- Email: [email protected]
Effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel
Author’s Accepted Manuscript Effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel J. Kang, F.C. Zhang, X.W. Yang, B. Lv, K.M. Wu www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)30053-9 http://dx.doi.org/10.1016/j.msea.2017.01.044 MSA34606
To appear in: Materials Science & Engineering A Received date: 5 August 2016 Revised date: 14 January 2017 Accepted date: 17 January 2017 Cite this article as: J. Kang, F.C. Zhang, X.W. Yang, B. Lv and K.M. Wu, Effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.01.044 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.
Effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel J. Kang1, F.C. Zhang1,2,, X.W. Yang1, B. Lv3, K.M. Wu4,5 1
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
2
National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao 066004, China
3
College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China 4
International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan 430081, China
5
Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK
Abstract: The effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel has been investigated through optical microscopy, electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction analyses. A nano-level microstructure containing plate-like bainitic ferrite and film-like retained austenite is obtained by isothermal transformation at Ms+10 oC followed by tempering within 240 oC–450 oC. Results show that the sample tempered at 340 oC occupies the optimal balance of strength and toughness by maintaining a certain level of plasticity; samples tempered at 320 oC and 360 oC with low and high yield ratio come second. The microstructure of the steel is not sensitive to tempering temperatures before 360 oC. When the temperature is increased to 450 oC, the significantly coarsened bainitic ferrite plate and the occurrence of a small quantity of carbide precipitation account for its low toughness. The amount of retained austenite increases with the tempering temperature before 400 o
C, followed by decreasing with further increase in the temperature. This behavior is related to the
competition between retained austenite further transforming into bainite and decomposing into carbide during tempering. Keywords: Carbide-free bainite; Tempering; Microstructure; Mechanical properties
Corresponding author: E-mail: [email protected], Tel: 0086 335 8063949, Fax: 0086 335 8074568 (F.C. Zhang) 1
1 Introduction Carbide-free bainitic steel with low cost and excellent performance has been widely used in many industries, such as railway frogs, bearings, and automobile sheets [1–4]. The bainitic ferrite plates with nano- or sub-micron size and high density of dislocation in the microstructure provide high strength [5, 6]. The soft retained austenite embedded in the hard matrix of the bainitic ferrite contributes to suitable plasticity and toughness [7, 8]. However, structural steel must be welded under certain conditions. There are problems during welding such as coalescence and coarsening of the bainitic ferrite plates and cementite precipitation in carbide-free bainitic steels, which will ruin the mechanical properties [9–11]. Therefore, the effect of tempering on the microstructure and properties of carbide-free bainitic steel must be investigated. The carbide-free bainite transformation is mainly dominated by displacive mechanism [12–14]. The plastic relaxation accompanied with the shape deformation during bainite transformation leads to local residual stress [15]. It can be released by tempering. As a metastable phase, the amount of the retained austenite decreases with increasing tempering temperature caused by transformation or decomposition [16, 17]. The thin film-like retained austenite possesses high mechanical stability because of the size effect and high carbon content within it [18–20]. However, the high carbon content in the retained austenite with nano-size increased the driving force for carbide precipitation and thus led to poor thermal stability, which was confirmed by the high-energy X-ray diffraction testing [19, 21, 22]. Podder studied the tempering behavior of low carbon carbide-free bainitic steel at 450°C and found that minute quantities of cementite precipitated at the early stage of tempering. This process rendered the austenite sufficiently unstable in certain local regions with carbon poor to martensitic transformation on cooling and further increased the quantities of carbides with prolonged tempering [23]. In nano-bainitic steel, the retained austenite with carbon-rich was decomposed to intense carbide precipitation at lath boundaries during tempering and prevented the coarsening of the bainitic ferrite [24]. Hasan et al. quantitatively analyzed the effect of solid solution, grain refinement, dislocation, and precipitation on the strength of nano-bainitic steel after tempering. The decrease in strength was mainly due to slight coarsening and low dislocation density of the bainitic plate [25]. This study aims to investigate the effect of tempering on the microstructure and properties of a medium carbon carbide-free bainitic steel. 2
2 Experimental material and procedures The chemical composition of the studied steel is listed in Table 1. Si can inhibit the precipitation of cementite as the obstacle of cementite growth [26] and enhance the tempering stability of the nano-bainite [27]. Addition of Al can accelerate bainitic transformation [28, 29] besides hindering cementite precipitation [30]. The studied steel was smelted in a vacuum furnace and forged into a round bar (Φ65 mm). Samples with a size of 25 mm × 25 mm × 115 mm were cut along the axial direction of the round bar and used for subsequent heat treatments. Austenization and salt bath isothermal treatments were performed, and the carbide-free bainitic microstructure was consequently obtained. Table 1 Ac1 and Ac3 temperatures of the studied steel were determined as 753 oC and 795 oC by the phase change analyzer. The Ms temperature was tested to be 260 oC by Gleeble 3500-type thermal simulation test machine. The samples were austenitized at 930 oC, followed by isothermal transformation at 270 oC for 2 h and then air cooling to the room temperature to obtain carbide-free bainite microstructure. Finally, after the samples were completely cooled down to the room temperature for 6 h, the tempering treatments were performed by varying the tempering temperature within 240 oC–450 oC. In this paper, the samples tempered at different temperatures were represented to 240 oC-sample, 280oC-sample, 320 oC-sample, 340 oC-sample, 360 oC-sample, 400 o
C-sample and 450 oC-sample. Fig. 1 shows the expansion volume constantly changes during
isothermal transformation at 270 °C in the studied steel. As we can see, isothermal transformation for 2 h is sufficient for bainite transformation in the studied medium carbon steel. Fig. 1 Tensile testing was conducted on a MTS universal hydraulic testing machine. The sample size used had 25 mm gauge and 5 mm diameter. The strain rate was 4 × 10−4 s−1. Charpy U-notch specimens with dimensions of 10 mm × 10 mm × 55 mm were used at room temperature. We use the U-notch sample to test the toughness due to the requirement of the engineering subject related to developing the steels used for rail and frog. The low temperature toughness at -40 oC also needs to be tested and the toughness is sometimes very low on the engineering background, so the U-notch sample is chosen for comparing the toughness at room temperature in this paper. The impact testing 3
was according to the China standard GB/T229-2007 (Charpy pendulum impact test of the metal materials). Three samples were tested in each state to obtain the average value. Optical microscopy (OM) was used to observe the bainitic microstructure at low magnification. Transmission electron microscopy (TEM) was utilized to observe and analyze the microstructure and the sub-structure at high magnification as well as precipitation. TEM specimens were fabricated on a TenuPol-5 type twin-jet electropolishing instrument at room temperature. The electrolytic polishing solution was composed of 7% perchloric acid and 93% ethanol alcohol solutions. The morphology and orientation of the bainitic sheaves were characterized by electron-backscattered diffraction microscopy (EBSD). The morphology of the impact fracture surface was examined by scanning electron microscopy (SEM). Phase components and relative volumes were tested using a Rigaku D/max-2500/PC X-ray diffractometer with a radiation target of Cu–Kα in step scanning, with 0.2°/step and each step lasting for 2 s. The samples were mechanical polished delicately with1 μm diamond paste under a rotating rate of 80 r/min and then etched. The process of polishing and etching was repeated several times. Finally, the samples were polished in colloidal silica to obtain underformed surface. The volume fraction of the retained austenite (Vγ) is determined according to Eq. (1) [31]. Carbon concentration in the retained austenite (C) is determined according to Eq. (2) [32] by using the lattice parameters of the retained austenite.
(1/ n) j 1 Ij Rj n
V
(1/ n) j 1 ( Ij Rj ) (1/ n) j 1 ( Ij Rj ) n
n
(1)
a( A) 3.578 0.033C
(2)
Where n is the number of peaks examined, and I is the integrated intensity of the diffraction peak. R is the material scattering factor, that is, R (1/ v 2 )[ F P((1 cos 2 2 ) / sin sin 2 )] e2 M . v is 2
volume of the unit cell, F is the structure factor, P is the multiplicity factor, e−2M is the temperature
factor, is the diffraction angle, and a ( A) is the austenite lattice parameter.
