by MAEKAWA Akira, NODA Michiyasu, TAKAHASHI Shigeru, OUMAYA Toru, SERIZAWA Hisashi and MURAKAWA Hidekazu

Transcription

1 [ 27 p. 240s-244s (2009)] by MAEKAWA Akira, NODA Michiyasu, TAKAHASHI Shigeru, OUMAYA Toru, SERIZAWA Hisashi and MURAKAWA Hidekazu This paper reports measured and estimated results of residual stress distributions of butt-welded austenitic stainless steel pipe in order to improve estimation accuracy of welding residual stress. Neutron diffraction and strain gauge method were employed for the measurement of the welding residual stress and its detailed distributions on inner and outer surface of the pipe as well as the distributions within the pipe wall were obtained. Finite element method was employed for the estimation. Transient and residual stresses in 3D butt-welded joint model were computed by employing Iterative Substructure Method and also commercial FEM code ABAQUS for a reference. The measured and estimated distributions presented typical characteristic of straight butt-welded pipe which had decreasing trend along the axial direction and bending type distributions through wall of the pipe. Both results were compared and the accuracy of measurement and estimation was discussed. Key Words: Residual stress, Austenitic stainless steel pipe, Butt-welded joint, Neutron diffraction method, Iterative substructure method 1. Introduction Residual stress due to welding of nuclear power plant components is one of the most important factors in SCC or fatigue crack initiation/propagation. There have been many researches 1) on measurements and theoretical predictions of the residual stress. However, their accuracies are not yet sufficient enough to evaluate the magnitude of residual stress as well as its distribution. Recently, neutron diffraction method is becoming popular as an effective method for the measurement of residual stress in Japan owing to the development of Neutron diffractometer 2) for Residual Stress Analysis at the Japan Research Reactor number 3 (JRR-3) by the Japan Atomic Energy Agency (JAEA). X-ray can only penetrate 20 m from steel surface and measurement inside the steel is impossible, while neutron can penetrate up to 50mm and the measurement of residual stress inside the thick plate becomes possible. Therefore, neutron diffraction technique to welded region of vessels or piping is actively studied. For estimation of residual stress, finite element method employing commercially developed analysis code such as ABAQUS, ANSYS and SYSWELD has been widely used. The uncertainty associated with the accuracy of analytical estimation and extremely long computation time are still yet unresolved. Iterative Substructure Method (ISM) has recently been proposed as a fast computing method by Murakawa et al. 3). The computational time to estimate the welding residual stress can be greatly reduced with keeping almost equivalent accuracy as the conventional finite element method. Noting that the welding *Received: ** Institute of Nuclear Safety System, Inc. ***Member, Joining and Welding Research Institute, Osaka University exhibits strong nonlinearity only in a small area close to the welding torch, the model to be analyzed is separated into two regions, namely weakly nonlinear and strongly nonlinear regions. Only the latter is solved by updating the stiffness at every increment, while the former is solved without updating the stiffness unless necessary. In this way, the computational time is greatly reduced especially when the scale of the model is large. The continuity on the boundary between two regions can be maintained through iterations. This paper reports the comparison of measured and estimated residual stresses in the straight butt-welded pipe made of stainless steel. The measurements were performed employing neutron diffraction and strain gauge technique and the estimation was performed employing ISM. For a reference, estimation with ABAQUS is also shown. 2. Experimental Procedure 2.1 Test specimen The 200mm length pipes made of SUS316TP with 165.2mm in outer diameter and 18.2mm in thickness were butt-welded together with 5 layers of welding by 5 passes. The first layer was welded by TIG welding and the second to the fifth layers were welded with Shielded Metal Arc Welding (SMAW) method. SMAW were performed from 0 to 360 in circumferential direction. The welding conditions are summarized in Table 1. Outer surface temperature of the test specimen was continuously measured by thermocouples and infrared thermography during the welding and the subsequent cooling until it reached 100 also. Thermocouples were placed on both inner and outer surface. At 0 and 180 positions, it was placed 5mm from the weld toe. At 90 and 270 positions, it was placed 10 and 20mm from the weld toe. After the welding, 400mm test specimen was cut at both ends to make the 100 mm specimen in total length for the stress

