Microstructural Evaluation of an Austempered Cast Iron Alloy

January 13, 2022 ADI are the result of graphite nodules and ausferrite microstructure (acicular ferrite + retained austenite). Over the past few years, ADI has become an important material for engineering due to its excellent mechanical properties (outstanding ductility, high mechanical strength and good toughness) and low cost. It is known the discussion of process variables such as austempering time and temperature are extremely important for the microstructural and hardness study of these materials. Thus, in the present work, ADI cast iron was investigated under twelve different austenitic conditions, aiming to characterize the influence of the amount of austenite on the ferritic matrix on the mechanical property of hardness. The heat treatment parameters vary from three different times (40 min, 90 min and 180 min) under four temperatures (280ºC, 300 ºC, 320 ºC and 370ºC). Results show there is a decreasing of hardness linked to the increasing of retained austenite % volume. The greater amount of retained austenite % volume in the matrix was presented by sample A14, treated during 40 minutes under 370ºC and presenting 32.30% of this microconstituint. The amounts of each phase present in the studied materials were raised by quantitative metallography through the software Fiji-ImageJ, allowing a comparison of the results obtained by these two methods. Time differences on the austempering heating treatment did not show several implications on ADI microstructure. In addition, analyzes of the graphite


Introduction
Austempered Ductile Cast Iron (ADI) oustands due to its carbon stabilized austenite alongside acicular ferrite. Some of the advantages of ADI are the combination of high mechanical strength and ductility and the possibility of increasing hardness and yield stress due to induced deformation. This kind of cast iron has been investigated through decades, and due its outstanding performance, researchs still being produced. Fernandez-Valdivieso et al. 1 came up with an investigation about the casting machinability of ADI, turning ADI with ceramic tools by optimizing processes with ceramic inserts.
Boneti et al. 2 analyzed the microstructure and mechanical properties of an ADI, where austempering was performed in melted metal baths through a range of temperatures of 300 °C and 370 °C, for 30 minutes. The study shows the microstructure found in these materials was ausferrite microstructure (acicular ferrite + retained austenite). Furthermore, it is noteworthy the same authors reports that higher austemper temperatures, e.g. 370 °C, can produce a thicker ferrite, but smaller regarding its volume fraction. This is accompanied by a decreasing in yield strength and hardness. For lower austempering temperatures, for example, 280 °C, result in finer ferrite fractions with greater volume and greater flow resistance. Finally, the authors concluded that yield strength, tensile strength and hardness decrease alongside the increase of austempering temperature 2 .
Wang et al. 3 brings ADI wear behaviour and mechanisms in question, comparing its performance in face of single-step and dual-step austempering processes 3 .
Tissi 4 , aimed to evaluate the influence of austempering time and temperature in austempered nodular cast iron samples. The authors analyzed changes in the microstructure and mechanical properties of these materials. The samples were subjected to an austempering heat treatment with austempeting time 90 minutes for 910°C. During the austempering bath, the samples were subjected to different temperatures 300°C, 320°C, 340°C, 360°C and 380°C for four different times for each temperature (75, 110, 145 and 180 minutes). The authors concluded all austempered samples presented graphite nodules, acicular ferrite and austenite with high carbon content in their microstructure. The study also shows that samples treated at higher austempering temperatures had lower hardness, as higher austempering temperatures lead to higher volumes of retained austenite 4 . *e-mail: leonardouffsilva@gmail.com As temperature increases, retained austenite also increases, due to greater carbon diffusion, causing hardness decrease as well. Therefore, the higher the temperature, the higher the percentage of retained austenite and therefore the lower the hardness. Finally, the study reports that the austempering time did not show significant results and, therefore, the influence of the austempering temperature on the microstructure and hardness in ADI is much more significant than the influence of time 4 .
The present article aims at the microstructural and mechanical characterization of nodular cast irons through metallographic analysis, hardness testing, and behavioral analysis of graphite nodules under different austempering treatment conditions. Also, authors aim to investigate the influence of austempering time on ADI microstructure and hardness, varying its treatment temperature and submitting the material to unconventional treatment conditions.

Ductile or nodular cast irons
Ductile or nodular cast irons are also Fe-C-Si alloys, characterized by presenting graphite in the form of spheres. Figure 1 shows the micrograph 200X of magnification nodular cast iron sample.
In Figure 1, it is possible to observe the spheroidal shape of the graphite (hence the nodular name due to the graphite nodules) without interrupting the continuity of the matrix and for this reason its mechanical properties such as ductility, toughness and strength are better than those of lamellar cast iron, because the graphite lamellae weaken the material 5,6 .

