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Prediction and Monitoring Systems of Creep-Fracture Behavior of 9Cr- 1Mo Steels for Reactor Pressure Vessels

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Prediction and Monitoring Systems of Creep-Fracture Behavior of 9Cr- 1Mo Steels for Reactor Pressure Vessels Project No. 09-835 Prediction and Monitoring Systems of Creep-Fracture Behavior of 9Cr- 1Mo Steels for Reactor Pressure Vessels Reactor Concepts RD&D Dr. Gabriel Potirniche University of Idaho Sue Lesica, Federal POC Richard Wright, Technical POC PREDICTION AND MONITORING SYSTEMS OF CREEP- FRACTURE BEHAVIOR OF 9Cr-1Mo STEELS FOR REACTOR PRESSURE VESSELS NEUP 2009 Project 09-835 (09-458) Final Report PI: Gabriel Potirniche [email protected]; tel.: 208-885-4049 Co-PIs: Dr. Indrajit Charit, Dr. Karl Rink, Dr. Fred Barlow Mechanical Engineering Department, University of Idaho, October 31, 2013 1 ABSTRACT Modified 9Cr-1Mo (Grade 91) steel is currently considered as a candidate material for reactor pressure vessels (RPVs) and reactor internals for the Very High Temperature Reactor (VHTR) and in fossil-fuel fired power plants at higher temperatures and stresses. The goal of this project was to provide a fundamental understanding of the creep-fracture behavior of modified 9Cr-1Mo steel welds for RPVs through modeling and experimentation, and to recommend a design for a monitoring system for the structural health of RPVs. Several tasks were originally proposed and fully accomplished. A description of the project tasks, the main findings and conclusions are given next. A list of the publications and presentations resulted from this research are listed in the Appendix at the end of this report. Task 1. Creep-fracture testing of base material and welds of Grade 91 steel specimens Research activities This task involed the manufacture of the base material testing specimens, the welded specimens, metallurgical characterization by means of optical, transmission and scanning electron microscopy, energy dispersive spectroscopy measurements, hardness measurements, characterization of the residual stress fields after welding process, and creep testing of base material specimens and of welded specimens. The tensile creep behavior of Grade 91 steel was studied in the temperature range of 600oC to 750oC and stresses between 35 MPa and 350 MPa. Heat treatment of Grade 91 steel was studied by normalizing and tempering the steel at various temperatures and times. Thermo-CalcTM calculations were used to predict the precipitate stability and their evolution, and construct carbon isopleths of Grade 91 steel. Residual stress distribution across gas tungsten arc welds (GTAW) in Grade 91 steel was measured by the time-of-flight neutron diffraction using the Spectrometer for Materials Research at Temperature and Stress (SMARTS) diffractometer at Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM, USA. Findings and conclusions Analysis of creep results yielded power stress exponents of n ~9-11 in the higher stress regime and n ~1 in the lower stress regime. The creep behavior of Grade 91 steel was described by the modified Bird-Mukherjee-Dorn relation. The rate-controlling creep deformation mechanism in the high stress regime was identified as the edge dislocation climb with a stress exponent of n = 5. On the other hand, the deformation mechanism in the Newtonian viscous creep regime (n = 1) was identified as the Nabarro-Herring creep. Creep rupture data were analyzed in terms of Monkman-Grant relation and Larson-Miller parameter. Creep damage tolerance factor and stress exponent were used to identify the cause of creep damage. The fracture surface morphology of the ruptured specimens was studied by scanning electron microscopy to elucidate the failure mechanisms. Fracture mechanism map for Grade 91 steel was developed based on the available material parameters and experimental observations. The microstructural evolution of heat treated steel was correlated with the differential scanning calorimetric study. The combination of microstructural studies with optical microscopy, scanning 2 and transmission electron microscopy, microhardness profiles, and calorimetric plots helped in the understanding of the evolution of microstructure and precipitates in Grade 91 steel. The residual stresses were determined at the mid-thickness of the Grade 91 steel plate, 4.35 mm and 2.35 mm below the surface of the as-welded and post-weld heat treated plate. The residual stresses of the as-welded plate were compared with the post-weld heat treated plate. The post- weld heat treatment significantly reduced the residual stress in the base metal, heat affected zone, and the weld zone. Vickers microhardness profiles of the as-welded, and post-weld heat treated specimens were also determined and correlated with the observed residual stress profile and microstructure. Finally, an extensive analysis of the creep rupture properties of the welded Grade 91 alloy (welded by the Gas Tungsten Arc Welding) alloy was carried out and presented in detail in this report. Task 2: Testing of damage levels in aged