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Mechanical properties of friction stir butt welds of high nitrogen-containing austenitic stainless steel
Materials Science and Engineering A 528 (2011) 2917–2921
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties of friction stir butt welds of high nitrogen-containing austenitic stainless steel Yasuyuki Miyano a,∗ , Hidetoshi Fujii b , Yufeng Sun b , Yasuyuki Katada c , Shuji Kuroda c , Osamu Kamiya d a
Faculty of Educations and Human Studies, Akita University, 1-1 Tegata-gakuen-machi, Akita 010-8502, Japan Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki 567-0047, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan d Faculty of Engineering and Resources, Akita University, 1-1 Tegata-gakuen-machi, Akita 010-8502, Japan b c
a r t i c l e
i n f o
Article history: Received 25 September 2010 Received in revised form 20 December 2010 Accepted 20 December 2010 Available online 25 December 2010 Keywords: Friction stir welding Butt-welding Austenite Phase transformation Grain refinement Nitride
a b s t r a c t In this study, the friction stir butt welding of 2-mm-thick high nitrogen-containing stainless steel (HNS; Ni-free austenitic stainless steel containing 1 mass% nitrogen) plates was performed using a loadcontrolled friction stir welding (FSW) machine with a Si3 N4 -based tool at various welding speeds, i.e., 50 mm/min, 100 mm/min, 200 mm/min and 300 mm/min, and a constant tool rotating speed of 400 rpm. To determine the optimum welding conditions to create reliable HNS FSW joints, the effect of the heat input on the mechanical properties of the HNS FSW joints was studied. The mechanical properties were evaluated by the Vickers hardness test and the tensile strength test. Full-penetrated and defect-free butt welded joints were successfully produced, under all the applied welding conditions. The stir zones consisted of very fine grained structures and showed an increase in the Vickers hardness. These joints also showed a higher tensile strength and yield strength than the base metal. In particular, the FSW welds obtained at a welding speed of 100 mm/min, which showed the best mechanical properties, had a relatively higher Vickers hardness, which indicates a good relationship between the welding parameter (heat input) and the hardness profile due to the microstructure refinements. It was estimated that these welding conditions were optimal, and under these conditions both grain growth and ␣-phase formation were prevented. © 2010 Elsevier B.V. All rights reserved.
1. Introduction High nitrogen-containing austenitic stainless steel (HNS) is one of the superior materials [1–9] that utilizes nitrogen as an alternative alloying element instead of nickel. In this steel, nitrogen is utilized to enhance the formation and stability of austenite and increase the austenite phase range. The high nitrogen content and the characteristics of austenitic steels give this material some superior aspects. These include high strength, high ductility and high toughness. Because nitrogen can drastically increase both the yield strength and ultimate tensile strength without any loss in ductility or toughness, through solidification strengthening, fine grain size strengthening, deformation strengthening and precipitate strengthening. Another advantage is high corrosion resistance. Because the high nitrogen content can improve the local corrosion resistance of stainless steel, especially pitting and crevice corrosion resistance. In addition, given the lack of nickel resources, HNS can be considered one of the next generation austenitic stainless
∗ Corresponding author. Tel.: +81 18 889 2522; fax: +81 18 833 3049. E-mail address: [email protected] (Y. Miyano). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.12.071
steels. Moreover, high nitrogen contents in HNS avoid problems with nickel allergies. Although HNS is known as a superior material, some problems with this material remain, such as nitride desorption from the weld pool, weld defects (porosity) related to blowhole generation in the fusion zone and nitride precipitation in the heat affected zone, which is caused by the heat input during the welding process due to the high nitrogen content [10–15]. These phenomena can reduce the mechanical properties as well as the corrosion resistance of the weld joints, which is why HNS is well known as a poorly weldable material. Based on such information, the application of a solid-state joining process for HNS, which can be performed below its melting point has been sought [16,17]. Friction stir welding (FSW), a new solid-state welding process developed by TWI (The Welding Institute, England) in 1991 [18], is considered an effective technique. Because FSW is a low heat-input welding process, and thus the welding heat input can be controlled below the melting point of HNS to avoid chromium nitride precipitation. Although FSW was originally investigated for low melting materials, such as Al, Mg and Cu alloys, the application of FSW to steel and other higher temperature materials has recently achieved significantly progress with the development of welding tools than
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can endure higher welding temperatures. In the study of FSW for HNS, only two research studies have been reported. Park et al. tried to produce stir in plate friction stir welds of HNS using polycrystalline cubic boron nitride (PCBN) [17]. However, sound welds could not be produced because of a tunnel-type defect in the stir zone and the severe wear of the PCBN tool. Sato et al. also tried to produce stir in plate friction stir welds of HNS using a PCBN tool [19]. The precise microstructure analysis of the stir in plate welds and HAZ showed the possibility of applying FSW to HNS to produce relatively high-quality welds and the tendency of chromium nitride precipitation related to the welding conditions. However, friction stir butt welding joints of HNS have not yet been successfully obtained. In this study, friction stir butt welding was applied to 2mm-thick high nitrogen-containing stainless steel (HNS; Ni-free austenitic stainless steel containing 1 mass% nitrogen) plates utilizing a Si3 N4 -based tool at different welding speeds from 50 to 300 mm/min and a constant rotation speed of 400 rpm. To determine the optimum welding conditions to create reliable HNS FSW joints, the effect of the heat input on the mechanical properties of the HNS FSW joints was studied. In addition, the relationship of the welding temperature and the microstructure (grain size and phase in the stir zone) was investigated in detail.
