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Elemental distribution in the martensite–austenite constituent in intercritically reheated coarse-grained heat-affected zone of a high-strength pipeline steel
Scripta Materialia 139 (2017) 67–70
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular article
Elemental distribution in the martensite–austenite constituent in intercritically reheated coarse-grained heat-affected zone of a high-strength pipeline steel Xueda Li a,⁎, Chengjia Shang b,⁎, Xiaoping Ma c, Baptiste Gault d, S.V. Subramanian c, Jianbo Sun a, R.D.K. Misra e a
College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, China c Department of Materials Science and Engineering, McMaster University, Hamilton, Canada d Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany e Department of Metallurgical, Materials and Biomedical Engineering, University of Texas at El Paso, El Paso, TX, USA b
a r t i c l e
i n f o
Article history: Received 3 May 2017 Received in revised form 13 June 2017 Accepted 13 June 2017 Available online xxxx Keywords: Atom probe tomography Elemental distribution M-A constituent Heat-affected zone C/Mn enrichment
a b s t r a c t We have studied here the distribution of carbon and alloying elements in the martensite-austenite (M-A) constituent in intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) of a high-strength pipeline steel using atom probe tomography (APT). Notable enrichment of C (0.49 wt%) and Mn (2.32 wt%) was observed within the M-A constituent, which induced the formation of lath martensite and deteriorated the toughness. Elemental segregation in the interfacial region between M-A constituent and matrix may contribute to the debonding mechanism of M-A constituent and assist nucleation of cleavage cracks. Distribution of solute Nb indicated no apparent difference between the matrix and M-A constituent. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Coarse-grained HAZ (CGHAZ) and ICCGHAZ are regions that can induce poor toughness in the HAZ of traditional structural steels [1]. However, in modern high-strength low alloy (HSLA) steels, the microstructure of the CGHAZ generally consists of upper bainite or granular bainite that has acceptable toughness within a wide range of heat input [2,3]. Additionally, martensite is not likely to form in the HAZ because of its low carbon equivalent. Thus, it is believed that abnormal distribution of the M-A constituent in the HAZ, especially in the ICCGHAZ, is the main factor that deteriorates the toughness [3–8]. Large blocky M-A constituent can provide nucleation sites for cleavage cracks [9], whereas near-connected or necklace-type M-A constituent along the prior austenite grain boundaries (PAGBs) can change the cleavage fracture propagation mechanism [10] and further deteriorate the toughness. The internal structure of the M-A constituent in the HAZ has also been studied, and twinned martensite was frequently observed [5,11,12]. However, there is a need for quantitative understanding in terms of elemental composition and distribution, in particular for carbon, within the M-A constituent, because it determines subsequent phase transformation, internal structure and hardness of the M-A constituent [13]. A few studies on the chemical composition of M-A constituent using electron probe micro-analyzer (EPMA) were reported [11,12], but the carbon concentration obtained by EPMA lacks ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Li), [email protected] (C. Shang).
http://dx.doi.org/10.1016/j.scriptamat.2017.06.017 1359-6462/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
accuracy. APT is considered to be an accurate technique to determine the elemental content at the atomic scale [14]. In the present study, the composition and distribution of carbon and alloying elements in the MA constituent and its surrounding matrix in the ICCGHAZ of an X100 (690 MPa) pipeline steel was studied using APT and electron microscopy, and its influence on the toughness was discussed. The bulk composition (wt%) of the experimental ×100 pipeline steel was Fe–0.07C–0.25Si–1.94Mn–0.081Nb–0.28Cr–0.26Mo–0.014Ti. A 14.7 mm thick plate was made into pipe of 1219 mm outer diameter. Double pass submerged arc welding was applied with 30 kJ/cm heat input in each pass. The weld joint was cut from the as-welded pipe for experimental study (Fig. 1a). Several sub-zones, i.e. CGHAZ, fine-grained HAZ (FGHAZ), intercritically reheated HAZ (ICHAZ) and ICCGHAZ, were formed due to the heat effect of welding passes. Our previous study under identical conditions indicated that necklace-type M-A constituent in the ICCGHAZ induced lowest toughness (b50 J) among several HAZs of ×100 pipeline steel weld [3]. Microstructure of the ICCGHAZ was characterized by light microscope (LM) and scanning electron microscope (SEM), and the results are presented in Fig. 1b–c. The matrix microstructure of ICCGHAZ consisted of upper bainite and granular bainite. Necklace-type M-A constituent was formed along the PAGBs. Transmission electron microscope (TEM) studies (Fig. 1 in the Supplementary material) indicated that the M-A constituent in the ICCGHAZ primarily consisted of lath martensite, with small fraction of retained austenite
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X. Li et al. / Scripta Materialia 139 (2017) 67–70
Fig. 1. (a) ×100 pipeline steel weld joint, (b) optical micrograph of the ICCGHAZ, and (c) SEM micrograph of the ICCGHAZ.
