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Mechanical properties of advanced active-TIG welded duplex stainless steel and ferrite steel
Materials Science & Engineering A 620 (2014) 140–148
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties of advanced active-TIG welded duplex stainless steel and ferrite steel Ying Zou n, Rintaro Ueji, Hidetoshi Fujii Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2014 Received in revised form 3 October 2014 Accepted 7 October 2014 Available online 15 October 2014
Duplex stainless steel and ferrite steel were welded by the advanced active-TIG (AA-TIG) welding method at different oxygen contents in the shielding gas. A double-shielded gas flow system was used and the oxygen content in the outer shielding gas was altered to several levels. With the increase of the oxygen content in the shielding gas, the weld penetrations in both steels increased; whereas the change in the impact toughness of the weld metal was strongly dependent on the type of steel. The impact toughness of the duplex stainless steel remained almost constant, while that of the ferrite steel was significantly decreased with the increase in the oxygen content. This difference can be linked with the change in the fracture pass along with the difference in the microstructure. & 2014 Elsevier B.V. All rights reserved.
Keywords: TIG welding Duplex stainless steel Ferrite steel Weld penetration Microstructure Mechanical properties
1. Introduction The mechanical properties of the weld metal have a strong dependence on the weld microstructure which is changed by the welding conditions. The proper welding conditions are desired to obtain not only a sound joint but also a high welding efficiency which is represented by the depth of the penetration per single welding pass. It is already known that a deep weld penetration could be obtained by smearing a small amount of the oxide or the fluoride on the surface of the plate before welding, or by mixing some oxygen or carbon dioxide into the inert shielding gas [1–8]. This method is called active flux TIG (A-TIG) welding, but the oxidization of the tungsten electrode is a worrisome problem [9]. For preventing the oxidization of the tungsten electrode, the advanced A-TIG (AA-TIG) was invented using a double-shielding gas with some oxide or carbon dioxide in the outer inert gas, while the inner layer remains pure inert gas, and has been proved to be effective such that no oxidization damage was found on the electrode tip [10–12]. Although the usefulness of the A-TIG and AA-TIG welding method has been well confirmed for the welding process of steel with a high efficiency, the research studies on the change in the mechanical properties with the application of A-TIG/AA-TIG are limited. The addition of oxygen to the shielding gas would increase not only the density of the oxide particles in the weld metal [13], but also the interaction between the oxide and the microstructure of the weld
n
Corresponding author. Tel./fax: þ 81 6 6879 8663. E-mail address: [email protected] (Y. Zou).
http://dx.doi.org/10.1016/j.msea.2014.10.006 0921-5093/& 2014 Elsevier B.V. All rights reserved.
metal since the interface could be regarded as the preferable site for the fracture formation [12]. It could be predicted that this interaction becomes more complicated when the microstructure of the weld metal is composed of several kinds of substructure such as martensite. Martensite is a typical microstructure of the weld metal in steel and this has a high density of crystallographic defects introduced by the martensitic transformation. In addition to the displacive transformation, the diffusional transformation can also provide a fine substructure when the resultant structure maintains the parent phase. One of the typical examples is the duplex stainless steel in which the weld metal consists of large ferrite grains as the parent phase along with fine austenite particles [14]. These two types of transformations would cover the major range of the fine microstructures that evolve in the weld metal of steels, and the comparison between the weld metal with the martensite phase and that with the duplex phase would provide the basic concept to design the weld along with the microstructural aspect. Therefore, in this study, the applicability of the AA-TIG weld method for duplex stainless steel and ferritic steel with a martensite structure in the weld metal was investigated, including the weld penetration change, the oxide distribution in the fusion zone, and evaluation of the mechanical properties of the Vickers hardness distribution, the yield strength, and the impact toughness.
2. Experimental procedures Duplex stainless steel ASTM A240 and ferrite steel ASTM A353 were selected as the materials in this study. ASTM A353 is typically
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141
Table 1 Chemical compositions of ASTM A240 and A353 base materials. Alloy element
C
Cr
Ni
Mn
Mo
Si
Al
P
N
O
S
Fe
ASTM A240 (wt%) ASTM A353 (wt%)
0.02 0.05
24.9 –
7.05 10.4
0.75 0.55
3 –
0.5 0.097
– 0.03
– 0.004
0.113 0.0017
– 0.0013
0.0004 0.003
Bal. Bal.
