The IGSCC, sensitization, and microstructure study of Alloys 600 and 690

Both Alloys 600 and 690 were studied to understand the effect of heat treatment on the sensitization and SCC behavior of these alloys. The microstructural evolution and chromium depletion near the grain boundaries were carefully studied using analytical electron microscopy. The majority of the precipitates formed in Alloy 600 was found to be M₇C₃ with a hexagonal structure (a₀ = 1.398 nm, c₀ = 0.45 nm); whereas the carbides found in Alloy 690 were identified as M₂₃C₆ with an fcc structure (a₀ = 1.06 nm). Modified Huey test performed in boiling 40% HNO₃ was used to study the effect of heat treatment and degree of sensitization. Constant load tests and constant extension rate tests were performed in the solution containing sodium thiosulfate to study the SCC resistance of these alloys. The results of the constant load tests for Alloy 600 indicated that the susceptibility to SCC is sensitive to the chromium depletion depth at grain boundary, and the minimum value to prevent SCC failure is approximately 8 wt%. No SCC was observed for Alloy 690 tested using constand load and CERT in the same environments. All tests showed that Alloy 690 has a far better resistance to intergranular attack and SCC than Alloy 600, which is believed due to its high chromium content. It is therefore anticipated that Alloy 690 now a better substitute to Alloy 600 as a steam generator tubing material for pressurized water reactor will also offer a superior corrosion resistance when “sensitized” and in particular if exposed to sulfur containing media such as thiosulfate solutions.     

The effect of grain boundary engineering on the oxidation behavior of INCOLOY alloy 800H in supercritical water

Grain boundary engineering (GBE) was applied to INCOLOY alloy 800H by means of thermomechanical processing. The oxidation behavior of GBE-treated alloy 800H exposed in supercritical water (SCW) with 25 ppb dissolved oxygen at 500 °C and 25 MPa was significantly improved as compared to 800H in the annealed condition. Gravimetry, optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) were employed in this study to analyze the oxidation behavior of control (annealed) and GBE-treated samples. GBE improves the protective oxidation behavior by enhancing spallation resistance and reducing oxidation rate. Spallation resistance correlates with a reduction in texture of the oxide layers.     

Appling grain boundary engineering to Alloy 690 tube for enhancing intergranular corrosion resistance

The feasibility of applying the grain boundary engineering (GBE) processing to Alloy 690 tube manufacturing for improving the intergranular corrosion resistance was studied. Through small amount of deformation by cold drawing using a draw-bench on a production line and subsequent short time annealing at high temperature, the proportion of low Σ coincidence site lattice (CSL) grain boundaries of the Alloy 690 tube can be enhanced to about 75% which mainly were of Σ3ⁿ (n = 1, 2, 3, …) type. In this case, the grain boundary network (GBN) was featured by the formation of highly twinned large size grain-clusters produced by multiple twinning during recrystallization. All of the grains inside this kind of cluster had Σ3ⁿ mutual misorientations, and hence all the boundaries inside the cluster were of Σ3ⁿ type and formed many interconnected Σ3ⁿ type triple junctions. The weight losses due to grain dropping during intergranular corrosion for the samples with the modified GBN were much less than that with conventional microstructure. Based on the characterization by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) technique, it was shown that the highly twinned large size grain-cluster microstructure played a key role in enhancing the intergranular corrosion resistance: (1) the large grain-cluster can arrest the penetration of intergranular corrosion; (2) the large grain-cluster can protect the underlying microstructure.     

The dependence of carbide morphology on grain boundary character in the highly twinned Alloy 690

The dependence of morphology of grain boundary carbides on grain boundary character in Alloy 690 (Ni-30Cr-10Fe, mass fraction, %) with high fraction of low Σ coincidence site lattice (CSL) grain boundaries was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Some of the surface grains were removed by means of deep etching. It was observed that carbides grow dendritically at grain boundaries. The carbide bars observed near incoherent twin boundaries and twin related Σ9 grain boundaries are actually secondary dendrites of the carbides on these boundaries. Higher order dendrites could be observed on random grain boundaries, however, no bar-like dendrites were observed near Σ27 grain boundaries and random grain boundaries. The morphology difference of carbides precipitated at grain boundaries with different characters is discussed based on the experimental results in this paper.     

