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Variation in microstructures and mechanical properties in the coarse-grained heat-affected zone of low-alloy steel with boron content
Materials Science & Engineering A 559 (2013) 178–186
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Variation in microstructures and mechanical properties in the coarse-grained heat-affected zone of low-alloy steel with boron content Sanghoon Kim, Yongjoon Kang, Changhee Lee n Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133–791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 July 2011 Received in revised form 21 July 2012 Accepted 14 August 2012 Available online 23 August 2012
The correlation between mechanical properties and boron content in the coarse-grained heat-affected zone (CGHAZ) of low-alloy steel was investigated using 10 ppm and 30 ppm boron added low-alloy steels. A Gleeble system was used to simulate various CGHAZs as a function of heat input. The segregation behavior of boron in the CGHAZ was estimated through secondary ion mass spectrometry (SIMS) and particle tracking autoradiography (PTA). Vickers hardness and Charpy impact tests were performed in order to assess the mechanical properties of the CGHAZs. In the steel containing 30 ppm of boron, boron segregation was relatively high even with low heat input, and increased compared to 10 ppm boron added steel. Furthermore, the boron segregation level maintained a maximum at intermediate heat input conditions from 60 to 500 kJ/cm. This is believed that the boron segregation can be reached a maximum level for relatively low heat input despite the faster cooling rate because of an increase in the boron segregation rate. Also, impact toughness decreased with increased boron content at identical heat input conditions. It is believed that the effect of segregation and precipitation of boron on deterioration of impact toughness is insignificant. However, deterioration of impact toughness may be due to the remarkable increase in the fraction of a second phase, such as the martesite–austenite (M–A) constituent, and the hardening of the matrix with increased boron content. & 2012 Elsevier B.V. All rights reserved.
Keywords: Low-alloy steel Welding Coarse-grained heat-affected zone Non-equilibrium grain boundary segregation Particle tracking autoradiography (PTA)
1. Introduction Boron is known to be an effective alloying element used to increase the hardenability of high-strength low-alloys (HSLAs) [1–4]. Thus, the content of other alloying elements such as carbon, nickel and manganese, which can deteriorate the weldability, can be reduced by minute boron addition. Further, a decrease in the amount of alloying elements is also advantageous in terms of economic feasibility. Therefore, demand for the utilization of boron has been growing. Increased hardenability resulting from boron addition is strongly related to the segregation behavior of boron. Boron segregation can occur at a grain boundary and can be divided into equilibrium and non-equilibrium segregation [1,2,5–9]. Equilibrium segregation occurs thermodynamically due to the presence of a driving force to decrease the free energy of the grain boundary. In contrast, nonequilibrium segregation occurs during cooling, and is due to the formation of supersaturated vacancies occurring as temperature decreases. At this point, the grain boundary acts as a vacancy sink and the diffusion of vacancy and vacancy-boron complexes occurs according to differences in concentration near the grain boundary and in the interior of the grain. Thus, during fabrication and heat
n
Corresponding author. Tel.: þ82 2 2220 0388; fax: þ82 2 2299 0389. E-mail address: [email protected] (C. Lee).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.072
treatment processing, non-equilibrium segregation is dominant, and the segregation behavior of boron can be greatly affected by thermal and stress cycling. Welding is an indispensable process in fabrication of structural parts. Thermal and stress cycles in each weld (the fusion zone and various heat-affected zones) are very complicated and are severe during welding. Consequently, the segregation behavior of boron can be hugely affected by the welding cycle. However, investigation of boron segregation and its effect on the mechanical properties in the coarse-grained heat-affected zone (CGHAZ) have hardly ever been performed [2,8]. Thus, in this study, low alloy steels containing 10 ppm and 30 ppm boron were evaluated in order to determine the effect of boron segregation behavior on the microstructures and mechanical properties of the CGHAZ of low-alloy steel. In particular, the variation in segregation rate according to boron content and the effect of boron on the impact toughness were investigated.
2. Experimental procedures 2.1. Materials To estimate the effect of boron on microstructure and mechanical properties, 10 ppm (steel A1) and 30 ppm (steel A3) boroncontaining high strength low alloy steel was fabricated in a vacuum
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179
Table 1 Chemical composition (wt%) of base steel specimens. Base steel
C
Si
Mn
Ni
Cr
Mo
Nb
V
Ti
Al
B
N
A1 A3
0.05 0.05
0.25 0.25
1.90 1.90
0.50 0.50
0.20 0.20
0.25 0.25
0.04 0.04
0.04 0.04
0.015 0.015
0.03 0.03
0.001 0.003
0.003 0.003
P o 0.01, So 0.003
Fig. 1. Optical micrographs of base steel specimens.
