- Email: [email protected]
The filler powders laser welding of ODS ferritic steels
Journal of Nuclear Materials 456 (2015) 206–210
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
The filler powders laser welding of ODS ferritic steels Liang Shenyong ⇑, Lei Yucheng, Zhu Qiang School of Material Science and Engineering of Jiangsu University, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 19 May 2014 Accepted 20 September 2014 Available online 30 September 2014
a b s t r a c t Laser welding was performed on Oxide Dispersion Strengthened (ODS) ferritic steel with the self-designed filler powders. The filler powders were added to weld metal to produce nano-particles (Y–M–O and TiC), submicron particles (Y–M–O) and dislocation rings. The generated particles were evenly distributed in the weld metal and their forming mechanism and behavior were analyzed. The results of the tests showed that the nano-particles, submicron particles and dislocation rings were able to improve the micro-hardness and tensile strength of welded joint, and the filler powders laser welding was an effective welding method of ODS ferritic steel. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Oxide Dispersion Strengthened (ODS) ferritic steels are known to be the most promising structural materials for sodium-cooled fast reactors as well as for other Generation IV reactors in the world because of excellent radiation resistance and high-temperature capability [1–3]. ODS ferritic steels, advanced ferrous-base resistant high temperature alloys, are researched and developed, and are used in aerospace, energy and other fields. ODS ferritic steels with the dispersed nanoscaled oxide particles is usually preferred over other steel varieties as a structural material. The size and distribution of nanoscaled oxide particles as well as the nanocrystalline structure of the matrix are responsible for many excellent properties of the ODS ferritic steels [4–9]. In order to improve mechanical properties, many studies have been conducted to refine the oxide particles in the matrix by modifying chemical compositions and developing new material processing techniques [10–15]. Good results have been achieved in the production process of the ODS ferritic steels. The welding method and the welding joint microstructure of ODS ferritic steels have been discussed in several works [16,17]. ODS ferritic steels in non-equilibrium supersaturation state is difficult to be welded by using the traditional fusion welding method, because the non-equilibrium supersaturation state of ODS ferritic steels is changed in the welding process and the dispersed oxide particles gather in weld metal. As a result, the original composition and properties of base metal are damaged in weld metal. In order to get the microstructural modifications and improve the weldability ⇑ Corresponding author. Tel.: +86 15951289338. E-mail address: [email protected] (S. Liang). http://dx.doi.org/10.1016/j.jnucmat.2014.09.041 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
of ODS ferritic steels, it is necessary to restrain the original oxide from gathering and refine grain. However, the method of the filler powders laser welding of ODS ferritic steels is seldom studied in this area. In this paper, the method was studied by adding the filler powders in laser welding. The plates of ODS ferritic steels were butt-welded with a self-designed filler metal powders in laser welding. Tensile test and microhardness test were carried out on welded joints, and the microstructure of welded joints was analyzed. The objective of this study was to analyze the effectiveness of the filler powders applied in welded joints of ODS ferritic steel. The results and discussion were given in the following sections. 2. Experiment The base metals were 2.0 mm thick steel plates with V-groove whose composition were: Fe–0.18C–19.34Cr–5.06Al–0.5Ti–0.017 N–0.26O–0.51Y2O3 (wt.%, same as below). The nominal composition of the filler powders of J1–2 groups was shown in the following: J1 group: Fe–0.25C–2.00Y2O3–23.00Cr–30Ni–1.0Ti–0.35Si–0.4Al; J2 group: no filler powders. The microstructure of joints was observed through Optical Microscope (OM), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). Metallurgical observation of the test pieces was carried out at the cross-section of the joints. All of the test pieces were polished and etched in a chemical solution of 10%HNO3 + 10%HF + 80%H2O (wt.%). The 120 mm 10 mm tensile test specimens were obtained by wire electrical discharge machining (WEDM) after the 2.0 mm thick steels base metals had been welded. Tensile tests of the joints were also carried out. Micro-Vickers hardness tests were conducted by a hardness tester (HVS-1000) with testing load of 0.98 N.