2. Experimental procedure The material used in this study was an HNS, Ni-free austenitic stainless steel containing 1 mass% nitrogen, produced by a pressurized electro-slag remelting (ESR) method [20]. Its chemical composition is Fe–23Cr–0Ni–1Mo–1N (the composition is in mass%). 1-Groove butt welding was performed using a load-controlled FSW machine. The plates were 2.0 mm thick, 30 mm wide and 150 mm long. The FSW was performed at a constant tool rotating speed of 400 rpm in the clockwise direction. To assess the range of optimum FSW conditions, several welding speeds were adopted, i.e., 50, 100, 200 and 300 mm/min. The welding tool, made of a Si3 N4 -based material and equipped with a columnar shape without a thread, which had a 15-mm-dia shoulder and 6-mm-dia probe, was applied. The tool tilt angle was set at 3◦ from the plate nominal direction [21,22]. To prevent oxidation of the plates, a water-cooled holder was installed, and an argon shield gas was employed. The plate top surface and groove surface were degreased with acetone just prior to the welding. During the entire welding process, the temperature variation at the welding center was monitored using a K-type thermocouple fixed on the bottom surface at the centerline of the work pieces. The joints were evaluated on the basis of their surface appearance, macrostructure and microstructure observations. The metallurgical inspections were performed on the cross section of the FSW joints after polishing and etching with aqua regia for optical microscopic observations. The microstructure of the welds was examined by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). The grain size in the stir zone and the phase constituents in the stir zone center were analyzed by OIM. Samples for the OIM analysis were cut perpendicular to the welding direction, thinned to 0.2 mm by abrasion using SiC papers and electron polished in a 10% oxalic acid solution at a temperature at −20 ◦ C and 30 V using a Struers Tenupol twin-jet unit. The mechanical properties were evaluated using the Vickers hardness test and transverse tensile test. The hardness profiles of the transverse section of the joints were measured at the thickness center with a load of 4.9 N for 15 s. The tensile properties of each joint were measured using the tensile test specimens machined with an electrical-discharging machine with the tensile axis perpendicular to the FSW joint line. The required dimensions were,
Fig. 1. Configuration of the transverse tensile specimen used in this study: (a) specifications and (b) appearance.
total length: 50 mm, gage length: 13 mm, and gage width: 6 mm, which are shown in Fig. 1(a). The FSW zone was situated in the center of the gage length (Fig. 1(b)). The tensile tests were performed at a constant crosshead speed of 1.0 mm/s at room temperature. 3. Results and discussion 3.1. Metallurgical inspection Fig. 2 shows typical images of the transverse cross section of the FSW welds, which were processed at different welding speeds with a constant rotation speed of 400 rpm. In the present study, FSW of the HNS was successfully performed. Specifically, different fully penetrated FSW butt welds of the HNS were successfully obtained by varying the processing parameter (heat input). The welding center showed a high degree of continuity, an extremely fine grain structure and no defects. Regardless of the welding conditions, no porosity and/or defects were observed in both the weld top and root surfaces. However, the size of the joint structure, the size of the stir zone and the width of the bottom, increased with decreasing welding speed, which indicates a good relationship between the welding parameter (heat input) and the macrostructure. 3.2. Tensile strength test The stress–strain curves obtained from the tensile test of the base material and transverse weld samples are plotted in Fig. 3. In this test, the gage lengths of the tensile test specimens were almost the same as the area size of the stir zone; however, all the fractures after the test were observed in the HAZ regions (Fig. 4). Furthermore, the tensile properties of the joint showed a higher yield strength and tensile strength than those of the base material. As shown in this figure, despite the values of the yield strengths being roughly the same for all the weld samples, the global ductility of the weld samples changed depending on the welding parameter. Specifically, the highest value was observed at a welding speed of 100 mm/min, and the lowest value was observed at a welding speed of 300 mm/min. 3.3. Vickers hardness test Fig. 5 shows the hardness profiles along the centerline of weld cross sections under different welding conditions. The hardness within the weld region was higher than that of the base met-
Y. Miyano et al. / Materials Science and Engineering A 528 (2011) 2917–2921
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Fig. 2. Optical micrographs of a transverse section of the welded zone in 1 mass% nitrogen-containing austenitic stainless steel at various welding speeds and a constant rotating speed of 400 rpm.
1400 Base metal
Stress (MPa)
1200 1000 800 100 mm/min
300 mm/min
600
50 mm/min
400
200 mm/min Rotation speed : 400rpm
200 0 0
20
40
60
80
100
120
140
Strain (%) Fig. 3. Tensile strength of 1 mass% nitrogen-containing austenitic stainless steel at various welding speeds and a constant rotating speed of 400 rpm.
Fig. 5. Vickers hardness of 1 mass% nitrogen-containing austenitic stainless steel at various welding speeds and a constant rotating speed of 400 rpm.
Fig. 4. Failure in the transverse tensile specimen used in this study.
als, which is likely due to the generation of extremely fine grains induced by the severe plastic deformation during the FSW (the size of grain become smaller and become uniform approaching to the welding center), although the hardness profiles were roughly dependent on the welding conditions. As shown here, at a constant rotating speed of 400 rpm, the lower the welding speed is (the greater the heat input), the lower the value of the hardness in the stir zone, which indicates a good relationship between the welding parameter (heat input) and the hardness profile. In particular, the FSW welds obtained at a welding speed of 100 mm/min, which showed the best mechanical properties, had a relatively higher hardness.
Fig. 6. Thermal profiles measured at the back of HNS plates during FSW process with various welding speeds and a constant rotating speed of 400 rpm.
