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Interface microstructure evolution of dissimilar friction stir butt welded F82H steel and SUS304
Materials Science and Engineering A 528 (2011) 5812–5821
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Interface microstructure evolution of dissimilar friction stir butt welded F82H steel and SUS304 Young Dong Chung a , Hidetoshi Fujii a,∗ , Yufeng Sun a , Hiroyasu Tanigawa b a b
Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan Japan Atomic Energy Agency 2-4 Shirane Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
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Article history: Received 17 December 2010 Received in revised form 23 March 2011 Accepted 7 April 2011 Available online 15 April 2011 Keywords: Reduced-activation ferritic/martensitic steel (RAF/M) F82H SUS304 Dissimilar friction stir butt welding Microstructure
a b s t r a c t The dissimilar butt welded joint of reduced-activation ferritic/martensitic steel (RAF/M) F82H and austenite stainless steel (AISI304 (SUS304)) were studied by friction stir welding. The effect of the position of the steels and tool plunging was considered in order to prohibit the mixing of the F82H and SUS304. When the dissimilar butt welding was performed such that the F82H plate was on the advancing side and the tool was plunged on the F82H side, defect-free joints could be successfully fabricated. Optical microscopy and EDX analysis were used to characterize the dissimilar joint microstructures and the interface. It was confirmed that the dissimilar joint formed no mixed structure and inter-metallic compounds. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Reduced-activation ferritic/martensitic steel (RAF/M) F82H is the most promising candidate material for fusion power plant reactors. In addition, austenite stainless steel is one of the candidate materials which are used in the piping area of the International Thermonuclear Experimental Reactor (ITER) Test blanket modules (TBM) [1]. The SUS304 is one of candidate materials of austenite stainless steels which are the most commercial austenite stainless steels. The current material design of the back wall in Japan consists of F82H and austenite stainless steels. However, when the fusion welding was performed using the F82H and austenite stainless steels, the strength and toughness of the F82H steel side significantly decreased due to the generation of ı ferrite and inter-metallic compounds on the F82H side [2]. Moreover, when the fusion welding of the F82H steel is performed, the hardness of the heat-affected zone decreased, and a hard and brittle martensite phase was formed. Accordingly, welding at a low temperature is desired [3–5]. The friction stir welding (FSW) is a solid state joining method and has been widely used and investigated for low melting materials such as Al [6,7], Mg [8], Cu alloys [9]. Recently, many researchers have reported the dissimilar FSW of low melting materials to low melting materials [9–12], high melting materials [13,14], sev-
∗ Corresponding author. Tel.: +81 6 68798663; fax: +81 6 68798663. E-mail address: [email protected] (H. Fujii). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.04.023
eral types of steel sheets [15–18], and dissimilar friction stir spot welding [19–21]. In general, when dissimilar friction stir butt welding is performed for the low melting point materials to high melting point materials, the positioning of the materials and plunged tool are the main factors of making a sound joint due to their different deformation characteristics at high temperature. An FSW study showed that the tool plunged in the low melting point materials and position of the low melting point materials on the retreating side were important in order to prevent an over heating of the low melting point materials [14–18]. Their microstructures and mechanical properties have also been studied. However, these studies could not prevent the forming of a mixed structure and inter-metallic compounds, and no study on the dissimilar FSW of high melting point steels has been reported. In this study, a study on the dissimilar FSW of high melting point steels was performed in order to analyze the dissimilar joints microstructures and mechanical properties, and suggest a process to prevent the forming of a mixed structure. 2. Experimental procedure The base material was a reduced-activation ferritic/martensitic steel (RAF/M) F82H and SUS304, and the details of their chemical compositions are listed in Table 1. Dissimilar friction stir welded joints of the SUS304/F82H was conducted using rectangular plates with the dimensions of 1.5 mm thickness, 250 mm length and 50 mm width. The welding conditions involved four
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Fig. 1. Schematic drawing of the dissimilar butt friction stir welding of F82H and SUS304; (a), (b) F82H steel on the advancing side, (c), (d) F82H steel on the retreating side, (a), (c) tool plunge on the advancing side and (b) and (d) tool plunged on the retreating side. Table 1 Chemical composition of F82H and SUS304. Type
F82H Type
SUS304
Chemical composition in mass % C
Cr
W
V
Ta
–
–
–
0.1
8
2
0.2
0.04
–
–
–
Chemical composition in mass % C
Cr
Ni
Mn
Si
P
S
Mo
0.08
18
8
2
1
0.045
0.003
–
types, with positions of the F82H steel sheet on the advancing side (Fig. 1(a) and (b)), and on the retreating side (Fig. 1(c) and (d)), and the FSW tool was plunged into the F82H sheet (Fig. 1(a) and (d)) and into the austenite stainless steel (Fig. 1(b) and (c)). In this case, the tool probe was shifted a 0.1 mm toward the F82H sheet and SUS304, and did not penetrate into the other steel sheet side. A detailed schematic illustration of the butt FSW is shown in Fig. 1. The welding experiments were performed using a load-controlled FSW machine. The welding tool was made of a WC-based material [22–26] and the tool was tilted 3◦ from the plate normal welding direction. The welding temperature was measured during the dissimilar friction stir butt welding by K-type thermocouples which were embedded on the
interface of the F82H and SUS304, therefore, the measured temperature corresponds to the value at the interface part of the welded joints. Table 2 shows the tool size and FSW parameters. The dissimilar friction stir welded joints of the SUS304/F82H was achieved at a 100 rpm rotation speed and 100 mm/min welding speed. Optical microscopy (OM) and scanning electron microscopy (SEM, JEOL, JSM-7001FA) observations of the microstructure and interface of each FSW joint were carried out. The specimen for the optical microscopy observations was cut perpendicular to the welding direction. Two kinds of etching solutions were used in order to analyze the interface and welded joints. When using Kellers etch (1 vol.% hydrofluoric acid, 1.5 vol.% hydrochloric acid, and 2.5 vol.% nitric acid in solution), only the F82H could be etched, therefore, distribution of the F82H, SUS304 particles and composite material were easily confirmed in the joint. Electrochemical (in a solution of 10% oxalic acid and 90% water with a power supply set to 15 V for 60 s) etching of both the F82H and the SUS304 plate was done. Energy dispersive X-ray spectroscopy (EDX) was performed to determine of the level of diffusion of the elements in the welded joint and interface. The tensile test for the joints used three tensile specimens cut perpendicular to the welding direction. The Vickers microhardness tests were performed in three regions (top, center, bottom of joint) on the cross section perpendicular to the welding direction.