S. Liang et al. / Journal of Nuclear Materials 456 (2015) 206–210
3. Results and discussion 3.1. The generation and distribution of submicron-particles in weld metal When the base metal of ODS ferritic steel and the filler powders were welded in laser welding, the rare earth Y2O3, C and other alloy powders would be in a molten state. The Al2O3 and the Y–O particles in base metal would be decomposed as equation Eqs. (1) and (2), and the Y2O3 in filler powders would be decomposed as Eq. (3).
Al2 O3 ! 2½Al þ 3½O
ð1Þ
Y O ! ½Y þ ½O
ð2Þ
Y2 O3 ! 2½Y þ 3½O
ð3Þ
These decomposed high-energy ions could diffuse and collide in molten pool. When the temperature of molten pool decreased to the degree of composition undercooling, the molten pool started to crystallize and concrete, which provided the appropriate conditions for the combination of different alloy elements and the formation of high melt point compounds or particles. The process was shown in Eq. (4). The degree of the combination depended on their chemical reaction Gibbs free energy and the chance of diffusing and meeting in molten pool.
½Y þ ½M þ ½O ! Y M O
ð4Þ
In order to make the chemical reaction complete in molten pool and get enough high melting point particles in weld metal, we chose the filler powders which could easily form the high melting point compounds. When the filler powders were welded, a large number of high melting point compounds were generated. By
207
means of SEM, tests showed that there were two kinds of particles in different size generated in weld metal: one was nano-particles (50–100 nm), the other was submicron particles (0.1–1 lm), as was shown in Fig. 1(a) and (b). By means of EDS, tests showed that the diffuse submicron-particles were composed of Cr, Fe, Al, O, Ti, C and Y, as was shown in Fig. 2(a) and (b). These irregular Y–M–O compounds (M: Al, Fe, Cr, Ti) were the combination of the rare earth oxides and other metal oxides, which had an excellent thermal stability. The melting point of these compounds was as high as 2850 °C. Y–M–O with high melting point started to concrete and form nucleation in the process of molten pool crystallization, which helped decrease Gibbs free energy and increase the number of grain nucleation. These irregular Y–M–O particles evenly distributed in weld metal, and improved the properties of the welded joint. 3.2. The generation and distribution of nano-particles in weld metal The production process of ODS ferritic steels was that blended powders were sealed in a steel container degassed to a vacuum and consolidated by hot extrusion. ODS ferritic steels were being in desupersaturation state of non-equilibrium metallurgy and the dispersion oxide particles evenly distributed in ODS ferritic steels. However, the traditional fusion welding made dispersion oxide particles with high density and high melting point aggregate to form clusters. As a result, the original distribution of dispersion oxide particles was changed. The filler powders laser welding was able to alleviate this change.
½C þ ½Ti ! TiC ½C þ ½O ! CO "
Fig. 1. (a and b) Particles of J1 weld metal (SEM).
Fig. 2. (a and b) Microstructure of Y–M–O particles in J1 weld metal (EDS).
ð5Þ ð6Þ
208
S. Liang et al. / Journal of Nuclear Materials 456 (2015) 206–210
Fig. 3. (a and b) TiC particles distribution in J1 weld metal (TEM).
Fig. 4. (a and b) Y–M–O particles distribution in J1 weld metal (TEM).
