Simulated High-temperature Gas-cooled Reactor Research Articles

Long-Term Creep Properties of Hastelloy XR in Simulated High-Temperature Gas-Cooled Reactor Helium

Creep tests of Hastelloy XR (a modified version of the conventional Hastelloy X) were carried out in simulated high-temperature gas-cooled reactor (HTGR) helium at 800, 900 and 1,000°C. The test results up to about 50,000 h showed no significant degradation in creep properties. The creep-rupture strength obtained through long-term tests was above the level corresponding to the design allowable creep-rupture stress of the High-Temperature Engineering Test Reactor. The values of the stress exponent were 4.5 to 5.7 when the stress dependence of the steady-state creep rate was expressed in terms of the Norton equation. It is judged that dominant creep process is dislocation creep. Rupture lives could be estimated with sufficient accuracy using Larson- Miller parameter. Carburization during creep in simulated HTGR helium did not degrade creep properties of Hastelloy XR. Internally formed cracks were initiated at sites of precipitates at grain boundaries, growing nearly perpendicular to the stress axis. Two phases, Cr-rich carbide and Mo-rich carbide, co-existed in specimens after long-term creep tests.

Long-Term Creep Properties of Hastelloy XR in Simulated High-Temperature Gas-Cooled Reactor Helium.

Creep tests of Hastelloy XR (a modified version of the conventional Hastelloy X) were carried out in simulated high-temperature gas-cooled reactor (HTGR) helium at 800, 900 and 1,000°C. The test results up to about 50,000 h showed no significant degradation in creep properties. The creep-rupture strength obtained through long-term tests was above the level corresponding to the design allowable creep-rupture stress of the High-Temperature Engineering Test Reactor. The values of the stress exponent were 4.5 to 5.7 when the stress dependence of the steady-state creep rate was expressed in terms of the Norton equation. It is judged that dominant creep process is dislocation creep. Rupture lives could be estimated with sufficient accuracy using Larson- Miller parameter. Carburization during creep in simulated HTGR helium did not degrade creep properties of Hastelloy XR. Internally formed cracks were initiated at sites of precipitates at grain boundaries, growing nearly perpendicular to the stress axis. Two pha...

高温ガス炉近似ヘリウム雰囲気におけるハステロイＸＲのクリープおよび破断挙動

Creep and rupture tests were conducted on Hastelloy XR (a modified version of the conventional Hastelloy X) at 1, 073, 1, 173 and 1, 273K in simulated high-temperature gas-cooled reactor helium. The test section of creep testing machine was designed and constructed with special care for impurity chemistry in helium. A device for counting Moire patterns was developed for contamination-free creep strain measurement. The test results up to 10, 000h showed no significant degradation in creep properties. Comparing the data on the stresses to rupture, to cause 1% total strain and to the onset of tertiary creep, the stress to cause 1% total strain is judged to be the dominant factor for the allowable stress (St) in the ASME Code. Carbon analysis for ruptured specimen showed that caburization was limited to the region near the ruptured portion and slight carbon intrusion occurred only in the early stage of exposure. Surface cracks were not responsible for creep-rupture, because they were blunted at the depth of approximately 100μm from the specimen surface at 1, 173K and 200μm at 1, 273K. Internally formed cracks were initiated at the sites of carbide precipitation at grain boundary, growing nearly perpendicular to the stress axis. Recrystallization observed was localized at 1, 173K but became pronounced at 1, 273K. Creep deformation enhanced coasening of the grain boundary carbide under tensile stress.

Strain-time effects in low-cycle fatigue of nickel-base heat-resistant alloys at high temperature

A series of strain controlled low-cycle fatigue tests in the simulated high-temperature gas-cooled reactor (HTGR) helium environment were conducted at 900°C on Hastelloy X and its modified version, Hastelloy XR. In those tests the effects of strain rate and hold time on high-temperature low-cycle fatigue behavior were investigated. Decreasing the strain rate led to notable reductions in the fatigue life. In the tests with the trapezoidal strain waveform, the fatigue life was found to be reduced most effectively in tensile hold-time experiments. The tendency was interpreted through the feature of the crack morphology.

The Effect of Gaseous Environment on the Low Cycle Fatigue Behavior of Ni-Cr-W Alloys for HTGR

Low cycle fatigue behavior of two Ni-Cr-W alloys (KSN and 113MA) was investigated at 1 273K in the simulated High Temperature Gas-cooled Reactor (HTGR) helium, He-2. Both alloys showed little cyclic hardening in fatigue tests and failed with a completely intergranular manner. Although the fatigue life (Nf) of 113MA was a little longer than that of KSN, observed intergranular fractures indicated that the fatigue properties of both alloys were severely influenced by impurities in He-2.As well as in He-2, fatigue tests of KSN and 113MA were carried out in pure He, air and vacuum. The results clearly showed that, regardless of the concentration of reactive gases, both alloys failed intergranularly with short Nfs in gas environments, as compared with the transgranular fracture with long Nf in vacuum. A minor effect of impurity contents was shown as an increase in Nf when substituting air by He-2 and then by pure He. It is considered from these behaviors that the fatigue fractures were affected by residual oxygen gas in the environments penetrating into the specimens along the grain boundaries.

Effects of thermal aging on the low cycle fatigue behavior of Ni-15%Cr-25%W alloy in the simulated HTGR helium.

