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Evaluation of the heat-affected zone (HAZ) of a weld joint using nonlinear Rayleigh waves
Accepted Manuscript Evaluation of the Heat-affected Zone (HAZ) of a Weld Joint Using Nonlinear Rayleigh Waves Christoph Doerr, Alex Jin-Yeon Lakocy Kim, Preet M. Singh, James J. Wall, Jianmin Qu, Laurence J. Jacobs PII: DOI: Reference:
S0167-577X(17)30021-6 http://dx.doi.org/10.1016/j.matlet.2017.01.021 MLBLUE 21974
To appear in:
Materials Letters
Received Date: Accepted Date:
22 October 2016 5 January 2017
Please cite this article as: C. Doerr, A.J-Y. Lakocy Kim, P.M. Singh, J.J. Wall, J. Qu, L.J. Jacobs, Evaluation of the Heat-affected Zone (HAZ) of a Weld Joint Using Nonlinear Rayleigh Waves, Materials Letters (2017), doi: http:// dx.doi.org/10.1016/j.matlet.2017.01.021
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of the Heat-affected Zone (HAZ) of a Weld Joint Using Nonlinear Rayleigh Waves Christoph Doerr1, Alex Lakocy1 Jin-Yeon Kim1, Preet M. Singh2, James J. Wall3,4, Jianmin Qu5, and Laurence J. Jacobs1,4,* 1
School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332 2 School of Materials Science and Engineering. Georgia Institute of Technology, Atlanta, GA 30332 3 Electric Power Research Institute, Charlotte, NC 28262 4 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 5 Department of Mechanical Engineering, Tufts University, Medford, MA 02155 *Corresponding author: [email protected] 404-894-2344
Abstract Nonlinear ultrasound (NLU) has been shown to be sensitive to microstructural features including precipitates and dislocations. The heat-affected zone (HAZ) of welded austenitic stainless steels can be susceptible to intergranular stress corrosion cracking (IGSCC). This research uses NLU to evaluate the microstructure of the HAZ with an emphasis on dislocations, precipitates, grain size/morphology and sensitization, which is the formation of chromium carbide precipitates at the grain boundaries. The results show that there is a large increase in the measured acoustic nonlinearity parameter in the vicinity of the HAZ, and can be used to monitor changes in microstructure such as sensitization. Keywords: microstructure; welding; nonlinear ultrasonics; austenitic steels; sensitization 1. Introduction The heat-affected zone (HAZ) of welded austenitic stainless steels has a complicated microstructure that can be susceptible to intergranular stress corrosion cracking (IGSCC). Sensitization in austenitic stainless steel, a driving factor for IGSCC, is caused by the depletion of chromium near the grain boundaries due to the formation of chromium carbide precipitates when these steels are exposed to a temperature range from 450 to 850°C. The formation of the chromium carbide precipitates is strongly dependent on the carbon and chromium content of the 1
austenitic stainless steel, and other factors such as grain size/morphology and deformation have a strong effect on the sensitization process. Ashby et al. [1] examined the phase changes in the HAZ in welded austenitic stainless steel and showed that, depending on temperature, there is a melt and a γ-phase close to the weld centerline. Moving away from the weld centerline, the meltphase disappears and, due to decreasing temperature, an α-phase is precipitated. Furthermore, the welding process creates a complicated distribution of hoop and axial stresses near the weld centerline in butt welded pipe. Suarez et al. showed that the grain size decreases from about 130 µ m to 15 µm between 1.5 to 6 mm away from the weld centerline [2]. Recent research has demonstrated that nonlinear ultrasound (NLU) is sensitive to critical microstructure features such as precipitates [3-5], dislocations [6, 7] or sensitization [8,9], as well as the presence of cold work or residual stresses [10]. More research is needed to understand the influence of sensitization on the measured acoustic nonlinearity parameter in welded specimens. As a step in that direction, this research uses NLU to examine the microstructure of a welded 304 austenitic stainless steel specimen with an emphasis on the influence of the HAZ on the measured acoustic nonlinearity parameter. The NLU measurements were used to characterize acoustic nonlinearity as a function of distance from the weld centerline. Subsequently, the weld specimen was heated to induce additional changes in the microstructure of the specimen and determine the effect on the NLU measurements. 2. Experimental Procedure A welded, 15.88 mm thick austenitic stainless steel 304 plate was prepared with a Vnotch cut along the centerline, and then welded full thickness (50.8 mm crown width) with 308 stainless steel filler material while all four edges of the specimen were clamped to prevent
