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Metall. Res. Technol.
Volume 117, Number 1, 2020
Article Number 104
Number of page(s) 10
DOI https://doi.org/10.1051/metal/2019072
Published online 20 January 2020

© EDP Sciences, 2020

1 Introduction

Austempering heat treatment is an effective way to improve the mechanical properties of ductile iron which can vary in a wide range making ADI suitable for different applications. High hardness and low ductility ADI has proved be effective to ballistic applications [1] while high hardness and toughness are desired for wear applications. On the other hand, applications like connecting rods or crankshafts require good fatigue properties, strength and machinability [2] among many others. In the traditional austempering process ductile iron is austenized in a range between 850 and 950 °C during a certain time followed by a rapid cooling in to a salt bath typically between 235 and 450 °C, where the work piece is maintained to allow the isothermal transformation for a time that can range from 5 minutes to 4 h before being cooled to room temperature [35]. A fully transformed microstructure known as ausferrite which consists of a mixture of acicular ferrite and high carbon enriched retained austenite is responsible for the excellent combination of properties such as high strength, ductility as well as good wear resistance. The challenge to produce ADI lies in finding the appropriate parameters and controlling the different variables to obtain the desired results from the austempering process. Chemical composition must be adjusted to respond properly to the austempering heat treatment, through the addition of alloying elements [4]. These additions determine the as-cast microstructure which plays an important role on the austempering process mainly due to microsegregation [6] making these factors highly important for heavy-section castings. On the other hand processing variables such as austenitizing temperature, austempering temperature and holding time [5,7] should be properly selected and controlled increasing the complexity for castings of different thickness.

A wide variety of studies have been conducted recently to improve the mechanical properties of ADI through new processes [8]. Two-step austempering processes to increase the fracture toughness and yield strength [9], intercritical heat treatments to improve ductility [10] or cryogenic treatment to increase the wear resistance [11] are some of recent and innovative processes. However, regardless of the variations in the chosen processing route which can improve the properties of the alloy for certain applications, it is widely known that the best balance of properties are obtained during the period of time known as the Optimal Processing Window (OPW) [3,5,12] which is defined as the austempering time interval between the end of the stage I (γγhc + α) and the start of the stage II (γhcα + carbide (Fe3C) bainitic reaction). A criterion to determine the end and start of these stages has been defined by Yazdani and Elliott [13] as the time required for the unstable austenite volume to decrease to 3% (start of the window) and the austempering time at which the volume of retained austenite has decreased to 90% of its maximum value (end of the window). A wide OPW is window is required in order to get the complete transformation from austenite to ausferrite along the thick section before the start of bainite formation in the areas where the transformation is faster [7]. The length of the OPW is highly influenced by alloying additions required to increase hardenability of thick sections and avoid the austenite decomposition during cooling before the isothermal transformation takes place [8,14,15]. As has been reported by different studies, elements such as Cr, Mn, Mo, and V, tend to segregate at the end of solidification to the cell boundaries regions narrowing of the OPW which is highly influenced by segregation of elements that slow down the austempering reaction and increase the volume of unstable austenite reducing impact properties due to the formation of unstable austenite islands which transforms to martensite [3,12,1618]. In the same way, segregation of carbide forming elements produce negative effects on the OPW enhancing the bainitic transformation and promoting the carbide formation at cell boundaries as has been reported for high Mn and Mo additions [17,19] decreasing machinability and reducing elongation and impact properties. However, small Mo additions accompanied with Ni have been used to enlarge the OPW [20,21] although the start of the transformation is displaced towards longer times, increasing the processing time. Likewise, recent studies [7] have shown that small boron additions can extend the OPW towards shorter austempering times but the hardness and wear resistance are considerably diminished [22]. Cr additions have been used to increase the yield and tensile strength of ADI by increasing the ferrite strength and hardenability but as has been reported by different researchers [2325] produces higher degrees of segregation reducing the OPW. However, even when these studies agree in the reduction of the OPW they report contradictory results in regards to the displacement of the OPW, which are attributed to different Cr effects suggesting the development of a systematic study for different Cr additions over a time range to clarify this controversy. In the first study, Bartosiewicz et al. [23] analyzed a 0.5%Cr ADI and reported that this addition reduces the ausferritic transformation during the stage I of the reaction displacing the OPW to longer times, whereas at longer time periods, Cr addition helped in growth of more ferrite. Likewise, these researchers also report that Cr produced more segregation which may lead to the formation of carbides. Similar effects has been reported for high manganese additions reducing the length and displacing the beginning of the OPW to longer times due to delay in the transformation originated by segregation [26,27].

