Effect of Isothermal Heat Treatment Time on the Microstructure and Properties of 4.3% Al Austempered Ductile Iron

This study aimed to investigate the effect of replacing silicon with 4.3% aluminum on the kinetics of the bainitic transformation and its effect on the physical properties of cast iron with a chemical composition of 3.4% C, 4.3% Al, 0.32% Si, 0.04% Mg. First, the samples were prepared to the same dimensions followed by austenitization for 2 hours at 900 °C; Then, austempering was performed on the samples at four temperatures i.e., 375 °C, 400 °C, 425 °C, and 450 °C (high-temperature region) for different periods (1–512 min). Microstructural studies were performed by both optical and electron microscopes and it was found that with an increase in the austempering temperature, the thickness of the bainitic ferrite plates increased from 0.31 μm at 375 °C to 0.63 μm at 450 °C and a coarser microstructure was obtained. The results of this study showed that the austempering transformation in Al-alloyed ductile iron will result in the formation of an ausferritic structure.


Introduction
Replacing silicon with aluminum changes some of the properties of cast iron including wear resistance and heat transfer resistance. 1,2In general, aluminum cast irons are divided into two categories.In the first category, aluminum is present in cast iron along with silicon.In this category, the Al content can be as high as 25%.In this case, aluminum can normally act as a graphitizer.In the second category, aluminum is present rather than silicon.The Al content in this category is up to 6% and the amount of other elements (manganese, phosphorus, sulfur, etc.) is the same as conventional cast irons.Aluminum, similar to silicon, on the one hand, increases the formation of graphite during eutectic solidification and on the other hand, stabilizes pearlite during eutectic transformation.Aluminum increases the temperature of eutectic transformation.It causes the formation and growth of spheres at a higher temperature, thus increasing the carbon penetration rate in the melt and increasing the probability of forming more spheres. 3,4til about four decades ago, spherical graphite seemed to be only producible in the Fe-C-Si system.However, in 1983, Ghoreshi and Kondic reported that spherical graphite could be obtained in the Fe-C-Al system.In general, SG-Al cast irons have tremendous advantages over traditional conventional SG-Si cast irons, as follows: 1.A range of mechanical properties that is higher than that of silicon cast iron is achievable, 2. The fabrication of thin components without the formation of carbides is quite possible and the susceptibility to carbide formation in this type of cast iron is much lower, 3. The oxidation resistance at high temperatures in these cast irons is much higher than in conventional cast irons, 5 4. In the fabrication of austenitized ductile iron, the Fe-C-Al system is much more suitable because: a) Compared to SG-Si cast irons, SG-Al cast irons show higher strength, which is attributed to the finer microstructure of this type of cast iron, which is an important factor in increasing strength.
b) n SG-Al austenitic cast irons, a high amount of impact energy (about 110 J) is obtained after three hours of austempering, which is due to the greater stability of the untransformed austenite compared with that of SG-Si cast iron in which, the impact energy reduces drastically after one hour. 6 the Fe-C-Si system, there is no concentration range in which the carbon potential becomes negative and then positive again while in the Fe-C-Al system, there are two stability ranges for the formation of Fe 3 AlC and Al 4 C 3 compounds.However, in the case of Si, there is only one range for the formation of SiC. 4 The heat treatment cycle of ADI consists of four main stages: austenitizing, quenching to the austempering temperature, austempering, and cooling to the room temperature.All four stages are important in determining the precise microstructure and the choice of the heat treatment parameters is effective in determining the final properties. 7 the present study, the microstructure and hardness of the austempered aluminum ductile cast iron are investigated and compared with those of silicon cast irons.For this purpose, austempering treatment was performed at four temperatures including 375, 400, 425, and 450 °C for different times 1, 4, 16, 32, 64, 128, 256, and 512 min and the results were compared with those of silicon cast iron.

