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Morphological evolution of HAZ microstructures in low carbon steel during simulated welding thermal cycle
Micron 131 (2020) 102828
Contents lists available at ScienceDirect
Micron journal homepage: www.elsevier.com/locate/micron
Morphological evolution of HAZ microstructures in low carbon steel during simulated welding thermal cycle
T
Liangyun Lana,b,*, Guoqing Shaoa a b
School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110819, China Key Laboratory of Vibration and Control of Aero-Propulsion Systems, Ministry of Education of China, Northeastern University, Shenyang 110819, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Low carbon steel HAZ microstructure Intragranular nucleation Granular bainite Bainitic ferrite
It is necessary to reveal the derivation of bainite microstructures with different appearance because these complex appearances are closely related to the transformation nature and their final mechanical properties. In this work, the morphological evolution of HAZ microstructures during simulated welding thermal cycle was investigated in a low carbon steel that was intentionally treated with combined addition of Ti and Mg elements. The inclusions present in the matrix have a complex core/shell composite structure composed of Al-Ca-Mg-Ti-O and MnS. However, only very few inclusions with large size (about 2∼6μm) can stimulate the intragranular nucleation of ferritic laths. Thus, this behavior did not show an obvious refinement effect on HAZ microstructure. Partial bainite microstructure revealed the ferritic lath structures always first form at the beginning of the transformation irrespective of the final morphologies of microstructure (granular bainite or bainitic ferrite), implying that the granular bainite is actually derived from the ferritic lath structures although these lath structures will become gradually obscure at the later stages of the transformation.
1. Introduction For solid-state phase transformation in steels and their welds, middle temperature transformation microstructures (collectively called bainite) exhibit a variety of appearances when viewed in the optical microscope. These appearances are not only related to the chemical compositions (including non-metallic inclusions) but also depend on heating and cooling schedules (Grong and Matlock, 1986; Ohmori et al., 1994; Abson, 2018; Kumar et al., 2016). There are several different classifications on bainite microstructure to depict its various appearances with different terminologies (Grong and Matlock, 1986; Krauss and Thompson, 1995; Thewlis, 2004). Grong and Matlock, 1986 considered that the microstructure formed in weld metal will be arranged in approximately decreasing order of transformation temperature: grain boundary ferrite, polygonal ferrite, Widmanstättan ferrite, acicular ferrite, upper bainite, lower bainite and martensite. Krauss and Thompson (1995) reviewed various bainite and ferrite classification systems and showed about 9 different terminologies to identify microstructural constituents formed during continuous cooling process for modern low-carbon high-strength steels. Thewlis (2004) even proposed the identification of 19 different microstructural constituents, with a flow diagram for both grain-boundary-nucleated and intragranually nucleated transformation products. ⁎
Because the bainite classification systems are not unified, some of the terms used are still in dispute. For example, acicular ferrite is expected for the first time to indicate the intergranular ferrite nucleated on the inclusions in the weld metal (Farrar and Harrison, 1987). However, it is often confused with bainitic ferrite that is formed below bainite start temperature, as reviewed by Abson (2018). This term is also widely used to describe the small non-aligned ferrite formed within prior austenite grains for hot-rolled steels (Kim et al., 2008; Pereloma et al., 1999; Zhao et al., 2018; Liu et al., 2016). Recently, Yin et al. (2017) showed that no morphological difference was found between Widmanstätten ferrite and bainitic ferrite at the early stage of the transformation, although their formation is traditionally regarded by different transformation mechanisms (Bhadeshia, 2015). Pereloma et al. (2014) substantiated that the difference between the bainitic ferrite and granular bainite exists not only in the morphology but also in the crystallography even if they are formed in the same steel. These disputes may be partly derived from the fact that the microstructural identification is only based on the final morphology of bainite. In fact, transformation behaviors change with decreasing temperature and/or holding time, which leads to continuous change in morphology of products during transformation within the scale of individual prior austenite grains. Our previous works showed the crystallography and kinetics of bainite microstructure vary with the
Corresponding author at: School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110819, China. E-mail address: [email protected] (L. Lan).
https://doi.org/10.1016/j.micron.2020.102828 Received 19 November 2019; Received in revised form 13 January 2020; Accepted 13 January 2020 Available online 15 January 2020 0968-4328/ © 2020 Elsevier Ltd. All rights reserved.
Micron 131 (2020) 102828
L. Lan and G. Shao
Fig. 1. Microstructural morphology with different t8/5 cooling times: (a) 30 s, (b) 50 s, (c) 120 s (BF: bainitic ferrite; GB: granular bainite; IBF: intragranular bainite ferrite; white arrows showing the prior austenite grain boundaries, the inclusions without assisting intragranular nucleation are marked with grids and the circled inclusion seems to stimulate the intragranular nucleation for several ferritic laths).
diagram that has been published in Ref. Lan et al. (2018). Metallographic samples were taken from the cross-section of heat treatment regions and examined using an optical microscope after these samples were subjected to conventional preparation, such as manual and mechanical polishing, 3 % nital etching, and hot air drying. As microstructure inhomogeneity can be produced by temperature difference across the cross-section or the surface energy near to surface regions of specimens, the region of interest for microstructure observation was focused on the center of specimens (about 4 mm in diameter). To reveal the complex composition of non-metallic inclusions in two-dimensional observation plane, micro-area scanning analysis on distribution of elements was conducted using a field-emission electron probe microscope (EPM). Besides, a field-emission transmission electron microscope (TEM) was employed to display the refinement structures of microstructure. TEM foils, discs 3 mm in diameter, were mainly cut from the partial bainite transformation specimens and ground to a thickness of about 0.05 mm, and then twin-jet electropolished at a temperature of −30 °C with a voltage of 35 V.