When C and Ti in filler powders were welded, they would dissolve in molten pool. The [C] in molten pool had an important role in chemical reaction and was able to combine with other metallic or non-metallic elements to form high melting point particles or gas, as was shown in Eqs. (5) and (6). [C] and [Ti] in molten pool reacted to TiC, as was shown in Fig. 3. Fig. 3(a) shows that the dispersion sub-micro particles were generated in molten pool and were widely distributed in J1 weld metal. Ti and C were found to be the major components of particles by energy spectrum, as was shown in Fig. 3(b). The generated TiC particles had a stable structure, a high melting point and high hardness. The tiny TiC particles could promote properties of welded joint, such as the tensile strength and the microstructure hardness. [C] and [O] in molten pool reacted to CO, which spilled out of the molten pool and then into the atmosphere. Such process reduced the solubility of O in the molten pool, and the generation chances of the low melting point particles such as FeO were decreased. Therefore, the C in filler powders played an important role in decreasing the generation of low melting point particles. The C in filler powders made the molten pool boiling, which accelerated the rise of the gas bubbles (CO or N2) out of the molten pool. As a result, C in filler powders made the particles distribute evenly and reduced the formation of pores in weld metal, which promoted the properties of the welded joint. Fig. 4(a) shows that the dispersion nano-particles generated were widely and evenly distributed in J1 weld metal. Al, O and
Y were found to be the major components of particles by energy spectrum, as was shown in Fig. 4(b). The special organizational structure was ascribed to effect of rare earth oxides Y2O3 in molten pool on welded joint. In the high temperature molten pool, Y2O3 was decomposed into Y and O. Y atom in Y–O had 6 coordinate bonds. Such special microstructure made Y easily combine with other oxides to form the complex particles (Y–M–O). This was why the rare earth oxides were always found to be coexisted with other oxides instead of existing alone. The tests showed that there were a large number of nano-particles Y–M–O generated in the weld metal, about 50–100 nm in size or even smaller. However, the chemical activation energy of Y2O3 oxides related to the temperature in which the reaction took place. Accompanying the cooling of molten pool, the chemical activation energy of the rare earth oxides increased. Therefore, the Y2O3 oxides with such excellent chemical activation energy were able to combine with other metal oxides to form complex oxides. The appropriate welding heat input could not make the alloy elements diffuse over a long distance, and the growth of the grains was inhibited. However, too much welding heat input could make the alloy elements diffuse over a long distance and make the complex oxides grow up. The coarse grain could make the tensile strength of welded joint decrease. Therefore, it was very necessary to control laser welding heat input and inhibit the growth of grains in weld metal.
209
S. Liang et al. / Journal of Nuclear Materials 456 (2015) 206–210
3.3. The generation and distribution of dislocation in weld metal The ODS ferritic steels were welded by the filler powders laser welding, and an abundant of nano-particles and dislocations were discovered in weld metal. The dislocations could be easily found in Fig. 5(a), while, dislocations were not obvious in Fig. 5(b). When weld metal appeared plastic deformation, these dislocations started to slip and form dislocation ring, dislocation blocks and dislocation networks. A large number of nano-particles existed alongside dislocations. A mass of dislocations and nano-particles generated in the weld metal with filler powders. The dislocations and nano-particles could bring benefit to the properties of weld metal. The smaller reinforcing particles and the more dislocation networks, the better the properties of weld metal would be. However, these microstructures were rarely found in weld metal without filler powders.
(a) J1: filler powders laser welding
3.4. Microhardness test and tensile test The welds by filler powders laser welding were shown in Fig. 6(a). The tests of the welded joint showed that under the combined influence of composition and welding heat cycle, there were different intensities in different positions as well as uneven distribution of hardness in welded joint. The weld was perfect except for a small amount of welding slag found on weld. The cross-sections of welded joint specimens were polished and etched. As a result, the welded region for the gage region of tensile specimen could be observed by OM and then could be picked out. micro-hardness of cross-section of welded joint was tested according to the following order: weld metal, heat affecting zone, and base metal. The average interval between two adjacent dots was 0.5 mm, and each dot was measured three times to seek its average. The results were shown in Fig. 6(b). The result of the micro-hardness tests showed that the welded joints without filler powders had a lower hardness than that of the filler powders. In general, the wear resistance property of weld metal was closely related to the hardness of weld metal. The higher the hardness of the welded joint was, the better the resistance properties would be. Compared with the traditional fusion welding without filler powders, the filler powders laser welding was able to obtain welded joint of ODS ferritic steel with higher hardness. The reason for this was that nano-particles and dislocations generated in weld metal were able to enhance the hardness of the welded joint by filler powders laser welding. However, it was hard for the traditional fusion welding to generate these nano-particles and dislocations. Therefore, the filler powders played an importable role in the enhancement of the hardness of the welded joint. The tensile test measurements were performed at room temperature. Table 1 shows that the welded joint with filler powders
(a) J1
(b) Hardness distribution on Cross-section of J1 Fig. 6. (a and b) The filler powders laser welding and hardness distribution.
Table 1 Tensile strength of J1 and J2 welded joint. No.