3.4. Welding temperature measurement Fig. 6 shows the thermal profiles measured at the back of the material during the FSW process. The measured temperature increased sharply to its maximum value when the rotating tools passed through the locations where the thermocouples were fixed in the centerline on the bottom surface of the material. The temperature then decreased gradually to room temperature. The maximum temperature increases strongly depended on the welding speed. The higher the welding speed is, the lower the
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Fig. 7. The results of the thermodynamic calculation of the calculated isopleths for the Fe–23Cr–0Ni–1Mo–N alloy system and the superimposed plots of the measured peak welding temperature under various conditions.
temperature increase and the lower the inclination of the plots growth. The maximum measured temperature increase reached about 1110 ◦ C when the rotation speed was 50 mm/min, 1050 ◦ C when it was 100 mm/min, 1010 ◦ C when it was 200 mm/min, and 950 ◦ C when it was 300 mm/min, which indicates a good relationship between the welding parameter (heat input) and the macrostructure. 3.5. Thermodynamic calculation and welding temperature Fig. 7 is the calculated phase diagram for the Fe–23Cr–1Mo–N alloy and the variation in the distribution of phases in equilib-
rium with the nitrogen content and temperature. This diagram was calculated using the Pandat software package [23,24] and the ssol2, thermodynamic database. The vertical (dashed) line denotes the chemical composition of the investigated alloy, and the plots along with this line denote the variation in the peak temperature measured by the K-type thermocouple for each welding condition, which were derived from the results shown in Fig. 6. The data must be read in the following way. The welding speed of 50 mm/min is only located in the ␥ single-phase region (FCC). The other conditions, 100, 200 and 300 mm/min, are located in the ␥ + Cr2 N (FCC + HCP(Cr2 N)), ␥ + Cr2 N + ␣ (FCC + HCP(Cr2 N) + BCC) and ␣ + Cr2 N (BCC + HCP(Cr2 N)) phase regions, respectively. From this point of view, the FSW should be performed at a temperature located in the ␥ single-phase region (FCC), because the ␣ phase (BCC) formation or Cr2 N (HCP) precipitation is considered to reduce the mechanical and material properties of the weld. In this study, except for the welding speed of 50 mm/min, the welding was performed in regions that could form the ␣ phase or precipitate Cr2 N. However, the FSW joint obtained at a welding speed of 100 mm/min showed the highest yield strength and the highest strain. Concerning this point, the authors postulate the following ideas. In this study, the thermocouples for measuring the temperature were embedded on the bottom surface at the centerline [25]. Therefore, it could be assumed that the actual peak temperature was quite close to the ␥ single-phase region, because the heat input in the upper zone should be estimated as a higher value. In other words, if the welding temperatures were precisely controlled by the welding conditions, the friction stir welding was performed in the austenite single phase region and could produce sound friction stir weld joints on the HNS.
Fig. 8. Orientation imaging maps of the FSW welds center region of the 1 mass% nitrogen-containing austenitic stainless steel at various welding speed and a constant rotating speed of 400 rpm. (a) Austenite phase identifications and (b) Ferrite phase identification.
Y. Miyano et al. / Materials Science and Engineering A 528 (2011) 2917–2921
3.6. OIM analysis OIM was used to examine the grain size and identify the phases. Fig. 8(a) shows grain boundary maps with the crystallographic orientations of the these four welding conditions that produced the joints studied, namely the welding speeds of 50 mm/min (a), 100 mm/min (b), 200 mm/min (c) and 300 mm/min (d). The OIM sample was obtained from the center part of the stir zone. In these maps, only high-angle boundaries with misorientations higher than 15◦ are indicated by black lines. From these observations, the grain size in the stir zone became smaller with increasing welding speed (i.e., decreasing heat input). This result shows a good correlation between the ultrarefinements of the grains and the grain size growth by heat input into the stir zone. In particular, the remarkable grain growth observed at the welding speed of 50 mm/min and the remarkable ultrarefinements observed at the welding speed of 300 mm/min are in good agreement with this observation. The average grain size in the conditions of 50, 100, 200 and 300 mm/min, detected by the line interception method, are 3.957, 2.593, 2.2572 and 2.594 m, respectively. In addition, as can be seen in these figures, black spot precipitations are present around the grain boundaries. Based on the results of additionally conducted ferrite phase identifications by OIM mapping (Fig. 8(b)), almost all of these black spots were identified as the ␣ phase shown in Fig. 8(a). The increase in the area distribution of the black spots is clearly confirmed with increasing welding speed (i.e., decreasing heat input). For the lowest heat input condition (400 rpm, 300 mm/min), the area distribution of the black spots is relatively large; however, for the highest heat input condition (400 rpm, 50 mm/min), the area distribution of the black spots is hardly seen. These results are in good agreement with the phase diagram and the experimentally acquired peak heat temperatures shown in Fig. 7. In this study, the FSW joint, which has the highest mechanical properties, was produced at a welding speed of 100 mm/min and a rotation speed of 400 rpm. In other words, this set of conditions is one of the sets of optimum conditions for obtaining good HNS FSW joints, because these conditions can sustain the austenite single phase, prevent ␣-phase formation and induce the ultrarefinement to improve the mechanical properties of the joints. In contrast, at a lower welding speed (higher heat input), when the temperature measured was more 1100 ◦ C, grain growth, which reduces the hardening in the stir zone, was induced. Furthermore, at higher welding speeds (lower heat input), the temperature measured was less 950 ◦ C, and thus, the ␥–␣ transformation, which reduces the mechanical properties of the joints, was strongly expected, for the sake of ␣ precipitations in the ␥ single phase. 