Influence of microstructure on the hydrogen permeability of 9%Cr–1%Mo ferritic steel

The influence of microstructure of 9%Cr–1%Mo steel on the hydrogen diffusivity, solubility and hence the permeability was investigated using electrochemical permeation technique. This steel was austenitised and cooled at various cooling rates to produce different microstructures. Tempering behaviour was also studied by heat treating for different durations at 1023 K. Characterisation of microstructures was carried out using scanning electron microscopy and analytical transmission electron microscopy. A fully martensitic product was obtained during fast cooling and a mixture of proeutectoid ferrite and martensite during slow cooling. Tempering the normalised steel resulted in the formation of fine intragranular M₂X precipitates and M₂₃C₆ on the boundaries. The hydrogen diffusivity and solubility showed a regular trend with the amount of strain in the lattice. Lattice defects and precipitates act as trap sites for hydrogen. Increase in lattice strain either due to increase in defect density, substructure or coherent precipitates resulted in decrease in diffusivity due to increase in trap sites. Martensite structure offered the maximum resistance to hydrogen diffusivity and tempered martensite the least resistance due to the annihilation of defects during tempering.     

Development of 9Cr-ODS Martensitic Steel Claddings for Fuel Pins by means of Ferrite to Austenite Phase Transformation

For use as fuel cladding of liquid metal fast reactors, Fe-0.12C-9Cr-2W ODS martensitic steel claddings were developed by cold-rolling under the softened ferrite phase induced by slow cooling from austenite phase, subsequently by ferrite to austenite phase transformation to break up substantially elongated grains produced by cold-rolling at the final heat-treatment. The produced claddings showed noticeable improvement in tensile and creep rupture strength that are considerably superior to PNC-FMS and even austenitic PNC316 at higher temperature and extended time to rupture. The strength improvement is mainly attributed to titanium addition in ODS martensitic steels through its reduction of Y₂O₃ particle size and shortening inter-particles spacing. The behavior of oxide particle size reduction is associated with stoichiometry between Y₂O₃ and TiO₂.     

Precipitation and void-swelling in nickel-manganese austenitic stainless steels

The precipitation and void-swelling characteristics of austenitic stainless steels in which nickel is partially replaced by manganese have been investigated. Alloy compositions were chosen on the basis of manganese being half as effective as nickel in stabilizing austenite, and steels with “nickel equivalent” contents of 25–37% were examined. The steels were irradiated with 46 MeV Ni⁶⁺ ions to 60 dpa at 625°C and also aged for 1000 h at 600°C. The high-Mn alloys (20–30% Mn) were very susceptible to the formation of intermetallic phases during thermal ageing but less so in the shorter-duration irradiation experiment. Irradiation promoted the formation of Ni- and Si-rich phases—the suicide G phase (in which Mn can replace Ti) and in one instance M₆C. The Cr-rich carbide M₂₃C₆ formed in both the aged and irradiated steels. Among the high-Mn alloys, void-swelling decreased with increasing Ni and (Ni+Mn) contents, although a 25Ni-1Mn steel showed no swelling at 625°C.     

The effects of silicon and titanium on void swelling and phase stability in 12Cr-15Ni austenitic alloys irradiated with 46 MeV nickel ions

The structure of 20% cold-worked 12 wt%Cr-15 wt%Ni austenitic alloys with Si and Ti contents in the ranges 0.14-1.42 and <0.02-0.27 wt%, respectively, has been investigated by TEM following irradiation with 46 MeV Ni⁶⁺ ions to 60 dpa at 525, 575 or 625° C in the Harwell VEC. Increasing the Si content progressively reduced the void concentrations and swelling to zero at all three temperatures and the addition of Ti further reduced the void concentration and swelling. The higher Si alloys were structurally unstable during irradiations at 525 and 575°C, when γ'(Ni₃Si) particles, M₂₃X₆ type precipitates and grains of a bcc phase were formed. Small amounts of the γ' and bcc phases were observed following irradiation at 625° C, the principal precipitates being of the M₂₃X₆ type. Voids were present in both austenite and bcc phases in the lower Si alloys and segregation of an unidentified fcc phase was observed around voids. It was concluded that all the void nucleation occurred in the austenite phase and that the bcc phase resulted from the partial transformation of the austenite to α'-martensite during cooling from the irradiation temperatures. This occurred as a consequence of the M_s(α') transformation temperature being raised to well above ambient owing to elemental depletions of the alloy matrices resulting from precipitation and segregation.     