J1
J2
rb (MPa)
708
312
had a higher tensile strength than that without filler powders in laser welding. The higher tensile strength showed that C and Y2O3 promoted the generation of nano-particles and even distribution of dislocation rings in weld metal by filler powders laser welding. The evenly distributed nano-particles and dislocation rings in the weld metal inhibited the dislocation motion and resulted in the strengthening effect. The tensile strength of welded joint with filler powders had increased twice compared with that of the welded
(b) J2
Fig. 5. (a and b) Dislocations and reinforcing particles in J1 and J2 weld metal (TEM).
210
S. Liang et al. / Journal of Nuclear Materials 456 (2015) 206–210
Fig. 7. (a and b) The fractured surface micrograph of J1 welded joint (SEM).
joint without the filler powders. The increased tensile strength attributed to the filler powders added in the laser welding of ODS ferritic steel. Appropriate filler powders content was able to help get the welded joint of ODS ferritic steels with high hardness and high tensile strength. Fig. 7 shows that the fracture surfaces of welded joint appeared cleavage river pattern as well as a small quantity of dimples. The ratio between the fracture surface and dimple surface depended on the test temperature. Two groups of specimens were tested at room temperature. The increasing dimples in group J1 showed that fracture formation and mechanism could be changed and the welded joints with filler powders had a better fracture toughness than those without the filler powders. 4. Conclusion The microstructure and mechanical properties of welded joint with and without filler powders were studied by laser welding of ODS ferritic steels. The main conclusions could be summarized as follows: The filler powders laser welding was an effective and workable method to weld ODS ferritic steel with non-equilibrium metallurgy characters. In weld metal of the filler powders laser welding, there generated an abundant of nano-particles (TiC and Y–M–O), submicron particles (Y–M–O) and dislocation rings which improved the tensile strength and micro-hardness of welded joints of ODS ferritic steel.
Acknowledgements This work was supported by the China National Natural Science Foundation with Grant No. 51075191, and the Graduate Innovation Program of Jiangsu province in China with the Grant No. CXZZ11_0556. References [1] Mohamed S. El-Genk, Jean-Michel Tournier, J. Nucl. Mater. 340 (2005) 93–112. [2] R.L. Klueh, J.P. Shingledecker, R.W. Swindeman, D.T. Hoelzer, J. Nucl. Mater. 341 (2-3) (2005) 103–114. [3] K.L. Murty, I. Charit, J. Nucl. Mater. 383 (2008) 189–195. [4] S.K. Samanta, S.K. Mitra, T.K. Pal, J. Nucl. Mater. 386–388 (2009) 561–563. [5] V. de Castro, T. Leguey, A. Munoz, J. Nucl. Mater. 386–388 (2009) 449–452. [6] Zbigniew Oksiuta, Malgorzata Lewandowska, J. Nucl. Mater. 442 (2013) S84– S88. [7] R. Gao, Zhang Wang Fang, C.S. Liu, J. Nucl. Mater. 444 (2014) 462–468. [8] Yuhai Doua, Yong Liu, Feng Liu, J. Nucl. Mater. 434 (2013) 129–132. [9] D. Sakuma, S. Yamashita, K. Oka, J. Nucl. Mater. 329–333 (2004) 392–396. [10] H. Sakasegawa, L. Chaffron, F. Legendre, J. Nucl. Mater. 384 (2009) 115–118. [11] Xiaodong Mao, Tae Kyu Kim, J. Nucl. Mater. 428 (2012) 82–89. [12] Man Wang, Zhangjian Zhou, Hongying Sun, Mater. Sci. Eng., A 559 (2013) 287– 292. [13] Jan Hoffmann, Michael Klimenkov, Rainer Lindau, Michael Rieth, J. Nucl. Mater. 428 (2012) 165–169. [14] Luke L. Hsiung, Michael J. Fluss, Akihiko Kimura, Mater. Lett. 64 (2010) 1782– 1785. [15] M. Klimenkov, R. Lindau, A. Molang, J. Nucl. Mater. 386–388 (2009) 557–560. [16] W.T. Han, Wan, Sci. Technol. Weld. Join. 16 (2011) 690–696. [17] Kiyohiro Yabuuch, Naoto Tsuda, Akihiko Kimura, Mater. Sci. Eng., A 595 (2014) 291–296.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
The filler powders laser welding of ODS ferritic steels Liang Shenyong ⇑, Lei Yucheng, Zhu Qiang School of Material Science and Engineering of Jiangsu University, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 19 May 2014 Accepted 20 September 2014 Available online 30 September 2014