between the martensite laths. No twinned martensite or ε-martensite was found in the present case. Quantitative study on the chemical composition of M-A constituent in the ICCGHAZ is necessary to improve the understanding of the determining role of M-A constituent on toughness. The specimens for APT were fabricated using a site-specific scanning electron microscope/focused ion beam (SEM/FIB, Zeiss NVision 40) instrument. The bulk sample was slightly etched using 4% nital to outline necklace-type M-A constituent. One M-A particle (marked in Fig. 1c) was cut in-situ and then milled to needle-like with tip size below 50 nm. APT analyses were performed on a Cameca LEAP 4000X HR, at a base temperature of 55 K in laser pulsing mode (λ = 355 nm), with pulse energy of 60 pJ and pulse repetition rate of 250 kHz. The detection rate was maintained at 4 ions per 1000 pulses on average. Reconstruction and quantitative evaluation of APT datasets were performed using software package IVAS 3.6.12–14 [15]. The APT analysis results are presented in Fig. 2. Fig. 2a shows the reconstruction map of multiple elements, and Fig. 2b–d are reconstruction maps of C, Mn + Nb, Cr + Mo, respectively. The carbon map in Fig. 2b indicated that the needle went across the carbon-depleted ferrite matrix, carbon-enriched M-A constituent and their interface. The results
revealed that there was notable segregation of elements in the interfacial region including C, Mn, Cr, Mo, while within the M-A constituent, only C and Mn were enriched. There was no apparent Nb enrichment either in the interfacial region or within the M-A constituent. The composition profile of carbon and alloying elements (Fig. 2e) across ferrite matrix, M-A constituent and their interface was obtained by averaging the atomic fraction in a column perpendicular to the interface as depicted in Fig. 2a. There is a sharp interface between the ferrite matrix and M-A constituent, and the interface is defined as the half composition from the carbon peak to the carbon-depleted plateau in the ferrite matrix. In addition, there are spikes of C and Mn in the composition profile in the interfacial region, with the peak composition of 7.0 at% and 5.1 at%, respectively. In our previous study [3], we proposed that the necklace-type M-A constituent was formed during the second pass intercritical reheating of the CGHAZ, at a slightly higher temperature than the Ac1 temperature (760 °C–800 °C). Part of the matrix reverted to austenite along the PAGBs. During the formation of reverted austenite, C and Mn partitioned into austenite from the surrounding ferritic matrix, resulting in significant enrichment of C and Mn in the interfacial region [16,17]. The spikes are expected to disappear when the growth of
Fig. 2. (a–d) APT reconstruction maps of carbon and alloying elements, (e) composition profile of carbon and alloying elements across ferrite, interface and M-A constituent, and (f) variation in carbon composition within the M-A constituent (distribution of Mn and Nb is also displayed).
