Effect of Boiler Operating Practice on Circumferential Crack Growth in Weld Overlays

Effect of Boiler Operating Practice on Circumferential Crack Growth in Weld Overlays Finite Element Modeling of Circumferential Cracking 1014248 Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S Export Administration Regulations As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication Effect of Boiler Operating Practice on Circumferential Crack Growth in Weld Overlays Finite Element Modeling of Circumferential Cracking 1014248 Technical Update, August 2007 EPRI Project Managers A Facchiano S Cardoso ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1338 ▪ PO Box 10412, Palo Alto, California 94303-0813 ▪ USA 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ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Welding Services, Inc Electric Power Research Institute This is an EPRI Technical Update report A Technical Update report is intended as an informal report of continuing research, a meeting, or a topical study It is not a final EPRI technical report NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail askepri@epri.com Electric Power Research Institute, EPRI, and TOGETHER…SHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc Copyright © 2007 Electric Power Research Institute, Inc All rights reserved CITATIONS This document was prepared by Welding Services, Inc 2225 Skyland Court Norcross, GA 30071 Principal Investigator T Scandroli Wate T Bakker, Consultant 9011 Village View Drive San Jose, CA 95135 Principal Investigator W Bakker This document describes research sponsored by the Electric Power Research Institute (EPRI) This publication is a corporate document that should be cited in the literature in the following manner: Effect of Boiler Operating Practice on Circumferential Crack Growth in Weld Overlays Finite Element Modeling of Circumferential Cracking; EPRI, Palo Alto, CA: 2007 1014248 iii REPORT SUMMARY The gas metal arc welding (GMAW) process, used to mitigate fireside corrosion on waterwalls due to Low NOx Burner operation in pulverized coal units, results in residual stresses in weld overlays that interact with stresses that occur during boiler operations This project used finite element (FE) analysis to model the residual stresses in weld overlays made of several alloys applied to waterwall tubes and to assess how thermal stress cycles during periodic sootblowing may affect crack growth in these components Background During the heating and cooling cycles that occur during the welding of boiler components, thermal strains in the metal and the base metal regions near the weld eventually produce a residual stress distribution in the product These stresses interact with the stresses that occur during boiler operation and may affect crack initiation and growth Objectives • To model the residual stresses resulting from GMAW welding of weld overlays in waterwall tubes • To assess the effect of thermal stress cycles caused by periodic sootblowing on crack growth in weld overlayed tubes Approach The project team developed a generalized plane strain FE model to simulate the weld overlay process as performed by gas metal arc welding (GMAW) on several alloys The team used Abaqus FE code to determine the thermal and mechanical stress conditions that would occur because of the periodic shedding of slag that results from sootblowing The team assessed the effects of these stresses on fatigue crack growth for the combinations of base and weld overlay metals previously modeled Results The modeling work presented in this report indicates that there are considerable residual tensile stresses in weld overlays in waterwalls prior to service Modeling of service conditions, including the thermal cycles due to slag buildup and removal by sootblowing, shows some reduction in the residual stresses However, after 3-6 cycles, little or no further reduction occurs; and considerable cyclic tensile stresses are experienced with stress ranges varying from 8,000-14,000 psi, depending on the weld overlay material In the temperature range most likely experienced by weld overlays, 1000-1050°F (538-566°C), Alloy 622 is predicted to be to 2.5 times more resistance to cracking than Alloy 309 This result agrees qualitatively with EPRI’s Laboratory Corrosion Fatigue Tests (EPRI reports 1009618 and 1012383) The model predicts that Alloy “Nimonic 86” should be about twice a resistant to cracking as Alloy 622 Unfortunately this alloy is not available as