Fig. 2. Microstructures of the base metal: (a) F82H and (b) SUS304.
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Table 2 Tool size and welding parameters. Tool size (mm)
Welding parameters
Shoulder diameter
Probe diameter
Probe length
Tool tilt (◦ )
Rotation speed (rpm)
Welding speed (mm/min)
Revolution pitch (mm/rev)
15
6
1.3
3
100
100
1
Fig. 3. Temperature dependence of 0.2% proof stress of the F82H steel and SUS304 steel.
Fig. 4. Effect of plates position on temperature hysteresis of the welded joints during the welding.
3. Results and discussion
3.3. Effect on the position of steels and plunged tool
3.1. Base metal
Fig. 3 shows the 0.2% proof stress of the F82H steel and SUS304 sheet. The 0.2% proof stress of the F82H steel and SUS304 is 530 and 240 MPa, respectively, at room temperature. The yield strength data of F82H was obtained on several heats by conventional experiment [28,29]. The least squares fitted equation for the yield strength over the temperature range of 20–750 ◦ C is
The surface appearances of the dissimilar friction stir welded joints are shown in Table 3. The positions of the F82H steels were on the advancing (a, b) and retreating sides (c, d) and the tool probe was plunged on the F82H (a, d) and the SUS304 (b, d). When the dissimilar friction stir welding of SUS304/F82H was performed with SUS304 on the retreating and advancing side and tool plunged on the SUS304, the tool was broken during the welding because the 0.2% proof stress of the SUS304 was still high. When the dissimilar friction stir welding of SUS304/F82H was performed with F82H on the advancing and retreating side and tool plunged on the F82H, these conditions was not broken during the welding because the both conditions was welded at the yield strength of F82H lower than SUS304 (Fig. 4). Therefore, in this study, the tool was plunged on the F82H side and the F82H steel was on the advancing side or on the retreating side, was used to discuss the optimum dissimilar friction stir welding of steels.
(MPa) = 531.4 − 0.38794 × T + 0.001482 × T 2 − 2.3965e
3.4. Low-magnification over view
The F82H base metal (Fig. 2(a)) shows a ferritic/martensitic steel with (CrFe)7 C3 precipitation in which the material was fabricated by applying the HIP (hot isostatic pressing) bonding method [27], and Fig. 2(b) shows that the typical microstructure of the base metal of SUS304 consists of equiaxed austenite grains. 3.2. Properties of materials over temperature
3
− 06 × T − 1.4506e − 10 × T
4
where the temperature (T) is in ◦ C. The correlation coefficient for the plotted data using this equation is R = 0.8835. The 0.2% proof stress of the F82H steel and SUS304 decreased with the increasing temperature and then these values reversed over than 710 ◦ C which the 0.2% proof stress of the SUS304 is greater than the F82H.
Fig. 5 shows cross sections of the dissimilar friction stir welded SUS304/F82H joint which the position of F82H is on the advancing side and the tool was plunged on the F82H plate. Fig. 5 shows that the dissimilar joint exhibited six distinct regions. No. 1 (Fig. 5(a)–(c)) cross section of the joint etched by Kellers, and No. 2 (Fig. 5(d)–(f)) cross section of the joint electrochemically etched.
Fig. 5. Optical macroscopic overview of the cross-section of the butt dissimilar friction stir welded SUS304 and F82H steels under conditions of Fig. 1(a).
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Table 3 Effect of the plate and tool positions on the dissimilar joint surfaces under conditions of Fig. 1. Welding condition
Probe position
As
RS
Fig. 1(a)
AS
F82H
SUS304
Fig. 1(b)
RS
F82H
SUS304
Fig. 1(c)
AS
SUS304
F82H
Fig. 1(d)
RS
SUS304
F82H
The Fig. 5(No. 1) dissimilar joint exhibited three distinct regions which were labelled (a) the stir zone in the SUS304, (b) the stir zone in the F82H, and (c) the below AC1 welded region in the F82H on the advancing side. The Fig. 5(No. 2) dissimilar joint exhibited three distinct regions which were labelled (d) the base metal of SUS304, (e) the base metal of F82H, and (f) the HAZ in the SUS304 on the retreating side. Fig. 6 shows cross sections of the dissimilar friction stir welded SUS304/F82H joint which the position of F82H is on the retreating side and the tool was plunged on the F82H plate. Fig. 6 shows that the dissimilar joint exhibited six distinct regions. No. 1 (Fig. 6(a)–(d)) cross section of the joints etched by Kellers, and No. 2 (Fig. 6(e)–(f)) cross section of the joint lectrochemically etched. The Fig. 6(No. 1) dissimilar joint exhibited four distinct regions which were labelled (a) the base metal of F82H, (b) the stir zone in the SUS304 and F82H mixed zone, (c) the stir zone in the bottom part, and (d) the base metal of SUS304. The Fig. 6(No. 2) dissimilar joint exhibited two distinct regions which were labelled (e) the HAZ in the F82H on the retreating side, and (f) the HAZ in the SUS304 on the advancing side. 3.5. Dissimilar friction stir welded joints phenomenon Fig. 7 shows the dissimilar friction stir welded joints of the SUS304/F82H phenomenon. The position of F82H is on the advancing side (a), F82H is on the retreating side (b) and the tool was plunged on the F82H plate. The samples were prepared by cut-
Dissimilar joined surface
ting 0.2 mm off the upper surface using a polishing technique and then the samples were etched by Kellers solution. When the joining begins, F82H (white) and SUS304 are mixed with each other, and the F82H and SUS304 was not mixed except a left black dot regions. Watanabe et al. [15] report this phenomenon which a probe plunged ed in the advancing side, the SUS304 interface has not been activated because the SUS304 contacted with plastic flowed F82H before moving of the probe. In this welding condition, the interfacial position depends on the strength of two steels in the welding temperature. The yield strength of F82H exceeds SUS304, and plasticity flowed F82H mixed with SUS304 side in the arrow (1) regions because the joining temperature is comparatively low at the beginning of joining. However, plastic flowed SUS304 is mixed with F82H side when the joining goes than an arrow (2) because the yield strength of SUS304 exceeds F82H. In Fig. 7(b) case, when the welding was begun, the dissimilar joint morphology indicates that all of the dissimilar joint microstructures were uniformly mixed. We had known that the welding phenomenon was different with F82H on the retreating side and advancing side. Watanabe et al. reported that the dissimilar friction stir welding was performed with soft material on the retreating side and tool plunged on the soft materials, the dissimilar materials a mixed because the hard material faying surface was activated by rotating probe [15]. When the dissimilar joining is performed with soften F82H on the retreating side and the tool plunged on the F82H side at the welding temperature, SUS304 fay-
Fig. 6. Optical macroscopic overview of the cross-section of the butt dissimilar friction stir welded SUS304 and F82H steels under conditions of Fig. 1(d).