The effects of thermal aging on the low cycle fatigue behavior of Ni-Cr-W alloys were investigated on KSN alloy (Ni-15wt%Cr-25wt%W) at 1 273K in the simulated helium (He-2) for High Temperature Gas-cooled Reactor (HTGR). Fully reversed strain control tests were made at a strain rate of 1×10-3 s-1 with holdtimes of 0, 6 and 60min. The thermal aging at 1 273K for 1 000hr initiated the precipitation of α2 phase (bcc-W) in the matrix and on grain boundaries of KSN.Although a degradation of fatigue lives was observed with increase of the period of holdtime for both solution annealed and aged KSN (KSN-SA and-AGE), the value of fatigue life was significantly longer for KSN-AGE than for KSN-SA. The morphology of intergranular fracture of two materials indicated that the existence of α2 particles at the grain boundaries increased both cycles to initiation and those to propagation of fatigue crack. The role of α2 precipitates was to reduce the weakening of grain boundaries and the oxidation rate by changing the corrosion products at a crack tip from Cr-oxides for KSN-SA to Ti/Al-oxides for KSN-AGE.

Low-Cycle Fatigue of Heat-Resistant Alloys in High-Temperature Gas-Cooled Reactor Helium

Strain controlled low-cycle fatigue tests were conducted on four nickel-base heat-resistant alloys at 900/sup 0/C in simulated high-temperature gas-cooled reactor (HTGR) environments and high vacuums of about 10/sup -6/ Pa. The observed behaviors of the materials were different and divided into two groups when tests were made in simulated HTGR helium, while all materials behaved similarly in vacuums. The materials that have relatively high ductility and compatibility with impure helium at test temperature showed considerable resistance to the fatigue damage in impure helium. On the other hand, the alloys qualified with their high creep strength were seen to suffer from the adverse effects of impure helium and the trend of intergranular cracking as well. The results were analyzed in terms of their susceptibility to the environmentenhanced fatigue damage by examining the ratios of the performance in impure helium to in vacuum. The materials that showed rather unsatisfactory resistance were considered to be characterized by their limited ductility partly due to their coarse grain structure and susceptibility to intergranular oxidation. Moderate carburization was commonly noted in all materials, particularly at the cracked portions, indicating that carbon intrusion had occurred during the crack growth stage.

Authors

Authors

Effect of Carburizing Service Environments on the Mechanical Properties of High-Temperature Alloys

Carburization of high-temperature alloys has been frequently observed during exposure to dry high-temperature gas-cooled reactor (HTGR) helium compositions. Therefore, the influence of carburization on mechanical properties of alloys that may be used for HTGR high-temperature components has been studied.In creep rupture tests on high-temperature alloys for up to 20 000 h, the data in air and in various simulated HTGR heliums lie in the same scatterband irrespective of carburization that has been observed in the contaminated helium atmospheres. The dependence of room temperature tensile properties and the impact strength in the 20 to 800 °C range on the carburization level has been measured so that the maximum carbon level for a given room temperature ductility and impact strength could be specified. The results showed that the minimum room temperature elongation fell to below 5% when the carbon content exceeded 0.5 wt% for Incoloy-800H and 0.2 wt% for lnconel-617. At these carbon levels, the alloys have impact strengths (ISO V-notch specimens) of ∼50 J or above at temperatures in the 25 to 800°C range.

Creep and Rupture Behavior of a Special Grade Hastelloy-X in Simulated HTGR Helium

Creep and rupture tests were conducted for Hastelloy-XR (a modified version of the conventional Hastelloy alloy X) at 800, 900, and 1000/sup 0/C in simulated high-temperature gas-cooled reactor helium. Creep testing machines with special control of helium chemistry were used. As a result, the scatter of creep-rupture data could be reduced, and the variability of creep-rupture behavior due to manufacturing history could be resolved. Results of metallography and carbon analysis of ruptured specimens showed that the material improved resistance to corrosion in the helium environment, and carbon intrusion during the steady-state creep stage was suppressed to a negligible level. Under refined test conditions combined with the quality controlled material, it was demonstrated that there was little significant difference between helium and air in the creep-rupture results obtained at 800 to 1000/sup 0/C up to about 10/sup 4/h. The importance of maintaining the protective function of the surface oxide film of alloys was stressed in securing reproducibility and predictability of long-time creep performance.

The Development of High-Strength Alloys Resistant to Corrosion in Impure Helium

The three completed stages of an alloy development program aimed at defining a nickel chromiumbased alloy resistant to carburization in simulated high-temperature gas-cooled reactor (HTGR) helium with high-temperature strength similar to alloy Inconel-617 show that alloys based on nickel and 10% chromium with 10 to 16% tungsten and 1.1 to 3.2% titanium exhibit good corrosion resistance in dry simulated HTGR helium and have short-time creep properties similar to Inconel-617. The alloys are readily hot and cold workable and show potentially good structural stability.

Oxygen potentials in the simulated high-temperature gas-cooled reactor helium gases

It is observed that the chemical reactions which occur among the impuritis in the simulated High-Temperature Gas-cooled Reactor (HTGR) helium gases, depend on temperature, gas composition, and gas flow rate. The direct measurement of the oxygen pressures in the gases using solid-oxide electrolytes has given that the oxygen pressures at high temperatures agree well with the equilibrium oxygen pressures which are calculated based on the chemical reactions. The results indicate that the oxidation potentials should be evaluated from the equilibrium oxygen pressures.

Carburization and decarburization of superalloys in the simulated htgr helium

Carburization and decarburization of superalloys due to the impurities in the High Temperature Gas-cooled Reactor (HTGR) helium influences the mechanical properties of the superalloys. The objective of this study is to investigate the carburization and decarburization behavior thermodynamically. The four commercial superalloys were exposed to the three simulated HTGR helium gases and the change in carbon concentration was examined. The following results were obtained: 1. (1) The carburization and decarburization depend strongly on the superalloys and the helium gases. 2. (2) The extent of the carburization or decarburization exhibits a linear relation against its thermodynamic driving force. The results make it possible to predict the carburization and decarburization in the HTGR helium environment.