Graphite formation in the solid state is both in ductile cast irons and in steels strongly promoted by high silicon contents above 3 wt.% Si. The matrix microstructure in austempered ductile iron can be further refined by secondary graphite if the austenitization, quench, and isothermal transformation into ausferrite are preceded by an austenitization at a slightly higher temperature followed by quench to martensite, resulting in higher carbon content than being soluble at the second austenitization temperature. Hypoeutectoid steels with high silicon contents can be rapidly graphitized, causing recrystallization of surrounding ferrite due to plastic deformation making room for less dense graphite. In rolled steels, the interface between manganese sulfide and steel matrix is the most common nucleation site. Voids are formed when graphite is partly or completely dissolved during austenitization in succeeding hardening heat treatments, but the mechanical properties can still be good if the graphite particles dissolved into voids are below 20 µm. Graphitized Si-solution strengthened ferritic steels may perform similar to free-cutting steels but with improved mechanical properties.

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Graphite Formation and Dissolution in Ductile Irons and Steels Having High Silicon Contents: Solid-State Transformations

Design and optimization of stepped austempered ductile iron using characterization techniques

This work presents the influence of austempering heat treatment carried out in one-step and two-step processes on the microstructures and mechanical properties of ductile cast iron. The samples were extracted from as-cast pieces and heat treated by austempering. For the one-step process the samples were heated at 910°C for 90 min for austenitization and cooled in salt bath at a temperature of 300°C for 30 min. For the two-step process the samples were cooled from 910°C to 245°C, kept at this temperature for 5 min in salt bath, then heated in another salt bath at a temperature of 300°C for 30 min. The samples were analyzed by optical microscopy and mechanical tests. After the one-step austempering, microscopic analysis of the samples showed ausferrite microstructure matrix and graphite in nodules surrounded by fine pearlite. For the two-step austempering, the presence of ausferrite matrix with graphite in nodules and retained austenite was observed. As to mechanical properties, the results showed that, wit...

Effect of Heat Treatments on Austempered Ductile Iron

Dilatometry has been extensively used to study the transformation behavior of steels. This paper outlines the use of Gleeble dilatometry to characterize the transformation kinetics and ferrite-pearlite ratios in a 65-45-12 ductile iron. Samples were heated to 900°C (1650°F) and cooled at constant cooling rates ranging from 2 to 2000°C/min (3.6 to 3600°Flmin). The dilatometer results were analyzed and transformation temperatures and the ferrite-pearlite ratio in the final microstructure were determined. The information was also used to estimate the continuous-cooling-transformation (CCT) curve for 65-45-12 ductile iron. The agreement between the ferrite-pearlite ratio predicted from the dilatometry results and metallographic analysis of dilatometry specimens was excellent.

Transformation Kinetics and Ferrite-Pearlite Ratios in a 65-45-12 Ductile Iron

/pdf/study-the-effect-of-austempering-temperature-on-the-kpptq7rz7j.pdf

Study The Effect Of Austempering Temperature On The Machinability Of Austempered Ductile Iron By Milling Process

An understanding of the kinetics of transformation during austenitization, cooling, and austempering of ductile iron is critical to achieving the desired microstructures and ultimately mechanical properties in austempered ductile iron (ADI). To this end, dilatometry experiments have been carried out to study the austenitization and cooling behavior of an unalloyed ductile iron. When a typical austenitization temperature of 900°C is used, unlike in steels, there is an initial expansion of the specimen, which levels off as the soaking time is increased. This occurs despite the fact that the temperature remains constant. This phenomenon, hitherto unreported, highlights the subtle differences between the austenitization of ductile irons and steels. The initial expansion is attributed to the increase in austenite lattice parameter, arising from the diffusion of carbon from the graphite nodules. The levelling off signals the saturation of austenite with carbon and can therefore be used as an indicator ...

A Dilatometry Study of the Austenitization and Cooling Behavior of Ductile Iron Meant for the Production of Austempered Ductile Iron (ADI)

Hot deformation of copper-containing microalloyed steels in the two-phase region was carried out to study the effect of copper on transformation and recrystallization behaviour of ferrite in HSLA steels. it was found that presence of copper could decrease the austenite to ferrite transformation temperature. The precipitation of epsilon-copper in ferrite could retard recovery and recrystallization by dislocation and grain boundary pinning in deformed ferrite. Retardation of transformation and ferrite recrystallization resulted in a less mixed structure consisting of fine transformed and recrystallized ferrite in the copper-containing steels.

Influence of two-phase region rolling on transformation and recrystallization behaviour of ferrite in copper-containing hsla steel

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A Dilatometry Study of the Austenitization and Cooling Behavior of Ductile Iron Meant for the Production of Austempered Ductile Iron (ADI)

Influence of two-phase region rolling on transformation and recrystallization behaviour of ferrite in copper-containing hsla steel