weld wire suitable for GMAW welding at this time v EPRI Perspective The foundation of this work was the analysis of the residual stress state after the welding process This evaluation is important for a successful study of the stress conditions during operation of a boiler In-service stresses such as the cyclic thermal stresses caused by sootblowing are critical because they can either assist or prevent the initiation and/or propagation of cracks under such mechanisms as stress-corrosion cracking or thermal fatigue Keywords Weld overlays Circumferential cracking Stainless steel Crack growth Sootblowing Waterwalls vi ABSTRACT The thermo mechanical modeling work presented in this report indicates that there are considerable residual tensile stresses in weld overlays or waterwalls prior to service Modeling of service conditions assuming thermal cycles due to slag build up and removal by sootblowing indicates that there will be some stress relief due to operating temperature, but that considerable tensile stresses remain The model predicts that thermal stress cycles caused by slagging and de-slagging are high enough to cause crack propagation in most commonly used weld overlay materials, especially if surface defects, possibly caused by corrosion, are present The model predicts that the thermo-mechanical crack resistance of Alloy 622 is to 2.5 times higher than that of Alloy 309 at 1000-1050°F (538-566°C) vii Figure 4-5 622 Inco Max Linearized Stress Distribution, Slag Reformed 4-6 Figure 4-6 309L SST Input Stress Amplitude Curve Figure 4-7 622 Inco Input Stress Amplitude Curve 4-7 Flaw Growth Analysis ASME Section XI, Article C-3000 provides the methodology for evaluation and describes the procedures to determine the flaw size at the end of the evaluation period Crack growth can be due to cyclic fatigue flaw growth, stress corrosion cracking (SCC) or a combination of both This study considered cyclic fatigue flaw growth as the condition in which the analysis was performed The fatigue flaw growth rate can be characterized in terms of the range of the applied stress intensity factor, Ki This characterization is in the form: da/dN = Co(ΔKi)n where n and Co are constants dependent on the material and environment conditions The fatigue crack growth behavior of austenitic stainless steels is affected by temperature, R ratio (kmin/kmax) and environments Reference fatigue crack growth behavior of cast and wrought austenitic stainless steels and their welds exposed to air environments are given by n = 3.3 and Co = C(S) where C is a scaling parameter to account for temperature S is a scaling parameter to account for R ratio and is given by S = 1.0 when R ≤0 S = 1.0 + 1.8R when < R ≤ 0.79 S = -43.35 + 57.97R when 0.79 < R < 1.0 The scaling constant Co produces fatigue crack growth rates in the units of inches/cycle when ΔK is in the units of ksi√in and is intended for use when data from actual product form is not available The flaw growth analysis assumed a uniform temperature equal to 900°F (482°C) for both 309L and Alloy 622 Inconel weld overlay applications Results and Discussion Figures 4-8 and 4-9 shows the accumulated incremental crack growth versus cycles for 309L SST and alloy 622 Inconel weld metal overlay applications Figure 4-10 shows the comparison of 309L SST versus Alloy 622 Inconel in relationship to the number of years for a flaw to grow with respect to thermal fatigue assuming a soot blowing cycle is times per day, hours per cycle The contours of normal stress in the near crack-tip region for the initial and final evaluation period crack advance positions are shown in Figures 4-11 through 4-14 4-8 Figure 4-8 309L SST/T11 Incremental Crack Growth Figure 4-9 622 Inco/T11 Incremental Crack Growth 4-9 Figure 4-10 309L SST/T11 versus 622 Inco/T11 Figure 4-11 309L SST/T11 Crack Growth Initial Advance Position 4-10 Figure 4-12 309L SST/T11 Crack Growth Final Advance Position Figure 4-13 622 Inco/T11 Crack Growth Initial Advance Position 4-11 Figure 4-14 622 Inco/T11 Crack Growth Final Advance Position 4-12 MODEL REFINEMENTS AND SENSITIVITY STUDIES Model Refinements The original model discussed in the previous chapters was simplified by assuming a constant, average temperature across the weld overlay of 900°F (482°C) This will make the results less conservative as the surface temperatures is up to 150°F (66°C) higher at the beginning of each cycle Thus the model was refined to simulate the actual temperature distribution across the weld