1. 70᪽ CONGRESO MUNDIAl
DE FUNDICIÓN
Pre-congreso 23 - 24 de Abril 2012, Saltillo, Coahuila
Congreso 25 -27 Abril 2012, Monterrey, Nuevo León
On The Austempering Behaviour and Toughness of Austempered Engineering Grade Ductile Iron Castings
S. E. Kisakurek, Professor, Istanbul University, Istanbul, Turkey
A. Ozel, Associate Professor, Sakarya University, Sakarya, Turkey
Y. Yalcin, Associate Professor, Kocetepe University, Afyon, Turkey
A. Turk, Associate Professor, Sakarya University, Sakarya Turkey
H. Akbulut, Professor, Sakarya University, Sakarya, Turkey
S. C. Okumus, Associate Professor, Sakarya University, Sakarya, Turkey
ABSTRACT
Austempering behaviour of engineering grade ductile iron castings have been studied by measuring the effects of the as-cast structure, the chemical composition, nodule count and the heat treatment parameters, austenitising temperature, austenitising time, austempering temperature and austempering time, on the hardness, impact touhness and microstructure developments in austempering process Method of factorial experimentation was employed to identify the relative effects of the heat treatment parameters on toughness variations, as well as, changes occurred in nodular characteristics during austempering. Finally, results of part of the studies on ductile/brittle transition behaviour of austempered GGG40 and GGG80 grade castings were briefly presented.
Keywords: Austempered ductile iron, austempering, bainite, hardenability, austemperability, austenite, bainitic ductile iron, toughness
INTRODUCTION
Over the past three decades intensive efforts have been expended by the industry and the academe to earn the Austempered Ductile Iron (ADI) its present status. Most of the studies of 1980s1-10 and of 90s11-23 were concerned with understanding the austempering process, paying particular attention to the effects of as-cast structure, chemical composition and heat treatment parameters on the structure and mechanical properties obtainable by austempering treatment. The concerns of the researchers of the last decade were mostly ware, surface hardening, corrosion, fatigue properties, etc., of ADIs.24

2. Austempering of ductile iron is a two stage process (Figure 1): (i) austenitising the iron in the temperature range 850-950
o C(1562-1742 o F) to change the as-cast matrix to austenite, (ii) quenching into a salt bath maintained at a temperature in the range 250-400 o C (392-752 o F) and hold isothermally at temperature until a satisfactory matrix structure is obtained. A requirement for the production of ADI with optimum combination of properties is that, pearlite formation should be avoided during quenching the iron into austempering bath after austenitisation, which can be achieved by making alloying element additions to the base metal compositions.1 Therefore, attentions in the past have been mostly directed to understand the austempering behaviour of low alloyed or alloyed ductile irons, in
particular to those containing Ni, Mo and Cu.
This paper is a brief report of some aspects of the authors’ studies of the austempering behaviour of engineering grade ductile iron castings produced in
accordance with Standard DIN 1693 specifications, namely GGG-40, 50, 60, 70 and 80, with special interests in the effects of the as-cast structure, chemical composition and heat treatment parameters on the hardness, impact toughness and microstructural changes occurring during austempering.
EXPERIMENTAL PROCEDURE
All castings were produced in local commercial foundries, Irons No.1, 2 and 3 by Ferrodokum A.S., all others by Doktas A.S., with chemical compositions given in Table 1 in the form of either 25mm Y or 25mm keel blocks. Test specimens, machined to final dimensions and embedded in piles of cast
iron chips, austenitised at different temperatures in the range 800-950 o C(1472-1742 o F) for 10-250 minutes in a

