasing the austenitising temperature increases the driving force for the stage I transformation reaction in which mother austenite transforms to high carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II reaction in which high carbon austenite transform...

Effect of silicon content on transformation kinetics of austempered ductile iron

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Effect of silicon content on transformation kinetics of austempered ductile iron