Materials Science, Vol. 40, No. 4, 2004

AUSTEMPERING OF SPHEROIDAL GRAPHITE CAST IRON

Z. M. El-Baradie, M. M. Ibrahim, I. A. El-Sisy, and A. A. Abd El-Hakeem

We study gray spherulitic cast iron, its microstructure, hardness ultimate strength in tension,elongation, and impact toughness as functions of the duration of isothermal austenitizing in thebainite region at 350 and 400°C after austenitizing at 900°C. As the temperature of quenchingincreases from 350 to 400°C, the microstructure of the analyzed cast iron changes from lower tohigher bainite and the amount of retained austenite increases (its maximum is attained after 1 h).At the same time, the ultimate strength in tension and hardness decrease, whereas the elongationand fracture toughness increase.

Austempered ductile cast iron (ADI) has a better combination of strength, ductility, toughness, and fatigueand wear resistance. Thus, it is considered as an important engineering material, which makes it useful in a greatnumber of applications [1 – 4].

ADI is obtained by subjecting the ductile cast iron to austempering heat-treatment cycles. The austemper-

ing reactions in ADI occur in two stages [5]. In stage I, the matrix austenite γ0 with carbon content Cγ0

isother-mally transforms into ausferrite, i.e., into a mixture of acicular ferrite with carbon-enriched stabilized austenite

γs with carbon content Csγ . In stage II, the stabilized austenite γs decomposes into ferrite and carbide.

The mechanical properties of ADI depend on the relative amounts of acicular ferrite and stabilized austeniteand on the morphology of the former. These, in turn, are affected by changing the austempering conditions

(temperatures TA and time tA ). Higher TA produce coarser matrices resulting in lower strength and higher duc-tility. On the other hand, transition carbides are formed inside ferrite if the austempering temperature is lowenough to slow down the diffusion of carbon from ferrite. The microstructure obtained as a result is bainitic[4–6].

In the present work, the effects of the time of austenitizing and temperature and time of austempering on themicrostructure, hardness, ultimate tensile strength, ductility, and toughness of ADI are carefully studied.

Experimental Procedure

Material and Heat Treatment. The ductile cast iron (DI) used in the present work has the followingchemical composition (wt. %): 3.58 C, 2.1 Si, 0.22 Mn, 0.039 Mg, 0.04 P, 0.006 S, balance Fe. It was sup-plied in the form of pipes with an inner diameter of 800 mm and a thickness of 15 mm. Specimens made of thisDI were subjected to two stages of heat treatment to produce austempered ductile cast iron: (a) austenitizing at

900°C for 30 min and (b) austempering at 350 and 400°C for different times at each temperature (15, 30, 60,120, and 180 min) in a salt bath with subsequent quenching in water rapid enough to prevent any transforma-tions of austenite into ferrite and/or pearlite.

Microscopy and X-Ray Analysis. Specimens were cut out from the as-cast and heat treated alloys. Theywere then polished by using standard metallographic techniques and etched with the Nital solution (3% nitric

Faculty of Engineering, Cairo University, Cairo, Egypt. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 4, pp. 79–83, July–August, 2004. Original article submitted January 12, 2004.

1068–820X/04/4004–0523 © 2005 Springer Science+Business Media, Inc. 523

524 Z. M. EL-BARADIE, M. M. IBRAHIM, I. A. EL-SISY, AND A. A. ABD EL-HAKEEM

acid solution in ethyl alcohol). An optical microscope was used for microstructural analysis. The phases andconstituents of the matrix were identified by using Co – Kα radiation with a Ni filter at 30 kV and 20 mA. Thecarbon content of retained austenite was calculated according to the angular positions of the FCC austenitepeaks, and the volume fraction of retained austenite was determined by using the Aranzabai [4] formula:

Xγ =

I

RI

R

I

R

γ

γ

γ

γ

α

α+

,

where Iγ and Iα are the integrated intensities from a given (hkl) plan from austenite and ferrite, respectively,

and Rγ and Rα are, respectively, the theoretical relative intensities for austenite and ferrite. The carbon content

Cγ was calculated from the equation:

Cγ = aγ − 3 555

0 044

.

.,

where aγ is the lattice parameter computed according to the angular position of the austenite peak [7].