a b s t r a c t Laser welding was performed on Oxide Dispersion Strengthened (ODS) ferritic steel with the self-designed filler powders. The filler powders were added to weld metal to produce nano-particles (Y–M–O and TiC), submicron particles (Y–M–O) and dislocation rings. The generated particles were evenly distributed in the weld metal and their forming mechanism and behavior were analyzed. The results of the tests showed that the nano-particles, submicron particles and dislocation rings were able to improve the micro-hardness and tensile strength of welded joint, and the filler powders laser welding was an effective welding method of ODS ferritic steel. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Oxide Dispersion Strengthened (ODS) ferritic steels are known to be the most promising structural materials for sodium-cooled fast reactors as well as for other Generation IV reactors in the world because of excellent radiation resistance and high-temperature capability [1–3]. ODS ferritic steels, advanced ferrous-base resistant high temperature alloys, are researched and developed, and are used in aerospace, energy and other fields. ODS ferritic steels with the dispersed nanoscaled oxide particles is usually preferred over other steel varieties as a structural material. The size and distribution of nanoscaled oxide particles as well as the nanocrystalline structure of the matrix are responsible for many excellent properties of the ODS ferritic steels [4–9]. In order to improve mechanical properties, many studies have been conducted to refine the oxide particles in the matrix by modifying chemical compositions and developing new material processing techniques [10–15]. Good results have been achieved in the production process of the ODS ferritic steels. The welding method and the welding joint microstructure of ODS ferritic steels have been discussed in several works [16,17]. ODS ferritic steels in non-equilibrium supersaturation state is difficult to be welded by using the traditional fusion welding method, because the non-equilibrium supersaturation state of ODS ferritic steels is changed in the welding process and the dispersed oxide particles gather in weld metal. As a result, the original composition and properties of base metal are damaged in weld metal. In order to get the microstructural modifications and improve the weldability ⇑ Corresponding author. Tel.: +86 15951289338. E-mail address: [email protected] (S. Liang). http://dx.doi.org/10.1016/j.jnucmat.2014.09.041 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
of ODS ferritic steels, it is necessary to restrain the original oxide from gathering and refine grain. However, the method of the filler powders laser welding of ODS ferritic steels is seldom studied in this area. In this paper, the method was studied by adding the filler powders in laser welding. The plates of ODS ferritic steels were butt-welded with a self-designed filler metal powders in laser welding. Tensile test and microhardness test were carried out on welded joints, and the microstructure of welded joints was analyzed. The objective of this study was to analyze the effectiveness of the filler powders applied in welded joints of ODS ferritic steel. The results and discussion were given in the following sections. 2. Experiment The base metals were 2.0 mm thick steel plates with V-groove whose composition were: Fe–0.18C–19.34Cr–5.06Al–0.5Ti–0.017 N–0.26O–0.51Y2O3 (wt.%, same as below). The nominal composition of the filler powders of J1–2 groups was shown in the following: J1 group: Fe–0.25C–2.00Y2O3–23.00Cr–30Ni–1.0Ti–0.35Si–0.4Al; J2 group: no filler powders. The microstructure of joints was observed through Optical Microscope (OM), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). Metallurgical observation of the test pieces was carried out at the cross-section of the joints. All of the test pieces were polished and etched in a chemical solution of 10%HNO3 + 10%HF + 80%H2O (wt.%). The 120 mm 10 mm tensile test specimens were obtained by wire electrical discharge machining (WEDM) after the 2.0 mm thick steels base metals had been welded. Tensile tests of the joints were also carried out. Micro-Vickers hardness tests were conducted by a hardness tester (HVS-1000) with testing load of 0.98 N.