3. muffle furnace in air atmosphere, then rapidly transferred into a salt bath mixture of 50/50 KNaNO3, maintained at a temperature in the range 225-450 o C(437-842 o F), to austemper isothermally. Austempered samples were subsequently allowed to cool in air.
Un-notched Charpy impact specimens were prepared according to ASTM A 327-72 Standard Specification, Impact and hardness tests were conducted on
Trebel Charpy Impact and Ernst Brinell HB30D2 test equipments, respectively. Metallographic examinations were done by an Olympus BHM 313U microscope, x-ray studies by a Phillips PW3710 diffractometer.
RESULTS AND DISCUSSION
STRUCTURE AND PROPERTIES OF DUCTILE IRONS IN AS-CAST CONDITON
The matrix structure of the castings were typical of the respective grades; namely, either ferritic, or ferritic-pearlitic, or pearlitic-ferritic or pearlitic. Nodule counts were in the range 106-175mm-2, and average nodule diameters in the range 25-44m; nodule shape factor was better than 0.90, except for iron No.1, which was in the range 0.59-0.80.
EFFECT OF HEAT TREATMENT PARAMETERS ON THE HARDNESS-AUSTEMPERING TIME AND IMPACT ENERGY-AUSTEMPERING TIME RELATIONSHIPS
Ductile irons No. 1(GGG-50), No.2(GGG-60) and No.3(GGG-60+1.19Ni+0.14Mo) were cast, with as-cast properties and microstructures as given in Table 2, to examine their austempering behaviour by hardness and unnotched impact toughness measurements through the time frame 30 -120 minutes in austempering time at 250, 300, 350 and 400 o C ( 482, 572, 662, 752 o F) after austenitising for 30 and 100 minutes at 850 and 900 o C(1562-1652 o F). The results are given in Figures 2-4 as hardness vs austempering time and
impact energy vs austempering time plots. As seen in Figures 2a to 2d, under the heat treatment conditions specified for this phase of the work, impact energy values Iron No.1 attained at the beginning of the time frame in question, which was the 30th minute of austempering time, were all collected in a narrow energy range, 10-30J. At this point, on the other hand, in all austempering treatments employed, the hardness was in the process of fall towards a minimum or a plateau (Figures 2e to 2h). Although not presented here, hardness and impact energy variations of Iron No.2 and of Iron No.3 at this stage of austempering were in similar transformations.
On a hardness-austempering time plot, point of time where the hardness falls to minimum is regarded as the time when the stage 1 reaction of the austempering process, i.e. transformation of matrix austenite to ferrite and high carbon austenite(known as the toughening reaction), is completed.6 Therefore, at this point, the retained austenite content of the structure would be expected to reach at the maximum

6. level that the treatment condition would allow.6,7 As the maximum toughness is correlated with the maximum amount of retained austenite in the matrix,7 the fall of the hardness to the minimum would also be expected to have occured in concurrence with the rise of the impact toughness to the maximum. However, in majority of the hardness and toughness vs austempering time plots presented in Figure 2 there were time lags in between the events of the fall of the hardness to the minimum and the reach of the toughness at the maximum. Suggesting that, retained austenite content of the structure develops to its maximum level not when the hardness falls to the minimum but some time later in the austempering time.
As seen by comparison between the impact energy- austempering plots in Figures 2a-2d and hardness- austempering time plots in Figures 2e-2h, under all austempering treatment conditions Iron No.2 entered the time frame, 30-120 minute austempering time, with higher impact toughness than Iron No.1, therefore, with greater amounts of retained austenite in its structure, as the impact toughness was related to the retained austenite content of the structure,7, 23 This also means, Iron 2 arrived at this stage of the treatment with a greater austempering rate than Iron No.1. Major differences between Iron No.1 and Iron No.2 were in chemical compositions and nodular characteristics. As seen in Table 1, carbon and silicon contents of these two irons were comparable. Mn and Cu contents of Iron No. 2 were nearly double the amounts of the same in Iron No.1; 0.69wt% Mn and 0.65wt% Cu in Iron No. 2 against 0.43wt% Mn and 0.34wt% Cu in Iron No.1. Available information on the role that manganese plays in austempering ductile irons were limited, however, it is well known that it segregates to cell boundaries during solidification of the ductile iron where it delays the transformation of austenite, and also that, increasing its concentration from 0.07wt% to 0.74wt% results in reductions in impact strength.7 Therefore, higher austempering rate of Iron No.2 compared to that of Iron No.1 at the 30. minute of austempering time couldn’t be accounted for simply by the Mn content difference between these two irons. As pointed out above, as moved from Iron No.1 to Iron No.2, not only the Mn content but, simultaneously, the copper content was also increased. Cu is one of the popular elements added into ductile irons to avoid pearlite formation during quenching the castings into the austempering bath after austenitisation,7 therefore, with the knowledge available, the austempering rate difference between Iron No.1 and Iron No.2 couldn’t have been related to the difference between the chemical compositions of these irons, either.
As to the austempering rates of Iron No.2 and that of Iron No.3, comparison between the data of Figure 3 and Figure 4 shows that, of the 16 austempering conditions applied in the experiments, in 15 of them Iron No.3 entered into the time frame 30-120 minute in austempering time with greater toughnesses than Iron No.1, suggesting that, again by deduction similar to that made above when the impact energies of Iron No.1 and Iron No.2 were related to the rate, Iron No.3 completed the first 30 minutes of the isothermal austempering treatment at a greater rate than Iron No.2. C, Si, Mn and Cu contents and nodular characteristics of these irons were very similar to each other, as could be seen in Table 1 and Table 2. The only major difference between them appeared to be the additional 1.19wt% Ni and 0.14wt% Mo contents of Iron 3, which was deliberately created to measure the effect of combined addition of these elements into the composition. Thus, the apparent rate difference between Iron No.2 and No.3 could be readily related to the Ni and Mo contents of Iron No.3.
Figures 2-4 indicate that there were differences in the maximum impact toughness Irons No.1, No.2 and No.3 achieved by austempering; the toughness of Iron No.2 and No.3 were both remarkably higher than that of Iron No.1, which could have arisen from the differences in chemical compositions and nodule counts. There were also differences between the maximum toughness values Iron No.2 and Iron No.3 attained during austempering. As seen in Figures 3 and 4, when austempered after the both were austenitised at 850 o C(1562 o F) for 30 minutes, the maximum toughness achieved by Iron No.2 was greater than that of Iron No.3. However, when austempered after austenitised for 100 minutes at 850 o C(1562 o F) and 30 and 100 minutes at 900 o C(1652 o F), Iron No.2 was ahead. Indicating that, 1.19wt% Ni and 0.14wt% Mo addition to ductile iron which