Experimental Procedure
In this work, an induction furnace with a 500 kg capacity rammed with silica-based refractory was charged with steel scrap, low-sulfur oil coke, and aluminum ingots.The final chemical composition was determined by optical spark spectroscopy.The optical spark spectroscopy showed the aluminum element in amounts above 0.4% with a high error; however, the inductively coupled plasma (ICP) method was also used to accurately determine the aluminum element.The sample was taken from the melt.
For molding, silica sand with a grain size of 171 (A.F.S. grain fineness number) was used.The cast iron was modified by the sandwich method at approximately 1460 °C.
The chemical analysis of the melt, after spheroidisation, is shown in Table 1.The melt was ladle inoculated with 0.4% of Coal powder at 1380°C and poured at 1345 °C.The casting design is shown in Figure 1.Samples were cut from the middle of the 4mm thickness belt.The dimensions of the samples were 4 mm 9 10 mm 9 10 mm.Several samples underwent standard metallographic preparations to investigate the microstructural properties including the volume percentages of graphite, ferrite, and pearlite before heat treatment.Then, using the Image J software, the volume percentages of graphite, ferrite, and pearlite phases were calculated.To calculate volume percentages of graphite used contrast in optical microstructure, in ImageJ software (In each case, 5 optical microstructures, were used for calculation).To perform the heat treatment cycle, the samples were first austenitized at 900 °C for 2 hours.They were then immediately transferred into a molten salt bath with a composition of 55% sodium nitrite and 45% potassium nitrate for austempering at temperatures of 375 °C, 400 °C, 425 °C, and 450 °C and kept at that temperature for various holding times of 1, 4, 16, 32, 64, 128, 256, and 512 min.They were then removed from the furnace after the heat treatment and cooled in the air.To determine and study the microstructure, after polishing, the samples were etched using the 5% nital solution.The microstructure of the samples was examined by both optical and electron microscopes and the thickness of the bainitic ferrite plates was measured with the Image J software.In addition, Vickers hardness (HV0.1)testing was performed on the samples.

Investigation of As-cast Microstructure
The microstructure of the non-heat-treated as-cast ductile cast iron sample before etching and after etching is shown in Figure 2. The post-etching microstructure included a pearlitic-ferritic matrix (dominant phase) with spherical graphite.Aluminum in high percentages, completely dissolved in ferrite and austenite, and partially dissolved in carbide.Aluminum increases the number of eutectic cells, which causes more nucleation in cast iron.In addition, the presence of aluminum stabilizes the pearlite phase in the eutectic transformation. 8,9The microstructural characteristics of this cast iron are presented in Table 2.