4. Conclusions In this study, 1-groove friction stir butt welding of 2-mm-thick high nitrogen-containing austenitic stainless steel (HNS) plates was successfully performed without welding defects or tool deformation utilizing Si3 N4 at different welding speeds from 50 to 300 mm/min and a constant rotation speed of 400 rpm. The following conclusions can be drawn from these experimental results. The stir zones obtained in this experiment consist of very fine grained structures and show an increase in the Vickers hardness. These joints also show a higher tensile strength and yield strength than the as-received joint. In particular, the FSW welds obtained at the welding speed of 100 mm/min, which shows the best mechan-
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ical properties, had a relatively higher Vickers hardness. This indicates a good relationship between the welding parameter (heat input) and the hardness profile. Based on the results of the thermal profiles measurement during the FSW process and the thermodynamic calculations of the experimental material, the FSW conditions, a welding speed of 100 mm/min and a tool rotation speed of 400 mm/min were estimated to be the most preferable conditions for obtaining the HNS FSW joints, which can sustain the ␥ single phase and prevent ␣phase formation. Acknowledgements The authors wish to thank Mr. M. Kato, Akita Industrial Research Institute, and Mr. R. Kato, Akita University, for their technical assistance. The financial support from the Japanese Ministry of Education, Science, Sports and Culture with a Grant-in-Aid for Young Scientists (Start-up, No. 20860019-00) and a Grant-in-Aid for Scientific Research (B) (No. 00247230) are gratefully acknowledged. This study was performed under the Cooperative Research Program of Institute for Joining and Welding Research Institute, Osaka University. References [1] L. Yuping, Z. Yong, R. Fan, C. Haitao, W. Yuqing, S. Jie, D. Han, The Conference Proceeding of HNS, 2009, pp. 195–201. [2] J.W. Simmons, Materials Science and Engineering A 207 (2) (1996) 159–169. [3] N. Nakada, N. Hirakawa, T. Tsuchiyama, S. Takaki, Scripta Materialia 57 (2) (2007) 153–156. [4] P.J. Uggowitzer, R. Magdowski, M.O. Speidel, ISIJ International 36 (7) (1996) 901–908. [5] M. Sagara, Y. Katada, T. Kodama, ISIJ International 43 (5) (2003) 714–719. [6] H. Baba, T. Kodama, Y. Katada, Corrosion Science 44 (10) (2002) 2393–2407. [7] H. Baba, Y. Katada, Corrosion Science 48 (9) (2006) 2510–2524. [8] W. Bal, H. Kozøowski, K.S. Kasprzak, Journal of Inorganic Biochemistry 79 (1–4) (2000) 213–218. [9] M. Vahter, M. Berglund, A. Åkesson, C. Lidén, Environmental Research 88 (3) (2002) 145–155. [10] O. Kamiya, Z.W. Chen, Y. Kikuchi, T. Ohyoshi. Proceedings of the International Conference on AMDP2002, SD-file, Metal-paper 3-10. [11] O. Kamiya, Y. Kikuchi, Z.W. Chen, Proceedings of ICMR 2001 (Akita), 2001, pp. 215–220. [12] I. Woo, Y. Kikuchi, ISIJ International 42-12 (2002) 1334–1343. [13] T. Ogawa, K. Hiraoka, Y. Katada, M. Sagara, C. Shiga, Quarterly Journal of Japan Welding Society 20-1 (2002) 96–104. [14] T. Ogawa, K. Hiraoka, Y. Katada, M. Sagara, S. Tsukamoto, C. Shiga, Quarterly Journal of Japan Welding Society 20-1 (2002) 107–113. [15] I. Woo, T. Horinouchi, Y. Miyano, Y. Kikuchi, Journal of Steel and Related Materials 2 (2004) 187–196. [16] I. Woo, M. Aritoshi, Y. Kikuchi, ISIJ International 42-4 (2002) 401–406. [17] S.H.C. Park, Y.S. Sato, H. Kokawa, K. Okamoto, S. Hirano, M. Inagaki, Materials Science Forum 539-543 (2007) 3757–3762. [18] W.M. Thomas, E.D. Nicholas, J.C. Needhman, M.G. Murch, P. Temple-Smith, C.J. Dawes. International Patent Application PCT/GB92/02203 and GB Patent Application 9125978.8, UK Patent Office, London, December 6, 1991. [19] Y.S. Sato, K. Nakamura, H. Kokawa, S. Narita, T. Shimizu. Metals and Material Society (2009) 67–73. [20] Y. Katada, M. Sagara, Y. Kobayashi, T. Kodama, Materials and Manufacturing Processes 19 (1) (2004) 19–30. [21] H. Fujii, R. Ueji, Y. Takada, H. Kitahara, N. Tsuji, K. Nakata, K. Nogi, Materials Transactions 47 (2006) 239–242. [22] H. Fujii, L. Cui, N. Tsuji, M. Maeda, K. Nakata, K. Nogi, Material Science Engineering A 429 (2006) 50–57. [23] W. Cao, S.-L. Chen, F. Zhang, K. Wu, Y. Yang, Y.A. Chang, R. Schmid-Fetzer, W.A. Oates, CALPHAD 33 (2) (2009) 328–342. [24] S.-L. Chen, S. Daniel, F. Zhang, Y.A. Chang, X.-Y. Yan, F.-Y. Xie, R. Schmid-Fetzer, W.A. Oates, CALPHAD 26 (2) (2002) 175–188. [25] L. Cui, H. Fujii, N. Tsuji, K. Nogi, Scripta Materialia 56 (2007) 637–640.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties of friction stir butt welds of high nitrogen-containing austenitic stainless steel Yasuyuki Miyano a,∗ , Hidetoshi Fujii b , Yufeng Sun b , Yasuyuki Katada c , Shuji Kuroda c , Osamu Kamiya d a
Faculty of Educations and Human Studies, Akita University, 1-1 Tegata-gakuen-machi, Akita 010-8502, Japan Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki 567-0047, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan d Faculty of Engineering and Resources, Akita University, 1-1 Tegata-gakuen-machi, Akita 010-8502, Japan b c
a r t i c l e
i n f o