Table 2 Welding parameters. Parameter
Value
Welding speed Welding current Arc length Shielding gas Electrode type Electrode diameter Electrode vertex angle
2 mm/s 150 A 3 mm Ar, Arþ O2 W–2%CeO2 2.4 mm 601
Fig. 2. Location and dimensions of the impact test sample.
analysis. The data for the fraction were average values taken from images at 10 different sites. The oxygen content in the weld metal was analyzed using an oxygen/nitrogen analyzer. The crack in the cross section near the fracture surface was also observed using the samples fractured by the impact test. Fig. 1. Shape and dimensions of the tensile test sample.
used in large vessels and the solidified microstructure is typical lath martensite [15]. It should be noted that this steel has low carbon content so that the effect of carbide is negligible. The chemical compositions of ASTM A240 and A353 base material are shown in Table 1. The thickness of the steel plate was 6.0 mm. A double-shielded condition was achieved using a plasma welding torch because this torch has a double-nozzle structure. The steel plates were welded by the AA-TIG welding method under the double-shielding gas with oxygen gas added to the outer shielding argon gas, while the inner shielding gas was pure argon [10]. The oxygen content in the fusion zone was altered by controlling the oxygen content in the outer shielding gas at three levels of 0%, 0.4% and 2.5% volume fractions. The bead-on-plate welding was performed at the set welding parameters listed in Table 2. The weld samples were mechanically polished and then electropolished in a CH3COOH þ HClO4 solution. The metallographic morphologies of the base metal and the fusion zone of ASTM A240 welds were revealed with 10% oxalic acid (electro-etched), while 4% nital was used for ASTM A353. The mechanical properties of the weld metal were determined by evaluating the Vickers hardness, the yield strength and the impact toughness. The Vickers hardness was tested at the middle part of the weld sample cross section with a 200 gf load for 15 s. A small size sample (Fig. 1) was used for the tensile test in order to cover only the fusion zone in the area of the gauge length. The tensile test was carried out at a displacement rate of 1.0 mm/min. The miniaturized impact test sample of 1 mm 1 mm 20 mm dimensions was machined from 0.5 mm beneath the surface of the weld metal, with a 0.2 mm depth and 301 angle notch in the center, as shown in Fig. 2. The impact test was conducted at a blade speed of 1.0 m/s. An optical microscope (OM) and a scanning electron microscope (SEM) were used for observing the weld shape and the distribution of the oxide. The austenite fraction in ASTM A240 was evaluated by an Electron Back Scattering Diffraction (EBSD)
3. Results and discussion 3.1. Weld shape and weld oxygen content analysis With the increase of the oxygen content in the shielding gas, the weld shape became narrow and deep in all the samples, as shown in Fig. 3. Because all the experimental parameters were the same except for the oxygen content in the shielding gas, it is credible that oxygen may have played a significant role in controlling the weld shape in both steels. In order to clarify the role that oxygen played in affecting the weld shape, the weld oxygen content was analyzed, and the result is shown in Fig. 4. The weld oxygen content in both steels increased with the increase of the oxygen content in the shielding gas. This indicates that oxygen affects the weld shape change. Meanwhile, the weld oxygen content of ASTM A353 was lower than that of A240 under the same oxygen content condition in the shielding gas. One of the possible reasons is the difference in the thickness of the oxide layer formed on the surface of the weld metal during welding due to the difference in the chemical composition. 3.2. Microstructures The microstructures of the base metal and the fusion zone of ASTM A240 and A353 were observed, and the results are shown in Figs. 5 and 6 respectively. The base metal of ASTM A240 has coarse austenite precipitated in the ferrite matrix, which is a typical morphology for the hot-deformed structure [16]. In ASTM A240 weld samples, the size of the austenite becomes smaller due to the relatively rapid cooling. Three kinds of austenite phases precipitated from the ferrite phase, i.e., the grain boundary austenite phase, the elongated Widmanstätten austenite phase and the scattered intragranular austenite phase. The fractions of the austenite phases were 14.5%, 13.4% and 18.3%, respectively, in the 0%, 0.4% and 2.5% O2 ASTM A240 weld samples, less than in the
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Fig. 3. Weld shapes for different oxygen contents in the shielding gas: (a)–(c) ASTM A240; (d)–(f) ASTM A353. Oxygen content in shielding gases: (a), (d) 0%; (b), (e) 0.4% and (c), (f) 2.5%.
oxygen content includes both the dissolved oxygen and the oxygen in the form of oxides, more and larger oxides indicate less dissolved oxygen in the fusion zone of ASTM A353. The difference in the oxygen solubility could be due to the difference of the chemical composition. For example, it was reported that the carbon content affects the solubility of oxygen [17]. The deeper weld penetration of ASTM A353 indicated that oxygen is more effective in promoting the weld penetration of A353 than A240. 3.3. Mechanical properties
Fig. 4. Weld oxygen contents for different oxygen contents in the shielding gas.
