Effect of matrix structure on the impact properties of an alloyed ductile iron

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Abstract

An investigation was performed to examine the influence of the matrix structure on the impact properties of a 1.03% Cu, 1.25% Ni and 0.18% Mo pearlitic ductile iron. Specimens were first homogenized at 925 °C for 7 h and a fully ferritic structure was obtained in all ductile iron samples. Then, various heat treatments were applied to the homogenized specimens in order to obtain pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic matrix structures. The unnotched charpy impact specimens were tested at temperatures between − 80 °C and + 100 °C; the tensile properties (ultimate tensile strength, 0.2% yield strength and elongation) and the hardnesses of the matrix structures were investigated at room temperature. The microstructures and the fracture surfaces of the impact specimens tested at room temperature were also investigated by optical and scanning electron microscope. The results showed that the best impact properties were obtained for the ferritic matrix structure that had the lowest hardness, yield and tensile strength. Ductile iron with a lower ausferritic matrix had the best combination of ultimate tensile strength, percent elongation and impact energies of all structures.

Introduction

Ductile irons are very unique engineering materials. They possess good castability, damping capacity and mechanical properties and fair machinability. Owing to these advantages, ductile irons have been used in many structural applications. Steering knuckles, hypoid rear axle gears [1], camshafts, crankshafts and disk-brake calipers are important examples of ductile iron used in vehicles [2].

The common feature that all the members of ductile iron share is the roughly spherical shape of graphite nodules [3]. In addition, considering that the volume percentage of the graphite phase embedded in ductile iron is only about 13 to 15 pct, the influence of the matrix structure on mechanical and physical properties should not be neglected [2]. Therefore, with a high percentage of graphite nodules present in the structure, the control of ductile iron matrix structure is of potential importance. There are many variables such as chemical composition and cooling rate that control the matrix structure of ductile iron. Heat treating of ductile iron is also another method to produce a family of materials offering a wide range of properties obtained through matrix microstructure control [4].

In the automotive industry, it is essential that a given cast component will stand up to actual conditions, like the impact from a succession of potholes on a cold, wintry day [5]. Considering the various types of ductile irons, their current and potential application and the numerous industries using these materials, the knowledge of the impact behavior of ductile iron at subzero temperatures is very useful. A literature review has revealed the fact that data on the impact properties of ductile iron, particularly at subzero temperatures, are insufficient. Riabov et al. [6] studied the impact properties of ASTM grades of austempered ductile iron (ADI) at low testing temperatures of − 40 °C and − 60 °C and compared the results with those of ferritic ductile iron (DI) grade 60-40-18 and pearlitic DI grade 100-70-03, which were also tested at the same temperatures. They reported that the impact toughness values of all ADI grades decreased with the reduction of the testing temperature from room temperature (RT) to − 60 °C and with the reduction of retained austenite content. Hafız [4] studied the effect of matrix structure with various pearlite contents on impact toughness at room temperature of unalloyed DI. Delia et al. [7] investigated the effect of austenitizing conditions on impact properties at room temperature of an austempered ductile iron containing 1.6% Cu and 1.6% Ni.

The main objective of the present research is to investigate the effect of the matrix structure (ferritic, ferritic/pearlitic, pearlitic, tempered martensitic, lower and upper ausferritic) of Cu, Ni and Mo alloyed ductile iron on the impact energies within − 80 °C and + 100 °C temperature range. Additionally, the research is aimed to identify the fracture morphologies of matrixes tested at room temperature.

Section snippets

Casting procedure

The alloyed ductile iron used in the present work was produced in a medium frequency induction furnace of 350 kg capacity. The charge was consisted of pig iron (220 kg), unalloyed nodular iron scrap (110 kg), steel sheet (10 kg), FeSi (3.3 kg), FeMn (1 kg), FeMo (1 kg), Cu (3.5 kg) and Ni (4.5 kg). After superheating to 1530 °C to ensure homogenization, the melt was treated with 1.8% of a Fe–Si–Mg alloy (44.5% Si, 1.27% Ca and 6.08% Mg) using the flotret method. Soon after the nodularization

Microstructure and room temperature properties

Fig. 4A–F displays the microstructures and Table 3 shows both the nodule characteristics and the hardness values of the matrixes obtained by the heat treatments given in Table 2. After the homogenization treatments, spheroidal graphite embedded in a fully ferritic matrix can be seen (Fig. 4A). More ferrite phase is seen in the specimen cooled at 2.4 °C/min to 660 °C (Fig. 4B) than in the specimen cooled at 5 °C/min to 650 °C (Fig. 4C and Table 2). The lower and upper ausferritic structures were

Conclusions

In conclusion, the results obtained during this study showed that the impact toughness values of all matrix structures decreased with the reduction of the testing temperature from + 100 °C to − 80 °C. The best and the worst impact energy values for all testing temperatures are obtained for the ferritic and the upper ausferritic structures respectively. The fracture modes of the ferritic, lower and upper ausferritic structures are in good agreement with their impact energies and percent

Acknowledgements

The authors would like to express their most sincere gratitude and appreciation to the Turkish Land Forces 6th Maintenance Central Commandership and Prof. Dr. Ali Bayram from Uludağ University for their valuable help and cooperation. This research was partially supported by the Research Project Fund of Balıkesir University.

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