Atomistic study on the microscopic mechanism of grain boundary embrittlement induced by small dense helium bubbles in iron

The helium bubbles induced by 14 MeV neutron irradiation can cause intergranular fractures in reduced activation ferritic martensitic steel,which is a candidate structural material for fusion reactors.In order to elucidate the susceptibility of different grain boundaries(GBs)to helium-induced embrittlement,the tensile fracture processes of 10 types of GBs with and without helium bubbles in body-centered cubic(bcc)iron at the relevant service temperature of 600 K were investigated via molecular dynamics methods.The results indicate that in the absence of helium bubbles,the GBs studied here can be classified into two distinct categories:brittle GBs and ductile GBs.The atomic scale analysis shows that the plastic deformation of ductile GB at high temperatures originates from complex plastic deformation mechanisms,including the Bain/Burgers path phase transition and deformation twinning,in which the Bain path phase transition is the most dominant plastic deformation mechanism.However,the presence of helium bubbles severely inhibits the plastic deformation channels of the GBs,resulting in a significant decrease in elongation at fractures.For bubble-decorated GBs,the ultimate tensile strength increases with the increase in the misorientation angle.Interestingly,the coherent twin boundary∑3{112}was found to maintain relatively high fracture strength and maximum failure strain under the influence of helium bubbles.

Technical Issues for the Fabrication of a CN-HCCB-TBM Based on RAFM Steel CLF-1

Reduced activation ferritic/martensitic steel （RAFM） is recognized as the primary candidate structural material for ITER＇s test blanket module （TBM）. To provide a material and property database for the design and fabrication of the Chinese helium cooled ceramic breeding TBM （CN HCCB TBM）, a type of RAFM steel named CLF-1 was developed and chaxacter^zed at the Southwestern Institute of Physics （SWIP）, China. In this paper, the R＆D status of CLF-1 steel and the technical issues in using CLF-1 steel to manufacture CN HCCB TBM were reviewed, including the steel manufacture and different welding technologies. Several kinds of property data have been obtained for its application to the design of the ITER TBM.

Strengthening mechanisms of reduced activation ferritic/martensitic steels:A review

This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic(RAFM)steels.High-angle grain boundaries,subgrain boundaries,nano-sized M_(23)C_(6),and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength.M23C6 carbides are easily coarsened under high temperatures,thereby weakening their ability to block dislocations.Creep properties are improved through the reduction of M23C6 carbides.Thus,the loss of strength must be compensated by other strengthening mechanisms.This review also outlines the recent progress in the development of RAFM steels.Oxide dispersion-strengthened steels prevent M23C6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength.Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel.The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries.This procedure increases the creep life of TMT(thermo-mechanical treatment)9Cr-1W-0.06Ta steel by~20 times compared with those of F82H and Eurofer 97 steels under 550℃/260 MPa.

Reduction of chromium oxides in stainless steel dust

The recovery of metal oxides from stainless steel dust using C（graphite）, SiFe, and Al as reductants was investigated under various conditions. The apparent distribution ratio of Cr（L Cr ′^m/s ） in the recovered metal and residual slag phases was defined as the major performance metric. The results show that the recovery ratio of metals increases as the ratio of CaO ：SiO2 by mass in the residual slag increases to 1.17. The residual content of metals in the slag decreases as the Al2O3 content of the slag is increased from approximately 8wt% to 10wt%. The recovery ratio of Cr increases with increasing L Cr ′^ m/s , and a linear relationship between L Cr ′^m/s and the activity coefficient ratio of CrO in the slag and the recovered metal phase is observed. The combination of C and SiFe or Al as the reducing agents reveals that Si is the more effective coreductant.

Yield Stress Prediction Model of RAFM Steel Based on the Improved GDM-SA-SVR Algorithm

With the development of society and the exhaustion of fossil energy,researcher need to identify new alternative energy sources.Nuclear energy is a very good choice,but the key to the successful application of nuclear technology is determined primarily by the behavior of nuclear materials in reactors.Therefore,we studied the radiation performance of the fusion material reduced activation ferritic/martensitic(RAFM)steel.The main novelty of this paper are the statistical analysis of RAFM steel data sets through related statistical analysis and the formula derivation of the gradient descent method(GDM)which combines the gradient descent search strategy of the Convex Optimization Theory to get the best value.Use GDM algorithm to upgrade the annealing stabilization process of simulated annealing algorithm.The yield stress performance of RAFM steel is successfully predicted by the hybrid model which is combined by simulated annealing(SA)with support vector machine(SVM)as the first time.The effect on yield stress by the main physical quantities such as irradiation temperature,irradiation dose and test temperature is also analyzed.The related prediction process is:first,we used the improved annealing algorithm to optimize the SVR model after training the SVR model on a training data set.Next,we established the yield stress prediction model of RAFM steel.The model can predict up to 96%of the data points with the prediction in the test set and the original data point in the 2range.The statistical test analysis shows that under the condition of confidence level=0.01,the calculation results of the regression effect significance analysis pass the T-test.