progress of transformation (Lan et al., 2015; Lan and Kong, 2018). However, the evolution of morphology of bainite was not analyzed in detail although some partial bainite transformation microstructures have been represented elsewhere (Lan et al., 2015; Lan and Kong, 2018; Lan et al., 2018). Here, the main focus is on the morphological evolution of bainite in the heat-affected zone (HAZ) of low carbon steel with the progress of phase transformation and the derivation of bainite with different appearances is revealed in detail. Because the steel was just subjected to high temperature welding thermal cycles but not melting, the terminologies by Krauss and Thompson (1995) will be mainly adopted. 2. Experimental methods The present work was carried out on an industrially manufactured plate of HSLA steel and its chemical composition is 0.053 C, 1.64 Mn, 0.22 Si, 1.02 (Cr + Ni + Mo), 0.06 (Nb + V), 0.015Al, 0.025 Ti, 0.0019 Ca, 0.002 Mg, and 0.0012B (wt.%) balanced by Fe. The aims of Ti and Mg added into steel are to form complex non-metallic inclusions that can stimulate the intragranular nucleation of acicular ferrite (Zhang et al., 2010). Specimens having cylindrical form (φ 6 mm × 55 mm) were cut from the steel plate and examined in a thermo- mechanical simulation tester. Temperatures were measured using very thin thermocouples (0.1 mm wires) spot-welded in the middle of the specimens and these were used to program the welding thermal cycles. In terms of a two dimensional Rykalin mathematical mode, the thermal cycles involved heating at 130 °C /s to 1350 °C, holding for 2 s and then cooling at prescribed rates through the temperature range from 800 to 500 °C corresponding to t8/5 times of 30, 50, and 120 s. The peak temperature of 1350 °C was employed to produce coarse austenite grain size. These coarse grains are in favor of assisting the intragranular nucleation of acicular ferrite (Thewlis, 2004). A dilatometer was also used to measure the change in sample diameter to represent the transformation behavior during cooling. To preserve partial bainite transformation microstructures during cooling, the welding thermal cycles were interrupted by water quenching when the temperature reached to the target temperature that is fixed based on the CCT
3. Results and discussion 3.1. Microstructure and the effect of inclusions According to the literature (Krauss and Thompson, 1995; Thewlis, 2004; Bhadeshia, 2015; Koo et al., 2003), bainitic ferrite is defined as the microstructure consisting of many highly organized and aligned ferritic crystals together with the parallel and elongated second phases such as martensite-austenite (MA) constituents that decorates along the lath boundaries; granular ferrite is characterized by a featureless ferritic matrix (sometimes coarse equiaxed morphology) with dispersions of granular MA constituents. Fig. 1 shows the microstructure of the sample after subjected to complete welding thermal cycles with different cooling times. It is obvious that the prior austenite grain boundaries are conserved in each specimens (signified with white arrows), implying that all product phases are confined to the prior austenite grains in which they grow. This phenomenon is sometimes regarded as a feature of shear transformation (Bhadeshia, 1999). The main microstructure 2
Micron 131 (2020) 102828
L. Lan and G. Shao
Fig. 2. EPM map scanning analysis on the composition distribution of complex inclusion.
inclusions have similar compositions, which are mainly attributed to combined addition of Mg and Ti into steel, and they would be expected to have an assisting effect on the intragranular nucleation of ferrite (Zhang et al., 2010; Sarma et al., 2009). However, this effect in the present study is very insignificant based on the microstructural observation (Fig. 1) and statistical analysis on the inclusions (see below). The size distribution of inclusions was statistically analyzed based on dozens of random fields (Fig. 3a). The maximum size of complex inclusions can reach about 6 μm, dominated by those in the range from about 1–2 μm. Both of these values are relatively larger than those for the inclusions formed in weld metal (Lan et al., 2016). However, only a few of inclusions with large size (about 2∼6 μm) have a promotion effect on the formation of intragranular bainitic ferrite. Fig. 3b and c present other two typical examples although these intragranular ferritic laths are not well-developed in the matrix. Most of inclusions are consumed by the growth of grain boundary bainitic ferrite. Zhang et al. (2016) also found that only inclusions with a large size of ∼5 μm can serve as heterogeneous nucleation sites for acicular ferrite. This means that the major role of complex inclusions on intragranular nucleation is still dependent on the reduction in interfacial energy by providing the inert substrate for ferrite nucleation. Here, microstructural refinement through intragranular nucleation is very insignificant because a very few amount of intragranular bainitic ferrite forms attached to these complex inclusions. It is probably attributed to high hardenability of
changes from bainitic ferrite to granular bainite with the increase in cooling time as the carbides or MA constituents are parallel to each other in each a packet (Fig. 1a), while in Fig. 1c these second phases do show more random distribution and only few amount of them still have a paralleling arrangement in the matrix. Meanwhile, it is expected to find that some black inclusions can be occasionally present in the matrix. However, most of them (marked with square grids) seem to not have any assisting effect on the nucleation of ferritic laths because they are located at the growth stage of ferritic laths shown by black arrow in Fig. 1a. By contrast, very few seem to assist the intragranular nucleation of ferritic laths, as circled in Fig. 1b. Close observation on these intragranular ferrite, it is more likely to be bainitic ferrite morphology rather than acicular ferrite because some of aligned carbides appear inside these ferritic crystals (Abson, 2018).