On the other hand, Rao et al. [24] reported in opposition to previous studies that Cr displaces the OPW to shorter austempering times and attributed this effect to the fact that Cr is a strong ferrite stabilizer but no systematic study over a time range was done and the observations were only supported by fracture toughness values and observations referenced from a study conducted by Muralidhara et al. [25]. In the case of the study performed by Muralidhara et al. [25] the effect of Cr additions in the range of 0.2 to 0.68% was studied, however, the results were only supported by the tensile strength values therefore, the results could be affected by the increase in ferrite strength produced with by Cr addition or other effects due to the high carbon equivalent content used in this study due that as has been reported by Boneti et al. [28] for hypereutectic ADIs, the non-uniform distribution of graphite nodules on the as-cast structure results in transformation gradients between intercellular areas and graphite-rich regions increasing the segregation negative effects.

In the present work, the effect of small Cr additions on the microstructure, mechanical properties and processing window of ADI is analyzed. The metallographic observations are correlated with the retained austenite quantification, hardness and tensile properties with and without a homogenization process prior austempering heat treatment.

2 Experimental procedure

Three different ductile iron melts were manufactured in an induction furnace of 30 kg capacity using high purity raw materials for chemical composition control. For nodulization Fe–Si–6%Mg was used applying the sandwich method. An addition of 0.2%Ca–Si was done into the same ladle for inoculation before pouring. The melts were poured into 25 mm thick T-Block green sand molds. Chill samples were taken for chemical analysis during pouring.

Once solidified, samples were cut for as-cast characterization as well as bars of 10 x 10 mm cross-section and 125 mm of length divided into two groups for heat treatment. The first group was homogenized at 1050 °C for 72 h and rapidly water quenched to avoid segregation. Subsequently, both groups were austenized at 1000 °C for 40 minutes into a tubular furnace and then rapidly cooled down to 350 °C into a salt bath where the samples were isothermally transformed for 5, 10, 30, 45, 60, 90, 120 and 150 minutes. Metallographic characterization was done by optical and scanning electron microscopy (SEM). The retained austenite volume fraction quantification was undertaken by x-ray diffraction (XRD) in a SIEMENS 5000 diffractometer by using Cu- radiation in a 2θ range of 30–90 and calculating the integrated areas under the austenite (100), (200), (211) and ferrite (111), (200), (220) peaks following the procedure described by Putatunda et al. [29]. Hardness measurements were carried out in a Rockwell hardness tester with 150 kg of load and tensile tests were performed according to the ASTM standard E-8 [30] in a Zwick Roell universal testing machine.

3 Results and discussion

Table 1 shows the chemical composition of the three experimental irons, where it can be seen, the only significant difference is the Cr content which is the element subject to study, these differences were carefully controlled and intended to evaluate the Cr effect in the range of 0 to 0.4 wt.%. Higher amounts were not used to avoid the eutectic carbide formation as reported by Bartosiewicz et al. [23].

Table 1

Chemical composition of the experimental irons (wt.%).

3.1 As-cast characterization

Figure 1 shows the as-cast microstructure of the three alloys after being etched with 2%Nital. The microstructure is composed by graphite nodules embedded in a ferrite/perilite matrix and there is no evidence of eutectic carbides formed during solidification due to Cr additions from the metallographic observations and only a slightly increase in the pearlite volume content is observed with the increase in Cr addition since this element is well known as a strong carbide former [31] that promotes the pearlite formation.

Table 2 shows the shape factor (nodularity), number, average diameter and volume of graphite nodules for the different alloys.