2
warping. The top surface of the welded plate was flattened, ground and then polished with sandpaper ranging from 50 to 800 grit. The measurement setup to generate and detect nonlinear Rayleigh waves was identical to the one used in [3] consisted of an air-coupled receiver, and a narrow band contact piezoelectric transducer (center frequency 2.25 MHz), which excited a longitudinal wave into a Plexiglas wedge, angle matched for Rayleigh wave generation in the 304 stainless steel specimen. A RITEC amplifier (as much as 60 dB) created second harmonic generation of the Rayleigh wave. The fundamental (A1 at 2.25 MHz) and second harmonic (A2 at 4.5 MHz) amplitudes were obtained using a fast Fourier transform (FFT) of the measured time-domain signal. The transducer/wedge source was held at a fixed position, while the air-coupled receiver was moved along the propagation path to determine the ratio of A2/A12 which is the relative acoustic nonlinearity parameter, β, and should be cumulative with increasing propagation distance. ASTM G108 [11] was used to perform complementary electrochemical potentiodynamic reactivation (EPR) measurements on the same specimens to estimate the degree of sensitization (DOS). DOS quantifies the extent of chromium depletion along the grain boundaries that is caused by M23C6 carbide precipitation. Previous work has correlated this chromium depletion along the grain boundaries to: coverage of the grain boundary by precipitates; width; and depth of the carbide precipitates [12]. Following an initial set of measurements on the “as-welded” specimen, it was held in an air furnace at 675° C for holding times of 30, 210 and 360 minutes to induce sensitization in the specimen. In addition, optical microscopy was used to locate the chromium carbide precipitates on the grain boundaries to determine if sensitization has occurred after each heating.
3
3. Results and Discussion Figure 1 shows the measured acoustic nonlinearity parameter, β, versus distance from the weld centerline for the as-welded, 30, 210 and 360 min specimens, Fig. 2 shows the measured EPR current density for the same specimens, at the same locations, while Fig. 3 are representative optical microscopy images at 30 mm from the weld centerline. All results in Fig. 1 are normalized to the results from the as-welded specimen measured at 130 mm. The as-welded specimen shows the measured β increases for decreasing distance from the weld centerline – the HAZ causes an almost 200% increase in β in comparison to the slightly heated material at 130 mm. The values of β are fairly consistent in the HAZ (the 30 and 50 mm locations) and then decrease through a transition zone from 50 to 110 mm, before stabilizing. The current density results shown in Fig. 2 are almost constant with distance from the weld centerline, and are much lower than the comparison values at 30, 210 and 360 minutes. Figure 3(a) shows no chromium carbide precipitates on the grain boundaries, confirming that no sensitization occurred in the as-welded specimen, even in the HAZ. It is thus likely that other microstructural phenomena such as dislocations, the γ and α-phases, grain size/morphology and macroscale residual stresses in the HAZ caused a 200% in the measured β, and that this large increase in β was in the absence of sensitization throughout the specimen. Now consider the 30-minute specimen, where the measured values of β decrease by approximately 60% (in the HAZ) to 25% (farthest from the weld centerline) in comparison to the as-welded results. It appears that heating at 675° C for 30 minutes anneals out some of the dislocations and partially relieves residual stresses present throughout the as-welded specimen. Similar results [3, 7, 9] have shown that a reduction in dislocation density by high temperature heating causes a decrease in β on the order of 30%, which is significantly less than the β decrease
4
in the HAZ. The EPR results of Fig. 2 and the absence of chromium carbide precipitates on the grain boundaries in Fig. 3(b) shows that no sensitization occurred during this 30 minutes of heating. Finally consider the 210 and 360 minute specimens, where there is a significant increase in β (between 10-30%) from 30 to 210 minutes, followed by a much smaller increase in β (on the order of 5%) from 210 to 360 minutes. The EPR results and microscopy show that the 210minute specimen was sensitized; there is a 500% increase in the measured current density from the 30-minute specimen and chromium carbide precipitates are clearly visible on the grain boundaries in Fig. 3(c). The very small increases in both β and the