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Influence of intercritical austempering on the microstructure and mechanical properties of austempered ductile cast iron (ADI)
Author’s Accepted Manuscript Influence of Intercritical Austempering on the Microstructure and Mechanical Properties of Austempered Ductile Cast Iron (ADI) Saranya Panneerselvam, Susil K. Putatunda, Richard Gundlach, James Boileau www.elsevier.com/locate/msea
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S0921-5093(17)30411-2 http://dx.doi.org/10.1016/j.msea.2017.03.096 MSA34879
To appear in: Materials Science & Engineering A Received date: 27 October 2016 Revised date: 23 March 2017 Accepted date: 24 March 2017 Cite this article as: Saranya Panneerselvam, Susil K. Putatunda, Richard Gundlach and James Boileau, Influence of Intercritical Austempering on the Microstructure and Mechanical Properties of Austempered Ductile Cast Iron ( A D I ) , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.03.096 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Intercritical Austempering on the Microstructure and Mechanical Properties of Austempered Ductile Cast Iron (ADI) Saranya Panneerselvam1, Susil K. Putatunda1*, Richard Gundlach2, James Boileau3 1
Wayne State University, Detroit, MI, USA.
2
Element Materials Technology, MI, USA.
3
Ford Motor Company, Dearborn, MI, USA.
*
Corresponding Author at: Department of Chemical Engineering and Materials
Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, MI 48202. Phone: +1 3135773808, Fax: +1 3135773810; e-mail address: [email protected]
Abstract The focus of this investigation was to examine the influence of intercritical austempering process on the microstructure and mechanical properties of low-alloyed austempered ductile cast iron (ADI). The investigation also examined the influence of intercritical austempering process on the plane strain fracture toughness of the material. The effect of both austenitization and austempering temperature on the microstructure and mechanical properties was examined. The microstructural analysis was carried out using optical microscopy, scanning electron microscopy and X-ray diffraction. The test results indicate that by intercritical austempering it is possible to produce proeutectoid ferrite in the matrix microstructure. Furthermore, the yield, tensile strength
1
and the fracture toughness of the ADI decreases with decrease in austenitizing temperature. Lower austenitizing temperature produces more proeutectoid ferrite in the matrix. A considerable increase in ductility was observed in the samples with higher proeutectoid ferrite content. The fracture surfaces of the ADI samples revealed that dimple ductile fracture produced higher fracture toughness of 60±5 MPa√m in this intercritically austempered ADI.
Keywords: austenitization, intercritical austempering, proeutectoid ferrite, ductility, fracture toughness.
1.0 INTRODUCTION Austempered ductile cast iron (ADI) has emerged as an important engineering material in recent years because of excellent combination of mechanical properties such as high yield and tensile strengths, good ductility [1-3], good fatigue strength [4,5] excellent wear resistance [6] and high fracture toughness [7]. ADI has low production costs due to its good castability and good machinability. Because of these attractive properties, ADI finds wide commercial applications in automotive, agricultural, earth moving machineries, railways [8-10] etc. The mechanical properties of ductile iron are entirely dependent on the volume fraction and distribution of phases in the microstructure [11-12]. A wide range of mechanical properties can be obtained in ductile cast iron by subjecting the as cast material to different heat treatment processes like quenching, normalizing, annealing and austempering.
2
Conventional austempering is an isothermal heat treatment process in which ductile cast iron is initially subjected to complete austenitization followed by austempering at a temperature above the Ms (martensite start temperature). During austempering ductile iron undergoes a two-stage phase transformation process [13-15]. In the first stage austenite (γ) decomposes into ferrite(α) and high carbon austenite(γHC).
γ= α+ γHC
(Stage 1)
If the ductile cast iron is held at this temperature for a prolonged period of time, a second reaction occurs, during the second stage, the γHC can further decompose into ferrite (α) and carbide (ε).
γHC= α+ ε
(Stage 2)
The optimum combination of strength and ductility in ADI is usually obtained after the completion of stage 1 reaction but before the onset of stage 2 reaction. The time period between the completion of stage 1 and the onset of stage 2 is referred to as the „process window‟. The processing window can be enlarged by adding suitable alloying elements like Ni, Mo and Cu. Austempering is usually performed at temperatures ranging between 500-750°F. Austempering of ductile cast iron at higher temperatures produces an ausferritic microstructure which is a combination of carbide free ferrite and austenite whereas at lower temperature it produces typical bainitic ferrite and austenite. The conventionally processed ADI contains large amounts of austenite. This higher austenitic content in the conventional ADI causes work hardening of the material and will affect the machinability of ADI. Another limitation of ADI processed by the
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conventional austempering is that it does not contain pro-eutectoid ferrite in the matrix and its transformed ferrite content (α) is very low which limits the ductility of ADI. To overcome these limitations of conventional ADI, in recent years there has been significant interests [16-18] in producing ADI with mixed microstructures consisting of pro-eutectoid ferrite and ausferrite to improve the machinability and ductility of ADI. This can be achieved by intercritical austempering. Intercritical austempering is a process in which the as-cast ductile cast iron is heated to an austenitizing temperature between the A1 and αT temperatures where (α) ferrite and austenite co-exist [19-22]. This is followed by subsequent austempering. The upper and lower intercritical austempering temperature of the ductile cast iron is strongly dependent on the chemical composition of the material. Suitable intercritical austempering will therefore produce a unique microstructure consisting of pro-eutectoid ferrite, bainitic ferrite and carbon enriched austenite [19-22]. This material is expected to exhibit much greater ductility than the conventionally austempered or quenched and tempered ductile cast iron [23-24]. The tensile and yield strengths of this material will be much higher than the pearlitic grades [19-22]. Machinability will also be enhanced [24] in ADI by this intercritical heat treatment process. Therefore, the intercritically austempered ductile cast iron will find significant applications in the production of automotive components (e.g. suspension parts) where a combination of high strength and high ductility is required [20]. By controlling the intercritical austenitizing temperature it is possible to control the volume fraction of austenite and its carbon content. The other advantages of intercritical austempering are as follows:
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1.The volume fraction of pro-eutectoid ferrite and the ferrite+ high carbon austenite content in the microstructure matrix can be controlled precisely to obtain the required strength and ductility in ADI. 2. A wide combination of intercritical austenitizing and austempering temperatures can be used to achieve targeted mechanical properties in ADI. 3.In the intercritical austempering process, austenitization is performed at a lower temperature than in conventional austempering, thus it is an energy saving process. While in recent years, a significant number of studies have been carried out to examine the effect of intercritical austempering on the microstructure [16-27], most studies were focused on the effect of intercritical austenitizing temperature and time [16], on the effect of volume fraction of phases [25], machinability [24], abrasive wear behaviour [17,26] and tensile properties [21-23] of ADI. However, so far no systematic investigation has been reported in the literature that examined the influence of both intercritical austenitizing and austempering temperature on the microstructure and mechanical properties including fracture toughness of unalloyed ADI. Therefore, this investigation was undertaken to examine the effects of both intercritical austenitizing as well as austempering temperatures on the microstructure, mechanical properties and fracture toughness of on low alloyed ductile cast iron. 2.0 EXPERIMENTAL PROCEDURE A low alloyed ductile cast iron with the chemical composition reported in the Table 1 was used in this investigation. The material was cast in the form of KEEL blocks according to ASTM A 536 of size 7.9"X 3.0"X4.0" and from these cast blocks cylindrical tensile samples and compact
5
tension samples for fracture toughness tests were machined as per ASTM standards E-8[27] and E-399[28] respectively. An equation [29] developed by the Ductile Iron Society was used to predict the intercritical temperature ranges for the ductile iron used in this study. The upper intercritical temperature (UCT) and lower intercritical temperatures (LCT) were thus determined to be 1520°F and 1404°F respectively. After fabrication, the machined cylindrical tensile samples and compact tension samples were divided into 8 batches (A, B, C, D, E, F, G and H) and heat treated as follows. The first batch of samples (batch A, B, C and D) was used to study the effect of intercritical austenitizing temperature on the microstructure and mechanical properties of ADI and, therefore, the first batch of samples was initially austenitized at an intercritical temperature (Tγ) between A1 and αT temperatures(where ferrite and austenite co-exist) at four different temperatures such as 1418°F, 1436°F, 1472°F and 1520°F, respectively, and then subsequently quenched in a salt bath maintained at 680°F. In order to determine the effect of austempering temperature, the remaining three batches of samples (E, F and G) were austenitized at an upper intercritical temperature of 1520°F, and then austempered at four different austempering temperatures (TA) such as 725°F, 680°F, 600°F and 550°F. Also for comparison purposes, one batch of samples (H) was austenitized just above the intercritical temperature of 1535°F and then austempered at 680°F.All the samples were austenitized for 3 hours and then austempered for another 3 hours and finally air cooled to room temperature.
