Nanoausferritic matrix of ductile iron

Introduction

Austempered ductile iron ADI is a structural material that constantly arouses the curiosity of researchers and interest of practitioners designers, users and foundrymen. This is probably the reason why new ideas and studies emerge all the time aiming mainly at an improvement of its properties.^1,2 This is particularly important in terms of the ADI implementation and use. It is enough to mention that since 2000 its production in the world has been increasing by an average of about 20000t per year.^3,4 This demonstrates the great interest of the users of castings made in ADI, which is a popular material for machine parts and equipment operating in the sectors of industry, such as automotive, railway, agriculture, defence, etc.

In this ductile iron, the ausferritic matrix is obtained by the austenitising and austempering treatment carried out under the specific conditions of time and temperature. The microstructural analysis of ausferrite morphology leads to the conclusion that there are several distinct features directly dependent on the time of isothermal transformation within the range of temperatures between Ar₁ and M_s. When the transformation time is very short, the ductile iron has an almost completely martensitic matrix with only traces of the lamellar ferrite precipitates Fig.1a. This is due to complete transformation of the thermally unstable austenite into martensite during final cooling to ambient temperature. The longer times of transformation increase the ferrite fraction in the form of packages of plates separated by austenite Fig.1b. Further transformation leads to austenite stabilisation. The result is the ductile iron matrix assuming a completely ausferritic structure and containing austenite in two forms austenite layers separating the plates of ferrite and blocks of austenite located between the ferriticaustenitic packages Fig.1c. The austenite stability is also affected by graphite nodularity as higher fraction of graphite nodules leads to greater stability of retained austenite. ⁵ A very long time of isothermal transformation, taking place within the most commonly used temperature range of 250400C, usually leads to the formation of typical bainitic microstructure.

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Figure 1

Microstructure of ausferritic ductile iron matrix after time of isothermal transformation amounting to a 5min, b 30min and c 120min , martenite, _b, bainitic ferrite, _f, austenite films, _b, blocky austenite conditions indicated in Fig.2 as A, B and C

The analysis shows that with the progressing time of isothermal transformation, austenite undergoes thermal stabilisation and the M_s temperature drops down. ^{6 7 68} Therefore, a diagram shown in Fig.2 can be sketched, allowing for all of the above mentioned microstructural conditions. The diagram neglects the values of the time and temperature of isothermal transformation, but as the results of experiments show, the general trend in M_s temperature changes is similar for all ADI types in the range of 250400C.

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Figure 2

Schematic representation of temperature changes during heat treatment shaping ductile iron matrix changes in M_s temperature during isothermal transformation

The time of isothermal transformation is one of the most important parameters controlling the ductile iron matrix during heat treatment. Calculating the best time of isothermal transformation, from the point of view of the desired cast iron properties, is a very difficult task. Typically, it is determined by carrying out a large number of the heat treatment operations and testing the mechanical properties in correlation with the timetemperaturetransformation TTT diagrams. ⁹

The increase in strength parameters of ADI may be obtained by formation of a proper content of phases and refinement of ausferritic matrix. ¹⁴ Moreover, to ensure the resistance to cracking, the ausferritic matrix should be free from the hard and brittle carbides or martensite. It means that the isothermal transformation should be carried out until austenite gets saturated with carbon to a degree such that it will reduce the M_s temperature to a point below room temperature. When this happens, the material will still contain a small amount of the mechanically unstable austenite, supporting the process of hardening during the experimental and technological operations. ^{10 11 12 13 1014}

It has been reported that in steels containing increased amount of carbon and silicon, it is possible to obtain a high strength and toughness by heat treatment inducing the isothermal transformation of austenite into a mixture of carbide free bainitic ferrite and carbon enriched austenite, ^{15 16 1517} both phases of nanometric dimensions.

The aim of this work is to obtain a nanocrystalline ausferritic structure in two types of ductile iron containing the additions of copper with molybdenum or molybdenum alone. For this, it was necessary to determine the proper time of isothermal transformation, in addition to proper undercooling in the first phase of the process, which are the critical factors for ultimate refinement of the ausferritic structure.

Methodology

Studies were carried out on ausferritic ductile iron with the chemical composition shown in Table1. The test samples of a cylindrical shape with a diameter of 3mm and a length of 10mm were cut out from the examined material by electrical discharge machining. Using a quenching dilatometer, the characteristic temperatures A₁₃ and M_s were determined, and phase transformations occurring at different temperatures during the process of austempering were examined for both tested cast irons. Using the obtained dilatometric data, the TTT diagrams were plotted for the range of bainitic transformation. Sections were prepared, from the samples treated at temperatures comprised in the range of the formation of upper and lower bainite, for metallographic examination by light microscopy and SEM. Additionally, from samples treated in the lower range of bainitic transformation, thin foils were cut out for TEM examinations in the bright and dark field conditions.

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Table 1

Chemical composition of examined ductile iron wt

Material	C	Si	Mn	S	P	Mg	Cu	Mo
ADICuMo	340	280	028	0015	0035	0055	072	027
ADIMo	350	254	016	0013	0041	0047	001	024

On the basis of the TEM images, the thickness of the ferrite plates and of the austenite layers was determined in accordance with the following stereological formula ¹⁸ 1 where d stands for the real size of the element of the microstructure in the analysed case the real thickness of a plate and L means the size of the microstructure element measured in the TEM image in this case, the plate width measured on the image. The plate widths L were measured perpendicularly to the interphase boundaries.