A model for the normal stress dependence of intergranular cracking of irradiated 316L stainless steel in supercritical water

The intergranular cracking behavior of irradiated 316L stainless steel in supercritical water (SCW) was found to be strongly dependent on the local grain boundary normal stress. Tensile specimens of 7 dpa proton irradiated 316L stainless steel were strained in 400 °C SCW to 2%, 5%, and 10% macroscopic plastic strain. The cracking behavior was characterized according to the distributions of cracked grain boundary plane surface trace inclinations to the tensile axis and the Schmid factors of the grains adjacent to the cracked boundaries. Cracks occurred preferentially along grain boundaries with trace inclinations perpendicular to the tensile axis and adjacent to grains with low Schmid factors. There was also a slight increase in cracking propensity for grain boundaries adjacent to grains with high Schmid factor mismatch. The normal stress dependence of intergranular cracking was confirmed by developing and applying the Schmid-Modified Grain Boundary Stress model which characterizes the normal stress acting on a grain boundary as a function of the inclination of the grain boundary plane to the tensile axis and the flow stress of the grain, as estimated from its Schmid factor. By applying this model, the Schmid factor dependence of intergranular cracking could be predicted from the distributions of cracked grain boundary trace inclinations to the tensile axis.     

Investigation on the formation of serrated grain boundaries with grain boundary characteristics in an AISI 316 stainless steel

The formation of serrated grain boundaries (GBs) depending on the GB characteristics has been investigated by using an electron backscattered diffraction (EBSD) technique and a transmission electron microscopy (TEM) in an AISI 316 stainless steel. It was observed that at the early stage of aging treatment, the GB morphology was changed from flat to wavy at random GBs without any indication of M₂₃C₆ carbide formation, and no GB serration at special GBs (lower than Σ29) was found. The comparison study on the misorientation angle between two neighboring grains indicated that the occurrence of GB serration at random GBs is attributed to the reduction of the total GB energy. Random GBs with high energy tend to be serrated, resulting in the formation of two segments with lower energies. On the other hands, the special GBs may be less likely to form serrated GBs due to their lower GB energy.     

Investigation of the corrosion behavior of 304L and 316L stainless steels at high-temperature borated and lithiated water

The effects of temperature and dissolved oxygen on the electrochemical behavior and the oxide film formation of grades 304L and 316L stainless steels at high-temperature borated and lithiated water were investigated by means of potentiodynamic polarization, scanning electron microscopy and X-ray photoelectron spectroscopy. The results revealed that increasing the solution temperature degrades the passivity of the oxide films formed on both grades of steel and shifts their corrosion potential toward more negative potentials. The oxide films formed on the steel samples immersed into the solution containing 20 ppb dissolved oxygen (DO) showed a duplex structure, in which the inner layer was mostly a composition of Cr oxides and the outer layer mainly was a Fe oxide and Ni–Fe spinel. Only a single layer of Cr-rich oxide was observed in the oxide films formed in the solutions with the DO concentrations higher than 20 ppb. Higher amount of Cr in the oxide films formed on the type 316L compared to 304L improves the passivity of the oxide film of this grade of steel and results in a wider passive region in its potentiodynamic polarization curves.     

The effect of grain boundary misorientation on the intergranular M23C6 carbide precipitation in thermally treated Alloy 690

The precipitation characteristics of chromium carbides on various types of grain boundaries in Alloy 690 thermally treated at 720 °C for 10 h were studied through transmission electron microscopy. Precipitation of the intergranular chromium carbides, identified as Cr-rich M₂₃C₆, was retarded on the low angle grain boundaries, compared to that on the random high angle grain boundaries on which coarse and discrete ones were found. They were rarely found on the coherent twin boundaries, however, needle-like ones were evolved on the incoherent twin and twin related Σ9 boundaries. Precipitation of the chromium carbides was also suppressed on the nearly exact coincidence site lattice boundaries such as Σ11 and Σ15, for which the Brandon criterion was fulfilled. The results of the intergranular M₂₃C₆ carbide precipitation were explained in terms of the influence of the grain boundary energy.     