6
Tensile tests and fracture toughness tests were carried out on a servo-hydraulic MTS test machine as per ASTM standards E-8 [27] and E-399 [28] respectively. Four samples were tested from each heat treated condition and the average values are reported. Microstructures of the compact tension samples were examined by optical microscopy and Scanning Electron Microscopy (SEM) after polishing and etching with 3% nital. The volume fraction of the proeutectoid ferrite and ausferrite in the intercritically austenitized ADI samples was determined by point counting in accordance with ASTM standard E-562 [30]. The fractured surface of all the heat treated ADI samples were examined by JEOL JSM 6510 LV LGS Scanning Electron microscope to determine the mode of fracture. X-ray diffraction (XRD) analysis was performed to estimate the austenite content and the carbon content of austenite following the procedure of Rundman and Klug [31]. XRD was conducted using monochromatic copper Kα radiation at 30 kV and 10 mA. A Bruker Phaser II diffractometer was used to scan angular 2θ range from 42°-46° and 72°-92°. The profiles were analyzed using Jade 5 software to obtain the peak positions and the integrated intensities of {111} and {220} planes of FCC austenite and {110} and {211} planes of BCC ferrite. The volume fractions of ferrite (Xα) and austenite (Xγ) were determined by the direct comparison method using the integrated intensities of the above planes [32]. The carbon content of the austenite was determined by the equation [33] a = 0.3548 + 0.00441 C Where a is the lattice parameter of austenite in nanometer and Cγ is the carbon content of austenite in wt%. The {111} and {220} planes of austenite were used to measure the lattice parameter. The ferritic cell sizes of the ausferritic structure were determined by the Scherrer
7
equation [32]. Three samples were examined from each heat treated condition and the data reported is the average from these samples. The proeutectoid ferrite size were determined using line intercept method. 3.0 RESULTS AND DISCUSSION 3.1 Influence of intercritical austempering on the microstructure of ADI The optical microstructure of the as cast ductile cast iron consisted of pearlite with a typical bull‟s eye ferrite structure and is shown in the figure 1. The microstructure consists of ferrite surrounding the graphite and pearlite which is a lamellar structure with alternating layers of ferrite and cementite. The graphite in the as-cast structure appears well rounded with 85% nodularity. Optical micrographs of the samples austenitized at 1520°F, 1472°F, 1436°F and austempered at 680°F are shown in the figures 2(a)-2(c). The optical micrographs show a mixed microstructure consisting of proeutectoid ferrite and ausferrite. There is no major difference in the optical micrograph between 1436ºF and 1418ºF austenitized samples, except for the increase in proeutectoid ferrite content. On heating the as cast sample in the intercritical austenitizing temperature region, where α + γ coexist, the pearlitic component of the as-cast microstructure converts to austenite and the amount of α-ferrite that remains depends on the austenitizing temperature. During austempering process, the austenite nucleates at the grain boundary of ferrite and then grows into ferrite by the nucleation and growth process. Nucleation produces ferritic matrix containing thin layers of austenite enriched with carbon. As the austenite continue to nucleate at prior ferrite grain boundaries, the austenite becomes enriched and saturated with carbon and the diffusion of carbon ahead of ferrite becomes difficult and the growth of ferrite is
8
arrested. Even though, unalloyed ductile cast iron is used in this study, appreciable amount of Ni, Si, Cu were present and they segregated around the graphite nodules and hence ferrite was found in the areas surrounding the graphite nodules of the intercritically austempered samples. Figure 3 show the effect of intercritical austenitizing temperature on the volume fraction of pro-eutectoid ferrite and ausferrite. The volume fraction of proeutectoid ferrite decreases with increase in austenitizing temperature, whereas the volume fraction of ausferrite increases with the increase in austenitizing temperature as predicted by the Lever rule [34]. The volume fraction of the austenite in the ausferrite was determined by X-ray analysis technique. Figures 4 and 5 show the influence of intercritical austenitizing temperature on the volume fraction of austenite and the percentage of dissolved carbon content in austenite respectively. The volume fraction of the austenite at the intercritical temperature is low for the ADI sample intercritically austenitized at 1418°F and its carbon content is also low. The volume fraction of austenite and its carbon content considerably increases with the increase in austenitizing temperature. This result is in good agreement with the existing literature [32, 33, 35]. The intercritical austenitizing temperature does not affect the ferritic cell size in the ausferrite of the samples as the ferritic cell size lies within a range of 22 nm to 27nm as shown in the figure 6. Figure 7 shows the influence of austenitizing temperature on the proeutectoid ferrite size. The proeutectoid ferrite cell size increases with the decrease in austenitizing temperature. The proeutectoid ferrite in the microstructure matrix restricts the growth of transformed ferritic needles. The optical microstructures of the samples intercritically austenitized at 1520°F and austempered at different austempering temperatures of 725°F, 600°F and 550°F are shown in the figures 8(a)8(c). The samples austempered at higher temperatures show a coarser ausferritic microstructure along with a considerable amount of proeutectoid ferrite in the matrix. As mentioned earlier,
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ferritic grain growth occurs by a nucleation and growth process upon heating above the lower critical temperature, pearlite decreases as proeutectoid ferrite nucleates and experiences grain growth. Since grain growth is a function of temperature, a lower austempering temperature produced more ferrite grains and at the same time lower volume fraction of austenite in the microstructure. Figure 9 shows the correlation of volume fraction of austenite (retained and reacted austenite) with austempering temperature. The volume fraction of the total reacted as well as retained austenite increases with the increase in austempering temperature. At lower austempering temperature (550°F), only retained austenite plus lower bainite is present because no ausferrite forms at this temperature. However, at higher temperature, the matrix contains reacted austenite only because of ausferrite reaction which occurs at the upper bainitic region. In addition, the carbon content of the austenite at the intercritical temperature decreases with temperature. Perhaps, the decrease in carbon content of austenite causes a shift in the transition temperature between upper and lower bainite. This may explain the variation of the reacted austenite content with temperature as represented in figure 9. Figure 10 shows the influence of austempering temperature on the dissolved carbon at the intercritical temperature. The carbon content of the transformed (reacted) austenite was more or less the same and its value was about 1.8%. The ferritic cell size tends to become finer as the austempering temperature decreases from 725°F to 550°F as shown in the figure 11. Therefore, as with the conventional austempering process, lower austempering temperature in the intercritically austenitized samples, produced a finer ferritic cell size. At traditional austempering temperature, a lower carbon austenite produces finer ferritic cell size. Based on this observation, it may be possible to determine the austempering process to create a 10
