Mechanical Property Variation within Inconel 82/182 Dissimilar Metal. Weld between Low Alloy Steel and 316 Stainless Steel

Transcription

1 Mechanical Property Variation within Inconel 82/182 Dissimilar Metal Weld between Low Alloy Steel and 316 Stainless Steel Changheui Jang, 1) * Jounghoon Lee, 1) Jong Sung Kim, 2) and Tae Eun Jin 2) 1) Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology 373-1, Guseong-dong, Yuseong-gu, Daejeon, , Rep. of Korea *Tel.: , Fax.: , chjang@kaist.ac.kr 2) Korea Power Engineering Company Mabuk-ri, Guseong-eup, Yongin-si, Gyeonggi-do, , Rep. of Korea Abstract In several locations of the pressurized water reactors, dissimilar metal welds using Inconel welding wires are used to join the low alloy steel components to stainless steel pipes. Because of the existence of different materials and chemistry variation within welds, the mechanical properties, such as tensile and fracture properties, are expected to show spatial variation. For design and integrity assessment of the dissimilar welds, these variations should be evaluated. In this study, dissimilar metal welds composed of low alloy steel, Inconel 82/182 weld, and stainless steel were prepared by gas tungsten arc welding and shielded metal arc welding techniques. Microstructures were observed using optical and electron microscopes. Typical dendrite structures were observed in Inconel 82/182 welds. Tensile tests using standard specimens and mini-sized specimens and micro-hardness tests were conducted to measure the variation in strength along the thickness of the weld as well as across the weld. In addition, fracture toughness specimens were taken at the bottom, middle, and top of the welds and tested to evaluate the spatial variation along the thickness. It was found that while the strength are about 50~70 MPa greater at the bottom of the weld than at the top of the weld, fracture toughness values at the top of the weld are about 70% greater than those at the bottom of the weld. 1

2 1. INTRODUCTION In pressurized water reactors, low alloy steels and stainless steels are widely used in the primary system because of good mechanical properties. For example, in Korean-designed OPR1000 and APR1400 plants, the main primary coolant piping is made of low alloy steels clad with stainless steel, but the major branch lines are made of stainless steel to provide proper strength and corrosion resistance. In most cases, Inconel welding wires are used to join the low alloy steel components to stainless steel pipes and form dissimilar metal welds. The Inconel welding wires are known to accommodate the differences in composition and thermal expansion of the two metals. The typical locations of the dissimilar metal welds in PWRs are shown in Fig. 1. As shown in the figure, quite many locations in primary piping systems are made of dissimilar metal welds. Recently, the concern and the interest in the integrity of the dissimilar metal welds have been raised since the cracking incident in V.C. Summer nuclear power plant [1]. In that incident, through wall cracks in Inconel 82/182 weld of the hot leg nozzle caused leak of primary water into the containment. It has been thought that the repair welding during the construction caused significant residual stress on the inner surface of the weld. For the integrity analysis of the dissimilar metal weld, it is essential to have enough materials property database. It is generally known that the materials properties within the weld show considerable scatter and spatial dependence. In case of the dissimilar metal welds which consist of different types of materials, in part due to the chemical composition gradient and the mixing of filler metals and base metals, the spatial variation and scatter in material properties could be even more significant. Therefore, it is important to know the variation of the mechanical properties in dissimilar weld area to properly analyze the integrity of the nozzles. However, even though there has been several attempts to measure the mechanical properties of Inconel 82/182 welds [2,3,4], the variation of mechanical properties were not properly investigated. In this paper, the spatial variations of the mechanical properties of the dissimilar metal weld were investigated by tensile tests, micro-hardness tests, and fracture toughness tests.. <Fig. 1> 2. MATERIALS AND EXPERIMENTS 2

