Low Activation Martensitic Steel Research Articles

Characterisation of multiphase ceramic coatings fabricated via laser in situ reaction technology

ABSTRACTCoatings are pre-fabricated on the China low activation martensitic (CLAM) steel via thermal spraying using Cr–Fe–Al–Ti mixed powders. Crystalline and amorphous multiphase ceramic coatings are formed by a new laser surface in situ prefabricated coatings in situ reaction technology under atmospheric environments based on the Standard Gibbs free-energy theory. The grain boundary of incomplete recrystallisation section and perfect crystallisation section can be observed clearly by high-resolution transmission electron microscopy. The Al2O3 and TiO2 are found to be formed in the crystallisation section. In addition, we adopt a new method to measure bonding strength of coating and base material, and the average shear strength is found to be 290 MPa. Further finite-element analysis shows that the shear stress distribution between the coatings and substrate is consistent with the experimental data. The average microhardness of the coatings is found to reach 1864.19 HV0.2.The coatings also exhibit the strongest liquid Pb corrosion resistance. This technique may be important in technological improvement of surfaces of the candidate structural materials for the accelerator-driven sub-critical system, which is usually applied to handle the nuclear waste under extreme conditions.

Corrosion behavior of as-weld CLAM steel after laser shock peening process and tempering in static molten lithium lead

China low activation martensitic steel was welded by gas tungsten arc welding, after that laser shock peening and tempering treatment are applied to as-weld material, its compatibility with molten lithium lead are studied. Tempering treatment weight loss is 1.32 mg/cm2 compared to LSP is 1.91 mg/cm2 after 3000 h. Tempered martensite is more excellent than coarse martensite, as-weld surface corrosion grooves are serious. Passivation layers protect the internal metal from destroy in molten metal and it firstly appeared in defective areas during LSP process. Main elements markedly reduced through dissolving into molten metal, microstructure and phase transformation took place along the whole corrosion surface.

Fracture toughness and fracture behavior of CLAM steel in the temperature range of 450 °C–550 °C

In order to analyze the fracture toughness and fracture behavior (J-R curves) of China Low Activation Martensitic (CLAM) steel under the design service temperature of Test Blanket Module of the International Thermonuclear Experimental Reactor, the quasi-static fracture experiment of CLAM steel was carried out under the temperature range of 450 °C–550 °C. The results indicated that the fracture behavior of CLAM steel was greatly influenced by test temperature. The fracture toughness increased slightly as the temperature increased from 450 °C to 500 °C. In the meanwhile, the fracture toughness at 550 °C could not be obtained due to the plastic deformation near the crack tip zone. The microstructure analysis based on the fracture topography and the interaction between dislocations and lath boundaries showed two different sub-crack propagation modes: growth along 45° of the main crack direction at 450 °C and growth perpendicular to the main crack at 500 °C.

Microstructure evolution and toughness degeneration of 9Cr martensitic steel after aging at 550 °C for 20000 h

The China Low Activation Martensitic (CLAM) steel is one of 9Cr martensitic steels, and it has been selected as the primary candidate structural material for future fusion reactors in China. It is essential to understand the thermal stability of CLAM steel to ensure the operation safety during exposures at operating temperatures. In this paper, the thermal aging behavior of CLAM steel at 550 °C for 20000 h was studied. Microstructure evolution and mechanical property change were investigated. The results showed that Laves phase was mainly precipitated along the martensitic lath boundaries and grain boundaries in CLAM samples after aging for more than 4000 h, and its size increased faster than that of M23C6. The size of TaC had no obvious changed but its number increased after aging. The microstructure evolution resulted to degradation of the mechanical property, especially the ductile-to-brittle transition temperature (DBTT), which was increased from − 92 to − 43 °C after aging for 20000 h. The possible mechanism of DBTT increasing had been analyzed based on the interaction between precipitations and grain boundaries.

Microstructure anisotropy and its effect on mechanical properties of reduced activation ferritic/martensitic steel fabricated by selective laser melting

Selective laser melting (SLM) is a promising way for the fabrication of complex reduced activation ferritic/martensitic steel components. The microstructure of the SLM built China low activation martensitic (CLAM) steel plates was observed and analyzed. The hardness, Charpy impact and tensile testing of the specimens in different orientations were performed at room temperature. The results showed that the difference in the mechanical properties was related to the anisotropy in microstructure. The planer unmelted porosity in the interface of the adjacent layers induced opening/tensile mode when the tensile samples parallel to the build direction were tested whereas the samples vertical to the build direction fractured in the shear mode with the grains being sheared in a slant angle. Moreover, the impact absorbed energy (IAE) of all impact specimens was significantly lower than that of the wrought CLAM steel, and the IAE of the samples vertical to the build direction was higher than that of the samples parallel to the build direction. The impact fracture surfaces revealed that the load parallel to the build layers caused laminated tearing among the layers, and the load vertical to the layers induced intergranular fracture across the layers.

