Creep-fatigue of High Temperature Materials for VHTR: Effect of ...

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Creep-fatigue of High Temperature Materials for VHTR: Effect of Cyclic Loading and Environment Proceedings of ICAPP 2011 Nice, France, May 2-5, 2011 Paper 11284 C. Cabet CEA, DEN, DPC, SCCME, Laboratoire d'Etude de la Corrosion Non Aqueuse, F-91191 Gif-sur-Yvette, France Tel: +33 169 081 615, Fax: +33 169 081 586, Email: celine.cabet@cea.fr L. Carroll Idaho National Laboratory Idaho Falls, ID, USA R. Madland Colorado School of Mines Denver, CO, USA R. Wright Idaho National Laboratory Idaho Falls, ID, USA Abstract – Alloy 617 is the one of the leading candidate materials for Intermediate Heat Exchangers (IHX) of a Very High Temperature Reactor (VHTR). System start-ups and shut-downs as well as power transients will produce low cycle fatigue (LCF) loadings of components. Furthermore, the anticipated IHX operating temperature, up to 950°C, is in the creep regime so that creep-fatigue interaction, which can significantly increase the fatigue crack growth, may be one of the primary IHX damage modes. To address the needs for Alloy 617 codification and licensing, a significant creep-fatigue testing program is underway at Idaho National Laboratory. Strain controlled LCF tests including hold times up to 1800s at maximum tensile strain were conducted at total strain range of 0.3% and 0.6% in air at 950°C. Creep-fatigue testing was also performed in a simulated VHTR impure helium coolant for selected experimental conditions. The creep-fatigue tests resulted in failure times up to 1000 hrs. Fatigue resistance was significantly decreased when a hold time was added at peak stress and when the total strain was increased. The fracture mode also changed from transgranular to intergranular with introduction of a tensile hold. Changes in the microstructure were methodically characterized. A combined effect of temperature, cyclic and static loading and environment was evidenced in the targeted operating conditions of the IHX. This paper reviews the data previously published by Carroll and coworkers in references 10 and 11 focusing on the role of inelastic strain accumulation and of oxidation in the initiation and propagation of surface fatigue cracks. I. INTRODUCTION Alloy 617 is the leading candidate material for Intermediate Heat eXchangers (IHX) of helium-cooled Very High Temperature Reactor (VHTR) systems. Conceptual design requires an outlet temperature of greater than 850°C to efficiently co-generate hydrogen and electricity, with a maximum expected temperature of 950°C 1 . The IHX will operate at the reactor outlet temperature of up to 950°C. Reactor start-ups and shut-downs as well as power transients will produce low cycle fatigue (LCF) loadings of components. Furthermore, the anticipated IHX operating temperature is in the range where creep deformation occurs so that interaction between a growing fatigue crack and the bulk creep damage, which can significantly increase the 312 fatigue crack growth rate, may be one of the primary IHX damage modes. However, in the draft code case that was developed in the 1980s to have Alloy 617 approved in ASME Code section III for nuclear use at high temperature 2 , creep-fatigue was not sufficiently addressed. This calls for a more complete database for creep-fatigue behavior of Alloy 617; for a better understanding of the coupled influence of aging, fatigue, creep and corrosion; and for a better life prediction of components under creepfatigue deformation. � A significant testing program is underway at Idaho National Laboratory to address the needs for codification and licensing. To reproduce the expected deformation in a laboratory setting, creep-fatigue testing introduces a hold time in a strain-controlled fatigue cycle. Previous work on Alloy 617 has suggested that a hold time during the tensile
