China Low Activation Martensitic Steel Research Articles

Effect of Y2Ti2O7 on microstructure and tensile properties of ODS-CLAM steel

Nano-Y2Ti2O7 is one of the phases used for improving the strength of oxide dispersion strengthened (ODS) steels, which is usually precipitated with Ti and Y2O3 added. The effect of synthesized Y2Ti2O7 on China low activation martensitic (CLAM) steel was studied. Two kinds of ODS-CLAM samples with nano-Y2Ti2O7 and amorphous Y2Ti2O7 additions were prepared using mechanical alloying (MA) and spark plasma sintering (SPS). The results indicates that amorphous Y2Ti2O7 powders had both amorphous and crystal oxides after milling with CLAM powders. Y2Ti2O7 powders were refined to below 10 nm in size with almost no structural change, and randomly distributed in the matrix. The initial size of added Y2Ti2O7 had a smaller impact on the size of precipitates, and the initial structure or the transform of the oxides during milling process were more important. The added Y2Ti2O7 particles were refined from ∼32 nm to 9.2 nm in size, while the size of amorphous Y2Ti2O7 particles was reduced from micron size to 6.9 nm in the sintered sample. The structure of Y2Ti2O7 in ODS powders affected the microstructure and strength of the sample after sintering. The sample with Y2Ti2O7 had higher strength and lower plasticity at both room temperature and 650 °C because of the higher dislocation density, while the sample with amorphous Y2Ti2O7 had higher plasticity with almost the same yield strength due to its higher dispersion strengthening. This study reveals the changes in Y2Ti2O7 structure during preparation process and suggests that the expected properties of ODS steels can be obtained by controlling the oxide structure.

Effect of pre-precipitation thermomechanical treatment on precipitation behavior of CLAM steel

China Low Activation Martensitic steel (CLAM) is a cladding material used in thermonuclear fusion reactors. Its high temperature performance is closely related to precipitation strengthening. The MX phase with strong thermal stability is the key to improving the high temperature performance of CLAM steel, and the M23C6 phase which is easy to coarsen under long-term high temperature service conditions is one of the important factors causing creep failure. In order to increase the content of MX phase and reduce the content of M23C6 phase, this paper first pre-precipitates near the precipitation temperature of M23C6 phase (which is also close to the maximum precipitation temperature of MX phase), which limits the precipitation of M23C6 phase and promotes the maximum precipitation of MX phase. Then the subsequent thermomechanical treatment (TMT) is carried out. The results show that the content of M23C6 phase in the samples after pre-precipitation TMT is greatly reduced, and the size is slightly reduced compared with the samples with only thermal deformation. The size of the MX phase is reduced by 10.6%, the volume density is increased by 88%, the volume fraction is increased by 34.1%, and the dislocation density is slightly increased. At the same time, it was found that a large number of MX phases precipitated at the lath boundary and pinned the boundary, which delayed the coarsening of the lath during tempering. The changes of these microstructures improve the precipitation strengthening, dislocation strengthening and boundary strengthening, so the room temperature tensile properties and high temperature tensile properties are improved, but the work hardening makes the total elongation and room temperature impact properties decrease.

Correlation between microstructure and hardening of 9Cr ferritic/martensitic steels irradiated in spallation neutron source

Significant hardening can be caused in ferritic/martensitic (FM) steels because of intensive displacement damage and transmutation helium/hydrogen when irradiated with high-energy protons and spallation neutrons. It is important but difficult to correlate microstructure and corresponding hardening of the FM steels irradiated in the Swiss spallation neutron source (SINQ). Based on the results of the China low activation martensitic (CLAM) steel and comparison with other 9Cr FM steels irradiated in SINQ up to 20.8 dpa/328 °C, total hardening of 9Cr FM steels was considered to be made up of helium bubbles, defect clusters/loops and α′ phase. Barrier strength of helium bubbles and clusters/loops in the CLAM specimens were obtained and investigated. Results show that the hardening of 9Cr FM specimens irradiated in SINQ increased with dose up to 20.8 dpa. After annealing at 600 °C, the hardening of FM specimens approximately showed a linear relationship with dose. Annealing made the clusters/loops in FM steels almost completely disappear under all irradiation conditions. In contrast, annealing changed the bubble size slightly and changed the bubble density to some degree. Barrier strength of helium bubbles in the CLAM specimens approximately increased linearly with size below 1.79 nm. Barrier strength of clusters/loops in the CLAM specimens also increased from 0.23 to 0.48 when their size increased from 3.5 nm to 19.1 nm. The obtained barrier strength of helium bubbles and loops in the CLAM steel is useful to explain the composition of irradiation hardening of other FM specimens irradiated in SINQ.