Mechanical Testing. Brinell macrohardness was measured under a load of 200 N and determined as the

average value of ten readings. The scattering was less than ± 2%. Tensile tests were conducted to failure onspecimens (30 mm in gauge length and 5 mm in gauge diameter with threaded end) in different conditions byusing a 10 kN universal testing machine with a cross-head speed of 10 mm / min. The Charpy V-notch impacttests were also carried out at room temperature on the as-cast and as-heat-treated specimens. The average ofthree observations was taken in this case.

Results and Discussion

Microstructures. As Received Material. The microstructure of the as-received (DI) material consists ofgraphite nodules embedded in a ferrite matrix (Fig. 1). These nodules are distributed homogeneously and the

number of nodules per an area of 1 mm2 is 1200–1500. In the production of good pipes, the number of graphitenodules may range from 500–1000 depending on the mold size and the thickness of the pipe.

Prior to austempering, the material was subjected to an austenitizing treatment at 900°C for 30 min. Thetemperature for the start of the martensitic transformation of this austenite Ms is determined by the followingequation [7]:

Ms = 400 – 260 Cγ and Cγ = Tγ

420 – 0.17 (% Si) – 0.95.

Hence, Cγ = 0.785 and Ms = 175°C.The temperature and time of isothermal transformation during the austempering treatment of ductile irons

have a marked influence on the relative amounts of bainite and austenite produced as a result of this transforma-tion.

AUSTEMPERING OF SPHEROIDAL GRAPHITE CAST IRON 525

Fig. 1. Microstructure of the as-received ductile cast iron.

Fig. 2. Microstructures of ADI austempered at 350°C (upper row) and 400°C (bottom row) for different times; × 500.

Austempering at 350°C (Fig. 2). For short periods of austempering, the matrix consists of martensite,lower bainite, and retained austenite. As shown in [8], the lower bainite takes the form of bundles of bainitic fer-rite plates. The higher Si content of ductile iron suppresses the formation of cementite and, therefore, in thecourse of subsequent isothermal holding, we observe thickening of ferrite plates and enrichment of the austenitewith carbon. After a certain period of time, the first stage of the austempering reaction is completed and the re-sulting austenite has a temperature Ms below room temperature. In this case, it is thermally very stable [9].Thus, as the austempering time increases, the amount of martensite in the matrix decreases and the amount ofretained austenite increases. After 60 min, the martensite disappears from the matrix, which contains only lowerbainite and retained austenite. However, for longer periods of time, the amount of retained austenite decreasesdue to its transformation into lower bainite [10].

Austempering at 400°C (Fig. 2). The present phases are upper bainite, retained austenite, martensite, andnodules of graphite. The amount of transformed upper bainite increases with the duration of austempering. Forshorter times, the nontransformed austenite forms martensite as a result of quenching. For longer times, no mar-tensite appears due to the elevated stability of the nontransformed austenite enriched with carbon.

526 Z. M. EL-BARADIE, M. M. IBRAHIM, I. A. EL-SISY, AND A. A. ABD EL-HAKEEM

(a) (b)

Fig. 3. Effects of the time of austempering on the amount of austenite in ADI (a) and the hardness of DI (b) (900°C).

This carbon is rejected from ferrite due to the higher silicon content of ductile cast iron. The formation ofcarbides is suppressed [11].

The amount of austenite increases with the time of austempering, reaches its maximum value, and then de-

creases (Fig. 3a). The procedure of austempering at 400°C produces higher austenite content than that at 350°C.

According to the Aranzabai formula [4], the amount of retained austenite at 400°C for 60 min (the optimumproperties) is 48.4% and the carbon content of austenite Cγ is 1.3 wt. %. The higher carbon content of austeniteis explained by higher diffusion rates of carbon from the regions transforming ferrite into the surrounding auste-nite at the higher austempering temperature as well as by the slower kinetics of ferrite formation in the process ofsupercooling [3].

This enables us to conclude that, for the same austempering time, the amount of stabilized austenite in thematrix at room temperature increases with the temperature of isothermal transformation (Fig. 3d). This increase

can be attributed to a higher carbon content of austenite at 400°C just prior to quenching at room temperature.

Lee et al. [9] indicated that the material austempered at 400°C contains more retained austenite than that treated

at 340°C. The diffusion of carbon into the austenite is much more rapid in the course of austempering at thehigher temperature.