nanoscale structure in ADI. For conventional ADI, when austempered from temperature above upper critical, generally results in much larger ferritic cell size. The plot (figure 9) substantiates the fact that not all the austempering temperatures produce ausferritic structure. At lower austempering temperatures (e.g. 550°F, where lower bainite form), it is anticipated that there will be unreacted austenite. This should evident by the broadening of the austenite peak in x-ray diffraction profile as shown in the figure 12, where the austenite peak is much broader and the volume fraction of austenite is lower than the finer ferrite grains. It also should affect the interpretation of carbon content of austenite. 3.2 Influence of Intercritical austempering on the mechanical properties of ADI Table 2 compares the mechanical properties of ADI samples as a function of intercritical austenitizing temperature. The yield strength and ultimate tensile strength of the intercritically austenitized ADI decreased with the decrease in austenitizing temperature. The transformed ferrite and austenite both coarsen as the austenitizing temperature decreases. The increase in the volume fraction of proeutectoid ferrite and the presence of coarser ferritic structures at lower austenitizing temperature decreased the strength of the ADI but progressively improved the ductility (% elongation) of the ADI. The ductility in terms of the % elongation of the intercritically austenitized ADI increased with the decrease in austenitizing temperature. Thus, by proper choice of austenitization temperature, it is possible to control the mechanical properties of ADI by controlling the amounts of pro-eutectoid ferrite and ausferrite in the matrix microstructure. Figure 13 shows the influence of austenitizing temperature on the hardness of ADI. The hardness value decreases with the decrease of austenitizing temperature. The presence of higher volume fraction of proeutectoid ferrite in the intercritically austenitized samples resulted in very low 11
hardness in these samples. Table 3 details the mechanical properties of the samples austenitized at 1520°F as a function of austempering temperature. The yield strength and ultimate tensile strength are higher for the samples austempered at lower austempering temperature of 550°F and 600°F, however, the ductility is lower. As the austempering temperature increases, the yield and ultimate tensile strength decreases while an improvement in the ductility was observed. This can be attributed to the presence of coarser ausferritic microstructures produced at the higher austempering temperatures. The finer ausferritic structure will contribute to higher strength as predicted by Hall-Petch equation [34]. As expected, hardness also decreases with increasing austempering temperature as shown in figure 14 due to coarser ausferritic structure. The strain hardening exponent values of all the samples were obtained through tensile testing. The results indicate that the strain hardening exponent (n) of the intercritically austempered ductile iron samples lies within a range of 0.09 to 0.14 which is a typical range for ADI. The austenitization temperature does not appear to have any significant effect on the strain hardening exponent of the material even though the austenite volume fraction decreased with austempering temperature. 3.3
Influence
of
intercritical
austempering
on
the
fracture
toughness
of
ADI:
The fracture surface of the compact tension samples subjected to different heat treatments was examined to determine the mechanism of fracture and this can be associated with the fracture toughness of the material. The fracture toughness of ADI austenitized in the intercritical region is shown in figure 15 which shows that the fracture toughness of the ADI decreased proportionally with the decrease in austenitizing temperature. This can be associated with the variation in the fractured surface of the samples with the change in austenitizing temperature. The fractographs of the sample austenitized at 1535°F is shown in the figure 16(a). The ADI austenitized just 12
above the intercritical temperature (1535°F) showed dimple ductile fracture throughout the fracture surface. From a previous investigation [36], it is clear that the ductile fracture produces a fracture toughness of 60 ± 5 MPa√m in ADI. As the austenitizing temperature is reduced, the transgranular cleavage type of fracture is also observed along with the dimple ductile fracture. As shown in the figures 16(b) and 16(c), at lower intercritical temperatures of 1436°F and 1418°F, the fractured surface is predominantly transgranular cleavage rather than dimple ductile and this has caused lower fracture toughness in these samples. This can be attributed to the coarse grained proeutectoid ferrite in the intercritically austempered samples. By the way, annealed ferrite ductile iron has a fracture toughness of only 45±5 MPa√m. The maximum fracture toughness obtained in ADI by the conventional single step austempered process (as reported in the literature) is between 60 and 66 MPa√m [37-40] Figure 17 shows the influence of austempering temperature on the fracture toughness of intercritically austenitized ADI. The fracture toughness of the samples increases with an increase in austempering temperature to a maximum at 680°F before decreasing again at 725°F. This can be due to the influence of ferritic grain size and the volume fraction of austenite [41-44]. The samples austempered at the higher temperature of 725°F and the lowest temperature of 550°F showed the presence of both dimple ductile and cleavage type of fracture as shown in the figure 18(a). The samples austempered at intermediate temperature of 600°F showed trans granular cleavage fracture throughout the fractured surface as shown in figure 18(b). The samples austempered at 680°F showed predominantly dimpled ductile fracture and that appears to be the reason for very high fracture toughness in these samples. Higher strength at lower temperature promotes transgranular cleavage. Also very coarse ausferrite favor transgranular cleavage fracture. This is consistent with the data reported in literatures. 13
4.0 Conclusions 1. The intercritical austempering of ductile cast iron produced a unique microstructure consisting of pro-eutectoid ferrite, bainitic ferrite and high carbon austenite in ADI.
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2. The mechanical properties including the yield strength, ultimate tensile strength of ADI gradually reduced with the decrease in austenitizing temperature due to an increase in proeutectoid ferrite content. 3. A progressive increase in the ductility (% elongation) of the ADI samples was observed with the decrease in austenitizing temperature due to the larger volume fraction of proeutectoid ferrite in the microstructure of ADI. 4. While the ductility and strength increased with decreasing austenitizing temperature, the fracture toughness decreased. This can be attributed to the presence of increasing amount of proeutectoid (polygonal) ferrite in the matrix microstructure. 5. The strain hardening exponent of ADI was not significantly affected by the intercritical austenitizing and austempering temperatures. 6. Intercritical austempering close to the upper critical temperature resulted in good mechanical properties (high strength and ductility combined with good fracture toughness). Intercritical austenitizing well below upper critical did not result in exceptional properties. The reason for this appears to be the coarse grained proeutectoid ferrite. 7. The austempering at lower temperature (550°F) appear to produce lower bainite combined with low retained austenite content. It appears that low carbon austenite produced by austenitizing near the upper critical temperature produced fine ferrite cell size.
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38. E. Dorazil, M.Holzman, “Fracture behavior of austempered ductile iron Proceedings of the World Conference on Austempered Ductile Iron”, Bloomingdale, IL, March 1991,32–66. 39. M. Grech, P.Bowen, J.M.Young, “Effect of austempering temperature on the fracture toughness and tensile properties of an ADI alloyed with copper and nickel”, Proceedings of the World Conference on Austempered Ductile Iron, Bloomingdale, IL, March 1991, 338–374. 40. P.P. Rao, S.K.Putatunda, “Influence of microstructure on fracture toughness of austempered ductlie cast iron” Metallurgical and Materials Transactions 28A, 7, 1997, 1457–147. 41. S.Panneerselvam, C.J.Martis, S.K.Putatunda and
J.Boileau, “An Investigation on the
Stability of Austenite in Austempered ductile Cast Iron”, Materials Science and Engineering A, 626, 2015, 237-246. 42. P.P.Rao, S.K.Putatunda, “ Comparative study of fracture toughness of austempered ductile iron with lower and upper ausferritic microstructures, Materials science and Technology, 14, 1998, 1257-1265. 43. J.L. Doong and C. Chen, "Fracture Toughness of Bainitic-Nodular Cast iron," Fatigue Fracture of Engineering Material and Structure, 12, 1989, 155-165. 44. E. Dorazil and M. Holzman, "Fracture Behavior of Austempered Ductile Iron," Proceeding World Conference on Austempered Ductile Iron, Bloomingdale, IL, 1991, 3266.
