Abstract
The influence of the austempering temperatures on the microstructure and mechanical properties of austempered ductile cast iron (ADI) was investigated. ADI is nodular graphite cast iron, which owing to higher strength and elongation, exceeds mechanical properties of conventional spheroidal graphite cast iron. Such a combination of properties is achieved by the heat treatment through austenitization, followed by austempering at different temperatures. The austenitization conditions were the same for all the samples: temperature 890 °C, duration 30 min, and quenching in a salt bath. The main focus of this research was on the influence of the austempering temperatures (270 °C, 300 °C, and 330 °C) on the microstructure evolution, elongation, toughness, and fatigue resistance of ADI modified by certain amounts of Ni, Cu, and Mo. The Vickers and Rockwell hardness decreased from 535.7 to 405.3 HV/1 (55.7 to 44.5 HRC) as the austempering temperature increased. Optical images showed the formation of graphite nodules and a matrix composed of ausferrite; the presence of these phases was confirmed by an XRD diffraction pattern. A fracture surface analysis revealed several types of the mechanisms: cleavage ductile, transgranular, and ductile dimple fracture. The stress-controlled mechanical fatigue experiments revealed that a 330 °C austempering temperature ensures the highest fatigue life of ADI.
Highlights
IntroductionAustempered ductile cast iron (ADI) is a heat-treatable engineering material due to a good compatibility of high strength, fluidity, toughness, and ductility, as well as machinability, wear and fatigue resistance (the last three more suitable for steels), along with excellent design flexibility [1] and low cost that result in it appearing as a significant material in industry nowadays [2]
◦ C, 300 ◦ C, and 330 ◦ C showed satisfactory austempered at the different temperatures austempered at the different temperatures 270 °C, 300 °C, and 330 °C showed satisfactory mechanical properties
The Vickers (HV/1) and Rockwell (HRC) hardness values gradually reduced with an increase of the austempering temperature from 644.1 to 441.5 HV/1 and from 55.7 to 44.5 HRC
Summary
Austempered ductile cast iron (ADI) is a heat-treatable engineering material due to a good compatibility of high strength, fluidity, toughness, and ductility, as well as machinability, wear and fatigue resistance (the last three more suitable for steels), along with excellent design flexibility [1] and low cost that result in it appearing as a significant material in industry nowadays [2]. The formed structure differs from that of austempered steel, where the microstructure is composed of ferrite and carbides With such a structure, the cast iron is able to reach very high tensile strength (over 1600 MPa) and very good toughness (over 100 J/cm2 ), combined with excellent fatigue resistance [4]. From a machinability point of view, this material possesses a high work hardening coefficient alongside a low thermal conductivity [5] These properties can be explained by a higher number of sliding planes in the austenite, which assures better conditions for being deformed. High fluidity of ADI eliminates unnecessary hammering and assembling operations, thereby saving on the cost and weight of the final product
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