3.2. Composition and size distribution of the inclusions Using EPM micro-area scanning analysis, Fig. 2 represents the distribution of alloy elements on the inclusion of Fig. 1b. It can be found that the main elements of the inclusion are Al, Ca, Mn, Ti, O, S, and Mg. Their distribution in two-dimensional plane shows that it has a core/ shell composite structure with Al-Ca-Mg-Ti oxides at the core and MnS at the shell (although Mn distribution does not represent). Other 3
Micron 131 (2020) 102828
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Fig. 3. Size distribution of complex inclusions (a) and other examples of inclusions (b and c) assisting the intragranular nucleation of ferritic laths.
Nishiyama-Wassermann relationship) with parent phase in low carbon steel (Bhadeshia, 2015; Lan et al., 2015; Morito et al., 2015). Close observation on the nucleation sites marked with red arrows in Fig. 4a, it is interesting to find that two different bainitic packets nucleate at the same or neighboring sites and then grow with different directions. This nucleation mode is different from any mode during isothermal transformation described by Yin et al. (2017). This may be largely attributed to the welding cooling mode that continuously increasing transformation driving force promotes the nucleation of different variants at the same nucleation sites (probably including sympathetic nucleation). With further cooling, partial bainite microstructure exhibits a feature of interlocked growth for two groups of morphologically parallel ferritic laths (one group is arrowed in Fig. 4b). This arrangement is commonly observed using in-situ laser confocal scanning microscope (Terasaki and Komizo, 2013; Mao et al., 2017) and it is deemed as a result of the selfaccommodation of transformation strain to reduce the stored energy
this steel (Gan et al., 2019) and the transformation start temperature is so low (∼545 °C when t8/5 is 120 s cooling time (Lan et al., 2015)) that the intragranular acicular ferrite is hard to nucleate at the inclusions. Therefore, in the following paragraphs, we only concentrated on the morphology evolution of bainite that was nucleated on the prior austenite grain boundaries. 3.3. Morphologies of partially transformed microstructure Fig. 4. shows the partial bainite transformation microstructure with t8/5 cooling time of 30 s and water quenching temperature of 480 and 450 °C. Majorities of newly formed bainite directly nucleate on the prior grain boundaries and exhibit a feature of fine lath structure (Fig. 4a). Each of prior austenite grains seems to be broken into four different bainite packets based on their habit planes because the products have a specific orientation relationship (Kurdjumov-Sachs or
Fig. 4. Partially transformed bainite microstructure with t8/5 cooling time of 30 s and water quenching temperature of 480 °C (a), where the numbers 1–4 largely indicate four different bainite packets newly formed in one prior austenite grain before quenching, and (b) quenching temperature is 450 °C. 4
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Fig. 5. Partial bainite transformation microstructure with t8/5 cooling time of 120 s and different interrupted temperatures by water quenching: 540 °C (a), 500 °C (b), and 470 °C (c).
to the final microstructure (Fig. 1c) that the granular bainite is predominant. From these sequential images formed during a single processing route, it is ascertained that the morphology of granular bainite, in nature, is derived from the lath structure at the later stage of phase transformation. It is a good explanation that the granular bainite is supposed to occur only in steels that have been cooled continuously, and it cannot be produced by isothermal transformation (Bhadeshia, 2015). Lambert-Perlade et al. (2004) also found that the bainite transformation appears to occur in two stages: in the first stage, highly intricate and straight groups of ferritic laths form; the second stage is thickening of these groups. In combination with the present results, the ferritic lath structures have become somewhat obscure after the formation of interlocked packets. The lath structure will disappear if the MA constituents formed no longer decorate the lath boundaries and the distribution of MA constituents will be discrete due to shrinkage of residual austenite from different direction, leading to the thickening of ferritic laths. The partial bainite microstructure of Fig. 5c was selected to reveal its intricate structure under TEM observation (Fig. 6). Several elongated MA constituents depict the position of lath boundaries (arrowed in Fig. 6a) although these boundaries seem to be indistinguishable due to low misorientation angle between them (Lambert-Perlade et al., 2004). The feature of lath structure is clearer under a low magnification (Fig. 6b). Meanwhile, it is more frequently observed that numerous massive MA constituents randomly distribute in the featureless ferritic matrix (Fig. 6c), which agrees with the definition of granular bainite (Krauss and Thompson, 1995). According to our previous study (Lan et al., 2019), these MA constituents with high carbon enrichment always have an intricate structure of high dislocation density martensite with some of twin martensite, leading to much higher hardness than the matrix. The featureless ferritic matrix also contains relatively high
(Lambert-Perlade et al., 2004). When the specimen was subjected to the heat treatment with t8/5 cooling time of 120 s and water quenching at 540 °C, a very few amount of bainite microstructure has occurred (Fig. 5a) as the quenching temperature is close to the transformation start temperature (Lan et al., 2018). The morphology of newly formed bainite is characterized by lath structure although the ferrite at the periphery of nucleation sites is more likely to be polygonal (or massive) ferrite (Krauss and Thompson, 1995), as circled area. Ohmori et al. (1994) proposed that the ferritic laths formed on the pre-existing grain boundary polygonal ferrite is regarded as Widmanstätten ferrite rather than bainitic ferrite. Here, although we do not distinguish these two morphologies, it is sure that these ferritic lath boundaries are very straight, decorated by the elongated MA constituents and their growth direction is well defined, at least at this quenching temperature. However, this morphology is obviously opposite to the final appearance of granular bainite (Fig. 1c). With the decrease in quenching temperature to 500 °C, the volume fraction transformed is ∼40 % and the parallel lath structure is still predominant (Fig. 5b). However, in some areas (marked with dotted lines) several sets of ferritic packets are interlocked by each other and this phenomenon makes their growth direction less well defined. Compared with the MA constituents in Fig. 1c, it can be inferred that these retained austenite between these ferritic laths at 500 °C will further decompose and the lath structures will occur to tempering effect if the cooling was not interrupted by quenching. Thus, the lath structures in these pre-transformed areas will become more obscure on subsequent cooling. When the quenching temperature decreases to 470 °C, the microstructure appears to be more granular as the MA constituents become a massive shape and their distribution is more random, and the matrix also exhibits irregular-shape ferrite (Fig. 5c). This morphology is similar 5
Micron 131 (2020) 102828
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Fig. 6. TEM images showing the intricate microstructure after the specimen was subjected to partial welding thermal cycle with t8/5 cooling time of 120 s and interrupted temperature by water quenching is 470 °C (a) elongated MA constituents, (b) lath structure, (c) blocky MA constituents distributed on the featureless ferritic matrix, (d) dislocation structures in ferritic matrix.