Article history: Received 25 September 2010 Received in revised form 20 December 2010 Accepted 20 December 2010 Available online 25 December 2010 Keywords: Friction stir welding Butt-welding Austenite Phase transformation Grain refinement Nitride
a b s t r a c t In this study, the friction stir butt welding of 2-mm-thick high nitrogen-containing stainless steel (HNS; Ni-free austenitic stainless steel containing 1 mass% nitrogen) plates was performed using a loadcontrolled friction stir welding (FSW) machine with a Si3 N4 -based tool at various welding speeds, i.e., 50 mm/min, 100 mm/min, 200 mm/min and 300 mm/min, and a constant tool rotating speed of 400 rpm. To determine the optimum welding conditions to create reliable HNS FSW joints, the effect of the heat input on the mechanical properties of the HNS FSW joints was studied. The mechanical properties were evaluated by the Vickers hardness test and the tensile strength test. Full-penetrated and defect-free butt welded joints were successfully produced, under all the applied welding conditions. The stir zones consisted of very fine grained structures and showed an increase in the Vickers hardness. These joints also showed a higher tensile strength and yield strength than the base metal. In particular, the FSW welds obtained at a welding speed of 100 mm/min, which showed the best mechanical properties, had a relatively higher Vickers hardness, which indicates a good relationship between the welding parameter (heat input) and the hardness profile due to the microstructure refinements. It was estimated that these welding conditions were optimal, and under these conditions both grain growth and ␣-phase formation were prevented. © 2010 Elsevier B.V. All rights reserved.
1. Introduction High nitrogen-containing austenitic stainless steel (HNS) is one of the superior materials [1–9] that utilizes nitrogen as an alternative alloying element instead of nickel. In this steel, nitrogen is utilized to enhance the formation and stability of austenite and increase the austenite phase range. The high nitrogen content and the characteristics of austenitic steels give this material some superior aspects. These include high strength, high ductility and high toughness. Because nitrogen can drastically increase both the yield strength and ultimate tensile strength without any loss in ductility or toughness, through solidification strengthening, fine grain size strengthening, deformation strengthening and precipitate strengthening. Another advantage is high corrosion resistance. Because the high nitrogen content can improve the local corrosion resistance of stainless steel, especially pitting and crevice corrosion resistance. In addition, given the lack of nickel resources, HNS can be considered one of the next generation austenitic stainless
∗ Corresponding author. Tel.: +81 18 889 2522; fax: +81 18 833 3049. E-mail address: [email protected] (Y. Miyano). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.12.071
steels. Moreover, high nitrogen contents in HNS avoid problems with nickel allergies. Although HNS is known as a superior material, some problems with this material remain, such as nitride desorption from the weld pool, weld defects (porosity) related to blowhole generation in the fusion zone and nitride precipitation in the heat affected zone, which is caused by the heat input during the welding process due to the high nitrogen content [10–15]. These phenomena can reduce the mechanical properties as well as the corrosion resistance of the weld joints, which is why HNS is well known as a poorly weldable material. Based on such information, the application of a solid-state joining process for HNS, which can be performed below its melting point has been sought [16,17]. Friction stir welding (FSW), a new solid-state welding process developed by TWI (The Welding Institute, England) in 1991 [18], is considered an effective technique. Because FSW is a low heat-input welding process, and thus the welding heat input can be controlled below the melting point of HNS to avoid chromium nitride precipitation. Although FSW was originally investigated for low melting materials, such as Al, Mg and Cu alloys, the application of FSW to steel and other higher temperature materials has recently achieved significantly progress with the development of welding tools than
2918
Y. Miyano et al. / Materials Science and Engineering A 528 (2011) 2917–2921
can endure higher welding temperatures. In the study of FSW for HNS, only two research studies have been reported. Park et al. tried to produce stir in plate friction stir welds of HNS using polycrystalline cubic boron nitride (PCBN) [17]. However, sound welds could not be produced because of a tunnel-type defect in the stir zone and the severe wear of the PCBN tool. Sato et al. also tried to produce stir in plate friction stir welds of HNS using a PCBN tool [19]. The precise microstructure analysis of the stir in plate welds and HAZ showed the possibility of applying FSW to HNS to produce relatively high-quality welds and the tendency of chromium nitride precipitation related to the welding conditions. However, friction stir butt welding joints of HNS have not yet been successfully obtained. In this study, friction stir butt welding was applied to 2mm-thick high nitrogen-containing stainless steel (HNS; Ni-free austenitic stainless steel containing 1 mass% nitrogen) plates utilizing a Si3 N4 -based tool at different welding speeds from 50 to 300 mm/min and a constant rotation speed of 400 rpm. To determine the optimum welding conditions to create reliable HNS FSW joints, the effect of the heat input on the mechanical properties of the HNS FSW joints was studied. In addition, the relationship of the welding temperature and the microstructure (grain size and phase in the stir zone) was investigated in detail.