overlay This refinement resulted in a reduction in the number of cycles needed for cracks to grow from to 75 mils as shown in the Table 5-1 Table 5-1 Predicted Crack Growth Time 5-75 Mils Max Heat Flux Max Surface Temp ΔT Btu/hr/ft2 °F °F Cycles Years Cycles Years 150,000 1050 160 7,342 6.7 17,199 15.7 309 622 Comparison with previous results indicates that the time for the crack to reach 75 mils is drastically reduced for Alloy 309, (6.7 years vs 10 years), but has remained almost the same for Alloy 622 Two other alloys were also evaluated under the same conditions: Alloy 52 (Ni 58, Cr 29, Fe 9%) and Nimonic 86 (Ni 65, Cr 25, Mo 10, Ce 0.03%) Results are given in the Table 5-2 Table 5-2 Predicted Crack Growth Time 5-75 Mils Max Heat Flux Max Surface Temp ΔT Btu/hr/ft2 °F °F Cycles Years Cycles Years 150,000 1050 160 15,480 14.1 28,710 26.2 Alloy 52 Nimonic 86 The modeling results indicate that Alloy 52 has about the same thermal fatigue resistance as Alloy 622, while Nimonic 86 should have a much higher resistance to cracking Therefore efforts are being made to test this alloy in EPRI’s corrosion fatigue test 5-1 Sensitivity Studies The effect of heat flux, ΔT, and maximum tube surface temperature on crack growth were investigated using the improved model For all cases, it was assumed that the heat flux in the slagged conditions was 70,000 Btu/hr/ft2 This gives a surface temperature of 887°F (475°C) Table 5-3 gives the surface temperature, ΔTs and crack growth rates when the heat flux ranges from 100,000-200,000 Btu/hr/ft2 for Alloy 309 Table 5-4 gives the same data for Alloy 622 Figure 5-1 gives the number of cycles needed for a crack to grow from mils to 75 mils in a function of surface temperature in the de-slagged condition The data indicate that 622 has a much greater resistance to thermal fatigue than Alloy 309 at all temperatures However, the difference between the two alloys decreases with increasing temperature Table 5-3 Effect of Heat Flux on Crack Growth for Alloy 309 Overlay Alloy Initial Flaw Depth Crack Growth Final Advance Position Overlay Surface Temperature DegF with Slag Layer, DegF 309L 0.005 0.075 887 Max Overlay Surface Temperature During Sootblowing/De-Slagging, DegF 946 Temperature Spike Delta T, DegF Number of Years of Cycles Service 59 11,427 10.4 100,000 996 109 9,196 8.4 125,000 1,047 160 7,342 6.7 150,000 1,098 211 5,592 5.1 175,000 1,149 262 4,385 4.0 200,000 Table 5-4 Effect of Heat Flux on Crack Growth for Alloy 622 Overlay Alloy 622 Inco Initial Flaw Depth 0.005 Crack Growth Final Advance Position 0.075 Overlay Surface Temperature with Slag Layer DegF 5-2 893 Reference Fireside Heat Flux Btu/Hr, Sq Ft Max Overlay Surface Temperature During Sootblowing De-Slagging, DegF 954 Temperature Spike Delta T, DegF 61 29,346 26.8 100,000 1005 112 21,281 19.4 125,000 1,056 163 15,883 14.5 150,000 1,107 214 11,421 10.4 175,000 1,158 265 8,078 7.4 200,000 5-3 Number of Years of Cycles Service Reference Fireside Heat Flux Btu/Hr, Sq Ft Figure 5-1 Effect of Tube Surface Temperature in the De-Slagged Condition on Crack Growth Rates of Alloys 309 and 622 Crack Growth Temperature Sensitivity 35000 No of Cycles 30000 Alloy 309L 25000 Alloy 622 20000 15000 10000 5000 900 950 1000 1050 1100 Overlay Surface Temperature, DegF 5-4 1150 1200 DISCUSSION AND CONCLUSION Discussion The modeling work presented in this report indicates that there are considerable residual tensile stresses in weld overlays in waterwalls prior to service Modeling of service conditions, assuming thermal cycles due to slag buildup and removal by sootblowing, shows some reduction in the residual stresses but after 3-6 cycles, little or no further reduction occurs, so that considerable cyclic tensile stresses are experienced with stress ranges varying from 8,000-14,000 psi, depending on the weld overlay material A model, based on state of the art fracture mechanics procedures, was therefore developed to determine if thermal fatigue cracks would propagate from pre-existing defects, caused by corrosion For calculation purposes a mil (0.125 mm) defect was assumed The number of thermal cycles needed to grow a crack to 75 mils (1.875 mm) was calculated for various thermal cycles, based on the difference between the temperature of the tube crown in the slagged and de-slagged condition The differences depend on the local heat flux (heat absorption) in the de-slagged condition, assuming the heat flux in the fully slagged condition is constant A heat flux of 70,000 