3 2.1. Materials and welding Base materials used to construct dissimilar metal welds are SA508 Cl.3 low alloy steel and F316 stainless steel. SA508 Cl.3 is a typical low alloy steel used for large vessels and some of nozzles in OPR1000 plants. F316 SS is one of the materials for the branch lines connected to those nozzles. The chemical compositions of the base metals are summarized in Table 1. Both base metals are provided as forged and heat treated condition and prepared as 40 mm thick plate before the welding procedure. The plates are welded manually, closely simulating the welding procedures used for the nozzle to pipe welding in OPR1000. The key welding parameters are summarized in Table 2. The schematics of weld design and procedure are shown in Fig. 2. Before the welding, 2-passes of buttering were applied on the machined edges of the SA508 plate by manual gas-tungsten arc welding (GTAW) with Inconel 82 bare wire. The resulting thickness of the buttering was about 5 mm. After the buttering, the pieces were post weld heat treated at 615 o C to relieve the residual stress. Before the groove welding, to provide the torch initiation and proper constraint, pieces of dummy metal blocks were welded at both ends of the base metals. The first 2-3 passes (about 5 mm) of V-groove welding were completed by GTAW with Inconel 82 bare wire, then the remaining thickness was filled by shielded-metal arc welding (SMAW) with Inconel 182 flux coated wires. During the welding, the argon gas was continuously supplied at the front and back sides of the plates as a shielding gas. The finished weld blocks were not heat treated. After the welding, the completed weld blocks were cut in pieces for microstructural observation and mechanical tests. After machining off a few centimeters at both ends of the blocks, the whole welded blocks were examined by ultrasonic test to check the soundness of the welds. To reveal the microstructural features in dissimilar weld and base metals, the electrolytic etching and chemical etching methods were applied. The microstructures were observed using optical and scanning electron microscopes. <Table 1> <Fig. 2> <Table 2> 2.2 Tensile and micro-hardness tests Small size round bar tensile specimens and mini-sized sheet type tensile specimens shown in Fig. 3 were machined from the welded plates. Specimens were taken from SA508 low alloy steel (LAS) region, Inconel 3

4 weld region, and stainless steel (SS) regions along the welding direction as shown in Fig. 4. The specimens were tested at room temperature at strain rate of 5x10-4 /sec. Micro-hardness measurements were done across the dissimilar metal welds at the locations where the mini-sized tensile specimens were taken, too. <Fig. 3> <Fig. 4> 2.3 Fracture toughness test Compact tension (CT) specimens were taken from the welded blocks. To measure the spatial variation of the fracture characteristics of the dissimilar metal weld, the top and the bottom of the welds were electron beam welded to the stainless steel blocks and the specimens were machined to place the machined notches along the thickness of the weld as shown in Fig. 5. Also shown in the Fig. 5 is the specimen geometry used in the study. The specimen thickness (B) was 12.7 mm, width (W) was 38.1 mm. Initially, the machined crack depth (a) was 16.9 mm. The specimens were fatigue pre-cracked to reach a/w ~ 0.55 before the fracture toughness tests. The tips of the final fatigue precrack were placed at the bottom, middle, and the top of the welds. During the fracture tests, the crack growth was measured using direct-current potential drop method (DCPD). After the test, the specimens were fatigue loaded to split in half to reveal the fracture surfaces. The fracture surfaces were examined using canning electron microscope. <Fig. 5> 3. RESULTS AND DISCUSSION 3.1 Weld microstructure The cross section of the finished weld is shown in Fig. 6. In the figure, the distinctive region of dissimilar metal welds, such as, low alloy steel base metal, Inconel 82 buttering, Inconel 82/182 fusion zone, and stainless steel base metal are clearly visible. Also, the dendrite structures within the fusion zone and theirs orientation showing the maximum cooling direction during the welding are evident. Microstructure observation of the base metals revealed that SA508 Cl.3 is composed of tempered bainite structure, and F316 is composed of well 4