measured current density from 210 to 360 minutes, plus the images in Fig. 3(d) all show that sensitization is basically saturated after 210 minutes, and additional precipitate nucleation at the grain boundaries is limited with longer holding times. Similar results were seen in oven sensitized 304 specimens in [9], where the measured β increased by 25% due to sensitization alone, reaching a constant once the specimen was fully sensitized. Liu et al. [10] saw an increase in β on the order of 100% due to cold work and residual stresses in shot peened 7075 aluminum specimens, while phase changes in duplex stainless steel caused an initial 60% increase in β [4], and precipitate nucleation and growth in 9%Cr steel caused a 50% increase in β [3]. So the 200% increase in β in the HAZ of the as-welded specimen was most likely caused by a combination of macroscale residual stresses developed during welding, and microstructural changes such as the formation of γ and α-phases. After some stress and cold work relief by the initial heating for 30 minutes, increases in β in the HAZ are then due to a competition between further relaxation of internal stresses (decreasing β), precipitation of Cr23C6 carbides on the grain boundaries (sensitization, increasing β) and other microstructural
5
changes. Since the increase in β from 30 to 360 minutes is larger in the HAZ (~30%) than in the region farther away from the weld centerline (~20%), it appears that the initial microstructure and macroscale residual stresses in the HAZ can give additional support to chromium carbide nucleation. Most importantly, initial changes in the HAZ have a much greater effect (as high as 200%) on the measured acoustic nonlinearity β, then just sensitization alone (25%). 4. Conclusions This research demonstrates that the acoustic nonlinearity parameter, β, is sensitive to changes in the microstructure and macroscale residual stresses associated with welding, showing a 200% increase in the measured β in the HAZ. Two independent measures, EPR and optical microscopy, show that sensitization did not initially occur in the HAZ, meaning this increase in β is in the absence of chromium carbide precipitates on the grain boundaries. A 30-minute initial heating then causes a decrease in measured β, most likely due to internal stress relaxation, while additional, longer term (up to 360 minutes) heating causes a smaller (15-30%) increase in β which can be directly attributed to sensitization. The increase in β from 30 to 360 minutes is larger closer to the weld centerline, which could indicate that the HAZ and associated internal stresses give additional support to sensitization. Overall, these results illustrate the potential of NLU to monitor changes in the microstructure of welded components such as sensitization.
Acknowledgements Partial financial support was provided by the Electric Power Research Institute (EPRI) and the Deutscher Akademischer Austauschdienst (DAAD). References [1] Ashby, M. and Easterling, K. E., Acta Metallurgica, 30 (11), 1969-1978, 1982. [2] Suárez, J., Molleda, F., and de Salazar, J. G., Materials Characterization, 28 (1), 3-13, 1992. 6
[3] Marino, D., Kim, J.-Y., Ruiz, A., Joo, Y.-S., Qu, J., and Jacobs, L. J., NDT & E Int, 79, 46-52, 2016 [4] Ruiz, A., Ortiz, N., Medina, A., Kim, J- Y, and Jacobs, L.J., NDT&E Int., 54, 19-26, 2013. [5] Matlack, K.H., Bradley, H.A., Thiele, S, Kim, Jin-Yeon, Wall, J.J., Jung, Hee Joon, Qu, Jianmin, and Jacobs, L.J., Vol. 71, pp. 8-15, 2015. [6] Walker, S. V., Kim, J.-Y., Qu, J., and Jacobs, L. J., NDT & E Int., 48, 10-15, 2012. [7] Matlack, K.H., Kim, J.-Y., Wall, J.J., Qu, J., Jacobs, L.J., Sokolov, M.A., J. Nucl. Mater. 448, 26–32 (2014). [8] Abraham, S. T., Albert, S., Das, C., Parvathavarthini, N., Venkatraman, B., Mini, R., and Balasubramaniam, K., Acta Metallurgica Sinica (English Letters), 26 (5), 545-552, 2013. [9] Doerr, C., Kim, Jin-Yeon, Singh, Preet, Wall, J.J., and Jacobs, L.J., NDT&E Int., submitted, 2016. [10] Liu, M., Kim, J.-Y., Jacobs, L.J., Qu, J., NDT&E Int. 44, 67–74, 2011. [11] ASTM E407-07 1994. [12] Parvathavarthini, N, Mudali, UK Corrosion Reviews, 32 (5-6) 183-225, 2014.
Fig 1. Acoustic nonlinearity parameter, β versus distance from the weld centerline for the aswelded and different holding times specimens.
7
Fig 2. Normalized current density versus distance from the weld centerline for the as-welded and the different holding times specimens.
Fig 3. Optical microscopy at 30 mm from weld centerline: (a) as-welded, (b) 30, (c) 210 and (d) 360 minutes.
8
•
Demonstrate acoustic nonlinearity parameter, β is sensitive to microstructure.
•
A 200% increase in the measured β in the heat-affected zone (HAZ).