density dislocations (Fig. 6d) due to lower transformation temperature at the later stage of the transformation. These intricate structures correspond to two morphologies of bainite as stated above. Thus, although granular bainite and bainitic ferrite have some difference in the morphology and crystallography (Pereloma et al., 2014), from a morphological evolution point of view, they are actually regarded as the same middle temperature transformation product that only represents at the different stage of the transformation.
Declaration of Competing Interest
4. Conclusions
References
The morphological evolution of microstructure during simulation welding thermal cycle was investigated in detail for a low carbon multimicroalloyed steel. The main microstructural morphology changed from bainitic ferrite to granular bainite with the increase in cooling time (from 30 s to 120 s) after the specimens underwent a complete welding thermal cycle. However, the lath structures of ferrite were first formed at the beginning stage of the transformation under any an experimental cooling condition. More specifically, the granular bainite in final microstructure is actually derived from the lath structures and the feature of “granular” is predominant after the interlocked ferritic laths are thickened. In addition, although some complex Ti-Mg oxide inclusions can be found in the matrix, they do not exhibit a notable promotion effect on the intragranular nucleation of ferrite.
Abson, D.J., 2018. Acicular ferrite and bainite in C-Mn and low-alloy steel arc weld metals. Sci. Technol. Weld. Joining 23, 635–648. Bhadeshia, H.K.D.H., 1999. The bainite transformation: unresolved issues. Mater. Sci. Eng. A 273–275, 54–56. Bhadeshia, H.K.D.H., 2015. Bainite in Steels, 3rd ed. Maney Publishing. Farrar, R.A., Harrison, P.L., 1987. Acicular ferrite in carbon-manganese weld metals: an overview. J. Mater. Sci. 22, 3812–3820. Gan, X.L., Wan, X.L., Zhan, Y.J., Wang, H.H., Li, G.Q., Xu, G., Wu, K.M., 2019. Investigation of characteristic and evolution of fine-grained bainitic microstructure in the coarse-grained heat-affected zone of super-high strength steel for offshore structure. Mater. Charact. 157, 109893. Grong, O., Matlock, D.K., 1986. Microstructural development in mild and low-alloy steel weld metals. Int. Metals Rev. 31, 27–48. Kim, Y.M., Lee, H., Kim, N.J., 2008. Transformation behavior and microstructural characteristics of acicular ferrite in linepipe steels. Mater. Sci. Eng. A 478, 361–370. Koo, J.Y., Luton, M.J., Bangaru, N.V., Petkovic, R.A., Fairchild, D.P., Petersen, C.W., et al., 2003. Metallurgical design of ultra-high strength steels for gas pipelines. In: Proceedings of The Thirteenth International Offshore and Polar Engineering Conference. Honolulu, Hawaii, USA, May 25–30.
The authors declare that they have no conflict of interest. Acknowledgement This work is supported by the National Natural Science Foundation of China (No. 51605084) and the Fundamental Research Funds for the Central Universities of China (N170304019 and N170308028).
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martensite and bainite. Mater. Today Proc. 2S, S913–S916. Ohmori, Y., Ohtsubo, H., Jung, Y.C., et al., 1994. Morphology of bainite and widmanstätten ferrite. Metall. Mater. Trans. A 25A, 1981–1990. Pereloma, E.V., Timokhina, I.B., Hodgson, P.D., 1999. Transformation behavior in thermomechanically processed C-Mn-Si TRIP steels with and without Nb. Mater. Sci. Eng. A 273–275, 448–452. Pereloma, E.V., Ai-Harbi, F., Gazder, A.A., 2014. The crystallography of carbide-free bainites in thermos-mechanically processed low Si transformation-induced plasticity steels. J. Alloys Comp. 615, 96–110. Sarma, D.S., Karasev, A.V., Jonsson, P.G., 2009. On the role of non-metallic inclusions in the nucleation of acicular ferrite in steels. ISIJ Int. 49, 1063–1074. Terasaki, H., Komizo, Y.I., 2013. Morphology and crystallography of bainite transformation in a single prior-austenite grain of low-carbon steel. Metall. Mater. Trans. A 44, 2683–2689. Thewlis, G., 2004. Classification and quantification of microstructures in steels. Mater. Sci. Technol. 20, 143–160. Yin, J.Q., Hillert, M., Borgenstam, A., 2017. Morphology of proeutectoid ferrite. Metall. Mater. Trans. A 48A, 1425–1443. Zhang, D., Terasaki, H., Komizo, Y.I., 2010. In situ observation of the formation of intragranular acicular ferrite at non-metallic inclusions in C-Mn steel. Acta Mater. 58, 1369–1378. Zhang, J., Feng, P.H., Pan, Y.C., Hwang, W.S., Su, Y.H., Lu, M.J., 2016. Effects of heat treatment on the microstructure and mechanical properties of low-carbon steel with magnesium-based inclusions. Metall. Mater. Trans. A 47A, 5049–5057. Zhao, H., Wynne, B.P., Palmiere, E.J., 2018. Conditions for the occurrence of acicular ferrite transformation in HSLA steels. J. Mater. Sci. 53, 3785–3804.