7. also contained some amount of Mn and Cu can benefit from austempering tretment by significant amount, provided that appropriate conditions were selected.
The magnitude of the relative effects of heat treatment parameters on the toughnes variations recorded by austempering heat treatment could be estimated by factorial analysis of the experimental results.25 Table 3 shows the factorial design of experiments and the results of analysis of the effects of austenitising temperature(factor A), austenitising time(factor B), austempering temperature(factor C) and austempering time(factor D), by varying each factor at two levels, as 800-900 o C(1562-1652 o F), 30-100 minutes, 250-350 o C(482-662 o F) and 30-120 minutes, respectively, on the maximum impact toughness of Iron No.2.
Results and effects are given in the fifth and sixth columns of Table 3 show that the most effective factors in increasing the impact energy of austempered Iron No.2 were austempering temperature and austempering time. Increasing the austenitising temperature and/or the austenitising time decreased the toughness. In decreasing order of effectiveness, austenitising temperature was the third factor in the queue, increasing the level of this factor resulted in significant reduction in toughness. The least effective factor was austenitising time, increase of which also caused reduction in toughness, but to a lesser extent. Therefore, the optimum condition to develop an ADI with maximum toughness was to austemper Iron No.2 at 350 o C for 120 minutes after austenitising at 850 o C for 30 minutes.
EFFECT OF COPPER CONTENT
The effect of Cu content on the development of retained austenite proportion of the austempered structure was studied on Irons No.4, No.5 and No.6, cast to meet the Standard Specifications for Grade-50, Grade-60 and Grade- 60 + 1.27 Cu ductile irons, respectively. As the castings were poured from a single melt by simply modifying the copper content of the melt between the taps, carbon, silicon and manganese contents could be kept constant in all members of the group. Chemical composition of each casting was given in Table 1, hardness and structural characteristics in the as-cast state, as
measured by auto-image analyser, are outlined in Table 4. All three irons were given equal austempering treatments; austenitised for 60 minutes at 850 o C(1472 o 2F) and 900 o C(1562 o F), and then austempered at 350 o C(662 o F) for 10 to 100 minutes. Results are presented in Table 5.
Data shows, when irons were austenitised at 850 o C(1562 o F), largest fraction of retained austenite in the structure was obtained within the first 25-50 minutes of the austempering process, however, when austenitised at 900 o C(1652 o F) prior to austempering, retained austenite proportion of the structure rised to its highest level at a later point in austempering time, in a time frame starting at,

8. approximately, the 50. minute of the treatment. The increase with austenitising temperature in the amount of retained austenite was, on the average, 26vol%. Other than that, there was no evidence to suggest that retained austenite content increased with copper content.
EFFECT OF NODULE COUNT AND NODULE SIZE
Irons No.7 and No. 8, cast to meet the Standard DIN 1693 Specificaitons for GGG-50 grade ductile iron, were planned to investigate the effects of nodular characterisitics, e.g. nodule count and nodule size, on austempering behaviour. They were both poured from a single melt: Iron No.8, however, in addition to the
standard laddle treatment applied to both, was also given in- mold treatment to modify its nodular characteristics. As seen
in Table 1, only difference in the chemical compositions of these two irons were in their silicon contents; there was 2.33wt% Si in Iron No.7 and 3.25 wt% Si in Iron No.8, which stemmed purely from the additional in-mold treatment applied to Iron 8. Table 6 lists the hardness,