S. Liang et al. / Journal of Nuclear Materials 456 (2015) 206–210
3. Results and discussion 3.1. The generation and distribution of submicron-particles in weld metal When the base metal of ODS ferritic steel and the filler powders were welded in laser welding, the rare earth Y2O3, C and other alloy powders would be in a molten state. The Al2O3 and the Y–O particles in base metal would be decomposed as equation Eqs. (1) and (2), and the Y2O3 in filler powders would be decomposed as Eq. (3).
Al2 O3 ! 2½Al þ 3½O
ð1Þ
Y O ! ½Y þ ½O
ð2Þ
Y2 O3 ! 2½Y þ 3½O
ð3Þ
These decomposed high-energy ions could diffuse and collide in molten pool. When the temperature of molten pool decreased to the degree of composition undercooling, the molten pool started to crystallize and concrete, which provided the appropriate conditions for the combination of different alloy elements and the formation of high melt point compounds or particles. The process was shown in Eq. (4). The degree of the combination depended on their chemical reaction Gibbs free energy and the chance of diffusing and meeting in molten pool.
½Y þ ½M þ ½O ! Y M O
ð4Þ
In order to make the chemical reaction complete in molten pool and get enough high melting point particles in weld metal, we chose the filler powders which could easily form the high melting point compounds. When the filler powders were welded, a large number of high melting point compounds were generated. By
207
means of SEM, tests showed that there were two kinds of particles in different size generated in weld metal: one was nano-particles (50–100 nm), the other was submicron particles (0.1–1 lm), as was shown in Fig. 1(a) and (b). By means of EDS, tests showed that the diffuse submicron-particles were composed of Cr, Fe, Al, O, Ti, C and Y, as was shown in Fig. 2(a) and (b). These irregular Y–M–O compounds (M: Al, Fe, Cr, Ti) were the combination of the rare earth oxides and other metal oxides, which had an excellent thermal stability. The melting point of these compounds was as high as 2850 °C. Y–M–O with high melting point started to concrete and form nucleation in the process of molten pool crystallization, which helped decrease Gibbs free energy and increase the number of grain nucleation. These irregular Y–M–O particles evenly distributed in weld metal, and improved the properties of the welded joint. 3.2. The generation and distribution of nano-particles in weld metal The production process of ODS ferritic steels was that blended powders were sealed in a steel container degassed to a vacuum and consolidated by hot extrusion. ODS ferritic steels were being in desupersaturation state of non-equilibrium metallurgy and the dispersion oxide particles evenly distributed in ODS ferritic steels. However, the traditional fusion welding made dispersion oxide particles with high density and high melting point aggregate to form clusters. As a result, the original distribution of dispersion oxide particles was changed. The filler powders laser welding was able to alleviate this change.
½C þ ½Ti ! TiC ½C þ ½O ! CO "
Fig. 1. (a and b) Particles of J1 weld metal (SEM).
Fig. 2. (a and b) Microstructure of Y–M–O particles in J1 weld metal (EDS).
ð5Þ ð6Þ
208
S. Liang et al. / Journal of Nuclear Materials 456 (2015) 206–210
Fig. 3. (a and b) TiC particles distribution in J1 weld metal (TEM).
Fig. 4. (a and b) Y–M–O particles distribution in J1 weld metal (TEM).
