Low Activation Martensitic Steel Research Articles

Microstructures and Mechanical Properties of Laser Welding Joint of a CLAM Steel with Revised Chemical Compositions

To suppress the tendency to form delta ferrite in weld metal (WM) of China low activation martensitic (CLAM) steel joint, a CLAM steel with revised chemical compositions was designed. Laser welding of the CLAM steel was investigated. The microstructures of the WM and heat-affected zone were analyzed. The impact toughness of the WM was evaluated by a Charpy impact test method with three V notches. The influence of temper temperature on mechanical properties was analyzed. It was found that the delta ferrite was eliminated almost completely in laser WM of CLAM steel with revised chemical compositions which has lower tendency to form delta ferrite than original chemical compositions. The joint has higher tensile strength than the parent metal. With increasing the heat input, the impact toughness of the joint is approximatively equal with that of parent metal first and then decreases obviously. Temper treatment could effectively improve mechanical property of the joint. When the temper temperature exceeds 600 °C, the impact toughness of the joint is higher than that of the parent metal.

Effect of tantalum content on microstructure and tensile properties of CLAM steel

Four ingots of China Low Activation Martensitic (CLAM) steel with different Tantalum (Ta) contents of 0.027wt%, 0.078wt%, 0.15wt% and 0.18wt%, respectively, were produced by vacuum induction furnace. Microstructure observation and tensile tests were performed to make clear the effect of Ta content on the microstructure and properties of CLAM steel. Experimental results showed that the content of Ta-riched MX particles increased with Ta content increasing from 0.027wt% to 0.18wt%, and the Cr-riched M23C6 carbides reversed. Meanwhile, the grain size became finer with the increase of Ta content. However, the effect of Ta content on grain size refinement weakened when Ta content was higher than 0.15wt% in this study. Both the grain size and precipitates could affect the tensile properties of the four ingots. The precipitation strengthening of Cr-riched M23C6 carbides was considered to be the main possible reason that CLAM steel with Ta content of 0.027wt% had the highest strength. In general, there was little difference in strength of the four ingots with the change of Ta content.

Effects of strain rate on the hot deformation behavior and dynamic recrystallization in China low activation martensitic steel

To investigate the effects of strain rate on dynamic recrystallization (DRX) behavior on China low activation martensitic steel, hot uniaxial compression tests with strain rates ranging from 0.1s−1 to 10s−1 and deformations amounts of 40% and 70% where conducted. The true stress–true strain curves were analyzed for the occurrence of DRX under the different strain rates and compressive deformation amounts. The steel microstructures were examined and linked to the observed stress-strain diagrams to study DRX. Results show that DRX was responsible for refining the grain structure over a wide range of strain rates under 70% deformation. However, significant DRX occurred only at the relatively low strain rate of 0.1s−1 under 40% deformation. The original elongated microstructure of the rolled plate from which the specimens were taken was replaced by dynamic recrystallization grains. At 70% deformation, the average grain size was 4.2μm at a strain rate of 0.1s−1, 2.5μm at a strain rate of 5s−1, 1.8μm at a strain rate of 10s−1. In conclusion, with increasing strain rate, the recrystallized grain size decreased and the peak stress increased.

Diffusive transport parameters of deuterium through China reduced activation ferritic-martensitic steels

Reduced Activation Ferritic/Martensitic (RAFM) steels have been considered as the most promising candidate structure materials for a fusion reactor. In the recent decades, two new types of RAFM steels, called China Low Activation Martensitic (CLAM) steel and China Low-activation Ferritic (CLF-1) steel, have been developed. The gas evolution permeation technique has been used to investigate diffusive transport parameters of deuterium through CLAM and CLF-1 over the temperature range 623 ∼ 873 K at deuterium pressure of 105 Pa. The resultant transport parameters are: Φ (mol. m−1 s−1 Pa−1/2) = 5.40 × 10−8 exp (−46.8 (kJ. mol−1)/RT), D(m2 s−1) = 3.81 × 10−7 exp(−24.0(kJ. mol−1)/RT) and S (mol. m−3 Pa−1/2) = 1.42 × 10−1 exp(−22.8(kJ. mol−1)/RT) for CLAM; while Φ(mol m−1 s−1 Pa−1/2) = 1.76 × 10−8 exp(−43.9(kJ. mol−1)/RT), D(m2. s−1) = 1.02 × 10−7 exp(−16.9(kJ. mol−1)/RT) and S(mol. m−1 Pa−1/2) = 1.73 × 10−1 exp(−27.0(kJ. mol−1) /RT) for CLF-1. The results show that CLAM is more permeable than CLF-1, thus it is easier for hydrogen isotopes to transport and be removed.