Deuterium retention removal in China reduced activation ferritic-martensitic steels through thermal desorption and hydrogen isotope exchange

Effective removal of deuterium and tritium trapping in fusion reactor materials is of great importance both to recycle fuel and prevent the performance degradation. Experiments on deuterium removal in China Low Activation Martensitic (CLAM) steel and China Low-activation Ferritic (CLF-1) steel through thermal desorption and hydrogen isotope exchange have been performed. The results indicate that CLAM steel has only one desorption peak at 578K with an activation energy of 21kJ/mol, while CLF-1 steel has two desorption peaks, whose corresponding activation energy is 31kJ/mol at 563K and 94kJ/mol at 797K respectively. After 2h thermal desorption, deuterium removal efficiency for CLAM is more than 90% above 373K. Yet for CLF-1, deuterium removal efficiency is less than 90% below 573K. Hydrogen isotope exchange can slightly enhance deuterium release. Overall, Deuterium retention in both materials can be removed effectively at relatively low temperature (<673K) through thermal desorption due to the high hydrogen diffusivity, especially for CLAM steels.

Growth of inter-metallic compound layers on CLAM steel by HDA and preparation of permeation barrier by oxidation

Hot-dip aluminizing (HDA) on China Low Activation Martensitic (CLAM) steel and followed by oxidation to obtain an Al2O3/FexAly layer covering was considered to be a promising method to resist the harsh environments in fusion reactors. In the present study, CLAM steel was coated in molten pure Al and Al-0.3Ce (wt.%) melt. The growth behavior of the inter-metallic compound (IMC) layers after different immersion time and temperature was investigated and described. The IMC layer mainly consists of Fe2Al5 and FeAl3 after hot-dipping. And the addition of Ce in the molten pure Al could promote the growth of IMC layers, which provided the idea of obtaining a certain thickness of the IMC layer at a lower temperature. After two independent oxidation processes (normalized heat-treatment and 760°C for 15h) in air, the Fe2Al5 and FeAl3 of IMC layers had been transformed into ductile phases (FeAl2 and FeAl), and Al2O3 layers were detected on the coating surface. The oxidation of 760°C for 15h was considered to form a denser Al2O3 film. However, cracks and pores were observed in the IMC layer after oxidation due to the heat-treatment and cooling methods

Cavitation erosion behavior of CLAM steel weld joint in liquid lead-bismuth eutectic alloy

The cavitation erosion of weld joint and base metal of China low activation martensitic (CLAM) steel in liquid lead-bismuth eutectic alloy (LBE) at 550 °C was investigated to simulate the cavitation erosion of the first wall and the nuclear main pump impeller in the accelerator driven sub-critical system (ADS). A suit of ultrasonic cavitation facility was self-designed to study the cavitation erosion. By studying the surface micro topography, roughness and mean pit depth of the tested specimens, it was found that some crater clusters and large scale cracks appeared on the tested specimen surface after the formation of numerous single craters, and the base metal exhibited much better cavitation erosion resistance than the weld bead due to the difference in their mechanical properties and microstructures. In addition, by comparing the results of static corrosion and cavitation erosion, it could be concluded that the cavitation erosion and the dissolution and oxidation corrosion in liquid LBE would accelerate mutually.

Status and improvement of CLAM for nuclear application

A program for China low activation martensitic steel (CLAM) development has been underway since 2001 to satisfy the material requirements of the test blanket module (TBM) for ITER, China fusion engineering test reactor and China fusion demonstration reactor. It has been undertaken by the Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences under wide domestic and international collaborations. Extensive work and efforts are being devoted to the R&D of CLAM, such as mechanical property evaluation before and after neutron irradiation, fabrication of scaled TBM by welding and additive manufacturing, improvement of its irradiation resistance as well as high temperature properties by precipitate strengthening to achieve its final successful application in fusion systems. The status and improvement of CLAM are introduced in this paper.

Experiments on the magnetostriction properties of the China low activation martensitic steel

In this paper, the magnetostriction properties of China Low Activation Martensitic (CLAM) steel, a typical Reduced-activation Ferritic/Martensitic steel developed for fusion reactor application, have been experimentally investigated. With a magnetic field from 0 to 1T and tensile stress from 0 to 130MPa, the correlations of strain-magnetic field intensity in several experimental conditions have been measured. Stress was found giving significant influence on the magnetostriction properties of the CLAM steel. A constitutive model is proposed then for describing the magnetostriction property of the CLAM steel based on the experimental results. The proposed constitutive relation between the magnetic field and the magnetostriction strain gives good description of the observed phenomena and is in good agreement with the experimental results in the middle strength range of applied magnetic field. This work provides a basic constitutive relation for the magneto-mechanical coupling analysis of structures of the CLAM steel in Tokamak fusion reactors.

