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Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel
J. Mater. Sci. Technol., 2012, 28(4), 336–342.
Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel Khodamorad Abbaszadeh, Hassan Saghafian† and Shahram Kheirandish School of Metallurgy and Materials Engineering, Iran University of Science and Technology , Tehran 1684613114, Iran [Manuscript received March 16, 2011, in revised form November 10, 2011]
The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910 ◦ C for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 ◦ C and the remaining samples were quenched in salt bath at 425 ◦ C for various holding times. All samples were tempered at 200 ◦ C for 2 h. Microstructures were examined by optical microscopy (OM) and scanning electron microscopy (SEM). Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch (CVN) impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior. KEY WORDS: Upper bainite; Lower bainite; Microstructure; D6AC; Mechanical properties
1. Introduction Medium carbon low alloy Cr-Mo-Ni-V steels are used widely for pressure vessels and automotive applications[1,2] . However, the use of these steels is limited by their poor ductility at the highest strength levels[3,4] . The ductility and toughness of these steels can be improved by replacing conventional quenching and tempering with austempering, through which a bainitic or mixed bainite-martensite microstructure forms instead of a fully martensitic microstructure[5–7] . Two morphologies of bainite may form depending on austempering temperature. At temperatures above and close to MS , the lower bainite forms while at temperatures close and be† Corresponding author. Assoc. Prof., Ph.D.; Tel.: +98 21 77459151; Fax: +98 21 77240480; E-mail address: [email protected] (H. Saghafian).
low the pearlite transformation temperature, the upper bainite forms[8] . Therefore, bainite morphology and its mechanical properties vary with austempering temperature[8] . Sajjadi and Zebarjad[9] reported that the lower bainite exhibited superior mechanical properties because of the finer and more uniform distributed carbides, higher amount of dissolved carbon in bainitic ferrite and fine bainitic ferrite grains. They demonstrated that all of these characteristics are related to the lower transformation temperatures at which the lower bainite forms. Bakhtiari and Ekrami[10] reported that for 4340 steel increasing the austempering temperature from 300 to 400 ◦ C (bainite morphology varies from the lower to upper bainite) leads to a reduction in yield and ultimate tensile strength, hardness, elongation and impact energy. Tomita[11,12] suggested that the lower bainite
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Table1 Chemical compositions of D6AC steel (wt%) C 0.47
Si 0.26
Mn 0.76
P 0.009
S 0.004
which appears in acicular form and divides the prior austenite grain, in the presence of martensite, provided a better combination of mechanical properties in AISI 4340 steel. This was not the case about the mixed microstructure of the upper bainite-martensite showing much lower mechanical properties compared to the martensitic microstructure. Contrary to the results of Tomita, Rao et al.[13] suggested that at equivalent strength, the toughness and ductility of mixed upper bainite-martensite microstructure in AISI 4330V are superior to those of the tempered martensite one. However, no beneficial effect of the lower bainite on mechanical properties was observed in the lower bainite-martensite microstructure. The above survey of literature implies that the effects of the mixed bainite-martensite microstructures on mechanical properties of ultra-high strength low alloy steels are complicated. Therefore, further investigations are needed to obtain a conclusive understanding in this regard. Thus the current work was undertaken to study the effect of bainite morphology on the mechanical properties of the mixed bainite-martensite microstructure in D6AC medium carbon low alloy ultra-high strength steel. 2. Experimental The D6AC steel was received as the forged bars of 85 mm in diameter with chemical compositions given in Table 1. Test plates of 60 mm×130 mm with a thickness of 6 and 12 mm were cut from the steel bars. The length of the plates was in the longitudinal direction of the bar. Each plate was first stress relieved at 650 ◦ C and then fully annealed at 850 ◦ C for 2 h. Austenitizing was carried out in an argon atmosphere furnace at 910±4 ◦ C for 40 min. Following austenitizing, samples were quenched in three different methods as follows: some of the samples were directly oil-quenched to produce martensitic microstructure. Some others were quenched in salt bath at 330 ◦ C above the MS and held for 3.5, 5, 7.5, 10, 12.5, 15, 20 and 180 min to obtain the mixed microstructures of lower bainite-martensite with different volume fractions of the lower bainite. The remaining specimens were quenched in salt bath at 425 ◦ C and held for 5, 10, 15, 20, 25 and 120 min to obtain the mixed microstructures of the upper bainitemartensite with different volume fractions of the upper bainite. Fully lower bainite and upper bainite microstructures were also obtained by quenching the specimens in salt baths at 330 (24 h) and 425 ◦ C (4 h), respectively. Finally, all specimens were tempered at 200 ◦ C for 2 h. After heat treatment, the plates were machined
Cr 0.99
Mo 0.93
Ni 0.54
V 0.11
Fe Bal.
