Dragan Rajnovic

Austempering study of unalloyed and alloyed ductile irons

Abstract Austempered ductile iron (ADI) proved to be an advanced material as it possesses remarkable combination of high strength, ductility and toughness with good wear resistance and machinability. These properties can be achieved upon adequate heat treatment which yields optimum microstructure for a given chemical composition. In the present paper, an investigation has been conducted on unalloyed ADI and alloyed with 0·47%Cu and 1·6%Cu + 1·5%Ni, austempered at 350°C for the time up to the 6 h. The microstructure and fracture mode developed throughout these treatments have been studied by means of light, transmission and scanning electron microscopy, as well as X-ray diffraction. It was shown that the strength, elongation and impact energy strongly depend on amount of ausferritic ferrite and stable, high carbon enriched retained, reacted austenite. Based on the results, optimal processing windows for alloyed ductile irons used, have been established. In addition, for alloyed ADIs with maximum amount of reacted, retained austenite, transition temperature was also obtained.

Transition temperature and fracture mode of as‐castand austempered ductile iron

The ductile to brittle transition temperature is a very important criterion that is used for selection of materials in some applications, especially in low‐temperature conditions. For that reason, in this paper transition temperature of as‐cast and austempered copper and copper–nickel alloyed ductile iron (DI) in the temperature interval from −196 to +150°C have been investigated. The microstructures of DIs and ADIs were examined by light microscope, whereas the fractured surfaces were observed by scanning electron microscope. The ADI materials have higher impact energies compared with DIs in an as−cast condition. In addition, the transition curves for ADIs are shifted towards lower temperatures. The fracture mode of Dls is influenced by a dominantly pearlitic matrix, exhibiting mostly brittle fracture through all temperatures of testing. By contrast, with decrease of temperature, the fracture mode for ADI materials changes gradually from fully ductile to fully brittle.

Austempering kinetics of Cu-Ni alloyed austempered Ductile Iron

The aim of the paper was to investigate the effect of austempering parameters (time and temperature) on the microstructure and mechanical properties of ADI alloyed with 1.5% Cu and 1.6% Ni (in wt.%) in order to establish the optimal processing window. It was shown that the strength, elongation and impact energy strongly depend on the amounts of ausferritic ferrite and retained austenite. A processing window was established according to the results of the kinetics of the isothermal transformation. The results show that the processing window for ADI alloyed with Cu and Ni at 350 °C was relatively wide, while the processing window for the isothermal transformation at 400 °C becomes narrower and shifted to the left. The processing window of ADI austempered at 300 °C is also narrower, but shifted to the right towards the longer times compared to the processing window of ADI austempered at 350 °C.

Microstructure and cavitation behaviour of alloyed austempered ductile irons

ABSTRACT In this paper, the study of cavitation behaviour of austempered ductile iron (ADI) alloyed with copper, as well as copper and nickel with a fully ausferritic microstructure, is presented. The ADI materials used were austenitized at 900 °C and austempered at 350 °C having an ausferrite microstructure with 16 and 19% of austenite, respectively. The experimental investigations were conducted using the ultrasonically induced cavitation test method. The results show that the cavitation damage was initiated at graphite nodules, as well as in the interface between a graphite nodule and an ausferrite matrix. The cavitation rate revealed that the ADI material alloyed with Cu + Ni austempered at 350 °C/3 h has a higher cavitation resistance in water than ADI alloyed with Cu. An increased cavitation resistance of the ADI material alloyed with Cu and Ni is due to the matrix hardening by stress assisted phase transformation of austenite into the martensite (SATRAM) phenomenon.

Effect of Cerium and Lanthanum Addition on Mechanical Properties of Bismuth Titanate Ceramics

Cerium- and lanthanum- substituted bismuth titanate (Bi4-xAxTi3O12; where A=La or Ce, and x=0, 0.5 and 1) ceramics were prepared from nanopowders synthesized by coprecipitation method. The as-synthesized powders were calcined, uniaxially pressed and finally sintered at 1050°C. It was shown that sintering behaviour, phase composition and grain morphology of the obtained ceramics were influenced by the presence of lanthanum and especially cerium ions in the titanate structure. Mechanical properties (hardness and fracture toughness) were measured at room temperature on polished sample surfaces using a Vickers microhardness tester. The hardness values for of bismuth titanate based ceramics were in the range for some other important perovskite titanate, whereas their fracture toughness was somewhat higher.

Fracture Toughness of Alloyed Austempered Ductile Iron (ADI)

Austempered Ductile Iron (ADI) has been established as an advanced material because of its excellent mechanical properties based on a good combination of wear resistance, toughness and very high strength combined with a relatively high ductility. It has a unique acicular matrix structure that consists of high-carbon austenite and ferrite with graphite nodules dispersed in it [1]. Some previous studies [1, 2, 3, 4] have been carried out on the mechanical properties of alloyed ADI. However, very little information is available regarding the influence of microstructure on the tensile properties and fracture toughness of alloyed ADI. The present investigation was undertaken to examine the influence of microstructure on the tensile properties and the plane strain fracture toughness (KIC) of copper and copper nickel alloyed austempered ductile iron (ADI). Two ductile irons with chemical compositions (in wt.%): (a) 3.6C; 2.5Si; 0.28Mn; 0.04Cr; 0.45Cu; 0.01P; 0.014S; 0.066Mg, and (b) 3.07C; 2.15.Si; 0.26Mn; 0.04Cr; 1.6Cu; 1.5Ni; in both alloys balance was Fe, were produced in a commercial electro-induction foundry furnace. Specimens for mechanical testing were machined from the test section of the Y-block. Machined specimens austenitized in a protective argon atmosphere at 900°C for 2h were rapidly transferred to a salt bath at austempering temperature 350°C, held between 1 and 6h and then air-cooled to room temperature. The reported results represent the average values of three measurements. Fracture surfaces of specimens after tensile and fracture toughness tests were examined by a JOEL JSM-6460LV scanning electron microscope (SEM) operated at 25kV. Metallographic investigations were carried out by light microscope. Change in the volume fraction of retained austenite during austempering was determined by the X-ray diffraction technique on diffractometer “Siemens D-500” with Ni filtered CuK radiation.