X. Li et al. / Scripta Materialia 139 (2017) 67–70
reverted austenite reaches equilibrium with elapsed holding time [16]. Similar results were reported by Xie et al. using APT and DICTRA simulation [18]. But considering the short time reheating and continuous cooling process during welding, the growth of reverted austenite was far from equilibrium in the present case. Therefore, the spikes of C and Mn were retained in the interfacial region. Meanwhile, the composition of Cr and Mo showed a small peak in the interfacial region, but no evidence of any significant enrichment within the M-A constituent. The composition of Si in the ferrite is slightly higher than that in the M-A constituent. The Nb peak in Fig. 2e comes from a Nb/C-rich cluster. A number of studies reported elemental segregation at the interfaces or grain boundaries, especially B, C, N, Mn, Nb, P [19–22]. Lerchbacher et al. [19] reported that up to ~ 10 at% carbon enrichment at martensite interlath boundaries led to formation of austenite thin films in a quenched martensitic steel. While, Raabe et al. [20] reported that heavy segregation of Mn (~ 24 at%) at martensite–martensite grain boundaries also induced the formation of austenite thin films in a Fe– 9 at%Mn maraging steel. The formation of austenite at the interface is beneficial to the toughness. However, Kuzmina et al. [21] reported that, after annealing at 450 °C or 600 °C for only 10–60 s, ~11 at% Mn segregation at the PAGBs induced grain boundary decohesion and fully intergranular failure, which led to lowest toughness in a Fe–9 wt%Mn steel. In the present case, average composition of C and Mn spikes in the interfacial region was calculated to be ~5.59 at% (~1.20 wt%) and ~3.96 at% (~3.90 wt%), respectively. In comparison to the aforementioned studies, ~5.59 at% C and ~3.96 at% Mn enrichment in the interfacial region may not sufficiently stabilize the reverted austenite, especially when the ferrite stabilizing elements, Cr and Mo, simultaneously segregated in the interfacial region. This aspect was confirmed by TEM studies, and no austenite phase was found in the interfacial region. Instead, the debonding of M-A constituent from the matrix was observed in the ICCGHAZ region from the fracture surface of Charpy impact test specimen of ×100 steel weld, and is presented in Fig. 2 in the Supplementary material. The debonding holes of M-A constituent provided nucleation sites for cleavage cracks. Thus, elemental segregation in the interfacial region (C, Mn, Cr, Mo) may weaken the boundary between the M-A constituent and ferrite matrix [4], promote the debonding of M-A constituent from the matrix and assist the nucleation of cleavage cracks. The average compositions of carbon and alloying elements in the M-A constituent and ferrite matrix were calculated from the dataset and are listed in Table 1. Average composition of carbon and Mn within the MA constituent were 2.30 at% (0.49 wt%) and 2.36 at% (2.32 wt%), respectively. In our previous study [8], it was calculated that carbon had enough time to diffuse into the reverted austenite during the welding thermal cycle. The present results indicated that Mn also had adequate time to get enriched in the reverted austenite in spite of fast reheating and continuous cooling process. Martensite start (MS) temperature of the M-A constituent was calculated to be 262 °C using the classic carbon equivalent equation [23]. This further confirms that majority of the reverted austenite transformed into lath martensite upon cooling, and small fraction of austenite was retained between the martensite laths. No twinned or ε-martensite were formed because of relatively low C and Mn composition.
Table 1 Average composition of carbon and alloying elements in the M-A constituent and ferrite matrix.
M-A constituent Ferrite matrix Bulk material
at% wt% at% wt% at% wt%
C
Mn
Nb
Cr
Mo
Si
2.30 0.49 0.19 0.041 0.32 0.07
2.36 2.32 1.63 1.61 1.95 1.94
0.018a 0.030a 0.019a 0.032a 0.048 0.081
0.31 0.29 0.27 0.25 0.30 0.28
0.14 0.24 0.17 0.29 0.15 0.26
0.46 0.23 0.70 0.35 0.50 0.25
Nb values marked with a are solute Nb composition excluding the highly concentrated regions using DIAM method [27].
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As indicated by the black dashed arrow in Fig. 2b, there is a carbon depletion zone in the M-A constituent. Fig. 2f shows the C/Mn/Nb composition profile within the M-A constituent, which was obtained by placing a cylinder perpendicular to the depletion zone, as depicted in Fig.2b. The results revealed large variation in C composition within the M-A constituent, while the distribution of Mn was nearly homogenous. The Nb profile confirmed that the C variation was not caused by large NbC precipitates. The carbon composition at the peak was ~ 4 at% (~0.89 wt%) and at the trough was ~0.6–1 at% (~0.13–0.22 wt%). Large variation in carbon composition within the M-A constituent provided evidence for the occurrence of carbon partitioning during the formation of M-A constituent at slow cooling rate. Supersaturated carbon in martensite diffused into retained austenite upon cooling. The carbon peak in Fig. 2f may also correspond to the retained austenite between the martensite laths. Ping et al. [24] reported similar carbon variation induced by the formation of metastable