19
Fig 1: As-cast microstructure of Ductile Cast Iron
20
(a) Tγ=1520°F, TA=680°F
(b) Tγ=1472°F, TA=680°F
21
(c) Tγ=1436°F, TA=680°F
Fig 2: The Microstructure of the Ductile Iron after austenitizing at (a)1520°F, (b)1472°F, (c) 1436°F and austempering at 680°F
22
100 90
Volume fraction (%)
80 70 60 50 40 30 20 10 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing Temperature (°F) Volume fraction of proeutectoid ferrite Volume fraction of austenite Linear (Volume fraction of proeutectoid ferrite) Linear (Volume fraction of austenite)
Fig 3: Influence of austenitizing temperature on the volume fraction of proeutectoid ferrite and austenite
23
Volume fraction of austenite (%)
15 13 11 9 7 5 3 1 -11400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing temperature(°F) Fig 4: Influence of austenitizing temperature on the volume fraction of austenite
Dissolved carbon content in austenite(%)
3.5 3 2.5 2 1.5 1 0.5 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing temperature (°F)
Fig 5: Influence of austenitizing temperature on the austenitic carbon content
24
30
Ferritic cell size (nm)
25 20 15 10 5 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing temperature(°F)
Fig 6: Influence of austenitizing temperature on the ferritic cell size of ausferrite
Proeutectoid ferrite cell size(µm)
35 30 25 20 15 10 5 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing temperature (°F) Fig 7: Influence of austenitizing temperature on the proeutectoid ferritic cell size
25
(a) Tγ =1520°F, TA=725°F
(b) Tγ =1520°F, TA=600°F
26
(c) Tγ =1520°F, TA=550°F Fig 8: The Microstructure of the Ductile Iron after austenitizing at 1520°F and austempering at
Volume fraction of austenite (%)
(a)725°F, (b)600°F and (c) 550°F
25
20
15
10
5
Tγ=1520°F 0 500
550
600
650
700
750
Austempering temperature (°F) Fig 9: Influence of austempering temperature on the volume fraction of austenite in austempered structure 27
Dissolved carbon content in austenite(%)
3 2.5 2 1.5 1 0.5 0 500
550
600
650
700
750
Austempering temperature (°F) Fig 10: Influence of austempering temperature on austenitic carbon content
Ferritic cell size(nm)
30 25 20 15 10
Tγ= 1520°F
5
Tγ=1520°F
0 500
550
600
650
700
750
Austempering Temperature(°F)
Fig 11: Influence of austempering temperature on the ferritic cell size in ausferrite
28
Fig 12: X-ray diffraction pattern of ADI austenitized at 1520°F and austempered at 550°F
350
Brinell hardness (HB)
300 250 200 150 100 50 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing Temperature (°F) Figure 13: Influence of austenitizing temperature on the hardness of ADI
29
Brinell hardness (HB)
500
400
300
200
100
Tγ= 1520°F
0 500
550
600
650
700
750
Austempering Temperature (°F) Fig 14: Influence of austempering temperature on the hardness of ADI
Fracture Toughness (MPa√m)
75 70 65 60 55 50 45 40 35 30 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing Temperature( °F) Fig 15: Influence of austenitizing temperature on the fracture toughness of ADI austempered at 680°F
30
(a)Tγ =1535°F, TA=680°F
(b) Tγ =1436°F, TA=680°F
31
(c) Tγ =1418°F, TA=680°F
Fracture Toughness (MPa√m)
Fig 16: Fractographs of the ductile iron after austenitizing at different temperatures a) 1535°F, b) 1436 °F, c) 1418 °F and austempering at 680°F
66 64 62 60 58 56 54
Tγ=1520°F
52 50 500
550
600
650
700
750
Austempering Tempearture (°F)
Fig 17: Influence of austempering temperature on the fracture toughness of ADI
32
(a) Tγ =1520°F, TA=550°F
(b) Tγ =1520°F, TA=600°F Fig 18: The Fractograph of the Ductile Iron after austenitizing at 1520°F and austempering at (a)500°F(b) 600°F
33
Table 1: Chemical Composition of Ductile Cast Iron
Element
% Composition
C
3.44
Si
2.46
Ni
1.03
Cu
0.52
Mn
0.08
Cr
0.05
Mg
0.043
Al
0.018
V
0.017
P
0.016
Ce
0.013
Ti
0.010
Mo
<0.010
S
0.008
Sb
<0.005
Sn
<0.005
The carbon equivalent of the ductile iron alloy was 4.27
34
Table 2: Influence of austenitizing temperature on the mechanical properties of ADI Austenitizing Temperature
Austempering Temperature
Yield Strength (MPa)
Ultimate Tensile Strength (MPa)
Elongation %
Strain hardening exponent n
1535°F
680°F
722 ± 164
919 ±128
7.0 ± 0.9
0.12 ± 0.02
1520°F
680°F
683 ± 125
872 ±119
5.3 ±2.2
0.14 ± 0.01
1472°F
680°F
605 ± 95
823 ±87
8.2 ± 0.5
0.13± 0.01
1436°F
680°F
380 ± 1
490±6
12.3 ± 0.2
0.10±0.004
1418°F
680°F
374 ± 39
498 ±18
35
12.6 ± 0.1 0.11 ± 0.02
Table 3: Influence of austempering temperature on the mechanical properties of intercritically austenitized ADI
Austenitizing Temperature
Austempering Temperature
725°F
Yield Strength (MPa)
888± 34
Elongation %
Strain hardening exponent n
1052 ±6
6.0 ± 1.1
0.09 ± 0.01
Ultimate Tensile Strength (MPa)
680°F
683 ± 125
872 ±119
5.3 ±2.2
0.14 ± 0.01
600°F
1229 ± 15
1385 ± 34
3.1 ± 0.3
0.09 ± 0.01
550°F
1317 ± 41
1481 ± 13
2.0 ± 0.5
0.14 ± 0.03
1520°F
36
37
PII: DOI: Reference:
S0921-5093(17)30411-2 http://dx.doi.org/10.1016/j.msea.2017.03.096 MSA34879
To appear in: Materials Science & Engineering A Received date: 27 October 2016 Revised date: 23 March 2017 Accepted date: 24 March 2017 Cite this article as: Saranya Panneerselvam, Susil K. Putatunda, Richard Gundlach and James Boileau, Influence of Intercritical Austempering on the Microstructure and Mechanical Properties of Austempered Ductile Cast Iron ( A D I ) , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.03.096 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Intercritical Austempering on the Microstructure and Mechanical Properties of Austempered Ductile Cast Iron (ADI) Saranya Panneerselvam1, Susil K. Putatunda1*, Richard Gundlach2, James Boileau3 1
Wayne State University, Detroit, MI, USA.
2
Element Materials Technology, MI, USA.
3
Ford Motor Company, Dearborn, MI, USA.
*
Corresponding Author at: Department of Chemical Engineering and Materials
Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, MI 48202. Phone: +1 3135773808, Fax: +1 3135773810; e-mail address: [email protected]
Abstract The focus of this investigation was to examine the influence of intercritical austempering process on the microstructure and mechanical properties of low-alloyed austempered ductile cast iron (ADI). The investigation also examined the influence of intercritical austempering process on the plane strain fracture toughness of the material. The effect of both austenitization and austempering temperature on the microstructure and mechanical properties was examined. The microstructural analysis was carried out using optical microscopy, scanning electron microscopy and X-ray diffraction. The test results indicate that by intercritical austempering it is possible to produce proeutectoid ferrite in the matrix microstructure. Furthermore, the yield, tensile strength
1
and the fracture toughness of the ADI decreases with decrease in austenitizing temperature. Lower austenitizing temperature produces more proeutectoid ferrite in the matrix. A considerable increase in ductility was observed in the samples with higher proeutectoid ferrite content. The fracture surfaces of the ADI samples revealed that dimple ductile fracture produced higher fracture toughness of 60±5 MPa√m in this intercritically austempered ADI.
Keywords: austenitization, intercritical austempering, proeutectoid ferrite, ductility, fracture toughness.
1.0 INTRODUCTION Austempered ductile cast iron (ADI) has emerged as an important engineering material in recent years because of excellent combination of mechanical properties such as high yield and tensile strengths, good ductility [1-3], good fatigue strength [4,5] excellent wear resistance [6] and high fracture toughness [7]. ADI has low production costs due to its good castability and good machinability. Because of these attractive properties, ADI finds wide commercial applications in automotive, agricultural, earth moving machineries, railways [8-10] etc. The mechanical properties of ductile iron are entirely dependent on the volume fraction and distribution of phases in the microstructure [11-12]. A wide range of mechanical properties can be obtained in ductile cast iron by subjecting the as cast material to different heat treatment processes like quenching, normalizing, annealing and austempering.
2
Conventional austempering is an isothermal heat treatment process in which ductile cast iron is initially subjected to complete austenitization followed by austempering at a temperature above the Ms (martensite start temperature). During austempering ductile iron undergoes a two-stage phase transformation process [13-15]. In the first stage austenite (γ) decomposes into ferrite(α) and high carbon austenite(γHC).
γ= α+ γHC
(Stage 1)
If the ductile cast iron is held at this temperature for a prolonged period of time, a second reaction occurs, during the second stage, the γHC can further decompose into ferrite (α) and carbide (ε).