As can be seen in Table 2, the graphite nodule characteristics are not considerably affected by the Cr addition and only a slight decrease in the graphite volume content is observed. This decrease in the graphite content can be explained by the fact that during cooling carbon migrates to the nodules due the decrease in carbon solubility of austenite slightly. This increases the graphite volume content and leading to the ferrite formation in the carbon impoverished areas around the nodules which gives rise to the structure known as bull’s eye. In the case of the Cr added irons this effect is reduced due to the Cr capacity to decrease the carbon diffusivity in austenite [32] and the carbon diffusion to the nodules which also increases the pearlite volume content due to the high C–Cr affinity. Therefore, as shown in Table 2, the 0.4%Cr addition produces a decrease of the graphite volume content from 10.6 to 8.6% while the pearlite content increases from 37.4 to 45.7%. This effect has also been reported for vermicular irons [33] where a 0.75%Cr addition causes the complete pearlitization of the matrix.

thumbnail Fig. 1

2%Nital etched microstructure of the as-cast ductile iron castings. a: 0 wt.%Cr; b: 0.2 wt.%Cr; c: 0.4 wt.%Cr.

Table 2

Summary of graphite nodules characteristics and phase quantification.

3.2 Microstructure after homogenization and quenching

As long as the negative effects of Cr additions are attributed to segregation a long homogenization process was applied to analyze the Cr effect during the subsequent austempering process and compare with the non-homogenized irons. Segregation contributes to the shortening of the processing window delaying the transformation from austenite to ausferrite (stage I γγr + α) and at the same time accelerates the second stage of the reaction (γr→α + (Fe3C) bainitic reaction) due to the formation of microscopic carbides at high segregation zones. This negative effect has been observed with high amounts of manganese [15,18,34] and molybdenum [17], nevertheless, alloying elements additions are necessary to improve hardenability and in some cases these elements can also be present in the steel scrap from the metal charge as residual elements. Figure 2 shows the microstructure of the three alloys after homogenization and quenching.

As can be seen in Figure 2, the microstructure of the three alloys is composed by graphite nodules in a martensitic matrix and there is no evidence of carbides or other precipitated phases during holding at 1050 °C. However, by comparing the microstructures, a thickening of the martensite is observed with the increase in Cr. This is attributed to the Cr capacity to increase the hardenability [35]. A morphology transition from lath to lenticular is also seen which is related to the decrease in the martensite start temperature (Ms). This effect has been reported by different researchers [36,37].

thumbnail Fig. 2

Microstructure of the ductile iron castings after homogenization and quenching. a: 0 wt.%Cr; b: 0.2 wt.%Cr; c: 0.4 wt.%Cr.

3.3 Microstructure of non-homogenized ADIs

Figure 3 shows a sequence of micrographs of the non-homogenized irons austenized at 1000 °C for 40 minutes and austempered for 10, 60 and 120 minutes. The first, second and third columns corresponds to the 0%Cr iron, 0.2 and 0.4%Cr irons, respectively.

By comparing the a, d and g micrographs of Figure 3, it can be seen that at 10 minutes of austempering the increase in Cr addition decreases the rate of transformation to ausferrite and higher amounts of martensite are observed which is attributed to the Cr capability to increase the hardenability and decrease the carbon diffusivity. Likewise, it is well known that Cr is a highly segregating element in ductile iron [23,24,38], segregation delays the ausferritric transformation and increases the amount of unstable austenite which transforms to martensite during the subsequent cooling reducing the OPW and impact properties [1618,24,39].

As holding time increases, the transformation to ausferrite proceeds decreasing the martensite content. In the case of the base alloy an almost complete transformation was observed for 30 minutes of austempering. On the other hand for the Cr added irons some martensite and unreacted zones were still present for times up to 45 minutes. At 60 minutes of austempering, a fully ausferritic matrix is observed in the three alloys (see Figs. 3b, 3e and 3h) however, a slight decrease in the retained austenite areas is observed with the increase in Cr content. It also can be seen that as the Cr addition increases the retained austenite areas become more isolated. This is attributed to Cr segregation effects which lead to different rates of transformation during austempering depending on the local concentration of elements.