Table 2 Thermal cycles for CGHAZs simulation. Peak temperature (1C)
Heat input (kJ/cm)
Heating rate (1C/s)
Cooling rate (PT 800 1C, 1C/s)
Cooling rate (800 1C RT, 1C/s)
1350
30 60 100 300 500 1000
420 210 125 42 25 12.5
70 36 22 7 4.4 2.2
30 15 10 3 2 1
induction furnace. The chemical composition of the base steel is shown in Table 1. The base steel was manufactured using a thermomechanical controlled process (TMCP) by which very high strength and toughness can be obtained simultaneously in spite of low alloying element content due to the very fine microstructure. For fabrication of the base steel, plates were austenitized at 1150 1C for 1 h and then severely rolled from 1080 1C to 780 1C, representing a thickness reduction of 76%. Cooling was started from 600 1C down to room temperature at a 10 1C/s cooling rate. The base steel consisted of bainite and martensite structures, as indicated in Fig. 1; however, A3 steel (30 ppm boron added) had a larger fraction of martensite due to the higher boron content. As a result, A1 and A3 steel had very high tensile strengths of roughly 1150 MPa. 2.2. HAZ simulation and heat treatment Various CGHAZs with welding heat inputs of 30, 60, 100, 300, 500 and 1000 kJ/cm were simulated using a Gleeble system. Thermal cycles of simulated CGHAZs are shown in Table 2. These thermal cycles as a function of welding heat input were calculated using the Rosenthal equation for thick plates [10]. The peak temperature of the CGHAZs was 1350 1C.
carried out at the HANARO Ex-core Neutron-Irradiation Facility (ENF) at the Korea Atomic Energy Institute. Neutron irradiation was performed at a rate of 1 109 n/cm2s over 6 h. After neutron irradiation, a solid state nuclear track detector was chemically etched using 2.5 N NaOH in water at 55 1C over 9 min and was then examined using an optical microscope [11].
2.4. Microstructure observation and mechanical testing Base steel and simulated CGHAZs were observed using optical microscopy (OM), while crack propagation paths and M–A constituents were analyzed using scanning electron microscopy (SEM). A two-step etching method was used to observe the M–A constituents. First, chemical etching was carried out using mixed etchant which was made up solution A (Na2SO4 1 g þdistilled water) and B (picric acid 4 g þ100 ml ethanol). After that, electrical etching was performed using a solution of NaOH 25 g, picric acid 5 g and distilled water at 5 V during 140 s. For a more detailed analysis of microstructure, electron back-scatter diffraction (EBSD) was conducted using a field emission SEM (Quanta 200 FEG, FEI). Transmission electron microscopy (TEM, JEOL JEM 2000EX II) was used to confirm boron precipitation. Samples were prepared for TEM analysis by mechanical polishing to below 100 mm and then, by thin jet-polishing (Tenupol-3) using a solution of 5 vol% perchloric acid and 95 vol% methanol at 20 V and 40 1C. The sizes of the M–A constituents were measured by an image analyzer (Image-Pro Plus, Media Cybernetics). Vickers hardness tests (HMV-2, Shimadzu) and Charpy impact tests (Tokyo Testing Machine, 500 L) were performed for the base steel and CGHAZs. The results of hardness tests represent the average of ten measurements taken at a load of 0.2 HV (1.961 N). Charpy impact tests were performed using standard specimens 10 10 55 mm in size [12] over a temperature range of 196–20 1C (room temperature).
2.3. Analysis of boron segregation 3. Results Boron segregation behavior was analyzed by SIMS and PTA using simulated CGHAZs specimens. For SIMS analysis (Cameca IMS 6 F), 7.5 keV O2 þ was used as the primary ion and the current of the primary ions was 300 pA. Analysis was performed on a 200 mm 200 mm area and the detected ion was 11B þ . PTA analysis was
3.1. Analysis of boron distribution Figs. 2 and 3 provide SIMS analysis results for the CGHAZs of the A1 and A3 steel according to welding heat input, respectively.
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Fig.2. Boron distribution revealed by SIMS in the CGHAZs of A1 steel according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
Fig. 3. Boron distribution revealed by SIMS in the CGHAZs of A3 steel according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
As shown in these figures, more boron segregation occurred at prior austenite grain boundaries of A3 steel CGHAZs than in A1 steel at identical welding heat input conditions. Also, the amount of boron atoms existing in the grain interior was larger in the A3 steel CGHAZ. These results were also observed on PTA analysis, as
shown in Figs. 4 and 5. Boron segregation initially increased and then decreased with increased welding heat input. Further, with increased welding heat input, some points with a high boron intensity were detected at grain boundaries, and this high intensity may correspond to the presence of some types of boron
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181
Fig. 4. Boron distribution revealed by PTA in the CGHAZs of A1 steel according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
Fig. 5. Boron distribution revealed by PTA in the CGHAZs of A3 steel according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
Fig. 6. Segregation factor R¼ L/Lmax in CGHAZs according to welding heat input. (a) A1 and (b) A3 steel.
precipitates such as M23(C,B)6 and BN [3,13,14]. However, in order to identify the boron precipitation more clearly, more experimental analysis such as TEM is needed.