2 s Table 1 Welding conditions Fig. 2 Measurement by RESA Table 2 Young's modulus and Poisson's ratio for the measurement Fig. 1 Specimen measurement. The final specimen was photographed in Fig. 1 with the illustrated image showing dimensions. 2.2 Measurement employing neutron diffraction technique Residual stress of the specimen was measured with neutron diffractometer for REsidual Stress Analysis (RESA) 2) as shown in Fig. 2 at JRR-3 owned by JAEA. Monochromatic neutron with wave length of 0.18nm was radiated with the gauge volume of 3mm cube on the test section. Irradiated neutron diffracts on the specimen satisfying Bragg condition as shown in Eq.(1), = 2d sin (1) where, : wavelength of neutron, d: distance of lattice plane and : diffraction angle. The diffraction angle tilts corresponding to a difference of lattice plane distances without strain and under applied loading or residual stress. By differentiating Eq.(1), Eq.(2) is obtained. Thus by measuring the difference of diffraction angle, strain can be obtained. e = ( d d 0 ) / d 0 = cot 0 (2) Value of the diffraction angle was determined by the maximum value of Gaussian approximation of the obtained diffracted neutron profile. The lattice plane distance or the diffraction angle without strain needs to be determined. By measuring the diffraction angle of "d 0 coupon", sliced from the measured specimen, d 0 was determined. The "d 0 coupon" was sliced until its residual strain becomes almost negligible. Neutron diffraction method can obtain strain in three dimensional states since it can penetrate deep into the specimen. Circumferential (diffraction plane 111), radial (diffraction plane 311) and axial (diffraction plane 311) strains were measured Fig. 3 Locations of the measurement separately and residual stresses were determined based on Hooke's law. The Young's modulus and Poisson's ratio, which were computed by Kröner model, to calculate residual stress are listed in Table 2. The circumferetial strain based on diffraction plane of 111 was corrected for equivalent strain of diffraction plane of 311 accordingly to the method proposed 4), and residual stress was determined. The locations of measurement were shown in Fig. 3. Eight points in axial direction originating from the weld toe with four points each in radial direction were all measured at 0 longitudinal section. The innermost points neighboring the weld were set to be 14mm from outer surface instead of 16mm avoiding inner thinned region. 2.2 Measurement employing strain gauge technique Strain gauge was placed on both inner and outer surface of the pipe to measure both circumferential and axial surface stresses. The measured locations were shown in Fig. 4. The terminal size of cubic piece after nearly all strain was released

3 242s MAEKAWA et al.: Evaluation of Residual Stress Distribution in Austenitic Stainless Steel Pipe Butt-Welded Joint Fig. 5 Conceptual image of Iterative Substructure Method Fig. 4 Locations of the measurement was set to be 10mm 10mm 4mm. The residual stresses x, y were estimated from the following equation using the measured relaxed strain x, y for the both parallel and perpendicular to the welded line. ( ) E s x = Dex + ndey n Ô E s y = ey + n e - ( D D x) Ô 2 1 n where, E: Young's modulus which was 195GPa, and Poisson's ratio was set to be 0.3. For the weld joint of a pipe, x and y represents circumferential and axial direction respectively. (3) 3. Estimation method 3.1 Iterative Substructure Method (ISM) Since welding process is a transient nonlinear problem, extremely long computing time is required to solve large scale simultaneous equations over huge number of times. If number of solving large equation is reduced, the computational time would be reduced significantly. Iterative Substructure Method (ISM) 3) was developed based on this concept with taking advantage of the characteristics of welding process, in which the region exhibits strong nonlinearity was limited in a very small area B, and the remaining part A is mostly linear or area with small stiffness change. Figure 5 shows conceptual image of ISM. In ISM, the quasi linear region A and strongly nonlinear region B are separately solved and the continuity at the boundary is maintained through the iterations. When the region B changes into the nonlinear region B by movement of the welding torch, it is reasonable to solve the combination between the weaknonlinear problem of the region (A +B ) and the strong-nonlinear problem of the region B. Since the change in stiffness of region A is small, the previous stiffness of (A +B ), for which the inverse matrix is already computed, can be used as long as the convergence is maintained. So, the computing speed is improved by reducing the process to solve the large equation associated to region A. When deformation of the problem is small in which the geometrical nonlinearity is neglected, the initial elastic stiffness can be used until the end of welding process. Thus the Fig. 6 Conditions for the welding simulation computational time can be significantly reduced. The welding condition used for the specimen in the experiment was applied to the finite element model and the welding process was simulated to obtain the residual stress distribution after the welding using ISM. In the computation, the nonlinear region B was extracted as the region in which the temperature is higher than 400 to minimize the computational time while keeping the same accuracy as the conventional method. 3.2 Estimated conditions of ISM The model used for FEM analysis is shown in Fig. 6 (a). It was a pipe with 165.2mm in outer diameter and 18.2mm in thickness and its length was 400mm. 72 meshes were generated along the circumferential direction and the total number of nodes and elements were 29,592 and 25,344, respectively. Type of element was 8 nodes linear hexahedral element. Thermal conduction analysis was performed assuming moving heat source, and the heat input, welding speed and welding efficiency were taken from welding condition shown in Table 1. Initial