Evolution of microstructure and changes in hydrogen embrittlement resistance of CLAM steel after long-term aging

To investigate the changes in hydrogen embrittlement (HE) resistance of China Low Activation Martensitic (CLAM) steel during thermal aging, CLAM steels were aged at 550 °C and 600 °C up to 5000 h. The evolution of microstructure, including the evolution of subgrains, the growth of M23C6 carbides and MX carbonitrides, as well as the formation of Laves phase, were carefully characterized. The observed trend revealed an initial increase followed by a subsequent decrease in HE resistance upon prolonged aging durations. The mechanisms for changes in HE resistance were analyzed based on the orientation relationship between precipitates and matrix.

Evolution of interfacial microstructure and oxides of CLAM steel by hot compression bonding

The development of the joining process is essential for the application of China low activation martensitic (CLAM) steel as the structural material for fusion reactors. In the present study, the hot compression bonding (HCB) for CLAM steel was carried out under different parameters. The effects of different strain rates, temperatures, strains and holding time on the evolution of interfacial microstructure and oxides were investigated. Compressed at 1000 °C with 5% deformation, some micron-size continuous oxides distributed along the flat interface. The interfacial oxides were broken and decomposed as the deformation increased to 20%, their average size was reduced to 400 nm. With the bonding temperature increasing, more interfacial grain boundaries (IGBs) could cross the original bonding interface and flat interface became wavy. Compressed at 1000 °C/20%, the interfacial oxides consisted of fcc-structured spinel MnCr2O4 and enriched in Si and Ta oxides. The MnCr2O4 oxides would decompose preferentially after 0.5 h holding treatment at 1100 °C. Only a small number of oxides rich in Si and Ta elements remained at the original bonding area after 2 h holding treatment. Compressed at 1100 °C/20%, the bonding interface was completely replaced by the migrated grains after 2 h holding treatment and no large-size oxides could be observed. Moreover, the tensile strength and elongation at room temperature of the joint were comparable to those of the base material.

Ductile ultrastrong China low activation martensitic steel with lamellar grain structure

The low high-temperature mechanical properties of reduced activated ferrite martensitic (RAFM) heat-resistant steels limit the maximum operating temperature of nuclear fusion reactors. However, it is difficult to improve the strength of the material without reducing its plasticity. In this study we prepared a China low activation martensitic (CLAM) steel with a layered structure through hot rolling followed by low-temperature tempering, which successfully improved its strength and ductility. In addition, we elucidate the ductility mechanisms in such lamellar structural materials. When the strength of the grain boundaries exceeds that of the grains, cracks will continuously extend along the layered grain boundaries, eventually resulting in a delamination fracture mechanism. When the grain strength exceeds that of the grain boundaries, cracks will continuously form through the relative sliding of the grain boundaries, and a large number of small cracks will initiate and propagate longitudinally along the layered boundaries. Both forms of crack propagation cause longitudinal expansion along the layered grain boundaries. The resulting longitudinal extension of cracks markedly diminishes local stress concentration to promote plastic deformation, thereby significantly increasing the ductility of the material. Employing the combination of this delamination ductility mechanism with interface strengthening and precipitation strengthening, the strength of the CLAM steel for nuclear applications is shown to be enhanced by ∼30 % without loss of elongation. The current delamination ductility strategy provides a unique pathway to develop materials with ultrahigh strength and superior ductility at economical material costs.