Because a quantitative analysis of boron segregation is impossible using SIMS and PTA analysis, a parameter R was applied in order to semi-quantitatively estimate the segregation level of
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boron in steel samples containing an identical amount of boron [1,5]. The parameter R can be expressed as follows: R ¼ L=Lmax ,
ð1Þ
where Lmax is the length per unit area of continuous etch pitting revealed on the PTA film for which segregation attains a maximum at this boron content. L is the length per unit area of continuous etch pitting at other heat inputs. Fig. 6 provides the results of boron distribution analysis using the segregation factor R. The highest boron segregation level was observed at intermediate welding heat input conditions. Further, the boron segregation level was relatively high even at low heat input due to the increase in the boron content. Also, boron segregation was confirmed to maintain a maximum level at intermediate heat input conditions in the A3 steel CGHAZs. The highest boron level observed in the A1 steel was only 300 kJ/cm (intermediate heat
input condition), whereas a relatively high boron segregation level (almost identical to the maximum level) was observed in the A3 steel from 60 kJ/cm (relatively low heat input) to 500 kJ/cm (relatively high heat input). 3.2. Evolution of microstructure and mechanical properties with increased boron content Figs. 7 and 8 show the optical microscope observation results of the simulated CGHAZs specimens. As shown, the microstructures of specimens simulated with 30, 60, and 100 kJ/cm heat input were mainly martensite, regardless of boron content. However, granular bainite was observed with martensite in specimens simulated at heat input conditions over 300 kJ/cm. The phase fraction of granular bainite increased according to increased heat input. Grain size also increased with increasing heat input. Therefore, a large
Fig. 7. Optical micrographs of CGHAZs of A1 steel specimens according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
Fig. 8. Optical micrographs of CGHAZs of A3 steel specimens according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
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content. As indicated in Table 3, the deterioration of impact toughness with increasing boron content was not as severe at relatively low heat input conditions such as 30, 60, 100 kJ/cm. However, for high heat input conditions, the CGHAZs of A3 steel had very low upper shelf energies compared to those of A1. These results were consistent with the results of microstructure observation and hardness analysis; microstructure and hardness varied sharply due to the formation of granular bainite.
amount of granular bainite with large grain boundary ferrite without martensite was observed in specimens simulated at 1000 kJ/cm. Vickers hardness data is shown in Fig. 9. The hardness of A3 steel measured at 30, 60, 100 kJ/cm of welding heat input was slightly higher than that of A1 steel. A rapid drop in hardness occurred at 300 kJ/cm welding heat input. These hardness results were in good agreement with the OM observation results. Table 3 and Fig. 10 show the results of Charpy impact tests. Impact energy decreased with increasing welding heat input regardless of boron content. Specifically, the upper shelf energy rapidly declined for heat input conditions over 300 kJ/cm. In addition, impact toughness deteriorated with increasing the boron
4. Discussion 4.1. Boron distribution according to boron content and welding conditions The microstructure and mechanical properties of welded parts, including HAZs, are determined by thermal cycles such as heating and cooling rate during the welding process. Therefore, welding heat input, which can determine the welding thermal cycle, is a very important factor on the microstructure and mechanical properties [10]. Also, boron segregation (especially nonequilibrium segregation) can be largely affected by heat input. As indicated in Table 2, heating and cooling rates decreased with increased heat input, thus, the exposure time at high temperature can be increased with increased heat input. As shown in Fig. 6, the boron segregation level increased initially and then decreased with increasing heat input regardless of the type of base steel. That is, the boron segregation level
Fig. 9. Vickers hardness data in CGHAZs according to welding heat input.
Table 3 Charpy impact test data for the CGHAZs. 30 kJ/cm
60 kJ/cm
100 kJ/cm
300 kJ/cm
5000 kJ/cm
1000 kJ/cm
Base steel
A1 A3
USE (J)
ETT (1C)
USE (J)
ETT (1C)
USE (J)
ETT (1C)
USE (J)
ETT (1C)
USE (J)
ETT (1C)
USE (J)
ETT (1C)
206 174
50 30
141 139
38 36
135 130
29 16
68 21
14 20
44 15
18 16
25 17
2 13
Fig. 10. Charpy impact energy data in CGHAZs of (a) A1, 30 kJ/cm, (b) A1, 300 kJ/cm, (c) A3, 30 kJ/cm and (d) A3, 300 kJ/cm.
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reached a maximum with intermediate heat input, and this heat input condition was critical for boron segregation. These results are in good agreement with previous research and show that nonequilibrium boron segregation is closely related to cooling rate [8,15]. However, boron segregation behaviors are a bit different at various boron contents. For example, the boron segregation level in the CGHAZs of steel containing 30 ppm of boron was relatively high even at low heat input. These results show that the rate of boron segregation is increased by increased boron content. Further, the heat inputs resulting in the maximum level of boron segregation were retained from 60 to 500 kJ/cm. This result means that the hardenability increase conferred by boron addition may be maintained longer as a result of increased boron content.
According to some researchers, impact toughness can be affected by some strengthening factors and effective grain size, which is divided by misorientations of at least ten degrees [19,20]. The relationships between these strengthening factors and the grain boundary can be expressed as 0
V Trs ¼ Fðs0 þ sS þ sD þ sP Þ2k
D1=2 ,
ð2Þ
4.2. Deterioration of impact toughness As mentioned above, boron prefers to segregate at the grain boundary, and boron precipitate can form at grain boundaries where there are a large amount of boron atoms. Thus, segregation and precipitation of boron can affect impact toughness because the segregation can cause intergranular cracking and the precipitate can act as a crack initiation site [16–18]. For a more detailed analysis of fracture mode in impact toughness tests, the crack propagation path was observed by SEM. Fig. 11 represents crack propagation paths of specimens under 30 and 300 kJ/cm welding heat input. As indicated in Fig. 11, cracks propagated through the grain interior. Further, the direction of crack propagation curved at prior austenite grain and packet boundaries and at second phases such as M–A constituents. Therefore, the effect of segregation on the deterioration of impact toughness by intergranular cracking was not confirmed in this study. To confirm the effect of precipitation, boron precipitate was observed by TEM using the A3 1000 kJ/ cm condition specimen because the generability of boron precipitation was the highest at this condition due to the high boron content and slow cooling rate. However, as shown in Fig. 12, boron precipitate was not observed at the grain boundaries. Consequently, the effect of boron precipitation was negligible.