For austempering times longer than 60 minutes the microstructures do not show significant changes but as mentioned before, if the holding time during austempering exceeds the OPW length, the bainitic transformation takes place. Figures 3c, 3f and 3i show the microstructure after 120 minutes of austempering, but due to the difficulty to distinguish bainite by optical microscopy only a fully transformed matrix is observed. However, it is well known that Cr is a strong carbide former [32,40,41] which can enhance the bainitic transformation due to its great affinity to carbon and reduce the OPW. Therefore, as seen later; the decrease in retained austenite volume fraction and elongation as well as the increase in tensile strength and hardness are clear indicatives of the beginning of the bainitic reaction at this stage in the Cr added ADIs.

thumbnail Fig. 3

Sequence of micrographs of the non-homogenized austempered irons. a–c: 0%Cr; d–f: 0.2%Cr; g–i: 0.4%Cr, austempered for 10, 60 and 120 minutes, respectively.

3.4 Microstructure of homogenized ADIs

Figure 4 shows the microstructure of the homogenized irons for the different Cr contents austempered for 10, 60 and 120 minutes, respectively. The effect of the homogenization heat treatment onto the microstructure of the experimental ADIs can be clearly in this sequence of micrographs and by comparing with sequence of micrographs shown in Figure 3 for the non-homogenized ADIs.

By analyzing the differences between Figures 3 and 4, it can clearly be seen that the homogenization heat treatment increases the rate of transformation to ausferrite reducing the amount of martensite formed for short periods of austempering. In the chromium added irons a slight delay in the ausferritic transformation is observed which is attributed to the decrease in carbon diffusivity that increases the time required to saturate austenite with carbon to be stable.

After 60 minutes of exposure time, a fully transformed microstructure is obtained similarly to non-homogenized irons, however in this case, a more uniform transformed microstructure is observed and the differences become more evident as the Cr content is increased. It also can be seen that Cr addition produces a refining effect which is attributed to its capability of decrease the carbon diffusivity in austenite.

For austempering times longer than 60 minutes the microstructure does not show significant changes with the increase in holding time. Figures 4c, 4f and 4i show the microstructure after 120 minutes of austempering where only a slight reduction of the austenite content due to the growth of the acicular ferrite plates and is observed. By comparing with the non-homogenized irons, the difference in the homogeneity of the microstructure at this time stage is noticeable.

Based in the metallographic observations, a faster and more uniform transformation occurs as a result of a better element distribution after homogenization. The reduction in the rate of transformation with the increase in the Cr addition is still noticeable and it is attributed to the decrease in carbon diffusivity, increasing the time required to saturate austenite with carbon to be stable. The increase in acicular ferrite also suggests that Cr reduces the stability of austenite requiring a higher carbon content to be stable.

The above observations will be supported by the results of retained austenite quantification, hardness and tensile properties in the following sections.

thumbnail Fig. 4

Sequence of micrographs of homogenized and austempered ADIs. a–c: 0 wt.%Cr; d–f: 0.2 wt.%Cr; g–i: 0.4 wt.%Cr, austempered for 10, 60 and 120 minutes, respectively.

3.5 Retained austenite volume fraction, hardness and tensile properties (non-homogenized)

As mention in the previous section, there is no evidence of carbide formation in the experimental irons, in agreement with these observations Figure 5 shows the XRD pattern of the 0.4%Cr ductile iron austempered for 150 minutes. Even when at this time stage the bainitic transformation is expected for this alloy, the small amount of carbide in the bainite cannot be detected by XRD and only the ferrite and austenite peaks are identified.

The intensities of these peaks vary with the holding time of austempering and the integrated areas of these peaks were used to calculate the retained austenite volume fraction shown in the plot of Figure 6. Where the values represent the volume fraction calculated from the iron matrix and the remainder volume may be composed by ferrite, martensite and/or bainite depending of the austempering time.

In agreement with the metallographic observations, for short austempering times only a small amount of austenite is stabilized which increases the martensite content due to the unstable austenite transformation during cooling. This phenomenon has been widely reported by several researchers [7,9,16] and the results can be easily correlated with the low elongation and high hardness values of Figure 7 and the tensile properties of Figure 8 where the higher values of yield and tensile strength are observed at this stage in the experimental ADIs. In Figure 6, it also can be seen that the increase in Cr content produced a decrease in the rate of transformation during the stage I of reaction requiring a longer time to reach a full transformation. In the same way, the austenite volume fraction is decreased with increasing the Cr addition. As reported in the literature, Cr is classified as an α-stabilizer [40] that raises the austenite transformation temperatures [33] and decreases the austenite stability, therefore, a higher amount of ferrite needs to be formed to provide the austenite with enough carbon to be stable decreasing the maximum austenite volume fraction values.