3 Experimental results and analysis 3.1 Microstructure Fig. 2 shows the OM micrographs of the specimens after isothermal transformation at 270 oC followed by tempering at 240 oC, 320 oC and 450 oC, respectively. The bainitic sheaves take on a 4
needle-like appearance. At low magnification, the morphology of the bainitic sheaves differs minimally. Fig. 2 Typical TEM micrographs of the 240 oC-sample, 320 oC-sample, 400 oC-sample and 450 o
C-sample are shown in Figs. 3 and 4. As can be seen, the microstructures of the 240 oC-sample,
320 oC-sample and 400 oC-sample consist of thin film-like retained austenite embedded within bainitic ferrite plate. At 320 oC, there also exists the presence of the blocky retained austenite according to the diffraction pattern (Fig. 3c). When the temperature is increased to 400 oC, no carbide precipitation appears to occur (Fig. 4a). However, fine carbides with size of 25 ± 5 nm are observed in the matrix of 450 oC-sample. These carbides can not only be formed by the decomposition of the thin film shaped retained austenite between the lath interfaces (Fig. 4b), but also precipitate within the bainitic ferrite plates (Fig. 4c). Liu et al. thought that the orientation relationship between -carbide and the matrix within the bainitic ferrite was similar to that in the tempered martensite [33]. Caballero investigated solute atom redistribution during tempering in nano-bainitic steel by atom probe technology. The results showed that the retained austenite at the plate interface would decompose before achieving the total equilibrium, and cementite was formed within the bainitic ferrite with over-saturated carbon or along the ferrite-retained austenite interface through the para-equilibrium mechanism [34]. Si was repelled quickly into the parent phase from the new product during tempering [34], thereby inhibiting the transition from -carbide to cementite and delaying the nucleation and growth of the latter [26]. In addition, the dislocation density in the sample tempered at 450 °C decreases evidently, as shown in Figs. 4b and c. Fig. 3 Fig. 4 The average linear intercept ( LT ) is measured along a direction normal to the length direction of the slender bainitic ferrite plate. The relationship between LT and the actual thickness of bainitic ferrite plate (tBF) can be represented as follows [35, 36]:
LT t / 2 Confidence error is 95%, E 2 L / N
where T represents the standard deviation, and N is the measured quantity. Fig. 5 shows the change in bainitic ferrite plate thickness with tempering temperature. The size 5
of the plate differs little within 240 °C–320 °C. The bainitic ferrite plate of 360 °C-sample becomes slightly coarsening, but is still below 100 nm. When the tempering temperature is further increased to 450 oC, the bainitic ferrite plate is evidently coarsened, and the thickness increases to 120 ± 6 nm. This is because the amount of carbide precipitated at the bainitic lath boundary is extremely small, which cannot prevent the coarsening of the lath effectively. Therefore, the bainite microstructure in this case is not sensitive to tempering at low temperatures (≤ 360 °C) but appears to coarsen at high temperatures (> 360 °C). Fig. 5 Fig. 6a and b shows the change in the volume fraction of the retained austenite (V) and the carbon within it (C) with tempering temperature. Vof the samples with tempering decreases, in comparison with the sample without tempering. At temperatures before 400oC, the Vincreases linearly with tempering temperature and reduces slightly after 400 oC. This finding is inconsistent with the results of tempering reported in the literature, that the amount of retained austenite decreases with increasing tempering temperature [16, 17, 37]. The carbon in the retained austenite reaches the peak at 320 oC, and then decreases slightly with further increase in tempering temperature. Fig. 6 Bainite in general tempers much more gently because it autotempers during the course of transformation. This is because much of the carbon precipitates as carbides or partitions from the ferrite to the remaining austenite during bainite formation [38]. The transformation mechanism of carbide-free bainite is mainly dominated by diaplacive mechanism. The transformation will stop when the carbon content in the retained austenite reaches the T0′ curve [39, 40]. Based on the results of TEM and XRD analyses, competition exists between retained austenite further transforming into bainite and decomposing into carbide during tempering, in addition to the dislocations rearrangement in the microstructure. Bainite transformation during tempering is related to the driving force ( Gmax ) of the bainitic transformation from retained austenite under different tempering temperature. This parameter can be calculated based on the average carbon content in the retained austenite after isothermal transformation at 270 oC by using the the Public Domain Research Software and Data Library of the Materials Algorithms Project (MAP) [41], as is shown in Fig. 7. The driving force exhibits a decreasing trend with increasing temperature. There is a 6
thermodynamic condition to be met in order for carbide-free bainite formation to take place at a transformation temperature T for a given alloy carbon concentration C. CT
after 320 oC is mainly caused by the increased V. At the tempering temperature of 450 oC,
the retained austenite, especially the thin-film shaped austenite, preferentially decomposes into
7
carbide because of the significantly increased diffusion coefficient of carbon and silicon, which leads to the reduction in V and C in the 450 oC-sample. Fig. 7 Fig. 8 Moreover, the bainitic ferrite is carbon over-saturated after isothermal transformation. The carbon will further partition to the parent austenite from the ferrite during the further tempering process, eventually leading to slow broadening of the bainitic ferrite plates [47]. The carbon diffusion coefficient under different tempering temperature can be calculated according to the equation- DC 0.12 e 32,000/ RT [44]. The variation of bainite ferrite plate thickness with carbon diffusion coefficient under different tempering temperature is shown in Fig. 9. As we can see, when tempering temperature is lower than 320 oC, carbon diffusion coefficient is very low and the thickness of bainitic ferrite plate is unchanged. As the carbon diffusion coefficient increases (tempering temperature >320 oC), the thickness of bainitic ferrite plate exhibits obvious increasing trend. This finding indicates that the coarsening degree of ferrite is related to the diffusion activity of carbon. Hence, the bainitic ferrite coarsening of 450 oC-sample is also associated with high diffusion coefficient of carbon at high tempering temperature. Fig. 9 Fig. 10 shows EBSD microstructure characterization of the 240 oC-sample, 320 oC-sample and 450 oC-sample. Large grain boundary misorientation angle (>15o) has a larger proportion among the bainitic sheaves. This finding is in good agreement with the feature of grain boundary misorientation angle distribution in the lower bainite [48–50]. Lower bainite transformation is accompanied by large plastic relaxation due to its low transformation temperature. The orientation of bainitic sheaves is mainly the N-W/N-W relationship without twinning relationship and the probability of large angle grain boundary with random distribution is relatively higher [48, 49]. In this case, the misorientation angle distribution in the 240 oC-sample (Fig. 10b) is similar with that in the 320 oC-sample (Fig. 10d). They both exhibit the peaks of number fraction at 40–50° and 55° misorientation angles whereas the peak of number fraction at 40–50° in the 320oC-sample exhibits a slight reduction. However, when the sample is tempered at 450 oC (Fig. 10f), the number fraction of 55° misorientation angle keeps unchanged. The proportion of 40-50° misorientation angles reduces evidently, whereas the proportion of 30–40° misorientation angles increases. It implies that the 8
bainitic sheaves with 40–50° misorientation angles in the studied steel are unstable and the number fraction decreases slightly at high tempering temperature (450 oC). This change in high angle grain boundary distribution may be associated with the coalescence of the bainitic ferrite plate caused by the decomposition of the retained austenite in the 450 oC-sample. Fig. 10 3.2 Property analysis Fig. 11 shows the engineering stress against engineering strain curves of the studied steel with and without tempering. Fig. 12 shows the change in hardness with tempering temperature. The variation range of the hardness is relatively narrow, changing within the range of ～2 HRC. Hardness remains at a higher level in the samples tempered within the range of 240 oC–360 oC and then decreases to the lowest in the 400 oC-sample. Finally, it exhibits a minor increase in the 450 o
C-sample. Fig. 11 Fig. 12 Fig. 13 shows the evolution of ultimate tensile strength (b), yield strength (s) and yield ratio
(sb) with tempering temperature. As the tempering temperature increasing, b tends to decrease firstly, then followed by increase, which is similar with the variation of hardness with tempering temperature, as shown in Fig. 11. By contrast, the change trend of s is virtually opposite to that of
b. The s shows a valley value at 320 oC and reaches the peak value at 400oC. The evolution of yield ratio with the tempering temperature is similar to that of s. Fig. 13 Fig. 14 shows the change in total elongation (t) and uniform elongation (u) with tempering temperature. t shows a parabolic curve and reaches the peak value at 400 °C followed by a sharp reduction at 450 °C. u continuously increases with increasing tempering temperature. Fig. 14 Tensile properties correspond to the microstructure features. High strength of 240oC-sample originates from its finer bainitic ferrite plate, while its poor plasticity mainly results from a minute amount of the retained austenite. 320 oC-sample possesses high ultimate tensile strength and medium plasticity, but relatively low yield strength. The high carbon content in the retained 9
austenite means a high mechanical stability. It is very difficult to undergo the stress induced martensitic transformation under small strain condition and thus cannot improve yield strength. However, the medium amount of retained austenite ensures medium plasticity in the 320 oC-sample. When tempered at 360 oC, the sample shows a reduced ultimate tensile strength and increased yield strength; and the elongation reaches a peak value. The retained austenite with low carbon content of the 360 oC-sample indicates low mechanical stability. Tensile testing with 3% strain was carried out in the 360 oC-sample, and the TEM micrograph in the gauge distance is shown in Fig. 15. Martensite can be observed in the 360 oC-sample after small straining. The enhanced yield strength in the 360
o
C-sample is probably attributed to stress induced martensitic transformation,
compensating for the negative effect of the dislocation recovery on yield strength. Meanwhile, the high amount of retained austenite and the decreased density of dislocations increase the work hardening capacity, which is beneficial for plasticity. Fig. 15 Although carbide is not observed in the 400 oC-sample through TEM analysis, we cannot disregard the existing possibility of carbide precipitates because of significant increase in the diffusion coefficient of carbon at 400 oC [41]. This may probably result in the enhanced yield strength of the 400 oC-sample. Several researchers showed that at tempering temperature of 400 oC, the thermal stability of the retained austenite declined which promoted the austenite decomposition into carbide or cementite [27, 51]. The fine dispersed carbide can interact with dislocation, refining the microstructure and improving the yield strength of the material [50]. High uniform elongation and total elongation in the 400 oC sample can be provided by higher volume fraction of the retained austenite with lower carbon content within it. More mechanically unstable austenite can transform into martensite continuously during tensile deformation, which can delay plastic instability and improve elongation [52-54]. When the tempering temperature is further increased to 450 oC, the marked coarsening of bainitic ferrite plate accounts for the lower yield strength, whereas carbide precipitation within the bainitic ferrite or along the ferrite–austenite interface ensures high tensile strength and high hardness, but deteriorates elongation. Fig. 16 shows the changing in impact toughness with tempering temperature. The variation of impact toughness with tempering temperature exhibits a wave curve. Samples tempered at medium temperatures (320 oC–360 oC) possess relatively higher values while the samples tempered at low 10
temperatures (240 oC and 280 oC) and high temperatures (400 oC and 450 oC) have low impact toughness. Fig. 16 High carbon content in the retained austenite of samples tempered at 320 oC-360 oC is associated with high mechanical stability of the retained austenite. The relatively stable austenite can enhance crack propagation resistance and decrease the crack propagation rate during the stage II which is beneficial for toughness [55]. The toughness damage of the 240 oC-sample is caused by the relatively more untempered bainitic ferrite with high carbon and extremely low amount of the soft retained austenite. The low toughness of 450 oC-sample is mainly related to the coarsened bainitic ferrite plate (>100 nm) and carbide precipitation. Crack propagation resistance is associated with the size of bainitic ferrite plate. Fine plates of bainitic ferrite provide more interfaces and thus improve toughness [56]. In contrast, the blocky bainitic ferrite plates promote crack initiation and fracture due to high stress concentration [57]. As the obstacle for the sliding, carbide provides the potential fracture path. The crack source can form at the carbide–matrix interface and deteriorate toughness [37]. In addition, the decreased fraction of the high-angle grain boundary with 40°–50o also performs a negative function in the toughness of the 450 oC-sample. Fig. 17 shows the fracture surfaces of the 240 oC-sample, 320 oC-sample and 450 oC-sample after impact testing. The white round points are the splashed oxide during cutting the samples. The fracture appearance in the 240 oC-sample and 320 oC-sample is the quasi-cleavage fracture consisting of small flat facets and intense dimples distributed on the tearing ridge around the facets. The fracture surface of 320 oC-sample is characterized by more quantity of dimples and smaller facets, compared to that of the 240 oC-sample. However, there is almost no dimple in the 450 o
C-sample and the mode is brittle cleavage fracture consisting of flat facets. The fracture feature
corresponds to the lowest impact toughness in the 450 oC-sample. Fig. 17 Yield strength and yield ratio increase, whereas the change in hardness, ultimate strength, elongation, and toughness is non-monotonic in the samples after tempering, in comparison with those of the sample before tempering. 340 °C-sample occupies the optimal balance of strength and toughness on the basis of maintaining certain plasticity; the 320 °C-sample and 360 °C-sample come second. The two samples show different yield ratio. Yield ratio represents the capacity of 11
continuous work hardening capacity of the material. During large plastic deformation under working conditions, a material with low yield ratio is preferred. When the lighter weight is required under the working condition, the material with high yield ratio is the priority. Therefore, the material with different yield ratios obtained by varying the tempering process can increase the usefulness and the range of the materials in applications.