5 developed austenitic grains with a few ferritic stringers. In the fusion zone, the dendrite microstructure is clear in Inconel fusion zone and buttering area. However, the direction and the spacing of the dendrites vary depending on the locations. The optical microstructures of several regions of dissimilar metal welds are shown in Fig. 7. As shown in the figure, the dendrites are more closely spaced at the bottom part of the weld compared to the top part. The recrystallized features with extensive grain boundary migration [5] shown in Fig. 7-(b) are observed in all part of the weld. Within the area between the dendrites, significant segregation and secondary phase precipitations are observed, though not clearly shown in the figure. Therefore, it is anticipated that large number of fine inclusions are present the Inconel 182 weld metal as reported by Sireesha [5]. The fusion boundary between Inconel weld and stainless steel base metal is shown in Fig. 7-(d). It is clear that a portion of the base metal was melted and resolidified during the welding process. <Fig. 6> <Fig. 7> 3.2. Tensile property variation The results of the round bar tensile tests are shown in Fig. 8. The test results show a little different property depending on the test position, across the dissimilar metal weld and along the thickness of weld. As a whole, SA508 Cl.3 base metals show higher yield strength than Inconel 82/182 weld and F316 base metals. However, UTS values are similar in both of base metals and Inconel weld metal. Within the Inconel fusion weld, a quite large tensile property variation is present, such that the yield strength and UTS are about 50~70 MPa larger at the bottom of the weld than at the top of the weld. The average elongations are about 0.21~0.23 in SA 508 Cl.3, 0.38~0.45 in Inconel 82/182 weld, and 0.61~0.78 in F316. The mini-sized tensile test results are shown in Fig. 9 and the average values are summarized in Table 3. It is shown in the figure that, for the base metals, the overall values of tensile properties measured using mini-sized specimens are compatible with those measured using round bar specimens. However, for the Inconel weld metal, the UTS values from the mini-sized specimens are substantially lower than those measured from the round bar specimens. The large and elongated grains in the weld may have contributed the under-estimation of UTS values for the mini-sized specimens with thickness of about 0.28 mm. To confirm the effects of the specimen thickness, mini-specimens with increased thickness are being tested. Nevertheless, the tensile property variation along the thickness is also clear in mini-sized tensile specimen tests, but the strength differences are 5

6 reduced to 40~50 MPa. A significant increase in strength is observed in F316 base metal near the fusion boundary. As shown in Fig. 9, the increases in yield strength in the heat affected zone of F316 are as large as 100MPa at the middle and bottom of the welded block. The size of the hardened zone is larger at the bottom of the welded block. Hardening of the heat affected zone of the stainless steels were previously reported by others [4,6], but the reported size of the hardened zone was a few millimeters for the thin plates. The sources of the hardening in stainless steels are grain recrystallization and carbide precipitation [4]. Therefore, the large hardened zone observed in our study may have been caused by the multiple welding passes to complete the 40 mm thick blocks. Accordingly, the amount of hardening and size of the hardened zone decrease at the top of the welded block, as shown in Fig. 9. <Fig. 8> <Fig. 9> <Table 3> 3.3. Micro-hardness variation across dissimilar metal welds The micro-hardness test results are summarized in Fig. 10. Despite of the large fluctuation within the Inconel weld, the strength variations observed in tensile tests are verified in micro-hardness test, but in much finer scale. The hardness values are higher at the bottom of the weld and decrease in middle and top of the weld, which is consistent with the tensile test results. The hardening behaviors in the heat affected zone of stainless steel base metals are also observed in hardness tests. The gradual increase in hardness in the heat affected zone of stainless steels approaching the fusion boundary was reported in other dissimilar metal welds, too [4,6]. Through micro-hardness tests, the narrow heat affected zone of the SA508 Cl.3 is also identified by as shown in Fig. 10. This could explain why the heat affected zone of SA508 Cl.3 is not evident in mini-sized tensile test results shown in Fig. 9. That is, because the heat affected zone of SA508 Cl.3 is less than the width of the mini-sized specimen, it is likely that the heat affected zone is completely or partially missed during specimen preparation. <Fig. 10> 3.4. Fracture toughness variation 6