and ground to the final thicknesses of 4 and 10 mm to eliminate any decarburized layer. The sub size tensile specimens which are 4 mm thick with a gauge length of 25 mm were wire cut from a plate which is 4 mm thick in accordance with ASTM E8M[14] . Charpy V-notch (CVN) impact specimens were wirecut from a plate which is 10 mm thick in accordance with ASTM E23[15] . Tensile tests were carried out using a 200 kN electromechanical universal CMT5205H machine at constant cross-head speed of 5 mm/min. The CVN impact properties were determined using a 300 J metal pendulum impact ZBC2152 machine. A minimum of seven impact and three tensile specimens were tested in each case. The microstructures were examined by optical and scanning electron microscopys (SEM). Metallography samples were cut from the impact test pieces. In order to reveal the lower and upper bainites presented in the mixed microstructures, the samples were etched by using a solution of 4 wt% picral plus 1 wt% HCl[16] . Volume fractions of the lower and upper bainites were determined by using clemex vision image analysis software based on the difference in color between lower bainite (dark) and martensite (white). At least five representative areas in each sample were studied through metallographic evaluations. The X-ray diffraction technique was employed by using CuKα radiation to detect retained austenite[17] . The fracture surfaces of the impact samples were examined by SEM in order to characterize the impact fracture mode. 3. Results and Discussion 3.1 Morphology and microstructure Fig. 1(a) and (b) shows typical optical micrographs of the mixed lower bainite-martensite and the upper bainite-martensite microstructures, respectively. In these figures, the dark etched regions correspond to bainite and the white regions correspond to martensite. Micro hardness measurements showed that the hardness values of the lower bainite, the upper bainite and martensite are 480, 390 and 740– 750 HV, respectively, further confirming the metallographic observations. Fig. 2(a) and (b) shows typical SEM images of the mixed lower bainite-martensite and the upper bainite-martensite microstructures, respectively. It can be seen from Fig. 2(a) that the lower bainite in mixed microstructures of lower bainitemartensite appears in acicular form and partitions the prior austenite grains, but Fig. 2(b) shows the upper bainite fills the prior austenite grains. The upper and lower bainite have different growth mechanism because they form at different temperatures. The growth of the upper bainite can be described as
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Fig. 1 Optical micrographs of mixed microstructure: (a) lower bainite (BL )-martensite (M), (b) upper bainite (BU )-martensite (M)
Fig. 2 SEM micrographs of mixed microstructure: (a) lower bainite (BL )-martensite (M), (b) upper bainite (BU )martensite (M)
the movement of a low coherent ferrite/austenite interface which necessitates some degree of self-diffusion and leads to rather immobility of the interface[18] . On the contrary, in the case of the lower bainite the propagation of a more coherent interface becomes possible and therefore, little or no self-diffusion required. This results in more rapid movement of the interface[19] . Thus, the reason for the different morphology of bainite has been attributed to the different mechanisms based on which growth and nucleation occur[20] . 3.2 Mechanical properties In order to study the effect of the lower bainitemartensite mixed microstructure on the mechanical properties of the D6AC steel, tensile and impact tests were conducted. Volume fractions (%) of the lower bainite in the mixed microstructures were 0 (a fully martensitic microstructure), 12, 28, 37, 41, 43, 45, 49, 75 and 100 (a fully bainitic microstructure) and those for the upper bainite were 0 (a fully martensitic microstructure), 6, 17, 30, 38, 42, 70 and 100 (a fully bainitic microstructure). Variations of yield and tensile strengths in terms of the lower and upper bainite volume fractions are shown in Figs. 3 and 4, respectively. The average errors for yield and ultimate tensile strengths are ±10 and ±15 MPa, respectively. According to the rule of mixtures (Eq. (1)), yield strength of the mixed bainite-martensite microstructure decreases linearly with increasing the volume fraction of bainite. This is obvious, because the
yield strength of martensite is higher than the yield strength of bainite[12] . σyMix = σyM (1 − VB ) + σyB VB
(1)
where σ Mix is yield strength of the bainite-martensite y B mixed microstructure, σ M y and σ y are yield strength of martensite and bainite microstructure, respectively, VB is volume fraction of lower bainite. In comparidevison, in the present experimental results, σ Mix y ates more from Eq. (1). As shown in Fig. 3(a), by increasing the volume fraction of the lower bainite up to 28 vol.%, the yield strength increases and attains about 1869 MPa and then decreases to the strength level of a fully lower bainitic sample. As seen in Fig. 2(a), the lower bainite partitions the prior austenite grains and therefore, decreases the martensite packet size. According to the HallPetch relationship, Eq. (2), decreasing the martensite packet size increases the strength of the martensite microstructure. −0.5 σyM = σi + kSm
(2)
where σ i is frictional stress, k is constant, and Sm is martensite packet size[12] . In addition, the strength of the lower bainite in the mixed microstructure was increased by the plastic constraint induced by the surrounding stronger martensite. Young and Behadeshia[21] demonstrated that the yield strength is increased by the plastic constraint, upon which a soft thin layer can be constrained by the hard matrix