Fig. 12. TEM micrographs of CGHAZs of A3 steel specimens simulated with a peak temperature of 1350 1C and 1000 kJ/cm.
Fig. 13. EBSD analysis data of cross-sections beneath fracture surfaces of CGHAZs: (a, b) inverse pole figures; (c, d) phase fraction maps of the A1 and A3 at 300 kJ/cm.
Fig. 11. Crack propagation path in CGHAZs of (a) A1, 30 kJ/cm, (b) A1, 300 kJ/cm, (c) A3, 30 kJ/cm and (d) A3, 300 kJ/cm.
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Fig. 14. (a, b) SEM images, (c, d) size distribution of M–A constituents in CGHAZs of the A1 and A3 specimens at 300 kJ/cm.
where s0, sS, sD, sP and D indicate the friction strengthening, the solid solution strengthening, the dislocation strengthening, the precipitation strengthening and the effective grain size or facet size for cleavage, respectively. EBSD analysis was performed in order to estimate the effects of effective grain size; also, the phase distribution can be obtained through this analysis. Effective grain size decreased with increased boron content. At 30 kJ/cm, the effective grain size of A1 and A3 were 9.6(71.8) and 7.0(71.3) mm and at 300 kJ/cm they were 38.0(72.6) and 15.8(72.0) mm, respectively. This suggests that boron segregation can suppress grain growth. However, the CGHAZs of A3 had very low impact toughness compared to that of A1 despite the noticeable decrease in effective grain size. Thus, the strengthening effect of the matrix due to increased boron content was much greater than the effect of decreased grain size. This impact toughness improvement due to the decrease in grain size was confirmed only for all the welding heat input conditions in the individual base steel specimens. The impact toughness deteriorated remarkably due to increased boron content at 300 kJ/cm. This result may be due to the increase in the second phase, i.e., M–A constituent, as shown in Fig. 13(c) and (d). In the figure it is apparent that the austenite fraction increased with increased boron content, and the detected austenite appeared to be present in the M–A constituent. Thus, the numbers and size distribution of M–A constituents was assessed by SEM observation and image analysis. The numbers and size of M–A constituents increased remarkably with increasing boron content (Fig. 14). Specifically, higher numbers of large M–A constituents above 0.6 mm, which can act as crack initiation sites, were observed in A3 [21].
5. Conclusions In this study, microstructural variation and changes in the mechanical properties of the CGHAZs of low-alloy steel as a function of boron content were investigated using steel specimens containing 10 and 30 ppm of boron under various welding
heat input conditions. Through these investigations the following conclusions were obtained: (1). The segregation of boron increased initially and decreased with increased heat input, suggesting that non-equilibrium grain boundary boron segregation is closely related to cooling rate, which is decided by heat input. (2). Boron segregation level was relatively high even at low heat input according to the increase in boron content. Thus, boron segregation reached a maximum level at a low heat input, which has a faster cooling rate, because of the increase in boron segregation rate. (3). Impact toughness deteriorated due to an increase in boron content under identical heat input conditions. It is believed that the deterioration of impact toughness may be due to the remarkable increase in the fraction of a second phase, such as M–A constituents, and the hardening effect of the matrix resulting from increased boron content. However, the effect of segregation and precipitation of boron is insignificant.
Acknowledgments This study was supported by a grant from the Fundamental R&D Program for the Core Technology of Materials, funded by the Ministry of Knowledge Economy, Republic of Korea. The authors would also like to thank Dr. Eunjoo Shin of the Korea Atomic Energy Institute (KAERI) for her help with the boron distribution analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
X.L. He, Y.Y. Chu, J.J. Jonas, Acta Metall. 37 (1989) 147. X.M. Wang, X.L. He, ISIJ Int. 42 (2002) S38. T. Hara, H. Asahi, R. Uemori, H. Tamehiro, ISIJ Int. 44 (2004) 1431. F. Han, B. Hwang, D. Suh, Z. Wang, D.K. Lee, S. Kim, Met. Mater. Int. 14 (2008) 667. X. Tingdong, C. Buyuan, Prog. Mater. Sci. 49 (2004) 109. L. Karlsson, H. Norde´n, H. Odelius, Acta Metall. 36 (1988) 1–12. X. Tingdong, S. Shenhua, Acta Metall. 37 (1989) 2499–2506. X. Huang, M.C. Chaturvedi, N.L. Richards, J. Jackman, Acta Mater. 45 (1997) 3095. M. Jahazi, J.J. Jonas, Mater. Sci. Eng. A 335 (2002) 49.
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[10] K. Easterling, Introduction to the physical metallurgy of welding, Butterworth, London, 1983, pp. 17–45. [11] E. Shin, The analysis method of the boron distribution for boron added steels by neutron PTA (Particle Tracking Auto-radiography), KAERI technical report, Korea, 2009, pp. 1–31. [12] ASTM E 23-07a, ASTM International, 2007. [13] H. Asahi, ISIJ Int. 42 (2002) 1150. [14] T. Iwamoto, T. Hoshino, A. Matsuzaki, K. Amano, ISIJ Int. 42 (2002) S77. [15] L. Karlsson, H. Norde´n, H. Odelius, Acta Metall. 36 (1988) 1. [16] M.A.V. Chapman, R.G. Faulkner, Acta Metall. 31 (1983) 677.