4 Conclusions Nano-bainitic microstructure containing bainitic ferrite plate and retained austenite film has been obtained by isothermal transformation at Ms+10 oC followed by tempering within the range of 240 oC-450 oC in a medium carbon carbide-free bainitic steel. The effect of tempering temperature on microstructure and properties has been investigated. Conclusions can be drawn as follows. 1. Among the tempering temperatures, 340 oC-sample occupies the optimal balance of strength and toughness on the basis of maintaining a certain plasticity. 320 oC-sample and 360 oC-sample come second. 2. When the studied steel is tempered at low and medium temperature levels (<360 oC), the microstructure is not sensitive to tempering. When the temperature is increased to 450 oC, the bainitic ferrite plate shows significant corarsening trend and the number fraction of bainitic sheave with 40-50° misorientation angle decreases slightly. Fine carbide can also be observed in the 450 oC-sample. These changes in microstructure are responsible for the low toughness in the 450 oC-sample. 3. Driving force of bainitic transformation generally decreases with the increase in tempering temperature according to the carbon content in the retained austenite of the sample without tempering. Before 400
o
C, the amount of retained austenite increases with tempering
temperature. The baintic transformation from the retained austenite dominates the tempering process due to high driving force and low diffusion rate of carbon and silicon. At tempering temperature of 450oC, the thin film-like retained austenite with higher carbon content preferentially decomposes to carbide because of the significantly increased diffusion coefficient of carbon, eventually leading to the decreased volume fraction of retained austenite and lower carbon content.
Acknowledgement 12
This work was supported by the Natural Science Foundation of China (No. 51471146), the Nat ional High Technology Research and Development Program of China (863 Program) (No. 2012AA 03A504), the Natural Science Foundation of Hebei Province of China (No. E2013203258).
References [1] F.C. Zhang, Z.N. Yang, J. Kang, Progress in bainitic steel used for railway crossing. J. Yanshan University, 37 (2013) 1–7. [2] K. Sswley, J. Sun, Advanced rail steels: investigating the bainitic option, Railway Track & Struct., 3 (1997) 22–27. [3] C. Garci-Mateo, F.G. Cabllero, H.K.D.H. Bhadeshia. Development of Hard Bainite. ISIJ Int. 43 (2003) 1238–1243. [4] F.C. Zhang, T.S. Wang, P. Zhang, C.L. Zheng, B. Lv, M. Zhang, Y.Z. Zheng. A novel method for development of nanobainitic microstructure in surface layer of low carbon steel. Scripta Mater. 59 (2008) 294–296. [5] J. Cornide, C. Garcia-Mateo, C. Capdevila, F.G. Caballero, An assessment of the contributing factors to the nanoscale structural refinement of advanced bainitic steels, J. Alloys Comp. 577S (2013) S43–S47. [6] F.G. Caballero, H.K.D.H. Bhadeshia, Very strong bainite, Current Opinion in Solid State and Materials Science 8 (2004) 251–257. [7] X.Y. Long, F.C. Zhang, J. Kang, B.Lv, X.B. Shi, Low-temperature bainite in low-carbon steel, Mater. Sci. Eng. A 594 (2014) 344–351. [8] L.H. Qian, Q. Zhou, F.C. Zhang, J.Y. Meng, M. Zhang, Y. Tian, Microstructure and mechanical properties of a low carbon carbide-free bainitic steel co-alloyed with Al and Si, Mater. Des. 39 (2012) 264–268. [9] K. Fang, J.G. Yang, D.L. Zhao, K.J. Song, Z.J. Yan and H.Y. Fang, Review of Nanobainite Steel Welding, Adv. Mater. Res. 482–484 (2012) 2405–2408. [10] J. H. Pak, H. K. D. H. Bhadeshia, L. Karlsson, E. Keehan, Coalesced bainite by isothermal transformation of reheated weld metal, Sci. Technol. Weld. Join. 13 (2008) 593–597. [11] H. K. D. H. Bhadeshia, Nanostructured bainite, Proc. R. Soc. A 466 (2113) 3–18. [12] F.G. Caballero, C. Garcia-Mateo, M.J. Santofimia, M.K. Miller, C. Garcı´a de Andre´s, New experimental evidence on the incomplete transformation phenomenon in steel, Acta Mater. 57 (2009) 8–17. [13] H.K.D.H. Bhadeshia, The bainite transformation: unresolved issues, Mater. Sci. Eng. A 273–275 (1999) 58–66. [14] H.K.D.H. Bhadeshia, D.V. Edmonds. The mechanism of bainite formation in steels, Acta Metall 28 (1980) 1265–1273. [15] H.K.D.H. Bhadeshia, Developments in martensitic and bainitic steels: role of the shape deformation, Mater. Sci. Eng. A 378 (2004) 34–39. [16] G.R. Speich, W.C. Leslie, Tempering of steel, Metall. Mater. Trans. B 4 (1973) 303–315. [17] A. Joarder, J.N. Ojha, D.S. Sarma, The tempering behavior of a plain carbon dual-phase steel, Mater. Charact. 25 (19) (1990) 9–209. [18] X.D. Wang, B.X. Huang, Y.H. Rong, L.Wang, Microstructures and stability of retained austenite in TRIP steels, Mater. Sci. Eng. A 438–440 (2006) 300–305. [19] C. Garcia-Mateo, F. G. Caballero, M. K. Miller, J. A. Jimenez, On measurement of carbon content in retained austenite in a nanostructured bainitic steel, J. Mater. Sci. 47 (2012) 1004–1010. [20] X.Y. Long, J. Kang,B.Lv , F.C. Zhang, Carbide-free bainite in medium carbon steel, Mater. Des. 64 (2014) 13
237–245. [21] A.S. Podder, I. Lonardelli, A. Molinari, H.K.D.H. Bhadeshia, Thermal stability of retained austenite in bainitic steel: an in situ study. Proc. R. Soc. A Phys. Eng. Sci. 467 (2011) 3141–56. [22] B. Avishan, C. Garcia-Mateo, S. Yazdani ,F.G. Caballero, Retained austenite thermal stability in a nanostructured bainitic steel, Mater. Charact. 81 (2013) 105–110. [23] P.A. Saha, H.K.D.H. Bhadeshia, Thermal stability of austenite retained in bainitic steels, Mater. Sci. Eng. A 527 (2010) 2121–8. [24] M. Peet, Transformation and tempering of low-temperature bainite (Ph.D. thesis), University of Cambridge, Cambridge, UK, 2010. [25] H.S. Hasan, M.J. Peet, M-N. Avettand-Fènoël, H.K.D.H. Bhadeshia, Effect of tempering upon the tensile properties of a nanostructured bainitic steel, Mater. Sci. Eng. A 615 (2014) 340–347. [26] B. Kim, C. Celada, D. San Martı´n, T. Sourmail, P.E.J. Rivera-Dı´az-del-Castillo, The effect of silicon on the nanoprecipitation of cementite, Acta Mater. 61 (2013) 6983–6992. [27] J.A.C. Junior, D.B. Santos, Effect of tempering temperature on isothermal decomposition product formed below Ms, J. Mater. Res. Technol. 2 (2013) 93–99. [28] C. Garcia-Mateo, F.G. Caballero, H.K.D.H. Bhadeshia, Acceleration of Low-temperature Bainite, ISIJ Inter., 43 (2003) 1821–1825. [29] E.V. Pereloma, I.B. Timokhina, M.K. Miller, P.D. Hodgson, Three-dimensional atom probe analysis of solute distribution in thermomechanically processed TRIP steels, Acta Materialia 55 (2007) 2587–2598. [30] B.C. De Cooman, Structure-properties relationship in TRIP steels containing carbide-free bainite, Curr. Opin. Solid Stat. Mater. Sci. 8 (2004) 285–303. [31] B. D. Cullity, S. R. Stock, Elements of X-ray diffraction. 3rd ed. New Jersey: Prentice Hall; 2001. p351. [32] D. J. Dyson, B. Holmes, Effect of alloying additions on the lattice parameter of austenite, J. Iron Steel Inst. 1970 (208) 469-474. [33] J. Liu, C.P. Luo, Precipitation behavior of the lower bainitic carbide in a medium-carbon steel containing Si, Mn and Mo, Materials Science and Engineering A 438–440 (2006) 153–157. [34] F.G. Caballeroa, Hung-Wei Yen, M.K. Miller, J. Cornide, Hsiao-Tzu Chang, C. Garcia-Mateo, Jer-Ren Yang, Three phase crystallography and solute distribution analysis during residual austenite decomposition in tempered nanocrystalline bainitic steels, Mater. Charact. 88 (2014) 15–20. [35] L.C. Chang, H.K.D.H. Bhadeshia, Austenite films in bainitic microstructures, Mater. Sci. Technol. (1995) 874–881. [36] H.K.D.H. Bhadeshia, Introduction to Quantitative Metallography, Part II Lecture Notes, 2013. [37] Y. Luo, J.M. Peng, H.B. Wang, X.C. Wu, Effect of tempering on microstructure and mechanical properties of a non-quenched bainitic steel, Mater. Sci. Eng. A 527 (2010) 3433–3437. [38] H. K. D. H. Bhadeshia, Bainite in steels, 2nd edn; 2001, London, The Institute of Materials. [39] H.K.D.H. Bhadeshia, D.V. Edmonds, The bainite transformation in a silicon steel, Metall. Trans. 10A (1979) 895–905. [40] H.K.D.H. Bhadeshia, Waugh AR. Bainite: an atom-probe study of the incomplete reaction phenomenon, Acta Metall. 30 (1982) 775–784. [41] the Public Domain Research Software and Data Library of the Materials Algorithms Project (MAP), Department of Materials Science and Metallurgy, University of Cambridge, http://www.msm.cam.ac.uk:map:map-main.html. [42] H. Huang, M.Y. Sherif, P.E.J. Rivera-Dıaz-del-Castillo, Combinatorial optimization of carbide-free bainitic nanostructures, Acta Materialia 61 (2013) 1639–1647. [43] Honeycombe R, Bhadeshia HKDH. Steels: microstructure and properties. 2nd ed. Edward Arnold; 1995. 14