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surrounding it. For example, a weak brazing alloy can be used effectively to bond much stronger specimens provided that the brazing material is thin enough to be constrained by the surrounding stronger matrix. Indeed the strength of the joint increases as the thickness of the brazing layer decreases. The same phenomenon occurs when the lower bainite plates form in the austenite which subsequently transforms to much stronger martensite. In other words the bainite plate plays the role of a soft layer. On the other hand, the presence of a peak at about 28 vol.% lower bainite in the curve of strength as a function of lower bainite volume fraction can be attributed to the improvement in the strength of martensite due to the refinement of its substructure induced by partitioning of the prior austenite grains by the lower bainite. In addition, the strength of lower bainite was increased by plastic constraint induced by the martensite. On the other hand, Fig. 3(b) shows that the yield strength of the upper bainite is 1304 MPa which is less than the yield strength of the lower bainite (1385 MPa). The higher yield strength of the lower bainite can be attributed to: (1) difficult diffusion of carbon to long distances in lower bainite because of low transformation temperature, causing low bainitic ferrite plate width. Relation of yield strength with plate width is accordance to Hall-Petch equation[22,23] ; (2) the lower bainite has more dislocation density and the probability of dislocation pile-up is higher; (3) in the lower bainite,
carbides are finer with more uniform distribution; so higher stresses are needed for dislocations to pass carbide[24] . In the mixed upper bainite-martensite microstructure, increasing the volume fraction of the upper bainite, the yield strength decreases and reaches the strength level of a fully upper bainitic sample and no peak is observed in the curve of the strength as a function of the upper bainite contents. This result is contrary to the beneficial effects of the upper bainitemartensite microstructure in AISI 4330V reported by Rao et al [13] . In the present work, optical and SEM images in Figs. 1(b) and 2(b) show the upper bainite fills the prior austenite grain without partitioning it. Hence, by comparison the Figs. 1(a) and 2(a) with Figs. 1(b) and 2(b), it can be concluded that the shape and distribution of bainite in the mixed bainitemartensite microstructure probably is the main controlling factor of the tensile properties. On the other hand, the lower bainite appears as acicular form and partitions the prior austenite grains and therefore at 28 vol.% the yield strength of the mixed lower bainitemartensite microstructure increases. But the upper bainite appears as a bulk and fills the prior austenite grain and therefore the yield strength of the mixed upper bainite-martensite microstructure decreases. Also similar result is found between tensile strength of the mixed bainite-martensite microstructure and bainite volume fraction (Fig. 4(a) and (b)). This result is in agreement with those reported by Tomita[12] .
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Fig. 3 Variation of yield strength with lower (a) and upper (b) bainite volume fraction 2100
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Fig. 4 Variation of tensile strength with lower (a) and upper (b) bainite volume fraction
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Fig. 5 Variation of elongation with lower (a) and upper (b) bainite volume fraction 30
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Fig. 6 Variation of CVN impact energy with lower (a) and upper (b) bainite volume fraction
The variations of elongation (EL) and CVN impact energy with the bainite volume fractions are shown in Figs. 5 and 6, respectively. As shown, elongation and impact toughness increased by increasing the lower bainite volume fraction. This improvement in ductility and impact toughness is not resulted from the retained austenite. X-ray diffraction patterns have not shown diffraction peaks at (200)γ , (220)γ , (113)γ , (222)γ and (004)γ . Hence, it is speculated that the amount of retained austenite was not sufficient enough to be detected[17] . Thus, this amount of retained austenite has no significant effect on the mechanical properties of various microstructures. On the other hand, it can be seen from Figs. 5 and 6 that the elongation and impact toughness decreased by increasing the upper bainite volume fraction. The superior impact toughness and ductility of the lower bainite compared with the upper bainite can be attributed to the presence of the finer carbide particles in the lower bainite, since probable cracks which formed by cracking of these particles are smaller and crack propegation does not occur easily. Furthermore, fracture strength is inversly related to carbide thickness[8] . Thus, the tensile and impact tests show that the lower bainitic microstructure is more ductile and tougher than the martensitic and upper bainitic microstructures. On
Fig. 7 SEM image of microcrack being initiated during charpy impact testing of specimen having 49 vol.% lower bainite
the other hand, in mixed lower bainite-martensite microstructure, a crack propagating within the martensite will be blunt when reaching the more flexible lower bainite plates and therefore the impact energy increases. This is because the lower bainite brings into full play the arresting effect of the crack and stress-relief, as a result of deformation associated with martensite[11] . 3.3 Fractography The fracture surfaces of the impact specimen are shown in Figs. 7 and 8. From Fig. 7 (SEM image) microcracks are found behind the fracture surfaces
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Fig. 8 SEM images of impact fracture surface showing: (a) quasi-cleavage and dimple rupture in mixed 28 vol.% lower bainite-martensite microstructure, (b) mostly dimple rupture and some quasi-cleavage facets in mixed 49 vol.% lower bainite-martensite, (c) dimple rupture in fully lower bainitic sample, (d) transgranular cleavage in martensitic sample, (e) transgranular cleavage in upper bainitic sample