Krauss, G., Thompson, S.W., 1995. Ferritic microstructures in continuously cooled lowand ultralow- carbon steels. ISIJ Int. 35, 937–945. Kumar, S., Nath, S.K., Kumar, V., 2016. Continuous cooling transformation behavior in the weld coarse grained heat affected zone and mechanical properties of Nb-microalloyed and HY85 steels. Mater. Des. 90, 177–184. Lambert-Perlade, A., Gourgues, A.F., Pineau, A., 2004. Austenite to bainite phase transformation in the heat-affected zone of a high strength low alloy steel. Acta Mater. 52, 2337–2348. Lan, L.Y., Kong, X.W., 2018. Transformation stasis phenomenon of bainite formation in low-carbon, multicomponent alloyed steel. JOM 70, 666–671. Lan, L.Y., Kong, X.W., Qiu, C.L., 2015. Characterization of coarse bainite transformation in low carbon steel during simulated welding thermal cycles. Mater. Charater. 105, 95–103. Lan, L.Y., Chang, Z.Y., Fan, P.H., 2018. Exploring the difference in bainite transformation with varying the prior austenite grain size in low carbon steel. Metals 8, 988. Lan, L.Y., Kong, X.W., Hu, Z.Y., Qiu, C.L., Zhao, D.W., Du, L.X., 2016. Hydrogen permeation behavior in relation to microstructural evolution of low carbon bainitic steel weldments. Corr. Sci. 112, 180–193. Lan, L.Y., Yu, M., Qiu, C.L., 2019. On the local mechanical properties of isothermally transformed bainite in low carbon steel. Mater. Sci. Eng. A 742, 442–450. Liu, C.X., Shi, L., Liu, Y.C., Li, C., Li, H.J., Guo, Q.Y., 2016. Acicular ferrite formation during isothermal holding in HSLA steel. J. Mater. Sci. 51, 3555–3563. Mao, G.J., Cao, R., Guo, X.L., Jiang, Y., Chen, J.H., 2017. In situ observation of kinetic processes of lath bainite nucleation and growth by laser scanning confocal microscope in reheated weld metals. Metall. Mater. Trans. A 48A, 5783–5798. Morito, S., Pham, A.H., Hayashi, T., Ohba, T., 2015. Block boundary analyses to identify
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Contents lists available at ScienceDirect
Micron journal homepage: www.elsevier.com/locate/micron
Morphological evolution of HAZ microstructures in low carbon steel during simulated welding thermal cycle
T
Liangyun Lana,b,*, Guoqing Shaoa a b
School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110819, China Key Laboratory of Vibration and Control of Aero-Propulsion Systems, Ministry of Education of China, Northeastern University, Shenyang 110819, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Low carbon steel HAZ microstructure Intragranular nucleation Granular bainite Bainitic ferrite
It is necessary to reveal the derivation of bainite microstructures with different appearance because these complex appearances are closely related to the transformation nature and their final mechanical properties. In this work, the morphological evolution of HAZ microstructures during simulated welding thermal cycle was investigated in a low carbon steel that was intentionally treated with combined addition of Ti and Mg elements. The inclusions present in the matrix have a complex core/shell composite structure composed of Al-Ca-Mg-Ti-O and MnS. However, only very few inclusions with large size (about 2∼6μm) can stimulate the intragranular nucleation of ferritic laths. Thus, this behavior did not show an obvious refinement effect on HAZ microstructure. Partial bainite microstructure revealed the ferritic lath structures always first form at the beginning of the transformation irrespective of the final morphologies of microstructure (granular bainite or bainitic ferrite), implying that the granular bainite is actually derived from the ferritic lath structures although these lath structures will become gradually obscure at the later stages of the transformation.
