Elsevier

Wear

Volume 231, Issue 2, July 1999, Pages 293-300
Wear

Effects of successive austempering on the tribological behavior of ductile cast iron

https://doi.org/10.1016/S0043-1648(99)00163-5Get rights and content

Abstract

A 0.75 wt.% Mn ductile iron with different nodule counts was austempered by conventional and successive austempering processes at 315 and 375°C for different periods. Specimens with optimum mechanical properties were used to study the effect of austempering process on the wear behavior of austempered ductile iron (ADI). Sliding wear tests were performed in a pin-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a surface hardening tool steel ring at speeds of 0.6 and 1.28 m s−1 and normal loads of 100, 200 and 300 N. To study worn surfaces of test specimens, optical and scanning electron microscopy (SEM) were used. It was found that specimens austempered at 315°C (LAT, lower austempering temperature) show the highest wear resistance while specimens austempered at 375°C (HAT, higher austempering temperature) show the lowest. Specimens austempered by successive austempering process (HLAT, high–low austempering temperature) with hardness about equivalent to HAT specimens, in addition to improvement of mechanical properties, show higher wear resistance than that of HAT specimens. Duplex structure, upper and lower bainite, along with higher carbon content of retained austenite and reduction of UAV (untransformed austenite volume) can be the main factors which improve mechanical properties and wear resistance of HLAT specimens. The results showed that specimens with longer solidification time have lower wear rate than specimens with shorter solidification time, supposedly due to effect of longer solidification time on the reduction of nodule count and heavier segregation of alloying elements. SEM study of worn surfaces suggests that delamination could be the probable wear mechanism in this investigation.

Introduction

Austempered ductile iron (ADI) has received much publicity as more applications have been reported 1, 2. ADI offers a combination of strength and wear resistance with low cost and good toughness. To provide sufficient hardenability during austempering, ductile iron should be alloyed with alloying elements like, Mo, Mn, Cu or Ni. Mn an inexpensive hardenability promoter may be added to iron for this purpose. However, Mn segregates in the intercellular regions which has adverse effects on the mechanical properties of ADI. As a consequence of negative contribution of Mn, it was recommended to keep Mn content around 0.35% 3, 4. To overcome the negative impact of Mn, one of the authors for the first time, introduced the successive austempering process which improves mechanical properties of 1 wt.% Mn, well comparable with low alloy conventional austempering process and much higher than ASTM values 5, 6. Subsequently, Bayati et al. [7]also applied this process on the same material, and their results confirmed the beneficial effects of successive austempering.

The wear behavior of ADI has been studied by some researchers. Nili Ahmadabadi et al. studied wear characteristic of ADI, unalloyed and 3 wt.% P gray cast iron [8]. They observed a higher wear resistance in ADI than the others even if all samples had almost the same hardness. They concluded that higher wear resistance of ADI should be due to the presence of ferritic and retained austenitic (a carbon enriched austenite which is stable at room temperature) component of bainite. Prado et al. [9]examined the effects of austempering temperature on the dry sliding wear of ADI. They reported that when high loads (500 and 800 N) are applied, the best wear resistance is obtained for thermal treatments at 340°C. However, their conclusion is rather in question, since they austempered all of specimens for 60 min. It has been well-established that the kinetics of bainitic transformation is a function of austempering temperature 10, 11. Therefore, if one assumes that 60 min is sufficient for completion of austempering at higher austempering temperatures but in other specimens, in particular, specimens austempered at lower temperatures, this austempering period seems to be insufficient. As a consequence of short austempering process at lower austempering temperature some regions may remain untransformed. These regions transform to martensite following cooling to room temperature. The formation of martensite in the matrix will have an important impact on the mechanical properties and wear behavior of ADI.

The tribological behavior of austempered spherical graphite cast iron containing aluminum was studied by Boutorabi et al. [12]. They showed that the wear resistance of ADI is a complex function of austempering temperature and time.

Inasmuch as successive austempering process extensively improves mechanical properties of ADI as a consequence of formation of duplex structure and higher retained austenite in the matrix 5, 6, it is reasonable to assume that the process should also have some important impacts on the wear behavior of ADI.

This work is an attempt to investigate the effect of austempering process and solidification time on the wear behavior of 0.75 wt.% Mn ADI. Special attention is paid to study the effect of successive austempering process on the wear behavior of this alloy.

Section snippets

Experimental procedure

High Mn ductile iron of the composition given in Table 1 was prepared. The alloy was produced by a sandwich spheroidization treatment and 1-in. thick Y block castings were poured in green sand molds. Base irons were treated by 5.5% MgFeSi alloy for spheroidization followed by post-inoculation with 75% FeSi.

Fig. 1 shows a schematic diagram of the austempering treatment carried out on different specimens. The specimens were austenitized for 90 min in an atmosphere-controlled furnace at 900°C,

Metallography

Fig. 2 shows optimum mechanical properties of LAT, HAT and HLAT specimens. This figure shows that mechanical properties of HAT specimens is lower than ASTM standard while mechanical properties of HLAT specimens is higher. Comparison between mechanical properties of HAT and HLAT specimens clearly expresses the drastic impact of successive austempering on the mechanical properties of high Mn ADI. It was shown that this improvement is due to the reduction of UAV (untransformed austenite volume,

Discussion

The mechanical and wear test results show that the successive austempering process improves both mechanical properties and wear resistance of ADI in comparison with conventional austempering process. It has been concluded by some researchers that in conventional ADI, an increase of hardness and decrease of retained austenite increase wear resistance 8, 16. However, by considering results of wear resistance of HLAT and HAT specimens achieved in this work, a more precise investigation is

Conclusions

Sliding wear tests on specimens with optimum mechanical properties austempered by different processes lead to the following conclusions:

  • 1.

    The results of this study suggesting delamination mechanism as a dominant wear mechanism.

  • 2.

    The mechanical and wear test results show that successive austempering process improves both mechanical properties and wear resistance of ADI in comparison with conventional austempering process.

  • 3.

    The specimens with lower nodule count (longer solidification time) have lower

Acknowledgements

The authors would like to express their sincere thanks to the research council of University of Tehran for financial support of this work.

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