Austenite-Martensite Transformation in Adi by Plastic Deformation

MK0400027

AUSTENITE-MARTENSITE TRANSFORMATION IN ADI BY PLASTIC DEFORMATION

Rsrc.Asst. Cem AKCA*, Rsrc.Asst. Murat LUS*, Asst.Prof. Nilgun KUSKONMAZ* Yildiz Technical University, Department of Metallurgical and Materials Engineering, Da- vutpasa Campus, Istanbul, Turkey

Abstract: In this article, retained austenite-martensite transformation occurred after plastic deformation and parameters that affect martensitic transformation in ADI is observed. Beside the technical and commercial advan- tages of ADI, martensitic transformation provides lower production costs and superior mechanical properties such as hardness and wearing resistance. In order to study this phenomenon, austempered GGG60 specimens are subjected to a fatigue test that causes plastic deformation. Afterward the specimens are investigated with metallography, mechanical testing and x-ray diffraction techniques.

INTRODUCTION The most important recent advance in commercial ductile iron production is austempering. Austem- pering is a heat treatment process that depends on changing microstructure to bainite. Austempering steps are as follows: " Austenitizing around 900°C depending on the chemical composition, • Rapid cooling to isothermal holding temperature which is 235-450°C range, • Holding at austempering temperature depending on the desired composition, • Cooling to the room temperature. Austenitizing temperature and time, austempering temperature and time, amount of alloying elements affects the mechanical properties gained after austempering directly. As many of the authors indicate, austempering time and temperature is the most effective factor among all (4,5,6). At the first step, the matrix is transformed to austenite and the carbon solubility of austenite increases. After cooling to the isothermal holding temperature, the carbon-enriched austenite is transformed to bainite or bainite and carbides depending on the temperature. If the carbon content of the austenite is high enough so that a predictable amount of retained austenite can be held in the structure. Retained austenite is subjected to the austenite-martensite transformation by plastic deformation. Sur- faces of the most rotary parts of machines are subjected to wearing and some parts like wheels and earth moving machines' tips are subjected to plastic deformation. Martensitic transformation is a key property on the touching faces of these kind parts due to the need for extra strength (7). Bainitic microstructure with a maximum amount of 40% retained austenite is a very important property for the machining processes (1). Occurrence of martensitic transformation in normal working conditions is a factor for reducing production costs in a comparison to the alternative materials such as forged steel and railway steels (2). Martensitic transformation occurs in the highly plastically deformed sites of the parts. Amount of plastic deformation can be reached to 25% (3). Un- der the forces resulting plastic deformation, austenite-martensite transformation occurs especially in the sliding planes and the intersection points of twinning sites. These are the sites that are subjected to reduce the energy of the system quickly and this is done by two mechanisms: sliding and twinning. Potential energy held in these sites can easily be lowered by sliding or twinning. Sliding and/or the intersection of twinning planes act like martensite nucleation sites and martensitic transformation generally start along these sites (8).

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EXPERIMENTAL The preparation of the ductile iron in this study was carried out in an induction furnace (Inducto- therm™) of 30kW capacity, 9600 Hz working frequency and equipped with a silica crucible. Casts were poured as a 30mm diameter and 30cm length rod in GGG60 specification. Afterward each specimen is machined in 14 mm diameter and 10 mm height cylinderical wheel-like shape as in the Fig. 1.

Fig. 1 Solid-state drawing of specimens from front and back view. Chemical composition of the specimens is as follows: %c %Si %Mn %P %S %Cr %Cu %Mg Table 1 Chemical composi- tions of specimens. 3.67 2.7 0.38 0.51 0.10 0.084 0.26 0.045

All of the specimens were austenitized at 900°C and austempered at 375°C. As the austempering temperature rises, the percentage of retained austenite increases. But above 375°C a few amount of martensite is being formed during heat treatment process (1). Temperatures above 375°C were not used due to this reason. Austenitizing time was 30 minutes to provide austenite to enrich approx. 2% C (3). Austempering time was chosen as 55 minutes to provide maximum amount of retained austenite as approx. 40%. Each three specimens were tested with a rotary test apparatus under 25 kg/specimen load for plastic deformation. Fig.2 shows the test apparatus. Fig. 2 Test apparatus used plastically deforming specimens. Standard metallographic techniques were used in the specimen preparation for light- microscopy examination. Nital 2 was used as etchant and all micrographs were taken at x500 magnification. The x-ray powder diffraction data were collected on a Rigaku DMax 2200™.