1. Introduction For solid-state phase transformation in steels and their welds, middle temperature transformation microstructures (collectively called bainite) exhibit a variety of appearances when viewed in the optical microscope. These appearances are not only related to the chemical compositions (including non-metallic inclusions) but also depend on heating and cooling schedules (Grong and Matlock, 1986; Ohmori et al., 1994; Abson, 2018; Kumar et al., 2016). There are several different classifications on bainite microstructure to depict its various appearances with different terminologies (Grong and Matlock, 1986; Krauss and Thompson, 1995; Thewlis, 2004). Grong and Matlock, 1986 considered that the microstructure formed in weld metal will be arranged in approximately decreasing order of transformation temperature: grain boundary ferrite, polygonal ferrite, Widmanstättan ferrite, acicular ferrite, upper bainite, lower bainite and martensite. Krauss and Thompson (1995) reviewed various bainite and ferrite classification systems and showed about 9 different terminologies to identify microstructural constituents formed during continuous cooling process for modern low-carbon high-strength steels. Thewlis (2004) even proposed the identification of 19 different microstructural constituents, with a flow diagram for both grain-boundary-nucleated and intragranually nucleated transformation products. ⁎
Because the bainite classification systems are not unified, some of the terms used are still in dispute. For example, acicular ferrite is expected for the first time to indicate the intergranular ferrite nucleated on the inclusions in the weld metal (Farrar and Harrison, 1987). However, it is often confused with bainitic ferrite that is formed below bainite start temperature, as reviewed by Abson (2018). This term is also widely used to describe the small non-aligned ferrite formed within prior austenite grains for hot-rolled steels (Kim et al., 2008; Pereloma et al., 1999; Zhao et al., 2018; Liu et al., 2016). Recently, Yin et al. (2017) showed that no morphological difference was found between Widmanstätten ferrite and bainitic ferrite at the early stage of the transformation, although their formation is traditionally regarded by different transformation mechanisms (Bhadeshia, 2015). Pereloma et al. (2014) substantiated that the difference between the bainitic ferrite and granular bainite exists not only in the morphology but also in the crystallography even if they are formed in the same steel. These disputes may be partly derived from the fact that the microstructural identification is only based on the final morphology of bainite. In fact, transformation behaviors change with decreasing temperature and/or holding time, which leads to continuous change in morphology of products during transformation within the scale of individual prior austenite grains. Our previous works showed the crystallography and kinetics of bainite microstructure vary with the
Corresponding author at: School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110819, China. E-mail address: [email protected] (L. Lan).
https://doi.org/10.1016/j.micron.2020.102828 Received 19 November 2019; Received in revised form 13 January 2020; Accepted 13 January 2020 Available online 15 January 2020 0968-4328/ © 2020 Elsevier Ltd. All rights reserved.
Micron 131 (2020) 102828
L. Lan and G. Shao
Fig. 1. Microstructural morphology with different t8/5 cooling times: (a) 30 s, (b) 50 s, (c) 120 s (BF: bainitic ferrite; GB: granular bainite; IBF: intragranular bainite ferrite; white arrows showing the prior austenite grain boundaries, the inclusions without assisting intragranular nucleation are marked with grids and the circled inclusion seems to stimulate the intragranular nucleation for several ferritic laths).
diagram that has been published in Ref. Lan et al. (2018). Metallographic samples were taken from the cross-section of heat treatment regions and examined using an optical microscope after these samples were subjected to conventional preparation, such as manual and mechanical polishing, 3 % nital etching, and hot air drying. As microstructure inhomogeneity can be produced by temperature difference across the cross-section or the surface energy near to surface regions of specimens, the region of interest for microstructure observation was focused on the center of specimens (about 4 mm in diameter). To reveal the complex composition of non-metallic inclusions in two-dimensional observation plane, micro-area scanning analysis on distribution of elements was conducted using a field-emission electron probe microscope (EPM). Besides, a field-emission transmission electron microscope (TEM) was employed to display the refinement structures of microstructure. TEM foils, discs 3 mm in diameter, were mainly cut from the partial bainite transformation specimens and ground to a thickness of about 0.05 mm, and then twin-jet electropolished at a temperature of −30 °C with a voltage of 35 V.
progress of transformation (Lan et al., 2015; Lan and Kong, 2018). However, the evolution of morphology of bainite was not analyzed in detail although some partial bainite transformation microstructures have been represented elsewhere (Lan et al., 2015; Lan and Kong, 2018; Lan et al., 2018). Here, the main focus is on the morphological evolution of bainite in the heat-affected zone (HAZ) of low carbon steel with the progress of phase transformation and the derivation of bainite with different appearances is revealed in detail. Because the steel was just subjected to high temperature welding thermal cycles but not melting, the terminologies by Krauss and Thompson (1995) will be mainly adopted. 2. Experimental methods The present work was carried out on an industrially manufactured plate of HSLA steel and its chemical composition is 0.053 C, 1.64 Mn, 0.22 Si, 1.02 (Cr + Ni + Mo), 0.06 (Nb + V), 0.015Al, 0.025 Ti, 0.0019 Ca, 0.002 Mg, and 0.0012B (wt.%) balanced by Fe. The aims of Ti and Mg added into steel are to form complex non-metallic inclusions that can stimulate the intragranular nucleation of acicular ferrite (Zhang et al., 2010). Specimens having cylindrical form (φ 6 mm × 55 mm) were cut from the steel plate and examined in a thermo- mechanical simulation tester. Temperatures were measured using very thin thermocouples (0.1 mm wires) spot-welded in the middle of the specimens and these were used to program the welding thermal cycles. In terms of a two dimensional Rykalin mathematical mode, the thermal cycles involved heating at 130 °C /s to 1350 °C, holding for 2 s and then cooling at prescribed rates through the temperature range from 800 to 500 °C corresponding to t8/5 times of 30, 50, and 120 s. The peak temperature of 1350 °C was employed to produce coarse austenite grain size. These coarse grains are in favor of assisting the intragranular nucleation of acicular ferrite (Thewlis, 2004). A dilatometer was also used to measure the change in sample diameter to represent the transformation behavior during cooling. To preserve partial bainite transformation microstructures during cooling, the welding thermal cycles were interrupted by water quenching when the temperature reached to the target temperature that is fixed based on the CCT
3. Results and discussion 3.1. Microstructure and the effect of inclusions According to the literature (Krauss and Thompson, 1995; Thewlis, 2004; Bhadeshia, 2015; Koo et al., 2003), bainitic ferrite is defined as the microstructure consisting of many highly organized and aligned ferritic crystals together with the parallel and elongated second phases such as martensite-austenite (MA) constituents that decorates along the lath boundaries; granular ferrite is characterized by a featureless ferritic matrix (sometimes coarse equiaxed morphology) with dispersions of granular MA constituents. Fig. 1 shows the microstructure of the sample after subjected to complete welding thermal cycles with different cooling times. It is obvious that the prior austenite grain boundaries are conserved in each specimens (signified with white arrows), implying that all product phases are confined to the prior austenite grains in which they grow. This phenomenon is sometimes regarded as a feature of shear transformation (Bhadeshia, 1999). The main microstructure 2
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Fig. 2. EPM map scanning analysis on the composition distribution of complex inclusion.