ω phase as an intermediate of {112}〈111〉–type twinning structures in an oil-quenched Fe–0.6C–1.9Si–0.8Mn (wt%) spring steel. These observations suggest that the redistribution of carbon during martensitic transformation occurred at different cooling rates. Regarding to the results from the APT study, the detrimental effect of M-A constituent on toughness in the ICCGHAZ is attributed to the following aspects. Insufficient enrichment of austenite stabilizer (C and Mn) in the reverted austenite resulted in the martensitic transformation, which produced transformation-induced residual stress in the surrounding matrix [4]. Higher hardness of M-A constituent compared to the surrounding matrix introduced stress concentration during loading [13]. In addition, when the M-A constituent was near-connected or continuously distributed, the residual stress and stress concentration were further strengthened, which could notably promote the initiation of cleavage cracks and deteriorate the toughness [4,12]. Furthermore, for the blocky type M-A constituent, debonding of M-A constituent from the matrix is more likely to occur than self-cracking of the M-A constituent [9,12]. Elemental segregation at the interface may promote the debonding of M-A constituent from the matrix and assist nucleation of cleavage cracks. The distribution of Nb in the dataset was analyzed via isoconcentration surfaces, which highlighted the regions that contained ≥1.5 at% of Nb, and is presented in Fig. 3. Isoconcentration surfaces of ≥1.5 at% C in the interfacial region are also displayed. Fig. 3b is the 180° rotated view of Fig. 3a and C atoms are ignored to clearly show the distribution of solute Nb atoms in the dataset. The results indicated that Nb preferred to segregate along dislocations (referred as ‘Nb-Cottrell atmosphere’ in [25]) or agglomerate in the form of clusters or precipitates. In most cases, carbon simultaneously segregated with Nb at dislocations, or formed Nb/C-rich clusters or NbC precipitates as indicated by the solid arrows in Fig. 2b–c. It is difficult to estimate the composition of Nb/Crich clusters or NbC precipitates due to the trajectory aberrations of APT [14,26]. However, there was no apparent difference in solute Nb between the ferrite matrix and M-A constituent, as presented in Fig. 3b and Table 1. Average solute concentration of Nb in the dataset was calculated to be 0.019 at% using the DIAM method [27] (excluding the highly concentrated regions) in comparison to the overall Nb composition of 0.044 at%. Thus, it was estimated that ~60% of the Nb atoms segregated to the dislocations along with C or formed Nb/C-rich clusters or NbC precipitates within the probed volume. The results suggested that short time thermal cycle with peak temperature of 760 °C–800 °C during the second pass welding did not render significant diffusion of Nb atoms into the MA constituent or the interfacial region. Therefore, it is reasonable to conclude that the deterioration of toughness is not attributed to the Nb segregation during welding processes. In summary, APT analysis provided specific information on the distribution of carbon and alloying elements in the M-A constituent formed in the ICCGHAZ of a high-strength pipeline steel. There was notable enrichment of C and Mn in the M-A constituent (0.49 wt% and 2.32 wt%) compared to the ferritic matrix (0.041 wt% and 1.61 wt%), which induced formation of lath martensite and deteriorated the toughness. Large variation in the carbon composition within the M-A constituent provided
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Fig. 3. (a) Nb isoconcentration surfaces at% 1.5 at along with C and Nb atoms, and (b) 180° rotation view of (a) (only Nb atoms are displayed).
evidence of carbon partitioning during the formation of the M-A constituent. Elemental segregation in the interfacial region (C, Mn, Cr, Mo) may promote the debonding of M-A constituent from the matrix and assist the nucleation of cleavage cracks. However, no apparent enrichment of Nb within the M-A constituent and in the interfacial region was noted after the welding process. Thus, the deterioration of toughness in the ICCGHAZ was attributed to the C/Mn enrichment in the M-A constituent and elemental segregation in the interfacial region after welding, instead of Nb segregation. Acknowledgements The authors acknowledge the financial support from CBMM (Brazil), Chinese Postdoctoral Science Foundation (2015M582159), Postdoctoral Innovation Projects of Shandong Province (201602029), Qingdao Postdoctoral Research & Application Project (2015-305) and Applied Basic Research Program of Qingdao (16-5-1-84-jch). R.D.K. Misra also acknowledges continued collaboration with University of Science and Technology Beijing as Honorary Professor under Foreign Expert Program and support from Freeport McMoRan Fund (3020350). We thank Prof. Gianluigi Botton, Dr. Glynis de Silveira and Travis Casagrande of CCEM (McMaster University) for the technical support, and Dr. Zhenjia Xie and Xuelin Wang (University of Science and Technology Beijing) for the fruitful discussion. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scriptamat.2017.06.017. References [1] F. Matsuda, Y. Fukada, H. Okada, C. Shiga, K. Ikeuchi, Y. Horii, T. Shiwaku, S. Suzuki, Weld. World 37 (1996) 134–154.