When C and Ti in filler powders were welded, they would dissolve in molten pool. The [C] in molten pool had an important role in chemical reaction and was able to combine with other metallic or non-metallic elements to form high melting point particles or gas, as was shown in Eqs. (5) and (6). [C] and [Ti] in molten pool reacted to TiC, as was shown in Fig. 3. Fig. 3(a) shows that the dispersion sub-micro particles were generated in molten pool and were widely distributed in J1 weld metal. Ti and C were found to be the major components of particles by energy spectrum, as was shown in Fig. 3(b). The generated TiC particles had a stable structure, a high melting point and high hardness. The tiny TiC particles could promote properties of welded joint, such as the tensile strength and the microstructure hardness. [C] and [O] in molten pool reacted to CO, which spilled out of the molten pool and then into the atmosphere. Such process reduced the solubility of O in the molten pool, and the generation chances of the low melting point particles such as FeO were decreased. Therefore, the C in filler powders played an important role in decreasing the generation of low melting point particles. The C in filler powders made the molten pool boiling, which accelerated the rise of the gas bubbles (CO or N2) out of the molten pool. As a result, C in filler powders made the particles distribute evenly and reduced the formation of pores in weld metal, which promoted the properties of the welded joint. Fig. 4(a) shows that the dispersion nano-particles generated were widely and evenly distributed in J1 weld metal. Al, O and
Y were found to be the major components of particles by energy spectrum, as was shown in Fig. 4(b). The special organizational structure was ascribed to effect of rare earth oxides Y2O3 in molten pool on welded joint. In the high temperature molten pool, Y2O3 was decomposed into Y and O. Y atom in Y–O had 6 coordinate bonds. Such special microstructure made Y easily combine with other oxides to form the complex particles (Y–M–O). This was why the rare earth oxides were always found to be coexisted with other oxides instead of existing alone. The tests showed that there were a large number of nano-particles Y–M–O generated in the weld metal, about 50–100 nm in size or even smaller. However, the chemical activation energy of Y2O3 oxides related to the temperature in which the reaction took place. Accompanying the cooling of molten pool, the chemical activation energy of the rare earth oxides increased. Therefore, the Y2O3 oxides with such excellent chemical activation energy were able to combine with other metal oxides to form complex oxides. The appropriate welding heat input could not make the alloy elements diffuse over a long distance, and the growth of the grains was inhibited. However, too much welding heat input could make the alloy elements diffuse over a long distance and make the complex oxides grow up. The coarse grain could make the tensile strength of welded joint decrease. Therefore, it was very necessary to control laser welding heat input and inhibit the growth of grains in weld metal.
209
S. Liang et al. / Journal of Nuclear Materials 456 (2015) 206–210
3.3. The generation and distribution of dislocation in weld metal The ODS ferritic steels were welded by the filler powders laser welding, and an abundant of nano-particles and dislocations were discovered in weld metal. The dislocations could be easily found in Fig. 5(a), while, dislocations were not obvious in Fig. 5(b). When weld metal appeared plastic deformation, these dislocations started to slip and form dislocation ring, dislocation blocks and dislocation networks. A large number of nano-particles existed alongside dislocations. A mass of dislocations and nano-particles generated in the weld metal with filler powders. The dislocations and nano-particles could bring benefit to the properties of weld metal. The smaller reinforcing particles and the more dislocation networks, the better the properties of weld metal would be. However, these microstructures were rarely found in weld metal without filler powders.
(a) J1: filler powders laser welding
3.4. Microhardness test and tensile test The welds by filler powders laser welding were shown in Fig. 6(a). The tests of the welded joint showed that under the combined influence of composition and welding heat cycle, there were different intensities in different positions as well as uneven distribution of hardness in welded joint. The weld was perfect except for a small amount of welding slag found on weld. The cross-sections of welded joint specimens were polished and etched. As a result, the welded region for the gage region of tensile specimen could be observed by OM and then could be picked out. micro-hardness of cross-section of welded joint was tested according to the following order: weld metal, heat affecting zone, and base metal. The average interval between two adjacent dots was 0.5 mm, and each dot was measured three times to seek its average. The results were shown in Fig. 6(b). The result of the micro-hardness tests showed that the welded joints without filler powders had a lower hardness than that of the filler powders. In general, the wear resistance property of weld metal was closely related to the hardness of weld metal. The higher the hardness of the welded joint was, the better the resistance properties would be. Compared with the traditional fusion welding without filler powders, the filler powders laser welding was able to obtain welded joint of ODS ferritic steel with higher hardness. The reason for this was that nano-particles and dislocations generated in weld metal were able to enhance the hardness of the welded joint by filler powders laser welding. However, it was hard for the traditional fusion welding to generate these nano-particles and dislocations. Therefore, the filler powders played an importable role in the enhancement of the hardness of the welded joint. The tensile test measurements were performed at room temperature. Table 1 shows that the welded joint with filler powders
(a) J1
(b) Hardness distribution on Cross-section of J1 Fig. 6. (a and b) The filler powders laser welding and hardness distribution.
Table 1 Tensile strength of J1 and J2 welded joint. No.