9. impact toughness and microstructural characteristics, of the castings, Figure 5 compares the microstructures of both irons in unetched and etched conditions
Figures 6a and 6b show the variation of impact energies against time in austempering treatments at 250, 300, 350 and 400 o C(482, 572, 662, 752 o F) of Iron 7 and Iron 8,

10. respectively, after austenitised at 850 o C(1562 o F) for 60 minutes. A major difference between the austempering behaviours of these irons was the change of the optimum temperature to obtain the maximum toughness: In Iron No.7 maximum toughness was achieved at 325 o C(617 o F), in Iron No.8 at 350 o C(572 o F). Maximum toughness achieved by Iron No.8(175J) was remarkably higher than that obtained by Iron 7(125J). There are evidences that, increasing the silicon content increased the optimum temperature for maximum ductility, increased fracture toughness by increasing the retained austenite content of the matrix and increased the impact strength of ADI.7 Therefore, the increase in impact toughness, as moved from Iron No.7 to Iron No.8, was, at least partly, due to the higher silicon content of the latter. No published data
were available suggesting direct relationship between the nodule count and the impact toughness, or ductility, of ADIs, although works on ductile iron castings has shown that increasing the nodule count effectively improved the tensile elongation of ductile irons, but room temperature impact toughness was independent of the nodule count for all matrix structures .5 Therefore, the nodule count-impact toughness relationship requires further research to identify the individual effect of the nodule count.
CHANGES OCCURRED IN NODULAR CHARACTERISTICS DURING AUSTEMPERING
Significant changes were noted in nodule counts and average nodule diameters of ductile irons during austempering heat treatment process. Measurements on the broken impact test specimens of Irons No. 1, 2, and 3, suggested that, up to 27% reductions in nodule count and up to 45 % increases in nodule size were possible. Method of factorial experimentation was also applied to measure the relative magnitudes of the effects of the heat treatment parameters, austenitising temperature(factor A), austenitising time(factor B), austempering temperature(factor C) and austempering time(factor D), upon the % variation occurred in nodule count and in nodule size during austempering of
Iron No.2 with respect to the as-cast state. Results are presented in Tables 7a and 7b, indicated that, out of the four heat treatment factors studied, the most effective one in influencing the variation in the number and size of the graphite nodules during austempering was the austenitising temperature. Effectiveness of the factors decreased in the order: austenitising temperature, austenitising time, austempering temperature, austempering time.
Effect of the as-cast matrix structure on the amount of

11. changes occurred in nodule count and nodule size, with respect to the as-cast state, during austempering were also searched. Irons No.9 and No.10 were prepared to meet the Standard DIN 1693 specifications for GGG-40 (fully ferritic) and GGG-80 (fully pearlitic) ductile irons. One surface of each sample, of dimensions 10mmx10mmx110mm, was marked,
so that nodule count and nodule size measurements before and after the treatments could be done on the same surface of the same specimen. Irons were austenitised at 850 o C (1562 o C) and then austempered at 350 o C (572 o F) for different times. Results are presented in Figures 7a and 7b, as estimated number of graphite nodules per unit volume of casting vs austempering time, and