of impact samples having the mixed lower bainitemartensite microstructure. It can be seen from Fig. 7 that microcracks were stopped by dimple rigions (the lower bainite area). This suggests that the lower bainite is responsible for the increased impact toughness. Fig. 8(a) and (b) shows that the impact fracture morphology in the mixed lower bainite-martensite microstructure samples was a mixed mode of dimple rupture and quasi-cleavage[7] . However, by increasing the lower bainite volume fraction the samples exhibited more dimple rupture. On the other hand, the fracture morphology changes from the mixed mode of dimple rupture and quasi-cleavage in the samples containing the lower bainite and martensite(Fig. 8(a) and (b)) to fully dimple rupture in fully lower bainitic sample (Fig. 8(c)). Therefore, as shown in Fig. 6, by increasing the lower bainite volume fraction, the impact toughness is increaesed. Also, the fully lower bainitic sample has higher absorbed impact energy than the other samples. Typical fracture surface of impact sample containing martensite and upper bainite is shown in Fig. 8(d) and (e), respectively. These figures reveal that a crack in martensitic and upper bainitic
samples propegated in a brittle manner, producing cleavage and quasi- cleavage facets[7] . The results are consistent with the low impact toughness of martensitic and upper bainitic samples. The superior impact toughness and ductility of the lower bainite compared with martensite in Q&T condition samples were also found in the earlier studies[25] . But, it is noteworthy that this is only observed when the lower bainite is formed through an isothermal transformation. The mixed microstructures of bainite-martensite formed by continious cooling and slack quenching have lower toughness than martensitic microstructures[26] . 4. Conclusions (1) Austempering at 330 and 425 ◦ C resulted in the formation of the lower and upper bainites, respectively. (2) The yield strength, ultimate tensile strength, elongation and Charpy V-notch impact energy of the lower bainitic samples are superior to those of the upper bainitic samples. This can be mainly attributed to finer and more uniform distribution of carbides,
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lower bainitic ferrite plate width and higher dislocation density in the lower bainite. (3) A maximum point was observed in the curve of strength as a function of the lower bainite. This can be explained on the basis of two factors. The first is an increase in the strength due to the partitioning of the prior austenite grains by the lower bainite resulting in the refinement of martensite substructures. The second is a plastic constraint effect leading to an enhanced strength of the lower bainite induced by the surrounding relatively rigid martensite. (4) By increasing the lower bainite volume fraction in the mixed lower bainite-martensite microstructures, the elongation, reduction area and Charpy Vnotch impact energy increased because of significantly greater ductility and impact toughness of the lower bainite compared with martensite. (5) By increasing the upper bainite volume fraction in the mixed upper bainite-martensite microstructures, yield strength, ultimate tensile strength, elongation and Charpy V-notch impact energy decreased. These behaviors can be related to the shape and distributions of the upper bainite with a bulky morphology occupying the prior austenite. (6) The impact fracture surfaces of the martensitic and the upper bainitic samples have shown cleavage and qusi-cleavage morphologies, indicating tendency to the brittle behavior. The impact fracture surfaces of the lower bainitic samples exhibited a dimple rupture mode. However, the fracture morphology in the mixed lower bainite-martensite microstructure samples was a mixed mode of dimple rupture and qusicleavage. REFERENCES [1 ] S. Maropoulos, N. Ridley and S. Karagiannis: Mater. Sci. Eng. A, 2004, 380, 79. [2 ] A.R. Mirak and M. Nili-Ahmadabadi: Mater. Sci. Technol., 2004, 20, 897. [3 ] A. Salemi, A. Abdollah-zadeh, M. Mirzaei and H. Assadi: Mater. Sci. Eng. A, 2008, 492, 45. [4 ] A. Abdollah-zadeh, A. Salemi and H. Assadi: Mater. Sci. Eng. A, 2008, 483–484, 325. [5 ] J. Wen, Q. Li and Y. Long: Mater. Sci. Eng. A, 2006,
438–440, 251. [6 ] T. Wang, J. Yang, C. Shang, X. Li, B. Zhang and F. Zhang: Scripta Mater., 2009, 61, 434. [7 ] J. Tartaglia, K. Lazzari, G. Hui and K. Hayrynen: Met. Trans. A, 2008, 39, 559. [8 ] H.K.D.L. Bhadeshia: Bainite in Steels: Transformation, Microstructure and Properties, IOM Comunications Ltd., London, 2001. [9 ] S.A. Sajjadi and S.M. Zebarjad: J Mater. Process. Technol., 2007, 189, 107. [10] R. Bakhtiari and A. Ekrami: Mater. Sci. Eng. A, 2009, 525, 159. [11] Y. Tomita: Metall. Mater. Trans. A, 1987, 18, 1495. [12] Y. Tomita: Metall. Mater. Trans. A, 1983, 14, 2387. [13] T.V.L. Narasimha Rao, S.N. Dikshit, G. Malakondaiah and P. Rama Rao: Scripta Metall. Mater., 1990, 24, 1323. [14] ASTM Standards E8 M: Test Method for Tension Testing of Metallic Material, Vol.03.01. ASTM International, PA, American Society for Testing and materials, 1998. [15] ASTM Standards E23: Test Method for Notched Bar Impact Testing of Metallic Material, Vol.03.01. ASTM International, PA, American Society for Testing and materials, 1998. [16] B.L. Bramfitt and A. Benscoter: Metallographer’s Guide, Practices and Procedures for Iron and Steels, ASM International, 2002, 233. [17] S.H. Manger, R.J. Angelis, W.N. Wein and J.D. Makinson: Adv. X–Ray Anal., 2002, 45, 92. [18] V.F. Zackay and H.I. Aaronson: Decomposition of Austenite by Diffusional Processes, Interscience Publishers, New York, 1962. [19] D. Quidort and Y.J.M. Brechet: Acta Mater., 2001, 49, 4161. [20] O.P. Morozov, V.M. Schastlivtsev and I.L. Yakovleva: Fiz. Metal. Metalloved., 1990, 69(2), 146. [21] C.H. Young and H.K.D.L Bhadeshia: Mater. Sci. Technol., 1994, 10, 209. [22] J. Chakraborty, D. Bhattacharjee and I. Manna: Scripta Mater., 2009, 61, 604. [23] J. Chakraborty, D. Bhattacharjee and I. Manna: Scripta Mater., 2008, 59, 247. [24] C.A.N. Lanzillotto and F.B. Pickering: Met. Sci., 1982, 16, 371. [25] N. Saeidi and A. Ekrami: Mater. Sci. Eng. A, 2009, 523, 125. [26] X.Z. Zhang and J.F. Knott: Acta Mater., 1999, 47, 3483.
Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel Khodamorad Abbaszadeh, Hassan Saghafian† and Shahram Kheirandish School of Metallurgy and Materials Engineering, Iran University of Science and Technology , Tehran 1684613114, Iran [Manuscript received March 16, 2011, in revised form November 10, 2011]
The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910 ◦ C for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 ◦ C and the remaining samples were quenched in salt bath at 425 ◦ C for various holding times. All samples were tempered at 200 ◦ C for 2 h. Microstructures were examined by optical microscopy (OM) and scanning electron microscopy (SEM). Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch (CVN) impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior. KEY WORDS: Upper bainite; Lower bainite; Microstructure; D6AC; Mechanical properties
1. Introduction Medium carbon low alloy Cr-Mo-Ni-V steels are used widely for pressure vessels and automotive applications[1,2] . However, the use of these steels is limited by their poor ductility at the highest strength levels[3,4] . The ductility and toughness of these steels can be improved by replacing conventional quenching and tempering with austempering, through which a bainitic or mixed bainite-martensite microstructure forms instead of a fully martensitic microstructure[5–7] . Two morphologies of bainite may form depending on austempering temperature. At temperatures above and close to MS , the lower bainite forms while at temperatures close and be† Corresponding author. Assoc. Prof., Ph.D.; Tel.: +98 21 77459151; Fax: +98 21 77240480; E-mail address: [email protected] (H. Saghafian).
low the pearlite transformation temperature, the upper bainite forms[8] . Therefore, bainite morphology and its mechanical properties vary with austempering temperature[8] . Sajjadi and Zebarjad[9] reported that the lower bainite exhibited superior mechanical properties because of the finer and more uniform distributed carbides, higher amount of dissolved carbon in bainitic ferrite and fine bainitic ferrite grains. They demonstrated that all of these characteristics are related to the lower transformation temperatures at which the lower bainite forms. Bakhtiari and Ekrami[10] reported that for 4340 steel increasing the austempering temperature from 300 to 400 ◦ C (bainite morphology varies from the lower to upper bainite) leads to a reduction in yield and ultimate tensile strength, hardness, elongation and impact energy. Tomita[11,12] suggested that the lower bainite
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Table1 Chemical compositions of D6AC steel (wt%) C 0.47
Si 0.26
Mn 0.76
P 0.009
S 0.004
which appears in acicular form and divides the prior austenite grain, in the presence of martensite, provided a better combination of mechanical properties in AISI 4340 steel. This was not the case about the mixed microstructure of the upper bainite-martensite showing much lower mechanical properties compared to the martensitic microstructure. Contrary to the results of Tomita, Rao et al.[13] suggested that at equivalent strength, the toughness and ductility of mixed upper bainite-martensite microstructure in AISI 4330V are superior to those of the tempered martensite one. However, no beneficial effect of the lower bainite on mechanical properties was observed in the lower bainite-martensite microstructure. The above survey of literature implies that the effects of the mixed bainite-martensite microstructures on mechanical properties of ultra-high strength low alloy steels are complicated. Therefore, further investigations are needed to obtain a conclusive understanding in this regard. Thus the current work was undertaken to study the effect of bainite morphology on the mechanical properties of the mixed bainite-martensite microstructure in D6AC medium carbon low alloy ultra-high strength steel. 2. Experimental The D6AC steel was received as the forged bars of 85 mm in diameter with chemical compositions given in Table 1. Test plates of 60 mm×130 mm with a thickness of 6 and 12 mm were cut from the steel bars. The length of the plates was in the longitudinal direction of the bar. Each plate was first stress relieved at 650 ◦ C and then fully annealed at 850 ◦ C for 2 h. Austenitizing was carried out in an argon atmosphere furnace at 910±4 ◦ C for 40 min. Following austenitizing, samples were quenched in three different methods as follows: some of the samples were directly oil-quenched to produce martensitic microstructure. Some others were quenched in salt bath at 330 ◦ C above the MS and held for 3.5, 5, 7.5, 10, 12.5, 15, 20 and 180 min to obtain the mixed microstructures of lower bainite-martensite with different volume fractions of the lower bainite. The remaining specimens were quenched in salt bath at 425 ◦ C and held for 5, 10, 15, 20, 25 and 120 min to obtain the mixed microstructures of the upper bainitemartensite with different volume fractions of the upper bainite. Fully lower bainite and upper bainite microstructures were also obtained by quenching the specimens in salt baths at 330 (24 h) and 425 ◦ C (4 h), respectively. Finally, all specimens were tempered at 200 ◦ C for 2 h. After heat treatment, the plates were machined
Cr 0.99
Mo 0.93
Ni 0.54
V 0.11
Fe Bal.