[17] S.H. Song, J. Wu, Z.X. Yuan, L.Q. Weng, T.H. Xi, Mater. Sci. Eng. A 486 (2008) 675. [18] C.R. Brooks, F. Bogni, Mater. Charact. 38 (1997) 103. [19] J.Y. Koo, M.J. Luton, N.V. Bangaru, R.A. Petkovic, D.P. Fairchild, C.W. Petersen, H. Asahi, T. Hara, Y. Terada, M. Sugiyama, H. Tamehiro, Y. Komizo, S. Okaguchi, M. Hamada, A. Yamamoto, I. Takeuchi, in: Proceedings of the 13th International Offshore and Polar Engineering Conference, 2003, p. 10. [20] B. Hwang, C.G. Lee, T. Lee, Metall. Mater. Trans. A 41 (2010) 85. [21] F. Matsuda, K. Ikeuchi, Y. Fukada, Y. Horii, H. Okada, T. Shiwaku, C. Shiga, S. Suzuki, Trans. JWRI 24 (1995) 1.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Variation in microstructures and mechanical properties in the coarse-grained heat-affected zone of low-alloy steel with boron content Sanghoon Kim, Yongjoon Kang, Changhee Lee n Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133–791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 July 2011 Received in revised form 21 July 2012 Accepted 14 August 2012 Available online 23 August 2012
The correlation between mechanical properties and boron content in the coarse-grained heat-affected zone (CGHAZ) of low-alloy steel was investigated using 10 ppm and 30 ppm boron added low-alloy steels. A Gleeble system was used to simulate various CGHAZs as a function of heat input. The segregation behavior of boron in the CGHAZ was estimated through secondary ion mass spectrometry (SIMS) and particle tracking autoradiography (PTA). Vickers hardness and Charpy impact tests were performed in order to assess the mechanical properties of the CGHAZs. In the steel containing 30 ppm of boron, boron segregation was relatively high even with low heat input, and increased compared to 10 ppm boron added steel. Furthermore, the boron segregation level maintained a maximum at intermediate heat input conditions from 60 to 500 kJ/cm. This is believed that the boron segregation can be reached a maximum level for relatively low heat input despite the faster cooling rate because of an increase in the boron segregation rate. Also, impact toughness decreased with increased boron content at identical heat input conditions. It is believed that the effect of segregation and precipitation of boron on deterioration of impact toughness is insignificant. However, deterioration of impact toughness may be due to the remarkable increase in the fraction of a second phase, such as the martesite–austenite (M–A) constituent, and the hardening of the matrix with increased boron content. & 2012 Elsevier B.V. All rights reserved.
Keywords: Low-alloy steel Welding Coarse-grained heat-affected zone Non-equilibrium grain boundary segregation Particle tracking autoradiography (PTA)
1. Introduction Boron is known to be an effective alloying element used to increase the hardenability of high-strength low-alloys (HSLAs) [1–4]. Thus, the content of other alloying elements such as carbon, nickel and manganese, which can deteriorate the weldability, can be reduced by minute boron addition. Further, a decrease in the amount of alloying elements is also advantageous in terms of economic feasibility. Therefore, demand for the utilization of boron has been growing. Increased hardenability resulting from boron addition is strongly related to the segregation behavior of boron. Boron segregation can occur at a grain boundary and can be divided into equilibrium and non-equilibrium segregation [1,2,5–9]. Equilibrium segregation occurs thermodynamically due to the presence of a driving force to decrease the free energy of the grain boundary. In contrast, nonequilibrium segregation occurs during cooling, and is due to the formation of supersaturated vacancies occurring as temperature decreases. At this point, the grain boundary acts as a vacancy sink and the diffusion of vacancy and vacancy-boron complexes occurs according to differences in concentration near the grain boundary and in the interior of the grain. Thus, during fabrication and heat
n
Corresponding author. Tel.: þ82 2 2220 0388; fax: þ82 2 2299 0389. E-mail address: [email protected] (C. Lee).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.072
treatment processing, non-equilibrium segregation is dominant, and the segregation behavior of boron can be greatly affected by thermal and stress cycling. Welding is an indispensable process in fabrication of structural parts. Thermal and stress cycles in each weld (the fusion zone and various heat-affected zones) are very complicated and are severe during welding. Consequently, the segregation behavior of boron can be hugely affected by the welding cycle. However, investigation of boron segregation and its effect on the mechanical properties in the coarse-grained heat-affected zone (CGHAZ) have hardly ever been performed [2,8]. Thus, in this study, low alloy steels containing 10 ppm and 30 ppm boron were evaluated in order to determine the effect of boron segregation behavior on the microstructures and mechanical properties of the CGHAZ of low-alloy steel. In particular, the variation in segregation rate according to boron content and the effect of boron on the impact toughness were investigated.