RESULTS AND DISCUSSION

The light-microscopy metallography of the austempered ductile iron used in tests is depicted in Fig. 3, where a bainitic distribution and some typical nodular graphite morphology is shown.

100 Proceedings of 3rd BMC-2003-Ohrid, R. Macedonia AUSTENITE-MARTENSITE TRANSFORMATION IN ADI BY PLASTIC DEFORMATION

* Fig.3 Upper bainitic microstructure at x500 magnification (Nital

Fig.3 discloses the microstructure induced by the isothermal transformation of austenite to bainite. Dark platelets indicate the bainite and the light fields indicate retained austenite. Fig. 4 shows the microstructure after deformation. Finer martensite-bearing microstructure can be observed.

Fig. 4 Microstructure of martensite after deformation (x500, Nital 2). The existence of martensite phase in the microstructure was proven by means of X-ray diffraction techniques. Fig. 5 shows the X-ray diffraction pattern from three different section in the specimen powders: surface (most deformed), middle and center (least deformed).

Martensitic transformation was only observed from the surface sites. Martensitic transformation due to the machining was not observed in the inner parts of the specimens. This indicates that the transformation was induced by only the plastic deformation made by the test apparatus. Fig. 5 X-ray diffraction pattern showing the peaks from outer, middle and inner parts. The martensite peak can only be seen at 78 deg from the outer

20 (degree)

AFCC D Fig. 6 Closer look to the martensite peak on the pattern of the outer 0 Body-cenlered Tetiagonel pjjpf (Martensile) V

The peaks acquired from outer, middle and inner parts of the specimen are printed on the same pattern for the ease of ob- servation. Martensite peak is only seen on the first pattern at

7: eo 78 deg. In the image analysis stage, amount of the retained 28 austenite is measured by means of image analysis techniques

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on a Leica DMRX microscope and Leica image analysis software. Avarage 19.09% decrease was measured for the amount of retained austenite after deformation.

CONCLUSIONS 1. Retained austenite to martensite transformation under plastic deformation is proved. 2. Martensitic transformation during plastic deformation of retained austenite was achieved about 20%. 3. Nearly 50% of retained austenite was transformed to martensite due to the plastic deformation 4. The transformation can be occurred on ADI alloys within the range of plastic deformation before cracking.

ACKNOWLEDGEMENTS The authors wish to thank Yildiz Technical University Research Fund for the financial support for this research, Assoc.Prof.Dr. Orhan Sahin for the X-ray analysis and all of the BCACT (Balkan Cen- ter for Advanced Casting Technologies) team for their help in experiments.

References

1. E. Dorazil, High Strength Austempered Ductile Cast Iron, 1991 2. http://www.ductile.org 3. J.L. Garin, R.L. Strain-induced Martensite in ADI Alloys, Journal of Materials Processing Technology (2003) 4. H. Bayati, R. Elliot, Mater.Sci. Tech. 15 (1999) 265 5. H. Bayati, R. Elliot, Mater.Sci. Tech. 13 (1997) 319 6. M. Bahmani, R. Elliot, Mater.Sci. Tech. 10 (1994) 1050 7. J. Zimba, D.J. Simbi, E.Navara, Austempered Ductile Iron: An Alternative Material for Earth Moving Components, Cement and Concrete Composites Vol. 25 Issue 6 August 2003 p.643-649 8. Chih-Kuang Lin, Yi-Lin Pai, Low-Cycle Fatigue of Austempered Ductile Irons at Various Strains Ratios, International Journal of Fatigue, Volume 21, Issue 1, January 1999, p. 45-54

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