and ground to the final thicknesses of 4 and 10 mm to eliminate any decarburized layer. The sub size tensile specimens which are 4 mm thick with a gauge length of 25 mm were wire cut from a plate which is 4 mm thick in accordance with ASTM E8M[14] . Charpy V-notch (CVN) impact specimens were wirecut from a plate which is 10 mm thick in accordance with ASTM E23[15] . Tensile tests were carried out using a 200 kN electromechanical universal CMT5205H machine at constant cross-head speed of 5 mm/min. The CVN impact properties were determined using a 300 J metal pendulum impact ZBC2152 machine. A minimum of seven impact and three tensile specimens were tested in each case. The microstructures were examined by optical and scanning electron microscopys (SEM). Metallography samples were cut from the impact test pieces. In order to reveal the lower and upper bainites presented in the mixed microstructures, the samples were etched by using a solution of 4 wt% picral plus 1 wt% HCl[16] . Volume fractions of the lower and upper bainites were determined by using clemex vision image analysis software based on the difference in color between lower bainite (dark) and martensite (white). At least five representative areas in each sample were studied through metallographic evaluations. The X-ray diffraction technique was employed by using CuKα radiation to detect retained austenite[17] . The fracture surfaces of the impact samples were examined by SEM in order to characterize the impact fracture mode. 3. Results and Discussion 3.1 Morphology and microstructure Fig. 1(a) and (b) shows typical optical micrographs of the mixed lower bainite-martensite and the upper bainite-martensite microstructures, respectively. In these figures, the dark etched regions correspond to bainite and the white regions correspond to martensite. Micro hardness measurements showed that the hardness values of the lower bainite, the upper bainite and martensite are 480, 390 and 740– 750 HV, respectively, further confirming the metallographic observations. Fig. 2(a) and (b) shows typical SEM images of the mixed lower bainite-martensite and the upper bainite-martensite microstructures, respectively. It can be seen from Fig. 2(a) that the lower bainite in mixed microstructures of lower bainitemartensite appears in acicular form and partitions the prior austenite grains, but Fig. 2(b) shows the upper bainite fills the prior austenite grains. The upper and lower bainite have different growth mechanism because they form at different temperatures. The growth of the upper bainite can be described as
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Fig. 1 Optical micrographs of mixed microstructure: (a) lower bainite (BL )-martensite (M), (b) upper bainite (BU )-martensite (M)
Fig. 2 SEM micrographs of mixed microstructure: (a) lower bainite (BL )-martensite (M), (b) upper bainite (BU )martensite (M)
the movement of a low coherent ferrite/austenite interface which necessitates some degree of self-diffusion and leads to rather immobility of the interface[18] . On the contrary, in the case of the lower bainite the propagation of a more coherent interface becomes possible and therefore, little or no self-diffusion required. This results in more rapid movement of the interface[19] . Thus, the reason for the different morphology of bainite has been attributed to the different mechanisms based on which growth and nucleation occur[20] . 3.2 Mechanical properties In order to study the effect of the lower bainitemartensite mixed microstructure on the mechanical properties of the D6AC steel, tensile and impact tests were conducted. Volume fractions (%) of the lower bainite in the mixed microstructures were 0 (a fully martensitic microstructure), 12, 28, 37, 41, 43, 45, 49, 75 and 100 (a fully bainitic microstructure) and those for the upper bainite were 0 (a fully martensitic microstructure), 6, 17, 30, 38, 42, 70 and 100 (a fully bainitic microstructure). Variations of yield and tensile strengths in terms of the lower and upper bainite volume fractions are shown in Figs. 3 and 4, respectively. The average errors for yield and ultimate tensile strengths are ±10 and ±15 MPa, respectively. According to the rule of mixtures (Eq. (1)), yield strength of the mixed bainite-martensite microstructure decreases linearly with increasing the volume fraction of bainite. This is obvious, because the