Mechanical Properties

Hardness. Austenitization at 900°C for Different Times (Fig. 3b). For the first 30 min, hardness in-creases with time due to the uncompleted martensite transformation or austenite transformation. The completemartensite transformation is realized for 30 min (peak value). Then the level of hardness decreases with timedue to the decrease in the amount of martensite and increase in the amount of retained austenite.

Austempering at 350 and 400°C for Different Times (Fig. 4a). The level of hardness first rapidly de-creases as the time of treatment increases. Later, it reaches the minimum value and then slightly increases. Asthe austempering time increases, the nontransformed austenite is enriched with carbon as a result of the lower

bainite (at 350°C) or upper bainite (at 400°C) transformation. This leads to the formation of a more stable re-

tained austenite with lower temperature Ms , which may equal to – 80°C [12]. Thus, as the austempering timeincreases, the amount of martensite decreases and the amounts of both stabilized retained austenite and bainite

increase, which results in a decrease in hardness. The rate of decrease at the higher temperature (400°C) is low-

er than that at the lower temperature (350°C). The diffusion of carbon at the higher temperature is faster, and thematerial contains more retained austenite [13].

AUSTEMPERING OF SPHEROIDAL GRAPHITE CAST IRON 527

(a) (b)

(c) (d)

Fig. 4. Effects of the austempering time on the hardness (a), tensile strength (b) elongation ∆ l (c), and impact toughness (d) of ADI.

Table 1

Material UTS, MPa Elongation, % HB Toughness, J

As-received DI 420 16 185 115

ADI (350°C for 60 min) 1200 6.5 370 85

ADI (400°C for 60 min) 1000 10 304 110

The minimum value of hardness for both austempering temperatures is obtained after 60 min. This mini-mum hardness is attributed to the presence of the maximum amount of stabilized austenite at room temperature[13]. Since austenite is a softer phase as compared with upper bainite, lower bainite, or martensite, it is thereforeresponsible for the hardness valley appearing after 60 min and causes an increase in hardness for both austem-pering temperatures. This increase can be explained by the increase in the amount of lower bainite at the ex-

pense of the retained austenite at 350°C and the decomposition of austenite into upper bainite at 400°C. The

difference between the values of hardness attained at different austempering temperatures (350 and 400°C) was

also expected because lower bainite [the austempering product of the lower temperature (350°C)] is harder than

upper bainite [the product of the higher temperature (400°C)] [12].

Other Mechanical Properties. The behaviors of the ultimate tensile strength, percent elongation, and im-pact toughness are similar. They increase with the time of austempering, attain their maximum values, and final-ly, undergo a smooth decrease. The initial increase can be attributed to the decrease in the amount of retained

austenite and lower (at 350°C) or upper bainite (at 400°C) constituents. For longer periods of austempering,these mechanical characteristics decrease due to different causes: their reduction in high-carbon austenite, in-

528 Z. M. EL-BARADIE, M. M. IBRAHIM, I. A. EL-SISY, AND A. A. ABD EL-HAKEEM

crease in the precipitation carbides, and transformation of high-carbon martensite into ferrite. Contrary to the

percent elongation and impact energy, the UTS and impact toughness after austempering at 350°C are higher

than after austempering at 400°C . In this case, an important role is played by the amount of retained austenite

(smaller at 400°C) and the difference between the lower brittle bainite (350°C) and upper bainite (400°C).

CONCLUSIONS

The austempering treatments exert well-pronounced effects upon the microstructure and mechanical prop-

erties of ductile iron. The austempering carried out at 350°C produces lower bainite, whereas the austempering

carried out at 400°C produces upper bainite. The amount of retained austenite increases with the temperature ofaustempering. In addition, the amount of retained austenite exhibits a peak value corresponding to a period of

austempering of 60 min for both austempering temperatures (350 and 400°C). The optimum mechanical prop-

erties of ductile iron (presented in Table 1) are obtained through austempering at 350 and 400°C for 1 h. The ultimate tensile strength and hardness decrease and the impact toughness and elongation increase as the

austempering temperature increases. Long periods of austempering are not recommended for ADI, since re-tained austenite is transformed into heavy carbides, which strongly affect its mechanical properties. The presenceof carbides increases the value of hardness but decreases the UTS, impact toughness, and ductility.

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