[44] C. Wells, R.F. Mehl, Transactions 140 (1940) 279–306. [45] H.-S. Park, J.-B. Seol, N.-S. Limc, S.-Il Kimc, C.-G. Park, Study of the decomposition behavior of retained austenite and the partitioning of alloying elements during tempering in CMnSiAl TRIP steels, Mater. Des. 82 (2015) 173–180. [46] B. Kim, C. Celada, D. San Martın, T. Sourmail, P.E.J. Rivera-Dıaz-del-Castillo, The effect of silicon on the nanoprecipitation of cementite, Acta Materialia 61 (2013) 6983–6992. [47] F.G. Caballero, M.K. Miller, C. Garcia-Mateo, Carbon supersaturation of ferrite in a nanocrystalline bainitic steel, Acta Mater. 58 (2010) 2338–2343. [48] S. Zajac, V. Schwinn, K.H. Tacke, Characterization and quantification of complex bainitic microstructures in high and ultra-high strength linepipe steels, Mater. Sci. Forum 500–501 (2005) 387–94. [49] X.Y. Long , J. Kang, B. Lv, F.C. Zhang. Carbide-free bainite in medium carbon steel, Mater. Des. 64 (2014) 237–245. [50] S.-G. Park, K.-H. Lee, M.-C. Kimn, B.-S. Lee, Effects of boundary characteristics on resistance to temper embrittlement and segregation behavior of Ni–Cr–Mo low alloy steel, Mater. Sci. Eng. A 561 (2013) 277–284. [51] Z.M. Shi, W. Gong, Y.Tomota, S. Harjo, J. Li, B. Chi, J. Pu, Study of tempering behavior of lath martensite using in situ-neutron diffraction, Mater. Charact. 107 (2015) 29–32. [52] M. Sherif, C. Garcia-Mateo, T. Sourmail, H.K.D.H. Bhadeshia, Stability of retained austenite in TRIP-assited steels, Mater. Sci. Technol. 20 (2004) 319–322. [53] M.Y. Sherif, Characterisation and development of nanostructured, ultrahigh strength, and ductile bainitic steels (Ph.D.thesis), University of Cambridge, 2005. [54] H.K.D.H. Bhadeshia, Nanostructured bainite, Proc. R. Soc. Lond. A 466 (2010) 3–18. [55] T.V. Venkatasubramanian, T.J. Baker, Enhanced fracture resistance of high strength steel produced by extrusion of nickel coated steel powder, Metall. Mater. Trans. A 11 (1980) 200–203. [56] M.W. Tong, P.K.C. Venkatsurya, W.H. Zhou, R.D.K. Misra, B. Guo,K.G. Zhang,W.Fan, Structure–mechanical property relationship in a high strength microalloyed steel with low yield ratio: The effect of tempering temperature, Mater. Sci. Eng. A 609 (2014) 209–216. [57] Y. You, C.J. Shang, L. Chen, S. Subramanian, Investigation on the crystallography of the transformation products of reverted austenite in intercritically reheated coarse grained heat affected zone, Mater. Des. 43 (2013) 485–491.
15
Figure captions Fig. 1 Expansion volume constantly changes during isothermal transformation at 270 °C in the studied steel. Fig. 2 OM microstructures of the 240 oC-sample (a), 320 oC-sample (b) and 450 oC-sample (c). Fig. 3 TEM observations of the 240 oC-sample (a) and 320 oC-sample (b, c). Fig. 4 TEM observations of the 400 oC-sample (a) and 450 oC-sample (b, c). Fig. 5 Thickness of the bainitic ferrite plate changing with the tempering temperature. tBF0 is the thickness of the bainitic ferrite plate in the untempered sample. Fig.6 Volume fraction of retained austenite (V) (a) and the carbon content within it (C) (b) changing with tempering temperature. V0 and C0 represent the volume fraction of retained austenite and the carbon within it in the untempered sample. Fig. 7 Driving force ( Gmax ) of the bainitic transformation from retained austenite in the sample obtained by isothermal transformation at 270oC changing with the tempering temperatures. Fig. 8 Para-equilibrium phase diagram and the comparative results obtained from XRD. Fig. 9 Thickness of bainitic ferrite plate varying with carbon diffusion coefficient under different tempering temperature. Fig. 10 EBSD microstructure characterization of the 240 oC-sample (a, b), 320 oC-sample (c, d) and 450oC-sample (e, f): Orientation image (left) and misorientation angle distribution (right). Fig. 11 Engineering stress against engineering strain curves of the studied steel. Fig. 12 Hardness changing with tempering temperature. H0 is the average hardness of the untempered sample. Fig. 13 Evolution of ultimate tensile strength (b), yield strength (s) and yield ratio (sb) with tempering temperature. b0, s0 and s0b0 represent the ultimate tensile strength, yield strength and yield ratio, respectively. Fig. 14 Evolution of the total elongation (t) and uniform elongation (u) with tempering temperature. t0 and u0 represent the total elongation and uniform elongation, respectively. Fig. 15 Martensite observed in the 360 oC-sample after 3% tensile deformation by TEM. Fig. 16 Impact toughness varying with the tempering temperature. ku0 represents the impact toughness. Fig. 17 Fracture morphologies of the 240 oC-sample (a), 320 oC-sample (b) and 450 oC-sample (c) 16
after impact testing. Table 1 Chemical composition of the studied steel, wt% Element
C
Si
Mn
Cr
Ni
Mo
Al
P
S
Composition
0.46
1.55
1.59
1.24
0.81
0.40
0.62
0.003
0.002
17
Figure(s)
Fig. 1 Expansion volume constantly changes during isothermal transformation at 270 °C in the studied steel.
Figure 2
Fig. 2 OM microstructures of the 240oC-sample (a), 320oC-sample (b) and 450oC-sample (c).
Figure 3
Fig. 3 TEM observations of the 240oC-sample (a) and 320oC-sample (b, c).
Figure 4
Fig. 4 TEM observations of the 400oC-sample (a) and 450oC-sample (b, c).
Figure 5
Fig. 5 Thickness of the bainitic ferrite plate changing with the tempering temperature. tBF0 is the thickness of the bainitic ferrite plate in the untempered sample.
Figure 6
Fig. 6 Volume fraction of retained austenite (V) (a) and the carbon content within it (C) (b) changing with the tempering temperature. V0 and C0 represent the volume fraction of retained austenite and the carbon within it in the untempered sample.
Figure 7
Fig. 7 Driving force ( Gmax ) of the bainitic transformation from retained austenite in the sample obtained by isothermal quenching at 270oC changing with the tempering temperatures.
Figure 8
Fig. 8 Para-equilibrium phase diagram and the comparative results obtained from XRD.
Figure(s)
Fig. 9 Thickness of bainitic ferrite plate varying with carbon diffusion coefficient under different tempering temperature.
Figure10
(b)
(d)
(f)
Fig. 10 EBSD microstructure characterization of the 240oC-sample (a, b), 320oC-sample (c, d) and 450oC-sample (e, f): Orientation image (left) and misorientation angle distribution (right).
Figure11
Fig. 11 Engineering stress vs engineering strain curves of the studied steel.
Figure12
Fig. 12 Hardness changing with the tempering temperature. H0 is the average hardness of the untempered sample.
Figure13
Fig. 13 Evolution of ultimate tensile strength (b), yield strength (s) and yield ratio (sb) with the tempering temperature. b0, s0 and s0b0 represent the ultimate tensile strength, yield strength and yield ratio, respectively.
Figure14
Fig. 14 Evolution of the total elongation (t) and uniform elongation (u) with the tempering temperature. t0 and u0 represent the total elongation and uniform elongation, respectively.
Figure15
Fig. 15 Martensite observed in the 360oC-sample after 3% tensile deformation by TEM.
Figure16
Fig. 16 Impact toughness varying with the tempering temperature. ku0 represents the impact toughness.
Figure17
Fig. 17 Fracture morphologies of the 240oC-sample (a), 320oC-sample (b) and 450oC-sample (c) after impact testing.