base metal of about 50%. This result indicates a suppression of the ferrite–austenite phase transformation by the introduction of oxygen to the shielding gas, as previously discussed [13]. The base metal of ASTM A353 was totally the ferrite phase, and its weld metals show the lath martensite phase. The prior austenite grains become larger in the weld metals and seemed to be not affected by the oxygen content in the shielding gas. The black and white contrast looks weaker when the oxide content increases. Therefore, in order to show the change in the block size of the martensite phase with the increase of the oxygen content in the shielding gas, the orientation color maps of ASTM A353 weld metal samples shielded with the 0% and 2.5% O2 shielding gases were obtained by the EBSD, as shown in Fig. 7. This result shows that the block size of the martensite phase is not affected by the oxygen content in the shielding gas, so that the weak contrast seems to be due to the behavior of the chemical etching. The oxides in the weld metal were observed by SEM and are shown in Fig. 8. These are the cross-sectional images of the fusion zones, and the white particles indicate the oxides. According to the EDS analysis result, the oxide formed by AA-TIG mainly consists of SiO2, Al2O3, MnO and FeO. The average diameter and the number density of the oxide in the two steels were measured and listed in Table 3. Both the average diameter and the number density of the oxide increased with the increase in the oxygen content (0%, 0.4%, 2.5%) of the shielding gas. At the oxygen content level of 2.5%, the oxides formed in ASTM A353 weld were larger and of a higher percentage than in A240. Because the analysis result of the weld
The Vickers hardness values at different positions along the horizontal central line of the cross sections of the weld metals were tested. The results are shown in Fig. 9(a) for ASTM A240 and in Fig. 9(b) for A353 steel. There was hardly difference among the samples of the different oxygen contents in the shielding gas in both weld metals. However, in both metals, the Vickers hardness value of the fusion zone was higher than that of the base metal. The width of the area where the Vickers hardness increased was consistent with the weld metal as shown in Fig. 3. The Vickers hardness of the weld metal of ASTM A353 was higher than that in A240. This is because in A240, both the base metal and the fusion zone were composed of the ferrite phase and the austenite phase. The slight increase in Vickers hardness of ASTM 240 weld metal was probably due to the refinement of the ferrite grain after the welding process. However, as shown in Fig. 6 for ASTM A353, the base metal was the ferrite phase, while the weld metal was the martensite phase, and thus the martensite phase transformation should provide an increase in Vickers hardness. The strength of the low carbon steel is usually characterized by the yield strength, thus the yield strengths of both steel were tested. The yield strength of the weld metals of both steels was tested with the results shown in Fig. 10. Compared to the base metal, the yield strengths of the fusion zone of both steels increased, and the degree of the increase was more significant in ASTM A353 than in A240. However, with the increase of the oxygen content in the shielding gas, the yield strength of both steels decreased, and the degree of decrease was approximately the same. The impact test clarified a significant difference between the duplex stainless steel and the ferrite steel. The load–displacement curves of the impact test are plotted in Fig. 11 and the values of the impact toughness are listed in Table 4. With the increase of the
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Fig. 5. Metallographic morphologies of the base metal and the fusion zones of ASTM A240 for different oxygen contents in the shielding gas: (a) base metal; (b) 0%; (c) 0.4%; and (d) 2.5%.
Fig. 6. Metallographic morphologies of the base metal and the fusion zones of ASTM A353 for different oxygen contents in the shielding gas: (a) base metal; (b) 0%; (c) 0.4% and (d) 2.5%.
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Fig. 7. Orientation color maps obtained from the EBSD measurement of the block size of the martensite phase in ASTM A353 weld metal for shielding gases of different oxygen contents. Oxygen content in shielding gases: (a) 0% and (b) 2.5%.
Fig. 8. Oxide distribution in the fusion zone for different oxygen contents in the shielding gas: (a)–(c) ASTM A240; (d)–(f) ASTM A353. Oxygen content in shielding gases: (a), (d) 0%; (b), (e) 0.4% and (c), (f) 2.5%.
Y. Zou et al. / Materials Science & Engineering A 620 (2014) 140–148
oxygen content in the shielding gas, the impact toughness of ASTM A240 gradually decreased, while that of A353 dramatically decreased. In order to clarify the reason for the impact toughness change with the oxygen content in the shielding gas, the microstructures near the fracture surfaces of these four samples were observed by SEM, and the results are shown in Fig. 12. In both steels, the morphologies of the fracture surface of the joint without oxygen and that with oxygen were different. However, all of them showed a dimple-like pattern, indicating a significant plastic deformation at the fracture surfaces. Therefore, the change in the oxygen content dependence could not be understood only by the observation of the fracture surface. Back scattered electron images of the cross sections near the fracture surface were taken by SEM, and the results are shown in Fig. 13. The outlines of the fracture surfaces are indicated by yellow lines, and the cracks that occurred in the impact test were pointed out by the red arrows. For ASTM A240, the white part in the back scattered electron images is the austenite phase, and the black part is the ferrite phase. The duplex stainless steel weld showed some voids near the fracture surface. Although the size of the void looks bigger with the addition of oxygen, the void was preferentially observed at the ferrite/austenite phase boundary. On the other hand, in ASTM A353, the void was hardly found in the joint without oxygen, while the joint with the oxygen had many voids. Some of these voids were found around the oxide particles. This observation result implied the difference in the preferential site for void formation. According to the observed results as already described, the models of the crack occurrence in ASTM A240 and A353 weld samples in the impact test at different oxygen contents in the shielding gas are demonstrated in Fig. 14. The number of cracks in the 0% and 2.5% O2 samples was almost the same for ASTM A240
due to the ferrite/austenite phase boundary no matter whether or not the oxide exists. In other words, the interphase boundary between the austenite and ferrite is weaker than the oxide surface. On the other hand, in ASTM A353, probably some of the boundaries induced by the phase transformation are preferable for the void formation. However, the interface between the oxide and weld metal should be more preferable for the fracture so that ASTM A353 joint with oxygen had a lower peak load for fracture due to the weaker nucleation site for void formation. Consequently, these results have clarified that the A-TIG/AA-TIG is very useful when the material has some interphase boundaries weaker than the oxide/matrix interface since the fracture in the weld metal is determined by the nucleation of the voids.