Microstructure and mechanical properties of China low activation martensitic steel joint by TIG multi-pass welding with a new filler wire

Tungsten Inner Gas (TIG) welding is employed for joining of China low activation martensitic (CLAM) steel. A new filler wire was proposed, and the investigation on welding with various heat input and welding passes were conducted to lower the tendency towards the residual of δ ferrite in the joint. With the optimized welding parameters, a butt joint by multi-pass welding with the new filler wire was prepared to investigate the microstructure and mechanical properties. The microstructure of the joint was observed by optical microscope (OM) and scanning electron microscope (SEM). The hardness, Charpy impact and tensile tests of the joint were implemented at room temperature (25 °C). The results revealed that almost full martensite free from ferrite in the joints were obtained by multipass welding with the heat input of 2.26 kJ/mm. A certain degree of softening occurred at the heat affected zone of the joint according to the results of tensile and hardness tests. The as welded joints showed brittle fracture in the impact tests. However, the joints showed toughness fracture after tempering and relatively better comprehensive performance were achieved when the joints were tempered at 740 °C for 2 h.

Synergistic Effect on Formation of Radiation Damage in CLAM Steel Studied by Triple Beam Irradiation

The synergistic effect associated with displacement damage, hydrogen and heliumin the China Low Activation Martensitic (CLAM) steel has been investigated using the triple ion beamirradiation. Triple ion beams, an iron beam of 109 MeV degraded by a tantalum foil of 7.45 μm thick, the100 keV hydrogen and 200 keV helium, were injected into the CLAM steel samples simultaneously or sequentially.The radiation damage examinations were carried out by the slow positron Doppler broadening technique. Themeasured S parameters indicate that the radiation damage is different for different irradiationprocedures with same dpa and concentrations of H and He. The sample suffers most severe damage in the simultaneoustriple beam irradiation. The present experimental results support the molecular dynamics simulation result that the H facilitates the He-bubble nucleation and growth.

The Corrosion Behavior of Multiphase Ceramic Protective Coatings on China Low Activation Martensitic Steel Surface in Flowing Liquid Lead-Bismuth Eutectic

China low activation martensitic (CLAM) steel is the preferred candidate structural material for the Chinese international thermonuclear experimental reactor test blanket module (ITER TBM). Surface coating technology is often adopted to alter or improve the performance of materials. In CLAM steel, surface treatment plays a key role in rendering them appropriate for application in accelerator driven sub-critical system (ADS). Here, the authors prepare multiphase ceramic coatings on CLAM steel via laser in-situ reaction technology. The prepared multiphase ceramic coatings exhibit optimal corrosion resistance. The liquid lead-bismuth eutectic (LEB) corrosion resistance can be attributed to inferior wettability of interface between the LEB and multiphase ceramic coatings, faint shear force produced by LEB, and stable chemical and physical property of the coatings. Moreover, the irradiated specimens also display excellent corrosion resistance against helium ions irradiation. The CLAM steel is found to suffer significant corrosion in the flowing LEB. Such multiphase ceramic coatings as a protective layer on the ADS candidate structural materials CLAM steel surface exhibit the strongest LEB corrosion resistance, which can be applied to handle nuclear waste under extreme conditions.

Thermal–Mechanical Responses of the First Wall in CFETR Under Transient Events

In this paper, the thermal–mechanical responses of the first wall (FW) composed of pure tungsten (W) and China Low Activation Martensitic steel in China Fusion Engineering Test Reactor are predicted under the normal state and the transient events, i.e., major disruption (MD), vertical displacement events (VDEs) and edge localized modes (ELMs), using finite element method (FEM). The temperature distribution in the armor material and structural material is predicted and analyzed, and the temperature induced thermal stress is obtained through a fully coupled thermal-stress analysis module. The comparative simulation results reveal that the short duration time of MD and ELMs have less thermal/mechanical effects on the FW than that of the long deposition time of VDEs.

Effect of thermal aging on grain structural characteristic and Ductile-to-Brittle transition temperature of CLAM steel at 550 °C

In this work, electron backscatter diffraction (EBSD) was used to investigate the grain structure evolution of China low activation martensitic (CLAM) steel samples which were aged at 550°C for 0h, 2000h, 4000h and 10,000h. The results showed that the prior austenitic grain size increased with the aging time, which led to the decrease of grain boundary length. The fraction of misorientation angle in a range from about 4 to 10° increased obviously after thermal aging for 10,000h, and it indicated that the fine subgrains formed in the CLAM steel during the long-term thermal exposure. Furthermore, Charpy impact experiments were carried out to analyze the toughness of the CLAM steel before and after aging, particularly the Ductile-to-Brittle Transition Temperature (DBTT). Though amounts of fine subgrians formed in matrix, a substantial increase in DBTT (∼40.1°C) had been noticed after aging for 10,000h. The results showed that the high angle boundaries such as prior austenitic grain boundaries are more effective in retarding the propagation of cleavage crack than subgrain boundaries.