Austempered Nodular cast iron (ADI)
The ADI is obtained through heat treatment of austempering in nodular cast iron. Eltaggaz et al. 7 argues that its unique ausferrite microstructure contributes to several properties. This resulting ausferrite (acicular ferrite in the middle of high carbon austenite) results in high values of mechanical strength, ductility, impact resistance and wear resistance 2,8 . Thus, the material provides great flexibility in the design and manufacture of parts [9][10][11][12] .
The austempering heat treatment was initially developed for steels by Davenport and Bain in 1930. Subsequently, the resulting microstructure -carbide ferrite -was named bainite in honor of Bain 13,14 .
According to Kovacs 15 when the structure of ADI was observed for the first time, it was thought to be bainite, as in steels, and, therefore, many articles and patents related to ADI called it bainite. A few years later, X-ray analysis proved that the structure was not bainite. However, today it is known that ausferrite and bainite are completely different constituents, since ausferrite is composed of ferrite and austenite, while bainite is composed of ferrite and carbides.
After several years of research to identify the microstructure and give it a correct name, in 1992, the ausferrite name was recognized 10,16,17 .
According to ASTM A 897M-1997 18 the microstructure resulting from this heat treatment is formed by acicular ferrite and austenite. Figure 2a and b illustrate ausferritic microstructures, produced with austemper temperatures around 370 °C and 260 °C respectively. Figure 2a shows a coarse ausferrite, composed of approximately 60% acicular ferrite (dark region) and 40% high carbon austenite (light region). However, Figure 2b shows a refined ausferrite, composed of approximately 85% of acicular ferrite (dark region) and 15% of high carbon austenite (light region). However, some authors believe that at a temperature of 260 °C, carbide precipitation already occurs, even in short maintenance times 16 .

Ferrite (α iron)
Ferrite (from the Latin "ferrum"), a solid solution of carbon and iron in the allotropic state of pure iron, contains traces of carbon in solution, stable below 910ºC and crystallizes in the centered body cubic system. Also called alpha iron. Compared with other constituents, ferrite is not very hard, with Brinell hardness equal to 80 6 .

Austenite
Austenite is a solid solution of carbon and gamma iron, has a CFC crystalline structure, existing between temperatures of 727 ° and 1495 °C, and with maximum carbon solubility in iron of 2.11% at 1148 °C. It has good mechanical resistance and appreciable toughness and is not magnetic 6 .

Graphite
Unlike the two constituents previously mentioned, also found in common steels, this is typically characteristic of cast irons. It is formed, practically, by pure carbon arranged in layers, being able to assume different shapes, dispositions and sizes. It has low mechanical resistance and hardness between 1 and 2 Mohs 6 .

Mechanical properties
Due to the excellent combination of mechanical properties, such as tensile strength, fatigue strength, ductility, toughness and wear resistance. It has competitive mechanical properties as compared with conventional cast irons 19 . These mechanical properties are due to its singular microstructure of acicular ferrite and retained austenite 20 . In addition, ADI has a density 10% lower than steel, because of the graphite nodules and has the best weight ratio by yield limit 16 .
Through the Table 1 below, we can obtain a better view of the main characteristics of the ADI. It shows the results of the tensile, impact and hardness tests of various classes of ADI using the ASTM standard.
Thus, in the present work, the characterization of phases present in the matrix of twelve samples of ADI was carried out, submitted to different conditions of austempering. This characterization was obtained through optical micrograph, with a qualitative and quantitative analysis of the phases present in each samplewith. Quantitative characterization was performed using the Fiji-ImageJ software. In addition, an analysis of the graphite nodules was also carried out according to their twelve austempering conditions.

Materials and Methods
The ADI samples used in this work were received but were already heat treated in 12 different conditions of time and temperature, and one of them in the raw state of fusion (BF). A range of temperatures was chosen in order to promote amounts of retained austenite between 20% and 40% 21 . The different conditions of the respective austempêra treatments were: with temperatures and times varying, respectively, between 280 ° C and 370 ° C and 40 min at 3 hours, are shown in Table 2.
The samples, Figure 3, were subjected to analysis by optical micrography, hardness tests and the software Fiji -ImageJ, aiming at the identification and quantification of the phases present at the end of each heat treatment and its consequent influence on the hardness of each material.