γHC= α+ ε
(Stage 2)
The optimum combination of strength and ductility in ADI is usually obtained after the completion of stage 1 reaction but before the onset of stage 2 reaction. The time period between the completion of stage 1 and the onset of stage 2 is referred to as the „process window‟. The processing window can be enlarged by adding suitable alloying elements like Ni, Mo and Cu. Austempering is usually performed at temperatures ranging between 500-750°F. Austempering of ductile cast iron at higher temperatures produces an ausferritic microstructure which is a combination of carbide free ferrite and austenite whereas at lower temperature it produces typical bainitic ferrite and austenite. The conventionally processed ADI contains large amounts of austenite. This higher austenitic content in the conventional ADI causes work hardening of the material and will affect the machinability of ADI. Another limitation of ADI processed by the
3
conventional austempering is that it does not contain pro-eutectoid ferrite in the matrix and its transformed ferrite content (α) is very low which limits the ductility of ADI. To overcome these limitations of conventional ADI, in recent years there has been significant interests [16-18] in producing ADI with mixed microstructures consisting of pro-eutectoid ferrite and ausferrite to improve the machinability and ductility of ADI. This can be achieved by intercritical austempering. Intercritical austempering is a process in which the as-cast ductile cast iron is heated to an austenitizing temperature between the A1 and αT temperatures where (α) ferrite and austenite co-exist [19-22]. This is followed by subsequent austempering. The upper and lower intercritical austempering temperature of the ductile cast iron is strongly dependent on the chemical composition of the material. Suitable intercritical austempering will therefore produce a unique microstructure consisting of pro-eutectoid ferrite, bainitic ferrite and carbon enriched austenite [19-22]. This material is expected to exhibit much greater ductility than the conventionally austempered or quenched and tempered ductile cast iron [23-24]. The tensile and yield strengths of this material will be much higher than the pearlitic grades [19-22]. Machinability will also be enhanced [24] in ADI by this intercritical heat treatment process. Therefore, the intercritically austempered ductile cast iron will find significant applications in the production of automotive components (e.g. suspension parts) where a combination of high strength and high ductility is required [20]. By controlling the intercritical austenitizing temperature it is possible to control the volume fraction of austenite and its carbon content. The other advantages of intercritical austempering are as follows:
4
1.The volume fraction of pro-eutectoid ferrite and the ferrite+ high carbon austenite content in the microstructure matrix can be controlled precisely to obtain the required strength and ductility in ADI. 2. A wide combination of intercritical austenitizing and austempering temperatures can be used to achieve targeted mechanical properties in ADI. 3.In the intercritical austempering process, austenitization is performed at a lower temperature than in conventional austempering, thus it is an energy saving process. While in recent years, a significant number of studies have been carried out to examine the effect of intercritical austempering on the microstructure [16-27], most studies were focused on the effect of intercritical austenitizing temperature and time [16], on the effect of volume fraction of phases [25], machinability [24], abrasive wear behaviour [17,26] and tensile properties [21-23] of ADI. However, so far no systematic investigation has been reported in the literature that examined the influence of both intercritical austenitizing and austempering temperature on the microstructure and mechanical properties including fracture toughness of unalloyed ADI. Therefore, this investigation was undertaken to examine the effects of both intercritical austenitizing as well as austempering temperatures on the microstructure, mechanical properties and fracture toughness of on low alloyed ductile cast iron. 2.0 EXPERIMENTAL PROCEDURE A low alloyed ductile cast iron with the chemical composition reported in the Table 1 was used in this investigation. The material was cast in the form of KEEL blocks according to ASTM A 536 of size 7.9"X 3.0"X4.0" and from these cast blocks cylindrical tensile samples and compact
5
tension samples for fracture toughness tests were machined as per ASTM standards E-8[27] and E-399[28] respectively. An equation [29] developed by the Ductile Iron Society was used to predict the intercritical temperature ranges for the ductile iron used in this study. The upper intercritical temperature (UCT) and lower intercritical temperatures (LCT) were thus determined to be 1520°F and 1404°F respectively. After fabrication, the machined cylindrical tensile samples and compact tension samples were divided into 8 batches (A, B, C, D, E, F, G and H) and heat treated as follows. The first batch of samples (batch A, B, C and D) was used to study the effect of intercritical austenitizing temperature on the microstructure and mechanical properties of ADI and, therefore, the first batch of samples was initially austenitized at an intercritical temperature (Tγ) between A1 and αT temperatures(where ferrite and austenite co-exist) at four different temperatures such as 1418°F, 1436°F, 1472°F and 1520°F, respectively, and then subsequently quenched in a salt bath maintained at 680°F. In order to determine the effect of austempering temperature, the remaining three batches of samples (E, F and G) were austenitized at an upper intercritical temperature of 1520°F, and then austempered at four different austempering temperatures (TA) such as 725°F, 680°F, 600°F and 550°F. Also for comparison purposes, one batch of samples (H) was austenitized just above the intercritical temperature of 1535°F and then austempered at 680°F.All the samples were austenitized for 3 hours and then austempered for another 3 hours and finally air cooled to room temperature.
6
Tensile tests and fracture toughness tests were carried out on a servo-hydraulic MTS test machine as per ASTM standards E-8 [27] and E-399 [28] respectively. Four samples were tested from each heat treated condition and the average values are reported. Microstructures of the compact tension samples were examined by optical microscopy and Scanning Electron Microscopy (SEM) after polishing and etching with 3% nital. The volume fraction of the proeutectoid ferrite and ausferrite in the intercritically austenitized ADI samples was determined by point counting in accordance with ASTM standard E-562 [30]. The fractured surface of all the heat treated ADI samples were examined by JEOL JSM 6510 LV LGS Scanning Electron microscope to determine the mode of fracture. X-ray diffraction (XRD) analysis was performed to estimate the austenite content and the carbon content of austenite following the procedure of Rundman and Klug [31]. XRD was conducted using monochromatic copper Kα radiation at 30 kV and 10 mA. A Bruker Phaser II diffractometer was used to scan angular 2θ range from 42°-46° and 72°-92°. The profiles were analyzed using Jade 5 software to obtain the peak positions and the integrated intensities of {111} and {220} planes of FCC austenite and {110} and {211} planes of BCC ferrite. The volume fractions of ferrite (Xα) and austenite (Xγ) were determined by the direct comparison method using the integrated intensities of the above planes [32]. The carbon content of the austenite was determined by the equation [33] a = 0.3548 + 0.00441 C Where a is the lattice parameter of austenite in nanometer and Cγ is the carbon content of austenite in wt%. The {111} and {220} planes of austenite were used to measure the lattice parameter. The ferritic cell sizes of the ausferritic structure were determined by the Scherrer
7
equation [32]. Three samples were examined from each heat treated condition and the data reported is the average from these samples. The proeutectoid ferrite size were determined using line intercept method. 3.0 RESULTS AND DISCUSSION 3.1 Influence of intercritical austempering on the microstructure of ADI The optical microstructure of the as cast ductile cast iron consisted of pearlite with a typical bull‟s eye ferrite structure and is shown in the figure 1. The microstructure consists of ferrite surrounding the graphite and pearlite which is a lamellar structure with alternating layers of ferrite and cementite. The graphite in the as-cast structure appears well rounded with 85% nodularity. Optical micrographs of the samples austenitized at 1520°F, 1472°F, 1436°F and austempered at 680°F are shown in the figures 2(a)-2(c). The optical micrographs show a mixed microstructure consisting of proeutectoid ferrite and ausferrite. There is no major difference in the optical micrograph between 1436ºF and 1418ºF austenitized samples, except for the increase in proeutectoid ferrite content. On heating the as cast sample in the intercritical austenitizing temperature region, where α + γ coexist, the pearlitic component of the as-cast microstructure converts to austenite and the amount of α-ferrite that remains depends on the austenitizing temperature. During austempering process, the austenite nucleates at the grain boundary of ferrite and then grows into ferrite by the nucleation and growth process. Nucleation produces ferritic matrix containing thin layers of austenite enriched with carbon. As the austenite continue to nucleate at prior ferrite grain boundaries, the austenite becomes enriched and saturated with carbon and the diffusion of carbon ahead of ferrite becomes difficult and the growth of ferrite is
8