Results and discussion

The TTT diagrams obtained through dilatometric study Figs.3 and 4 show various phase transformations taking place in the examined cast iron, enabled tracing changes in the cast iron microstructure during heat treatment. From Figs.3 and 4, it is clearly visible that for both examined ADI irons, two overlapping C curves are present within the temperature range between Ar₁ and M_s in TTT diagrams. The upper and lower C curves correspond to formation of upper and lower ausferrite respectively.

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Figure 3

TTT diagram for ADIMo

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Figure 4

TTT diagram for ADICuMo

The most interesting results were obtained in the analysis of microstructure images of cast iron matrix in both upper and lower ausferrite range Figs.5 and 6. The detailed observations by the use of TEM revealed that the upper range of transformation is characterised by an increasing amount of the ferrite plates of an irregular structure Fig.7, which after certain time undergo transformation into a typically bainitic structure Fig.5b. In the lower range, the plates of ferrite are similar in their morphology to needles. The transformation at low temperature with a sufficiently long time promote the formation of structure with a nanometric thickness of the ferrite plates Figs.8 and 9.

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Figure 5

a proper microstructure of ADICuMo matrix austenite and ferrite b bainite in ADIMo iron matrix after long time of isothermal transformation

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Figure 6

ADI_Mo matrix microstructure after very long time of transformation in lower ausferrite range (72 h)

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Figure 7

a bright field b dark field austenite reflexes

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Figure 8

a bright field b dark field austenite reflexes

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Figure 9

a bright field b dark field austenite reflexes

The thickness of ferrite plates in ADICuMo iron austempered in 250C for 10h Fig.8 varies from 15 to 434nm, with the mean value of 1178nm. The plates were separated by retained austenite films with thickness varied from 8 to 258nm mean thickness value was 524nm. Small blocks of austenite were also observed in microstructure. Overall volume fraction of austenite determined by stereological analysis is 37146. Ausferrite structure of ADIMo austempered at 320C for 36h Fig.9 is distinctly finer. Mean ferrite plates thickness is 636nm within range from 26 to 173nm. The austenite appeared in form of layers between ferrite plates, no blocks were observed. The thickness of austenite varied from 8 to 166nm, with a mean value of 505nm. Volume fraction of austenite is 32650. Results of the stereological measurements of the size of phases present in both kinds of cast iron subjected to austempering at various temperatures are summarised in Table2. Microscopic observations confirm that in low range of austempering, temperatures with increasing time of the treatment to certain critical time has beneficial effect to the fine, typical ausferrite structure formation.

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Table 2

Size of phases present in both types of cast iron subjected to austempering at various temperatures

	Width of bainitic ferrite platesnm		Width of retained austenite layersnm		Austenite blocks aream2	Austenite volume fraction
	Range	Average	Range	Average	Range	Average
ADIMo 320C36h	26173	636	8166	505	Lack	3265
ADICuMo 250C10h	15434	1178	8268	524	035366	37146

In both types of cast iron, it is easy to notice differences in the duration of bainitic transformation. In the lower range of ausferrite, in the cast iron containing copper and molybdenum and in the cast iron containing molybdenum alone, the times of transformation are 166 and 7h respectively. Moreover, in the lower range, the time to end the bainitic transformation is shortest at about 260C for ADICuMo and 300C for ADICuM. On the other hand, in the upper range of ausferrite, the minimum time to stop the transformation is similar for both ADICuMo and ADIMo, and amounts to 17min. This minimum time is acquired for 480C in both alloys studied. In this range, the decrease of temperature prolongs the time necessary to end the transformation.

An important fact to which attention should be paid is the different M_s temperatures for the two types of cast iron. For the copper and molybdenum containing cast iron, the M_s point is 225C, while for the cast iron with 027 molybdenum alone, it is 180C. This indicates that, in the case of cast iron with the addition of molybdenum alone, substantial undercooling in the first step of austempering, even down to 180C, is not able to initiate the martensitic transformation. Conducting the process of austempering at a temperature so low for a time sufficiently long to complete the transformation with stabilisation of remaining austenite, could promote the formation of an extremely fine mixture of ferrite and austenite.

Conclusions

The results presented hereby show that it is possible to obtain in cast iron an ausferritic structure with nanometric thickness of the ferrite plates 100nm and significant amount of retained austenite over 30. This nanocrystalline ausferritic structure was obtained during the heat treatment consisting in the first step on rapid undercooling of cast iron to the temperature slightly above the M_s, and then by conducting isothermal transformation for a sufficiently long time within the lower ausferrite transformation range. The values of temperature and time of isothermal holding must be adjusted to chemical composition of cast iron. The selection of appropriate heat treatment parameters for given cast iron was made on the basis of precise studies of phase transformations assisted by microstructure observations.

Acknowledgements

The results presented in this paper have been supported by the project Production of nanocrystalline steels using phase transformations NANOSTAL contract no. POIG 0101021410009 with the Polish Ministry of Science and Higher Education. The project is cofinanced by the European Union from the European Regional Development Fund within Operational Programme Innovative Economy 20072013.