pressure vessels with the leak rate method Research activities The objective of this task was to characterize the creep damage in welded pressure vessels by identifying the Rb deposition resulting from Kr-85 decay at damage sites and measure leak rates of Kr-85 from. The pressure vessels were manufactured by two welding methods, i.e. inertia welding and TIG welding. The main outcome of this task was to compare the creep damage reliability of the two welding processes. Small Grade 91 pressure vessels were manufactured by the TIG and inertia welding techniques. The two halves of a cylindrical pressure vessel were machined out of a round bar of Grade 91 steel. Internal volumes in the two halves were formed by a simple boring process. Each vessel featured a specially-designed fill port through which gases could be introduced into the pressure vessels and then sealed using a ball weld process. The internal geometry of the inertia- welded units differed from that of the TIG welded specimens due to the nature of the weld process. The TIG welded parts were bored to an internal diameter and fillet welds preceded by the proper preheat treatment followed by adherence to the specified interpass and cooling temperatures. Pressure vessels were first filled with a mixture of the Kr-85 and air (0.01% Kr-85, molar basis) at very low pressures (2.0 mm Hg) followed by a high pressure (approximately 6000 to 7500 psi) mixture of argon and helium (80% Ar / 20%He, molar basis). The weight of each pressure vessel was recorded after filling and an initial reading of the Kr-85 content was recorded. In this manner, leaks from the pressure vessel may be detected by mass loss (a gross or large leak check) or by reduction in Kr-85 greater than that expected by inherent radioactive decay. Leak detection by monitoring the reduction in Kr-85 content through gamma ray counting is considered the most sensitive leak detection technique available. Hardness and microstructure were measured across the inertia weld zones of two specimens. Hardness values were measured using a Vickers indenter with a 500 gram load. Vickers Hardness values, HV, are noted in cross section views of the weld zones in the locations where they were measured. Microstructure photos were taken at various noted locations across the weld zones. 3 Findings and conclusions Gaseous Kr-85 decays to form solid rubidium (Rb) as a decomposition product. From the EDS analysis, it was indeed found that the Kr-85 decays to Rb. Rb presence on the interior surface of the pressure vessels indicated the extent of gas penetration within the pressure vessels walls, and the possible evolution of damage (cracks, voids, porosity, etc.) within the parent materials. EDS study was done on a control sample that was not subjected to radioactive Kr-85, and on samples that were subjected to radioactive Kr-85. In samples subjected to radioactive Kr-85, non-radioactive Rb was detected in cracks. Rb was detected mainly in cracks and voids on the inner face of the pressure vessels. However, Rb was not detected in the cracks of the control sample that was not subjected to radioactive Kr-85. The inertia welded specimens were found to be of better quality with no large crack present, and no significant localization of Rb at damage sites. Kr-85 leaked out of the pressure vessels as a result of creep damage induced in the welds of the pressure vessels. Radiation readings of the TIG and inertia-welded pressure vessels fabricated were taken periodically. Results indicated that the reduction in gamma radiation from the pressure vessels manufactured by inertia welding over time agreed with the expected decay of Kr-85 over this time period. Thus, the leak rates from inertia welded pressure vessels appear to be in the range 1x10-10 atm cc/s and perhaps even lower. This indicates a good quality of the inertia welded in regard to the leak rates as a result of creep loading. The leak rates from the pressure vessels manufactured by TIG welding indicated leak rates slightly greater than those recorded from the inertia welded pressure vessels. To measure such extremely fine leak rates, a novel calibration methodology was developed to better understand the limit of detectibility of gamma radiation using our experimental instrumentation for our geometries and materials. The activity of oil saturated with Kr-85 was first determined using the scintillation crystal detection equipment. The activity of the oil was determined on a mass basis in units of microcuries/gm of (vacuum pump) oil. Then known masses of the oil were carefully introduced in empty TIG and inertia welded specimens so that the attenuation of the gamma ray signal due to the wall thickness of the different test vessels could be determined. As a result, the quantity of Kr-85 in a vessel can be known very accurately - much more accuratley than using other calibration techniqies. From the findings of this task, it was concluded that inertia welding is a much better welding technique compared to TIG welding for components that are intended to withstand creep conditions.
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