19 th International Symposium on Zirconium in the Nuclear Industry Full Scale Fracture Toughness Behavior of Zr-2.5Nb Pressure Tubes with High Hydrogen Concentrations and Different Hydride Morphologies May 20, 2019 Jun Cui and Gordon K. Shek, Kinectrics Inc, Toronto, Canada 1
OUTLINE • Introduction • Experimental Program • Results and Discussion • Conclusions • Acknowledgements 2
INTRODUCTION • Hydrogen Embrittlement in CANDU Pressure Tubes ❑ Cold-worked Zr-2.5Nb pressure tubes are used in CANDU reactors. ❑ Hydrogen ❑ Hydrides OD Radial Hoop Hoop Stress Stress Circumferential (or transverse) Circumferential hydrides Radial hydrides formed without applied stress formed at 350 MPa 3
INTRODUCTION • Hydrogen Embrittlement in CANDU Pressure Tubes ❑ The detrimental effects of reoriented hydrides for lowing ductility and fracture toughness of Zirconium alloys are known for years. ❑ Recently, there is a renewed interest on this issue because: ➢ As the pressure tubes age and approach their design end-of-life, the hydrogen concentration in the pressure tubes becomes significantly higher than before. ➢ For in-service evaluation for fracture initiation and leak-before-break, there is a need to determine the fracture toughness properties of the pressure tubes with high hydrogen concentrations and different hydride morphologies. ➢ Such information is essential to support continued reactor safe operation and reactor life extension. 4
INTRODUCTION • Hydrogen Embrittlement in CANDU Pressure Tubes ❑ An extensive experimental program has been initiated in the CANDU industry to provide information to support development of fracture toughness models. The test parameters include: ➢ Irradiated versus unirradiated materials ➢ Small CCT specimen versus full-scale burst test specimen ➢ High hydrogen concentration and different hydride morphologies ➢ Hydride reorientation cycles ➢ Test temperature 5
INTRODUCTION • Scope of This Paper ❑ This paper presents results obtained from rising pressure burst tests performed on sixteen full-scale burst test specimens over a range of temperatures to characterize the fracture toughness properties of hydrided, unirradiated Zr-2.5Nb pressure tube material in the lower shelf, transition and at the onset of the upper shelf fracture regimes. ❑ The test variables include: ➢ Material variability ➢ Hydrogen concentration and hydride morphology ➢ Test temperature 6
EXPERIMENTAL PROGRAM • Material ❑ Two unirradiated, cold-worked Zr-2.5Nb pressure tubes Y041 and C022 were selected for testing from a total of 10 candidate tubes. 7
EXPERIMENTAL PROGRAM • Specimen Axial through-wall notch Illustration of a burst test specimen prior to fatigue pre-cracking 8
EXPERIMENTAL PROGRAM • Hydriding ❑ For each tube, burst test specimens with four different hydrogen concentrations were prepared for testing: 6 ppm (as-fabricated condition), 60 ppm, 100 ppm and 130 ppm. ❑ For specimens that require hydriding, an electrolytic hydriding technique was used to deposit a hydride layer on both inner and outer surfaces of each specimen. 9
EXPERIMENTAL PROGRAM • Hydride Reorientation 450 170 160 400 Heatup: 1˚C/min; Cooldown: 0.7˚C/min. 150 350 140 302°C/20h 300°C/1h Applied Hoop Stress (MPa) 300 130 Temperature (°C) 120 250 110 200 100 150 90 80 100 70 50 60°C 60 0 50 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Time (h) Temperature Applied Hoop Stress Illustration of hydride reorientation cycle for 60 ppm specimens 10
EXPERIMENTAL PROGRAM • Fatigue Pre-Cracking ❑ Prior to burst test, the EDM notch was sharpened by pressure cycling in de-ionized (DI) water to form ~5 mm crack at both ends of the notch using a decreasing K procedure with a final K of ~ 15 MPa√m . • Burst Test ❑ Argon gas was used to perform rising pressure burst test. ❑ For testing at elevated temperatures, an internal cartridge heater was used to heat the specimen. ❑ The specimen was pressurized to failure while the crack extension was measured using the potential drop technique. ➢ This is referred to as “ Rising Pressure Burst Test ” . 11
EXPERIMENTAL PROGRAM • Characterization of Burst Test Fracture Toughness ❑ Two approaches are generally used to characterize fracture toughness of a burst test specimen. 1 st approach uses K c , the plane stress critical stress intensity factor ❑ at the onset of flaw instability. ➢ This is calculated using the burst pressure and the initial crack length. 2 nd approach uses J-R curve, the J-integral versus crack extension ❑ curve. 12