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Fig. 7. Effect of plate position on flow near the surface of the dissimilar friction stir butt welded joints; (a) F82H on the advancing side and (b) F82H on the retreating side.
ing side was activated, and then the interface was generated from the back of the rotating tool and SUS304 and F82H were mixed each other. 3.6. Microstructural evolution Fig. 8 shows the microstructure of the dissimilar friction stir welded SUS304/F82H joints which the position of F82H is on the advancing side and the tool was plunged on the F82H plate. Fig. 8(a) shows that the microstructure of the stir zone (SUS304) has a fine equiaxed grain structure. Fig. 8(b) shows the microstructures of the stir zone on the F82H side and the microstructure consists of a mixture of the fine ferrite with transformed martensite and more detail analyses were performed in Fig. 9. Fig. 8(c) shows the microstructure of the below AC1 welded region in the F82H, and the microstructure of this part under went a grain refinement and exhibited the same composition as the base metal. Fig. 8(d) shows the interface microstructure of SUS304/F82H, and the interface microstructure has a fine equiaxed grain structure (SUS304) and fine ferrite structure which was the below AC1 welded. The interface of SUS304/F82H is completely welded and separated and no defects could be found. Fig. 8(e) shows the stir zone microstructure of SUS304/F82H, and
the stir zone did not have a mixed microstructure and it was completely separated. The following SEM results (Fig. 9) show the detailed microstructure and the distribution of the precipitation of the F82H side stir zone which corresponds to Fig. 8(b). The microstructure of the stir zone has exactly the same structure from top to bottom which means that the welding condition is sufficient as the dissimilar welding conditions for the SUS304 and F82H steels and the grain refined ferrite and transformed martensite structures with Cr-rich precipitation (M23 C6 ) were uniformly distributed in the ferrite matrix. The welding temperature exceed AC1 (840 ◦ C) which was the ␥ region, and the ferrite was grain refined by dynamic recrystallization and then ␥ phase transformed into martensite with the decreasing temperature. The grain sizes of the ferrite are less than 1.5 m from top to bottom of the joint and the mean interval of the martensite lath boundaries are 300 nm. The sizes of M23 C6 particles are a mixture with some particles larger than 100 nm and others less than 100 nm. Fig. 10 shows the microstructure of the dissimilar friction stir welded SUS304/F82H joints which the position of F82H is on the retreating side and the tool was plunged on the F82H plate. Fig. 10(a) shows the microstructure of the stir zone in the upper black dotted line as related to Fig. 6(b). This welded joint region
Fig. 8. Microstructures of FSW joint under condition of Fig. 1(a): (a) stir zone in SUS304, (b) stir zone in F82H, (c) HAZ on the advancing side (F82H), (d) interface in SUS304/F82H and (e) microstructure of SUS304/F82H.
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Fig. 9. The SEM images of stir zone in F82H under condition of Fig. 1(a): (a) Top part, (b) center part, (c) bottom part and the distribution of the precipitation of the top (a-1), center (b-1) and bottom part (c-1).
exhibits a mixed structure of the F82H steel and SUS304, and the shape of the mixed structure was complex and contained both lamellar and coarse microstructures. The mixed structure existed in the entire upper part of the welded joint because the upper part of the SUS304 advancing side was stirred with the F82H steel. Fig. 10(b) shows the microstructure of the stir zone (F82H, bottom black dotted line) and these microstructures consisted of a fine ferrite with transformed martensite and more detail analyses were performed in Fig. 10(c) shows the microstructure of the below AC1 (840 ◦ C) welded region in the F82H on the retreating side, and the microstructure of this part became grain refined and exhibited the same composition as the base metal. The real region of Fig. 10(d) is the SUS304 side, but the microstructure showed the same as the SZ in F82H. Therefore, the advancing side of SUS304 was stirred with the F82H steel. Fig. 10(e) shows the interface of F82H/SUS304 on the advancing side, and the interface of SUS304/F82H is
completely welded and separated and no defects could be found. The following SEM results (Fig. 11) show the detailed microstructure and the distribution of the precipitation of the F82H side stir zone which corresponds to Fig. 10(b). Fig. 11(a, a-1) show that the grain refined ferrite and transformed martensite structures with Cr-rich precipitation (M23 C6 ) were uniformly distributed in the ferrite matrix. The welding temperature should exceed AC1 (840 ◦ C) which was the ␥ region, the ferrite was grain refined and then ␥ phase transformed into martensite with the decreasing temperature. The grain sizes of the ferrite are less than 1.5 um and the mean interval of the martensite lath boundaries are 200 nm. The sizes of M23 C6 particles are a mixture with some particles larger than 100 nm and others less than 100 nm. Fig. 11(b) shows that the grain refined ferrite and (CrFe)7 C3 precipitation were uniformly distributed in the ferrite matrix. The welding temperature should
Fig. 10. Microstructures of FSW joint under condition Fig. 1(d): (a) stir zone in the SUS304 and F82H mixed zone, (b) stir zone in F82H, (c) HAZ on the retreating side, (d) HAZ in SUS304 and (e) interface on the advancing side.
