Radiation-induced segregation in desensitized type 304 austenitic stainless steel

Radiation-induced segregation (RIS) in desensitized type 304 stainless steel (SS) was investigated using a combination of electrochemical potentiokinetic reactivation (EPR) test and atomic force microscopy (AFM). Desensitized type 304 SS was irradiated to 0.43 dpa (displacement per atom) using 4.8 MeV protons at 300 °C. The maximum attack in the EPR test for the irradiated desensitized SS was measured at a depth of 70 μm from the surface. Grain boundaries and twin boundaries got attacked and pit-like features within the grains were observed after the EPR test at the depth of 70 μm. The depth of attack, as measured by AFM, was higher at grain boundaries and pit-like features as compared to twin boundaries. It has been shown that the chromium depletion due to RIS takes place at the carbide-matrix as well as at the carbide-carbide interfaces at grain boundaries. The width of attack at grain boundaries after the EPR test of the irradiated desensitized specimen appeared larger due to the dislodgement of carbides at grain boundaries.     

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.     

Embrittlement susceptibility induced by impurities segregation to grain boundaries in martensitic steels candidates to be used in ADS

Grain boundary microchemical characterization, by Auger electron spectroscopy, has been performed in the martensitic steel EM-10 doped with relevant spallation elements that are expected to be produced at the spallation target window in future Accelerator Driven Systems (ADS). A heat treatment of step-cooling was performed in all doped materials to accelerate impurity segregation. The results indicate that step-cooling promotes chromium and molybdenum enrichment at the grain boundaries in the four materials studied. Step-cooling promotes phosphorus segregation to grain boundaries in the reference material, in the material doped with titanium, and in the material doped with titanium, phosphorus and sulphur. Step-cooling also promotes titanium enrichment in the materials doped with this element. A relation among chromium, molybdenum and phosphorus has been observed in the alloy doped with titanium, phosphorus and sulphur suggesting co-segregation of these elements to grain boundaries or segregation of phosphorus at the interface of the matrix and the carbides rich in chromium and molybdenum.     

Cracking in stabilized austenitic stainless steel piping of German boiling water reactors—characteristic features and root cause

Cracks have been found in the welds of piping systems made from stabilized austenitic stainless steels in German boiling water reactors (BWR). In the course of the intensive failure analysis metallographic examinations, microstructural investigations by electron microscopy, corrosion experiments and welding tests have been performed. The results show that cracking under the given medium conditions is due to intergranular stress corrosion cracking (IGSCC) in those parts of the heat affected zone (HAZ) which are overheated during welding and where solution of titanium carbides and subsequent precipitation of chromium carbides and depletion of chromium along the affected grain boundaries could occur.     

The role of niobium carbide in radiation induced segregation behaviour of type 347 austenitic stainless steel

The effect of niobium carbide precipitates on radiation induced segregation (RIS) behaviour in type 347 stainless steel was investigated. The material in the as-received condition was irradiated using double-loop 4.8 MeV protons at 300 °C for 0.43 dpa (displacement per atom). The RIS in the proton irradiated specimen was characterized using double-loop electrochemical potentiokinetic reactivation (DL-EPR) test followed by atomic force microscopic examination. The nature of variation of DL-EPR values with the depth matched with the variation of the calculated irradiation damage (dpa) with the depth. The attack on grain boundaries during EPR tests was negligible indicating absence of chromium depletion zones. The interface between niobium carbide and the matrix acts as a sink for point defects generated during irradiation and this had reduced point defect flux toward grain boundaries. The attack was noticed at a few large cluster of niobium carbide after the DL-EPR test at the depth of maximum attack for the irradiated specimen. Pit-like features were not observed within the matrix indicating the absence of chromium depletion regions within the matrix.     

Estimation of stress corrosion cracking sensitivity of type 304 stainless steel by magnetic force microscope

An application of a magnetic force microscope (MFM) to the measurement of the chromium depleted regions of type 304 stainless steel is proposed to enable more effective evaluation of the material sensitization to stress corrosion cracking than the conventional methods. The MFM images of sensitized materials show that the magnetizations are induced along grain boundaries by the chromium depletion. The dependence of the magnetization on the sensitization condition conforms to the expected one from the behavior of chromium depletion. Furthermore, the phase identification was performed by electron backscattered pattern technique to reveal the magnetization mechanism due to sensitization. Then, it was found that the magnetization is caused by the transformation from austenite phase to martensite phase. From the discussion on the temperature at which martensitic transformation starts, we see that it seems to be possible to detect regions where the chromium concentration is under 14% by using an MFM.     