Sample preparation
In order to perform sanding and polishing of the samples was used STRUERS polishing/sanding machine, model KNUTH-ROTOR 2 and Polishing Teclago model PL02EP.

Metallographic analysis
For the evaluation of the microstructure and the recording of images, the optical micrograph technique was used.
The micrographs were obtained with the aid of the Nikon Eclipse LV 150 optical microscope.
Microstructure evaluations were carried out without and with chemical attack. The metallographic analysis without chemical attack aimed at identifying the graphite nodules generated in each heat treatment, their sizes and distributions, and changes in the graphite nodules in relation to the received raw melting material. The metallographic analysis with chemical attack aimed at the identification of the present phases and their quantification.
The initial idea of the work was to carry out the chemical attack with a 3% solution of Nital, by immersion for four   seconds, but due to unwanted results it was discarded. The solution found for this problem was to carry out the colored chemical attack with the Beraha solution, of composition: 5 ml of distilled water, 5 ml of hydrochloric acid and 0.1 gram of potassium metabisulfite, the attack was carried out by immersion for 15 seconds.

Microscopic and quantitative analyzes
The necessary procedures for performing the optical micrograph were performed in the Microstructural Characterization laboratory of the Federal Fluminense University -EEIMVR.

Hardness test
The Brinell scale was used to determine the hardness of the samples in this work, and the test was carried out at the Mechanical Testing Laboratory of Universidade Federal Fluminense -EEIMVR. The samples were previously sanded in order to obtain a flat and smooth surface and free from oxidation, and tested 3 times each, with a 2.5 mm steel ball penetrator, load equal to 187.5 Kgf for 15 seconds, according to NBR NM 187-1 22 , on a Brinell HECKERT HPO 250 WPM hardness machine.
The determination of hardness by the Brinell scale presents some inaccuracy due to the mechanical handling of the machine and the reading is not direct, thus requiring an interpretation by the observer to measure the values of the printing diameters and from there to treat the data to obtain the value of hardness.

Microstructural analysis
The microstructural characterization of the samples was carried out using the techniques of optical micrography (OM), Brinell hardness test and phase quantification. The results are grouped by sample, where each sample has already been identified in the previous section. Figure 4 shows micrographs of the samples chemically attacked with 3% Nital for 3 and 4 seconds, respectively.
In Figure 4, it is possible to observe that this type of chemical attack was not successful for the samples in question, because the chemical attack burned the samples, making it difficult to identify and quantify the phases, requiring another chemical attack.

BF sample
The sample in the raw state of fusion (BF), presents a reasonable amount of nodules, with different sizes, as we can see in Figure 5.
The morphology of the nodules in this sample does not entirely present uniform geometries, as can be seen in Figure 5.

Sample A2 and A17
In order to evaluate the different properties that occurred in the samples at different temperature variations, the properties observed in samples A2 and A17 were presented. Sample A2 It is a nodular cast iron sample that has undergone austemperature heat treatment under the conditions of 280ºC and 0.67 hours and the sample A17 is a nodular cast iron sample, which has undergone austempering heat treatment under the conditions of 370ºC and 3.0 hours, as indicated in Table 2 of the previous section.
After performing the heat treatment on the sample, it can be seen through Figure 6 that the graphite nodules apparently have an increase in size when compared to the BF sample.
Analyzing the nodules in a higher magnification lens, as in Figure 7, it is observed a homogenization in the format of the graphite nodules of samples A2 and A17 in relation to the graphite nodules of the sample BF. It is also possible to

Identification and quantification of matrix phases
The Beraha colored chemical attack was used in order to reveal and quantify the phases present in samples A2 and A17.
Through optical microscopy analysis it was possible to verify that the material matrix is constituted by acicular ferrite, which is presented in blue and by retained austenite represented in white, as it is possible to observe through Figure 8.
The percentages of the ferrite and retained austenite phases resulting from the austempering treatment in samples A2 and A17, are shown in Tables 3.
In addition, as can be seen also seen in Figure 8, some regions are brown, these are not phases in question, but a burning principle in the matrix resulting from the chemical attack.
This color profile is maintained for all samples of ADI cast iron attacked by the Beraha reagent in the present work.