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Fig. 11. SEM images of stir zone in F82H: (a) top part of unmixed zone, (a-1) distribution of the precipitation of (a) and (c) bottom part of unmixed zone.
below the AC1 (840 ◦ C), the ferrite was grain refined and then ␥ phase transformation not occurred. The grain sizes of the ferrite are larger than 1um and others less than 1um. The sizes of (CrFe)7 C3 particles are a mixture with some particles larger than 300 nm and others less than 300 nm. 3.6.1. Concentrations distribution The following SEM and EDX mapping results showed the concentrations distribution of the dissimilar friction stir welded SUS304/F82H joints. Fig. 12 shows an SEM image and EDX result of the interface for the dissimilar friction stir welded SUS304/F82H which the position of F82H on the advancing side and the tool
plunged on the F82H steel side. The concentrations of Cr are 16.9 and 8.1 which is approximately the same concentration of each base metal. Fig. 12(b) shows the concentration profiles of Cr and Ni across the region from the EDX analysis, and the concentration profiles of Cr and Ni at the interface between the F82H and the SUS304 suggest that the concentrations of these elements are uniform and then significantly decreased between the interface of the F82H and SUS304. These results indicated that the welding conditions of the F82H steel on the advancing side and tool plunged F82H side are a more suitable for the dissimilar butt FSW SUS304 to F82H steel because the dissimilar butt joined steels were not mixed and perfectly separated from each other.
Fig. 12. SEM image and EDX result of interface of dissimilar butt FSW joint; F82H on the advancing side and tool plunged F82H steel side; (a) point analysis and (b) line analysis.
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Fig. 13. SEM images and EDX mapping of the mixed region of the dissimilar friction stir butt welded joint; F82H on the retreating side and tool plunged on the F82H side.
Fig. 13 shows SEM images (a) and EDX mapping results (b) of the mixed structure of the dissimilar friction stir welded SUS304/F82H joint which the position of F82H on the retreating side and tool plunged F82H side. The concentrations of Cr are 16.1–16.3 in the SUS316 region and 8.5–8.6 in the F82H region both of which are approximately the same concentration in each base metal. This result indicates that the SUS304 and F82H are simply mixed, and the two materials have not diffused from each other. Fig. 13(c) shows the high magnification image of SEM and (d) EDX result of the stir zone in the SUS304 and F82H steel. The mixed zone showed a different concentration of Cr which indicates that the SUS304 and F82H steel were totally mixed in the upper part of the stir zone.
3.7. Mechanical properties evolution The following tensile strength results and the appearance of the joints after the tensile test are shown in Table 4; F82H steel on advancing side (Fig. 13(a)), F82H steel on retreating side (Fig. 13(b)). Fig. 13(a) joint is fractured at the F82H side base metal and Fig. 13(b) fractured at the SUS304/F82H joint interface. Therefore, F82H on the advancing side and tool plunged on the F82H plate is more appropriate for the dissimilar friction stir welding conditions. The tensile strength of the dissimilar friction stir welded SUS304/F82H joints (F82H on the advancing side) is same with the base metal because the dissimilar welded SUS304/F82H joints have fractured on the F82H side base metal. Accordingly, the dissimilar FSW joint shows a stable elongation. Regarding the tensile strength of the dissimilar friction stir welded SUS304/F82H (F82H on the retreating side),
Fig. 14. Hardness profile of cross-section of the dissimilar friction stir butt welded joints; (a) F82H on the advancing and (b) retreating side.
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is less than half of the F82H base metal because the dissimilar welded SUS304/F82H joint suddenly fractured at stir zone. The hardness profiles of the dissimilar friction stir welded SUS304/F82H joints with the F82H on the advancing (a) and retreating sides (b) are shown in Fig. 14. The hardness of the SUS304 and F82H steel base metal is approximately 210 HV. Fig. 14(a) shows that the hardness profile of the upper, center and bottom lines of the dissimilar friction stir welded SUS304/F82H joint. The dissimilar joint had a similar trend in which the hardness profiles have two distinct regions; i.e., the SUS304 (retreating side) and F82H steel (advancing side). The hardness value slightly increased toward the SZ (SUS304 side), dramatically increased in the joint center (F82H stir zone) and then decreased in the F82H base metal on the advancing side. The maximum hardness value of the bottom line is less than the upper and center regions because of the different cooling rates. The maximum hardness of the upper and center regions of the welded joint is greater than 500 HV because the welding temperature exceeded AC1 . Fig. 14(b) shows the hardness profile of the upper, center and bottom lines of the dissimilar friction stir welded SUS304/F82H joint. The upper part of the stir zone showed a discontinuous and fluctuating profile because the F82H and SUS304 are mixed. The maximum hardness value of the center and bottom lines is slightly higher than the base metal because of the refined ferrite and equiaxed grain. 4. Conclusions In conclusion, this study clarified that the joining of high melting point dissimilar steels without a mixed structure and no formed inter-metallic compounds can be achieved when the FSW process is conducted under the appropriate conditions. This study focused on the dissimilar friction stir welding of high melting point steels which is a process to prevent the forming of a mixed structure and inter-metallic compounds. When the dissimilar butt welding of high melting point steels was performed by FSW, the properties of two steels at the welding temperature should be considered. The dissimilar steels have totally different properties at the room temperature in which the F82H steel is harder than SUS304. However, the F82H steel is softened with the increasing temperature and then the SUS304 became harder than the F82H at the welding temperature. The yield strength of two steels at the welding temperature is related to the position of the steels and plunged tool. When the dissimilar FSW was performed with soft materials on the advancing side and the tool plunged on the soft materials side at the welding temperature, the interface was clearly divided into the soft and hard materials and no mixed structure was observed. When the dissimilar FSW was performed with soft materials on the retreating side and the tool plunged on the soft materials side at the welding temperature, the stir zone
microstructure exhibited a mixed microstructure of the soft and hard materials.
Acknowledgements This work was supported by the Japan Atomic Energy Agency under the Joint Work contract #21K079. The authors wish to acknowledge the financial support of a Grant-in-Aid for Science Research from the Japan Society for the Promotion of Science and Technology of Japan, the Global COE Programs, a Grant-in-Aid for the Cooperative Research Project of Nationwide Joint-Use Research Institutes from the Ministry of Education, Sports, Culture, Science and Toray Science Foundation, ISIJ Research Promotion Grant, Iketani Foundation and Collaborative Research Based on Industrial Demand.