yield strength of martensite is higher than the yield strength of bainite[12] . σyMix = σyM (1 − VB ) + σyB VB
(1)
where σ Mix is yield strength of the bainite-martensite y B mixed microstructure, σ M y and σ y are yield strength of martensite and bainite microstructure, respectively, VB is volume fraction of lower bainite. In comparidevison, in the present experimental results, σ Mix y ates more from Eq. (1). As shown in Fig. 3(a), by increasing the volume fraction of the lower bainite up to 28 vol.%, the yield strength increases and attains about 1869 MPa and then decreases to the strength level of a fully lower bainitic sample. As seen in Fig. 2(a), the lower bainite partitions the prior austenite grains and therefore, decreases the martensite packet size. According to the HallPetch relationship, Eq. (2), decreasing the martensite packet size increases the strength of the martensite microstructure. −0.5 σyM = σi + kSm
(2)
where σ i is frictional stress, k is constant, and Sm is martensite packet size[12] . In addition, the strength of the lower bainite in the mixed microstructure was increased by the plastic constraint induced by the surrounding stronger martensite. Young and Behadeshia[21] demonstrated that the yield strength is increased by the plastic constraint, upon which a soft thin layer can be constrained by the hard matrix
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surrounding it. For example, a weak brazing alloy can be used effectively to bond much stronger specimens provided that the brazing material is thin enough to be constrained by the surrounding stronger matrix. Indeed the strength of the joint increases as the thickness of the brazing layer decreases. The same phenomenon occurs when the lower bainite plates form in the austenite which subsequently transforms to much stronger martensite. In other words the bainite plate plays the role of a soft layer. On the other hand, the presence of a peak at about 28 vol.% lower bainite in the curve of strength as a function of lower bainite volume fraction can be attributed to the improvement in the strength of martensite due to the refinement of its substructure induced by partitioning of the prior austenite grains by the lower bainite. In addition, the strength of lower bainite was increased by plastic constraint induced by the martensite. On the other hand, Fig. 3(b) shows that the yield strength of the upper bainite is 1304 MPa which is less than the yield strength of the lower bainite (1385 MPa). The higher yield strength of the lower bainite can be attributed to: (1) difficult diffusion of carbon to long distances in lower bainite because of low transformation temperature, causing low bainitic ferrite plate width. Relation of yield strength with plate width is accordance to Hall-Petch equation[22,23] ; (2) the lower bainite has more dislocation density and the probability of dislocation pile-up is higher; (3) in the lower bainite,
carbides are finer with more uniform distribution; so higher stresses are needed for dislocations to pass carbide[24] . In the mixed upper bainite-martensite microstructure, increasing the volume fraction of the upper bainite, the yield strength decreases and reaches the strength level of a fully upper bainitic sample and no peak is observed in the curve of the strength as a function of the upper bainite contents. This result is contrary to the beneficial effects of the upper bainitemartensite microstructure in AISI 4330V reported by Rao et al [13] . In the present work, optical and SEM images in Figs. 1(b) and 2(b) show the upper bainite fills the prior austenite grain without partitioning it. Hence, by comparison the Figs. 1(a) and 2(a) with Figs. 1(b) and 2(b), it can be concluded that the shape and distribution of bainite in the mixed bainitemartensite microstructure probably is the main controlling factor of the tensile properties. On the other hand, the lower bainite appears as acicular form and partitions the prior austenite grains and therefore at 28 vol.% the yield strength of the mixed lower bainitemartensite microstructure increases. But the upper bainite appears as a bulk and fills the prior austenite grain and therefore the yield strength of the mixed upper bainite-martensite microstructure decreases. Also similar result is found between tensile strength of the mixed bainite-martensite microstructure and bainite volume fraction (Fig. 4(a) and (b)). This result is in agreement with those reported by Tomita[12] .