Investigation of Effect of Austempering Time on Microstructure
Figures 3, 4, 5 and 6 show the microstructure of the samples austempered at 375 °C, 400 °C, 425 °C, and 450 °C.
Since the austempering transformation is a process based on nucleation and growth and consists of three stages, 10,11 it is expected that at an appropriate austempering temperature, more carbon from the bainitic ferrite diffuses into the austenite with increasing the austempering time.Moreover, the austenite present between the bainitic ferrite plates is enriched with carbon to achieve thermal and then mechanical stability.Finally, by decomposing the highcarbon austenite to ferrite and carbide, the third stage of the transformation begins.According to Figure 3, at 375 °C and short austempering times, (1 min and 4 min), there are some martensite, unreacted austenite as well as bainitic ferrite.With increasing the transformation time, the martensite phase content decreases and is replaced by untransformed austenite and bainitic ferrite so that after 32 min, martensite is completely removed from the microstructure and the matrix consists of bainitic ferrite and untransformed austenite.In the early stages of transformation, there was not enough time for the transformation to progress, and insufficient growth of the bainitic ferrite plates causes the lack of saturation of the untransformed austenite with carbon.Therefore, there is some martensite in the microstructure and most of the untransformed austenite in the micrographs is untransformed austenite.The micrographs of the samples austempered at 400 °C (Figure 4) show that the microstructure mainly consists of bainitic ferrite plates, untransformed austenite films, and some blocky austenite.Blocky austenite is caused by the geometric shape of the primary austenite restricted by the bainitic clusters while austenite films are imprisoned between subunits of the bainitic ferrite units.Figures 5 and 6 show the microstructure of the samples austenized at 425 °C and 450 °C at different times.At these temperatures, at 1 and 4 min, it is clear that the austempering transformation has progressed significantly, and with increasing the austempering time to 16 min, the microstructure has become completely ausferritic.Ausferrite is a microconstituent consisting of bainitic ferrite (i.e.acicular ferrite) and high-carbon untransformed austenite.The results are consistent with those reported by Eric on ductile aluminum cast iron. 12ic et al. showed that with increasing the austempering temperature from 300 °C to 400 °C, due to significant changes in carbon diffusion rate and the reduction of the nucleation driving force of bainitic ferrite, a high amount of ausferrite is formed and bainitic ferrite is formed in the form of plates.The microstructure in aluminum cast irons in the upper bainite range is in the form of coarse ferrite plates with untransformed austenite, which is mostly blocky-shaped and similar to other austempered ductile cast irons. 12snjak et al. performed austempering on conventional ductile iron containing copper, molybdenum, and nickel stated that austempering at high temperatures (350 °C and 400 °C) reduced the bainitic ferrite nuclei while the growth rate of bainitic ferrite became faster.As a result, the matrix   As can be seen in all the micrographs, bainitic ferrite nucleates from the austenite grain boundaries as well as the interface between the graphite spheres and austenite.The concentration of graphitizer elements around graphite spheres has the highest value while it is the lowest at the boundary of eutectic cells. 14,15At short times (1 min), a    the greater transformation progress.These micrographs are consistent with SEM images presented by Perez et al. about silicon cast iron. 16,17They stated that when austempering is performed at high temperatures (above 350 °C), a coarse structure consisting of bainitic ferrite and untransformed austenite is observed.The results of SEM images of cast iron containing 2.11% Al at 350 °C presented by Naziftoosi et al. also showed that a small part of the matrix was converted to bainitic ferrite after 1 min. 18Subsequently, with increasing the austempering time, a larger volume fraction of austenite was transformed to bainitic ferrite.They found that the austempering time of 100 min was sufficient to achieve a microstructure with the best combination of mechanical properties and the minimum amount of martensite or carbide.

Investigation of the Effect of Austempering Temperature on Microstructure
Figure 11 shows the microstructure of the samples austempered at 375 °C, 400 °C, 425 °C, and 450 °C for 1 min.As shown in Figure 10a, the microstructure of the sample austempered at 375 °C has thinner plates of bainitic ferrite as well as some martensite.According to Figure 12, which shows the average thickness of the bainitic ferrite plates, the bainitic ferrite plates become thicker with increasing austempering temperature, and martensite is removed from the structure.Panneerselvam et al. also stated that a coarser microstructure can be obtained by increasing the austempering temperature. 19At lower austempering temperatures, the difference between the austenitization and austempering temperatures is higher, resulting in a higher driving force for the nucleation of ferrite and higher nucleation of this phase.However, due to the lower austempering temperature, both the diffusion of carbon and the growth rate is lower.According to the SEM and optical images and Figure 13, which shows the average thickness of the bainitic ferrite plates, it is found that with increasing the austempering temperature, the average thickness of the bainitic ferrite plates increases from 0.31 lm at 375 °C to 0.63 lm at 450 °C.Also, with increasing temperature, the number of bainitic ferrite plates decreases, and these plates appear shorter, thicker, and more distinct in structure.In fact, according to the images depicted, the growth rate of ferrite plates in upper bainite is higher than the nucleation rate in terms of transformation kinetics.Therefore, the ferrite plates are also thicker in the transverse direction.Due to the higher diffusion rate of carbon in the upper bainite region, more carbon is able to diffuse to the outside while the bainitic ferrite plates are growing, which leads to the enrichment of the austenite phase between the plates with carbon.