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Interface microstructure evolution of dissimilar friction stir butt welded F82H steel and SUS304 Young Dong Chung a , Hidetoshi Fujii a,∗ , Yufeng Sun a , Hiroyasu Tanigawa b a b
Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan Japan Atomic Energy Agency 2-4 Shirane Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
a r t i c l e
i n f o
Article history: Received 17 December 2010 Received in revised form 23 March 2011 Accepted 7 April 2011 Available online 15 April 2011 Keywords: Reduced-activation ferritic/martensitic steel (RAF/M) F82H SUS304 Dissimilar friction stir butt welding Microstructure
a b s t r a c t The dissimilar butt welded joint of reduced-activation ferritic/martensitic steel (RAF/M) F82H and austenite stainless steel (AISI304 (SUS304)) were studied by friction stir welding. The effect of the position of the steels and tool plunging was considered in order to prohibit the mixing of the F82H and SUS304. When the dissimilar butt welding was performed such that the F82H plate was on the advancing side and the tool was plunged on the F82H side, defect-free joints could be successfully fabricated. Optical microscopy and EDX analysis were used to characterize the dissimilar joint microstructures and the interface. It was confirmed that the dissimilar joint formed no mixed structure and inter-metallic compounds. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Reduced-activation ferritic/martensitic steel (RAF/M) F82H is the most promising candidate material for fusion power plant reactors. In addition, austenite stainless steel is one of the candidate materials which are used in the piping area of the International Thermonuclear Experimental Reactor (ITER) Test blanket modules (TBM) [1]. The SUS304 is one of candidate materials of austenite stainless steels which are the most commercial austenite stainless steels. The current material design of the back wall in Japan consists of F82H and austenite stainless steels. However, when the fusion welding was performed using the F82H and austenite stainless steels, the strength and toughness of the F82H steel side significantly decreased due to the generation of ı ferrite and inter-metallic compounds on the F82H side [2]. Moreover, when the fusion welding of the F82H steel is performed, the hardness of the heat-affected zone decreased, and a hard and brittle martensite phase was formed. Accordingly, welding at a low temperature is desired [3–5]. The friction stir welding (FSW) is a solid state joining method and has been widely used and investigated for low melting materials such as Al [6,7], Mg [8], Cu alloys [9]. Recently, many researchers have reported the dissimilar FSW of low melting materials to low melting materials [9–12], high melting materials [13,14], sev-
∗ Corresponding author. Tel.: +81 6 68798663; fax: +81 6 68798663. E-mail address: [email protected] (H. Fujii). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.04.023
eral types of steel sheets [15–18], and dissimilar friction stir spot welding [19–21]. In general, when dissimilar friction stir butt welding is performed for the low melting point materials to high melting point materials, the positioning of the materials and plunged tool are the main factors of making a sound joint due to their different deformation characteristics at high temperature. An FSW study showed that the tool plunged in the low melting point materials and position of the low melting point materials on the retreating side were important in order to prevent an over heating of the low melting point materials [14–18]. Their microstructures and mechanical properties have also been studied. However, these studies could not prevent the forming of a mixed structure and inter-metallic compounds, and no study on the dissimilar FSW of high melting point steels has been reported. In this study, a study on the dissimilar FSW of high melting point steels was performed in order to analyze the dissimilar joints microstructures and mechanical properties, and suggest a process to prevent the forming of a mixed structure. 2. Experimental procedure The base material was a reduced-activation ferritic/martensitic steel (RAF/M) F82H and SUS304, and the details of their chemical compositions are listed in Table 1. Dissimilar friction stir welded joints of the SUS304/F82H was conducted using rectangular plates with the dimensions of 1.5 mm thickness, 250 mm length and 50 mm width. The welding conditions involved four
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Fig. 1. Schematic drawing of the dissimilar butt friction stir welding of F82H and SUS304; (a), (b) F82H steel on the advancing side, (c), (d) F82H steel on the retreating side, (a), (c) tool plunge on the advancing side and (b) and (d) tool plunged on the retreating side. Table 1 Chemical composition of F82H and SUS304. Type
F82H Type
SUS304
Chemical composition in mass % C
Cr
W
V
Ta
–
–
–
0.1
8
2
0.2
0.04
–
–
–
Chemical composition in mass % C
Cr
Ni
Mn
Si
P
S
Mo
0.08
18
8
2
1
0.045
0.003
–
types, with positions of the F82H steel sheet on the advancing side (Fig. 1(a) and (b)), and on the retreating side (Fig. 1(c) and (d)), and the FSW tool was plunged into the F82H sheet (Fig. 1(a) and (d)) and into the austenite stainless steel (Fig. 1(b) and (c)). In this case, the tool probe was shifted a 0.1 mm toward the F82H sheet and SUS304, and did not penetrate into the other steel sheet side. A detailed schematic illustration of the butt FSW is shown in Fig. 1. The welding experiments were performed using a load-controlled FSW machine. The welding tool was made of a WC-based material [22–26] and the tool was tilted 3◦ from the plate normal welding direction. The welding temperature was measured during the dissimilar friction stir butt welding by K-type thermocouples which were embedded on the
interface of the F82H and SUS304, therefore, the measured temperature corresponds to the value at the interface part of the welded joints. Table 2 shows the tool size and FSW parameters. The dissimilar friction stir welded joints of the SUS304/F82H was achieved at a 100 rpm rotation speed and 100 mm/min welding speed. Optical microscopy (OM) and scanning electron microscopy (SEM, JEOL, JSM-7001FA) observations of the microstructure and interface of each FSW joint were carried out. The specimen for the optical microscopy observations was cut perpendicular to the welding direction. Two kinds of etching solutions were used in order to analyze the interface and welded joints. When using Kellers etch (1 vol.% hydrofluoric acid, 1.5 vol.% hydrochloric acid, and 2.5 vol.% nitric acid in solution), only the F82H could be etched, therefore, distribution of the F82H, SUS304 particles and composite material were easily confirmed in the joint. Electrochemical (in a solution of 10% oxalic acid and 90% water with a power supply set to 15 V for 60 s) etching of both the F82H and the SUS304 plate was done. Energy dispersive X-ray spectroscopy (EDX) was performed to determine of the level of diffusion of the elements in the welded joint and interface. The tensile test for the joints used three tensile specimens cut perpendicular to the welding direction. The Vickers microhardness tests were performed in three regions (top, center, bottom of joint) on the cross section perpendicular to the welding direction.