Effect of prior thermal treatment on the microchemistry and crack propagation of proton-irradiated AISI 304 stainless steels

The effect of prior thermal treatment on crack growth was investigated on proton-irradiated Type 304 stainless steel (SS) of initially solution annealed (SA) and thermally sensitized (SEN) conditions. The Cr depletion profiles were measured by field emission gun transmission electron microscopy/energy dispersive spectroscopy (FEGTEM/EDS) in an attempt to correlate grain boundary chromium composition with the measured crack growth rate. The results showed that the crack growth of the 1-dpa-irradiated SEN 304SS is substantially higher than that of SA 304SS with the same irradiation dose. The unirradiated SEN material initially started with a shallow Cr depletion profile near grain boundary. After 1 dpa irradiation with proton, the Cr depletion profile becomes narrower and deeper. In contrast, the grain boundary Cr concentration in the SA specimen at the same irradiation dose was higher than that of the SEN specimen, mainly due to an initial Cr enriched condition. Consequently, the irradiated SEN specimen exhibited a higher degree of sensitization in electrochemical potentiokinetic reactivation test and faster crack growth rate in the stress corrosion crack test. The absence of irradiation enhanced crack growth in heavily thermal-sensitized 304SS is probably attributed to slower radiation-induced Cr depletion as a result of pre-existing thermally induced grain boundary Cr depletion. It is a clear indication that the inverse Kirkendall effect was hampered by the back diffusion of Cr due to initially depleted Cr concentration gradient near grain boundary.     

Interaction between borosilicate melt and Inconel

Understanding the mode of interaction between borosilicate melt and Inconel is important for long time usage of melter pot in vitrification plant. The present study shows that significant elemental exchanges take place across the borosilicate melt/Inconel interface resulting in the development of (Fe, Ni)CrO₄ needle and (Fe, Ni)Cr₂O₄ cubic phases. This results in significant depletion of Cr within Inconel near the interface. Beside these, CrB precipitates formed along the Inconel grain boundaries.     

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.     

The release of fission gas from nuclear fuels during temperature transients

The fractional release of rare gas atoms from a spherical grain of uranium dioxide containing a uniform concentration of gas atoms and intragranular bubbles is calculated for short-duration temperature transients and post-irradiation annealing. The released fraction is shown to reach a small maximum value which is dependent only on the grain size, gas atom concentration and intragranular bubble distribution. The analysis therefore may be applied to any fuel material providing these parameters are known. The conclusion therefore is that during a brief temperature transient, only a very small fraction of gas will be released from grain interiors to grain boundaries. Consequently the majority of gas released into the free volume of a pin will come from gas already residing on grain boundaries and released by mechanical cracking; the fractional gas release is small and should not cause undue concern. The calculations do not cover the case of fuel melting.     

Re-solution effects and fission gas swelling in UO2

A fission gas swelling model is proposed which enables one to calculate swelling in the vicinity of grain boundary networks and in imperfection-free regions. The grain boundary swelling requires a knowledge of the gas accumulation and the reaction-rate at the boundary. The gas accumulation was calculated by deriving a modified form of Fick's second law wherein it was assumed that because of re-solution effects the in-pile diffusion coefficient can be described as a function of the gas concentration but is independent of the actual operating time. Reaction-rates for bubbles at grain boundaries were derived in the manner discussed by de Jong and Koehler in their treatment of vacancy clustering. The results indicate that there is a grain size of about 10⁻⁴ cm for which the swelling is a maximum, which increases somewhat with irradiation temperature and with depletion at a constant temperature. The results enable one to predict the swelling and the mean radii of both intergranular and intragranular bubbles. Mean bubble radii predicted using the re-solution swelling model are in reasonable agreement with radii obtained from electron micrographs of irradiated UO₂ fuel samples. It is argued that gas bubble migration is the predominant means by which gas atoms arrive at grain boundaries at irradiation temperatures above about 900°C.     

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.     

Cyclic stress-strain response of textured Zircaloy-4

In this paper, the cyclic stress-strain response of textured Zircaloy-4 is investigated at room temperature in an incremental step test using fully reversed tension-compression loading under strain control. The material exhibits an asymmetry of stress response in both rolling and transverse directions, and the corresponding cyclic stress-strain curves can be expressed by a power law relation. Furthermore, phenomenological friction and back stresses are derived from an analysis of hysteresis loop shapes using the Cottrell scheme. It has been shown that the magnitude of the phenomenological friction stress in compression is always higher than that in tension for either rolling or transverse direction. While the magnitude of the phenomenological back stress, being independent of the loading direction, increases much more rapidly in transverse direction than that in rolling direction with increasing the plastic strain amplitude, and the trend in both directions can be expressed by a logarithmic relation. A further discussion suggests that (i) the intergranular thermal stress in the material is responsible for the difference in the phenomenological friction stress between tension and compression, thus leading to the asymmetry of stress response; (ii) the increase of the saturated stress with the plastic strain amplitude stems from the back stress that is primarily a direct consequence of the plastic strain incompatibilities between grains; (iii) the different performance between rolling and transverse directions results from the texture effect.     

Microstructural evolution of the China Low Activation Martensitic (CLAM) steel irradiated by H and He ion beams

China Low Activation Martensitic (CLAM) steel was irradiated at room temperature with different doses of He⁺ and H⁺ ion beams. TEM indicated that the microstructure of unirradiated CLAM steel consisted of laths, grain boundaries, dislocations and carbides. Electron diffraction patterns revealed that the microstructure of carbides at grain boundaries was primarily dominated by M₂₃C₆ carbide. Vacancy clusters were induced into the matrix after irradiation. TEM-EDX of carbides and matrices of unirradiated and post-irradiated samples were performed to investigate the composition of carbides and the effect of irradiation on the composition of carbides. Carbides from unirradiated and irradiated specimens at grain boundaries were found to be enriched with Cr. For irradiated specimens, concentrations of Cr increased as the irradiation dose was increased. Cr enrichment could lead to precipitation of additional phase.     