Investigation on the martensitic transformation and the associated intermediate phase in U-2 wt%Zr alloy

In most of the binary uranium alloy systems, different metastable phases can be retained at room temperature depending on the rate of cooling of the gamma phase. On the other hand, in U-rich U-Zr system, the gamma phase transforms into either martensitic α or Widmanstatten α depending on the cooling rate. It has not been ascertained so far if any intermediate phase is involved prior to the martensitic transformation in U-rich U-Zr system and, therefore, nature of the intermediate phase remains undetermined. Present investigation shows the existence of an intermediate phase prior to the final martensitic transformation. Although the equilibrium beta phase is known to undergo martensitic transformation, quenching from the γ phase appears to suppress the β phase formation. In fact the present experimental evidence suggests that the most likely intermediate phase is the monoclinic distortion of alpha phase (α^″), and not the beta, when gamma phase of U-2 wt%Zr alloy is quenched rapidly.     

Effect of stress and plastic deformation on the transformation of the delta phase in a Pu-lat % Ga alloy

The influence of elastic and plastic deformation on the transformation of the fcc δ phase in a Pu-1 at % Ga alloy was investigated. Undeformed, this phase could be retained without decomposition for over 17 months at 23°C. An M_s at 8°C is indicated, below which the δ to α transformation starts instantaneously. Transformation to the monoclinic α phase above 8°C could be induced by compressive elastic loading. At room temperature, transformation was induced only by plastic deformation; both the α and orthorhombic γ phases formed with up to a 20% decrease in volume. Different martensite morphologies and variations in the amount of transformation and in the transformation kinetics were obtained depending upon the temperature and condition of loading.     

Compatibility of FBR structural materials with supercritical carbon dioxide

A key problem in the application of a supercritical carbon dioxide (CO₂) turbine cycle to a fast breeder reactor is the corrosion of structural materials brought about by supercritical CO₂ at high temperatures. In this study, long-term (8000 h) compatibility tests on candidate materials, two high-chromium martensitic steels (12Cr- and 9Cr-steels) and an austenitic stainless steel (316FR), were performed at 400-600 °C in supercritical CO₂ pressurized at 20 MPa, and corrosion allowances for the steels were proposed for application to preliminary reactor design.Although high temperature oxidation was measured in all steels, the behavior differed greatly. For martensitic steels, weight gain exhibited parabolic growth as exposure time increased at each temperature. Neither exfoliation of the oxide nor the breakage was observed during the 8000 h of exposure. The corrosion behavior was equivalent to that seen in supercritical CO₂ at 10 MPa, and it was confirmed that no effects of CO₂ pressure were present under the CO₂ turbine cycle operation conditions. Based on the results, corrosion allowances for temperature-dependant parabolic growth were proposed. For 316FR steel, weight gain was significantly lower than that of martensitic steels, with a maximum value of 6.2 g/m² at 600 °C for 8000 h. Since no dependency of temperature and immersion time on weight gain such as the martensitic steels was noted, corrosion allowances proportional to time was proposed. Estimated corrosion allowances for the martensitic and austenitic steels were 380 μm and 220 μm, respectively, for reactors, whose design life is rated at 60 years.     

Compatibility of reduced activation ferritic martensitic steel JLF-1 with liquid metals Li and Pb–17Li

The corrosion of reduced activation ferritic martensitic steel, JLF-1 (Fe–9Cr–2W–0.1C), in high-purity Li was quite small. However, carbon in the steel matrix was depleted by the immersion to the Li. The depletion caused the phase transformation of the steel surface in which the morphology of the steel surface changed to ferrite structure from initial martensite structure. The phase transformation degraded the mechanical property of the steel. However, the carbon depletion and the phase transformation of the steel were suppressed in carbon doped Li. The carbon in the steel was chemically stable and did not dissolve into the Li when the carbon potential in the Li was high. The concentration of nitrogen and oxygen must be kept as low as possible because the corrosion was larger when the concentration of oxygen or nitrogen in the Li was higher. The chemical reaction between the steel and the Li compounds of Li₃N and Li₂O was also investigated. The corrosion of the JLF-1 steel in Pb–17Li was summarized as the dissolution of Fe and Cr from the steel into the melt. The corrosion of the specimen with Er₂O₃ coating fabricated by metal organic decomposition process in the Li and the Pb–17Li was investigated. The coating was deformed, cracked and partially exfoliated in the liquid metals, though the oxide itself was chemically stable in the liquid breeders. The damage was probably made by the stress, which was generated by a large difference of the thermal expansion ratio between the solidified Li or Pb–17Li and the coating during a heat up and a cool down process of the corrosion test.     

