In this paper, the wear rate of ferritic and pearlitic ductile iron, as well as three unalloyed ADI (Austempered Ductile Iron) materials with different ausferritic microstructure morphologies, was studied. Due to excellent combination of properties, the ductile irons and the ADIs are used for a number of applications, some of them relating to equipment exposed to abrasive wear in mining and agricultural industries.

The metal matrix microstructure of un-alloyed ductile iron (DI-F) was fully ferritic (Fig. 1a), while alloyed ductile iron (DI-P), as result of alloying with of Cu and Ni, had fully pearlitic microstructure (Fig. 1b). The ADI materials were obtained by austempering of unalloyed ductile iron at 300, 350 or 400°C for 1 hour (ADI-300, ADI-350, ADI-400, respectively). The microstructure of all ADI materials was fully ausferritic with 16, 24.9 and 31.4% of retained austenite. The ausferrite morphology changes from needle-like (acicular) at lower austempering temperatures to a more plate-like (feathery) at higher temperatures. Furthermore, due to microstructure the hardness of ADI-300 was highest, while ADI-400 was lowest. In order to determine an abrasive wear behaviour, the pin on disc wear tests were performed by SiC grinding paper with grit size P240, P500 and P800, and under 0.5, 1.3 and 2 kg loads. For microstructural characterization a “Leitz-Orthoplan” metallographic microscope was used.

After the wear test, in all cases, larger or smaller degree of plastic flow and tongue formation i.e. metal overlapping nodules is observed, Fig. 1c. As graphite nodules are very soft, they are easily covered by plastically deform material. The microstructure of ADI-300 and ADI-350 has not change after wear test, Fig 2a,b. In the case of the ADI-400 after wear testing at 2 kg loading and P240 grit paper a martensite is present in the microstructure, Fig 2c. The martensite was formed as a result of local pressure induced by the coarsest grit abrasive particles and maximal loading through stress assisted phase transformation (SATRAM) [2]. The results of wear rate represented by average weight loss of different material tested, as a function of grinding paper grit and loading are shown in Fig. 3a-c. The highest wear rate was obtained with the ferrite ductile iron (DI-F), while the lowest was for hardest ADI-300. However, ADI-400, in case when martensite form, exhibits better wear resistance.

It was found that the wear resistance primarily depends on materials’ microstructure, corresponding hardness and transformation during wear. In case of ADI materials, the SATRAM phenomenon play a major role in wear behaviour. However, the SATRAM phenomenon occurs only if appropriate conditions are fulfilled, namely: presence of metastable, low carbon-enriched, retained austenite; and local pressure on the metal matrix is sufficient, i.e. the SATRAM was detected only for ADI-400 at loads of 1.3 and 2 kg and at the coarsest abrasive grain size (P240). As a consequence, the wear rate of ADI austmpered at 400°C (the softest ADI tested) is equivalent to ADI austempered at 300°C (the hardest) [3].

Acknowledgment:

The authors gratefully acknowledge research funding from The Ministry of Education, Science and Technological Development of The Republic of Serbia under grant number TR34015.    

References:

[1] L. Sidjanin, D. Rajnovic, O. Eric and R. E. Smallman, Mater. Sci. Tech.-Lond., 26/5, (2010), 567-571

[2] C.Z. Wu, Y.J. Chen and T.S. Shih, Mater. Charact. 48, (2002) 43–54

[3] S. Balos, D. Rajnovic, M. Dramicanin, D. Labus, O. Eric-Cekic, J. Grbovic-Novakovic, L. Sidjanin, International Journal of Cast Metals Research, published online, doi: 10.1080/13640461.2015.1125982

Figures:

Fig. 1 - LM microstructures: a) DI-F – ferritic microstructure; b) DI-P – pearlitic microstructure; c) overlapping of graphite nodules and delamination after wear testing

Fig. 2 - Microstructures after the wear testing at 2 kg with P240 paper: a) ADI-300; b) ADI-350; c) ADI-400 (M-martensite)

Fig. 3 - Wear rates in relation to applied loads: a) 0.5 kg; b) 1.3 kg; c) 2 kg

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