Elsevier

Wear

Volume 255, Issues 1–6, August–September 2003, Pages 410-416
Wear

Effect of graphite morphologies on the tribological behavior of austempered cast iron

https://doi.org/10.1016/S0043-1648(03)00156-XGet rights and content

Abstract

Three different cast irons, gray, nodular and compact iron, with different chemical compositions were subjected to austempering to investigate the effects of graphite morphology, composition and austempering on tribological behavior. The austempering process was carried out at 375 °C for different periods to achieve optimum mechanical properties. The austempered cast iron specimens with optimum mechanical properties were used for wear tests. For comparison, wear tests were also carried out on a pearlitic gray iron specimen made from railway brake shoes.

Sliding wear tests were performed in a block-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a ring made from a railway wheel at speeds of 250 and 950 rpm with nominal stress of 2 MPa. A non-contacting capacitance transducer with a computer interface was used to measure the coefficient of friction of the specimens.

The impact test results show that austempered nodular iron with 105 J/cm2 has the highest impact energy while austempered gray iron with 7–9 J/cm2 shows the lowest. The optimum impact energy of the austempered compact cast iron was 14–22 J/cm2 as a function of chemical composition. The wear test results indicate that austempered cast iron shows superior wear resistance than pearlitic gray iron, particularly at the lower speed. For austempered cast iron specimens, compact iron shows the highest wear resistance while gray and nodular cast iron have lower wear resistance.

Introduction

Many studies have been concerned with austempered iron because there are some excellent characteristics of austempered ductile iron (ADI). ADI offers a combination of strength, ductility and toughness and wear resistance. ADI has been used for many applications such as impact plates, jaw crusher components, hammers, excavator teeth, drills and rolls [1]. In addition, it has been reported that austempering would also improve mechanical and wear resistance of other types of cast irons [2], [3]. Nili Ahmadabadi et al. [4] studied wear characteristics of ADI and 3 wt.% phosphorous pearlitic gray cast iron. They observed that ADI shows about 2.5 times higher wear resistance than gray iron. They concluded that the higher wear resistance of ADI is likely due to the presence of bainitic ferrite and retained austenite components of bainite. The tribological behavior of ADI containing aluminium was studied by Boutorabi et al. [5]. They showed that the wear resistance of ADI is a complex function of austempering temperature and time. Nili Ahmadabadi et al. [6] have shown that successive austempering increases mechanical properties and wear resistance of austempered ductile iron. The results have shown that the wear of austempered iron increases as speed and normal load increase [6]. However, Fordyce and Allen [7] have reported that a severe wear mechanism changes to a mild one as running speed increases.

Although the tensile strength of compact iron which has a graphite shape intermediate between spheroidal and flake (ASTM-A247) [8], and gray iron is lower than that of nodular (ductile) iron, they have superior characteristics such as castability, damping capacity, and machinability [9]. The effect of graphite morphology on wear behavior of iron has been studied by Zhang et al. [10] and Hatate et al. [9]. Zhang et al. studied pearlitic iron with different graphite morphologies in sliding wear conditions. Their results have shown that graphite morphology affects thermal fatigue peeling resistance. They also showed that the higher strength and good heat transfer ability of the compacted graphite iron as compared to other irons resulted in its higher wear resistance. Dry slip-rolling contact wear and wet slip-rolling contact fatigue of several austempered cast irons with various graphite shapes were studied by Hatate et al. [9]. They showed that changing graphite nodularity resulted in greater weight loss at the initial wear stage due to shorter intergraphite distance and also a larger stress concentration factor at the graphite tips.

In the present work, the effects of austempering and graphite morphology on the tribological behavior of five different cast irons have been investigated. Furthermore, the results are compared with a pearlitic gray cast iron commercially used in railway brake shoe materials.

Section snippets

Experimental procedure

The chemical composition of the six cast iron specimens used in this study are given in Table 1. The first five irons with different graphite morphologies and chemical compositions were used for austempering while the last one was prepared from a railway brake shoe.

The materials were melted in a 50 kg induction furnace with medium frequency. Some melts were compacted with Fe–Ti 70% and Fe–Si–Mg 5%, and one melt was spherodized with Fe–Si–Mg 5% by a sandwich treatment. They were all refined with

Results and discussion

Fig. 1, Fig. 2 show the graphite structure of specimens G1 and C2, respectively. The hardness results are given in Table 2. This table shows that the hardness decreases as austempering time increases due to full transformation of austenite to bainite. In the case of shorter austempering, some austenite remains untransformed, which upon cooling to room temperature, transforms to martensite.

The results of impact test are given in Table 3. The specimens austempered for 120 min were used for wear

Conclusions

Austempered and wear tests on the different cast irons lead to the following conclusions:

  • 1.

    Austempered ductile iron shows the highest impact energy followed by compact and gray iron.

  • 2.

    Austempering extensively improves wear resistance of iron as compared with pearlitic gray iron.

  • 3.

    Austempered compact iron shows the highest wear resistance probably due to its graphite morphology which controls crack propagation and thermal conductivity.

Acknowledgements

The authors would like to express their thanks to Iranian National Railway Company for financial support of this work under grant no. 18/14-27400.

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This paper was unable to be presented by the author at the 14th International Conference on Wear of Materials due to the prevailing political situation at the time.

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