4. Conclusions The duplex stainless steel ASTM A240 and the ferrite steel ASTM A353 were welded by the advanced A-TIG (AA-TIG) welding method for investigation of the weld penetration, the oxide distribution in the fusion zone, and the mechanical properties of the Vickers hardness, the yield strength and the impact toughness. The mechanisms of the crack occurrence in the impact test of both steels were also discussed. The main conclusions are summarized as follows: (1) The weld oxygen content played a significant role in affecting the weld shape of both steels, which could be controlled by adjusting the oxygen content in the shielding gas of the AA-TIG welding process.
Table 3 Average diameter and number density of oxide in ASTM A240 and A353 welds. Oxygen content in shielding gas (%)
0 0.4 2.5
Average diameter (μm)
Number density ( 103/mm2)
ASTM A240
ASTM A353
ASTM A240
ASTM A353
0.6 1.1 1.3
0.4 1.0 3.2
1.4 1.5 2.6
0.2 1.7 3.6
145
Fig. 10. Yield strength of ASTM A240 and A353.
Fig. 9. Vickers hardness distributions in the steel welds for different oxygen contents in the shielding gas: (a) ASTM A240 and (b) ASTM A353.
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Fig. 11. Load–displacement curves of the impact test for different oxygen contents in the shielding gas: (a) ASTM A240 and (b) ASTM A353.
Table 4 Impact toughness values of ASTM A240 and A353 welds. Oxygen content in shielding gas (%)
ASTM A240 ( 10 3 J)
ASTM A353 ( 10 3 J)
Base metal 0 0.4 2.5
351.7 312.1 283.9 277.8
319.5 315.1 219.6 124.4
Fig. 12. Fracture surfaces of the tensile tested samples observed by the SEM: (a), (b) ASTM A240 and (c), (d) ASTM A353. Oxygen content in shielding gases: (a), (c) 0% and (b), (d) 2.5%.
(2) With the increase in the weld oxygen content, the weld shape became narrow and deep for both steels. (3) The Vickers hardness of the weld metals of both steels was not affected by the oxygen content in the shielding gas. (4) With the increase of the oxygen content in the shielding gas, the decrease degrees of the yield strength of both steels were
the same; the impact toughness of the duplex stainless steel gradually decreased, while that of the ferrite steel dramatically decreased. (5) In the impact test, a crack occurred at the austenite/ferrite phase boundary in the duplex stainless steel, while it started from the oxide in the ferrite steel.
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Fig. 13. Reflected electron images of the cross sections near the fracture surface of the tensile tested samples observed by SEM: (a), (b) ASTM A240; (c), (d) ASTM A353. Oxygen content in shielding gases: (a), (c) 0%; (b), (d) 2.5%. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 14. Crack occurrence models in ASTM A240 and A353 weld samples in the impact test for different oxygen contents in the shielding gas: (a) ASTM A240 of 0% O2; (b) ASTM A240 of 2.5% O2; (c) ASTM A353 of 0% O2; and (d) ASTM A353 of 2.5% O2. Black dot: oxide; red short stick: crack. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Acknowledgements The authors wish to acknowledge the financial support of the Global COE Programs from the Ministry of Education, Sports, Culture, Science, and a Grant-in-Aid for Science Research from the Japan Society for Promotion of Science and Technology of Japan, ISIJ Research Promotion Grant.
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[6] S. Leconte, P. Paillard, P. Chapelle, G. Henrion, J. Saindrenan, Sci. Technol. Weld. Join. 11 (2006) 389–397. [7] K.C. Mills, B.J. Keene, Int. Mater. Rev. 35 (1990) 185–216. [8] S. Leconte, P. Paillard, J. Saindrenan, Sci. Technol. Weld. Join. 11 (2006) 43–47. [9] S.P. Lu, H. Fujii, H. Sugiyama, M. Tanaka, K. Nogi, ISIJ Int. 43 (2003) 1590–1595. [10] H. Fujii, T. Sato, S.P. Lu, K. Nogi, Mater. Sci. Eng. A 495 (2008) 296–303. [11] S.P. Lu, H. Fujii, K Nogi, Mater. Sci. Eng. A 26 (2010) 170–176. [12] S.P. Lu, M.P. Qin, W.C. Dong, J. Mater. Proc. Technol. 213 (2013) 229–237.