PII: DOI: Reference:
S0921-5093(17)30053-9 http://dx.doi.org/10.1016/j.msea.2017.01.044 MSA34606
To appear in: Materials Science & Engineering A Received date: 5 August 2016 Revised date: 14 January 2017 Accepted date: 17 January 2017 Cite this article as: J. Kang, F.C. Zhang, X.W. Yang, B. Lv and K.M. Wu, Effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.01.044 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.
Effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel J. Kang1, F.C. Zhang1,2,, X.W. Yang1, B. Lv3, K.M. Wu4,5 1
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
2
National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao 066004, China
3
College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China 4
International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan 430081, China
5
Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK
Abstract: The effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel has been investigated through optical microscopy, electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction analyses. A nano-level microstructure containing plate-like bainitic ferrite and film-like retained austenite is obtained by isothermal transformation at Ms+10 oC followed by tempering within 240 oC–450 oC. Results show that the sample tempered at 340 oC occupies the optimal balance of strength and toughness by maintaining a certain level of plasticity; samples tempered at 320 oC and 360 oC with low and high yield ratio come second. The microstructure of the steel is not sensitive to tempering temperatures before 360 oC. When the temperature is increased to 450 oC, the significantly coarsened bainitic ferrite plate and the occurrence of a small quantity of carbide precipitation account for its low toughness. The amount of retained austenite increases with the tempering temperature before 400 o
C, followed by decreasing with further increase in the temperature. This behavior is related to the
competition between retained austenite further transforming into bainite and decomposing into carbide during tempering. Keywords: Carbide-free bainite; Tempering; Microstructure; Mechanical properties
Corresponding author: E-mail: [email protected], Tel: 0086 335 8063949, Fax: 0086 335 8074568 (F.C. Zhang) 1
1 Introduction Carbide-free bainitic steel with low cost and excellent performance has been widely used in many industries, such as railway frogs, bearings, and automobile sheets [1–4]. The bainitic ferrite plates with nano- or sub-micron size and high density of dislocation in the microstructure provide high strength [5, 6]. The soft retained austenite embedded in the hard matrix of the bainitic ferrite contributes to suitable plasticity and toughness [7, 8]. However, structural steel must be welded under certain conditions. There are problems during welding such as coalescence and coarsening of the bainitic ferrite plates and cementite precipitation in carbide-free bainitic steels, which will ruin the mechanical properties [9–11]. Therefore, the effect of tempering on the microstructure and properties of carbide-free bainitic steel must be investigated. The carbide-free bainite transformation is mainly dominated by displacive mechanism [12–14]. The plastic relaxation accompanied with the shape deformation during bainite transformation leads to local residual stress [15]. It can be released by tempering. As a metastable phase, the amount of the retained austenite decreases with increasing tempering temperature caused by transformation or decomposition [16, 17]. The thin film-like retained austenite possesses high mechanical stability because of the size effect and high carbon content within it [18–20]. However, the high carbon content in the retained austenite with nano-size increased the driving force for carbide precipitation and thus led to poor thermal stability, which was confirmed by the high-energy X-ray diffraction testing [19, 21, 22]. Podder studied the tempering behavior of low carbon carbide-free bainitic steel at 450°C and found that minute quantities of cementite precipitated at the early stage of tempering. This process rendered the austenite sufficiently unstable in certain local regions with carbon poor to martensitic transformation on cooling and further increased the quantities of carbides with prolonged tempering [23]. In nano-bainitic steel, the retained austenite with carbon-rich was decomposed to intense carbide precipitation at lath boundaries during tempering and prevented the coarsening of the bainitic ferrite [24]. Hasan et al. quantitatively analyzed the effect of solid solution, grain refinement, dislocation, and precipitation on the strength of nano-bainitic steel after tempering. The decrease in strength was mainly due to slight coarsening and low dislocation density of the bainitic plate [25]. This study aims to investigate the effect of tempering on the microstructure and properties of a medium carbon carbide-free bainitic steel. 2
2 Experimental material and procedures The chemical composition of the studied steel is listed in Table 1. Si can inhibit the precipitation of cementite as the obstacle of cementite growth [26] and enhance the tempering stability of the nano-bainite [27]. Addition of Al can accelerate bainitic transformation [28, 29] besides hindering cementite precipitation [30]. The studied steel was smelted in a vacuum furnace and forged into a round bar (Φ65 mm). Samples with a size of 25 mm × 25 mm × 115 mm were cut along the axial direction of the round bar and used for subsequent heat treatments. Austenization and salt bath isothermal treatments were performed, and the carbide-free bainitic microstructure was consequently obtained. Table 1 Ac1 and Ac3 temperatures of the studied steel were determined as 753 oC and 795 oC by the phase change analyzer. The Ms temperature was tested to be 260 oC by Gleeble 3500-type thermal simulation test machine. The samples were austenitized at 930 oC, followed by isothermal transformation at 270 oC for 2 h and then air cooling to the room temperature to obtain carbide-free bainite microstructure. Finally, after the samples were completely cooled down to the room temperature for 6 h, the tempering treatments were performed by varying the tempering temperature within 240 oC–450 oC. In this paper, the samples tempered at different temperatures were represented to 240 oC-sample, 280oC-sample, 320 oC-sample, 340 oC-sample, 360 oC-sample, 400 o
C-sample and 450 oC-sample. Fig. 1 shows the expansion volume constantly changes during
isothermal transformation at 270 °C in the studied steel. As we can see, isothermal transformation for 2 h is sufficient for bainite transformation in the studied medium carbon steel. Fig. 1 Tensile testing was conducted on a MTS universal hydraulic testing machine. The sample size used had 25 mm gauge and 5 mm diameter. The strain rate was 4 × 10−4 s−1. Charpy U-notch specimens with dimensions of 10 mm × 10 mm × 55 mm were used at room temperature. We use the U-notch sample to test the toughness due to the requirement of the engineering subject related to developing the steels used for rail and frog. The low temperature toughness at -40 oC also needs to be tested and the toughness is sometimes very low on the engineering background, so the U-notch sample is chosen for comparing the toughness at room temperature in this paper. The impact testing 3
was according to the China standard GB/T229-2007 (Charpy pendulum impact test of the metal materials). Three samples were tested in each state to obtain the average value. Optical microscopy (OM) was used to observe the bainitic microstructure at low magnification. Transmission electron microscopy (TEM) was utilized to observe and analyze the microstructure and the sub-structure at high magnification as well as precipitation. TEM specimens were fabricated on a TenuPol-5 type twin-jet electropolishing instrument at room temperature. The electrolytic polishing solution was composed of 7% perchloric acid and 93% ethanol alcohol solutions. The morphology and orientation of the bainitic sheaves were characterized by electron-backscattered diffraction microscopy (EBSD). The morphology of the impact fracture surface was examined by scanning electron microscopy (SEM). Phase components and relative volumes were tested using a Rigaku D/max-2500/PC X-ray diffractometer with a radiation target of Cu–Kα in step scanning, with 0.2°/step and each step lasting for 2 s. The samples were mechanical polished delicately with1 μm diamond paste under a rotating rate of 80 r/min and then etched. The process of polishing and etching was repeated several times. Finally, the samples were polished in colloidal silica to obtain underformed surface. The volume fraction of the retained austenite (Vγ) is determined according to Eq. (1) [31]. Carbon concentration in the retained austenite (C) is determined according to Eq. (2) [32] by using the lattice parameters of the retained austenite.
(1/ n) j 1 Ij Rj n
V
(1/ n) j 1 ( Ij Rj ) (1/ n) j 1 ( Ij Rj ) n
n
(1)
a( A) 3.578 0.033C
(2)
Where n is the number of peaks examined, and I is the integrated intensity of the diffraction peak. R is the material scattering factor, that is, R (1/ v 2 )[ F P((1 cos 2 2 ) / sin sin 2 )] e2 M . v is 2
volume of the unit cell, F is the structure factor, P is the multiplicity factor, e−2M is the temperature
factor, is the diffraction angle, and a ( A) is the austenite lattice parameter.