•
This increase in β is in the absence of sensitization.
•
Induce additional microstructure changes to determine effect on β.
•
Illustrate potential of nonlinear ultrasonics to monitor microstructure changes.
9
S0167-577X(17)30021-6 http://dx.doi.org/10.1016/j.matlet.2017.01.021 MLBLUE 21974
To appear in:
Materials Letters
Received Date: Accepted Date:
22 October 2016 5 January 2017
Please cite this article as: C. Doerr, A.J-Y. Lakocy Kim, P.M. Singh, J.J. Wall, J. Qu, L.J. Jacobs, Evaluation of the Heat-affected Zone (HAZ) of a Weld Joint Using Nonlinear Rayleigh Waves, Materials Letters (2017), doi: http:// dx.doi.org/10.1016/j.matlet.2017.01.021
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of the Heat-affected Zone (HAZ) of a Weld Joint Using Nonlinear Rayleigh Waves Christoph Doerr1, Alex Lakocy1 Jin-Yeon Kim1, Preet M. Singh2, James J. Wall3,4, Jianmin Qu5, and Laurence J. Jacobs1,4,* 1
School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332 2 School of Materials Science and Engineering. Georgia Institute of Technology, Atlanta, GA 30332 3 Electric Power Research Institute, Charlotte, NC 28262 4 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 5 Department of Mechanical Engineering, Tufts University, Medford, MA 02155 *Corresponding author: [email protected] 404-894-2344
Abstract Nonlinear ultrasound (NLU) has been shown to be sensitive to microstructural features including precipitates and dislocations. The heat-affected zone (HAZ) of welded austenitic stainless steels can be susceptible to intergranular stress corrosion cracking (IGSCC). This research uses NLU to evaluate the microstructure of the HAZ with an emphasis on dislocations, precipitates, grain size/morphology and sensitization, which is the formation of chromium carbide precipitates at the grain boundaries. The results show that there is a large increase in the measured acoustic nonlinearity parameter in the vicinity of the HAZ, and can be used to monitor changes in microstructure such as sensitization. Keywords: microstructure; welding; nonlinear ultrasonics; austenitic steels; sensitization 1. Introduction The heat-affected zone (HAZ) of welded austenitic stainless steels has a complicated microstructure that can be susceptible to intergranular stress corrosion cracking (IGSCC). Sensitization in austenitic stainless steel, a driving factor for IGSCC, is caused by the depletion of chromium near the grain boundaries due to the formation of chromium carbide precipitates when these steels are exposed to a temperature range from 450 to 850°C. The formation of the chromium carbide precipitates is strongly dependent on the carbon and chromium content of the 1
austenitic stainless steel, and other factors such as grain size/morphology and deformation have a strong effect on the sensitization process. Ashby et al. [1] examined the phase changes in the HAZ in welded austenitic stainless steel and showed that, depending on temperature, there is a melt and a γ-phase close to the weld centerline. Moving away from the weld centerline, the meltphase disappears and, due to decreasing temperature, an α-phase is precipitated. Furthermore, the welding process creates a complicated distribution of hoop and axial stresses near the weld centerline in butt welded pipe. Suarez et al. showed that the grain size decreases from about 130 µ m to 15 µm between 1.5 to 6 mm away from the weld centerline [2]. Recent research has demonstrated that nonlinear ultrasound (NLU) is sensitive to critical microstructure features such as precipitates [3-5], dislocations [6, 7] or sensitization [8,9], as well as the presence of cold work or residual stresses [10]. More research is needed to understand the influence of sensitization on the measured acoustic nonlinearity parameter in welded specimens. As a step in that direction, this research uses NLU to examine the microstructure of a welded 304 austenitic stainless steel specimen with an emphasis on the influence of the HAZ on the measured acoustic nonlinearity parameter. The NLU measurements were used to characterize acoustic nonlinearity as a function of distance from the weld centerline. Subsequently, the weld specimen was heated to induce additional changes in the microstructure of the specimen and determine the effect on the NLU measurements. 2. Experimental Procedure A welded, 15.88 mm thick austenitic stainless steel 304 plate was prepared with a Vnotch cut along the centerline, and then welded full thickness (50.8 mm crown width) with 308 stainless steel filler material while all four edges of the specimen were clamped to prevent
2