[13] Y. Zou, R. Ueji, H. Fujii, Mater. Charact. 91 (2014) 42–49. [14] J.W.A. Menezes, H. Abreu, S. Kundu, H.K.D.H. Bhadeshia, P.M. Kelly, Sci. Technol. Weld. Join. 14 (2009) 4–9. [15] K. Zhang, H.B. Wu, D. Tang, J. Iron Steel Res. Int. 19 (2012) 58–62. [16] A.A. Iris, D.M. Suzanne, Duplex Stainless Steels, first ed., ISTE Ltd., London (2009) 3. [17] M.A. Ziegler, Trans. Am. Soc. Steel Treat. 20 (1932) 73–83.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties of advanced active-TIG welded duplex stainless steel and ferrite steel Ying Zou n, Rintaro Ueji, Hidetoshi Fujii Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2014 Received in revised form 3 October 2014 Accepted 7 October 2014 Available online 15 October 2014
Duplex stainless steel and ferrite steel were welded by the advanced active-TIG (AA-TIG) welding method at different oxygen contents in the shielding gas. A double-shielded gas flow system was used and the oxygen content in the outer shielding gas was altered to several levels. With the increase of the oxygen content in the shielding gas, the weld penetrations in both steels increased; whereas the change in the impact toughness of the weld metal was strongly dependent on the type of steel. The impact toughness of the duplex stainless steel remained almost constant, while that of the ferrite steel was significantly decreased with the increase in the oxygen content. This difference can be linked with the change in the fracture pass along with the difference in the microstructure. & 2014 Elsevier B.V. All rights reserved.
Keywords: TIG welding Duplex stainless steel Ferrite steel Weld penetration Microstructure Mechanical properties
1. Introduction The mechanical properties of the weld metal have a strong dependence on the weld microstructure which is changed by the welding conditions. The proper welding conditions are desired to obtain not only a sound joint but also a high welding efficiency which is represented by the depth of the penetration per single welding pass. It is already known that a deep weld penetration could be obtained by smearing a small amount of the oxide or the fluoride on the surface of the plate before welding, or by mixing some oxygen or carbon dioxide into the inert shielding gas [1–8]. This method is called active flux TIG (A-TIG) welding, but the oxidization of the tungsten electrode is a worrisome problem [9]. For preventing the oxidization of the tungsten electrode, the advanced A-TIG (AA-TIG) was invented using a double-shielding gas with some oxide or carbon dioxide in the outer inert gas, while the inner layer remains pure inert gas, and has been proved to be effective such that no oxidization damage was found on the electrode tip [10–12]. Although the usefulness of the A-TIG and AA-TIG welding method has been well confirmed for the welding process of steel with a high efficiency, the research studies on the change in the mechanical properties with the application of A-TIG/AA-TIG are limited. The addition of oxygen to the shielding gas would increase not only the density of the oxide particles in the weld metal [13], but also the interaction between the oxide and the microstructure of the weld
n
Corresponding author. Tel./fax: þ 81 6 6879 8663. E-mail address: [email protected] (Y. Zou).
http://dx.doi.org/10.1016/j.msea.2014.10.006 0921-5093/& 2014 Elsevier B.V. All rights reserved.
metal since the interface could be regarded as the preferable site for the fracture formation [12]. It could be predicted that this interaction becomes more complicated when the microstructure of the weld metal is composed of several kinds of substructure such as martensite. Martensite is a typical microstructure of the weld metal in steel and this has a high density of crystallographic defects introduced by the martensitic transformation. In addition to the displacive transformation, the diffusional transformation can also provide a fine substructure when the resultant structure maintains the parent phase. One of the typical examples is the duplex stainless steel in which the weld metal consists of large ferrite grains as the parent phase along with fine austenite particles [14]. These two types of transformations would cover the major range of the fine microstructures that evolve in the weld metal of steels, and the comparison between the weld metal with the martensite phase and that with the duplex phase would provide the basic concept to design the weld along with the microstructural aspect. Therefore, in this study, the applicability of the AA-TIG weld method for duplex stainless steel and ferritic steel with a martensite structure in the weld metal was investigated, including the weld penetration change, the oxide distribution in the fusion zone, and evaluation of the mechanical properties of the Vickers hardness distribution, the yield strength, and the impact toughness.
2. Experimental procedures Duplex stainless steel ASTM A240 and ferrite steel ASTM A353 were selected as the materials in this study. ASTM A353 is typically
Y. Zou et al. / Materials Science & Engineering A 620 (2014) 140–148
141
Table 1 Chemical compositions of ASTM A240 and A353 base materials. Alloy element
C
Cr
Ni
Mn
Mo
Si
Al
P
N
O
S
Fe
ASTM A240 (wt%) ASTM A353 (wt%)
0.02 0.05
24.9 –
7.05 10.4
0.75 0.55
3 –
0.5 0.097
– 0.03
– 0.004
0.113 0.0017
– 0.0013
0.0004 0.003
Bal. Bal.