4 s temperature was set at 20 and heat transfer coefficient from the pipe surface was set to be W/mm 2 K and the inter-pass temperature was 100. The thermal expansion coefficient and the thermal conductivity and the specific heat and the density were obtained from ASME code. In the simulation employing ISM, stress-strain relation of the welded region was considered to be the same as the base metal and the strain hardening is assumed to be zero. These assumptions may lead to small residual stress. In the first step, the residual stress due to the welding was computed as thermal-elastic-plastic process. Then, the stress distribution after the shortening of the welded pipe was calculated as the second step and compared with the measured values. 3.3 ABAQUS For a reference, the commercial FEM code ABAQUS was also employed. The measured stress-strain relations shown in Fig. 6 (b) were used for the base metal and the weld metal, respectively. The other estimation conditions were set to be the same as in the case of ISM. 4. Result and discussion 4.1 Comparison of measurement by neutron diffraction method with estimations The residual stresses measured by neutron diffraction method and those estimations by ISM and ABAQUS are shown in Fig. 7. The residual stress is plotted against distance from the weld toe (9.59mm from the weld center). Positive value represents tensile stress and negative value represents compressive stress. Left, center and right figure shows circumferential, axial and radial stress, respectively. Qualitative trends of estimated circumferential stress distributions are consistent with the measured values. Measured values shifted about 100MPa in tensile side comparing with the estimated distributions. Suzuki et al. 5) reported that the welding residual stress in austenitic stainless steel measured by neutron diffraction method may produce over 150MPa of scatter in some case due to "type-2 strain". The values shown in Fig. 7 may have been under the same influence and it is possible to say that it was within the error band of measurement by the neutron diffraction method. On the other hand, the estimated result may include errors due to meshing or stress-strain relations of weld metal. Comparing estimated results by ISM and ABAQUS, both of them had similar values although the result by ABAQUS produced smaller values near the weld toe. Qualitative trends of estimated axial stress distributions by ISM were consistent with the measured values by neutron diffraction method. Measured axial stress also shifted about 100MPa to tensile side as well as circumferential stress in comparison with the estimated distributions. Radial stress had the same trend as Fig. 7 Measured (Neutron diffraction method) and estimated stress distributions

5 244s MAEKAWA et al.: Evaluation of Residual Stress Distribution in Austenitic Stainless Steel Pipe Butt-Welded Joint Fig. 8 Measured (Strain gauge method) and estimated stress distributions the circumferential and the axial stress. Measured values were 100MPa larger than the estimated one. 4.2 Comparison of the measurement by strain gauge method with the estimation The stresses at 0 measured using strain gauge method and estimated using ISM and ABAQUS are compared in Fig. 8 (a) and (b), for outer surface and inner surface respectively. Qualitative trends as well as magnitudes of estimated stress distributions matched well with the measured values except for the axial stress further than 10mm from the weld toe, which was tensile in measured values, contrary to the fact that the estimated value was compressive. Comparing estimated results by ISM and ABAQUS, both of them had similar trend and values. 5. Conclusion The residual stress distributions of butt-welded austenitic stainless steel pipe were measured by neutron diffraction and strain gauge method, and compared with the values estimated by ISM and ABAQUS. The measured values by strain gauge method matched well with the estimated values. In case of the residual stress measured by neutron diffraction method, the distribution showed good agreement with the computed results but its value were shifted approximately 100MPa in the tensile direction. The cause of difference can be attributed to influence of "type-2 strain" in the measurement, and the meshing and material properties of the weld used in the computations. Acknowledgements The residual stress measurement by neutron diffraction method was performed under the sponsorship of Ministry of Education, Culture, Sports, Science and Technology. References 1) S. K. Bate, P. Hurrell, J. A. Francis and M. Turski: UK Research Programme on Residual Stresses A Review of Progress, Proc. of ASME PVP, PVP , (2008). 2) M. Hayashi, S. Ohkido, Y. Morii and N. Minakawa: Measurement of Residual Stress in Structural Components by Neutron Diffraction, Mat. Sci. Res. Int., Special Technical Publication-1, (2001), ) H. Murakawa, I. Oda, S. Ito, H. Serizawa, M. Shibahara and H. Nishikawa: Iterative Substructure Method for Fast Computation of Thermal Elastic Plastic Welding Problems, J. Kansai Soc. N. A., Japan, 243 (2005), (in Japanese) 4) S. Okido, M. Hayashi, Y. Akiniwa, K. Tanaka, N. Minakawa and Y. Morii: Residual Stress Measurement of Shrink Fitted Component in Textured Al Alloy by Neutron Diffraction Method, J. Soc. Mat. Sci., Japan, 54-3 (2005), (in Japanese) 5) H. Suzuki and T. M. Holden: Neutron Diffraction Measurements of Stress in an Austenitic Butt Weld, J. Strain Analysis, 41-8 (2006),