J1
J2
rb (MPa)
708
312
had a higher tensile strength than that without filler powders in laser welding. The higher tensile strength showed that C and Y2O3 promoted the generation of nano-particles and even distribution of dislocation rings in weld metal by filler powders laser welding. The evenly distributed nano-particles and dislocation rings in the weld metal inhibited the dislocation motion and resulted in the strengthening effect. The tensile strength of welded joint with filler powders had increased twice compared with that of the welded
(b) J2
Fig. 5. (a and b) Dislocations and reinforcing particles in J1 and J2 weld metal (TEM).
210
S. Liang et al. / Journal of Nuclear Materials 456 (2015) 206–210
Fig. 7. (a and b) The fractured surface micrograph of J1 welded joint (SEM).
joint without the filler powders. The increased tensile strength attributed to the filler powders added in the laser welding of ODS ferritic steel. Appropriate filler powders content was able to help get the welded joint of ODS ferritic steels with high hardness and high tensile strength. Fig. 7 shows that the fracture surfaces of welded joint appeared cleavage river pattern as well as a small quantity of dimples. The ratio between the fracture surface and dimple surface depended on the test temperature. Two groups of specimens were tested at room temperature. The increasing dimples in group J1 showed that fracture formation and mechanism could be changed and the welded joints with filler powders had a better fracture toughness than those without the filler powders. 4. Conclusion The microstructure and mechanical properties of welded joint with and without filler powders were studied by laser welding of ODS ferritic steels. The main conclusions could be summarized as follows: The filler powders laser welding was an effective and workable method to weld ODS ferritic steel with non-equilibrium metallurgy characters. In weld metal of the filler powders laser welding, there generated an abundant of nano-particles (TiC and Y–M–O), submicron particles (Y–M–O) and dislocation rings which improved the tensile strength and micro-hardness of welded joints of ODS ferritic steel.
Acknowledgements This work was supported by the China National Natural Science Foundation with Grant No. 51075191, and the Graduate Innovation Program of Jiangsu province in China with the Grant No. CXZZ11_0556. References [1] Mohamed S. El-Genk, Jean-Michel Tournier, J. Nucl. Mater. 340 (2005) 93–112. [2] R.L. Klueh, J.P. Shingledecker, R.W. Swindeman, D.T. Hoelzer, J. Nucl. Mater. 341 (2-3) (2005) 103–114. [3] K.L. Murty, I. Charit, J. Nucl. Mater. 383 (2008) 189–195. [4] S.K. Samanta, S.K. Mitra, T.K. Pal, J. Nucl. Mater. 386–388 (2009) 561–563. [5] V. de Castro, T. Leguey, A. Munoz, J. Nucl. Mater. 386–388 (2009) 449–452. [6] Zbigniew Oksiuta, Malgorzata Lewandowska, J. Nucl. Mater. 442 (2013) S84– S88. [7] R. Gao, Zhang Wang Fang, C.S. Liu, J. Nucl. Mater. 444 (2014) 462–468. [8] Yuhai Doua, Yong Liu, Feng Liu, J. Nucl. Mater. 434 (2013) 129–132. [9] D. Sakuma, S. Yamashita, K. Oka, J. Nucl. Mater. 329–333 (2004) 392–396. [10] H. Sakasegawa, L. Chaffron, F. Legendre, J. Nucl. Mater. 384 (2009) 115–118. [11] Xiaodong Mao, Tae Kyu Kim, J. Nucl. Mater. 428 (2012) 82–89. [12] Man Wang, Zhangjian Zhou, Hongying Sun, Mater. Sci. Eng., A 559 (2013) 287– 292. [13] Jan Hoffmann, Michael Klimenkov, Rainer Lindau, Michael Rieth, J. Nucl. Mater. 428 (2012) 165–169. [14] Luke L. Hsiung, Michael J. Fluss, Akihiko Kimura, Mater. Lett. 64 (2010) 1782– 1785. [15] M. Klimenkov, R. Lindau, A. Molang, J. Nucl. Mater. 386–388 (2009) 557–560. [16] W.T. Han, Wan, Sci. Technol. Weld. Join. 16 (2011) 690–696. [17] Kiyohiro Yabuuch, Naoto Tsuda, Akihiko Kimura, Mater. Sci. Eng., A 595 (2014) 291–296.