Table 2 Welding parameters. Parameter
Value
Welding speed Welding current Arc length Shielding gas Electrode type Electrode diameter Electrode vertex angle
2 mm/s 150 A 3 mm Ar, Arþ O2 W–2%CeO2 2.4 mm 601
Fig. 2. Location and dimensions of the impact test sample.
analysis. The data for the fraction were average values taken from images at 10 different sites. The oxygen content in the weld metal was analyzed using an oxygen/nitrogen analyzer. The crack in the cross section near the fracture surface was also observed using the samples fractured by the impact test. Fig. 1. Shape and dimensions of the tensile test sample.
used in large vessels and the solidified microstructure is typical lath martensite [15]. It should be noted that this steel has low carbon content so that the effect of carbide is negligible. The chemical compositions of ASTM A240 and A353 base material are shown in Table 1. The thickness of the steel plate was 6.0 mm. A double-shielded condition was achieved using a plasma welding torch because this torch has a double-nozzle structure. The steel plates were welded by the AA-TIG welding method under the double-shielding gas with oxygen gas added to the outer shielding argon gas, while the inner shielding gas was pure argon [10]. The oxygen content in the fusion zone was altered by controlling the oxygen content in the outer shielding gas at three levels of 0%, 0.4% and 2.5% volume fractions. The bead-on-plate welding was performed at the set welding parameters listed in Table 2. The weld samples were mechanically polished and then electropolished in a CH3COOH þ HClO4 solution. The metallographic morphologies of the base metal and the fusion zone of ASTM A240 welds were revealed with 10% oxalic acid (electro-etched), while 4% nital was used for ASTM A353. The mechanical properties of the weld metal were determined by evaluating the Vickers hardness, the yield strength and the impact toughness. The Vickers hardness was tested at the middle part of the weld sample cross section with a 200 gf load for 15 s. A small size sample (Fig. 1) was used for the tensile test in order to cover only the fusion zone in the area of the gauge length. The tensile test was carried out at a displacement rate of 1.0 mm/min. The miniaturized impact test sample of 1 mm 1 mm 20 mm dimensions was machined from 0.5 mm beneath the surface of the weld metal, with a 0.2 mm depth and 301 angle notch in the center, as shown in Fig. 2. The impact test was conducted at a blade speed of 1.0 m/s. An optical microscope (OM) and a scanning electron microscope (SEM) were used for observing the weld shape and the distribution of the oxide. The austenite fraction in ASTM A240 was evaluated by an Electron Back Scattering Diffraction (EBSD)
3. Results and discussion 3.1. Weld shape and weld oxygen content analysis With the increase of the oxygen content in the shielding gas, the weld shape became narrow and deep in all the samples, as shown in Fig. 3. Because all the experimental parameters were the same except for the oxygen content in the shielding gas, it is credible that oxygen may have played a significant role in controlling the weld shape in both steels. In order to clarify the role that oxygen played in affecting the weld shape, the weld oxygen content was analyzed, and the result is shown in Fig. 4. The weld oxygen content in both steels increased with the increase of the oxygen content in the shielding gas. This indicates that oxygen affects the weld shape change. Meanwhile, the weld oxygen content of ASTM A353 was lower than that of A240 under the same oxygen content condition in the shielding gas. One of the possible reasons is the difference in the thickness of the oxide layer formed on the surface of the weld metal during welding due to the difference in the chemical composition. 3.2. Microstructures The microstructures of the base metal and the fusion zone of ASTM A240 and A353 were observed, and the results are shown in Figs. 5 and 6 respectively. The base metal of ASTM A240 has coarse austenite precipitated in the ferrite matrix, which is a typical morphology for the hot-deformed structure [16]. In ASTM A240 weld samples, the size of the austenite becomes smaller due to the relatively rapid cooling. Three kinds of austenite phases precipitated from the ferrite phase, i.e., the grain boundary austenite phase, the elongated Widmanstätten austenite phase and the scattered intragranular austenite phase. The fractions of the austenite phases were 14.5%, 13.4% and 18.3%, respectively, in the 0%, 0.4% and 2.5% O2 ASTM A240 weld samples, less than in the
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Fig. 3. Weld shapes for different oxygen contents in the shielding gas: (a)–(c) ASTM A240; (d)–(f) ASTM A353. Oxygen content in shielding gases: (a), (d) 0%; (b), (e) 0.4% and (c), (f) 2.5%.