2. Experimental procedure The material used in this study was an HNS, Ni-free austenitic stainless steel containing 1 mass% nitrogen, produced by a pressurized electro-slag remelting (ESR) method [20]. Its chemical composition is Fe–23Cr–0Ni–1Mo–1N (the composition is in mass%). 1-Groove butt welding was performed using a load-controlled FSW machine. The plates were 2.0 mm thick, 30 mm wide and 150 mm long. The FSW was performed at a constant tool rotating speed of 400 rpm in the clockwise direction. To assess the range of optimum FSW conditions, several welding speeds were adopted, i.e., 50, 100, 200 and 300 mm/min. The welding tool, made of a Si3 N4 -based material and equipped with a columnar shape without a thread, which had a 15-mm-dia shoulder and 6-mm-dia probe, was applied. The tool tilt angle was set at 3◦ from the plate nominal direction [21,22]. To prevent oxidation of the plates, a water-cooled holder was installed, and an argon shield gas was employed. The plate top surface and groove surface were degreased with acetone just prior to the welding. During the entire welding process, the temperature variation at the welding center was monitored using a K-type thermocouple fixed on the bottom surface at the centerline of the work pieces. The joints were evaluated on the basis of their surface appearance, macrostructure and microstructure observations. The metallurgical inspections were performed on the cross section of the FSW joints after polishing and etching with aqua regia for optical microscopic observations. The microstructure of the welds was examined by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). The grain size in the stir zone and the phase constituents in the stir zone center were analyzed by OIM. Samples for the OIM analysis were cut perpendicular to the welding direction, thinned to 0.2 mm by abrasion using SiC papers and electron polished in a 10% oxalic acid solution at a temperature at −20 ◦ C and 30 V using a Struers Tenupol twin-jet unit. The mechanical properties were evaluated using the Vickers hardness test and transverse tensile test. The hardness profiles of the transverse section of the joints were measured at the thickness center with a load of 4.9 N for 15 s. The tensile properties of each joint were measured using the tensile test specimens machined with an electrical-discharging machine with the tensile axis perpendicular to the FSW joint line. The required dimensions were,
Fig. 1. Configuration of the transverse tensile specimen used in this study: (a) specifications and (b) appearance.
total length: 50 mm, gage length: 13 mm, and gage width: 6 mm, which are shown in Fig. 1(a). The FSW zone was situated in the center of the gage length (Fig. 1(b)). The tensile tests were performed at a constant crosshead speed of 1.0 mm/s at room temperature. 3. Results and discussion 3.1. Metallurgical inspection Fig. 2 shows typical images of the transverse cross section of the FSW welds, which were processed at different welding speeds with a constant rotation speed of 400 rpm. In the present study, FSW of the HNS was successfully performed. Specifically, different fully penetrated FSW butt welds of the HNS were successfully obtained by varying the processing parameter (heat input). The welding center showed a high degree of continuity, an extremely fine grain structure and no defects. Regardless of the welding conditions, no porosity and/or defects were observed in both the weld top and root surfaces. However, the size of the joint structure, the size of the stir zone and the width of the bottom, increased with decreasing welding speed, which indicates a good relationship between the welding parameter (heat input) and the macrostructure. 3.2. Tensile strength test The stress–strain curves obtained from the tensile test of the base material and transverse weld samples are plotted in Fig. 3. In this test, the gage lengths of the tensile test specimens were almost the same as the area size of the stir zone; however, all the fractures after the test were observed in the HAZ regions (Fig. 4). Furthermore, the tensile properties of the joint showed a higher yield strength and tensile strength than those of the base material. As shown in this figure, despite the values of the yield strengths being roughly the same for all the weld samples, the global ductility of the weld samples changed depending on the welding parameter. Specifically, the highest value was observed at a welding speed of 100 mm/min, and the lowest value was observed at a welding speed of 300 mm/min. 3.3. Vickers hardness test Fig. 5 shows the hardness profiles along the centerline of weld cross sections under different welding conditions. The hardness within the weld region was higher than that of the base met-
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Fig. 2. Optical micrographs of a transverse section of the welded zone in 1 mass% nitrogen-containing austenitic stainless steel at various welding speeds and a constant rotating speed of 400 rpm.
1400 Base metal
Stress (MPa)
1200 1000 800 100 mm/min
300 mm/min
600
50 mm/min
400
200 mm/min Rotation speed : 400rpm
200 0 0
20
40
60
80
100
120
140
Strain (%) Fig. 3. Tensile strength of 1 mass% nitrogen-containing austenitic stainless steel at various welding speeds and a constant rotating speed of 400 rpm.
Fig. 5. Vickers hardness of 1 mass% nitrogen-containing austenitic stainless steel at various welding speeds and a constant rotating speed of 400 rpm.
Fig. 4. Failure in the transverse tensile specimen used in this study.
als, which is likely due to the generation of extremely fine grains induced by the severe plastic deformation during the FSW (the size of grain become smaller and become uniform approaching to the welding center), although the hardness profiles were roughly dependent on the welding conditions. As shown here, at a constant rotating speed of 400 rpm, the lower the welding speed is (the greater the heat input), the lower the value of the hardness in the stir zone, which indicates a good relationship between the welding parameter (heat input) and the hardness profile. In particular, the FSW welds obtained at a welding speed of 100 mm/min, which showed the best mechanical properties, had a relatively higher hardness.
Fig. 6. Thermal profiles measured at the back of HNS plates during FSW process with various welding speeds and a constant rotating speed of 400 rpm.