warping. The top surface of the welded plate was flattened, ground and then polished with sandpaper ranging from 50 to 800 grit. The measurement setup to generate and detect nonlinear Rayleigh waves was identical to the one used in [3] consisted of an air-coupled receiver, and a narrow band contact piezoelectric transducer (center frequency 2.25 MHz), which excited a longitudinal wave into a Plexiglas wedge, angle matched for Rayleigh wave generation in the 304 stainless steel specimen. A RITEC amplifier (as much as 60 dB) created second harmonic generation of the Rayleigh wave. The fundamental (A1 at 2.25 MHz) and second harmonic (A2 at 4.5 MHz) amplitudes were obtained using a fast Fourier transform (FFT) of the measured time-domain signal. The transducer/wedge source was held at a fixed position, while the air-coupled receiver was moved along the propagation path to determine the ratio of A2/A12 which is the relative acoustic nonlinearity parameter, β, and should be cumulative with increasing propagation distance. ASTM G108 [11] was used to perform complementary electrochemical potentiodynamic reactivation (EPR) measurements on the same specimens to estimate the degree of sensitization (DOS). DOS quantifies the extent of chromium depletion along the grain boundaries that is caused by M23C6 carbide precipitation. Previous work has correlated this chromium depletion along the grain boundaries to: coverage of the grain boundary by precipitates; width; and depth of the carbide precipitates [12]. Following an initial set of measurements on the “as-welded” specimen, it was held in an air furnace at 675° C for holding times of 30, 210 and 360 minutes to induce sensitization in the specimen. In addition, optical microscopy was used to locate the chromium carbide precipitates on the grain boundaries to determine if sensitization has occurred after each heating.
3
3. Results and Discussion Figure 1 shows the measured acoustic nonlinearity parameter, β, versus distance from the weld centerline for the as-welded, 30, 210 and 360 min specimens, Fig. 2 shows the measured EPR current density for the same specimens, at the same locations, while Fig. 3 are representative optical microscopy images at 30 mm from the weld centerline. All results in Fig. 1 are normalized to the results from the as-welded specimen measured at 130 mm. The as-welded specimen shows the measured β increases for decreasing distance from the weld centerline – the HAZ causes an almost 200% increase in β in comparison to the slightly heated material at 130 mm. The values of β are fairly consistent in the HAZ (the 30 and 50 mm locations) and then decrease through a transition zone from 50 to 110 mm, before stabilizing. The current density results shown in Fig. 2 are almost constant with distance from the weld centerline, and are much lower than the comparison values at 30, 210 and 360 minutes. Figure 3(a) shows no chromium carbide precipitates on the grain boundaries, confirming that no sensitization occurred in the as-welded specimen, even in the HAZ. It is thus likely that other microstructural phenomena such as dislocations, the γ and α-phases, grain size/morphology and macroscale residual stresses in the HAZ caused a 200% in the measured β, and that this large increase in β was in the absence of sensitization throughout the specimen. Now consider the 30-minute specimen, where the measured values of β decrease by approximately 60% (in the HAZ) to 25% (farthest from the weld centerline) in comparison to the as-welded results. It appears that heating at 675° C for 30 minutes anneals out some of the dislocations and partially relieves residual stresses present throughout the as-welded specimen. Similar results [3, 7, 9] have shown that a reduction in dislocation density by high temperature heating causes a decrease in β on the order of 30%, which is significantly less than the β decrease
4
in the HAZ. The EPR results of Fig. 2 and the absence of chromium carbide precipitates on the grain boundaries in Fig. 3(b) shows that no sensitization occurred during this 30 minutes of heating. Finally consider the 210 and 360 minute specimens, where there is a significant increase in β (between 10-30%) from 30 to 210 minutes, followed by a much smaller increase in β (on the order of 5%) from 210 to 360 minutes. The EPR results and microscopy show that the 210minute specimen was sensitized; there is a 500% increase in the measured current density from the 30-minute specimen and chromium carbide precipitates are clearly visible on the grain boundaries in Fig. 3(c). The very small increases in both β and the measured current density from 210 to 360 minutes, plus the images in Fig. 3(d) all show that sensitization is basically saturated after 210 minutes, and additional precipitate nucleation at the grain