An assessment of carburization-decarburizatton behaviour of Fe-9Cr-Mo steels in a sodium environment

A critical assessment is made of the carburization-decarburization kinetics of Fe-9Cr-Mo steels exposed to a sodium environment, using the available information on carbide phase morphology, chromium activity in a ferrite matrix, chromium carbide activity in mixed carbides, carbon solubility in Cr-Mo ferritic steels, and activity-concentration relationships based on α-phase/M₆C or M₂₃C₆carbide equilibrium. Experimental data are presented on the decarburization of Fe-5Cr-Mo and Fe-9Cr-Mo steels at 973 K in a sodium environment to ascertain the long-term behaviour of these steels. The analysis shows that the decarburization of ferritic steels is largely dependent on the chemical reaction at the carbide/α interface and that at carbon activities <0.04, the rate is predominantly determined by the dissolution of (Fe, Mo)₆C carbides.     

Development of Oxide Dispersion Strengthened Steels for FBR Core Application, (II)

Previously manufactured oxide dispersion strengthened (ODS) ferritic steel cladding tubes had inferior internal creep rupture strength in the circumferential hoop direction. This unexpected feature of ODS cladding tubes was substantially ascribed to the needle-like grain structure aligned with the forming direction. In this study, the grain morphology was controlled by using the martensite transformation in ODS martensitic steels to produce an equi-axial grain structure. A major improvement in the strength anisotropy was successfully achieved. The most effective yttria addition was about 1 mass% in improving the strength of the ODS martensitic steels. A simple addition of titanium was particularly effective in increasing the strength level of the ODS martensitic steels to that of ODS ferritic steels.     

Correlation of the thermodynamic calculation and the experimental observation of Ni-Mo-Cr low alloy steel changing Ni, Mo, and Cr contents

SA508 Gr.4N Ni-Mo-Cr low alloy steel has improved fracture toughness and strength compared to commercial low alloy steels such as SA508 Gr.3 Mn-Mo-Ni low alloy steel, which has less than 1% Ni. Higher strength and fracture toughness of low alloy steels can be achieved by increasing the Ni and Cr contents. In this study, the effects of the alloying elements of Ni and Cr on the microstructural characteristics and mechanical properties of SA508 Gr.4N Ni-Mo-Cr low alloy steel are evaluated. Changes in the stable phases of SA508 Gr.4N low alloy steel with these alloying elements were evaluated using thermodynamic calculation software. These values were then compared with the observed microstructural results. Additionally, tensile tests and Charpy impact test were carried out to evaluate the mechanical properties. The thermodynamic calculations show that Ni mainly affects the change of the matrix phase of γ and α rather than the carbide phase. Contrary to the Ni effect, Cr and Mo primarily affect the precipitation behavior of the carbide phases of Cr₂₃C₆, Cr₇C₃ and Mo₂C. In the microscopic observations, the lath martensitic structure becomes finer as the Ni content increases without affecting the carbides. When the Cr content decreases, the Cr carbide becomes unstable and carbide coarsening occurs. Carbide Mo₂C in the form of fine needles were observed in the high-Mo alloy. Greater strength was obtained after additions of Ni and Mo and the transition properties were improved as the Ni and Cr contents increased. These results were correlated with the thermodynamic calculation results.     

Precipitation in AISI 316L(N) during creep tests at 550 and 600 °C up to 10 years

The precipitation behaviour in the gauge lengths and in the heads of initially solution annealed type 316L(N) austenitic stainless steel specimens tested in creep at 550 and 600 °C for periods of up to 85 000 h has been studied using several metallographic techniques. Three phases were detected: M₂₃C₆, Laves, and sigma phase. The volume fraction of the precipitated sigma phase was significantly higher than that of carbides and the Laves phase. M₂₃C₆ carbide precipitation occurred very rapidly and was followed by the sigma and Laves phases formation in the delta ferrite islands. Sigma and Laves phases precipitated at grain boundaries after longer times. Two different mechanisms of sigma phase precipitation have been proposed, one for delta ferrite decomposition and another for grain boundary precipitation. Small quantities of the Laves phase were detected in delta ferrite, at grain boundaries and inside the grains.     