2. Experimental procedures 2.1. Materials To estimate the effect of boron on microstructure and mechanical properties, 10 ppm (steel A1) and 30 ppm (steel A3) boroncontaining high strength low alloy steel was fabricated in a vacuum
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179
Table 1 Chemical composition (wt%) of base steel specimens. Base steel
C
Si
Mn
Ni
Cr
Mo
Nb
V
Ti
Al
B
N
A1 A3
0.05 0.05
0.25 0.25
1.90 1.90
0.50 0.50
0.20 0.20
0.25 0.25
0.04 0.04
0.04 0.04
0.015 0.015
0.03 0.03
0.001 0.003
0.003 0.003
P o 0.01, So 0.003
Fig. 1. Optical micrographs of base steel specimens.
Table 2 Thermal cycles for CGHAZs simulation. Peak temperature (1C)
Heat input (kJ/cm)
Heating rate (1C/s)
Cooling rate (PT 800 1C, 1C/s)
Cooling rate (800 1C RT, 1C/s)
1350
30 60 100 300 500 1000
420 210 125 42 25 12.5
70 36 22 7 4.4 2.2
30 15 10 3 2 1
induction furnace. The chemical composition of the base steel is shown in Table 1. The base steel was manufactured using a thermomechanical controlled process (TMCP) by which very high strength and toughness can be obtained simultaneously in spite of low alloying element content due to the very fine microstructure. For fabrication of the base steel, plates were austenitized at 1150 1C for 1 h and then severely rolled from 1080 1C to 780 1C, representing a thickness reduction of 76%. Cooling was started from 600 1C down to room temperature at a 10 1C/s cooling rate. The base steel consisted of bainite and martensite structures, as indicated in Fig. 1; however, A3 steel (30 ppm boron added) had a larger fraction of martensite due to the higher boron content. As a result, A1 and A3 steel had very high tensile strengths of roughly 1150 MPa. 2.2. HAZ simulation and heat treatment Various CGHAZs with welding heat inputs of 30, 60, 100, 300, 500 and 1000 kJ/cm were simulated using a Gleeble system. Thermal cycles of simulated CGHAZs are shown in Table 2. These thermal cycles as a function of welding heat input were calculated using the Rosenthal equation for thick plates [10]. The peak temperature of the CGHAZs was 1350 1C.
carried out at the HANARO Ex-core Neutron-Irradiation Facility (ENF) at the Korea Atomic Energy Institute. Neutron irradiation was performed at a rate of 1 109 n/cm2s over 6 h. After neutron irradiation, a solid state nuclear track detector was chemically etched using 2.5 N NaOH in water at 55 1C over 9 min and was then examined using an optical microscope [11].
2.4. Microstructure observation and mechanical testing Base steel and simulated CGHAZs were observed using optical microscopy (OM), while crack propagation paths and M–A constituents were analyzed using scanning electron microscopy (SEM). A two-step etching method was used to observe the M–A constituents. First, chemical etching was carried out using mixed etchant which was made up solution A (Na2SO4 1 g þdistilled water) and B (picric acid 4 g þ100 ml ethanol). After that, electrical etching was performed using a solution of NaOH 25 g, picric acid 5 g and distilled water at 5 V during 140 s. For a more detailed analysis of microstructure, electron back-scatter diffraction (EBSD) was conducted using a field emission SEM (Quanta 200 FEG, FEI). Transmission electron microscopy (TEM, JEOL JEM 2000EX II) was used to confirm boron precipitation. Samples were prepared for TEM analysis by mechanical polishing to below 100 mm and then, by thin jet-polishing (Tenupol-3) using a solution of 5 vol% perchloric acid and 95 vol% methanol at 20 V and 40 1C. The sizes of the M–A constituents were measured by an image analyzer (Image-Pro Plus, Media Cybernetics). Vickers hardness tests (HMV-2, Shimadzu) and Charpy impact tests (Tokyo Testing Machine, 500 L) were performed for the base steel and CGHAZs. The results of hardness tests represent the average of ten measurements taken at a load of 0.2 HV (1.961 N). Charpy impact tests were performed using standard specimens 10 10 55 mm in size [12] over a temperature range of 196–20 1C (room temperature).
2.3. Analysis of boron segregation 3. Results Boron segregation behavior was analyzed by SIMS and PTA using simulated CGHAZs specimens. For SIMS analysis (Cameca IMS 6 F), 7.5 keV O2 þ was used as the primary ion and the current of the primary ions was 300 pA. Analysis was performed on a 200 mm 200 mm area and the detected ion was 11B þ . PTA analysis was
3.1. Analysis of boron distribution Figs. 2 and 3 provide SIMS analysis results for the CGHAZs of the A1 and A3 steel according to welding heat input, respectively.
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Fig.2. Boron distribution revealed by SIMS in the CGHAZs of A1 steel according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
Fig. 3. Boron distribution revealed by SIMS in the CGHAZs of A3 steel according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
As shown in these figures, more boron segregation occurred at prior austenite grain boundaries of A3 steel CGHAZs than in A1 steel at identical welding heat input conditions. Also, the amount of boron atoms existing in the grain interior was larger in the A3 steel CGHAZ. These results were also observed on PTA analysis, as
shown in Figs. 4 and 5. Boron segregation initially increased and then decreased with increased welding heat input. Further, with increased welding heat input, some points with a high boron intensity were detected at grain boundaries, and this high intensity may correspond to the presence of some types of boron
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Fig. 4. Boron distribution revealed by PTA in the CGHAZs of A1 steel according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
Fig. 5. Boron distribution revealed by PTA in the CGHAZs of A3 steel according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
Fig. 6. Segregation factor R¼ L/Lmax in CGHAZs according to welding heat input. (a) A1 and (b) A3 steel.
precipitates such as M23(C,B)6 and BN [3,13,14]. However, in order to identify the boron precipitation more clearly, more experimental analysis such as TEM is needed.