boundaries is limited with longer holding times. Similar results were seen in oven sensitized 304 specimens in [9], where the measured β increased by 25% due to sensitization alone, reaching a constant once the specimen was fully sensitized. Liu et al. [10] saw an increase in β on the order of 100% due to cold work and residual stresses in shot peened 7075 aluminum specimens, while phase changes in duplex stainless steel caused an initial 60% increase in β [4], and precipitate nucleation and growth in 9%Cr steel caused a 50% increase in β [3]. So the 200% increase in β in the HAZ of the as-welded specimen was most likely caused by a combination of macroscale residual stresses developed during welding, and microstructural changes such as the formation of γ and α-phases. After some stress and cold work relief by the initial heating for 30 minutes, increases in β in the HAZ are then due to a competition between further relaxation of internal stresses (decreasing β), precipitation of Cr23C6 carbides on the grain boundaries (sensitization, increasing β) and other microstructural
5
changes. Since the increase in β from 30 to 360 minutes is larger in the HAZ (~30%) than in the region farther away from the weld centerline (~20%), it appears that the initial microstructure and macroscale residual stresses in the HAZ can give additional support to chromium carbide nucleation. Most importantly, initial changes in the HAZ have a much greater effect (as high as 200%) on the measured acoustic nonlinearity β, then just sensitization alone (25%). 4. Conclusions This research demonstrates that the acoustic nonlinearity parameter, β, is sensitive to changes in the microstructure and macroscale residual stresses associated with welding, showing a 200% increase in the measured β in the HAZ. Two independent measures, EPR and optical microscopy, show that sensitization did not initially occur in the HAZ, meaning this increase in β is in the absence of chromium carbide precipitates on the grain boundaries. A 30-minute initial heating then causes a decrease in measured β, most likely due to internal stress relaxation, while additional, longer term (up to 360 minutes) heating causes a smaller (15-30%) increase in β which can be directly attributed to sensitization. The increase in β from 30 to 360 minutes is larger closer to the weld centerline, which could indicate that the HAZ and associated internal stresses give additional support to sensitization. Overall, these results illustrate the potential of NLU to monitor changes in the microstructure of welded components such as sensitization.
Acknowledgements Partial financial support was provided by the Electric Power Research Institute (EPRI) and the Deutscher Akademischer Austauschdienst (DAAD). References [1] Ashby, M. and Easterling, K. E., Acta Metallurgica, 30 (11), 1969-1978, 1982. [2] Suárez, J., Molleda, F., and de Salazar, J. G., Materials Characterization, 28 (1), 3-13, 1992. 6
[3] Marino, D., Kim, J.-Y., Ruiz, A., Joo, Y.-S., Qu, J., and Jacobs, L. J., NDT & E Int, 79, 46-52, 2016 [4] Ruiz, A., Ortiz, N., Medina, A., Kim, J- Y, and Jacobs, L.J., NDT&E Int., 54, 19-26, 2013. [5] Matlack, K.H., Bradley, H.A., Thiele, S, Kim, Jin-Yeon, Wall, J.J., Jung, Hee Joon, Qu, Jianmin, and Jacobs, L.J., Vol. 71, pp. 8-15, 2015. [6] Walker, S. V., Kim, J.-Y., Qu, J., and Jacobs, L. J., NDT & E Int., 48, 10-15, 2012. [7] Matlack, K.H., Kim, J.-Y., Wall, J.J., Qu, J., Jacobs, L.J., Sokolov, M.A., J. Nucl. Mater. 448, 26–32 (2014). [8] Abraham, S. T., Albert, S., Das, C., Parvathavarthini, N., Venkatraman, B., Mini, R., and Balasubramaniam, K., Acta Metallurgica Sinica (English Letters), 26 (5), 545-552, 2013. [9] Doerr, C., Kim, Jin-Yeon, Singh, Preet, Wall, J.J., and Jacobs, L.J., NDT&E Int., submitted, 2016. [10] Liu, M., Kim, J.-Y., Jacobs, L.J., Qu, J., NDT&E Int. 44, 67–74, 2011. [11] ASTM E407-07 1994. [12] Parvathavarthini, N, Mudali, UK Corrosion Reviews, 32 (5-6) 183-225, 2014.
Fig 1. Acoustic nonlinearity parameter, β versus distance from the weld centerline for the aswelded and different holding times specimens.
7
Fig 2. Normalized current density versus distance from the weld centerline for the as-welded and the different holding times specimens.
Fig 3. Optical microscopy at 30 mm from weld centerline: (a) as-welded, (b) 30, (c) 210 and (d) 360 minutes.
8
•
Demonstrate acoustic nonlinearity parameter, β is sensitive to microstructure.
•
A 200% increase in the measured β in the heat-affected zone (HAZ).
•
This increase in β is in the absence of sensitization.
•
Induce additional microstructure changes to determine effect on β.
•
Illustrate potential of nonlinear ultrasonics to monitor microstructure changes.
9