International Journal of Minerals, Metallurgy and Materials Volume 19, Number 6, June 2012, Page 506 DOI: 10.1007/s12613-012-0587-1
Formation kinetics of austenite in pearlitic ductile iron Payam Abdollahi, Alborz Amirsadeghi, Shahram Kheirandish, and Shamsoddin Mirdamadi Department of Materials Science and Engineering, Iran University of Science and Technology, Tehran 4513863833, Iran (Received: 26 June 2011; revised: 19 July 2011; accepted: 23 July 2011)
Abstract: This work evaluated the isothermal transformation of austenite in unalloyed pearlitic ductile iron and drew the isothermal phase diagram of austenitization in the ductile iron. Austenite forms at grain boundaries and then grows up to graphite regions during austenitization. The formation kinetics of austenite complies with the Avrami equation, in which the parameter (n) ranges from 4.71 to 4.99. The start time and finish time of transformation can be calculated at each temperature using the Avrami equation. Keywords: cast iron; ductility; pearlite; austenitization; kinetics; isothermal transformations
1. Introduction Due to appropriate mechanical properties, such as resistance to abrasion and fatigue, machining-ability, strength, and low cost, ductile cast iron is considered as an appropriate substitute for steel components [1]. Austenitization is required to be done before austempering, annealing, or normalizing, and has a determinant effect on the microstructure and properties of the product. Therefore, an optimization of austenitization temperature and time is of high importance. It is obvious from the kinetic perspective that the size of austenite grains is influenced by time and temperature. The kinetics of austenite nucleation and growth is determinant to the final microstructure [2]. Austenite transformation, which is diffusion control, is affected by the initial microstructure of cast iron, chemical composition, austenitization temperature and time, and segregation of alloying elements [3-4]. Alloying elements influence the kinetics of austenitization through the carbon diffusion coefficient. Ferrite stabilizing elements, such as Si, hinder the austenite transformation upon heating, while austenite stabilizing elements, such as Mn and Ni, lower the start temperature of austenite transformation. Segregation of alloying elements is inevitable during non-equilibrium solidification. Therefore, it is obvious that austenite formers, such as Mn, which segregate in the intracellular boundaries Corresponding author: Payam Abdollahi
[5-6], make these areas appropriate for austenite nucleation. Besides, the grain boundaries have a lower transformation energy barrier since they act as fast paths for diffusion. Therefore, it is the reason that austenite first nucleates at the grain boundary and grows up continuously, until the austenite grains meet each other [7]. The carbon content of austenite is also determined by carbon diffusion from graphite into the austenitic matrix, and therefore is dependent on the austenitization time and temperature. Decreasing the austenitization temperature lowers the carbon content of austenite [8]. Besides, increasing the austenitization time and temperature increase the carbon content of austenite. The high carbon content of austenite hinders the transformation of austenite to martensite, giving rise to an increased amount of remaining austenite upon quenching [4]. Owing to the short diffusion path in a pearlitic structure for carbon diffusion from cementite (Fe3C) into ferrite, pearlitic structures are appropriate for the formation and growth of austenite [9]. In a pearlitic structure, nucleation can also occur inside the pearlite colonies as well as the intercellular boundaries [7]. Austenitization transformation follows the Avrami equation as X=1−exp(−Ktn)
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© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2012
(1)
P. Abdollahi et al., Formation kinetics of austenite in pearlitic ductile iron
where X is the volumetric fraction of austenite formed, K the velocity constant which is temperature-dependent, n the exponent which is dependent on the transformation mechanism, and t the austenitization time. To obtain the values of n and K at each temperature, Eq. (2) is shown as the following. lnln(1/(1−X))=nlnt+lnK
(2)
According to experimental data, the values of n and K can be obtained by plotting. Later, by assuming X=0.01 as the transformation initiation point and X=0.99 as the completion of transformation, the start time and end time for each temperature can also be readily obtained [9].
2. Experimental Table 1 shows the chemical composition of cast iron obtained by spectrophotometer. Cylindrical bars with 20 mm in diameter were cast in sand molds. Magnesium ferrosilicon was used as a nucleant and ferrosilicon was used as a deoxidizer. The bars were cut to round pieces with 10 mm thick, which were divided into four equal parts. Then, the samples were normalized at 950°C for 1 h and cooled at ambient temperature to obtain full-pearlitic microstructures. Before austenitization, the samples were pre-heated at 600°C for 1 min to homogenize the temperature throughout the thickness. Pre-heating was performed to shorten the time to reach the austenitization temperature. The samples were eventually austenitized in a sodium chloride molten bath, and immediately quenched in cold water. The temperature precision of the furnace used was ±3°C.
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3. Results and discussion Prior to austenitization, the primary structure of the sample is thoroughly pearlitic. Figs. 1 and 2 depict the microstructures of ductile iron before and after etching by the sodium-meta-bisulphate solution. From Fig. 2, it can be seen that the pearlitic matrix occupies more than 98% of the matrix. The test samples have 200 m−2 graphite with 98% sphering in shape and an initial hardness of Hv 290. Figs. 2 to 5 show the microstructures of the austenitized samples at 920°C in different intervals of time. According to the figures, the transformation of pearlite to austenite starts in the regions far away from the graphite nodules, and the regions near the graphite nodules undergo the austenitic transformation last. This is due to the high silicon content segregating in these regions, which results in the decrease of carbon diffusivity in austenite [3]. Compared with Figs. 3 and 5, the increase of austenitization time results in an increase of martensite fraction. Considering the relatively high temperature of austenitization, the increase in martensite fraction by temperature is so significant. Compared Fig. 5 with Fig. 6, the martensite contents are the same in both the sam-
Table 1. Chemical composition of elements in cast iron wt% C
Si
Mn
P
S
3.9
2
0.2
0.05
0.01
Owing to the instability of austenite phase at ambient temperature during quenching, austenite formed at the austenitization stage may be transformed into martensite. After polishing the samples, 11vol% sodium-meta-bisulphate was used as an etchant to distinguish martensite phase from pearlite. The volume fraction of austenite formed was done by the investigation of martensite phase formed upon quenching. The volume fraction of martensite was measured according to ASTM E-562 by optical microscopy and the dot-counting method. Meanwhile, to investigate the change in hardness, the samples underwent hardness tests by the Vickers method. The standard deviation of Vickers hardness tests was Hv ±5.