Fig. 2. Microstructures of the base metal: (a) F82H and (b) SUS304.
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Table 2 Tool size and welding parameters. Tool size (mm)
Welding parameters
Shoulder diameter
Probe diameter
Probe length
Tool tilt (◦ )
Rotation speed (rpm)
Welding speed (mm/min)
Revolution pitch (mm/rev)
15
6
1.3
3
100
100
1
Fig. 3. Temperature dependence of 0.2% proof stress of the F82H steel and SUS304 steel.
Fig. 4. Effect of plates position on temperature hysteresis of the welded joints during the welding.
3. Results and discussion
3.3. Effect on the position of steels and plunged tool
3.1. Base metal
Fig. 3 shows the 0.2% proof stress of the F82H steel and SUS304 sheet. The 0.2% proof stress of the F82H steel and SUS304 is 530 and 240 MPa, respectively, at room temperature. The yield strength data of F82H was obtained on several heats by conventional experiment [28,29]. The least squares fitted equation for the yield strength over the temperature range of 20–750 ◦ C is
The surface appearances of the dissimilar friction stir welded joints are shown in Table 3. The positions of the F82H steels were on the advancing (a, b) and retreating sides (c, d) and the tool probe was plunged on the F82H (a, d) and the SUS304 (b, d). When the dissimilar friction stir welding of SUS304/F82H was performed with SUS304 on the retreating and advancing side and tool plunged on the SUS304, the tool was broken during the welding because the 0.2% proof stress of the SUS304 was still high. When the dissimilar friction stir welding of SUS304/F82H was performed with F82H on the advancing and retreating side and tool plunged on the F82H, these conditions was not broken during the welding because the both conditions was welded at the yield strength of F82H lower than SUS304 (Fig. 4). Therefore, in this study, the tool was plunged on the F82H side and the F82H steel was on the advancing side or on the retreating side, was used to discuss the optimum dissimilar friction stir welding of steels.
(MPa) = 531.4 − 0.38794 × T + 0.001482 × T 2 − 2.3965e
3.4. Low-magnification over view
The F82H base metal (Fig. 2(a)) shows a ferritic/martensitic steel with (CrFe)7 C3 precipitation in which the material was fabricated by applying the HIP (hot isostatic pressing) bonding method [27], and Fig. 2(b) shows that the typical microstructure of the base metal of SUS304 consists of equiaxed austenite grains. 3.2. Properties of materials over temperature
3
− 06 × T − 1.4506e − 10 × T
4
where the temperature (T) is in ◦ C. The correlation coefficient for the plotted data using this equation is R = 0.8835. The 0.2% proof stress of the F82H steel and SUS304 decreased with the increasing temperature and then these values reversed over than 710 ◦ C which the 0.2% proof stress of the SUS304 is greater than the F82H.
Fig. 5 shows cross sections of the dissimilar friction stir welded SUS304/F82H joint which the position of F82H is on the advancing side and the tool was plunged on the F82H plate. Fig. 5 shows that the dissimilar joint exhibited six distinct regions. No. 1 (Fig. 5(a)–(c)) cross section of the joint etched by Kellers, and No. 2 (Fig. 5(d)–(f)) cross section of the joint electrochemically etched.
Fig. 5. Optical macroscopic overview of the cross-section of the butt dissimilar friction stir welded SUS304 and F82H steels under conditions of Fig. 1(a).
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Table 3 Effect of the plate and tool positions on the dissimilar joint surfaces under conditions of Fig. 1. Welding condition
Probe position
As
RS
Fig. 1(a)
AS
F82H
SUS304
Fig. 1(b)
RS
F82H
SUS304
Fig. 1(c)
AS
SUS304
F82H
Fig. 1(d)
RS
SUS304
F82H
The Fig. 5(No. 1) dissimilar joint exhibited three distinct regions which were labelled (a) the stir zone in the SUS304, (b) the stir zone in the F82H, and (c) the below AC1 welded region in the F82H on the advancing side. The Fig. 5(No. 2) dissimilar joint exhibited three distinct regions which were labelled (d) the base metal of SUS304, (e) the base metal of F82H, and (f) the HAZ in the SUS304 on the retreating side. Fig. 6 shows cross sections of the dissimilar friction stir welded SUS304/F82H joint which the position of F82H is on the retreating side and the tool was plunged on the F82H plate. Fig. 6 shows that the dissimilar joint exhibited six distinct regions. No. 1 (Fig. 6(a)–(d)) cross section of the joints etched by Kellers, and No. 2 (Fig. 6(e)–(f)) cross section of the joint lectrochemically etched. The Fig. 6(No. 1) dissimilar joint exhibited four distinct regions which were labelled (a) the base metal of F82H, (b) the stir zone in the SUS304 and F82H mixed zone, (c) the stir zone in the bottom part, and (d) the base metal of SUS304. The Fig. 6(No. 2) dissimilar joint exhibited two distinct regions which were labelled (e) the HAZ in the F82H on the retreating side, and (f) the HAZ in the SUS304 on the advancing side. 3.5. Dissimilar friction stir welded joints phenomenon Fig. 7 shows the dissimilar friction stir welded joints of the SUS304/F82H phenomenon. The position of F82H is on the advancing side (a), F82H is on the retreating side (b) and the tool was plunged on the F82H plate. The samples were prepared by cut-
Dissimilar joined surface
ting 0.2 mm off the upper surface using a polishing technique and then the samples were etched by Kellers solution. When the joining begins, F82H (white) and SUS304 are mixed with each other, and the F82H and SUS304 was not mixed except a left black dot regions. Watanabe et al. [15] report this phenomenon which a probe plunged ed in the advancing side, the SUS304 interface has not been activated because the SUS304 contacted with plastic flowed F82H before moving of the probe. In this welding condition, the interfacial position depends on the strength of two steels in the welding temperature. The yield strength of F82H exceeds SUS304, and plasticity flowed F82H mixed with SUS304 side in the arrow (1) regions because the joining temperature is comparatively low at the beginning of joining. However, plastic flowed SUS304 is mixed with F82H side when the joining goes than an arrow (2) because the yield strength of SUS304 exceeds F82H. In Fig. 7(b) case, when the welding was begun, the dissimilar joint morphology indicates that all of the dissimilar joint microstructures were uniformly mixed. We had known that the welding phenomenon was different with F82H on the retreating side and advancing side. Watanabe et al. reported that the dissimilar friction stir welding was performed with soft material on the retreating side and tool plunged on the soft materials, the dissimilar materials a mixed because the hard material faying surface was activated by rotating probe [15]. When the dissimilar joining is performed with soften F82H on the retreating side and the tool plunged on the F82H side at the welding temperature, SUS304 fay-
Fig. 6. Optical macroscopic overview of the cross-section of the butt dissimilar friction stir welded SUS304 and F82H steels under conditions of Fig. 1(d).