Btu/hr/ft2 was assumed for the fully slagged condition This results in a temperature of about 890°F (477°C), if the water temperature inside the tube is 750°F (399°C) For the de-slagged condition heat fluxes varying from 100,000 to 200,000 Btu/hr/ft2 were assumed These heat fluxes depend on the cleanliness of the tubes and the flue gas temperature adjacent to the wall Lower heat fluxes result in lower crown temperatures and smaller temperature difference It was found that crack growth occurred under all conditions As expected, the rate of crack growth increased with increasing surface temperatures/temperature differences and heat flux (See Figure 5-1) For Alloy 309, the number of cycles required to cause cracks to grow from to 75 mils deceased from approximately 10,000 to less than 5,000 cycles when the base tube surface temperature increased from 950 to 1150°F (510 to 621°C) Similarly for Alloy 622, the number of cycles decreased from approximately 30,000 to less than 10,000 Thus Alloy 622 is significantly more resistant to crack growth than Alloy 309, although the differences decreases with increasing surface temperature and heat flux In the temperature range most likely experienced by weld overlays, 1000-1050°F (538-566°C), Alloy 622 is predicted to be to 2.5 times more resistance to cracking near Alloy 309 This is in qualitative agreement with field experience and EPRI’s Laboratory Corrosion Fatigue Tests (Ref 9, 10) It is finally of interest to note that the model predicts that Alloy “Nimonic 86” should be about twice a resistant to cracking as Alloy 622 Unfortunately this alloy is not available as weld wire suitable for GMAW welding at this time 6-1 Conclusions A thermo mechanical model was developed to predict crack growth in GMAW weld overlays on waterwalls The model predicts that Alloy 622 is to 2.5 times more resistant to cracking than Alloy 309 At this point, the model does not take the effect of corrosion on crack initiation and growth Correlation with service experience and results from laboratory corrosion fatigue testing is needed to include the effect of corrosion in the model 6-2 REFERENCES B.Taljat, T Zacharia, X.L Wang, J.R.Keiser, R.W Swindeman, Z Feng and M.J Jirinec, Numerical Analysis of Residual Stress Distribution in Tubes with Spiral Weld Cladding C.M Adams, Jr Heat Flow in Welding, Chapter 3, Welding Handbook, Volume One, Seventh Edition Chon L Tsai and Chin M Tso, The Ohio State University, Heat Flow in Fusion Welding 1995 ASME Boiler & Pressure Vessel Code, Part D – Properties Michaleris and A DeBiccari, Prediction of Welding Distortion J K Hong, C L Tsai and P Dong, Assessment of Numerical Procedures for Residual Stress Analysis of Multipass Welds Metals Handbook, Desk Edition, ASM Metals Park, OH 1985 John F Harvey, P.E Pressure Component Construction EPRI Report 1009618 April 2005, “Material Solutions for Waterwall Wastage.” 10 EPRI Report 1012383, December 2006, “Corrosion Fatigue Testing of GMAW and Laser Weld Overlays.” 7-1 Export Control Restrictions The Electric Power Research Institute (EPRI) Access to and use of EPRI Intellectual Property is granted with the specific understanding and requirement that responsibility for ensuring full compliance with all applicable U.S and foreign export laws and regulations is being undertaken by you and your company This includes an obligation to ensure that any individual receiving access hereunder who is not a U.S citizen or permanent U.S resident is permitted access under applicable U.S and foreign export laws and regulations In the event you are uncertain whether you or your company may lawfully obtain access to this EPRI Intellectual Property, you acknowledge that it is your obligation to consult with your company’s legal counsel to determine whether this access is lawful Although EPRI may make available on a 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askepri@epri.com • www.epri.com 1014248 ... Effect of Boiler Operating Practice on Circumferential Crack Growth in Weld Overlays Finite Element Modeling of Circumferential Cracking 1014248 Technical Update,... affect crack growth in these components Background During the heating and cooling cycles that occur during the welding of boiler components, thermal strains in the metal and the base metal regions... Practice on Circumferential Crack Growth in Weld Overlays Finite Element Modeling of Circumferential Cracking; EPRI, Palo Alto, CA: 2007 1014248 iii REPORT SUMMARY The gas metal arc welding (GMAW)