In Figures 7 and 8, it also can be seen that for short austempering times the increase in Cr addition increases the hardness and strength values due to the decrease in austenite stability and the lower volume transformed into ausferrite producing the extensive martensite formation. In the case of the 0.4%Cr ADI, a maximum tensile strength of 880 MPa is reached at 5 minutes of austempering which also corresponds to the higher hardness and lower retained austenite values observed.

As the holding time in austempering increases, austenite is stabilized with the carbon from the growing ferrite plates and suppresses the transformation to martensite increasing the austenite volume fraction. The increase in the volume transformed to ausferrite and the reduction of the martensite content produces a decrease in hardness, yield and tensile strength while at the same time improving the elongation due to the increase in austenite volume fraction.

As can be seen in Figure 6, in the case of the 0.4 wt.%Cr added iron the maximum austenite volume content corresponds to the 0.41 of the iron matrix which is reached after 60 minutes of austempering compared with a 0.44 in the base iron with the same holding time. This behavior can be explained by the fact that as long as austenite is stabilized with carbon from the growing ferrite plates and the austenite stability decreases with the Cr addition, a higher amount of ferrite formed during austempering is required to provide to the austenite with enough carbon to be stable decreasing the maximum austenite volume content.

In the case of the base iron, the maximum austenite values are reached between 30 and 150 minutes of austempering with a difference of less than a 3.4% and the hardness and tensile properties show no significant differences during this time range, therefore, according with the definition of Yazdani and Elliott [13] this period of time falls into the OPW for this alloy. Previous studies and others researchers have shown that this period of time can be extended up to 180 minutes for the base iron.

On the other hand, for the 0.2%Cr iron the maximum austenite volume contents are reached between 45 and 120 minutes, reducing the OPW to this time range since a considerable decrease in austenite volume fraction and an increase in hardness and strength are observed at 150 minutes of austempering. A considerable reduction on length of the OPW is observed in the case of the 0.4%Cr iron. In Figure 6, it can be seen that austempering times slightly longer than 45 minutes are required to decrease the unstable austenite volume content to less than 3% which determines the end of the stage I (start of OPW) and at 90 minutes of austempering the austenite content decreases to 91.5% of its maximum value which is close to the end of the OPW. For austempering times longer than 90 minutes a substantial reduction in austenite content and elongation is observed accompanied by a slight increase in hardness, yield and tensile strength. As mentioned before, these results are clear indicators of the bainitic reaction in which the high carbon austenite transforms to bainite (α + Fe3C). This effect is attributed to the high Cr–C affinity that can promote the bainite formation in the higher Cr concentration areas due to segregation during solidification accelerating the stage II reaction of austempering.

From above observations, Cr addition increases the hardenability of the ADI and decreases the rate of transformation to ausferrite displacing the beginning of the OPW to longer times and due to the high Cr–C affinity as strong carbide former accelerates the beginning of the stage II reaction enhancing the bainitic transformation producing a considerable reduction of the OPW length in the non-homogenized ADIs.

thumbnail Fig. 5

XRD pattern of the 0.4 wt.%Cr ductile iron austempered for 150 minutes.

thumbnail Fig. 6

Austenite volume fraction as a function of austempering time for the non-homogenized ADIs.

thumbnail Fig. 7

HRC hardness and elongation as a function of the austempering time for the non-homogenized ADIs.

thumbnail Fig. 8

Ultimate tensile strength and yield strength as a function of the austempering time for the non-homogenized ADIs.

3.6 Retained austenite volume fraction, hardness and tensile properties (homogenized)

Figure 9 shows the results of the austenite volume fraction of the homogenized ADIs. As it can be seen, austenite volume fractions between 0.36 and 0.4 are reached at only 5 minutes of austempering which completely agrees with the metallographic observations where an increase in the rate of transformation was observed in the three alloys and only a small delay is observed with the increase in Cr.

The increase in the austenite content over short periods of austempering compared with the non-homogenized ADIs can be explained by the fact that long periods of homogenization develop a complete austenization and a uniform carbon distribution, then, the time required to reach the maximum retained austenite volume fraction is reduced increasing the austenite stability, these observations agree with results obtained by other researchers [6,16].

