Abstract
Austempered ductile cast irons (ADI) have received great attention in last years because their combined properties of good ductility, high strength and fracture toughness, good fatigue strength, good wear properties and low production cost. Such combination of properties can be reached because of their microstructures consist of a mixture acicular ferrite (bainite), residual austenite with a high carbon content and nodular graphite. In this work, the effect of austempering heat treatment on the microstructure of a commercial alloy to produce three different grades of ADI, with different strength level, is analyzed. Microstructure characterization has been performed using techniques of optical microscopy, scanning electron microscopy and x-ray diffraction. Mechanical properties were evaluated from tensile and impact tests at room temperature. In addition, the residual stress due to heat treatment was evaluated. The results of this study show that there is a strong relationship between austempering temperatures and mechanical properties. The highest tensile and yield strength obtained were 1599 and 1427 MPa, respectively, for the sample austempered at 280°C. The sample austempered at 320°C presented the highest Charpy absorption energy (99,90 J) and highest volume fraction of austenite (27%).
Highlights
The as-cast mechanical properties of ductile iron can be significantly improved through an austempering heat treatment
This has led to a new grade of cast irons, the austempered ductile iron (ADI), with its unique microstructure composed by an ausferritic matrix with spheroidal graphite [1]
ADI has emerged as an important engineering material in recent years because of the combination of properties such as high strength, good ductility, good wear resistance, high fatigue strength and fracture toughness [2]
Summary
The as-cast mechanical properties of ductile iron can be significantly improved through an austempering heat treatment. ADI is 10% less dense than steel [3], producing high specific strength when compared to other classes of materials [4]. Because of these advantages, allied with low production costs, ADI has been commonly used in many commercial engineering applications, such as in automotive components [5]. The production of ADI consists in the casting of ductile iron, followed by a heat treatment that begins with heating above the transformation temperature range, maintained long enough to create a fully austenitic matrix (γ) saturated with carbon.
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