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Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular article
Elemental distribution in the martensite–austenite constituent in intercritically reheated coarse-grained heat-affected zone of a high-strength pipeline steel Xueda Li a,⁎, Chengjia Shang b,⁎, Xiaoping Ma c, Baptiste Gault d, S.V. Subramanian c, Jianbo Sun a, R.D.K. Misra e a
College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, China c Department of Materials Science and Engineering, McMaster University, Hamilton, Canada d Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany e Department of Metallurgical, Materials and Biomedical Engineering, University of Texas at El Paso, El Paso, TX, USA b
a r t i c l e
i n f o
Article history: Received 3 May 2017 Received in revised form 13 June 2017 Accepted 13 June 2017 Available online xxxx Keywords: Atom probe tomography Elemental distribution M-A constituent Heat-affected zone C/Mn enrichment
a b s t r a c t We have studied here the distribution of carbon and alloying elements in the martensite-austenite (M-A) constituent in intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) of a high-strength pipeline steel using atom probe tomography (APT). Notable enrichment of C (0.49 wt%) and Mn (2.32 wt%) was observed within the M-A constituent, which induced the formation of lath martensite and deteriorated the toughness. Elemental segregation in the interfacial region between M-A constituent and matrix may contribute to the debonding mechanism of M-A constituent and assist nucleation of cleavage cracks. Distribution of solute Nb indicated no apparent difference between the matrix and M-A constituent. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Coarse-grained HAZ (CGHAZ) and ICCGHAZ are regions that can induce poor toughness in the HAZ of traditional structural steels [1]. However, in modern high-strength low alloy (HSLA) steels, the microstructure of the CGHAZ generally consists of upper bainite or granular bainite that has acceptable toughness within a wide range of heat input [2,3]. Additionally, martensite is not likely to form in the HAZ because of its low carbon equivalent. Thus, it is believed that abnormal distribution of the M-A constituent in the HAZ, especially in the ICCGHAZ, is the main factor that deteriorates the toughness [3–8]. Large blocky M-A constituent can provide nucleation sites for cleavage cracks [9], whereas near-connected or necklace-type M-A constituent along the prior austenite grain boundaries (PAGBs) can change the cleavage fracture propagation mechanism [10] and further deteriorate the toughness. The internal structure of the M-A constituent in the HAZ has also been studied, and twinned martensite was frequently observed [5,11,12]. However, there is a need for quantitative understanding in terms of elemental composition and distribution, in particular for carbon, within the M-A constituent, because it determines subsequent phase transformation, internal structure and hardness of the M-A constituent [13]. A few studies on the chemical composition of M-A constituent using electron probe micro-analyzer (EPMA) were reported [11,12], but the carbon concentration obtained by EPMA lacks ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Li), [email protected] (C. Shang).
http://dx.doi.org/10.1016/j.scriptamat.2017.06.017 1359-6462/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
accuracy. APT is considered to be an accurate technique to determine the elemental content at the atomic scale [14]. In the present study, the composition and distribution of carbon and alloying elements in the MA constituent and its surrounding matrix in the ICCGHAZ of an X100 (690 MPa) pipeline steel was studied using APT and electron microscopy, and its influence on the toughness was discussed. The bulk composition (wt%) of the experimental ×100 pipeline steel was Fe–0.07C–0.25Si–1.94Mn–0.081Nb–0.28Cr–0.26Mo–0.014Ti. A 14.7 mm thick plate was made into pipe of 1219 mm outer diameter. Double pass submerged arc welding was applied with 30 kJ/cm heat input in each pass. The weld joint was cut from the as-welded pipe for experimental study (Fig. 1a). Several sub-zones, i.e. CGHAZ, fine-grained HAZ (FGHAZ), intercritically reheated HAZ (ICHAZ) and ICCGHAZ, were formed due to the heat effect of welding passes. Our previous study under identical conditions indicated that necklace-type M-A constituent in the ICCGHAZ induced lowest toughness (b50 J) among several HAZs of ×100 pipeline steel weld [3]. Microstructure of the ICCGHAZ was characterized by light microscope (LM) and scanning electron microscope (SEM), and the results are presented in Fig. 1b–c. The matrix microstructure of ICCGHAZ consisted of upper bainite and granular bainite. Necklace-type M-A constituent was formed along the PAGBs. Transmission electron microscope (TEM) studies (Fig. 1 in the Supplementary material) indicated that the M-A constituent in the ICCGHAZ primarily consisted of lath martensite, with small fraction of retained austenite
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X. Li et al. / Scripta Materialia 139 (2017) 67–70
Fig. 1. (a) ×100 pipeline steel weld joint, (b) optical micrograph of the ICCGHAZ, and (c) SEM micrograph of the ICCGHAZ.