Effect of Thermal Aging on Microstructure and Mechanical Properties of China Low-Activation Martensitic Steel at 550 °C

Abstract The thermal aging effects on mechanical properties and microstructures in China low-activation martensitic steel have been tested by aging at 550 °C for 2,000 hours, 4,000 hours, and 10,000 hours. The microstructure was analyzed by scanning and transmission electron microscopy. The results showed that the grain size and martensitic lath increased by about 4 μm and 0.3 μm, respectively, after thermal exposure at 550 °C for 10,000 hours. MX type particles such as TaC precipitated on the matrix and Laves-phase was found on the martensitic lath boundary and grain boundary on aged specimens. The mechanical properties were investigated with tensile and Charpy impact tests. Tensile properties were not seriously affected by aging. Neither yield strength nor ultimate tensile strength changed significantly. However, the ductile–brittle transition temperature of China low-activation martensitic steel increased by 46 °C after aging for 10,000 hours due to precipitation and grain coarsening.

Microstructure and microhardness of CLAM steel irradiated up to 20.8 dpa in STIP-V

Specimens of China low activation martensitic (CLAM) steel were irradiated in the fifth experiment of SINQ target irradiation program (STIP-V) up to 20.8 dpa/1564 appm He. Microhardness measurements and transmission electron microscope (TEM) observations have been performed to investigate irradiation induced hardening effects. The results of CLAM steel specimens show similar trend in microhardness and microstructure changes with irradiation dose, compared to F82H/Optimax-A steels irradiated in STIP-I/II. Defects and helium bubbles were observed in all specimens, even at a very low dose of 5.4 dpa. For defects and bubbles, the mean size and number density increased with increasing irradiation dose to 13 dpa, and then the mean size increased and number density decreased with the increasing irradiation dose to 20.8 dpa.

Tensile properties of CLAM steel irradiated up to 20.1 dpa in STIP-V

Specimens of China low activation martensitic steel (CLAM) were irradiated in the fifth experiment of SINQ Target Irradiation Program (STIP-V) up to 20.1 dpa/1499 appm He/440 °C. Tensile tests were performed at room temperature (R.T) and irradiation temperatures (Tirr) in the range of 25–450 °C. The tensile results demonstrated strong effect of irradiation dose and irradiation temperature on hardening and embrittlement. With Tirr below ∼314 °C, CLAM steel specimens tested at R.T and Tirr showed similar evolution trend with irradiation dose, compared to other reduced activation ferritic/martensitic (RAFM) steels in similar irradiation conditions. At higher Tirr above ∼314 °C, it is interesting that the hardening effect decreases and the ductility seems to recover, probably due to a strong effect of high irradiation temperature.

Transient liquid phase bonding of CLAM/CLAM steels with Ni-based amorphous foil as the interlayer

Transient liquid phase bonding is employed to join China low activation martensitic (CLAM) steels, with Ni-based amorphous foil as interlayer. The effects of the Ni-based amorphous foil on the microstructure and mechanical performance of the joint are investigated. During bonding, as the temperature reaches to the start temperature of austenite transformation, boron, a melting point depressant in amorphous foil, can diffuse along austenite boundaries, resulting into compounds containing boron and the fine-grain zones adjacent to the interlayer. Owing to the melting of interlayer and the dissolution of substrate material, the final width of the interlayer is increased. Moreover, at the interlayer close to the joint interface, small voids can be observed, associated with the difference between Fe and Ni in diffusion rates. It should also be noted that the bonded CLAM/CLAM steels always fracture at the interlayer.

High cycle fatigue properties of CLAM steel at 723 K and 823 K

This paper highlights the results of a study on the high cycle fatigue strength and fracture mechanism of China Low Activation Martensitic (CLAM) steel. The high cycle fatigue test results showed that the fatigue strength of CLAM steel decreased with the temperature, and the condition fatigue strengths (N=107) were 275MPa and 235MPa at 723K and 823K, respectively. The fractograph results indicated that the fractures were mainly initiated from the surface of the specimen.

Effect of Tantalum Content on Creep Properties of CLAM Steel

This paper presents the results of an experimental study about the effect of tantalum content on the creep properties of China low activation martensitic steel. Four heats of the steel with tantalum content ranging from 0.027 to 0.18 wt% have been fabricated and the creep tests have been carried out at 550 °C over a stress range of 230–240 MPa. The results indicated that the elevation of tantalum content caused an obvious increase in the rupture time of one order of magnitude and a dramatically decrease in creep rate for one order of magnitude. Both the creep rupture time and minimum creep rate revealed good linear relationships versus tantalum content on log–log scale, and empirical models were built up. The improvement of creep properties could be attribute to the change of microstructure caused by the increase of tantalum content. The microstructure observation indicated that the average grain size of the prior austenite decreased and the precipitation of Ta-riched MX was promoted with the addition of tantalum.