Helium irradiation-induced segregation and mechanical response in China low activation martensitic (CLAM) steel

The China Low Activation Martensitic (CLAM) steel was irradiated by high energy helium ions under different ion doses, the microstructure evolution was studied by using a transmission electron microscope (TEM) and Energy Dispersive Spectrometer (EDS) mapping. The mechanical response of this material before and after irradiation was investigated under micropillar compression test. The results indicate that radiation leads to Cr segregation at the grain boundary and facilitates the growth of M23C6 precipitate. The pillar compression demonstrate that the strength of the CLAM steel increased due to irradiation-induced defects strengthening. The deformed micropillar is analyzed by TEM observation, indicating the cracks nucleated around the M23C6 precipitate. Furthermore, the effect of the M23C6 on mechanical behavior is discussed.

Benefits of high entropy alloy interlayer in the vacuum diffusion bonding of W-CLAM steel

In this paper, an equaimolar FeCoCrNi high entropy alloy (HEA) interlayer was used to join W and China Low Activation Martensitic steel (CLAM).One-step and two-steps diffusion bonding crafts were applied. It was found that reliable interfacial bonding was achieved through atoms diffusion between substrates and interlayer, and no Fe–W, Ni–W intermetallics formed at W/FeCoCrNi interface. Theoretical calculation indicated that W, Fe, Co, Cr and Ni elements tend to form single solid solution rather than intermetallics at certain percentage. Highest shear strength of the joints reached 224 ± 6 MPa which obtained using two steps bonding craft.

Deterioration of irradiation resistance of ODS-F/M steel under high concentration of helium

Simultaneous Fe+He ion irradiations of China Low Activation Martensitic (CLAM) steel and oxide-dispersion-strengthened ferritic/martensitic (ODS-F/M) steel were carried out at 350 °C-550 °C to the damage dose of 11 dpa with different He concentrations to investigate the effect of oxide particles on the evolution of He bubbles and dislocation loops. The microstructures of all irradiated samples were observed by transmission electron microscopy (TEM). The average size of oxide particles decreased and the density increased after irradiation. The aggregation of He bubbles was observed at the edge of oxide particles. As the He concentration increased, the aggregation of helium bubbles appeared at the edge of more oxide particles. Furthermore, He bubbles were not observed inside ODS-F/M steel grains or at grain boundaries, which suggests that the oxide particle interface is more capable of capturing helium atoms than other sinks. The average dislocation loop size of ODS-F/M steels is only 58% of that in CLAM steel at 450 °C, showing that ODS-F/M steel has excellent inhibition on the growth of dislocation loops. Nevertheless, with the increase of temperature and He concentration, the number density and size of dislocation loops in ODS-F/M steels gradually approach those in CLAM steels. The ability of ODS-F/M steels to inhibit irradiation hardening relative to CLAM steels increases with increasing temperature and decreases with increasing He concentration. The result of first-principles calculations shows that the formation energy of interstitial Fe atom at the interface increases with the increasing of the number of He atoms, which means that the aggregation of excessive He atoms at the interface will inhibit superior irradiation resistance of ODS-F/M steel.

Hydrogen isotope plasma-driven permeation through RAFM steel: isotope exchange and helium irradiation effect