between the martensite laths. No twinned martensite or ε-martensite was found in the present case. Quantitative study on the chemical composition of M-A constituent in the ICCGHAZ is necessary to improve the understanding of the determining role of M-A constituent on toughness. The specimens for APT were fabricated using a site-specific scanning electron microscope/focused ion beam (SEM/FIB, Zeiss NVision 40) instrument. The bulk sample was slightly etched using 4% nital to outline necklace-type M-A constituent. One M-A particle (marked in Fig. 1c) was cut in-situ and then milled to needle-like with tip size below 50 nm. APT analyses were performed on a Cameca LEAP 4000X HR, at a base temperature of 55 K in laser pulsing mode (λ = 355 nm), with pulse energy of 60 pJ and pulse repetition rate of 250 kHz. The detection rate was maintained at 4 ions per 1000 pulses on average. Reconstruction and quantitative evaluation of APT datasets were performed using software package IVAS 3.6.12–14 [15]. The APT analysis results are presented in Fig. 2. Fig. 2a shows the reconstruction map of multiple elements, and Fig. 2b–d are reconstruction maps of C, Mn + Nb, Cr + Mo, respectively. The carbon map in Fig. 2b indicated that the needle went across the carbon-depleted ferrite matrix, carbon-enriched M-A constituent and their interface. The results
revealed that there was notable segregation of elements in the interfacial region including C, Mn, Cr, Mo, while within the M-A constituent, only C and Mn were enriched. There was no apparent Nb enrichment either in the interfacial region or within the M-A constituent. The composition profile of carbon and alloying elements (Fig. 2e) across ferrite matrix, M-A constituent and their interface was obtained by averaging the atomic fraction in a column perpendicular to the interface as depicted in Fig. 2a. There is a sharp interface between the ferrite matrix and M-A constituent, and the interface is defined as the half composition from the carbon peak to the carbon-depleted plateau in the ferrite matrix. In addition, there are spikes of C and Mn in the composition profile in the interfacial region, with the peak composition of 7.0 at% and 5.1 at%, respectively. In our previous study [3], we proposed that the necklace-type M-A constituent was formed during the second pass intercritical reheating of the CGHAZ, at a slightly higher temperature than the Ac1 temperature (760 °C–800 °C). Part of the matrix reverted to austenite along the PAGBs. During the formation of reverted austenite, C and Mn partitioned into austenite from the surrounding ferritic matrix, resulting in significant enrichment of C and Mn in the interfacial region [16,17]. The spikes are expected to disappear when the growth of
Fig. 2. (a–d) APT reconstruction maps of carbon and alloying elements, (e) composition profile of carbon and alloying elements across ferrite, interface and M-A constituent, and (f) variation in carbon composition within the M-A constituent (distribution of Mn and Nb is also displayed).