Grain boundary diffusion of tritium in 304- and 316-stainless steels

Grain boundary diffusion of tritium in 304- and 316-stainless steel was studied over the temperature range -78 °C to 185 °C through direct measurement of tritium concentration profiles. Tritium was injected through transmutation of a surface blanket of ⁶LiF, specimens were heated isothermally to establish a tritium diffusion gradient, and the specimen layers containing the rapidly-diffusing grain boundary component were removed with a lathe and/or electropolishing. The data were analyzed by both Fisher's and Suzuoka's models for grain boundary diffusion and similar diffusion coefficients were obtained. Grain boundary diffusion coefficients (G) and their standard deviation are given by: (Fisher) G = exp (8.85 ± 1.2) × exp (? 0.45 ± 0.03 eV/kT) cm²-sec¹, (Suzuoka) G = exp (8.55 ± 0.85) × exp (? 0.43 ± 0.02 eV/kT) cm²-sec^{? 1}.     

Radiation-induced stress relaxation in high temperature water of type 316L stainless steel evaluated by neutron diffraction

Weld beads on plate specimens made of type 316L stainless steel were neutron-irradiated up to about 2.5 × 10²⁵ n/m² (E > 1 MeV) at 561 K in the Japan Material Testing Reactor (JMTR). Residual stresses of the specimens were measured by the neutron diffraction method, and the radiation-induced stress relaxation was evaluated. The values of σ_x residual stress (transverse to the weld bead) and σ_y residual stress (longitudinal to the weld bead) decreased with increasing neutron dose. The tendency of the stress relaxation was almost the same as previously published data, which were obtained for type 304 stainless steel. From this result, it was considered that there was no steel type dependence on radiation-induced stress relaxation. The neutron irradiation dose dependence of the stress relaxation was examined using an equation derived from the irradiation creep equation. The coefficient of the stress relaxation equation was obtained, and the value was 1.4 (×10⁻⁶/MPa/dpa). This value was smaller than that of nickel alloy.     

The sintering of grain boundary cavities in uranium dioxide

The final stage sintering of uranium dioxide has been investigated in the temperature range 1623–1973 K where the residual porosity is situated mainly on grain boundaries.     

辐照316L不锈钢的相变研究

运用穆斯堡尔效应和X射线衍射方法对辐照前后的316L不锈钢样品中的穆斯堡尔参数和相变进行了研究。实验表明316L不锈钢经能量为54MeV的碳离子辐照后,在结构和微观参数上都发生了重大变化。同时也对样品中碳的分布和相变的类型进行了探讨。     

Interferometric measurements of gas diffusivity in supercritical water

This study constitutes a part of attempt to develop a clean and high efficiency combustion system utilizing reactions in supercritical water. This study is aimed at establishing the measuring techniques of transport properties and observing techniques of reaction zone in supercritical water. First, the refractive index of water was measured because previous values were not available in supercritical region. The measurements were made at the temperatures from ambient temperature to 420°C and pressures from atmospheric pressure to 27MPa. The results show that the refractive index is constant in the experimental conditions within experimental errors. Next, the mass diffusivity of gas in supercritical water was measured. Nitrogen was used in this experiment to avoid the effects of reaction. The mass diffusivity of nitrogen in supercritical water was calculated from the fringe shift. The value of the coefficient was intermediate between gas and liquid's one.     

Grain boundary engineering of austenitic steel PNC316 for use in nuclear reactors

Austenitic stainless steel PNC316 was subjected to grain boundary engineering (GBE). It was found that the grain boundary engineered PNC316 (PNC316-GBEM) had a coincidence site lattice (CSL) fraction of 86% and that the network of random grain boundaries was perfectly divided by the CSL boundaries. The thermal stability and the void swelling behavior of PNC316-GBEM were investigated by means of SEM and TEM analyses. After thermal aging at 973 K for 100 h, structural changes were observed neither in the grain boundary networks of PNC316-GBEM nor in another sample of PNC316-GBEM subjected to 20% additional cold rolling, PNC316-GBEM20%CW. PNC316-GBEM showed a higher void swelling rate than as-received PNC316 (PNC316-AS). However, with additional 20% cold rolling after GBE, the void swelling rate decreased to as low as that of PNC316-AS.     