3 Experimental results and analysis 3.1 Microstructure Fig. 2 shows the OM micrographs of the specimens after isothermal transformation at 270 oC followed by tempering at 240 oC, 320 oC and 450 oC, respectively. The bainitic sheaves take on a 4
needle-like appearance. At low magnification, the morphology of the bainitic sheaves differs minimally. Fig. 2 Typical TEM micrographs of the 240 oC-sample, 320 oC-sample, 400 oC-sample and 450 o
C-sample are shown in Figs. 3 and 4. As can be seen, the microstructures of the 240 oC-sample,
320 oC-sample and 400 oC-sample consist of thin film-like retained austenite embedded within bainitic ferrite plate. At 320 oC, there also exists the presence of the blocky retained austenite according to the diffraction pattern (Fig. 3c). When the temperature is increased to 400 oC, no carbide precipitation appears to occur (Fig. 4a). However, fine carbides with size of 25 ± 5 nm are observed in the matrix of 450 oC-sample. These carbides can not only be formed by the decomposition of the thin film shaped retained austenite between the lath interfaces (Fig. 4b), but also precipitate within the bainitic ferrite plates (Fig. 4c). Liu et al. thought that the orientation relationship between -carbide and the matrix within the bainitic ferrite was similar to that in the tempered martensite [33]. Caballero investigated solute atom redistribution during tempering in nano-bainitic steel by atom probe technology. The results showed that the retained austenite at the plate interface would decompose before achieving the total equilibrium, and cementite was formed within the bainitic ferrite with over-saturated carbon or along the ferrite-retained austenite interface through the para-equilibrium mechanism [34]. Si was repelled quickly into the parent phase from the new product during tempering [34], thereby inhibiting the transition from -carbide to cementite and delaying the nucleation and growth of the latter [26]. In addition, the dislocation density in the sample tempered at 450 °C decreases evidently, as shown in Figs. 4b and c. Fig. 3 Fig. 4 The average linear intercept ( LT ) is measured along a direction normal to the length direction of the slender bainitic ferrite plate. The relationship between LT and the actual thickness of bainitic ferrite plate (tBF) can be represented as follows [35, 36]:
LT t / 2 Confidence error is 95%, E 2 L / N
where T represents the standard deviation, and N is the measured quantity. Fig. 5 shows the change in bainitic ferrite plate thickness with tempering temperature. The size 5
of the plate differs little within 240 °C–320 °C. The bainitic ferrite plate of 360 °C-sample becomes slightly coarsening, but is still below 100 nm. When the tempering temperature is further increased to 450 oC, the bainitic ferrite plate is evidently coarsened, and the thickness increases to 120 ± 6 nm. This is because the amount of carbide precipitated at the bainitic lath boundary is extremely small, which cannot prevent the coarsening of the lath effectively. Therefore, the bainite microstructure in this case is not sensitive to tempering at low temperatures (≤ 360 °C) but appears to coarsen at high temperatures (> 360 °C). Fig. 5 Fig. 6a and b shows the change in the volume fraction of the retained austenite (V) and the carbon within it (C) with tempering temperature. Vof the samples with tempering decreases, in comparison with the sample without tempering. At temperatures before 400oC, the Vincreases linearly with tempering temperature and reduces slightly after 400 oC. This finding is inconsistent with the results of tempering reported in the literature, that the amount of retained austenite decreases with increasing tempering temperature [16, 17, 37]. The carbon in the retained austenite reaches the peak at 320 oC, and then decreases slightly with further increase in tempering temperature. Fig. 6 Bainite in general tempers much more gently because it autotempers during the course of transformation. This is because much of the carbon precipitates as carbides or partitions from the ferrite to the remaining austenite during bainite formation [38]. The transformation mechanism of carbide-free bainite is mainly dominated by diaplacive mechanism. The transformation will stop when the carbon content in the retained austenite reaches the T0′ curve [39, 40]. Based on the results of TEM and XRD analyses, competition exists between retained austenite further transforming into bainite and decomposing into carbide during tempering, in addition to the dislocations rearrangement in the microstructure. Bainite transformation during tempering is related to the driving force ( Gmax ) of the bainitic transformation from retained austenite under different tempering temperature. This parameter can be calculated based on the average carbon content in the retained austenite after isothermal transformation at 270 oC by using the the Public Domain Research Software and Data Library of the Materials Algorithms Project (MAP) [41], as is shown in Fig. 7. The driving force exhibits a decreasing trend with increasing temperature. There is a 6
thermodynamic condition to be met in order for carbide-free bainite formation to take place at a transformation temperature T for a given alloy carbon concentration C. CT
after 320 oC is mainly caused by the increased V. At the tempering temperature of 450 oC,
the retained austenite, especially the thin-film shaped austenite, preferentially decomposes into
7
carbide because of the significantly increased diffusion coefficient of carbon and silicon, which leads to the reduction in V and C in the 450 oC-sample. Fig. 7 Fig. 8 Moreover, the bainitic ferrite is carbon over-saturated after isothermal transformation. The carbon will further partition to the parent austenite from the ferrite during the further tempering process, eventually leading to slow broadening of the bainitic ferrite plates [47]. The carbon diffusion coefficient under different tempering temperature can be calculated according to the equation- DC 0.12 e 32,000/ RT [44]. The variation of bainite ferrite plate thickness with carbon diffusion coefficient under different tempering temperature is shown in Fig. 9. As we can see, when tempering temperature is lower than 320 oC, carbon diffusion coefficient is very low and the thickness of bainitic ferrite plate is unchanged. As the carbon diffusion coefficient increases (tempering temperature >320 oC), the thickness of bainitic ferrite plate exhibits obvious increasing trend. This finding indicates that the coarsening degree of ferrite is related to the diffusion activity of carbon. Hence, the bainitic ferrite coarsening of 450 oC-sample is also associated with high diffusion coefficient of carbon at high tempering temperature. Fig. 9 Fig. 10 shows EBSD microstructure characterization of the 240 oC-sample, 320 oC-sample and 450 oC-sample. Large grain boundary misorientation angle (>15o) has a larger proportion among the bainitic sheaves. This finding is in good agreement with the feature of grain boundary misorientation angle distribution in the lower bainite [48–50]. Lower bainite transformation is accompanied by large plastic relaxation due to its low transformation temperature. The orientation of bainitic sheaves is mainly the N-W/N-W relationship without twinning relationship and the probability of large angle grain boundary with random distribution is relatively higher [48, 49]. In this case, the misorientation angle distribution in the 240 oC-sample (Fig. 10b) is similar with that in the 320 oC-sample (Fig. 10d). They both exhibit the peaks of number fraction at 40–50° and 55° misorientation angles whereas the peak of number fraction at 40–50° in the 320oC-sample exhibits a slight reduction. However, when the sample is tempered at 450 oC (Fig. 10f), the number fraction of 55° misorientation angle keeps unchanged. The proportion of 40-50° misorientation angles reduces evidently, whereas the proportion of 30–40° misorientation angles increases. It implies that the 8
bainitic sheaves with 40–50° misorientation angles in the studied steel are unstable and the number fraction decreases slightly at high tempering temperature (450 oC). This change in high angle grain boundary distribution may be associated with the coalescence of the bainitic ferrite plate caused by the decomposition of the retained austenite in the 450 oC-sample. Fig. 10 3.2 Property analysis Fig. 11 shows the engineering stress against engineering strain curves of the studied steel with and without tempering. Fig. 12 shows the change in hardness with tempering temperature. The variation range of the hardness is relatively narrow, changing within the range of ～2 HRC. Hardness remains at a higher level in the samples tempered within the range of 240 oC–360 oC and then decreases to the lowest in the 400 oC-sample. Finally, it exhibits a minor increase in the 450 o
C-sample. Fig. 11 Fig. 12 Fig. 13 shows the evolution of ultimate tensile strength (b), yield strength (s) and yield ratio
(sb) with tempering temperature. As the tempering temperature increasing, b tends to decrease firstly, then followed by increase, which is similar with the variation of hardness with tempering temperature, as shown in Fig. 11. By contrast, the change trend of s is virtually opposite to that of
b. The s shows a valley value at 320 oC and reaches the peak value at 400oC. The evolution of yield ratio with the tempering temperature is similar to that of s. Fig. 13 Fig. 14 shows the change in total elongation (t) and uniform elongation (u) with tempering temperature. t shows a parabolic curve and reaches the peak value at 400 °C followed by a sharp reduction at 450 °C. u continuously increases with increasing tempering temperature. Fig. 14 Tensile properties correspond to the microstructure features. High strength of 240oC-sample originates from its finer bainitic ferrite plate, while its poor plasticity mainly results from a minute amount of the retained austenite. 320 oC-sample possesses high ultimate tensile strength and medium plasticity, but relatively low yield strength. The high carbon content in the retained 9
austenite means a high mechanical stability. It is very difficult to undergo the stress induced martensitic transformation under small strain condition and thus cannot improve yield strength. However, the medium amount of retained austenite ensures medium plasticity in the 320 oC-sample. When tempered at 360 oC, the sample shows a reduced ultimate tensile strength and increased yield strength; and the elongation reaches a peak value. The retained austenite with low carbon content of the 360 oC-sample indicates low mechanical stability. Tensile testing with 3% strain was carried out in the 360 oC-sample, and the TEM micrograph in the gauge distance is shown in Fig. 15. Martensite can be observed in the 360 oC-sample after small straining. The enhanced yield strength in the 360
o
C-sample is probably attributed to stress induced martensitic transformation,
compensating for the negative effect of the dislocation recovery on yield strength. Meanwhile, the high amount of retained austenite and the decreased density of dislocations increase the work hardening capacity, which is beneficial for plasticity. Fig. 15 Although carbide is not observed in the 400 oC-sample through TEM analysis, we cannot disregard the existing possibility of carbide precipitates because of significant increase in the diffusion coefficient of carbon at 400 oC [41]. This may probably result in the enhanced yield strength of the 400 oC-sample. Several researchers showed that at tempering temperature of 400 oC, the thermal stability of the retained austenite declined which promoted the austenite decomposition into carbide or cementite [27, 51]. The fine dispersed carbide can interact with dislocation, refining the microstructure and improving the yield strength of the material [50]. High uniform elongation and total elongation in the 400 oC sample can be provided by higher volume fraction of the retained austenite with lower carbon content within it. More mechanically unstable austenite can transform into martensite continuously during tensile deformation, which can delay plastic instability and improve elongation [52-54]. When the tempering temperature is further increased to 450 oC, the marked coarsening of bainitic ferrite plate accounts for the lower yield strength, whereas carbide precipitation within the bainitic ferrite or along the ferrite–austenite interface ensures high tensile strength and high hardness, but deteriorates elongation. Fig. 16 shows the changing in impact toughness with tempering temperature. The variation of impact toughness with tempering temperature exhibits a wave curve. Samples tempered at medium temperatures (320 oC–360 oC) possess relatively higher values while the samples tempered at low 10
temperatures (240 oC and 280 oC) and high temperatures (400 oC and 450 oC) have low impact toughness. Fig. 16 High carbon content in the retained austenite of samples tempered at 320 oC-360 oC is associated with high mechanical stability of the retained austenite. The relatively stable austenite can enhance crack propagation resistance and decrease the crack propagation rate during the stage II which is beneficial for toughness [55]. The toughness damage of the 240 oC-sample is caused by the relatively more untempered bainitic ferrite with high carbon and extremely low amount of the soft retained austenite. The low toughness of 450 oC-sample is mainly related to the coarsened bainitic ferrite plate (>100 nm) and carbide precipitation. Crack propagation resistance is associated with the size of bainitic ferrite plate. Fine plates of bainitic ferrite provide more interfaces and thus improve toughness [56]. In contrast, the blocky bainitic ferrite plates promote crack initiation and fracture due to high stress concentration [57]. As the obstacle for the sliding, carbide provides the potential fracture path. The crack source can form at the carbide–matrix interface and deteriorate toughness [37]. In addition, the decreased fraction of the high-angle grain boundary with 40°–50o also performs a negative function in the toughness of the 450 oC-sample. Fig. 17 shows the fracture surfaces of the 240 oC-sample, 320 oC-sample and 450 oC-sample after impact testing. The white round points are the splashed oxide during cutting the samples. The fracture appearance in the 240 oC-sample and 320 oC-sample is the quasi-cleavage fracture consisting of small flat facets and intense dimples distributed on the tearing ridge around the facets. The fracture surface of 320 oC-sample is characterized by more quantity of dimples and smaller facets, compared to that of the 240 oC-sample. However, there is almost no dimple in the 450 o
C-sample and the mode is brittle cleavage fracture consisting of flat facets. The fracture feature
corresponds to the lowest impact toughness in the 450 oC-sample. Fig. 17 Yield strength and yield ratio increase, whereas the change in hardness, ultimate strength, elongation, and toughness is non-monotonic in the samples after tempering, in comparison with those of the sample before tempering. 340 °C-sample occupies the optimal balance of strength and toughness on the basis of maintaining certain plasticity; the 320 °C-sample and 360 °C-sample come second. The two samples show different yield ratio. Yield ratio represents the capacity of 11
continuous work hardening capacity of the material. During large plastic deformation under working conditions, a material with low yield ratio is preferred. When the lighter weight is required under the working condition, the material with high yield ratio is the priority. Therefore, the material with different yield ratios obtained by varying the tempering process can increase the usefulness and the range of the materials in applications.