Hydrogen isotope exchange in China low activation martensitic (CLAM) steel was investigated by plasma-driven permeation (PDP) measurements in the temperature range of ∼670–920 K. It was found that as H was introduced into D plasma, the permeation flux of HD increased while that of D2 decreased. The total D permeation flux, however, remained almost unchanged. Such a result allowed the analysis of isotope replacement and showed that the reduction of D2 is balanced by the rise of HD. In addition, the formation of HD was found to be closely related to the H and D content in plasma. Hydrogen isotopic effects on H and D plasma dissociation/ionization rate and on incident flux were evaluated by measuring plasma parameters including electron temperature and electron density using a triple Langmuir probe. The correlation between isotope effects on ionization rate in the plasma and isotope effects in the permeation flux was discussed. He plasma pre-irradiation and seeding during D PDP were also conducted to investigate the He effects on D diffusion. The total D permeation flux was found to be reduced by He seeding. Data analysis of Langmuir probe showed that ionization rate of D plasma was influenced by He seeding. Besides, under simultaneous D + He irradiation, He atoms were preferentially trapped at the defects resulting in a shallower D concentration gradient that described the observed decrease in D permeation rate. He plasma pre-irradiation was found to reduce D permeation flux with a much slower breakthrough to reach steady-state. The amount of D trapped at He-induced defects was derived quantitatively from the transient permeation curves. It is believed that He bubbles formed at the near surface act as trapping sites of D atoms, leading to a less effective diffusion coefficient and an enhanced retention in the material.

Effect of Ce on microstructure evolution of oxide scale for CLAM steel exposed to LBE containing 10−6 wt% oxygen at 500 ℃

Lead-cooled fast reactor (LFR) is one of the six types of fourth-generation advanced nuclear reactors, and the corrosion with liquid lead-bismuth eutectic (LBE) to structural materials is regarded as a major obstacle restricting the development of LFR. This study focused on the surface morphology of oxide scale, aiming to illustrate the evolution law of oxide scale and the mechanism of Ce effect on oxide scale. China low activation martensitic (CLAM) and Ce-containing CLAM (Ce-CLAM) specimens generated roughly circular and slat-like magnetite islands respectively, but their magnetite pellets had similar growth trends from small to large sizes. Ce delayed the evolution of magnetite, enhanced the adhesion, continuity and integrity of oxide scale for Ce-CLAM specimen. The mechanism model of Ce effect on the evolution of magnetite and the scale growth model of the two kinds of specimens were proposed.

Effect of heat treatment on microstructure and mechanical property of ODS-CLAM-Zr steel

Oxide dispersion strengthening (ODS) is one of the effective methods to improve the high temperature strength of many reduced activation ferritic/martensitic (RAFM) steels, alloys and even intermetallic compounds. This manuscript is focused on the ODS experiments for the improvement of China low activation martensitic (CLAM) steel with Zr addition. The tempered ODS-CLAM-Zr samples were prepared by mechanical alloying (MA) and hot isostatic pressing (HIP) followed with different heat treatments (HTs). The results showed that the samples after HT had a typical bimodal grain structure consisting of residual ferrite and martensite structures. The inhomogeneous structure of the samples may be caused by MA, and the ultrafine grains had good thermal stability at 1150°C. Higher tensile properties were obtained under normalizing at 1050°C and tempering at 750°C. The coarsening of the rod-shape ZrO2 phase at the grain boundaries with the increase of temperature was the main reason for the decrease of the strength of the samples. This work provides a reasonable HT for tempering ODS martensitic steels.

Establishment of multi-beam irradiation facility at Wuhan University

A multiple ion beam irradiation facility has been established at Wuhan University to simulate severe irradiation conditions to materials in advanced nuclear energy systems. The platform consists of a 2 × 1.7 MV tandem accelerator, a 200 kV ion implanter, and a 50 kV ion implanter. A single high-energy irradiation chamber and a triple-beam irradiation chamber were set up. China Low Activation Martensitic Steel (CLAM) samples were irradiated with single beam of iron ions and simultaneous dual-beam of iron ions and helium ions at 450 °C. A large number of dislocation loops were observed in the samples after both irradiation conditions. The number density and average size of dislocation loops in the dual-beam irradiated samples were significantly larger than those in the single-beam irradiated samples. Irradiation hardening in dual-beam irradiated samples is more severe than that in single-beam irradiated samples.