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Fig. 7. Effect of plate position on flow near the surface of the dissimilar friction stir butt welded joints; (a) F82H on the advancing side and (b) F82H on the retreating side.
ing side was activated, and then the interface was generated from the back of the rotating tool and SUS304 and F82H were mixed each other. 3.6. Microstructural evolution Fig. 8 shows the microstructure of the dissimilar friction stir welded SUS304/F82H joints which the position of F82H is on the advancing side and the tool was plunged on the F82H plate. Fig. 8(a) shows that the microstructure of the stir zone (SUS304) has a fine equiaxed grain structure. Fig. 8(b) shows the microstructures of the stir zone on the F82H side and the microstructure consists of a mixture of the fine ferrite with transformed martensite and more detail analyses were performed in Fig. 9. Fig. 8(c) shows the microstructure of the below AC1 welded region in the F82H, and the microstructure of this part under went a grain refinement and exhibited the same composition as the base metal. Fig. 8(d) shows the interface microstructure of SUS304/F82H, and the interface microstructure has a fine equiaxed grain structure (SUS304) and fine ferrite structure which was the below AC1 welded. The interface of SUS304/F82H is completely welded and separated and no defects could be found. Fig. 8(e) shows the stir zone microstructure of SUS304/F82H, and
the stir zone did not have a mixed microstructure and it was completely separated. The following SEM results (Fig. 9) show the detailed microstructure and the distribution of the precipitation of the F82H side stir zone which corresponds to Fig. 8(b). The microstructure of the stir zone has exactly the same structure from top to bottom which means that the welding condition is sufficient as the dissimilar welding conditions for the SUS304 and F82H steels and the grain refined ferrite and transformed martensite structures with Cr-rich precipitation (M23 C6 ) were uniformly distributed in the ferrite matrix. The welding temperature exceed AC1 (840 ◦ C) which was the ␥ region, and the ferrite was grain refined by dynamic recrystallization and then ␥ phase transformed into martensite with the decreasing temperature. The grain sizes of the ferrite are less than 1.5 m from top to bottom of the joint and the mean interval of the martensite lath boundaries are 300 nm. The sizes of M23 C6 particles are a mixture with some particles larger than 100 nm and others less than 100 nm. Fig. 10 shows the microstructure of the dissimilar friction stir welded SUS304/F82H joints which the position of F82H is on the retreating side and the tool was plunged on the F82H plate. Fig. 10(a) shows the microstructure of the stir zone in the upper black dotted line as related to Fig. 6(b). This welded joint region
Fig. 8. Microstructures of FSW joint under condition of Fig. 1(a): (a) stir zone in SUS304, (b) stir zone in F82H, (c) HAZ on the advancing side (F82H), (d) interface in SUS304/F82H and (e) microstructure of SUS304/F82H.
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Fig. 9. The SEM images of stir zone in F82H under condition of Fig. 1(a): (a) Top part, (b) center part, (c) bottom part and the distribution of the precipitation of the top (a-1), center (b-1) and bottom part (c-1).
exhibits a mixed structure of the F82H steel and SUS304, and the shape of the mixed structure was complex and contained both lamellar and coarse microstructures. The mixed structure existed in the entire upper part of the welded joint because the upper part of the SUS304 advancing side was stirred with the F82H steel. Fig. 10(b) shows the microstructure of the stir zone (F82H, bottom black dotted line) and these microstructures consisted of a fine ferrite with transformed martensite and more detail analyses were performed in Fig. 10(c) shows the microstructure of the below AC1 (840 ◦ C) welded region in the F82H on the retreating side, and the microstructure of this part became grain refined and exhibited the same composition as the base metal. The real region of Fig. 10(d) is the SUS304 side, but the microstructure showed the same as the SZ in F82H. Therefore, the advancing side of SUS304 was stirred with the F82H steel. Fig. 10(e) shows the interface of F82H/SUS304 on the advancing side, and the interface of SUS304/F82H is
completely welded and separated and no defects could be found. The following SEM results (Fig. 11) show the detailed microstructure and the distribution of the precipitation of the F82H side stir zone which corresponds to Fig. 10(b). Fig. 11(a, a-1) show that the grain refined ferrite and transformed martensite structures with Cr-rich precipitation (M23 C6 ) were uniformly distributed in the ferrite matrix. The welding temperature should exceed AC1 (840 ◦ C) which was the ␥ region, the ferrite was grain refined and then ␥ phase transformed into martensite with the decreasing temperature. The grain sizes of the ferrite are less than 1.5 um and the mean interval of the martensite lath boundaries are 200 nm. The sizes of M23 C6 particles are a mixture with some particles larger than 100 nm and others less than 100 nm. Fig. 11(b) shows that the grain refined ferrite and (CrFe)7 C3 precipitation were uniformly distributed in the ferrite matrix. The welding temperature should
Fig. 10. Microstructures of FSW joint under condition Fig. 1(d): (a) stir zone in the SUS304 and F82H mixed zone, (b) stir zone in F82H, (c) HAZ on the retreating side, (d) HAZ in SUS304 and (e) interface on the advancing side.
