• Journal of the Korean Society for Heat Treatment
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• v.30 no.2
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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.

• Nuclear Engineering and Technology
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• v.49 no.8
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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.

• Nuclear Engineering and Technology
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• v.52 no.5
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• pp.1008-1012
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• 2020

In order to make reasonable design for the improvement of comprehensive mechanical properties of RAFM steels, the design system with both machine learning and high-throughput optimization algorithm was established. As the basis of the design system, a dataset of RAFM steels was compiled from previous literatures. Then, feature engineering guided random forests regressors were trained by the dataset and NSGA II algorithm were used for the selection of the optimal solutions from the large-scale solution set with nine composition features and two treatment processing features. The selected optimal solutions by this design system showed prospective mechanical properties, which was also consistent with the physical metallurgy theory. This efficiency design mode could give the enlightenment for the design of other metal structural materials with the requirement of multi-properties.

• Nuclear Engineering and Technology
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• v.49 no.3
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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 $25-500^{\circ}C$.

• Nuclear Engineering and Technology
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• v.54 no.7
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• pp.2376-2385
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• 2022

Two groups of oxide dispersion-strengthened reduced-activation ferritic/martensitic steels (A and B) were prepared by adding Y, Ti, and Zr into steels through vacuum induction melting to investigate the inclusions, microstructures, mechanical properties of the alloys. Results showed that particles with Y, Ti, and Zr easily formed. Massive, Zr-rich inclusions were found in B steel. Density of micron inclusions in A steel was 1.42 × 10¹⁴ m^-3, and density of nanoparticles was 3.61 × 10¹⁶ m^-3. More and finer MX carbides were found in steel tempered at 650 ℃, and yield strengths (YS) of A and B steel were 714±2 and 664±3.5 MPa. Thermomechanical processing (TMP) retained many dislocations, which improved the mechanical properties. YSs of A and B treated by TMP were 725±3 and 683±4 MPa. The existence of massive Zr-rich inclusions in B steels interrupted the continuity of the matrix and produced microcracks (fracture), which caused a reduction in mechanical properties. The presence of fine prior austenite grain size and inclusions was attributed to the low DBTTs of the A steels; DBTTs of A650 and A700 alloy were -79 and -65 ℃. Tempering temperature reduction and TMP are simple, readily useable methods that can lead to a superior balance of strength and impact toughness in industry applications.

• Nuclear Engineering and Technology
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• v.55 no.6
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• pp.2139-2146
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• 2023

During reactor operation, the divertor must withstand unprecedented simultaneous high heat fluxes and high-energy neutron irradiation. The extremely severe service environment of the divertor imposes a huge challenge to the bonding quality of divertor joints, i.e., the joints must withstand thermal, mechanical and neutron loads, as well as cyclic mode of operation. In this paper, potassium-doped tungsten (KW) is selected as the plasma facing material (PFM), oxygen-free copper (OFC) as the interlayer, oxide dispersion strengthened copper (ODS-Cu) alloy as the heat sink material, and reduced activation ferritic/martensitic (RAFM) steel as the structural material. In this study, a vacuum brazing technology is proposed and optimized to bond Cu and ODS-Cu alloy with the silver-free brazing material CuSnTi. The most appropriate brazing parameters are a brazing temperature of 940 ℃ and a holding time of 15 min. High-quality bonding interfaces have been successfully obtained by vacuum brazing technology, and the average shear strength of the as-obtained KW/Cu and ODS-Cu alloy joints is ~268 MPa. And a fabrication route for manufacturing the flat-type divertor target based on brazing technology is set. For evaluating the reliability of the fabrication technologies under the reactor relevant condition, the high heat flux test at 20 MW/m² for the as-manufactured flat-type KW/Cu/ODS-Cu/RAFM mockup is carried out by using the Electron-beam Material testing Scenario (EMS-60) with water cooling. This paper reports the improved vacuum brazing technology to connect Cu to ODS-Cu alloy and summarizes the production route, high heat flux (HHF) test, the pre and post non-destructive examination, and the surface results of the flat-type KW/Cu/ODS-Cu/RAFM mockup after the HHF test. The test results demonstrate that the mockup manufactured according to the fabrication route still have structural and interfacial integrity under cyclic high heat loads.

• Nuclear Engineering and Technology
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• v.50 no.1
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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.

• Journal of Welding and Joining
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• v.34 no.6
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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.