arrested. Even though, unalloyed ductile cast iron is used in this study, appreciable amount of Ni, Si, Cu were present and they segregated around the graphite nodules and hence ferrite was found in the areas surrounding the graphite nodules of the intercritically austempered samples. Figure 3 show the effect of intercritical austenitizing temperature on the volume fraction of pro-eutectoid ferrite and ausferrite. The volume fraction of proeutectoid ferrite decreases with increase in austenitizing temperature, whereas the volume fraction of ausferrite increases with the increase in austenitizing temperature as predicted by the Lever rule [34]. The volume fraction of the austenite in the ausferrite was determined by X-ray analysis technique. Figures 4 and 5 show the influence of intercritical austenitizing temperature on the volume fraction of austenite and the percentage of dissolved carbon content in austenite respectively. The volume fraction of the austenite at the intercritical temperature is low for the ADI sample intercritically austenitized at 1418°F and its carbon content is also low. The volume fraction of austenite and its carbon content considerably increases with the increase in austenitizing temperature. This result is in good agreement with the existing literature [32, 33, 35]. The intercritical austenitizing temperature does not affect the ferritic cell size in the ausferrite of the samples as the ferritic cell size lies within a range of 22 nm to 27nm as shown in the figure 6. Figure 7 shows the influence of austenitizing temperature on the proeutectoid ferrite size. The proeutectoid ferrite cell size increases with the decrease in austenitizing temperature. The proeutectoid ferrite in the microstructure matrix restricts the growth of transformed ferritic needles. The optical microstructures of the samples intercritically austenitized at 1520°F and austempered at different austempering temperatures of 725°F, 600°F and 550°F are shown in the figures 8(a)8(c). The samples austempered at higher temperatures show a coarser ausferritic microstructure along with a considerable amount of proeutectoid ferrite in the matrix. As mentioned earlier,
9
ferritic grain growth occurs by a nucleation and growth process upon heating above the lower critical temperature, pearlite decreases as proeutectoid ferrite nucleates and experiences grain growth. Since grain growth is a function of temperature, a lower austempering temperature produced more ferrite grains and at the same time lower volume fraction of austenite in the microstructure. Figure 9 shows the correlation of volume fraction of austenite (retained and reacted austenite) with austempering temperature. The volume fraction of the total reacted as well as retained austenite increases with the increase in austempering temperature. At lower austempering temperature (550°F), only retained austenite plus lower bainite is present because no ausferrite forms at this temperature. However, at higher temperature, the matrix contains reacted austenite only because of ausferrite reaction which occurs at the upper bainitic region. In addition, the carbon content of the austenite at the intercritical temperature decreases with temperature. Perhaps, the decrease in carbon content of austenite causes a shift in the transition temperature between upper and lower bainite. This may explain the variation of the reacted austenite content with temperature as represented in figure 9. Figure 10 shows the influence of austempering temperature on the dissolved carbon at the intercritical temperature. The carbon content of the transformed (reacted) austenite was more or less the same and its value was about 1.8%. The ferritic cell size tends to become finer as the austempering temperature decreases from 725°F to 550°F as shown in the figure 11. Therefore, as with the conventional austempering process, lower austempering temperature in the intercritically austenitized samples, produced a finer ferritic cell size. At traditional austempering temperature, a lower carbon austenite produces finer ferritic cell size. Based on this observation, it may be possible to determine the austempering process to create a 10
nanoscale structure in ADI. For conventional ADI, when austempered from temperature above upper critical, generally results in much larger ferritic cell size. The plot (figure 9) substantiates the fact that not all the austempering temperatures produce ausferritic structure. At lower austempering temperatures (e.g. 550°F, where lower bainite form), it is anticipated that there will be unreacted austenite. This should evident by the broadening of the austenite peak in x-ray diffraction profile as shown in the figure 12, where the austenite peak is much broader and the volume fraction of austenite is lower than the finer ferrite grains. It also should affect the interpretation of carbon content of austenite. 3.2 Influence of Intercritical austempering on the mechanical properties of ADI Table 2 compares the mechanical properties of ADI samples as a function of intercritical austenitizing temperature. The yield strength and ultimate tensile strength of the intercritically austenitized ADI decreased with the decrease in austenitizing temperature. The transformed ferrite and austenite both coarsen as the austenitizing temperature decreases. The increase in the volume fraction of proeutectoid ferrite and the presence of coarser ferritic structures at lower austenitizing temperature decreased the strength of the ADI but progressively improved the ductility (% elongation) of the ADI. The ductility in terms of the % elongation of the intercritically austenitized ADI increased with the decrease in austenitizing temperature. Thus, by proper choice of austenitization temperature, it is possible to control the mechanical properties of ADI by controlling the amounts of pro-eutectoid ferrite and ausferrite in the matrix microstructure. Figure 13 shows the influence of austenitizing temperature on the hardness of ADI. The hardness value decreases with the decrease of austenitizing temperature. The presence of higher volume fraction of proeutectoid ferrite in the intercritically austenitized samples resulted in very low 11
hardness in these samples. Table 3 details the mechanical properties of the samples austenitized at 1520°F as a function of austempering temperature. The yield strength and ultimate tensile strength are higher for the samples austempered at lower austempering temperature of 550°F and 600°F, however, the ductility is lower. As the austempering temperature increases, the yield and ultimate tensile strength decreases while an improvement in the ductility was observed. This can be attributed to the presence of coarser ausferritic microstructures produced at the higher austempering temperatures. The finer ausferritic structure will contribute to higher strength as predicted by Hall-Petch equation [34]. As expected, hardness also decreases with increasing austempering temperature as shown in figure 14 due to coarser ausferritic structure. The strain hardening exponent values of all the samples were obtained through tensile testing. The results indicate that the strain hardening exponent (n) of the intercritically austempered ductile iron samples lies within a range of 0.09 to 0.14 which is a typical range for ADI. The austenitization temperature does not appear to have any significant effect on the strain hardening exponent of the material even though the austenite volume fraction decreased with austempering temperature. 3.3
Influence
of
intercritical
austempering
on
the
fracture
toughness
of
ADI:
The fracture surface of the compact tension samples subjected to different heat treatments was examined to determine the mechanism of fracture and this can be associated with the fracture toughness of the material. The fracture toughness of ADI austenitized in the intercritical region is shown in figure 15 which shows that the fracture toughness of the ADI decreased proportionally with the decrease in austenitizing temperature. This can be associated with the variation in the fractured surface of the samples with the change in austenitizing temperature. The fractographs of the sample austenitized at 1535°F is shown in the figure 16(a). The ADI austenitized just 12
above the intercritical temperature (1535°F) showed dimple ductile fracture throughout the fracture surface. From a previous investigation [36], it is clear that the ductile fracture produces a fracture toughness of 60 ± 5 MPa√m in ADI. As the austenitizing temperature is reduced, the transgranular cleavage type of fracture is also observed along with the dimple ductile fracture. As shown in the figures 16(b) and 16(c), at lower intercritical temperatures of 1436°F and 1418°F, the fractured surface is predominantly transgranular cleavage rather than dimple ductile and this has caused lower fracture toughness in these samples. This can be attributed to the coarse grained proeutectoid ferrite in the intercritically austempered samples. By the way, annealed ferrite ductile iron has a fracture toughness of only 45±5 MPa√m. The maximum fracture toughness obtained in ADI by the conventional single step austempered process (as reported in the literature) is between 60 and 66 MPa√m [37-40] Figure 17 shows the influence of austempering temperature on the fracture toughness of intercritically austenitized ADI. The fracture toughness of the samples increases with an increase in austempering temperature to a maximum at 680°F before decreasing again at 725°F. This can be due to the influence of ferritic grain size and the volume fraction of austenite [41-44]. The samples austempered at the higher temperature of 725°F and the lowest temperature of 550°F showed the presence of both dimple ductile and cleavage type of fracture as shown in the figure 18(a). The samples austempered at intermediate temperature of 600°F showed trans granular cleavage fracture throughout the fractured surface as shown in figure 18(b). The samples austempered at 680°F showed predominantly dimpled ductile fracture and that appears to be the reason for very high fracture toughness in these samples. Higher strength at lower temperature promotes transgranular cleavage. Also very coarse ausferrite favor transgranular cleavage fracture. This is consistent with the data reported in literatures. 13
4.0 Conclusions 1. The intercritical austempering of ductile cast iron produced a unique microstructure consisting of pro-eutectoid ferrite, bainitic ferrite and high carbon austenite in ADI.