7 The fracture toughness test results of the Inconel welds are shown in Fig. 11 and summarized in Table 4. The tests were completed with fully ductile fracture and stopped when crack extension reached about 3 mm. Two specimens were tested at each weld location, but one of the tests for the middle of weld was interrupted and not successful. As shown in Fig. 11, for the specimens taken at the same location, the J-R curves are similar. The J-R curves of the specimens taken at the top of weld are much greater than those at the bottom of weld. From the J-R curves, the fracture toughness values were determined following ASTM E [7] and are summarized in Table 4. All of the fracture toughness values satisfied the thickness and other requirements and therefore considered valid J IC values. In our tests, Inconel 82/182 welds exhibit fracture toughness values of around 100 ~ 220 kj/m 2. These values are considerably lower than the values reported for ENH82 welds [2]. It is clear in Table 4 that the fracture toughness strongly depends on the location of the weld along the thickness direction. It is quite natural considering that because of large number of welding pass and repeated heat cycles during the welding process as well as compositional differences, the microstructure and the thermo-mechanical history can not be uniform within the weld. Therefore, the mechanical property variation within the weld should be considered in the structural integrity evaluation of the dissimilar metal welds. <Fig. 11> <Table 4> 3.5. Fracture Surface Observation The fracture surfaces of round bar tensile specimens were observed under scanning electron microscope and the results are shown in Fig. 12. The fracture surface shows the typical ductile dimple fracture surface. The dendrite morphology is clearly visible even on the fracture surface, in which the microvoids are aligned along the primary dendrites. In some of the microvoids, the secondary particles are visible at the center. From the microstructural features, it is postulated that the interdendritic core with severe segregation or secondary particles would have been the microvoid initiation sites. From the fracture surface observation of the compact tension specimens, ductile fracture modes are observed with three operative cracking mechanisms, such as, primary microvoid coalescence, void-sheet formation, and shear-stretch formation [2]. In the shear stretched zone, well-defined slip offsets are typically observed, indicating that this mechanism requires extensive plastic deformation. Representative fracture-surface 7

8 morphologies of Inconel 82/182 welds are shown in Fig. 13. The dominant fracture mechanisms are different depending on the location within the weld. In the bottom of the weld (Figure 13-(a)), Primary microvoid coalescences are dominantly observed. In the middle of the weld (Figure 13-(b)), shear-stretch features are observed and primary dimples are also partially observed. In the top of the weld (Figure 13-(c, d)), shear-stretch features are dominantly observed, and the size of shear-stretch region are much larger than the size in the middle part. Void-sheet features are also observed surrounding the large shear-stretch region. As previously mentioned, the J IC fracture toughness is increased as the specimen location is moved from bottom to top of the weld. These facts demonstrate that the size of shear-stretch mode is related with J IC fracture toughness value. And the shear-stretch mode is more effective for increasing J IC fracture toughness rather than primary microvoid coalescence and void-sheet formation. 4. CONCLUSIONS The dissimilar metal welds joining the low alloy steel and stainless steel were fabricated and the microstructures were analyzed. Also the spatial variation in mechanical properties was investigated. Through the analysis, the following conclusions were drawn: 1. The dendritic structures are well developed in Inconel 82/182 weld. Within the area between the dendrites, significant segregation and secondary phase precipitations are observed. Also, the dendrites are more closely spaced at the bottom part of the weld compared to the top part 2. Within the Inconel fusion weld, the yield strength and tensile strength are about 50~70 MPa larger at the bottom of the weld than at the top of the weld. Such differences in strength along the thickness of the weld were also confirmed in min-sized tensile tests and micro-hardness tests. 3. In our tests, Inconel 82/182 welds exhibited fracture toughness values of around 100 ~ 220 kj/m 2 at room temperature. The fracture toughness strongly depended on the location of the weld along the thickness direction such that fracture toughness values at the top of the weld are about 70% greater than those at the bottom of the weld. This may have been caused by the spatial variation in the composition, the microstructure and the thermo-mechanical history during the multi-pass welding of thick plates. 4. The fracture surface of compact tension specimens showed general ductile fracture modes. However, dominant fracture modes were different depending on the location within the weld, such that the dominant 8