Effect of helium and its synergism with displacement damage on microstructure of CLAM steel investigated by single, sequential and simultaneous Fe and He ion irradiations

The reduced-activation ferritic/martensitic steel, China Low-Activation Martensitic (CLAM) steel, was irradiated with single Fe, sequential He/Fe, sequential Fe/He, and simultaneous Fe+He ion beam irradiations at 350 ℃–550 ℃ to a peak displacement damage dose of 15 dpa with 0 - 110 appm/dpa He to explore the effect of He and its synergism with displacement damage on microstructure evolution. The microstructures of all the irradiated specimens were investigated by transmission electron microscope (TEM). For Fe/He sequential irradiation, He inhibited the growth of dislocation loop induced by high-temperature He post-irradiation and reduced the number density of loops. Non-spherical He bubbles were observed only at post-injection He at 550 °C. The size of non-spherical He bubbles produced at 110 appm/dpa was about twice as large as 11 appm/dpa, and the number density increased about 6.4 times. The number density of dislocation loops induced by sequential He/Fe irradiation is the highest, and the average size of loops is comparable to that caused by simultaneous Fe+He irradiation. Moreover, the sequential Fe/He irradiation induced the lowest number density of loops of approximately 64% of the simultaneous Fe+He irradiation. The spherical bubble was observed in all irradiation with He injection above 350 ℃. The average size of spherical He bubbles induced by simultaneous Fe+He irradiation is the largest, and the number density is about 2.6 times that of the sequential Fe/He irradiation. The synergisms of He in different irradiation sequences were analyzed systematically, which helps to better understand the evolutionary processes of the synergistic effect of He and displacement damage.

Stabilities of the precipitates in CLAM steel during 30,000 h thermal aging

China Low Activation Martensitic steel (CLAM steel) was thermally aged for up to 30,000 h at 550 and 600 °C. Scanning electron microscopy and transmission electron microscopy were employed to characterize the evolutions of the Laves phase, the M23C6 phase and the MX phase. The relationships between the particle sizes of the three precipitation phases with time, and the Johnson-Mehl-Avrami (J-M-A) equation for the growth kinetics of the Laves phase during thermal aging at 600 °C were obtained. The results show that the relationship between the particle size of the Laves phase with time is different from those of the M23C6 phase and the MX phase; the Laves phase has the largest coarsening rate and the worst stability; the time exponent of the J-M-A equation of the Laves phase is large (n=2.8). In 9–12%Cr tempered ferritic/martensitic steels, the precipitation of the Laves phase tends to lag behind the M23C6 phase. The Laves phase has two nucleation modes: independent nucleation and dependent nucleation adhering to the M23C6 phase. There is no unified understanding of how the growths of the two phases in the assemblage affect each other after dependent nucleation. In this study, an enwrapping growth mechanism of the Laves phase was observed, which provides a new clue to this question. The dependent nucleation and enwrapping growth mechanism of the Laves phase is the reason for the linear growth of its particle size and the large time exponent.

Corrosion behavior and mechanism of CLAM and 316L steels in flowing Pb–17Li alloy under magnetic field

The liquid lead-lithium (Pb–17Li) blanket has many applications in fusion reactors due to its good tritium breeding performance, high heat transfer efficiency and safety. The compatibility of liquid Pb–17Li alloy with the structural material of blanket under magnetic field is one of the concerns. In this study, corrosion experiments China low activation martensitic (CLAM) steel and 316L steel were carried out in a forced convection Pb–17Li loop under 1.0 T magnetic field at 480 °C for 1000 h. The corrosion results on 316L steel showed the characteristic with a superficial porous layer resulted from selective leaching of high-soluble alloy elements and subsequent phase transformation from austenitic matrix to ferritic phase. Then the porous layers were eroded by high-velocity jet fluid. The main corrosion mechanism of CLAM steel was selective dissolution-base corrosion attack on the microstructure boundary regions and exclusively on high residual stress areas. CLAM steel performed a better corrosion resistance than that of 316L steel. The high Ni dissolution rate and the erosion of corroded layers are the main causes for the severe corrosion of 316L steel.