4 Conclusions Nano-bainitic microstructure containing bainitic ferrite plate and retained austenite film has been obtained by isothermal transformation at Ms+10 oC followed by tempering within the range of 240 oC-450 oC in a medium carbon carbide-free bainitic steel. The effect of tempering temperature on microstructure and properties has been investigated. Conclusions can be drawn as follows. 1. Among the tempering temperatures, 340 oC-sample occupies the optimal balance of strength and toughness on the basis of maintaining a certain plasticity. 320 oC-sample and 360 oC-sample come second. 2. When the studied steel is tempered at low and medium temperature levels (<360 oC), the microstructure is not sensitive to tempering. When the temperature is increased to 450 oC, the bainitic ferrite plate shows significant corarsening trend and the number fraction of bainitic sheave with 40-50° misorientation angle decreases slightly. Fine carbide can also be observed in the 450 oC-sample. These changes in microstructure are responsible for the low toughness in the 450 oC-sample. 3. Driving force of bainitic transformation generally decreases with the increase in tempering temperature according to the carbon content in the retained austenite of the sample without tempering. Before 400
o
C, the amount of retained austenite increases with tempering
temperature. The baintic transformation from the retained austenite dominates the tempering process due to high driving force and low diffusion rate of carbon and silicon. At tempering temperature of 450oC, the thin film-like retained austenite with higher carbon content preferentially decomposes to carbide because of the significantly increased diffusion coefficient of carbon, eventually leading to the decreased volume fraction of retained austenite and lower carbon content.
Acknowledgement 12
This work was supported by the Natural Science Foundation of China (No. 51471146), the Nat ional High Technology Research and Development Program of China (863 Program) (No. 2012AA 03A504), the Natural Science Foundation of Hebei Province of China (No. E2013203258).
References [1] F.C. Zhang, Z.N. Yang, J. Kang, Progress in bainitic steel used for railway crossing. J. Yanshan University, 37 (2013) 1–7. [2] K. Sswley, J. Sun, Advanced rail steels: investigating the bainitic option, Railway Track & Struct., 3 (1997) 22–27. [3] C. Garci-Mateo, F.G. Cabllero, H.K.D.H. Bhadeshia. Development of Hard Bainite. ISIJ Int. 43 (2003) 1238–1243. [4] F.C. Zhang, T.S. Wang, P. Zhang, C.L. Zheng, B. Lv, M. Zhang, Y.Z. Zheng. A novel method for development of nanobainitic microstructure in surface layer of low carbon steel. Scripta Mater. 59 (2008) 294–296. [5] J. Cornide, C. Garcia-Mateo, C. Capdevila, F.G. Caballero, An assessment of the contributing factors to the nanoscale structural refinement of advanced bainitic steels, J. Alloys Comp. 577S (2013) S43–S47. [6] F.G. Caballero, H.K.D.H. Bhadeshia, Very strong bainite, Current Opinion in Solid State and Materials Science 8 (2004) 251–257. [7] X.Y. Long, F.C. Zhang, J. Kang, B.Lv, X.B. Shi, Low-temperature bainite in low-carbon steel, Mater. Sci. Eng. A 594 (2014) 344–351. [8] L.H. Qian, Q. Zhou, F.C. Zhang, J.Y. Meng, M. Zhang, Y. Tian, Microstructure and mechanical properties of a low carbon carbide-free bainitic steel co-alloyed with Al and Si, Mater. Des. 39 (2012) 264–268. [9] K. Fang, J.G. Yang, D.L. Zhao, K.J. Song, Z.J. Yan and H.Y. Fang, Review of Nanobainite Steel Welding, Adv. Mater. Res. 482–484 (2012) 2405–2408. [10] J. H. Pak, H. K. D. H. Bhadeshia, L. Karlsson, E. Keehan, Coalesced bainite by isothermal transformation of reheated weld metal, Sci. Technol. Weld. Join. 13 (2008) 593–597. [11] H. K. D. H. Bhadeshia, Nanostructured bainite, Proc. R. Soc. A 466 (2113) 3–18. [12] F.G. Caballero, C. Garcia-Mateo, M.J. Santofimia, M.K. Miller, C. Garcı´a de Andre´s, New experimental evidence on the incomplete transformation phenomenon in steel, Acta Mater. 57 (2009) 8–17. [13] H.K.D.H. Bhadeshia, The bainite transformation: unresolved issues, Mater. Sci. Eng. A 273–275 (1999) 58–66. [14] H.K.D.H. Bhadeshia, D.V. Edmonds. The mechanism of bainite formation in steels, Acta Metall 28 (1980) 1265–1273. [15] H.K.D.H. Bhadeshia, Developments in martensitic and bainitic steels: role of the shape deformation, Mater. Sci. Eng. A 378 (2004) 34–39. [16] G.R. Speich, W.C. Leslie, Tempering of steel, Metall. Mater. Trans. B 4 (1973) 303–315. [17] A. Joarder, J.N. Ojha, D.S. Sarma, The tempering behavior of a plain carbon dual-phase steel, Mater. Charact. 25 (19) (1990) 9–209. [18] X.D. Wang, B.X. Huang, Y.H. Rong, L.Wang, Microstructures and stability of retained austenite in TRIP steels, Mater. Sci. Eng. A 438–440 (2006) 300–305. [19] C. Garcia-Mateo, F. G. Caballero, M. K. Miller, J. A. Jimenez, On measurement of carbon content in retained austenite in a nanostructured bainitic steel, J. Mater. Sci. 47 (2012) 1004–1010. [20] X.Y. Long, J. Kang,B.Lv , F.C. Zhang, Carbide-free bainite in medium carbon steel, Mater. Des. 64 (2014) 13
237–245. [21] A.S. Podder, I. Lonardelli, A. Molinari, H.K.D.H. Bhadeshia, Thermal stability of retained austenite in bainitic steel: an in situ study. Proc. R. Soc. A Phys. Eng. Sci. 467 (2011) 3141–56. [22] B. Avishan, C. Garcia-Mateo, S. Yazdani ,F.G. Caballero, Retained austenite thermal stability in a nanostructured bainitic steel, Mater. Charact. 81 (2013) 105–110. [23] P.A. Saha, H.K.D.H. Bhadeshia, Thermal stability of austenite retained in bainitic steels, Mater. Sci. Eng. A 527 (2010) 2121–8. [24] M. Peet, Transformation and tempering of low-temperature bainite (Ph.D. thesis), University of Cambridge, Cambridge, UK, 2010. [25] H.S. Hasan, M.J. Peet, M-N. Avettand-Fènoël, H.K.D.H. Bhadeshia, Effect of tempering upon the tensile properties of a nanostructured bainitic steel, Mater. Sci. Eng. A 615 (2014) 340–347. [26] B. Kim, C. Celada, D. San Martı´n, T. Sourmail, P.E.J. Rivera-Dı´az-del-Castillo, The effect of silicon on the nanoprecipitation of cementite, Acta Mater. 61 (2013) 6983–6992. [27] J.A.C. Junior, D.B. Santos, Effect of tempering temperature on isothermal decomposition product formed below Ms, J. Mater. Res. Technol. 2 (2013) 93–99. [28] C. Garcia-Mateo, F.G. Caballero, H.K.D.H. Bhadeshia, Acceleration of Low-temperature Bainite, ISIJ Inter., 43 (2003) 1821–1825. [29] E.V. Pereloma, I.B. Timokhina, M.K. Miller, P.D. Hodgson, Three-dimensional atom probe analysis of solute distribution in thermomechanically processed TRIP steels, Acta Materialia 55 (2007) 2587–2598. [30] B.C. De Cooman, Structure-properties relationship in TRIP steels containing carbide-free bainite, Curr. Opin. Solid Stat. Mater. Sci. 8 (2004) 285–303. [31] B. D. Cullity, S. R. Stock, Elements of X-ray diffraction. 3rd ed. New Jersey: Prentice Hall; 2001. p351. [32] D. J. Dyson, B. Holmes, Effect of alloying additions on the lattice parameter of austenite, J. Iron Steel Inst. 1970 (208) 469-474. [33] J. Liu, C.P. Luo, Precipitation behavior of the lower bainitic carbide in a medium-carbon steel containing Si, Mn and Mo, Materials Science and Engineering A 438–440 (2006) 153–157. [34] F.G. Caballeroa, Hung-Wei Yen, M.K. Miller, J. Cornide, Hsiao-Tzu Chang, C. Garcia-Mateo, Jer-Ren Yang, Three phase crystallography and solute distribution analysis during residual austenite decomposition in tempered nanocrystalline bainitic steels, Mater. Charact. 88 (2014) 15–20. [35] L.C. Chang, H.K.D.H. Bhadeshia, Austenite films in bainitic microstructures, Mater. Sci. Technol. (1995) 874–881. [36] H.K.D.H. Bhadeshia, Introduction to Quantitative Metallography, Part II Lecture Notes, 2013. [37] Y. Luo, J.M. Peng, H.B. Wang, X.C. Wu, Effect of tempering on microstructure and mechanical properties of a non-quenched bainitic steel, Mater. Sci. Eng. A 527 (2010) 3433–3437. [38] H. K. D. H. Bhadeshia, Bainite in steels, 2nd edn; 2001, London, The Institute of Materials. [39] H.K.D.H. Bhadeshia, D.V. Edmonds, The bainite transformation in a silicon steel, Metall. Trans. 10A (1979) 895–905. [40] H.K.D.H. Bhadeshia, Waugh AR. Bainite: an atom-probe study of the incomplete reaction phenomenon, Acta Metall. 30 (1982) 775–784. [41] the Public Domain Research Software and Data Library of the Materials Algorithms Project (MAP), Department of Materials Science and Metallurgy, University of Cambridge, http://www.msm.cam.ac.uk:map:map-main.html. [42] H. Huang, M.Y. Sherif, P.E.J. Rivera-Dıaz-del-Castillo, Combinatorial optimization of carbide-free bainitic nanostructures, Acta Materialia 61 (2013) 1639–1647. [43] Honeycombe R, Bhadeshia HKDH. Steels: microstructure and properties. 2nd ed. Edward Arnold; 1995. 14