3.4. Welding temperature measurement Fig. 6 shows the thermal profiles measured at the back of the material during the FSW process. The measured temperature increased sharply to its maximum value when the rotating tools passed through the locations where the thermocouples were fixed in the centerline on the bottom surface of the material. The temperature then decreased gradually to room temperature. The maximum temperature increases strongly depended on the welding speed. The higher the welding speed is, the lower the
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Fig. 7. The results of the thermodynamic calculation of the calculated isopleths for the Fe–23Cr–0Ni–1Mo–N alloy system and the superimposed plots of the measured peak welding temperature under various conditions.
temperature increase and the lower the inclination of the plots growth. The maximum measured temperature increase reached about 1110 ◦ C when the rotation speed was 50 mm/min, 1050 ◦ C when it was 100 mm/min, 1010 ◦ C when it was 200 mm/min, and 950 ◦ C when it was 300 mm/min, which indicates a good relationship between the welding parameter (heat input) and the macrostructure. 3.5. Thermodynamic calculation and welding temperature Fig. 7 is the calculated phase diagram for the Fe–23Cr–1Mo–N alloy and the variation in the distribution of phases in equilib-
rium with the nitrogen content and temperature. This diagram was calculated using the Pandat software package [23,24] and the ssol2, thermodynamic database. The vertical (dashed) line denotes the chemical composition of the investigated alloy, and the plots along with this line denote the variation in the peak temperature measured by the K-type thermocouple for each welding condition, which were derived from the results shown in Fig. 6. The data must be read in the following way. The welding speed of 50 mm/min is only located in the ␥ single-phase region (FCC). The other conditions, 100, 200 and 300 mm/min, are located in the ␥ + Cr2 N (FCC + HCP(Cr2 N)), ␥ + Cr2 N + ␣ (FCC + HCP(Cr2 N) + BCC) and ␣ + Cr2 N (BCC + HCP(Cr2 N)) phase regions, respectively. From this point of view, the FSW should be performed at a temperature located in the ␥ single-phase region (FCC), because the ␣ phase (BCC) formation or Cr2 N (HCP) precipitation is considered to reduce the mechanical and material properties of the weld. In this study, except for the welding speed of 50 mm/min, the welding was performed in regions that could form the ␣ phase or precipitate Cr2 N. However, the FSW joint obtained at a welding speed of 100 mm/min showed the highest yield strength and the highest strain. Concerning this point, the authors postulate the following ideas. In this study, the thermocouples for measuring the temperature were embedded on the bottom surface at the centerline [25]. Therefore, it could be assumed that the actual peak temperature was quite close to the ␥ single-phase region, because the heat input in the upper zone should be estimated as a higher value. In other words, if the welding temperatures were precisely controlled by the welding conditions, the friction stir welding was performed in the austenite single phase region and could produce sound friction stir weld joints on the HNS.
Fig. 8. Orientation imaging maps of the FSW welds center region of the 1 mass% nitrogen-containing austenitic stainless steel at various welding speed and a constant rotating speed of 400 rpm. (a) Austenite phase identifications and (b) Ferrite phase identification.
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3.6. OIM analysis OIM was used to examine the grain size and identify the phases. Fig. 8(a) shows grain boundary maps with the crystallographic orientations of the these four welding conditions that produced the joints studied, namely the welding speeds of 50 mm/min (a), 100 mm/min (b), 200 mm/min (c) and 300 mm/min (d). The OIM sample was obtained from the center part of the stir zone. In these maps, only high-angle boundaries with misorientations higher than 15◦ are indicated by black lines. From these observations, the grain size in the stir zone became smaller with increasing welding speed (i.e., decreasing heat input). This result shows a good correlation between the ultrarefinements of the grains and the grain size growth by heat input into the stir zone. In particular, the remarkable grain growth observed at the welding speed of 50 mm/min and the remarkable ultrarefinements observed at the welding speed of 300 mm/min are in good agreement with this observation. The average grain size in the conditions of 50, 100, 200 and 300 mm/min, detected by the line interception method, are 3.957, 2.593, 2.2572 and 2.594 m, respectively. In addition, as can be seen in these figures, black spot precipitations are present around the grain boundaries. Based on the results of additionally conducted ferrite phase identifications by OIM mapping (Fig. 8(b)), almost all of these black spots were identified as the ␣ phase shown in Fig. 8(a). The increase in the area distribution of the black spots is clearly confirmed with increasing welding speed (i.e., decreasing heat input). For the lowest heat input condition (400 rpm, 300 mm/min), the area distribution of the black spots is relatively large; however, for the highest heat input condition (400 rpm, 50 mm/min), the area distribution of the black spots is hardly seen. These results are in good agreement with the phase diagram and the experimentally acquired