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Fig. 11. SEM images of stir zone in F82H: (a) top part of unmixed zone, (a-1) distribution of the precipitation of (a) and (c) bottom part of unmixed zone.
below the AC1 (840 ◦ C), the ferrite was grain refined and then ␥ phase transformation not occurred. The grain sizes of the ferrite are larger than 1um and others less than 1um. The sizes of (CrFe)7 C3 particles are a mixture with some particles larger than 300 nm and others less than 300 nm. 3.6.1. Concentrations distribution The following SEM and EDX mapping results showed the concentrations distribution of the dissimilar friction stir welded SUS304/F82H joints. Fig. 12 shows an SEM image and EDX result of the interface for the dissimilar friction stir welded SUS304/F82H which the position of F82H on the advancing side and the tool
plunged on the F82H steel side. The concentrations of Cr are 16.9 and 8.1 which is approximately the same concentration of each base metal. Fig. 12(b) shows the concentration profiles of Cr and Ni across the region from the EDX analysis, and the concentration profiles of Cr and Ni at the interface between the F82H and the SUS304 suggest that the concentrations of these elements are uniform and then significantly decreased between the interface of the F82H and SUS304. These results indicated that the welding conditions of the F82H steel on the advancing side and tool plunged F82H side are a more suitable for the dissimilar butt FSW SUS304 to F82H steel because the dissimilar butt joined steels were not mixed and perfectly separated from each other.
Fig. 12. SEM image and EDX result of interface of dissimilar butt FSW joint; F82H on the advancing side and tool plunged F82H steel side; (a) point analysis and (b) line analysis.
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Fig. 13. SEM images and EDX mapping of the mixed region of the dissimilar friction stir butt welded joint; F82H on the retreating side and tool plunged on the F82H side.
Fig. 13 shows SEM images (a) and EDX mapping results (b) of the mixed structure of the dissimilar friction stir welded SUS304/F82H joint which the position of F82H on the retreating side and tool plunged F82H side. The concentrations of Cr are 16.1–16.3 in the SUS316 region and 8.5–8.6 in the F82H region both of which are approximately the same concentration in each base metal. This result indicates that the SUS304 and F82H are simply mixed, and the two materials have not diffused from each other. Fig. 13(c) shows the high magnification image of SEM and (d) EDX result of the stir zone in the SUS304 and F82H steel. The mixed zone showed a different concentration of Cr which indicates that the SUS304 and F82H steel were totally mixed in the upper part of the stir zone.
3.7. Mechanical properties evolution The following tensile strength results and the appearance of the joints after the tensile test are shown in Table 4; F82H steel on advancing side (Fig. 13(a)), F82H steel on retreating side (Fig. 13(b)). Fig. 13(a) joint is fractured at the F82H side base metal and Fig. 13(b) fractured at the SUS304/F82H joint interface. Therefore, F82H on the advancing side and tool plunged on the F82H plate is more appropriate for the dissimilar friction stir welding conditions. The tensile strength of the dissimilar friction stir welded SUS304/F82H joints (F82H on the advancing side) is same with the base metal because the dissimilar welded SUS304/F82H joints have fractured on the F82H side base metal. Accordingly, the dissimilar FSW joint shows a stable elongation. Regarding the tensile strength of the dissimilar friction stir welded SUS304/F82H (F82H on the retreating side),
Fig. 14. Hardness profile of cross-section of the dissimilar friction stir butt welded joints; (a) F82H on the advancing and (b) retreating side.
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Y.D. Chung et al. / Materials Science and Engineering A 528 (2011) 5812–5821 Table 4 Tensile strength and appearance of joints after tensile test.
is less than half of the F82H base metal because the dissimilar welded SUS304/F82H joint suddenly fractured at stir zone. The hardness profiles of the dissimilar friction stir welded SUS304/F82H joints with the F82H on the advancing (a) and retreating sides (b) are shown in Fig. 14. The hardness of the SUS304 and F82H steel base metal is approximately 210 HV. Fig. 14(a) shows that the hardness profile of the upper, center and bottom lines of the dissimilar friction stir welded SUS304/F82H joint. The dissimilar joint had a similar trend in which the hardness profiles have two distinct regions; i.e., the SUS304 (retreating side) and F82H steel (advancing side). The hardness value slightly increased toward the SZ (SUS304 side), dramatically increased in the joint center (F82H stir zone) and then decreased in the F82H base metal on the advancing side. The maximum hardness value of the bottom line is less than the upper and center regions because of the different cooling rates. The maximum hardness of the upper and center regions of the welded joint is greater than 500 HV because the welding temperature exceeded AC1 . Fig. 14(b) shows the hardness profile of the upper, center and bottom lines of the dissimilar friction stir welded SUS304/F82H joint. The upper part of the stir zone showed a discontinuous and fluctuating profile because the F82H and SUS304 are mixed. The maximum hardness value of the center and bottom lines is slightly higher than the base metal because of the refined ferrite and equiaxed grain. 4. Conclusions In conclusion, this study clarified that the joining of high melting point dissimilar steels without a mixed structure and no formed inter-metallic compounds can be achieved when the FSW process is conducted under the appropriate conditions. This study focused on the dissimilar friction stir welding of high melting point steels which is a process to prevent the forming of a mixed structure and inter-metallic compounds. When the dissimilar butt welding of high melting point steels was performed by FSW, the properties of two steels at the welding temperature should be considered. The dissimilar steels have totally different properties at the room temperature in which the F82H steel is harder than SUS304. However, the F82H steel is softened with the increasing temperature and then the SUS304 became harder than the F82H at the welding temperature. The yield strength of two steels at the welding temperature is related to the position of the steels and plunged tool. When the dissimilar FSW was performed with soft materials on the advancing side and the tool plunged on the soft materials side at the welding temperature, the interface was clearly divided into the soft and hard materials and no mixed structure was observed. When the dissimilar FSW was performed with soft materials on the retreating side and the tool plunged on the soft materials side at the welding temperature, the stir zone
microstructure exhibited a mixed microstructure of the soft and hard materials.
Acknowledgements This work was supported by the Japan Atomic Energy Agency under the Joint Work contract #21K079. The authors wish to acknowledge the financial support of a Grant-in-Aid for Science Research from the Japan Society for the Promotion of Science and Technology of Japan, the Global COE Programs, a Grant-in-Aid for the Cooperative Research Project of Nationwide Joint-Use Research Institutes from the Ministry of Education, Sports, Culture, Science and Toray Science Foundation, ISIJ Research Promotion Grant, Iketani Foundation and Collaborative Research Based on Industrial Demand.
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