Oxide layer stability in lead-bismuth at high temperature

Materials protection by ‘in situ’ oxidation has been studied in stagnant lead-bismuth, with different oxygen levels (H₂/H₂O ratios of 0.3 and 0.03), at temperatures from 535 °C to 600 °C and times from 100 to 3000 h. The materials tested were the martensitic steels F82Hmod, EM10 and T91 and the austenitic stainless steels, AISI 316L and AISI 304L. The results obtained point to the existence of an apparent threshold temperature above which corrosion occurs and the formation of a protective and stable oxide layer is not possible. This threshold temperature depends on material composition, oxygen concentration in the liquid lead-bismuth and time. The threshold temperature is higher for the austenitic steels, especially for the AISI 304L, and it increases with the oxygen concentration in the lead-bismuth. The oxide layer formed disappear with time and, after 3000 h all the materials, except AISI 304L, suffer corrosion, more severe for the martensitic steels and at the highest temperature tested.     

Effects of an intermediate heat treatment during a cold rolling on the tensile strength of a 9Cr–2W steel

The effects of an intermediate heat treatment during a cold rolling on the tensile strength of a 9Cr–2W steel were evaluated. Before a cold rolling, the steel was normalized at 1050 °C and tempered at 550 °C in order to avoid the formation of M₂₃C₆ and V-rich MX precipitates in the martensitic structure. A 75% cold rolling and a heat treatment at 750 °C for 30 min induced the formation of large M₂₃C₆ carbides in a fully recrystallized structure. However, three cold rollings with an intermediate heat treatment at 750 °C for 10 min after each cold rolling led to the formation of fine and uniform M₂₃C₆ carbides in a partially recrystallized structure, providing an enhanced tensile strength at 650 °C. It is thus concluded that an intermediate heat treatment during a cold rolling could be an effective procedure for fabricating a high strength 9Cr–2W steel at high temperatures.     

M2N nitride phases of 9% chromium steels for nuclear applications

M₂N nitride phases of 9% chromium steels with an extra-low carbon content have been investigated using a transmission electron microscope and an energy-dispersive X-ray (EDX) spectroscopy. The steel samples were normalized for 1 h at 1050 °C and then tempered at 600-780 °C for 30 min to 5 h followed by an air cooling. Through the analyses of the electron micro-diffraction patterns and EDX data for the precipitate particles on the extracted carbon replica, two types of Cr-rich M₂N nitride phases with the same hexagonal structure but totally different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the steels. Four types of Cr-rich M₂N phases with different lattice parameters probably existed in the steels. The M₂N phase revealed a decrease in its Cr content, an increase in its V content as the tempering temperature was increased, and no obvious change in its content for the metal fraction with an increasing tempering time.     

The effects of tungsten addition on the microstructural stability of 9Cr–Mo Steels

The effects of tungsten addition on the microstructure and high-temperature tensile strength of 9Cr–Mo steels have been investigated by using three different steels: M10 (9Cr–1Mo), W18 (9Cr–0.5Mo–1.8W), and W27 (9Cr–0.1Mo–2.7W) steels. The tungsten-added 9Cr steels have revealed better high-temperature tensile strength. Microchemical analysis for (Cr,Fe)₂ (C,N) revealed that the tungsten addition increased the Cr/Fe ratio, which resulted in the lattice expansion of (Cr,Fe)₂ (C,N), and then the enhanced pinning effect on the glide of dislocation. In addition, in M10 steel, the M₂₃C₆ carbides quickly grew and agglomerated, while the tungsten-added 9Cr steels revealed a fine and uniform distribution of M₂₃C₆ carbides. Dislocation recovery during tempering treatments was delayed in tungsten-added 9Cr steels, which was correlated with the stabilized precipitates and the decreased self-diffusivity of iron. It is, thus, believed that the excellent high-temperature tensile strength of tungsten-added 9Cr steels is attributed to the stabilized M₂X carbo-nitrides and M₂₃C₆ carbides and the decreased self-diffusivity of iron.     