oxygen content includes both the dissolved oxygen and the oxygen in the form of oxides, more and larger oxides indicate less dissolved oxygen in the fusion zone of ASTM A353. The difference in the oxygen solubility could be due to the difference of the chemical composition. For example, it was reported that the carbon content affects the solubility of oxygen [17]. The deeper weld penetration of ASTM A353 indicated that oxygen is more effective in promoting the weld penetration of A353 than A240. 3.3. Mechanical properties
Fig. 4. Weld oxygen contents for different oxygen contents in the shielding gas.
base metal of about 50%. This result indicates a suppression of the ferrite–austenite phase transformation by the introduction of oxygen to the shielding gas, as previously discussed [13]. The base metal of ASTM A353 was totally the ferrite phase, and its weld metals show the lath martensite phase. The prior austenite grains become larger in the weld metals and seemed to be not affected by the oxygen content in the shielding gas. The black and white contrast looks weaker when the oxide content increases. Therefore, in order to show the change in the block size of the martensite phase with the increase of the oxygen content in the shielding gas, the orientation color maps of ASTM A353 weld metal samples shielded with the 0% and 2.5% O2 shielding gases were obtained by the EBSD, as shown in Fig. 7. This result shows that the block size of the martensite phase is not affected by the oxygen content in the shielding gas, so that the weak contrast seems to be due to the behavior of the chemical etching. The oxides in the weld metal were observed by SEM and are shown in Fig. 8. These are the cross-sectional images of the fusion zones, and the white particles indicate the oxides. According to the EDS analysis result, the oxide formed by AA-TIG mainly consists of SiO2, Al2O3, MnO and FeO. The average diameter and the number density of the oxide in the two steels were measured and listed in Table 3. Both the average diameter and the number density of the oxide increased with the increase in the oxygen content (0%, 0.4%, 2.5%) of the shielding gas. At the oxygen content level of 2.5%, the oxides formed in ASTM A353 weld were larger and of a higher percentage than in A240. Because the analysis result of the weld
The Vickers hardness values at different positions along the horizontal central line of the cross sections of the weld metals were tested. The results are shown in Fig. 9(a) for ASTM A240 and in Fig. 9(b) for A353 steel. There was hardly difference among the samples of the different oxygen contents in the shielding gas in both weld metals. However, in both metals, the Vickers hardness value of the fusion zone was higher than that of the base metal. The width of the area where the Vickers hardness increased was consistent with the weld metal as shown in Fig. 3. The Vickers hardness of the weld metal of ASTM A353 was higher than that in A240. This is because in A240, both the base metal and the fusion zone were composed of the ferrite phase and the austenite phase. The slight increase in Vickers hardness of ASTM 240 weld metal was probably due to the refinement of the ferrite grain after the welding process. However, as shown in Fig. 6 for ASTM A353, the base metal was the ferrite phase, while the weld metal was the martensite phase, and thus the martensite phase transformation should provide an increase in Vickers hardness. The strength of the low carbon steel is usually characterized by the yield strength, thus the yield strengths of both steel were tested. The yield strength of the weld metals of both steels was tested with the results shown in Fig. 10. Compared to the base metal, the yield strengths of the fusion zone of both steels increased, and the degree of the increase was more significant in ASTM A353 than in A240. However, with the increase of the oxygen content in the shielding gas, the yield strength of both steels decreased, and the degree of decrease was approximately the same. The impact test clarified a significant difference between the duplex stainless steel and the ferrite steel. The load–displacement curves of the impact test are plotted in Fig. 11 and the values of the impact toughness are listed in Table 4. With the increase of the
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Fig. 5. Metallographic morphologies of the base metal and the fusion zones of ASTM A240 for different oxygen contents in the shielding gas: (a) base metal; (b) 0%; (c) 0.4%; and (d) 2.5%.
Fig. 6. Metallographic morphologies of the base metal and the fusion zones of ASTM A353 for different oxygen contents in the shielding gas: (a) base metal; (b) 0%; (c) 0.4% and (d) 2.5%.
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Fig. 7. Orientation color maps obtained from the EBSD measurement of the block size of the martensite phase in ASTM A353 weld metal for shielding gases of different oxygen contents. Oxygen content in shielding gases: (a) 0% and (b) 2.5%.
Fig. 8. Oxide distribution in the fusion zone for different oxygen contents in the shielding gas: (a)–(c) ASTM A240; (d)–(f) ASTM A353. Oxygen content in shielding gases: (a), (d) 0%; (b), (e) 0.4% and (c), (f) 2.5%.