peak heat temperatures shown in Fig. 7. In this study, the FSW joint, which has the highest mechanical properties, was produced at a welding speed of 100 mm/min and a rotation speed of 400 rpm. In other words, this set of conditions is one of the sets of optimum conditions for obtaining good HNS FSW joints, because these conditions can sustain the austenite single phase, prevent ␣-phase formation and induce the ultrarefinement to improve the mechanical properties of the joints. In contrast, at a lower welding speed (higher heat input), when the temperature measured was more 1100 ◦ C, grain growth, which reduces the hardening in the stir zone, was induced. Furthermore, at higher welding speeds (lower heat input), the temperature measured was less 950 ◦ C, and thus, the ␥–␣ transformation, which reduces the mechanical properties of the joints, was strongly expected, for the sake of ␣ precipitations in the ␥ single phase. 4. Conclusions In this study, 1-groove friction stir butt welding of 2-mm-thick high nitrogen-containing austenitic stainless steel (HNS) plates was successfully performed without welding defects or tool deformation utilizing Si3 N4 at different welding speeds from 50 to 300 mm/min and a constant rotation speed of 400 rpm. The following conclusions can be drawn from these experimental results. The stir zones obtained in this experiment consist of very fine grained structures and show an increase in the Vickers hardness. These joints also show a higher tensile strength and yield strength than the as-received joint. In particular, the FSW welds obtained at the welding speed of 100 mm/min, which shows the best mechan-
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ical properties, had a relatively higher Vickers hardness. This indicates a good relationship between the welding parameter (heat input) and the hardness profile. Based on the results of the thermal profiles measurement during the FSW process and the thermodynamic calculations of the experimental material, the FSW conditions, a welding speed of 100 mm/min and a tool rotation speed of 400 mm/min were estimated to be the most preferable conditions for obtaining the HNS FSW joints, which can sustain the ␥ single phase and prevent ␣phase formation. Acknowledgements The authors wish to thank Mr. M. Kato, Akita Industrial Research Institute, and Mr. R. Kato, Akita University, for their technical assistance. The financial support from the Japanese Ministry of Education, Science, Sports and Culture with a Grant-in-Aid for Young Scientists (Start-up, No. 20860019-00) and a Grant-in-Aid for Scientific Research (B) (No. 00247230) are gratefully acknowledged. This study was performed under the Cooperative Research Program of Institute for Joining and Welding Research Institute, Osaka University. References [1] L. Yuping, Z. Yong, R. Fan, C. Haitao, W. Yuqing, S. Jie, D. Han, The Conference Proceeding of HNS, 2009, pp. 195–201. [2] J.W. Simmons, Materials Science and Engineering A 207 (2) (1996) 159–169. [3] N. Nakada, N. Hirakawa, T. Tsuchiyama, S. Takaki, Scripta Materialia 57 (2) (2007) 153–156. [4] P.J. Uggowitzer, R. Magdowski, M.O. Speidel, ISIJ International 36 (7) (1996) 901–908. [5] M. Sagara, Y. Katada, T. Kodama, ISIJ International 43 (5) (2003) 714–719. [6] H. Baba, T. Kodama, Y. Katada, Corrosion Science 44 (10) (2002) 2393–2407. [7] H. Baba, Y. Katada, Corrosion Science 48 (9) (2006) 2510–2524. [8] W. Bal, H. Kozøowski, K.S. Kasprzak, Journal of Inorganic Biochemistry 79 (1–4) (2000) 213–218. [9] M. Vahter, M. Berglund, A. Åkesson, C. Lidén, Environmental Research 88 (3) (2002) 145–155. [10] O. Kamiya, Z.W. Chen, Y. Kikuchi, T. Ohyoshi. Proceedings of the International Conference on AMDP2002, SD-file, Metal-paper 3-10. [11] O. Kamiya, Y. Kikuchi, Z.W. Chen, Proceedings of ICMR 2001 (Akita), 2001, pp. 215–220. [12] I. Woo, Y. Kikuchi, ISIJ International 42-12 (2002) 1334–1343. [13] T. Ogawa, K. Hiraoka, Y. Katada, M. Sagara, C. Shiga, Quarterly Journal of Japan Welding Society 20-1 (2002) 96–104. [14] T. Ogawa, K. Hiraoka, Y. Katada, M. Sagara, S. Tsukamoto, C. Shiga, Quarterly Journal of Japan Welding Society 20-1 (2002) 107–113. [15] I. Woo, T. Horinouchi, Y. Miyano, Y. Kikuchi, Journal of Steel and Related Materials 2 (2004) 187–196. [16] I. Woo, M. Aritoshi, Y. Kikuchi, ISIJ International 42-4 (2002) 401–406. [17] S.H.C. Park, Y.S. Sato, H. Kokawa, K. Okamoto, S. Hirano, M. Inagaki, Materials Science Forum 539-543 (2007) 3757–3762. [18] W.M. Thomas, E.D. Nicholas, J.C. Needhman, M.G. Murch, P. Temple-Smith, C.J. Dawes. International Patent Application PCT/GB92/02203 and GB Patent Application 9125978.8, UK Patent Office, London, December 6, 1991. [19] Y.S. Sato, K. Nakamura, H. Kokawa, S. Narita, T. Shimizu. Metals and Material Society (2009) 67–73. [20] Y. Katada, M. Sagara, Y. Kobayashi, T. Kodama, Materials and Manufacturing Processes 19 (1) (2004) 19–30. [21] H. Fujii, R. Ueji, Y. Takada, H. Kitahara, N. Tsuji, K. Nakata, K. Nogi, Materials Transactions 47 (2006) 239–242. [22] H. Fujii, L. Cui, N. Tsuji, M. Maeda, K. Nakata, K. Nogi, Material Science Engineering A 429 (2006) 50–57. [23] W. Cao, S.-L. Chen, F. Zhang, K. Wu, Y. Yang, Y.A. Chang, R. Schmid-Fetzer, W.A. Oates, CALPHAD 33 (2) (2009) 328–342. [24] S.-L. Chen, S. Daniel, F. Zhang, Y.A. Chang, X.-Y. Yan, F.-Y. Xie, R. Schmid-Fetzer, W.A. Oates, CALPHAD 26 (2) (2002) 175–188. [25] L. Cui, H. Fujii, N. Tsuji, K. Nogi, Scripta Materialia 56 (2007) 637–640.