Fig. 1. Microstructure of ductile cast iron after normalizing without etching.
Fig. 2. Microstructure of ductile cast iron with the pearlitic matrix and nodular graphite.
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ples, but the austenitization time of the sample in Fig. 6 is longer due to a lower carbon diffusion in austenite.
Fig. 3. Microstructure change during austenitization at 920°C for 12 s.
The results from metallographic investigations on fully martensitic samples showed that the decrease of austenitization temperature led to a finer martensite with the increase of numbers and dispersal. This could be justified according to transformation regulations. In fact, with super cool (∆T) being increased, the critical radius of the nucleus shortened thermodynamically, which thereby increased the number of stable nucleants in the molt. This phenomenon resulted in the refinement of austenite grains, and therefore, the refinement of martensite.
4. Transformation kinetics
Fig. 4. Microstructure change during austenitization at 920°C for 14 s.
Fig. 7 shows the volume fraction of austenite formed as a function of austenitization time at different temperatures. It is observed that the start time of austenite transformation is much lower at higher temperatures. Meanwhile, considering the gradient of the curves, the transformation rate is much higher at higher temperatures, because the austenite transformation is basically diffusion controlled. The factors, such as temperature, can cause the increase of transformation rate.
Fig. 5. Microstructure change during austenitization at 920°C for 16 s. Fig. 7. Austenite volumetric fraction according to time and temperature.
Fig. 6. Microstructure change during austenitization at 880°C for 25 s.
As mentioned before, the austenitization treatment follows the Avrami equation. The plot of lnln(1/(1−X)) vs. lnt at each temperature is depicted in Fig. 8. The gradient of each line implies the amount of n, and the y-intercept of each line implies lnK. With the n and K coefficients being obtained at each temperature, the relevant Avrami equation is obtained. X=0.01 and X=0.99 are considered as the transformation start point and transformation completion, respectively. The transformation start time and finish time can be calculated for each temperature by using the Avrami equa-
P. Abdollahi et al., Formation kinetics of austenite in pearlitic ductile iron
tion. The n and K coefficients calculated at each temperature and the transformation start time and finish time calculated are shown in Table 2. The results obtained show that, with the temperature increasing, the value of K increases as well, yet n has limited changes. The reason for partial changes in n is its dependency on the transformation mechanism [7], and thus, n is of considerable changes only when the transformation mechanism changes; while austenitization at all temperatures has a diffusion-controlled mechanism. Yet, the dependency of K on temperature leads to the increasing with the rise of temperature. Fig. 9 shows the dependency of K on temperature.
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With the transformation start time and finish time obtained in terms of temperature, the isothermal transformation curves of austenitization (IT curves) are shown in Fig. 10. The curves show that the austenitization terminates in 20 s at 920°C, while with a 100°C decrease in temperature, the transformation requires 70 s to finish. It is interesting to compare the results with another study on austenitization of ferritic ductile Iron. Austenitization of ductile iron with the similar chemical composition but a ferritic matrix was studied by Schissler et al. [6]. Their results showed that the required time for austenitization to finish at 820 and 900°C reached 300 and 900 s [6], respectively, which was evidently longer than the pearlitic structure. The shorter transformation finish time in a pearlitic structure is due to the shorter diffusion path for carbon in the pearlitic structure. The laminarly distributed cementite (high carbon content) within ferrite (low carbon content) decomposes upon during heating and acts as a carbon source for ferrite.
Fig. 8. Kinetics of austenitization according to austenitization temperature. Table 2. Values of n and K calculated and the austenite formation start time and finish time (ts and tf) in terms of austenite temperature Temperature / °C
K
810
n
ts / s
tf / s
1.48×10−9
4.71
28
104
840
−9
9.02×10
4.86
18
62
880
1.41×10−7
4.88
10
34
920
1.01×10−6
4.98
6
22
Fig. 10. Isothermal transformation diagram of austenite formation during heating.
5. Conclusions Isothermal transformation of austenite in unalloyed pearlitic ductile cast iron was studied between 810 and 920°C, and the following results were obtained. (1) In pearlitic structure, grain boundaries are the most important places for austenite nucleation; then austenite grows towards to the graphite nodules. (2) With the austenitization time increasing and within the specified temperature limit, the austenite volume fraction increases. For example, at the thermal limit of 920°C, with the austenitization time increasing from 8 to 16 s, the austenite volume fraction increases from 3% to 6%.
Fig. 9. Values of K obtained at different temperatures.
(3) With the increase of austenitization temperature, austenitization transformation starts and finishes earlier. With the austenitization temperature increasing from 810 to
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920°C, the transformation start time and finish time decrease from 28 to 6 s and 104 to 22 s, respectively. (4) The austenitization reaction kinetics can be described by the Avrami equation, in which the exponential value of n for this cast iron falls between 4.71 and 4.99.
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