12. % increase in average diameter of volume distribution of graphite nodules vs austempering time plots. Increase in the % reduction in nodule count and % increase in average nodule size, as moved from pearlitic to ferritic iron was
noticable. Changes occurred in nodule counts and nodule size can well be accounted for, at least qualitatively, by Oswald Ripening mechanis,26 which is an observed phenomenon in solid solutions, which describes the change of an inhomogeneous structure over time. In other words, over time, small particles, graphite nodules in this case, dissolve, and redeposit onto larger particles.
DUCTILE/BRITTLE TRANSITON BEHAVIOUR OF AUSTEMPERED DUCTILE IRONS
In this phase of the studies ductile/brittle transition
behaviour of the whole series of engineering grade ductile iron castings, were examined. Specimens, following
austenitisation for 100 minutes at 900 o C (1652 o F), were isothermally austempered at 250, 300, 350 and 400 o C(482,
572, 662, and 752 o F) for 7 to 210 minutes. For below-zero impact tests, specimens were refrigerated in ethyl alcohol in
a Lab-Plant Refrigerated Immersion Probe, RP-100. For above-zero tests, specimens were heated in distilled water. Figures 8a and 8b, showing the variation of impact energies of Iron No. 11 (GGG40-fully ferritic) and Iron No.12(GGG80-fully pearlitic) with test temperature in as- cast and austempered conditions, demonstrate that, both irons were susceptible to temperature changes of the environment. As seen in Figure 8a, fully ferritic ductile iron(Iron No.11) gained no increments in toughness by austempering. Even after austempered at the optimum conditions to obtain maximum toughness, its room temperature impact energywas equal to that of the as-cast
state. With the fall of the test temperature to -40 o C(-40 o F), the toughness of austempered Iron No.11 fell to 25J, which was a very low toughness level for a fully ferritic ductile iron. On the other hand, austempering increased the room temperature toughness of the pearlitic iron(Iron No.12), from 5J in as-cast state to 80J. As the temperature of the media was lowered to -40 o C(-40 o F), toughness of this pearlitic ductile fell to 40J. A low value, but still higher than the room temperature toughness of its as-cast state. The subject will be discussed elsewhere.
CONCLUDING REMARKS
During isothermal austempering, maximum toughness at temperature is achieved at a later stage than the fall of the hardness to minimum, suggesting that ADI owes its high strength and high ductility to its carbide dispersed metal matrix (ferrite plus high carbon austenite
ADI can achieve high toughness in presence of manganese, even in amounts up to 0.65wt%, provided that it was added in combination with copper.
Austempering treatment increases the nodule shape factor to > 0.90, at the same time nodule count decreases and nodule size increases.
Nodule count increases the maximum toughness obtainable

13. from a given austempering treatment.
ADI is susceptible to ductile/brittle transition.
REFERENCES
1. Dorazil, E., Barta, B., Munsterova,E., Stransky,L, Huvar,A., AFS. Int. Cast Metals Research J., 1982, V.7, No.2, pp.52-60
2. Janowak, J. F., Gundlach, R. B., AFS Transactions, 1983, v.91, pp. 377-381.
3. Voigt, R.C., AFS Transactions, 1983, v.91, pp.253- 262.
4. Janowak, J.F., Morton, P.A., AFS Transactions, 1984, v.92, pp.489-492.
5. Voight, R.C. and Loper, C.R., J..Heat Treating, 1984, No.4, Vol.3, pp.291-309.
6. Moore, D.J., Rouns, T.N., Rundman, K.B., J. Heat Treat., 1985, V.4 , pp.7-23.
7. P.A.Blackmore, R.A.Harding, J.Heat.Treat., 1984, v.3, pp.310-325.
8. Moore, D.J., Rouns, T.N., Rundman, K.B., AFS Transactions, 85-103, pp.705-717.
9. Gilbert, G.N.J., Report No. 1666, BCIRA, July 1986.
10. Rouns, T.N. and Rundman, K.B., AFS Transactions, 1987, v.95, pp.851-875.
11. Viau, R., Gagne, M., Thibau, R., AFS Transactions, 87-77, pp.171-178.
12. Rundman, K.B., Moore, D.J., Hayrynen, K.L., Dubensky, W.J, Rouns, T.N., J.Heat Treat., 1988,v.5,pp.79-95.
13. White, P., Report No. 1787, BCIRA, pp.381
14. Grech, M and Young, J.M., Mater. Sci. and Tech., 1990, v.6, pp.415-421.
15. Krishnaraj, D., Narasimhan, H.N.L., Seshan,S., AFS Transactions, 92-100, pp.105-112.
16. Darwish, N and Elliott, R., Mater. Sci. and Tech., 1993, v.9, pp.572-585.
17. Darwish, N and Elliott, R., Mater. Sci. and Tech., 1993, v.9, pp.586-602.
18. Darwish, N and Elliott, R., Mater. Sci. and Tech., 1993, v.9, pp.882-889.
19. Bahmani, M., Elliott, R., Mater. Sci. Tech., 1994, v.10, pp. 1050-1056.
20. Korichi, J,.Priestner,R., Mater Sci.Tech., 1995, v.11, pp.901-907.
21. Krishnaraj, D., Seshan,S., AFS Trans., 95-119, pp.767-776.
22. Hamid Ali, A.S. and Elliott, R., Mater. Sci. and Tech., 1996, v.12, pp.780-787.
23. Hamid Ali, A.S., Elliott, R., Mater. Sci. Tech., 1997, v.13, pp.24-30
24. A.A. Nofal, Jekova,, L., Journal of the University of Chemical Technology and Metallurgy, 2009, vol.44, pp.213-228
25. Duckworth, W.E., “Statistical Techniques in Technological Researché, p.60-86, Methuen and Co., 1968, London.
26. Oriani, R.A., Acta Met., 1964, v.12, p.1399.,

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