Because a quantitative analysis of boron segregation is impossible using SIMS and PTA analysis, a parameter R was applied in order to semi-quantitatively estimate the segregation level of
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boron in steel samples containing an identical amount of boron [1,5]. The parameter R can be expressed as follows: R ¼ L=Lmax ,
ð1Þ
where Lmax is the length per unit area of continuous etch pitting revealed on the PTA film for which segregation attains a maximum at this boron content. L is the length per unit area of continuous etch pitting at other heat inputs. Fig. 6 provides the results of boron distribution analysis using the segregation factor R. The highest boron segregation level was observed at intermediate welding heat input conditions. Further, the boron segregation level was relatively high even at low heat input due to the increase in the boron content. Also, boron segregation was confirmed to maintain a maximum level at intermediate heat input conditions in the A3 steel CGHAZs. The highest boron level observed in the A1 steel was only 300 kJ/cm (intermediate heat
input condition), whereas a relatively high boron segregation level (almost identical to the maximum level) was observed in the A3 steel from 60 kJ/cm (relatively low heat input) to 500 kJ/cm (relatively high heat input). 3.2. Evolution of microstructure and mechanical properties with increased boron content Figs. 7 and 8 show the optical microscope observation results of the simulated CGHAZs specimens. As shown, the microstructures of specimens simulated with 30, 60, and 100 kJ/cm heat input were mainly martensite, regardless of boron content. However, granular bainite was observed with martensite in specimens simulated at heat input conditions over 300 kJ/cm. The phase fraction of granular bainite increased according to increased heat input. Grain size also increased with increasing heat input. Therefore, a large
Fig. 7. Optical micrographs of CGHAZs of A1 steel specimens according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
Fig. 8. Optical micrographs of CGHAZs of A3 steel specimens according to welding heat input; simulated with a peak temperature of 1350 1C and (a) 30 kJ/cm, (b) 60 kJ/cm, (c) 100 kJ/cm, (d) 300 kJ/cm, (e) 500 kJ/cm and (f) 1000 kJ/cm.
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content. As indicated in Table 3, the deterioration of impact toughness with increasing boron content was not as severe at relatively low heat input conditions such as 30, 60, 100 kJ/cm. However, for high heat input conditions, the CGHAZs of A3 steel had very low upper shelf energies compared to those of A1. These results were consistent with the results of microstructure observation and hardness analysis; microstructure and hardness varied sharply due to the formation of granular bainite.
amount of granular bainite with large grain boundary ferrite without martensite was observed in specimens simulated at 1000 kJ/cm. Vickers hardness data is shown in Fig. 9. The hardness of A3 steel measured at 30, 60, 100 kJ/cm of welding heat input was slightly higher than that of A1 steel. A rapid drop in hardness occurred at 300 kJ/cm welding heat input. These hardness results were in good agreement with the OM observation results. Table 3 and Fig. 10 show the results of Charpy impact tests. Impact energy decreased with increasing welding heat input regardless of boron content. Specifically, the upper shelf energy rapidly declined for heat input conditions over 300 kJ/cm. In addition, impact toughness deteriorated with increasing the boron
4. Discussion 4.1. Boron distribution according to boron content and welding conditions The microstructure and mechanical properties of welded parts, including HAZs, are determined by thermal cycles such as heating and cooling rate during the welding process. Therefore, welding heat input, which can determine the welding thermal cycle, is a very important factor on the microstructure and mechanical properties [10]. Also, boron segregation (especially nonequilibrium segregation) can be largely affected by heat input. As indicated in Table 2, heating and cooling rates decreased with increased heat input, thus, the exposure time at high temperature can be increased with increased heat input. As shown in Fig. 6, the boron segregation level increased initially and then decreased with increasing heat input regardless of the type of base steel. That is, the boron segregation level
Fig. 9. Vickers hardness data in CGHAZs according to welding heat input.
Table 3 Charpy impact test data for the CGHAZs. 30 kJ/cm
60 kJ/cm
100 kJ/cm
300 kJ/cm
5000 kJ/cm
1000 kJ/cm
Base steel
A1 A3
USE (J)
ETT (1C)
USE (J)
ETT (1C)
USE (J)
ETT (1C)
USE (J)
ETT (1C)
USE (J)
ETT (1C)
USE (J)
ETT (1C)
206 174
50 30
141 139
38 36
135 130
29 16
68 21
14 20
44 15
18 16
25 17
2 13
Fig. 10. Charpy impact energy data in CGHAZs of (a) A1, 30 kJ/cm, (b) A1, 300 kJ/cm, (c) A3, 30 kJ/cm and (d) A3, 300 kJ/cm.