Root causes of intergranular attack in an operating nuclear steam generator tube

This paper reports the secondary side intergranular attack of an Alloy 600 tube, which was located within sludge piles in the hot-leg side of an operating nuclear steam generator. Carbide distribution along the grain boundaries and chromium depletion were analyzed using optical microscopy and transmission electron microscopy. Local crevice chemistry in contact with the defect was also assessed from the hideout return test data and oxide film analysis results using energy dispersive spectroscopy. The main causes of this defect are discussed based on the microstructure, local chemistry and operation temperature.     

Inconel 617合金中第二相的析出规律研究

利用SEM和TEM对Inconel 617合金在700 ℃时效0~1 000 h后的M₂₃C₆碳化物和γ′-Ni₃(Ti,Al)相的析出规律及第二相析出对硬度的影响进行了研究。结果表明,700 ℃时效过程中样品在160 h时达到硬度峰值,且硬度的提高与晶界处析出的不规则M₂₃C₆碳化物、晶粒内部析出的棒状M₂₃C₆碳化物和球状γ′相有关,其中γ′相对材料的强化效果更明显。时效初期,第二相以M₂₃C₆碳化物为主,γ′相倾向在碳化物周围的富Al和Ti的区域形核长大。两种相互依附的析出相存在共格取向关系,限制彼此的生长。γ′相数量不断增多并向碳化物内部生长,导致碳化物逐渐分解。时效后期,晶粒内部的碳化物几乎消失,晶粒内部弥散分布着大量球状γ′相。根据实验结果,讨论了两种析出相的析出长大规律。     

Effects of fabrication process on microstructure and texture of Inconel 690 tubes for steam generator

The main goal of this research was to investigate the relationship between the grain boundary misorientation and the precipitation of intergranular M₂₃C₆ carbides during the pilgering process and the heat treatment of Inconel 690 tubes for steam generators. The M₂₃C₆ carbides behavior is obviously influenced by the grain boundary character and interfacial energy. The grain boundary misorientation of the Inconel 690 tubes was investigated by electron backscattered diffraction of carbide precipitates at these grain boundaries. Numerous M₂₃C₆ carbide precipitate at the large angle grain boundaries with high interfacial energy.     

Microstructure and its influence on mechanical properties of CLAM steel

The microstructure of China Low Activation Martensitic steel (CLAM) and its influence on mechanical properties were investigated. The tensile test showed that the strength of CLAM (HEAT 0603A) was higher than that of HEAT 0408B at room temperature, and the reverse results were obtained at elevated temperatures. The results indicated that the microstructure was composed of dispersived carbide particles and lath martensite with high dislocation density. The main precipitation phases were Cr-rich M₂₃C₆ carbides precipitated mainly along the lath boundaries and prior-austenite grain boundaries and Ta-rich MX particles precipitated mainly in the laths and lath boundaries. The finer lath was the main reason for the higher strength of HEAT 0603A compared with HEAT 0408B at room temperature; contrasted with the lower strength at high temperature. Heavier hot forging deformation degree was considered as the main possible reason for the decrease of martensite lath width in HEAT 0603A.     

Bending tests on T91 steel in Pb-Bi eutectic, Bi and Pb-Li eutectic

The influence of specific liquid metals on the behaviour of the ferritic/martensitic steel T91 are investigated to understand better the processes taking place at the metal surface. Of special interest is particularly if there is a penetration of selected elements out of the melt along grain boundaries. Metallurgical investigation, SEM and EDX analyses analysis were performed on bent T91 specimens exposed to static LBE, Bi and Pb-17Li at 300 °C for 1000 h. Steel T91 was used in the standard and specially annealed conditions. It is shown that the heat treatment of the steel has a strong influence on the corrosion resistance. Additionally the strain affected on the component is responsible for the occurrence of LME.     

Measurement of plastic strain of polycrystalline material by electron backscatter diffraction

It is important to know the degree of plastic strain in order to evaluate the susceptibility and crack growth rate of stress corrosion cracking (SCC) in stainless steel and nickel based alloy, because SCC is enhanced by the cold work and causes many problems in nuclear power plant components. In this study, electron backscatter diffraction in conjunction with scanning electron microscopy is applied to measure the plastic strain imposed to stainless steel by tensile load. A new parameter, which quantifies the spread of the crystal orientation within individual grains arising due to dislocation accumulation during plastic deformation, is correlated with imposed plastic strain. The new parameter is called ‘crystal deformation’ and is determined from the spread in misorientation from the central grain orientation. It is confirmed that this parameter has a good correlation with plastic strain and is not affected by the data density of the crystal orientation map. The dislocation density distribution is also evaluated from the misorientation from the central orientation. Relatively high dislocation density was observed near grain boundaries and grain boundary triple points, which was consistent with the observed deterioration of EBSD pattern quality in those locations.