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Fig. 4 Variation of tensile strength with lower (a) and upper (b) bainite volume fraction
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Fig. 5 Variation of elongation with lower (a) and upper (b) bainite volume fraction 30
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Fig. 6 Variation of CVN impact energy with lower (a) and upper (b) bainite volume fraction
The variations of elongation (EL) and CVN impact energy with the bainite volume fractions are shown in Figs. 5 and 6, respectively. As shown, elongation and impact toughness increased by increasing the lower bainite volume fraction. This improvement in ductility and impact toughness is not resulted from the retained austenite. X-ray diffraction patterns have not shown diffraction peaks at (200)γ , (220)γ , (113)γ , (222)γ and (004)γ . Hence, it is speculated that the amount of retained austenite was not sufficient enough to be detected[17] . Thus, this amount of retained austenite has no significant effect on the mechanical properties of various microstructures. On the other hand, it can be seen from Figs. 5 and 6 that the elongation and impact toughness decreased by increasing the upper bainite volume fraction. The superior impact toughness and ductility of the lower bainite compared with the upper bainite can be attributed to the presence of the finer carbide particles in the lower bainite, since probable cracks which formed by cracking of these particles are smaller and crack propegation does not occur easily. Furthermore, fracture strength is inversly related to carbide thickness[8] . Thus, the tensile and impact tests show that the lower bainitic microstructure is more ductile and tougher than the martensitic and upper bainitic microstructures. On
Fig. 7 SEM image of microcrack being initiated during charpy impact testing of specimen having 49 vol.% lower bainite
the other hand, in mixed lower bainite-martensite microstructure, a crack propagating within the martensite will be blunt when reaching the more flexible lower bainite plates and therefore the impact energy increases. This is because the lower bainite brings into full play the arresting effect of the crack and stress-relief, as a result of deformation associated with martensite[11] . 3.3 Fractography The fracture surfaces of the impact specimen are shown in Figs. 7 and 8. From Fig. 7 (SEM image) microcracks are found behind the fracture surfaces
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Fig. 8 SEM images of impact fracture surface showing: (a) quasi-cleavage and dimple rupture in mixed 28 vol.% lower bainite-martensite microstructure, (b) mostly dimple rupture and some quasi-cleavage facets in mixed 49 vol.% lower bainite-martensite, (c) dimple rupture in fully lower bainitic sample, (d) transgranular cleavage in martensitic sample, (e) transgranular cleavage in upper bainitic sample
of impact samples having the mixed lower bainitemartensite microstructure. It can be seen from Fig. 7 that microcracks were stopped by dimple rigions (the lower bainite area). This suggests that the lower bainite is responsible for the increased impact toughness. Fig. 8(a) and (b) shows that the impact fracture morphology in the mixed lower bainite-martensite microstructure samples was a mixed mode of dimple rupture and quasi-cleavage[7] . However, by increasing the lower bainite volume fraction the samples exhibited more dimple rupture. On the other hand, the fracture morphology changes from the mixed mode of dimple rupture and quasi-cleavage in the samples containing the lower bainite and martensite(Fig. 8(a) and (b)) to fully dimple rupture in fully lower bainitic sample (Fig. 8(c)). Therefore, as shown in Fig. 6, by increasing the lower bainite volume fraction, the impact toughness is increaesed. Also, the fully lower bainitic sample has higher absorbed impact energy than the other samples. Typical fracture surface of impact sample containing martensite and upper bainite is shown in Fig. 8(d) and (e), respectively. These figures reveal that a crack in martensitic and upper bainitic
samples propegated in a brittle manner, producing cleavage and quasi- cleavage facets[7] . The results are consistent with the low impact toughness of martensitic and upper bainitic samples. The superior impact toughness and ductility of the lower bainite compared with martensite in Q&T condition samples were also found in the earlier studies[25] . But, it is noteworthy that this is only observed when the lower bainite is formed through an isothermal transformation. The mixed microstructures of bainite-martensite formed by continious cooling and slack quenching have lower toughness than martensitic microstructures[26] . 4. Conclusions (1) Austempering at 330 and 425 ◦ C resulted in the formation of the lower and upper bainites, respectively. (2) The yield strength, ultimate tensile strength, elongation and Charpy V-notch impact energy of the lower bainitic samples are superior to those of the upper bainitic samples. This can be mainly attributed to finer and more uniform distribution of carbides,
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lower bainitic ferrite plate width and higher dislocation density in the lower bainite. (3) A maximum point was observed in the curve of strength as a function of the lower bainite. This can be explained on the basis of two factors. The first is an increase in the strength due to the partitioning of the prior austenite grains by the lower bainite resulting in the refinement of martensite substructures. The second is a plastic constraint effect leading to an enhanced strength of the lower bainite induced by the surrounding relatively rigid martensite. (4) By increasing the lower bainite volume fraction in the mixed lower bainite-martensite microstructures, the elongation, reduction area and Charpy Vnotch impact energy increased because of significantly greater ductility and impact toughness of the lower bainite compared with martensite. (5) By increasing the upper bainite volume fraction in the mixed upper bainite-martensite microstructures, yield strength, ultimate tensile strength, elongation and Charpy V-notch impact energy decreased. These behaviors can be related to the shape and distributions of the upper bainite with a bulky morphology occupying the prior austenite. (6) The impact fracture surfaces of the martensitic and the upper bainitic samples have shown cleavage and qusi-cleavage morphologies, indicating tendency to the brittle behavior. The impact fracture surfaces of the lower bainitic samples exhibited a dimple rupture mode. However, the fracture morphology in the mixed lower bainite-martensite microstructure samples was a mixed mode of dimple rupture and qusicleavage. REFERENCES [1 ] S. Maropoulos, N. Ridley and S. Karagiannis: Mater. Sci. Eng. A, 2004, 380, 79. [2 ] A.R. Mirak and M. Nili-Ahmadabadi: Mater. Sci. Technol., 2004, 20, 897. [3 ] A. Salemi, A. Abdollah-zadeh, M. Mirzaei and H. Assadi: Mater. Sci. Eng. A, 2008, 492, 45. [4 ] A. Abdollah-zadeh, A. Salemi and H. Assadi: Mater. Sci. Eng. A, 2008, 483–484, 325. [5 ] J. Wen, Q. Li and Y. Long: Mater. Sci. Eng. A, 2006,
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