9 feature changed from primary dimple mode to large shear-stretch mode as fracture toughness increased from the bottom to the top of the dissimilar metal welds. ACKNOWLEDGEMENTS This study was supported by the Korean Ministry of Commerce, Industry and Economy (MOCIE) under the Electric Power Research Program. Part of the funding was provided by the Second Phase BK21 Program of the Ministry of Education and Human Resource Development of Korea. REFERENCES 1. USNRC. Crack in weld area of reactor coolant system hot leg piping at V.C. Summer, IN00-17, October 18, Mills W. J., Brown C. M. Fracture toughness of alloy 600 and en82h weld in air and water. Metall Trans 2001;32A(5): Chopra O. K., Soppet W. K., Shack W. J. Effects of alloy chemistry, cold work, and water chemistry on corrosion fatigue and stress corrosion cracking of nickel alloys and welds. NUREG/CR-6721, Celik A., Asaran A. Mechanical and structural properties of similar and dissimilar steel joints. Materials Characterization 1999;43: Sireesha M., Albert S., Shankar V., Sundaresan S. Microstructural features of dissimilar welds between 316LN austenitic stainless steel and alloy 800. Mat Sci and Eng 2000;A292: Sireesha M., Albert S., Shankar V., Sundaresan S. A comparative evaluation of welding consumables for dissimilar welds between 316LN austenitic stainless steel and Alloy 800. J Nucl Mater 2000;279: ASTM. Standard test method for measurement of fracture toughness. ASTM E ,

10 Table captions Table 1. Chemical compositions of the base metals and welding wires Table 2. Welding parameters for dissimilar metal weld Table 3. Tensile properties in different part of the dissimilar welds measured by mini-sized specimens Table 4. Mean J IC fracture toughness properties in different part of the Inconel 82/182 welds at room temperature. 10

11 Figure captions Fig. 1. Locations of dissimilar metal welds in typical PWRs Fig. 2. Schematics of the dissimilar metal weld of single V-groove design. Fig. 3. Small size round bar specimen and mini-sized specimen for tensile test; (a) small size specimen, (b) mini-sized specimen. Fig. 4. Location where the tensile specimens were taken from the finished dissimilar weld; (a) locations for small size specimens, (b) sheet locations for mini-sized specimens. Fig. 5. Fracture test specimens and the orientation of the specimens; (a) EB welded block, (b) locations of the CT specimen. Fig. 6. Fig. 7. Cross-sectional view of the finished dissimilar weld Typical microstructure of Inconel 82/182 weld fusion zone; (a) root area, (b) middle (c) top of weld, and (d) 182/F316 fusion boundary Fig. 8. Tensile properties variation across dissimilar metal welds. Using round bar specimens; (a) top part of the weld, (b) middle part of the weld, (c) bottom part of the weld. Fig. 9. Tensile properties variation across dissimilar metal welds. Using mini-sized specimens; (a) top part of the weld, (b) middle part of the weld, (c) bottom part of the weld. Fig. 10. Micro-hardness variation across dissimilar metal welds; (a) top part of the weld, (b) middle part of the weld, (c) bottom part of the weld. Fig. 11. J-R curves variation across the Inconel 82/182 welds at room temperature. Fig. 12. Fracture surfaces of tensile specimen taken at the middle of Inconel 82/182 weld Fig. 13. Fracture surface morphology for Inconel 82/182 welds; (a) specimen No. 2 (Bottom), (b) specimen No. 3 (Middle), (c) specimen No.5 (TOP), (d) specimen No. 6 (TOP). 11