Behavior of steels in flowing liquid PbBi eutectic alloy at 420-600 °C after 4000-7200 h

This paper presents the results of steel exposure up to 7200 h in flowing LBE at elevated temperatures and is a follow-up paper of that with results of an exposure of up to 2000 h. The examined AISI 316 L, 1.4970 austenitic and MANET 10Cr martensitic steels are suitable as a structural material in LBE (liquid eutectic Pb₄₅Bi₅₅) up to 550 °C, if 10⁻⁶ wt% of oxygen is dissolved in the LBE. The martensitic steel develops a thick magnetite and spinel layer while the austenites have thin spinel surface layers at 420 °C and thick oxide scales like the martensitic steel at 550 °C. The oxide scales protect the steels from dissolution attack by LBE during the whole test period of 7200 h. Oxide scales that spall off are replaced by new protective ones. At 600 °C severe attack occurs already after 2000 and 4000 h of exposure. Steels with 8-15 wt% Al alloyed into the surface suffer no corrosion attack at all experimental temperatures and exposure times.     

Corrosion behaviors of US steels in flowing lead-bismuth eutectic (LBE)

Corrosion tests of several US martensitic and austenitic steels were performed in a forced circulation lead-bismuth eutectic non-isothermal loop at the Institute of Physics and Power Engineering (IPPE), Russia. Tube and rod specimens of austenitic steels 316/316L, D-9, and martensitic steels HT-9, T-410 were inserted in the loop. Experiments were carried out simultaneously at 460 °C and 550 °C for 1000, 2000 and 3000 h. The flow velocity at the test sections was 1.9 m/s and the oxygen concentration in LBE was in the range of 0.03-0.05 wppm. The results showed that at 460 °C, all the test steels have satisfactory corrosion resistance: a thin protective oxide layer formed on the steel surfaces and no observable dissolution of steel components occurred. At 550 °C, rod specimens suffered rather severe local liquid metal corrosion and slot corrosion; while tube specimens were subject to oxidation and formed double-layer oxide films that can be roughly described as a porous Fe₃O₄ outer layer over a chrome-rich spinel inner layer. Neglecting the mass transfer corrosion effects by the flowing LBE, calculations based on Wagner’s theory reproduce the experimental results on the oxide thickness, indicating that the oxide growth mechanism of steels in LBE is similar to that of steels in air/steam, with slight modification by dissolution and oxide dissociation at the liquid metal interface.     

TEM investigations of MN nitride phases in a 9% chromium ferritic/martensitic steel with normalization conditions for nuclear reactors

The investigations on the precipitate phases in a 9%Cr ferritic/martensitic steel under different normalization conditions have been made by using a transmission electron microscope and an energy-dispersive X-ray spectroscopy. Hot-rolled steel samples were normalized at 1050-1200 °C for 1-2 h followed by an air cooling to room temperature. MN vanadium nitride precipitates with a plate-like morphology and a chemical formula of about (V_0.4Nb_0.4Cr_0.2)N have been observed at triple junctions, grain boundaries and within matrix in the steel samples normalized at 1050-1150 °C for 1-2 h, but they were dissolved out at 1200 °C within 1 h. Vanadium nitride is a stable phase at 1050 °C according to thermocalc prediction of equilibrium phases in the steel. With increasing normalizing temperature and time, there was no a striking change in the chemical composition of metallic elements in the MN phase, but a considerable increase in the size of the MN precipitate.     

Defect structure of irradiated PH13-8Mo steel

PH13-8Mo bolts, which are considered for use in the ITER reactor, were irradiated up to doses of 0.5, 1 and 2 dpa. The microstructure was investigated with transmission electron microscopy and its evolution is discussed with reference to the mechanical properties. PH13-8Mo is a precipitation hardened martensitic steel, but a large amount of austenite has been observed as well. The precipitation hardening results from the formation of small coherent NiAl precipitates in the martensite phase. Their size, size distribution and density are found to be unaffected by neutron irradiation. The dislocations in the martensite phase are mainly a/2〈1 1 1〉 type screw dislocations, whereas in the austenite phase mainly a/2〈1 1 0〉 type screw dislocations are present. The line dislocation structure did not change during irradiation, but small irradiation induced defects were observed. Using the Orowan model, it is argued that the latter are responsible for the irradiation hardening.