• Nuclear Engineering and Technology
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• 제51권5호
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• pp.1365-1372
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• 2019

The effects of Y and Ti on the microstructure stability and tensile properties of the reduced activation ferritic/martensitic steel have been investigated. The addition of Y and Ti affected the prior austenite grain size due to the pinning of the inclusions. Ti addition of 0.008 wt% to the steel was intended to promote the precipitation of nano-sized carbides with a high resistance to coarsening. 8Ti14Y exhibited a higher yield strength and a lower DBTT than the other alloys due to the fine grain size and additional precipitation hardening by (Ti, Ta)-rich MX. After thermal exposure at $550^{\circ}C$ for 1500 h, yield strength was dropped significantly in exposed 0Ti13Y. On the contrary, a lower reduction of YS was observed in 8Ti14Y. The $M_{23}C_6$ in 0Ti13Y and 8Ti14Y and MX in 25Ti14Y and 39Ti15Y coarsened seriously during ageing, which could be responsible for the reduction of the tensile properties of alloys.

• Nuclear Engineering and Technology
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• 제50권1호
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• pp.132-139
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• 2018

Nanocrystalline reduced activation ferritic/martensitic (RAFM) steel samples were prepared using surface mechanical attrition treatment (SMAT). Un-SMATed and SMATed reduced activation ferritic/martensitic samples were irradiated by helium ions at $200^{\circ}C$ and $350^{\circ}C$ with 2 dpa and 8 dpa, respectively, to investigate the effects of grain boundaries (GBs) and temperature on the formation of He bubbles during irradiation. Experimental results show that He bubbles are preferentially trapped at GBs in all the irradiated samples. Bubble denuded zones are clearly observed near the GBs at $350^{\circ}C$, whereas the bubble denuded zones are not obvious in the samples irradiated at $200^{\circ}C$. The average bubble size increases and the bubble density decreases with an increasing irradiation temperature from $200^{\circ}C$ to $350^{\circ}C$. Both the average size and density of the bubbles increase with an increasing irradiation dose from 2 dpa to 8 dpa. Bubbles with smaller size and lower density were observed in the SMATed samples but not in the un-SMATed samples irradiated in the same conditions, which indicate that GBs play an important role during irradiation, and sink strength increases as grain size decreases.

• 한국해양공학회지
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• 제18권2호
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• pp.58-63
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• 2004

Reduced Activation Ferritic/Martensitic Steels (RAFs) are leading candidntes for structural materials of a D-T fusion reactor. One of the RAFs, JLF-l (9Cr-2W-V, Ta) has been developed and has shown to have good resistance against high-fluency neutrino irradiation and good phase stability. Recently, in order to clarify the strengthening mechanisms at high temperatures, a new scheme to improve high temperature mechanical properties is desired. Therefore, the test technique development of high temperature creep behaviors for this material is very important. In this paper, the creep properties and creep life prediction, using the Larson-Miler parameter method for JLF-l to be used for fusion reactor materials or other high temperature components, are presented at the elevated temperatures of 50$0^{\circ}C$, 55$0^{\circ}C$, $600^{\circ}C$, $650^{\circ}C$ and 704$^{\circ}C$. It was confirmed, experimentally and quantitatively, that a creep life predictive equation, at such various high temperatures, is well derived mr the LMP method.

• Nuclear Engineering and Technology
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• 제49권3호
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• pp.569-575
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• 2017

One of the important requirements for the application of reduced-activation ferritic/martensitic (RAFM) steel is to retain proper mechanical properties under irradiation and high-temperature conditions. To simulate the yield strength and stress-strain curve of steels during high-temperature and irradiation conditions, a multiscale simulation method consisting of both microstructure and strengthening simulations was established. The simulation results of microstructure parameters were added to a superposition strengthening model, which consisted of constitutive models of different strengthening methods. Based on the simulation results, the strength contribution for different strengthening methods at both room temperature and high-temperature conditions was analyzed. The simulation results of the yield strength in irradiation and high-temperature conditions were mainly consistent with the experimental results. The optimal application field of this multiscale model was 9Cr series (7-9 wt.%Cr) RAFM steels in a condition characterized by 0.1-5 dpa (or 0 dpa) and a temperature range of $25-500^{\circ}C$.

• Journal of Welding and Joining
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• 제34권6호
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• pp.47-54
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• 2016

Post-Weld Heat Treatment (PWHT) cracking susceptibility in the weld heat-affected zone (HAZ) of reduced activation ferritic-martensitic (RAFM) steels was evaluated through stress-rupture tests. 9Cr-1W based alloys including different C, Ta and Ti content were prepared. The coarse grained heat-affected zone (CGHAZ) samples were simulated with welding condition of 30 kJ/cm heat input. CGHAZ samples consisted of martensite matrix. Stress rupture experiments were carried out using a Gleeble simulator at temperatures of $650-750^{\circ}C$ and at stress levels of 125-550 MPa, corresponding to PWHT condition. The results revealed that PWHT cracking resistance was improved by Ti addition, i.e., Ti contributed to the formation of fine and stable MX precipitates and suppression of coarse M23C6 carbides, resulting in improvement of stress rupture ductility. Meanwhile, rupture strength increased with increasing solute C content.