X. Li et al. / Scripta Materialia 139 (2017) 67–70
reverted austenite reaches equilibrium with elapsed holding time [16]. Similar results were reported by Xie et al. using APT and DICTRA simulation [18]. But considering the short time reheating and continuous cooling process during welding, the growth of reverted austenite was far from equilibrium in the present case. Therefore, the spikes of C and Mn were retained in the interfacial region. Meanwhile, the composition of Cr and Mo showed a small peak in the interfacial region, but no evidence of any significant enrichment within the M-A constituent. The composition of Si in the ferrite is slightly higher than that in the M-A constituent. The Nb peak in Fig. 2e comes from a Nb/C-rich cluster. A number of studies reported elemental segregation at the interfaces or grain boundaries, especially B, C, N, Mn, Nb, P [19–22]. Lerchbacher et al. [19] reported that up to ~ 10 at% carbon enrichment at martensite interlath boundaries led to formation of austenite thin films in a quenched martensitic steel. While, Raabe et al. [20] reported that heavy segregation of Mn (~ 24 at%) at martensite–martensite grain boundaries also induced the formation of austenite thin films in a Fe– 9 at%Mn maraging steel. The formation of austenite at the interface is beneficial to the toughness. However, Kuzmina et al. [21] reported that, after annealing at 450 °C or 600 °C for only 10–60 s, ~11 at% Mn segregation at the PAGBs induced grain boundary decohesion and fully intergranular failure, which led to lowest toughness in a Fe–9 wt%Mn steel. In the present case, average composition of C and Mn spikes in the interfacial region was calculated to be ~5.59 at% (~1.20 wt%) and ~3.96 at% (~3.90 wt%), respectively. In comparison to the aforementioned studies, ~5.59 at% C and ~3.96 at% Mn enrichment in the interfacial region may not sufficiently stabilize the reverted austenite, especially when the ferrite stabilizing elements, Cr and Mo, simultaneously segregated in the interfacial region. This aspect was confirmed by TEM studies, and no austenite phase was found in the interfacial region. Instead, the debonding of M-A constituent from the matrix was observed in the ICCGHAZ region from the fracture surface of Charpy impact test specimen of ×100 steel weld, and is presented in Fig. 2 in the Supplementary material. The debonding holes of M-A constituent provided nucleation sites for cleavage cracks. Thus, elemental segregation in the interfacial region (C, Mn, Cr, Mo) may weaken the boundary between the M-A constituent and ferrite matrix [4], promote the debonding of M-A constituent from the matrix and assist the nucleation of cleavage cracks. The average compositions of carbon and alloying elements in the M-A constituent and ferrite matrix were calculated from the dataset and are listed in Table 1. Average composition of carbon and Mn within the MA constituent were 2.30 at% (0.49 wt%) and 2.36 at% (2.32 wt%), respectively. In our previous study [8], it was calculated that carbon had enough time to diffuse into the reverted austenite during the welding thermal cycle. The present results indicated that Mn also had adequate time to get enriched in the reverted austenite in spite of fast reheating and continuous cooling process. Martensite start (MS) temperature of the M-A constituent was calculated to be 262 °C using the classic carbon equivalent equation [23]. This further confirms that majority of the reverted austenite transformed into lath martensite upon cooling, and small fraction of austenite was retained between the martensite laths. No twinned or ε-martensite were formed because of relatively low C and Mn composition.
Table 1 Average composition of carbon and alloying elements in the M-A constituent and ferrite matrix.
M-A constituent Ferrite matrix Bulk material
at% wt% at% wt% at% wt%
C
Mn
Nb
Cr
Mo
Si
2.30 0.49 0.19 0.041 0.32 0.07
2.36 2.32 1.63 1.61 1.95 1.94
0.018a 0.030a 0.019a 0.032a 0.048 0.081
0.31 0.29 0.27 0.25 0.30 0.28
0.14 0.24 0.17 0.29 0.15 0.26
0.46 0.23 0.70 0.35 0.50 0.25
Nb values marked with a are solute Nb composition excluding the highly concentrated regions using DIAM method [27].
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As indicated by the black dashed arrow in Fig. 2b, there is a carbon depletion zone in the M-A constituent. Fig. 2f shows the C/Mn/Nb composition profile within the M-A constituent, which was obtained by placing a cylinder perpendicular to the depletion zone, as depicted in Fig.2b. The results revealed large variation in C composition within the M-A constituent, while the distribution of Mn was nearly homogenous. The Nb profile confirmed that the C variation was not caused by large NbC precipitates. The carbon composition at the peak was ~ 4 at% (~0.89 wt%) and at the trough was ~0.6–1 at% (~0.13–0.22 wt%). Large variation in carbon composition within the M-A constituent provided evidence for the occurrence of carbon partitioning during the formation of M-A constituent at slow cooling rate. Supersaturated carbon in martensite diffused into retained austenite upon cooling. The carbon peak in Fig. 2f may also correspond to the retained austenite between the martensite laths. Ping et al. [24] reported similar carbon variation induced by the formation of metastable ω phase as an intermediate of {112}〈111〉–type twinning