14
2. The mechanical properties including the yield strength, ultimate tensile strength of ADI gradually reduced with the decrease in austenitizing temperature due to an increase in proeutectoid ferrite content. 3. A progressive increase in the ductility (% elongation) of the ADI samples was observed with the decrease in austenitizing temperature due to the larger volume fraction of proeutectoid ferrite in the microstructure of ADI. 4. While the ductility and strength increased with decreasing austenitizing temperature, the fracture toughness decreased. This can be attributed to the presence of increasing amount of proeutectoid (polygonal) ferrite in the matrix microstructure. 5. The strain hardening exponent of ADI was not significantly affected by the intercritical austenitizing and austempering temperatures. 6. Intercritical austempering close to the upper critical temperature resulted in good mechanical properties (high strength and ductility combined with good fracture toughness). Intercritical austenitizing well below upper critical did not result in exceptional properties. The reason for this appears to be the coarse grained proeutectoid ferrite. 7. The austempering at lower temperature (550°F) appear to produce lower bainite combined with low retained austenite content. It appears that low carbon austenite produced by austenitizing near the upper critical temperature produced fine ferrite cell size.
References 15
1. J. Dodd, “High strength, high ductility ductile irons”, Modern Castings,68, 5, 1978, 60-66. 2. R.B.Gundlach and J.F.Janowak, “Development of a ductile iron for commercial austempering” 94, 1983, 377-388. 3. R.A.Harding and G.N.J. Gilbert, “Why the properties of ductile irons should interest engineers”,79, 1986, 489-496. 4. L.Bartosiewicz, A.R.Krause, F.A. Alberts, Iqbal Singh, S.K. Putatunda, "Influence of Microstructure on High Cycle Fatigue Behavior of Austempered Ductile Cast Iron," Materials Characterization, 30, 1993, 22l-234. 5. LBartosiewicz, A.R.Krause, A.Sengupta and S.K.Putatunda, "Relationship between Fatigue threshold and Fatigue Strength in Austempered Ductile Cast Iron." International Symposium for Testing and Failure Analysis, ISTFA, ASM, 16, 1990, 323-336. 6. I.Schmidt and A.Schuchert, “Unlubricated wear of austempered ductile cast iron”, Zeitschriftjbr Metalkunde,78, 1987, 871-875. 7. S.K.Putatunda and P.P.Rao, "Influence of Microstructure on Fracture Toughness of Austempered Ductlie Cast Iron," Metallurgical and Materials Transaction, 28A, 7, 1997, 1457-1470. 8. J.Panasiewicz, C.Grupke and J.Huth, "Chrysler's Experience with Austempered Ductile Iron," World Conference on Austempered Ductile Iron, Bloomingdale, IL, 1991, 176-194. 9. K.Okazaki, H.Asai, M.Tokuyoshi, H.Kusonoki and H.Sakahara, Proceeding World Conference on Austempered Ductile Iron, Bloomingdale, IL, 1991, 288-299. 10. B.V.Kovacs, "Austempered Ductile Iron, Facts and Fiction," Modern Casting,36, 1990, 38-41. 11. T.N.Rouns and K.B.Rundman, “Constitution of austempered ductile iron and kinetics of austempering” AFS Trans, 95, 1987, 851-874. 12. J.Yang and S.K.Putatunda, “Near Threshold Fatigue Crack Growth Behavior of Austempered Ductile Cast Iron (ADI) Processed by a Novel Two-Step Austempering Process”, Mater. Sci. and Eng. A, 393, 2005, 254-268. 13. J.F.Janowak and P.A.Norton, "A Guide to Mechanical Properties Possible by Austempering, 1 Spercent Ni, 0.3 percent Mo Iron," AFS Transaction,88, 1985, 23-135.
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14. R.B.Gundlach and J.F.Janowak, "Austempered Ductile Irons Combine Strength with Toughness and Ductility," Metal Progress,12, 1985, 231-236. 15. J.F.Janowak, R.B.Gundlach, G.T.Eldis and K.Rohrting, "Technical Advances in Cast Iron Metallurgy,"AFS International Cast Metal Journal, 6, 1982, 28-42. 16. A.Basso, R.Martínez, J.Sikora, “Influence of chemical composition and holding time on austenite (γ) → ferrite (α) transformation in ductile iron occurring within the intercritical interval”, Journal of Alloys and Compounds, 509, 2011, 9884– 9889. 17. Y.Sahina, M.Erdoganb, M.Cerahb, “Effect of martensite volume fraction and tempering time on abrasive wear of ferritic ductile iron with dual matrix”, Wear 265, 2008, 196–202. 18. S.C.Murcia, M.A.Paniagua, E.A.Ossa, “Development of as-cast dual matrix structure (DMS) ductile iron”, Materials Science & Engineering A, 566, 2013, 8–15. 19. T.Kobayashi and S.Yamada, “Effect of Holding Time in the (α + γ) Temperature Range on Toughness of Specially Austempered Ductile Iron”, Metallurgical and Materials Transactions A, 27, 7, 1996,1961-1971. 20. J.Aranzabal, G.Serramoglia, and D.Rousiere, “Development of a New Mixed (FerriticAusferritic) Ductile Iron for Automotive Suspension Parts”, International Journal of Cast Metals Research,16, 1-3, 2003, 185-190. 21. V.Kilicli and M.Erdogan, ''Tensile Properties of Partially Austenitized and Austempered Ductile Irons with Dual Matrix Structures'', Materials Science and Technology, 22, 8, 2006, 919-928. 22. M.Cerah, K.Kocatepe, and M.Erdogan, “Influence of Martensite Volume Fraction and Tempering Time on Tensile Properties of Partially Austenitized in the (α + γ) Temperature Range and Quenched + Tempered Ferritic Ductile Iron”, Journal of Material Science, 40, 13, 2005,3453-3459. 23. M.Erdogan, K.Kocatepe, and M.Cerah, “Influence of Intercritical Austenitizing and Tempering Time and Martensite Volume Fractions on Tensile Properties of Ferritic Ductile Iron with Dual Matrix Structure”, International Journal of Cast Metals Research,19,4, 2006, 248-253. 24. J.K.Chen,J.S.Tsai,B.T.Chen, “Toughening of ADI austenitized in intercritical region”,
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General Paper Selections, The Minerals, Metals and Materials Society, 3, 2011,701-708. 25. M.Erdogan, V.Kilicli, B.Demir, “The influence of the austenitic dispersion on phase transformation during the austempering of ductile cast iron having a dual matrix structure”, International Journal of Materials Research, Volume 99, 7, 2008, 751-760. 26. Y.Sahin,V.Kilicli,M.Ozer, M.Erdogan, “Comparison of abrasive wear behavior of ductile iron with different dual matrix structures”,Wear, 268,2010, 153-165. 27. ASTM E-8, Standard Test Method for Tensile Testingof Metallic Materials, Annual Book of ASTM Standards, ASTM 3.01,1992, 542-566. 28. ASTM E-399, Standard Test Method for Plane Strain fracture toughness Testing of Metallic Materials, Annual Book of ASTM Standards, ASTM 3.01, 1992, 745-756. 29. K.L. Harynen, J.R.Keough, B.V.Kovacs, “The effect of alloying elements on the critical temperatures in ductile iron”, Ductile Iron Society, 1990,16-23. 30. ASTM E-562, Standard Test Method for determining volume fraction by systemic manual point count, ASTM International, West Conshohocken, PA, 2011. 31. K.B.Rundman, R.C.Klug, An x-ray and metallographic study of austempered ductile cast iron, American Foundry Society Transactions, 90, 1982, 499-508. 32. B.D.Cullity, Elements of X-ray diffraction, Addison –Wesley, Reading, MA,1974. 33. C.S.Roberts, “Effect of carbon on the lattice parameter of austenite”, Transactions of AIME, 197,1982,203-204. 34. G.E.Dieter, Mechanical Metallurgy, third ed.,McGraw Hills,1986. 35. V.Kilicli and M.Erdogan, “ The strain hardening behavior of partially austenitized and the austempered ductile irons with dual matrix structures”, Journal of Materials Engineering and Performance, 17,2008, 240-249. 36. P.P.Rao, S.K.Putatunda, “ Investigations on the Fracture Toughness of austemnpered ductile irons austenitized at different temperatures”, Materials Science and Engineering A, 349, 2003, 136-149. 37. J.L. Doong, C.Chen, “Fracture toughness of bainitic nodular cast iron”Fatigue and Fracture of Engineering . Material Structure 12,1989, 155–165.