Fabrication and assessment of China low activation martensitic (CLAM) steels solid diffusion bonded joint

Solid diffusion bonding of China low activation martensitic (CLAM) steels was performed. The bonding process was designed based on the austenite transformation, and the austenite transformation information was obtained by using high-resolution dilatometer prior to diffusion bonding. The reliable joining between CLAM/CLAM steels was obtained by using a two-step diffusion bonding procedure. Ultrasonic non-destructive testing revealed no lack-of-bonding porosities at the diffusion bonding interface. Details on interfacial microstructure and mechanical properties of diffusion bonded joint were investigated. Microstructure analysis showed the uniform distribution of elements across the joint, and there was no significant microstructure gradient at the vicinity of the joint. The tensile properties of diffusion bonded samples are almost identical to those of base metal. However, the fracture position of the diffusion bonded samples was located on the base materials, which is related to the finer martensite near the joint zone.

Effect of thermomechanical treatment on MX phase precipitation behavior in CLAM steel

Reduced Activation Ferritic/Martensitic (RAFM) steels are the preferred first wall structural materials in nuclear fusion experimental reactors at this stage, which puts forward high requirements for its creep properties. Previous references point out that the main factor for the creep resistance of RAFM steel is the massively precipitated MX phases in the matrix. Thermomechanical treatment (TMT) technology can promote the precipitation of a large number of nano-sized MX phases, and has great potential for improving the creep strength of RAFM steels. In this paper, China Low Activation Martensitic (CLAM) steel is taken as the research object, and four different thermomechanical treatment processes are designed for comparison. The purpose is to deepen our understanding of the effects of austenitization temperature, deformation temperature and strain on the precipitation of MX phases. It is found that the number density of MX phase after TMT increases by two orders of magnitude, and the average particle size decreases by about 79% when the austenitization temperature increases from 1000 to 1150 °C, while the effect of deformation temperature and strain is relatively small; after TMT, the specimen with the largest number and the smallest size of MX phase has the best creep performance, and vice versa.

Optimization of Electron Beam Welding Joint of Water-Cooled Ceramic Breeder Blanket.

The water-cooled ceramic breeder (WCCB) blanket is a component of the China Fusion Engineering Test Reactor (CFETR). The Reduced Activation Ferrite/Martensite (RAFM) steels are the preferred structural materials for WCCB blanket. As a kind of RAFM steels, China low activation martensitic (CLAM) steel was welded by electron beam welding (EBW), and then quenched-tempered treatment was carried out. The results show that at the welding state, the welding seam was composed of large martensite and δ ferrite and the organization of the heat-affected zone (HAZ) was changed slightly with the different heat input. Moreover, the hardness of welded joints was higher than that of base material (BM), but the impact toughness was very low. After quenched-tempered treatment, the δ ferrite in the weld was eliminated, the residual stress of the test plate decreased as a whole, and the mechanical properties were improved significantly.

Effect of zirconium on inclusions and mechanical properties of China low activation martensitic steel

The effects of 0.01–0.11 wt.% Zr on the inclusions, microstructure, tensile properties, and impact toughness of the China low activation martensitic steel were investigated. Results showed that Zr exhibits good deoxidation and desulfurization abilities. The scanning electron microscope was used to examine the inclusions in the ingots. The main inclusions in the alloys were Zr–Ta–O, Zr–O, and Zr–O–S. However, some blocky Zr-rich inclusions appeared in Zr-2 and Zr-3 alloys. Typical martensitic structures were observed in the alloys, and average prior austenite grain sizes of 21.1, 15.7, and 14.8 µm were obtained for Zr-1, Zr-2, and Zr-3 steels, respectively. However, increasing Zr content of the steels deteriorated their mechanical property, owing to the blocky inclusions. The alloy with 0.01% Zr resulted in excellent mechanical properties due to the fine inclusions and the precipitation of Zr3V3C carbides. Values of 576 and 682 MPa were obtained for the yield strength and ultimate tensile strength of Zr-1 alloy, respectively. Furthermore, the ductile–brittle transition temperature of the alloy decreased to − 85 °C.