In Figure 9, it also can be seen that the maximum austenite values reached are close to the obtained without homogenization. Different researchers have reported that the maximum retained austenite volume fraction depends of the carbon content dissolved in austenite [29,42,43], which in this case is determined by the austenization temperature therefore, the homogenization heat treatment do not have influence on the maximum amount of retained austenite that can be obtained but it modifies the time at which these values are reached.

As seen in the non-homogenized irons, the maximum austenite volume content is decreased with the increase in Cr which was attributed to the decrease in austenite stability requiring a higher volume of ferrite formed to provide the austenite with enough carbon to be stable at room temperature.

An important difference compared with the non-homogenized ADIs is that in this case, the homogenization heat treatment suppresses the sudden reduction of the austenite volume fraction seen in the Cr added ADIs for holding times longer than 60 minutes. As shown for the non-homogenized irons and as reported by the literature, the increase in Cr content accelerates the stage II of the reaction due to a higher degree of segregation in the interdendritic regions produced during solidification, therefore, decreasing the local C–Cr concentration due to a better element distribution after homogenization produces a more uniform transformation extending the OPW for longer periods of time.

Figure 10 shows the hardness and elongation results for the homogenized irons, where a reduction in the hardness values is observed compared with the non-homogenized ADIs and these differences are more evident for short austempering times. As shown in the austenite quantification results and metallographic observations, the increase in the rate of transformation and the higher austenite volume fraction obtained at short austempering times suppresses the extensive formation of martensite which decreases the hardness and increases the elongation values extending the OPW for short austempering times compared with the non-homogenized ADIs.

In the case of the Cr added irons, the decrease in carbon diffusivity and austenite stability produced an increase in the martensite content and the consequent hardness increase for short austempering times with respect to the base iron. However, these values are still considerably lower compared with the non-homogenized ADIs.

For austempering times longer than 30 minutes there are not significant differences in hardness with the increase in holding time for the three irons but the overall hardness values are increased with the increase in Cr addition which is attributed to the finer microstructure obtained and the Cr capacity to increase the ferrite hardness and strength [23,44] and no evidence of the bainitic transformation is observed. In support to these observations Figure 10 shows that the elongation values are not drastically reduced after long austempering times conversely to the behavior seen in the Cr added non homogenized irons.

Figure 11 shows the tensile and yield strength values for the homogenized ADS and as can be seen, these are consistent with the austenite volume fraction, hardness and elongation values shown previously.

It also can be seen that the increase in Cr addition only produces a slight increase in yield and tensile strength due to the increase in acicular ferrite but these values do not show a substantial difference with respect to the austempering time as shown in the non-homogenized irons where the increase in hardness, yield and tensile strength was accompanied by a decrease in elongation and austenite volume fraction characteristic of the stage II of reaction in the Cr added irons.

In accordance with different researchers [12,1618,23,24,39], Cr like Mn causes segregation to intercellular regions which modifies the length of the OPW. Based in the results of the present study, the increase in Cr addition substantially reduces the length of the OPW by shifting the end and beginning of the stage I and stage II reactions to longer and shorter austempering times respectively. Specifically in the case of the 0.4%Cr added ADI which corresponds to the higher Cr content in used the present study, the beginning and end of the OPW is displaced to austempering times slightly longer than 45 and 90 minutes, respectively. On the other hand, homogenization heat treatment increases the rate of transformation to ausferrite and suppresses the negative Cr segregation effects, hence, only a slight delay in the transformation is observed with the increase in Cr addition as well as a hardening effect due to the increase in acicular ferrite but no evidence of the bainitic transformation is observed for holding times up to 150 minutes.

thumbnail Fig. 9

Austenite volume fraction as a function of austempering time for the homogenized ADIs.

thumbnail Fig. 10

HRC hardness and elongation as a function of the austempering time for the homogenized ADIs.

thumbnail Fig. 11

Ultimate tensile strength and yield strength as a function of the austempering time for the homogenized ADIs.

4 Conclusions

Nodularity, number and average diameter of graphite nodules are not considerably affected by the Cr addition. Only a small decrease in the graphite volume content with the increase in Cr addition is observed and at the same time the as-cast microstructure of the matrix experiences an increment in pearlite volume content from 37.4% in the base iron to 45.7% with the 0.4%Cr addition.