• Nuclear Engineering and Technology
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• 제49권8호
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• pp.1748-1751
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• 2017

Reduced activation ferritic/martensitic (RAFM) steels are widely applied as structural materials in the nuclear industry. To investigate hydrogen's effect on RAFM steels' elastic properties and the mechanism of that effect, a procedure of first principles simulation combined with experiment was designed. Density functional theory models were established to simulate RAFM steels' elastic status before and after hydrogen's insertion. Also, experiment was designed to measure the Young's modulus of RAFM steel samples with and without hydrogen charging. Both simulation and experiment showed that the solubility of hydrogen in RAFM steels would decrease the Young's modulus. The effect of hydrogen on RAFM steels' Young's modulus was more significant in water-quenched steels than it was in tempering steels. This indicated that defects inside martensite, considered to be hydrogen traps, could decrease the cohesive energy of the matrix and lead to a decrease of the Young's modulus after hydrogen insertion.

• 열처리공학회지
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• 제30권2호
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• pp.53-60
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• 2017

The aim of this study is to examine a feasibility to substitute Ti for Ta in reduced activation ferritic/martensitic (RAFM) steel by comparing a Ti-added RAFM steel with a conventional Ta-added RAFM steel. The microstructures and mechanical properties of Ta-, and Ti-added RAFM steels were investigated and a relationship between microstructures and mechanical properties was considered based on quantitative analysis of precipitates in two RAFM steels. Ta-, and Ti-added RAFM steels were normalized at $1000{\sim}1040^{\circ}C$ for 30 min and tempered at $750^{\circ}C$ for 2 hr. Both RAFM steels had very similar microstructures, that is, typical tempered martensite with relatively coarse $M_{23}C_6$ carbides at boundaries of grain and lath, and fine MX precipitates inside laths. The MX precipitates were identified as TaC in Ta-added RAFM steel and TiC or (Ti, W)C in Ti-added RAFM steel, respectively. It is believed that these RAFM steels show similar tensile and Charpy impact properties due to similar microstructures. Precipitate hardening and brittle fracture strength calculated with quantitative analysis of precipitates elucidated well the similar behaviors on the tensile and Charpy impact properties of Ta-, and Ti-added RAFM steels.

• Journal of Welding and Joining
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• 제33권6호
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• pp.42-48
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• 2015

Reduced activation ferritic/martensitic steels are recognized as the primary candidate structural materials for fusion blanket systems. Welding is an inevitable for breeding blanket for pressure tightness and radioisotope confinement. Especially, TIG welding was chosen for sealing because it has the largest gap allowance compared to the other welding methods, and its properties are controllable by feed wire and welding conditions. In this study, the low cycle fatigue test using two-type gage such as extensometer and strain gage was applied to the TIG welded joint of F82H steel, for evaluating fatigue damage accumulation behavior of the HAZs. As the result, the over-tempered HAZ have shown a higher fatigue damage accumulation compared with other materials at all the testing conditions.

• Nuclear Engineering and Technology
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• 제51권5호
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• pp.1251-1260
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• 2019

The water-cooled WCLL blanket is one of the possible candidates for the blanket of the fusion power reactors. The plasma-facing first wall manufactured from the reduced-activation ferritic-martensitic steel Eurofer97 will be cooled with water at a typical pressurized water reactor (PWR) conditions. According to new estimates, the first wall will be exposed to peak heat fluxes up to $7MW/m^2$ while the maximum operated temperature of Eurofer97 is set to $550^{\circ}C$. The performed analysis shows the capability of the designed flat first wall concept to remove heat flux without exceeding the maximum Eurofer97 operating temperature only up to $0.75MW/m^2$. Several heat transfer enhancement methods (turbulator promoters), structural modifications, and variations of parameters were analysed. The effects of particular modifications on the wall temperature were evaluated using thermo-hydraulic three-dimensional numerical simulation. The analysis shows the negligible effect of the turbulators. By the combination of the proposed modifications, the permitted heat flux was increased up to $1.69MW/m^2$ only. The results indicate the necessity of the re-evaluation of the existing first wall concepts.

• Nuclear Engineering and Technology
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• 제51권1호
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• pp.221-227
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• 2019

One of the important requirements for the application of reduced activation ferritic/martensitic steel is to retain proper mechanical properties in irradiation and high temperature conditions. In order to simulate the impact toughness with the effect of temperature and irradiation, a simulation model based on energy balance method consisted of crack initiation, plastic propagation and cleavage propagation stages was established. The effect of temperature on impact toughness was analyzed by the model and the trend of the simulation results was basicly consistent with the previous experimental results of CLAM steels. The load-displacement curve was simulated to express the low temperature ductile-brittle transition. The effect of grain size and inclusion was analyzed by the model, which was consistent with classical experiment results. The transgranular-intergranular transformation in brittle materials was also simulated.