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oxygen content in the shielding gas, the impact toughness of ASTM A240 gradually decreased, while that of A353 dramatically decreased. In order to clarify the reason for the impact toughness change with the oxygen content in the shielding gas, the microstructures near the fracture surfaces of these four samples were observed by SEM, and the results are shown in Fig. 12. In both steels, the morphologies of the fracture surface of the joint without oxygen and that with oxygen were different. However, all of them showed a dimple-like pattern, indicating a significant plastic deformation at the fracture surfaces. Therefore, the change in the oxygen content dependence could not be understood only by the observation of the fracture surface. Back scattered electron images of the cross sections near the fracture surface were taken by SEM, and the results are shown in Fig. 13. The outlines of the fracture surfaces are indicated by yellow lines, and the cracks that occurred in the impact test were pointed out by the red arrows. For ASTM A240, the white part in the back scattered electron images is the austenite phase, and the black part is the ferrite phase. The duplex stainless steel weld showed some voids near the fracture surface. Although the size of the void looks bigger with the addition of oxygen, the void was preferentially observed at the ferrite/austenite phase boundary. On the other hand, in ASTM A353, the void was hardly found in the joint without oxygen, while the joint with the oxygen had many voids. Some of these voids were found around the oxide particles. This observation result implied the difference in the preferential site for void formation. According to the observed results as already described, the models of the crack occurrence in ASTM A240 and A353 weld samples in the impact test at different oxygen contents in the shielding gas are demonstrated in Fig. 14. The number of cracks in the 0% and 2.5% O2 samples was almost the same for ASTM A240
due to the ferrite/austenite phase boundary no matter whether or not the oxide exists. In other words, the interphase boundary between the austenite and ferrite is weaker than the oxide surface. On the other hand, in ASTM A353, probably some of the boundaries induced by the phase transformation are preferable for the void formation. However, the interface between the oxide and weld metal should be more preferable for the fracture so that ASTM A353 joint with oxygen had a lower peak load for fracture due to the weaker nucleation site for void formation. Consequently, these results have clarified that the A-TIG/AA-TIG is very useful when the material has some interphase boundaries weaker than the oxide/matrix interface since the fracture in the weld metal is determined by the nucleation of the voids.
4. Conclusions The duplex stainless steel ASTM A240 and the ferrite steel ASTM A353 were welded by the advanced A-TIG (AA-TIG) welding method for investigation of the weld penetration, the oxide distribution in the fusion zone, and the mechanical properties of the Vickers hardness, the yield strength and the impact toughness. The mechanisms of the crack occurrence in the impact test of both steels were also discussed. The main conclusions are summarized as follows: (1) The weld oxygen content played a significant role in affecting the weld shape of both steels, which could be controlled by adjusting the oxygen content in the shielding gas of the AA-TIG welding process.
Table 3 Average diameter and number density of oxide in ASTM A240 and A353 welds. Oxygen content in shielding gas (%)
0 0.4 2.5
Average diameter (μm)
Number density ( 103/mm2)
ASTM A240
ASTM A353
ASTM A240
ASTM A353
0.6 1.1 1.3
0.4 1.0 3.2
1.4 1.5 2.6
0.2 1.7 3.6
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Fig. 10. Yield strength of ASTM A240 and A353.
Fig. 9. Vickers hardness distributions in the steel welds for different oxygen contents in the shielding gas: (a) ASTM A240 and (b) ASTM A353.
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Fig. 11. Load–displacement curves of the impact test for different oxygen contents in the shielding gas: (a) ASTM A240 and (b) ASTM A353.
Table 4 Impact toughness values of ASTM A240 and A353 welds. Oxygen content in shielding gas (%)
ASTM A240 ( 10 3 J)
ASTM A353 ( 10 3 J)
Base metal 0 0.4 2.5
351.7 312.1 283.9 277.8
319.5 315.1 219.6 124.4
Fig. 12. Fracture surfaces of the tensile tested samples observed by the SEM: (a), (b) ASTM A240 and (c), (d) ASTM A353. Oxygen content in shielding gases: (a), (c) 0% and (b), (d) 2.5%.
(2) With the increase in the weld oxygen content, the weld shape became narrow and deep for both steels. (3) The Vickers hardness of the weld metals of both steels was not affected by the oxygen content in the shielding gas. (4) With the increase of the oxygen content in the shielding gas, the decrease degrees of the yield strength of both steels were
the same; the impact toughness of the duplex stainless steel gradually decreased, while that of the ferrite steel dramatically decreased. (5) In the impact test, a crack occurred at the austenite/ferrite phase boundary in the duplex stainless steel, while it started from the oxide in the ferrite steel.
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Fig. 13. Reflected electron images of the cross sections near the fracture surface of the tensile tested samples observed by SEM: (a), (b) ASTM A240; (c), (d) ASTM A353. Oxygen content in shielding gases: (a), (c) 0%; (b), (d) 2.5%. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 14. Crack occurrence models in ASTM A240 and A353 weld samples in the impact test for different oxygen contents in the shielding gas: (a) ASTM A240 of 0% O2; (b) ASTM A240 of 2.5% O2; (c) ASTM A353 of 0% O2; and (d) ASTM A353 of 2.5% O2. Black dot: oxide; red short stick: crack. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Acknowledgements The authors wish to acknowledge the financial support of the Global COE Programs from the Ministry of Education, Sports, Culture, Science, and a Grant-in-Aid for Science Research from the Japan Society for Promotion of Science and Technology of Japan, ISIJ Research Promotion Grant.
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