After homogenization and quenching the increase in Cr addition produced a thickening effect on the martensite as well as the transition from lath to lenticular attributed to the Cr capacity of increase the hardenability and decrease the Ms temperature.

The increase in Cr addition produced a delay in the rate of transformation to ausferrite, retaining a higher amount of unstable austenite which transforms to martensite under the subsequent cooling shifting the beginning of the OPW for later austempering times.

The maximum austenite volume fraction decreases with the Cr addition due to the decrease in austenite stability requiring a higher volume of ferrite formed to provide enough carbon to be stable at room temperature.

The retained austenite quantification, hardness and tensile properties showed that Cr accelerates the stage II reaction promoting the formation of bainite and narrowing the optimum processing window.

For the 0.4%Cr ADI the end of the stage I and beginning of stage II are displaced to austempering times slight longer than 45 and 90 minutes, respectively.

Yield and tensile strength values increased with Cr addition due to the Cr capacity of increase the ferrite strength and reduce the retained austenite volume fraction increasing the acicular ferrite content.

Homogenization enlarges the OPW by increasing the rate of transformation to ausferrite and suppressing the extensive martensite formation for short austempering times.

A more uniform transformed ausferritic microstructure is observed with the homogenization heat treatment and these microstructures are refined with the increase in Cr addition.

For the homogenized Cr added ADIs only a slightly delay in the first stage of transformation is produced with the increase in Cr addition as well as a hardening effect due to the increase in acicular ferrite and the refinement of the microstructure.

Homogenization avoids the Cr segregation negative effect extending the length of the OPW for longer austempering times since no evidence of the bainitic transformation was observed for holding times up to 150 minutes.

Acknowledgments

The authors acknowledge the CIC-UMSNH grant: 019 and CONACyT grant: 057197 for the financial support and FV. Guerra acknowledges CONACyT for the scholarship during his postdoctoral stay at the University of Iowa.

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Cite this article as: Arnoldo Bedolla-Jacuinde, Román Angel Hernandez-Hernandez, Francisco Vapeani Guerra, Ignacio Mejia, The role of chromium during austempering of ductile iron, Metall. Res. Technol. 117, 104 (2020)

All Tables

Table 1

Chemical composition of the experimental irons (wt.%).

Table 2

Summary of graphite nodules characteristics and phase quantification.

All Figures

thumbnail Fig. 1

2%Nital etched microstructure of the as-cast ductile iron castings. a: 0 wt.%Cr; b: 0.2 wt.%Cr; c: 0.4 wt.%Cr.

In the text
thumbnail Fig. 2

Microstructure of the ductile iron castings after homogenization and quenching. a: 0 wt.%Cr; b: 0.2 wt.%Cr; c: 0.4 wt.%Cr.

In the text
thumbnail Fig. 3

Sequence of micrographs of the non-homogenized austempered irons. a–c: 0%Cr; d–f: 0.2%Cr; g–i: 0.4%Cr, austempered for 10, 60 and 120 minutes, respectively.

In the text
thumbnail Fig. 4

Sequence of micrographs of homogenized and austempered ADIs. a–c: 0 wt.%Cr; d–f: 0.2 wt.%Cr; g–i: 0.4 wt.%Cr, austempered for 10, 60 and 120 minutes, respectively.

In the text
thumbnail Fig. 5

XRD pattern of the 0.4 wt.%Cr ductile iron austempered for 150 minutes.

In the text
thumbnail Fig. 6

Austenite volume fraction as a function of austempering time for the non-homogenized ADIs.

In the text
thumbnail Fig. 7

HRC hardness and elongation as a function of the austempering time for the non-homogenized ADIs.

In the text
thumbnail Fig. 8

Ultimate tensile strength and yield strength as a function of the austempering time for the non-homogenized ADIs.

In the text
thumbnail Fig. 9

Austenite volume fraction as a function of austempering time for the homogenized ADIs.

In the text
thumbnail Fig. 10

HRC hardness and elongation as a function of the austempering time for the homogenized ADIs.

In the text
thumbnail Fig. 11

Ultimate tensile strength and yield strength as a function of the austempering time for the homogenized ADIs.

In the text

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