12 Table 1. Chemical compositions of the base metals and welding wires C Mn P S Si Ni Cr Mo Cu Al F SA508 class Ni+Co C Mn Fe S Si Cu Cr Ti Nb+Ta P Others Alloy82 Alloy min min Table 2. Welding parameters for dissimilar metal weld Layer X Filler Metal Type SFA5.14, ERNiCr-3 SFA5.14, ERNiCr-3 SFA5.11, ENiCrFe-3 Size (mm) Welding Process Current Polarity Current (A) Voltage (V) Travel Speed (cm/min) Shield Gas, Ar (l/min) Interpass Temp. () Heat Input (kj/cm) 2.4 GTAW DCSP GTAW DCSP SMAW DCRP Table 3. Tensile properties in different part of the dissimilar welds measured by mini-sized specimens Location Yield Strength, MPa Ultimate Tensile Strength, MPa SA508 Cl.3 Inconel 82/182 weld F316 SA508 Cl.3 Inconel 82/182 weld Top ± ± ± ± ± ± 16.3 Middle ± ± ± ± ± ± 14.5 Bottom ± ± ± ± ± ± 12.0 F316 12

13 Table 4. Mean J IC fracture toughness properties in different part of the Inconel 82/182 welds at room temperature. Position ID J IC (kj/m 2 ) Mean value J IC (kj/m 2 ) Top Middle Bottom

14 Cold-leg Thermal Sleeve Dissimilar-metal weld Spray pipe (3 in.) Thermal sleeve SB-168 (Alloy 600) Safe end, SA-182 F-316 (stainless steel forging) Pressurizer 2-in. connects to charging pipe Charging pipe-tonozzle juncture Dissimilar metal weld Cladding Safe end SA-182 F-1 (low alloy steel nozzle forging) Stainless steel cladding Thermal sleeve SB-168 (Alloy600) Dissimilar-metal Weld RPV nozzle Stainless steel or NiCrFe alloy weld Type 316 stainless steel safe end forging Hot leg 12 in. Surge Piping Typical PWR Cladding Stainless steel or NiCrFe buttering Stainless steel cladding Safe end SA-351 GR.CF8M SA-105 Gr. II (stainless steel casting) (carbon steel) RPV nozzle Cladding Fig. 1. Locations of dissimilar metal welds in typical PWRs Fig. 2. Schematics of the dissimilar metal weld of single V-groove design. 14

15 a) small size specimen b) mini-sized specimen Fig. 3. Small size round bar specimen and mini-sized specimen for tensile test a) locations for small size specimens b) sheet locations for mini-sized specimens Fig. 4. Location where the tensile specimens were taken from the finished dissimilar weld. 15

16 a) EB welded block b) location of the CT specimen Fig. 5. Fracture test specimens and the orientation of the specimens Fig. 6. Cross-sectional view of the finished dissimilar weld 16

17 (a) top (b) middle c) root d) 182/F316 interface Fig. 7. Typical microstructure of Inconel 82/182 weld fusion zone; (a) root area, (b) middle (c) top of weld, and (d) 182/F316 fusion boundary 17

18 a) top part of the weld b) middle part of the weld c) bottom part of the weld Fig. 8. Tensile properties variation across dissimilar metal welds. Using round bar specimens 18

19 a) top part of the weld b) middle part of the weld Fig. 9. c) bottom part of the weld Tensile properties variation across dissimilar metal welds. Using mini-sized specimens 19

20 a) top part of the weld b) middle part of the weld Fig. 10. c) bottom part of the weld Micro-hardness variation across dissimilar metal welds. 20

21 J-integral, KJ/m J-R curve of Inconel 82/182 weld at room temperature J integral_top 6 power fit_top 6 J integral_top 5 power fit_top 5 J integral_middle 3 power fit_middle 3 J integral_bottom 2 power fit_bottom 2 J integral_bottom 1 power fit_bottom crack extension, mm Fig. 11. J IC fracture toughness properties variation across the Inconel 82/182 welds at room temperature. 21

22 Specimen from the mid-center Fig. 12. Fracture surfaces of tensile specimen taken at the middle of Inconel 82/182 weld (a) specimen No. 2 (Bottom) (b) specimen No. 3 (Middle) 22

23 (c) specimen No.5 (TOP) (d) specimen No. 6 (TOP) Fig. 13. Fracture surface morphology for Inconel 82/182 welds. 23