Effect of post-weld heat treatment on the mechanical properties of CLAM/316L dissimilar joint

Dissimilar welding between China low activation martensitic (CLAM) steel and 316L austenitic stainless steel was investigated to achieve the reliable connection between test blanket modules (TBMs) and piping system in the international thermonuclear experimental reactor (ITER). The dissimilar joints were welded by tungsten inert gas (TIG) welding process with a filler material type-309. In order to stabilize the microstructure and improve the strength and toughness, post-weld heat treatments (PWHTs) of tempering at 740°C, 780°C and 820°C, respectively, for 2h were performed. The microstructure observation showed that tempering at 740°C for 2h was the preferable PWHT rule in this work. After the treatment, the hardening in heat affected zone (HAZ) on the CLAM steel side decreased remarkably. The tensile strength of the joint was roughly the same as that of the base metal. The impact toughness of HAZ on the CLAM steel side was 77% of that of the base metal. The absorbed energy of HAZ of 316L steel decreased by 93J, and that of weld metal (WM) was 110J after the treatment.

Small Specimen Test Techniques Applied to Evaluate the Mechanical Properties of CLAM Steel

A study of small specimen test techniques applied to examine the mechanical properties of fusion candidate structural material was presented. Tensile and instrumented impact tests were performed with sub-size and standard size specimens, respectively, to evaluate the mechanical properties of China low activation martensitic steel. The ultimate strength and uniform elongation of sub-size specimens were almost equal to those of the standard size specimens at all the three test temperatures. But the sub-size specimens showed slightly higher yield strength than the standard size specimens. With Sokolov method, the calibrated DBTTs obtained for the sub-size Charpy specimens were almost equal to those of the standard size specimens. These results of the sub-size specimens had a good agreement with those of the standard size specimens.

Formation of α-alumina scales in the Fe–Al(Cr) diffusion coating on China low activation martensitic steel

To study the formation mechanism of stable α-Al2O3 scales, the oxidation behavior of Fe–Al(Cr) diffusion coating on China low activation martensitic steel has been investigated under the oxygen partial pressure ranging from 1 to 20,000Pa at 1253K. A single, continuous Al2O3 scale with the maximum thickness of about 2000nm was formed on the Fe–Al(Cr) diffusion layer. The phase transformation of alumina scales on the surface of Fe–Al(Cr) layer was studied at different oxidation times ranging from 3 to 180min. With the increase in oxygen partial pressure, the phase transformation time of α-Al2O3 is decreased. The metastable γ-Al2O3 and transition α-(Al0.948Cr0.052)2O3 phases were formed in the earlier oxidation process and finally transformed to the stable α-Al2O3 phase, which were detected by grazing incidence angle X-ray diffraction and confirmed by transmission electron microscopy. This implies that Cr shows the third element effect and serves as a template for the nucleation of the stable α-Al2O3.

Effect of Vanadium on Microstructure and Mechanical Properties of TIG Weld Joint of China Low Activation Martensitic Steel

The microstructure and mechanical properties on weld metal of China low activation martensitic steel were investigated in this paper. Welding wire of various Vanadium content was used and Tungsten Inert Gas welding was taken in this research. Microstructure and microhardness distributions along weld sample were detected. The component of weld metal and precipitated phases were observed by SEM and EDS. The results indicate that the addition of Vanadium can preferentially generate VC with Carbon. The precipitation of VC can enhance the hardness and tensile strength of the welding line, but decrease the impact toughness. At the same time the precipitation of Cr23C6 can provide a certain improvement on intergranular corrosion.

Tensile properties and fracturing behavior of weld joints in the CLAM at high temperatures

The tensile properties and fracturing behavior of weld joints in the Chinese low activation martensitic steel (CLAM) at high temperatures were studied. The result revealed that the cracks of weld joints in the base metal would appear in the heat-affected zone, after post-weld heat treatment for the high-temperature tensile test. The microstructure in the fractured frontier had different deformation and directions, and the fractured surface had different angles, a result associating with the normal faulting and shear fracturing. The tri-axial theory of stress can well explain the deformation and fracturing behavior of weld joints in the high-temperature tensile.