[44] C. Wells, R.F. Mehl, Transactions 140 (1940) 279–306. [45] H.-S. Park, J.-B. Seol, N.-S. Limc, S.-Il Kimc, C.-G. Park, Study of the decomposition behavior of retained austenite and the partitioning of alloying elements during tempering in CMnSiAl TRIP steels, Mater. Des. 82 (2015) 173–180. [46] B. Kim, C. Celada, D. San Martın, T. Sourmail, P.E.J. Rivera-Dıaz-del-Castillo, The effect of silicon on the nanoprecipitation of cementite, Acta Materialia 61 (2013) 6983–6992. [47] F.G. Caballero, M.K. Miller, C. Garcia-Mateo, Carbon supersaturation of ferrite in a nanocrystalline bainitic steel, Acta Mater. 58 (2010) 2338–2343. [48] S. Zajac, V. Schwinn, K.H. Tacke, Characterization and quantification of complex bainitic microstructures in high and ultra-high strength linepipe steels, Mater. Sci. Forum 500–501 (2005) 387–94. [49] X.Y. Long , J. Kang, B. Lv, F.C. Zhang. Carbide-free bainite in medium carbon steel, Mater. Des. 64 (2014) 237–245. [50] S.-G. Park, K.-H. Lee, M.-C. Kimn, B.-S. Lee, Effects of boundary characteristics on resistance to temper embrittlement and segregation behavior of Ni–Cr–Mo low alloy steel, Mater. Sci. Eng. A 561 (2013) 277–284. [51] Z.M. Shi, W. Gong, Y.Tomota, S. Harjo, J. Li, B. Chi, J. Pu, Study of tempering behavior of lath martensite using in situ-neutron diffraction, Mater. Charact. 107 (2015) 29–32. [52] M. Sherif, C. Garcia-Mateo, T. Sourmail, H.K.D.H. Bhadeshia, Stability of retained austenite in TRIP-assited steels, Mater. Sci. Technol. 20 (2004) 319–322. [53] M.Y. Sherif, Characterisation and development of nanostructured, ultrahigh strength, and ductile bainitic steels (Ph.D.thesis), University of Cambridge, 2005. [54] H.K.D.H. Bhadeshia, Nanostructured bainite, Proc. R. Soc. Lond. A 466 (2010) 3–18. [55] T.V. Venkatasubramanian, T.J. Baker, Enhanced fracture resistance of high strength steel produced by extrusion of nickel coated steel powder, Metall. Mater. Trans. A 11 (1980) 200–203. [56] M.W. Tong, P.K.C. Venkatsurya, W.H. Zhou, R.D.K. Misra, B. Guo,K.G. Zhang,W.Fan, Structure–mechanical property relationship in a high strength microalloyed steel with low yield ratio: The effect of tempering temperature, Mater. Sci. Eng. A 609 (2014) 209–216. [57] Y. You, C.J. Shang, L. Chen, S. Subramanian, Investigation on the crystallography of the transformation products of reverted austenite in intercritically reheated coarse grained heat affected zone, Mater. Des. 43 (2013) 485–491.
15
Figure captions Fig. 1 Expansion volume constantly changes during isothermal transformation at 270 °C in the studied steel. Fig. 2 OM microstructures of the 240 oC-sample (a), 320 oC-sample (b) and 450 oC-sample (c). Fig. 3 TEM observations of the 240 oC-sample (a) and 320 oC-sample (b, c). Fig. 4 TEM observations of the 400 oC-sample (a) and 450 oC-sample (b, c). Fig. 5 Thickness of the bainitic ferrite plate changing with the tempering temperature. tBF0 is the thickness of the bainitic ferrite plate in the untempered sample. Fig.6 Volume fraction of retained austenite (V) (a) and the carbon content within it (C) (b) changing with tempering temperature. V0 and C0 represent the volume fraction of retained austenite and the carbon within it in the untempered sample. Fig. 7 Driving force ( Gmax ) of the bainitic transformation from retained austenite in the sample obtained by isothermal transformation at 270oC changing with the tempering temperatures. Fig. 8 Para-equilibrium phase diagram and the comparative results obtained from XRD. Fig. 9 Thickness of bainitic ferrite plate varying with carbon diffusion coefficient under different tempering temperature. Fig. 10 EBSD microstructure characterization of the 240 oC-sample (a, b), 320 oC-sample (c, d) and 450oC-sample (e, f): Orientation image (left) and misorientation angle distribution (right). Fig. 11 Engineering stress against engineering strain curves of the studied steel. Fig. 12 Hardness changing with tempering temperature. H0 is the average hardness of the untempered sample. Fig. 13 Evolution of ultimate tensile strength (b), yield strength (s) and yield ratio (sb) with tempering temperature. b0, s0 and s0b0 represent the ultimate tensile strength, yield strength and yield ratio, respectively. Fig. 14 Evolution of the total elongation (t) and uniform elongation (u) with tempering temperature. t0 and u0 represent the total elongation and uniform elongation, respectively. Fig. 15 Martensite observed in the 360 oC-sample after 3% tensile deformation by TEM. Fig. 16 Impact toughness varying with the tempering temperature. ku0 represents the impact toughness. Fig. 17 Fracture morphologies of the 240 oC-sample (a), 320 oC-sample (b) and 450 oC-sample (c) 16
after impact testing. Table 1 Chemical composition of the studied steel, wt% Element
C
Si
Mn
Cr
Ni
Mo
Al
P
S
Composition
0.46
1.55
1.59
1.24
0.81
0.40
0.62
0.003
0.002
17
Figure(s)
Fig. 1 Expansion volume constantly changes during isothermal transformation at 270 °C in the studied steel.
Figure 2
Fig. 2 OM microstructures of the 240oC-sample (a), 320oC-sample (b) and 450oC-sample (c).
Figure 3
Fig. 3 TEM observations of the 240oC-sample (a) and 320oC-sample (b, c).
Figure 4
Fig. 4 TEM observations of the 400oC-sample (a) and 450oC-sample (b, c).
Figure 5
Fig. 5 Thickness of the bainitic ferrite plate changing with the tempering temperature. tBF0 is the thickness of the bainitic ferrite plate in the untempered sample.
Figure 6
Fig. 6 Volume fraction of retained austenite (V) (a) and the carbon content within it (C) (b) changing with the tempering temperature. V0 and C0 represent the volume fraction of retained austenite and the carbon within it in the untempered sample.
Figure 7
Fig. 7 Driving force ( Gmax ) of the bainitic transformation from retained austenite in the sample obtained by isothermal quenching at 270oC changing with the tempering temperatures.
Figure 8
Fig. 8 Para-equilibrium phase diagram and the comparative results obtained from XRD.
Figure(s)
Fig. 9 Thickness of bainitic ferrite plate varying with carbon diffusion coefficient under different tempering temperature.
Figure10
(b)
(d)
(f)
Fig. 10 EBSD microstructure characterization of the 240oC-sample (a, b), 320oC-sample (c, d) and 450oC-sample (e, f): Orientation image (left) and misorientation angle distribution (right).
Figure11
Fig. 11 Engineering stress vs engineering strain curves of the studied steel.
Figure12
Fig. 12 Hardness changing with the tempering temperature. H0 is the average hardness of the untempered sample.
Figure13
Fig. 13 Evolution of ultimate tensile strength (b), yield strength (s) and yield ratio (sb) with the tempering temperature. b0, s0 and s0b0 represent the ultimate tensile strength, yield strength and yield ratio, respectively.
Figure14
Fig. 14 Evolution of the total elongation (t) and uniform elongation (u) with the tempering temperature. t0 and u0 represent the total elongation and uniform elongation, respectively.
Figure15
Fig. 15 Martensite observed in the 360oC-sample after 3% tensile deformation by TEM.
Figure16
Fig. 16 Impact toughness varying with the tempering temperature. ku0 represents the impact toughness.
Figure17
Fig. 17 Fracture morphologies of the 240oC-sample (a), 320oC-sample (b) and 450oC-sample (c) after impact testing.