inclusions have similar compositions, which are mainly attributed to combined addition of Mg and Ti into steel, and they would be expected to have an assisting effect on the intragranular nucleation of ferrite (Zhang et al., 2010; Sarma et al., 2009). However, this effect in the present study is very insignificant based on the microstructural observation (Fig. 1) and statistical analysis on the inclusions (see below). The size distribution of inclusions was statistically analyzed based on dozens of random fields (Fig. 3a). The maximum size of complex inclusions can reach about 6 μm, dominated by those in the range from about 1–2 μm. Both of these values are relatively larger than those for the inclusions formed in weld metal (Lan et al., 2016). However, only a few of inclusions with large size (about 2∼6 μm) have a promotion effect on the formation of intragranular bainitic ferrite. Fig. 3b and c present other two typical examples although these intragranular ferritic laths are not well-developed in the matrix. Most of inclusions are consumed by the growth of grain boundary bainitic ferrite. Zhang et al. (2016) also found that only inclusions with a large size of ∼5 μm can serve as heterogeneous nucleation sites for acicular ferrite. This means that the major role of complex inclusions on intragranular nucleation is still dependent on the reduction in interfacial energy by providing the inert substrate for ferrite nucleation. Here, microstructural refinement through intragranular nucleation is very insignificant because a very few amount of intragranular bainitic ferrite forms attached to these complex inclusions. It is probably attributed to high hardenability of
changes from bainitic ferrite to granular bainite with the increase in cooling time as the carbides or MA constituents are parallel to each other in each a packet (Fig. 1a), while in Fig. 1c these second phases do show more random distribution and only few amount of them still have a paralleling arrangement in the matrix. Meanwhile, it is expected to find that some black inclusions can be occasionally present in the matrix. However, most of them (marked with square grids) seem to not have any assisting effect on the nucleation of ferritic laths because they are located at the growth stage of ferritic laths shown by black arrow in Fig. 1a. By contrast, very few seem to assist the intragranular nucleation of ferritic laths, as circled in Fig. 1b. Close observation on these intragranular ferrite, it is more likely to be bainitic ferrite morphology rather than acicular ferrite because some of aligned carbides appear inside these ferritic crystals (Abson, 2018).
3.2. Composition and size distribution of the inclusions Using EPM micro-area scanning analysis, Fig. 2 represents the distribution of alloy elements on the inclusion of Fig. 1b. It can be found that the main elements of the inclusion are Al, Ca, Mn, Ti, O, S, and Mg. Their distribution in two-dimensional plane shows that it has a core/ shell composite structure with Al-Ca-Mg-Ti oxides at the core and MnS at the shell (although Mn distribution does not represent). Other 3
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Fig. 3. Size distribution of complex inclusions (a) and other examples of inclusions (b and c) assisting the intragranular nucleation of ferritic laths.
Nishiyama-Wassermann relationship) with parent phase in low carbon steel (Bhadeshia, 2015; Lan et al., 2015; Morito et al., 2015). Close observation on the nucleation sites marked with red arrows in Fig. 4a, it is interesting to find that two different bainitic packets nucleate at the same or neighboring sites and then grow with different directions. This nucleation mode is different from any mode during isothermal transformation described by Yin et al. (2017). This may be largely attributed to the welding cooling mode that continuously increasing transformation driving force promotes the nucleation of different variants at the same nucleation sites (probably including sympathetic nucleation). With further cooling, partial bainite microstructure exhibits a feature of interlocked growth for two groups of morphologically parallel ferritic laths (one group is arrowed in Fig. 4b). This arrangement is commonly observed using in-situ laser confocal scanning microscope (Terasaki and Komizo, 2013; Mao et al., 2017) and it is deemed as a result of the selfaccommodation of transformation strain to reduce the stored energy
this steel (Gan et al., 2019) and the transformation start temperature is so low (∼545 °C when t8/5 is 120 s cooling time (Lan et al., 2015)) that the intragranular acicular ferrite is hard to nucleate at the inclusions. Therefore, in the following paragraphs, we only concentrated on the morphology evolution of bainite that was nucleated on the prior austenite grain boundaries. 3.3. Morphologies of partially transformed microstructure Fig. 4. shows the partial bainite transformation microstructure with t8/5 cooling time of 30 s and water quenching temperature of 480 and 450 °C. Majorities of newly formed bainite directly nucleate on the prior grain boundaries and exhibit a feature of fine lath structure (Fig. 4a). Each of prior austenite grains seems to be broken into four different bainite packets based on their habit planes because the products have a specific orientation relationship (Kurdjumov-Sachs or
Fig. 4. Partially transformed bainite microstructure with t8/5 cooling time of 30 s and water quenching temperature of 480 °C (a), where the numbers 1–4 largely indicate four different bainite packets newly formed in one prior austenite grain before quenching, and (b) quenching temperature is 450 °C. 4
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Fig. 5. Partial bainite transformation microstructure with t8/5 cooling time of 120 s and different interrupted temperatures by water quenching: 540 °C (a), 500 °C (b), and 470 °C (c).