A Preliminary Study on the CLAM Steel Composition Optimization Based on Extreme Learning Machine

Material problem is one of the key issues for the realization of fusion energy. Reduced Activation Ferritic/Martensitic steels have been considered to be the primary candidate structural material for fusion DEMO reactors and the first fusion power plants. China low activation martensitic steel (CLAM) has been chosen as the primary structure material in the designs of FDS series PbLi blankets for fusion reactors, CN helium cooled ceramic breeder test blanket module for ITER, liquid blanket of China fusion engineering test reactor (CFETR). In this work, optimization of CLAM steel composition was performed based on extreme learning machine method. The results showed that CLAM steel with Ta content of 0.18–0.20 % had better tensile property at the temperature of 350–550 °C, which provided references for the optimization of CLAM steel composition.

Y 2 O 3 颗粒弥散强化低活化钢的组织稳定性研究

Oxide dispersion strengthened(ODS) steels are being developed as a promising structural material for next-generation nuclear energy systems, due to its excellent resistance to both irradiation damage and high-temperature creep. In this work, the mechanical alloying(MA) and hot isostatic pressing(HIP) technologies were used to prepare a ODS low- activation steel, based on the China low activation martensitic(CLAM) steel. SEM, XRD analysis and EPMA were used to examine the particle size, alloying element distribution and lattice distortion of the ball-milled powders. In order to obtain uniform powders, CLAM powders with 0.3%Y2O3particles should be milled with hard steel balls of 6 mm in diameter for 50 h in Ar protective atmosphere, and the ball- to- powder weight ratio at 10∶1. The microstructure of well-prepared ODS-CLAM steel was stable till 1200 ℃ for 1 h, with grain size of 50~60 mm and martensitic lath width of 200 nm, meanwhile, the Y2O3 particles could still be found in the steel matrix.

Microstructure and Stress Distribution on TIG Welded Joint of CLAM Steel

China low activation martensitic steel, 5 mm, were butt-welded in tungsten insert gas welding. Mechanical properties and microstructure in joints were studied in detail. The residual stress and deformation in the joints were numerically simulated using finite element method. Hardening at WM and softening in heat affected zone is detected in weld joint. The tensile specimen always undergoes localized necking and breaks in BM when the temperature of PWHT was lower than 800 °C. Until PWHT was done at 760 °C/30 min, the specimen ductile fractured in local area, but the absorbed energy of each CIT specimen is much lower than that of BM. The microstructure in WM is mainly tempered martensite with a little of delta ferrite. Tempered lath martensite containing dislocations and M23C6 carbides is the primary structural feature. According to the results of numerical studies, angular distortion is the main form of residual deformation, and caused by the transverse shrinkage of WM.

Effects of LSP on micro-structures and residual stresses in a 4 mm CLAM steel weld joints

The effects of laser shock processing (LSP) on the distribution of residual stress and micro-structure of China Low Activation Martensitic (CLAM) steel weldment were investigated via neutron diffraction and optical microscope (OM). A pair of 4mm CLAM steel plates joined by GTA welding. Special attention is paid to the generation of high level compressive residual stresses introduced by LSP. Residual stress in longitudinal, normal and transversal direction at weldment surface and longitudinal stress through thickness are evaluated via neutron diffraction. Compressive residual stress after LSP occurred at more than 90% areas within the weld joint, it is almost double the areas of compressive stress compare to weldment surface before LSP. The maximum compressive normal residual stress becomes to −183MPa after LSP from −63MPa before LSP. The Modification of surface micro-structures including weld zone (WZ), heat affected zone (HAZ) and base metal (BM) are also discussed. Results to date demonstrate that laser shock processing has been a great potential method for the improvement of mechanical performance of components.

Effect of Electron Beam Currents on Welding Quality of CLAM Steel for High Density Cooling Channels

The blanket is the key component for fusion reactor. The China Low Activation Martensitic steel is being developed under wide collaboration in China, and chosen as the candidate structure material of dual functional lithium–lead (DFLL) blanket of fusion design study series reactor designs. The cooling plate (CP) is one of the key components of the DFLL blanket, and it is embedded with high density cooling channels (HDCCs). The CP can be fabricated with grooved base plate, thin strips and thin cover plate by electron beam welding (EBW) and hot isostatic pressing methods. The welding of thin strips with grooved base plate is one of the key issues of HDCCs manufacturing. In this work, the effect of electron beam currents (EBCs) on the EBW quality of HDCCs was studied. The microstructure and the microhardness of the optimized EBC joint were tested at as-weld and post weld heat treatment to check its quality. The results showed that no obvious defect was observed and the quality of the joint was sound. Preliminary testing verified the effectiveness of the EBW for the HDCCs fabrication.