Hardness test
As previously mentioned, the hardness samples were tested 3 times each.
The result of the mean and standard deviation of the hardness measures of sample A2 was: The result of the mean and standard deviation of the hardness measures of sample A17 was: ( ) 334.12 11.12 The important factor to note is how the amount of austenite and the hardness of the material behaved with the variations in time and temperature of the heat treatment, as shown in Figure 9.  Analyzing the temperature variation, Figure 10, for each of the three austempering treatment times, it is observed that with the increase in the austempering treatment temperature, the percentage of retained austenite shows a growth trend. According to Du et al. 23 mechanical properties of ADI are affected by its phase constutuints due to the carbon saturated retained austenite.
It can be noticed a tendency in % volume of retained austenite. Excepting the samples treated under 320ºC, those treated on 280, 300 and 370ºC presents an increasing of % volume for retained austenite. According to Bendikien et al. 8 , higher austempering temperatures produces gross acicular ferrites. Also, the increasing of austempering temperature leads to a higher % volume of retained austenite, contributing to the hardness decrease 2,4 . Figure 11 shows the graph of hardness and percentage of retained austenite in the heat-treated materials for 180 minutes.
As seen through Figures 9, 10 and 11, for the same austempering time, the higher the percentage of retained austenite, the smaller amount of acicular ferrite, the harder    This result is also a consequence of the higher activation energy for the formation of inferior ausferrite compared to the superior ausferrite, as mentioned by Campos-Cambranis et al. 24 , which leads to an increase in the percentage of retained austenite with an increase in the treatment temperature for the same time.
Analyzing the time variation for each austempering treatment temperature, as shown in Figures 12 and 13, it is observed that the results suggest stability or a slight reduction in retained austenite contents with the increase in time for all temperatures.
It can be noticed that the austempering time did not show significant changes and, therefore, the influence of the austempering temperature on the microstructure and hardness in ADI is much more significant than the influence of time.
On the other hand, the hardness results indicate stability at 300 °C and 320 °C and increase at 370 ° C with longer austemper times, even with a slight reduction in the retained austenite content. These results may be linked to carbide precipitation for longer treatment times, but they would need to be further analyzed to prove this hypothesis. Results at 280 °C, with a significant increase in hardness for longer times, would need to be analyzed further.
Through Table 3, it can be seen that each sample tested underwent austempering treatment with variations in time and temperature. These variations directly influenced the properties of the material.
According to each austempering condition, the ADI matrix presented a different percentage of phases, thereby directly influencing the mechanical properties of the material.
The phase quantification was based on the quantification of retained austenite, due to the influences it has on the material, in particular the hardness. Theoretically, the higher the percentage of retained austenite, the lower the material's hardness should be, due to the fact that retained austenite is the softer phase of the matrix in question, as acicular ferrite is a phase of greater hardness due to its carbon saturation.

Effect of time and temperature variation
on the formation of ferrite and retained austenite Figure 14 shows the profiles of ferrite formation as a function of the variation in the treatment time and temperature of the samples. Figure 14 presented the Profile of ferrite formation as a function of the variation of time and temperature of treatment of the samples where it was possible to observe that in higher values of time in values of temperatures up to 350 0 C high values of ferrite formation were found.   In Figure 15, the retained austenite formation profile is presented as a function of the variation in time and temperature of treatment of the samples. Figure 15 presented the retained austenite formation profile as a function of the variation of time and temperature of the treatment of the samples, where it was possible to observe that higher values of the retained austenite content were found in high temperature values and it is also possible to observe that time values do not have great influence in the increase of retained austenite formation in the sample. Figure 16 shows the hardness profile as a function of the variation in time and temperature of treatment of the samples. Figure 16 presented the hardness profile as a function of the variation in time and temperature of treatment of the samples, where it was possible to observe that higher values of the hardness content were found in lower values of temperature throughout the time interval.

Conclusion
The microstructural characterization, phase quantification and the hardness test of the Austemperated Nodular Cast Iron, allowed to conclude that: • The Beraha attack requires a lot of care and speed to carry out its analysis, as it changes quickly over time; • The most favorable condition for a greater amount of retained austenite in the matrix was the condition of sample A14 (40 minutes and 370ºC), presenting 32.30%; • The greatest hardness occurred in sample A3 (1.5 hours and 300 ºC), which can also be proven because it is the sample with the highest percentage of ferrite; • As the heat treatment temperature increases, the percentage of retained austenite tends to increase and the hardness of the material tends to decrease for the same austempering time; • The increase in time for the same austempering temperature has inexpressive impact on the percentage of retained austenite and hardness, but this fact needs to be analyzed more deeply; • It was possible to verify tha an unusual austempering time such as 3h has a insignificant impact on the % volume of retained austenite.