国产改进型316L不锈钢的微结构研究

采用正电子湮没寿命方法研究了国产改进型316L不锈钢的微结构及其温度变化。该不锈钢含有单空位、双空位、位错和小空位团等缺陷。经400℃,600℃,800℃退火后,单空位、双空位和位错缺陷分别消失。小空位团是四空位和五空位构成的空位团,低于200℃退火,缺陷复合五空位团成分随退火温度升高而增大,高于400℃时空位团分裂,五空位团成分减少,800℃退火还存在较低浓度的四空位团。     

On the role of grain boundary diffusion in fission gas release

It is generally believed that thermal fission gas release from LWR fuel occurs mainly via interconnected grain boundary bubbles. Grain boundary diffusion is not considered to be a significant mechanism. We investigated this supposition by two methods; first, by assessing the distance a gas atom can migrate in a grain boundary containing perfectly absorbing traps. For areal number densities and fractional coverages by the traps observed in fuel irradiated to burnups exceeding ∼20 MWd/kg, gas atoms will be trapped after a migration distance equal to the size of a grain or less. This supports the supposition for medium-to-high burnups. However, the above-mentioned model is inapplicable for trace-irradiated specimens. In our second analysis, we examined Xe release from trace-irradiated UO₂. The measurements indicated that the liberation involves more than only lattice diffusion at the specimen surface, and that the data are consistent with sequential lattice and grain boundary diffusion unimpeded by intergranular traps. The analysis also provided rough estimates of the grain boundary diffusion coefficient in UO₂.     

Corrosion behavior of Alloy 625 in supercritical water environments

For a better understanding on the corrosion behavior of Alloy 625, samples fabricated from this alloy were exposed to supercritical water (SCW) environments with 8.3 ppm dissolved oxygen at 400 and 600 °C and 24.8 MPa (3600 psi) for various periods of time up to 1000 h. Pits were found on the surfaces of the samples after the corrosion tests, and the formation of these pits could be attributed to metal carbide inclusions in the as-received Alloy 625. Mass changes (w) in the samples as a function of test duration (t) could be fitted by an equation of w^2.21 = 1.4 × 10⁻⁵ t, indicating that the mass change approximately followed the parabolic law in the specified SCW environment. In addition, oxides with a double-layer structure were observed on the samples. The outer layer of the oxides consisted mainly of sub-micron spinels of Ni(Cr,Fe)₂O₄, and the compact inner layer was mixed Ni(Cr,Fe)₂O₄ and Cr₂O₃ with a grain size of tens of nanometer.     

Effects of impurity elements on SiC grain boundary stability and corrosion

Grain boundaries(GBs)have critical influences on the stability and properties of various materials.In this study,first-principles calculations were performed to determine the effects of four metallic impurities(Ni,Al,Bi,and Pb)and three nonmetallic impurities(H,O,and N)on the GBs of silicon carbide(SiC),using the Σ5(210)GBs as models.The GB energy and segregation energy(SE)were calculated to identify the effects of impurities on the GB stability.Electronic interactions considerably influ-enced the bonding effects of SiC.The formation of weak bonds resulted in the corrosion and embrittlement of GBs.The co-segregation of Bi,Pb,and O was also investigated in detail.     

Corrosion behavior of porous chromium carbide in supercritical water

The corrosion behavior of highly porous chromium carbide (Cr₃C₂) prepared by a reactive sintering process was characterized at temperatures ranging from 375 °C to 625 °C in a supercritical water environment with a pressure of 25–30 MPa. The test results show that porous chromium carbide is stable in SCW environments at temperatures under 425 °C, above which disintegration occurred. The porous carbide was also tested under hydrothermal conditions of pressures between 12 MPa and 50 MPa at constant temperatures of 400 °C and 415 °C, respectively. The pressure showed little effect on the stability of chromium carbide in the tests at those temperatures. The mechanism of disintegration of chromium carbide in SCW environments is discussed.     

Theoretical study of flashing and water hammer in a supercritical water cycle during pressure drop

During a loss of coolant accident (LOCA) the pressure of the coolant can drop significantly in the vicinity of the leak. It will be shown that unlike in pressurized water reactors (PWRs) where this pressure drop can cause only sudden vaporization - also called flashing - in supercritical water cooled reactors (SCWRs) it can cause sudden condensation (condensation-induced water hammer), too. The reason is that from supercritical state the system can go to metastable liquid as well as to metastable vapour state after LOCA. Relaxation from metastable fluid states is a fast process, followed by a local positive or negative pressure-jump, which might increase the damage around the leak. Conservative estimation will be given for the magnitude of these pressure jumps caused by the flashing or water hammer by assuming various initial pressure losses. In our calculations, three different equations of state are used: the simple van der Waals EoS; the Redlich-Kwong as an empirical development; and the more sophisticated non-cubic Deiters equation of state. These equations are able to describe metastable states qualitatively but with different accuracy. These calculations can help us to map the local immediate effect of any sudden pressure drop and therefore it can help to design better safety protocols.