EXPERIMENTAL PROGRAM • Post-Test Examination ❑ Fracture surface was examined to measure crack length and examine the influence of hydrides on crack growth behavior. ❑ Metallographic samples in the vicinity of the fracture surface were prepared to examine the hydride morphology in the radial-transverse plane. ❑ Hydride morphology was characterized by a parameter referred to as “Hydride Continuity Coefficient”, or HCC, see illustration on next slide. ❑ HCC varies between 0 and 1. HCC provides a measure of the extent to which the hydrides are reoriented towards the radial direction of the pressure tube wall, with a higher HCC corresponding to more reoriented hydrides. 13
EXPERIMENTAL PROGRAM • HCC Characterization Radial Transverse 14
EXPERIMENTAL PROGRAM • Test Matrix 15
RESULTS AND DISCUSSION • Stage 1 Results: Hydride Morphology Y041, 100ppm Y041, 130ppm C022, 100ppm C022, 130ppm HCC=0.19 HCC=0.15 HCC=0.66 HCC=0.67 Radial Transverse 16
RESULTS AND DISCUSSION • Stage 1 Results: Crack Growth Behavior and Kc Comparison of J-R curves from Stage 1 burst test specimens 17
RESULTS AND DISCUSSION • Stage 1 Results: Summary ❑ Fracture Toughness Ranking: ➢ S1-1 (Y041, 100 ppm) is the lowest toughness specimen. ➢ This is the Stage 2 material condition. ➢ S1-4 (C022, 130 ppm) is the highest toughness specimen. ➢ This is the Stage 3 material condition. ➢ S1-3 (Y041, 130 ppm) is the second lowest toughness specimen. ➢ This is the Stage 4 material condition. 18
RESULTS AND DISCUSSION • Stage 1 Results: Fracture Surface 19
RESULTS AND DISCUSSION • Stage 1 Results: Fracture Surface 20
RESULTS AND DISCUSSION • Stage 2 Results: Crack Growth Behavior 700.0 600.0 S2-1 (HCC: 0.54), 200 C 500.0 J-Integral (kJ/m2) 400.0 300.0 S2-3 (HCC: 0.47), 150 C 200.0 100.0 S2-2 (HCC: 0.45), RT S1-1 (HCC: 0.66), 100 C 0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 Average Crack Extension (mm) Comparison of J-R curves from Stage 2 burst test specimens (Y041, 100 ppm) 21
RESULTS AND DISCUSSION • Stage 2 Results: Fracture Surface 22
RESULTS AND DISCUSSION • Stage 2 Results: Fracture Toughness Transition Behavior 23
RESULTS AND DISCUSSION • Stage 3 Results: Crack Growth Behavior 600.0 S3-2 (HCC: 0.28), 150 C 500.0 400.0 J-Integral (kJ/m2) S3-3 (HCC: 0.30), 100 C 300.0 S1-4 (HCC: 0.15), 100 C 200.0 S3-1 (HCC: 0.27), RT 100.0 0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 Average Crack Extension (mm) Comparison of J-R curves from Stage 3 burst test specimens (C022, 130 ppm) 24
RESULTS AND DISCUSSION • Stage 3 Results: Fracture Toughness Transition Behavior 25
RESULTS AND DISCUSSION • Stage 4 Results: Crack Growth Behavior 1400.0 1200.0 S4-1 (HCC: 0.50), 150 C 1000.0 J-Integral (kJ/m2) 800.0 600.0 400.0 200.0 S4-2 (HCC: 0.36), RT S1-3 (HCC: 0.67), 100 C 0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 Average Crack Extension (mm) Comparison of J-R curves from Stage 4 burst test specimens (Y041, 130 ppm) 26
RESULTS AND DISCUSSION • Stage 4 Results: Fracture Toughness Transition Behavior 27
RESULTS AND DISCUSSION • Stage 5 and Stage 6 Results: Crack Growth Behavior Comparison of J-R curves from Stage 5 (Y041, 6 ppm [H], 4.1 ppm [Cl]) and Stage 6 (C022, 6 ppm [H], 1.4 ppm [Cl]) burst test specimens 28
RESULTS AND DISCUSSION • Stage 5 and Stage 6 Results: Fracture Surface SEM micrograph showing fracture SEM micrograph showing fracture surface of S5-1, Y041, 4.1 ppm [Cl] surface of S6-1, C022, 1.4 ppm [Cl] 29
RESULTS AND DISCUSSION • Stage 7 (Y041, 60 ppm [H], 4.1 ppm [Cl]) and Stage 8 (C022, 60 ppm [H], 1.4 ppm [Cl]) Results: Hydride Morphology HCC=0.19 HCC=0.15 30
RESULTS AND DISCUSSION • Stage 7 and Stage 8 Results: Crack Growth Behavior Comparison of J-R curves from Stage 7 (Y041, 60 ppm [H], 4.1 ppm [Cl]) and Stage 8 (C022, 60 ppm, 1.4 ppm [Cl]) burst test specimens 31
RESULTS AND DISCUSSION • Effect of Material Variability ❑ Y041 vs C022 with as-fabricated [H]. ➢ S5-1 vs S6-1 ❑ Y041 vs C022 with 60 ppm [H]. ➢ S7-1 vs S8-1 ❑ Y041 vs C022 with 100 ppm [H]. ➢ This refers to burst tests on S1-1 (Y041) and S1-2 (C022). Both tests were performed at 100˚C. Under nominally identical hydride reorientation cycles, there was significantly more hydride reorientation in S1-1 (HCC: 0.66) than that in S1-2 (HCC: 0.19). The K c of S1-1 was lower than that of S1-2. 32
RESULTS AND DISCUSSION • Effect of Material Variability ❑ Y041 vs C022 with 130 ppm [H]. 33
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