Investigation of the Effect of Austempering Time and Temperature on Hardness
Figure 14 shows the variation in the hardness of the samples with the austempering time at different austempering temperatures.According to this figure, the general trend of hardness changes with the austempering time, so that at the beginning and passing through the first stage of austempering and entering the second stage of austempering, the hardness value decreases significantly.Then, with increasing the austempering time and increasing the carbon content of the untransformed austenite in the second stage, the hardness increases slightly.The hardness increases for all four temperatures after the longest austempering temperature i.e., 512 min.This increase in hardness after long periods of austempering is probably due to the beginning of the third stage of austempering and the formation of carbides in the structure.According to the figure, the highest hardness is obtained at three temperatures of 375, 400, and 425 °C after austempering for 1 min.The higher hardness of the sample austempered for 1 min at 375 °C compared with that austempered at 450 °C is probably due to the presence of martensite in the microstructure.
The higher hardness achieved after austempering for 512 min at 450 °C compared with that austempered for 1 min at this temperature may be due to the higher amount of carbide resulting from the decomposition of austenite in the third stage of austempering caused by the higher  austempering temperature.These carbides are very fine and require a high-resolution electron microscope to observe them.In general, the hardness of the whole structure is a function of the thickness of the ferrite plates and the percentage of carbon in the ferrite plates, and the quality of the untransformed austenite, so that as the thickness of the plates increases, the hardness decreases.
At high temperatures, due to the faster diffusion of carbon into the austenite, this phase is stabilized faster, which reduces the amount of martensite and removes it, and decreases the hardness.Figure 15 shows an example of the variation of hardness with the austempering time for the silicon cast iron which is in consistent with the variation of hardness in the present study.
The results reported by Ghoroghi et al. showed that hardness decreases with increasing the austempering time at a constant austempering temperature in the ductile silicon cast iron containing nickel and copper at austempering temperatures of 350 °C and 390 °C. 21Subsequently, the hardness value increases with the onset of the third stage of austempering and the decomposition of the untransformed austenite to ferrite and carbide.Also, higher austempering temperatures lead to lower hardness values owing to both a coarser ausferrite structure and a higher amount of highcarbon austenite.

Conclusion
The major results of the present research are as follows: 1.With increasing the austempering temperature, due to increased diffusion speed, the thickness of the bainitic ferrite plates increased from 0.31 lm to 0.63 lm. 2. The thickness of the plates increased with the austempering increasing time at a certain temperature due to the opportunity for more growth and increasing the austempering temperature due to the higher diffusion rate.3. The maximum hardness was obtained after austempering at 375 °C for 1 min (612 HV).This high hardness is due to the presence of a martensite phase in the microstructure 4. With the passage of time at each temperature, the hardness first decreased due to the progress of the austempering process and the formation of ausferrite, and the removal of martensite.After that, it increased with the passage of time due to the start of the next step of the austemper and the decomposition of the untransformed austenite into carbide and ferrite. 5.By increasing the austempering temperature for 512 min, the hardness increases to 493 HV.The reason for this can be attributed to the greater decomposition of the untransformed austenite into ferrite and carbide at 450 °C compared to other temperatures.

Figure 2 .
Figure 2. Optical micrographs of the microstructure of the as-cast ductile cast iron, (a) before etching, (b) after etching.

Figures 7 ,
Figures 7, 8, 9 and 10 show the SEM images of samples austempered at 375 °C, 400 °C, 425 °C, and 450 °C.In these images, the lighter regions are austenite while the darker regions are bainitic ferrite.At 375 °C, the bainitic ferrite plates trapped between the austenite films are finer

Figure 14 .
Figure 14.Variation of hardness with the austempering time and temperature.

Table 1 .
Chemical Composition of the Cast Iron Studied in this Sample (wt%) P Cu Mg Ni Mn Si Al C Fe 0.02 0.4 0.04 0.72 0.36 0.32 4.3 3.4 Balance Figure 1.A schematic figure of the mold and position of samples.

Table 2 .
Characteristics of the Microstructure of As-Cast Ductile Cast Iron 13,14national Journal of Metalcasting/Volume 17, Issue 4, 2023 had a lower volume fraction of bainitic ferrite, but these bainitic ferrite plates were significantly coarser.Also, the optical micrographs are presented.Balos et al. showed a high amount of ausferrite at 400 °C including coarse ferrite plates with untransformed austenite as blocky austenite, which is in agreement with the results of the present study.13,14