18
38. E. Dorazil, M.Holzman, “Fracture behavior of austempered ductile iron Proceedings of the World Conference on Austempered Ductile Iron”, Bloomingdale, IL, March 1991,32–66. 39. M. Grech, P.Bowen, J.M.Young, “Effect of austempering temperature on the fracture toughness and tensile properties of an ADI alloyed with copper and nickel”, Proceedings of the World Conference on Austempered Ductile Iron, Bloomingdale, IL, March 1991, 338–374. 40. P.P. Rao, S.K.Putatunda, “Influence of microstructure on fracture toughness of austempered ductlie cast iron” Metallurgical and Materials Transactions 28A, 7, 1997, 1457–147. 41. S.Panneerselvam, C.J.Martis, S.K.Putatunda and
J.Boileau, “An Investigation on the
Stability of Austenite in Austempered ductile Cast Iron”, Materials Science and Engineering A, 626, 2015, 237-246. 42. P.P.Rao, S.K.Putatunda, “ Comparative study of fracture toughness of austempered ductile iron with lower and upper ausferritic microstructures, Materials science and Technology, 14, 1998, 1257-1265. 43. J.L. Doong and C. Chen, "Fracture Toughness of Bainitic-Nodular Cast iron," Fatigue Fracture of Engineering Material and Structure, 12, 1989, 155-165. 44. E. Dorazil and M. Holzman, "Fracture Behavior of Austempered Ductile Iron," Proceeding World Conference on Austempered Ductile Iron, Bloomingdale, IL, 1991, 3266.
19
Fig 1: As-cast microstructure of Ductile Cast Iron
20
(a) Tγ=1520°F, TA=680°F
(b) Tγ=1472°F, TA=680°F
21
(c) Tγ=1436°F, TA=680°F
Fig 2: The Microstructure of the Ductile Iron after austenitizing at (a)1520°F, (b)1472°F, (c) 1436°F and austempering at 680°F
22
100 90
Volume fraction (%)
80 70 60 50 40 30 20 10 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing Temperature (°F) Volume fraction of proeutectoid ferrite Volume fraction of austenite Linear (Volume fraction of proeutectoid ferrite) Linear (Volume fraction of austenite)
Fig 3: Influence of austenitizing temperature on the volume fraction of proeutectoid ferrite and austenite
23
Volume fraction of austenite (%)
15 13 11 9 7 5 3 1 -11400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing temperature(°F) Fig 4: Influence of austenitizing temperature on the volume fraction of austenite
Dissolved carbon content in austenite(%)
3.5 3 2.5 2 1.5 1 0.5 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing temperature (°F)
Fig 5: Influence of austenitizing temperature on the austenitic carbon content
24
30
Ferritic cell size (nm)
25 20 15 10 5 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing temperature(°F)
Fig 6: Influence of austenitizing temperature on the ferritic cell size of ausferrite
Proeutectoid ferrite cell size(µm)
35 30 25 20 15 10 5 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing temperature (°F) Fig 7: Influence of austenitizing temperature on the proeutectoid ferritic cell size
25
(a) Tγ =1520°F, TA=725°F
(b) Tγ =1520°F, TA=600°F
26
(c) Tγ =1520°F, TA=550°F Fig 8: The Microstructure of the Ductile Iron after austenitizing at 1520°F and austempering at
Volume fraction of austenite (%)
(a)725°F, (b)600°F and (c) 550°F
25
20
15
10
5
Tγ=1520°F 0 500
550
600
650
700
750
Austempering temperature (°F) Fig 9: Influence of austempering temperature on the volume fraction of austenite in austempered structure 27
Dissolved carbon content in austenite(%)
3 2.5 2 1.5 1 0.5 0 500
550
600
650
700
750
Austempering temperature (°F) Fig 10: Influence of austempering temperature on austenitic carbon content
Ferritic cell size(nm)
30 25 20 15 10
Tγ= 1520°F
5
Tγ=1520°F
0 500
550
600
650
700
750
Austempering Temperature(°F)
Fig 11: Influence of austempering temperature on the ferritic cell size in ausferrite
28
Fig 12: X-ray diffraction pattern of ADI austenitized at 1520°F and austempered at 550°F
350
Brinell hardness (HB)
300 250 200 150 100 50 0 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing Temperature (°F) Figure 13: Influence of austenitizing temperature on the hardness of ADI
29
Brinell hardness (HB)
500
400
300
200
100
Tγ= 1520°F
0 500
550
600
650
700
750
Austempering Temperature (°F) Fig 14: Influence of austempering temperature on the hardness of ADI
Fracture Toughness (MPa√m)
75 70 65 60 55 50 45 40 35 30 1400
1420
1440
1460
1480
1500
1520
1540
1560
Austenitizing Temperature( °F) Fig 15: Influence of austenitizing temperature on the fracture toughness of ADI austempered at 680°F
30
(a)Tγ =1535°F, TA=680°F
(b) Tγ =1436°F, TA=680°F
31
(c) Tγ =1418°F, TA=680°F
Fracture Toughness (MPa√m)
Fig 16: Fractographs of the ductile iron after austenitizing at different temperatures a) 1535°F, b) 1436 °F, c) 1418 °F and austempering at 680°F
66 64 62 60 58 56 54
Tγ=1520°F
52 50 500
550
600
650
700
750
Austempering Tempearture (°F)
Fig 17: Influence of austempering temperature on the fracture toughness of ADI
32
(a) Tγ =1520°F, TA=550°F
(b) Tγ =1520°F, TA=600°F Fig 18: The Fractograph of the Ductile Iron after austenitizing at 1520°F and austempering at (a)500°F(b) 600°F
33
Table 1: Chemical Composition of Ductile Cast Iron
Element
% Composition
C
3.44
Si
2.46
Ni
1.03
Cu
0.52
Mn
0.08
Cr
0.05
Mg
0.043
Al
0.018
V
0.017
P
0.016
Ce
0.013
Ti
0.010
Mo
<0.010
S
0.008
Sb
<0.005
Sn
<0.005
The carbon equivalent of the ductile iron alloy was 4.27
34
Table 2: Influence of austenitizing temperature on the mechanical properties of ADI Austenitizing Temperature
Austempering Temperature
Yield Strength (MPa)
Ultimate Tensile Strength (MPa)
Elongation %
Strain hardening exponent n
1535°F
680°F
722 ± 164
919 ±128
7.0 ± 0.9
0.12 ± 0.02
1520°F
680°F
683 ± 125
872 ±119
5.3 ±2.2
0.14 ± 0.01
1472°F
680°F
605 ± 95
823 ±87
8.2 ± 0.5
0.13± 0.01
1436°F
680°F
380 ± 1
490±6
12.3 ± 0.2
0.10±0.004
1418°F
680°F
374 ± 39
498 ±18
35
12.6 ± 0.1 0.11 ± 0.02
Table 3: Influence of austempering temperature on the mechanical properties of intercritically austenitized ADI
Austenitizing Temperature
Austempering Temperature
725°F
Yield Strength (MPa)
888± 34
Elongation %
Strain hardening exponent n
1052 ±6
6.0 ± 1.1
0.09 ± 0.01
Ultimate Tensile Strength (MPa)
680°F
683 ± 125
872 ±119
5.3 ±2.2
0.14 ± 0.01
600°F
1229 ± 15
1385 ± 34
3.1 ± 0.3
0.09 ± 0.01
550°F
1317 ± 41
1481 ± 13
2.0 ± 0.5
0.14 ± 0.03
1520°F
36
37