structures in an oil-quenched Fe–0.6C–1.9Si–0.8Mn (wt%) spring steel. These observations suggest that the redistribution of carbon during martensitic transformation occurred at different cooling rates. Regarding to the results from the APT study, the detrimental effect of M-A constituent on toughness in the ICCGHAZ is attributed to the following aspects. Insufficient enrichment of austenite stabilizer (C and Mn) in the reverted austenite resulted in the martensitic transformation, which produced transformation-induced residual stress in the surrounding matrix [4]. Higher hardness of M-A constituent compared to the surrounding matrix introduced stress concentration during loading [13]. In addition, when the M-A constituent was near-connected or continuously distributed, the residual stress and stress concentration were further strengthened, which could notably promote the initiation of cleavage cracks and deteriorate the toughness [4,12]. Furthermore, for the blocky type M-A constituent, debonding of M-A constituent from the matrix is more likely to occur than self-cracking of the M-A constituent [9,12]. Elemental segregation at the interface may promote the debonding of M-A constituent from the matrix and assist nucleation of cleavage cracks. The distribution of Nb in the dataset was analyzed via isoconcentration surfaces, which highlighted the regions that contained ≥1.5 at% of Nb, and is presented in Fig. 3. Isoconcentration surfaces of ≥1.5 at% C in the interfacial region are also displayed. Fig. 3b is the 180° rotated view of Fig. 3a and C atoms are ignored to clearly show the distribution of solute Nb atoms in the dataset. The results indicated that Nb preferred to segregate along dislocations (referred as ‘Nb-Cottrell atmosphere’ in [25]) or agglomerate in the form of clusters or precipitates. In most cases, carbon simultaneously segregated with Nb at dislocations, or formed Nb/C-rich clusters or NbC precipitates as indicated by the solid arrows in Fig. 2b–c. It is difficult to estimate the composition of Nb/Crich clusters or NbC precipitates due to the trajectory aberrations of APT [14,26]. However, there was no apparent difference in solute Nb between the ferrite matrix and M-A constituent, as presented in Fig. 3b and Table 1. Average solute concentration of Nb in the dataset was calculated to be 0.019 at% using the DIAM method [27] (excluding the highly concentrated regions) in comparison to the overall Nb composition of 0.044 at%. Thus, it was estimated that ~60% of the Nb atoms segregated to the dislocations along with C or formed Nb/C-rich clusters or NbC precipitates within the probed volume. The results suggested that short time thermal cycle with peak temperature of 760 °C–800 °C during the second pass welding did not render significant diffusion of Nb atoms into the MA constituent or the interfacial region. Therefore, it is reasonable to conclude that the deterioration of toughness is not attributed to the Nb segregation during welding processes. In summary, APT analysis provided specific information on the distribution of carbon and alloying elements in the M-A constituent formed in the ICCGHAZ of a high-strength pipeline steel. There was notable enrichment of C and Mn in the M-A constituent (0.49 wt% and 2.32 wt%) compared to the ferritic matrix (0.041 wt% and 1.61 wt%), which induced formation of lath martensite and deteriorated the toughness. Large variation in the carbon composition within the M-A constituent provided
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X. Li et al. / Scripta Materialia 139 (2017) 67–70
Fig. 3. (a) Nb isoconcentration surfaces at% 1.5 at along with C and Nb atoms, and (b) 180° rotation view of (a) (only Nb atoms are displayed).
evidence of carbon partitioning during the formation of the M-A constituent. Elemental segregation in the interfacial region (C, Mn, Cr, Mo) may promote the debonding of M-A constituent from the matrix and assist the nucleation of cleavage cracks. However, no apparent enrichment of Nb within the M-A constituent and in the interfacial region was noted after the welding process. Thus, the deterioration of toughness in the ICCGHAZ was attributed to the C/Mn enrichment in the M-A constituent and elemental segregation in the interfacial region after welding, instead of Nb segregation. Acknowledgements The authors acknowledge the financial support from CBMM (Brazil), Chinese Postdoctoral Science Foundation (2015M582159), Postdoctoral Innovation Projects of Shandong Province (201602029), Qingdao Postdoctoral Research & Application Project (2015-305) and Applied Basic Research Program of Qingdao (16-5-1-84-jch). R.D.K. Misra also acknowledges continued collaboration with University of Science and Technology Beijing as Honorary Professor under Foreign Expert Program and support from Freeport McMoRan Fund (3020350). We thank Prof. Gianluigi Botton, Dr. Glynis de Silveira and Travis Casagrande of CCEM (McMaster University) for the technical support, and Dr. Zhenjia Xie and Xuelin Wang (University of Science and Technology Beijing) for the fruitful discussion. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scriptamat.2017.06.017. References [1] F. Matsuda, Y. Fukada, H. Okada, C. Shiga, K. Ikeuchi, Y. Horii, T. Shiwaku, S. Suzuki, Weld. World 37 (1996) 134–154.
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