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reached a maximum with intermediate heat input, and this heat input condition was critical for boron segregation. These results are in good agreement with previous research and show that nonequilibrium boron segregation is closely related to cooling rate [8,15]. However, boron segregation behaviors are a bit different at various boron contents. For example, the boron segregation level in the CGHAZs of steel containing 30 ppm of boron was relatively high even at low heat input. These results show that the rate of boron segregation is increased by increased boron content. Further, the heat inputs resulting in the maximum level of boron segregation were retained from 60 to 500 kJ/cm. This result means that the hardenability increase conferred by boron addition may be maintained longer as a result of increased boron content.
According to some researchers, impact toughness can be affected by some strengthening factors and effective grain size, which is divided by misorientations of at least ten degrees [19,20]. The relationships between these strengthening factors and the grain boundary can be expressed as 0
V Trs ¼ Fðs0 þ sS þ sD þ sP Þ2k
D1=2 ,
ð2Þ
4.2. Deterioration of impact toughness As mentioned above, boron prefers to segregate at the grain boundary, and boron precipitate can form at grain boundaries where there are a large amount of boron atoms. Thus, segregation and precipitation of boron can affect impact toughness because the segregation can cause intergranular cracking and the precipitate can act as a crack initiation site [16–18]. For a more detailed analysis of fracture mode in impact toughness tests, the crack propagation path was observed by SEM. Fig. 11 represents crack propagation paths of specimens under 30 and 300 kJ/cm welding heat input. As indicated in Fig. 11, cracks propagated through the grain interior. Further, the direction of crack propagation curved at prior austenite grain and packet boundaries and at second phases such as M–A constituents. Therefore, the effect of segregation on the deterioration of impact toughness by intergranular cracking was not confirmed in this study. To confirm the effect of precipitation, boron precipitate was observed by TEM using the A3 1000 kJ/ cm condition specimen because the generability of boron precipitation was the highest at this condition due to the high boron content and slow cooling rate. However, as shown in Fig. 12, boron precipitate was not observed at the grain boundaries. Consequently, the effect of boron precipitation was negligible.
Fig. 12. TEM micrographs of CGHAZs of A3 steel specimens simulated with a peak temperature of 1350 1C and 1000 kJ/cm.
Fig. 13. EBSD analysis data of cross-sections beneath fracture surfaces of CGHAZs: (a, b) inverse pole figures; (c, d) phase fraction maps of the A1 and A3 at 300 kJ/cm.
Fig. 11. Crack propagation path in CGHAZs of (a) A1, 30 kJ/cm, (b) A1, 300 kJ/cm, (c) A3, 30 kJ/cm and (d) A3, 300 kJ/cm.
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Fig. 14. (a, b) SEM images, (c, d) size distribution of M–A constituents in CGHAZs of the A1 and A3 specimens at 300 kJ/cm.
where s0, sS, sD, sP and D indicate the friction strengthening, the solid solution strengthening, the dislocation strengthening, the precipitation strengthening and the effective grain size or facet size for cleavage, respectively. EBSD analysis was performed in order to estimate the effects of effective grain size; also, the phase distribution can be obtained through this analysis. Effective grain size decreased with increased boron content. At 30 kJ/cm, the effective grain size of A1 and A3 were 9.6(71.8) and 7.0(71.3) mm and at 300 kJ/cm they were 38.0(72.6) and 15.8(72.0) mm, respectively. This suggests that boron segregation can suppress grain growth. However, the CGHAZs of A3 had very low impact toughness compared to that of A1 despite the noticeable decrease in effective grain size. Thus, the strengthening effect of the matrix due to increased boron content was much greater than the effect of decreased grain size. This impact toughness improvement due to the decrease in grain size was confirmed only for all the welding heat input conditions in the individual base steel specimens. The impact toughness deteriorated remarkably due to increased boron content at 300 kJ/cm. This result may be due to the increase in the second phase, i.e., M–A constituent, as shown in Fig. 13(c) and (d). In the figure it is apparent that the austenite fraction increased with increased boron content, and the detected austenite appeared to be present in the M–A constituent. Thus, the numbers and size distribution of M–A constituents was assessed by SEM observation and image analysis. The numbers and size of M–A constituents increased remarkably with increasing boron content (Fig. 14). Specifically, higher numbers of large M–A constituents above 0.6 mm, which can act as crack initiation sites, were observed in A3 [21].
5. Conclusions In this study, microstructural variation and changes in the mechanical properties of the CGHAZs of low-alloy steel as a function of boron content were investigated using steel specimens containing 10 and 30 ppm of boron under various welding
heat input conditions. Through these investigations the following conclusions were obtained: (1). The segregation of boron increased initially and decreased with increased heat input, suggesting that non-equilibrium grain boundary boron segregation is closely related to cooling rate, which is decided by heat input. (2). Boron segregation level was relatively high even at low heat input according to the increase in boron content. Thus, boron segregation reached a maximum level at a low heat input, which has a faster cooling rate, because of the increase in boron segregation rate. (3). Impact toughness deteriorated due to an increase in boron content under identical heat input conditions. It is believed that the deterioration of impact toughness may be due to the remarkable increase in the fraction of a second phase, such as M–A constituents, and the hardening effect of the matrix resulting from increased boron content. However, the effect of segregation and precipitation of boron is insignificant.
Acknowledgments This study was supported by a grant from the Fundamental R&D Program for the Core Technology of Materials, funded by the Ministry of Knowledge Economy, Republic of Korea. The authors would also like to thank Dr. Eunjoo Shin of the Korea Atomic Energy Institute (KAERI) for her help with the boron distribution analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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