Microstructure evolution and yield strength of CLAM steel in low irradiation condition

The microstructure and yield strength of China low activation martensitic (CLAM) steel were observed and tested. The evolution of defects, including He bubbles and dislocation loops, and MC carbides were analyzed. According to the microstructure observation results, a two-step simulation model was established to analyze the relationship between microstructure and strength. For the first step, a rate theory model was used to simulate the evolution of defects (He bubbles and dislocation loops). The incubation, nucleation and growth stage of defects were shown by simulation results. For the second step, the simulation results of defect evolution were added into a superposition strengthening model, which combined different strengthening methods, to simulate the yield strength of CLAM steel in high temperature and irradiation condition. Based on both experimental and simulation results, the effect of high temperature and irradiation on microstructure and strength was discussed in CLAM steels. The simulation results of yield strength were basically consistent with experimental results.

Low cycle fatigue properties of CLAM steel at 450 °C and 550 °C

The low cycle fatigue behavior of China Low Activation Martensitic (CLAM) steel has been studied using a constant strain rate of 8×10−3/s with the strain amplitudes ranging from 0.3% to 0.8% at 450°C and 550°C. Cyclic stress response showed a gradual softening until complete failure. The fatigue life decreased with increasing test temperature, and the effect of temperature on fatigue life was more pronounced at lower strain amplitudes. The cyclic deformation behavior at different temperatures has been analyzed according to the hysteresis loop, and the mechanism of cyclic softening was interpreted in view of the changes taking place in dislocation density and lath structures. Evaluation of low cycle fatigue properties of CLAM steel at 450°C and 550°C can help in design of the Chinese Test Blanket Module (TBM) for the International Thermonuclear Experimental Reactor (ITER) and a future fusion power plant.

Effect of weld spacing on microstructure and mechanical properties of CLAM electron beam welding joints

China low activation martensitic (CLAM) steel has been chosen as the primary structural material in the designs of dual function lithium-lead (DFLL) blanket for fusion reactors, China helium cooled ceramic breeder (HCCB) test blanket module (TBM) for ITER and China fusion engineering test reactor (CFETR) blanket. The cooling components of the blankets are designed with high density cooling channels (HDCCs) to remove the high nuclear thermal effectively. Hence, the welding spacing among the channels are small. In this paper, the welded joints of CLAM steel with different weld spacings have been fabricated with electron beam welding (EBW). The weld spacing was designed to be 2mm, 3mm, 4mm, 6mm and 8mm. The microstructure and mechanical properties such as microhardness, impact and tensile were investigated at different welding spacing for both conditions of as-welded and post weld heat treatment (PWHT). The PWHT is tempering at 740°C for 120min. The results showed that the grain size in the heat affected zone (HAZ) increased with the increasing weld spacing, and the joint with small weld spacing had a better performance after PWHT. This work would give useful guidance to improve the preparation of the cooling components of blanket.

Microstructure evolution of the oxide dispersion strengthened CLAM steel during mechanical alloying process

Abstracts Oxide dispersion strengthened Ferritic/Martensitic steel is considered as one of the most potential structural material for future fusion reactor, owing to its high mechanical properties and good irradiation resistance. The oxide dispersion strengthened China Low Activation Martensitic (ODS-CLAM) steel was fabricated by mechanical alloying (MA) and hot isostatic pressing (HIP). The microstructural evolutions during the process of ball milling and subsequent consolidation were investigated by SEM, XRD and TEM. The results showed that increasing the milling time during the first 36 h milling could effectively decrease the grain size to a value of around 30 nm, over which grain sized remained nearly constant. Increasing the rotation speed promoted the solution of tungsten (W) element obviously and decreased the grain size to a certain degree. Observation on the consolidated and further heat-treated ODS-CLAM steel samples indicated that a martensite microstructure with a high density of nano-particles was achieved.

Effects of thermal aging on microstructure and hardness of China low activation martensitic steel welded joint

The aim of this paper is to investigate the microstructure evolution and the change in hardness distribution of China low activation martensitic steel welded joints after thermal aging at 550°C for 6000h. The joint was processed by electron beam welding. Compared to the base metal (BM) and heat affected zone (HAZ), Laves-phase was not formed in weld metal (WM) in the as-aged condition due to the higher tantalum content and less precipitation in WM before aging. The dislocation density decreased in HAZ and WM after aging for 6000h. The property results showed that hardness of WM and HAZ was decreased significantly after aging for 6000h due to the weakening of solution strengthening and dislocations strengthening. However, the change in the hardness of the base metal by aging remained at a minor level.