portion of the fatigue cycle is more damaging 3-5 than compressive holds or both a tensile and compressive hold. Therefore holds at peak tensile strain were investigated. The helium coolant in the primary circuit of a VHTR system is expected to contain low levels of impurities 6 such as H2, H2O, CO, CO2, CH4, etc, that can react toward metallic materials at high temperature. As far as corrosion of Alloy 617 is concerned, the optimum coolant chemistry will enable the alloy to be oxidized in service for the formation of a surface, continuous and dense, chromia scale which would protect the bulk alloy and has little influence on the mechanical properties 7 . This corrosion behavior is similar to oxidation in air at the same temperature 8,9 . Hence, experiments were performed in air as well as in controlled VHTR-simulated helium to account for any possible environmental effect. To enhance the understanding of creep-fatigue deformation mechanisms at 950°C, the fatigue and creep-fatigue behavior of Alloy 617 is presented with systematic microstructure characterization of the alloy surface, the specimen bulk and the vicinity of the cracks. This paper reviews the creep-fatigue data presented in two previous references by Carroll et al. 10, 11 with an emphasis on the role of inelastic strain accumulation and the effect of oxidation in the initiation and propagation of surface fatigue cracks. II. EXPERIMENTAL II.A. Material Cylindrical creep-fatigue specimens, 7.5 mm diameter in the reduced section and a gage length of 12 mm, were machined from Alloy 617 annealed plate; the long axis of the specimen aligned with the rolling direction. The composition of the investigated heat of Alloy 617 is given in Table I. TABLE I Alloy 617 chemical composition in wt.%. Ni C Cr Co Mo Fe Al Ti Si Mn Cu bal 0.08 21.9 11.4 9.3 1.7 1.0 0.3 0.1 0.1 0.04 II.B. Apparatus Cyclic testing was conducted on servo-hydraulic test machines in axial strain-control mode in accordance with the ASTM Standard E606-04 12 . Radio-frequency induction heating was used to heat the specimens. Temperature control was achieved using a combination of spot-welded thermocouples on a shoulder of the specimen and a thermocouple loop at the center of the gage section. The temperature gradient was measured with a specimen 313 Proceedings of ICAPP 2011 Nice, France, May 2-5, 2011 Paper 11284 instrumented with spot-welded thermocouples along the gage length and was found to deviate by less than 1% within the gage section. For the cyclic tests in controlled chemistry helium, a servo-hydraulic test frame equipped with a gas-tight chamber was used. A gas system was developed to feed VHTR simulated coolant to the chamber. It comprises a manifold containing bottles of helium with controlled partial pressures of CO, CH4, and H2; mass flow controller to regulate the helium flow; and pressure controllers. Small quantities of H2O are added to control the partial pressure of water vapor in the helium flow (P(H2O) as low as 5μbar). A gas chromatograph and two solid-state hygrometers record the gas chemistry at the inlet and outlet of the chamber. Feedback is employed on the inlet hygrometry. II.C. Testing methods Fully reversed strain controlled low cycle fatigue and creep-fatigue testing was completed on Alloy 617 at a total strain range of 0.3% and 0.6% in air and a simulated VHTR helium at 950°C. A triangular waveform and a ramp rate of 10 -3 /s were used for low cycle fatigue testing. Creep-fatigue testing followed a trapezoidal waveform with a tensile hold imposed at the maximum tensile strain. Tensile holds of 180, 600, and 1800 sec were investigated. The number of cycles to failure, Nf, is defined as the point at which the ratio of the peak tensile stress to the peak compressive stress initially deviated from its level slope and the point at which the ratio was 80% of the value at deviation 12 . Test completion was prior to actual specimen separation. For testing in helium, a gas mixture was selected to roughly simulate the controlled VHTR coolant. It contained 50 μbar CO and 350 μbar H2 in addition to approximately 8 μbar H2O at 1.1 bar total helium pressure to be oxidizing with respect to Alloy 617. II.D. Specimen characterization After creep-fatigue deformation, the gage sections of the deformed specimens were cut along the stress axis through the largest surface crack. The specimen was then coated with a thin layer of gold followed by nickel plating to ensure retention of the surface oxide. The coated specimens were mounted in phenolic, polished to a mirror finish, and then etched with a 2% bromine solution in methanol. Metallurgical evaluation was conducted with optical microscopy and Field Emission Gun-Scanning Electron Microscopy (FEG-SEM) equipped with an EDX spectrometer.
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