to the final microstructure (Fig. 1c) that the granular bainite is predominant. From these sequential images formed during a single processing route, it is ascertained that the morphology of granular bainite, in nature, is derived from the lath structure at the later stage of phase transformation. It is a good explanation that the granular bainite is supposed to occur only in steels that have been cooled continuously, and it cannot be produced by isothermal transformation (Bhadeshia, 2015). Lambert-Perlade et al. (2004) also found that the bainite transformation appears to occur in two stages: in the first stage, highly intricate and straight groups of ferritic laths form; the second stage is thickening of these groups. In combination with the present results, the ferritic lath structures have become somewhat obscure after the formation of interlocked packets. The lath structure will disappear if the MA constituents formed no longer decorate the lath boundaries and the distribution of MA constituents will be discrete due to shrinkage of residual austenite from different direction, leading to the thickening of ferritic laths. The partial bainite microstructure of Fig. 5c was selected to reveal its intricate structure under TEM observation (Fig. 6). Several elongated MA constituents depict the position of lath boundaries (arrowed in Fig. 6a) although these boundaries seem to be indistinguishable due to low misorientation angle between them (Lambert-Perlade et al., 2004). The feature of lath structure is clearer under a low magnification (Fig. 6b). Meanwhile, it is more frequently observed that numerous massive MA constituents randomly distribute in the featureless ferritic matrix (Fig. 6c), which agrees with the definition of granular bainite (Krauss and Thompson, 1995). According to our previous study (Lan et al., 2019), these MA constituents with high carbon enrichment always have an intricate structure of high dislocation density martensite with some of twin martensite, leading to much higher hardness than the matrix. The featureless ferritic matrix also contains relatively high
(Lambert-Perlade et al., 2004). When the specimen was subjected to the heat treatment with t8/5 cooling time of 120 s and water quenching at 540 °C, a very few amount of bainite microstructure has occurred (Fig. 5a) as the quenching temperature is close to the transformation start temperature (Lan et al., 2018). The morphology of newly formed bainite is characterized by lath structure although the ferrite at the periphery of nucleation sites is more likely to be polygonal (or massive) ferrite (Krauss and Thompson, 1995), as circled area. Ohmori et al. (1994) proposed that the ferritic laths formed on the pre-existing grain boundary polygonal ferrite is regarded as Widmanstätten ferrite rather than bainitic ferrite. Here, although we do not distinguish these two morphologies, it is sure that these ferritic lath boundaries are very straight, decorated by the elongated MA constituents and their growth direction is well defined, at least at this quenching temperature. However, this morphology is obviously opposite to the final appearance of granular bainite (Fig. 1c). With the decrease in quenching temperature to 500 °C, the volume fraction transformed is ∼40 % and the parallel lath structure is still predominant (Fig. 5b). However, in some areas (marked with dotted lines) several sets of ferritic packets are interlocked by each other and this phenomenon makes their growth direction less well defined. Compared with the MA constituents in Fig. 1c, it can be inferred that these retained austenite between these ferritic laths at 500 °C will further decompose and the lath structures will occur to tempering effect if the cooling was not interrupted by quenching. Thus, the lath structures in these pre-transformed areas will become more obscure on subsequent cooling. When the quenching temperature decreases to 470 °C, the microstructure appears to be more granular as the MA constituents become a massive shape and their distribution is more random, and the matrix also exhibits irregular-shape ferrite (Fig. 5c). This morphology is similar 5
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Fig. 6. TEM images showing the intricate microstructure after the specimen was subjected to partial welding thermal cycle with t8/5 cooling time of 120 s and interrupted temperature by water quenching is 470 °C (a) elongated MA constituents, (b) lath structure, (c) blocky MA constituents distributed on the featureless ferritic matrix, (d) dislocation structures in ferritic matrix.
density dislocations (Fig. 6d) due to lower transformation temperature at the later stage of the transformation. These intricate structures correspond to two morphologies of bainite as stated above. Thus, although granular bainite and bainitic ferrite have some difference in the morphology and crystallography (Pereloma et al., 2014), from a morphological evolution point of view, they are actually regarded as the same middle temperature transformation product that only represents at the different stage of the transformation.
Declaration of Competing Interest
4. Conclusions
References
The morphological evolution of microstructure during simulation welding thermal cycle was investigated in detail for a low carbon multimicroalloyed steel. The main microstructural morphology changed from bainitic ferrite to granular bainite with the increase in cooling time (from 30 s to 120 s) after the specimens underwent a complete welding thermal cycle. However, the lath structures of ferrite were first formed at the beginning stage of the transformation under any an experimental cooling condition. More specifically, the granular bainite in final microstructure is actually derived from the lath structures and the feature of “granular” is predominant after the interlocked ferritic laths are thickened. In addition, although some complex Ti-Mg oxide inclusions can be found in the matrix, they do not exhibit a notable promotion effect on the intragranular nucleation of ferrite.
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The authors declare that they have no conflict of interest. Acknowledgement This work is supported by the National Natural Science Foundation of China (No. 51605084) and the Fundamental Research Funds for the Central Universities of China (N170304019 and N170308028).
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