Thermal Activation Analyses of Dynamic FractureToughness of High Strength Low Alloy Steels

A formula is derived for determining the influence of temperature and loading rate on dynamic fracture toughness of a high strength low alloy steel (HQ785C) from thermal activation analysis of the experimental results of three-point bend specimens as well as introducing an Arrhenius formula. It is shown that the results obtained by the given formula are in good agreement with the experimental ones in the thermal activation region. The present method is also valuable to describe the relationship between dynamic fracture toughness and temperature and loading rate of other high strength low alloy steels.

Unsteady state precipitation of M_(23)C_(6)carbides during thermal cycling in reduced activation steel manufactured by laser melting deposition

The temperature field distribution and thermal history of Fe-9Cr2WVTa reduced activation steel prepared by laser melting deposition(LMD)have been calculated with Gaussian and Ring laser beams,and the nucleation and growth behaviors of M_(23)C_(6)precipitates in the 1st,7th and 19th layers have been calculated using the modified classical nucleation theory and Svoboda Fischer Fratzl Kozeschnik model.The energy distribution shows W-shape with Ring laser beam while it shows V-shape with Gaussian laser beam,which results in the more uniform M_(23)C_(6)size in the same layer with Ring laser beam.Precipitates in the bottom(i.e.,the 1st layer)have the minimum size and the size increases with the layer number with Gaussian and Ring laser beams.The temperature history,the instantaneous nucleation rate and the size evolution of M_(23)C_(6)have been systematically discussed.The results indicate that the nucleation,growth and re-dissolution of precipitates in reduced activation steel depend on the amount of energy absorbed in the thermal cycle during LMD.The continuous accumulation of energy during the thermal cycle leads to larger M_(23)C_(6)at the top area.The unsteady state precipitation dynamics of M_(23)C_(6)carbides during thermal cycling are consistent with the simulation results.

Dynamic and Static Recrystallization Behavior of Low Carbon High Niobium Microalloyed Steel

The recrystallization behavior of a low carbon high niobium microalloyed steel was investigated using continuous and interrupted hot compression tests. The results showed that the initiation of dynamic recrystallization （DRX） could be detected from inflection in the plot of the strain hardening rate against stress regardless of the stress peak appearance. According to the Zener-Hollomon parameter equation, the activity energy of DRX （Qad） was ob tained and a new modified expression calculating Qdef was proposed in consideration of the chemical composition of experimental steel. Applying the 2% offset method, the static softening fraction was determined. The graphic representation of the softening fraction vs interruption time gave the information about the non-static reerystallization temperature （about 1 000℃ ） and the relationship of precipitation time temperature. Static recrystallization kinetics followed the Avrami＇s law at high deformation temperature, and different values of the exponent m were given to illustrate the effect of niobium element on static recrystallization at different deformation temperatures.

Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel

In this study,the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic（RAFM） steel friction stir welded joints were investigated.When a W-Re tool is used,the recommended welding parameters are 300 rpm rotational speed,60 mm/min welding speed and10 kn axial force.In stir zone（SZ）,austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths.The microstructure in SZ contains lath martensite with high dislocation density,a lot of nano-sized MX and M3C phase particles,but almost no M23C6 precipitates.In thermal mechanically affect zone（TMAZ）and heat affect zone（HAZ）,refinement of prior austenite and martensitic laths and partial dissolution of M（23）C6 precipitates are obtained at relatively low rotational speed.However,with the increase of heat input,coarsening of martensitic laths,prior austenite grains,and complete dissolution of M23C6 precipitates are achieved.Impact toughness of SZ at-20℃ is slightly lower than that of base material（BM）,and exhibits a decreasing trend with the increase of rotational speed.

First Results of Characterization of 9Cr-3WVTiTaN Low Activation Ferritic/Martensitic Steel

Ferritic/martensitic steels with Cr of 9%-12% （in mass percent） are favourable candidates for fuel cladding tube and in-core components of supercritical water-cooled reactor. 9Cr-3WVTiTaN low activation ferritic/martensitic steel, designated as China Nuclear Steel- I （CNS- I ）, was patterned after T91 steel （modified 9Cr-lMo） for the reactor. The idea of low activation material and microalloy technology was introduced into the design of the steel. The hardening, tempering and transformation behaviour of CNS- I steel was investigated. The steel has advantages in tensile properties at elevated temperature relative to zircaloy that has been widely used as cladding material for conventional light water reactors. CNS- I steel exhibits tensile properties and impact toughness comparable to T91 steel which exhibits availability in the present fission reactors and fast breeder reactor but includes undesired radioactive elements such as molybdenum and niobium.

Effect of thermal cycles on microstructure of reduced activation steel fabricated using laser melting deposition

Reduced activation steel was successfully fabricated by laser melting deposition employing a Gaussian and a ring-shaped laser.The microstructure evolution of the reduced activation steel was investigated using the scanning electron microscope,transmission electron microscope and electron backscatter diffraction.The experimental results showed that the grains close to the substrate were smaller than the grains in the upper layers.Compared to those deposited using a Gaussian laser,the samples deposited using a ring-shaped laser showed a more homogeneous microstructure.Furthermore,a finite element analysis(FEA)model was applied to reveal the thermal history during laser melting deposition.The simulation results were well validated with the experimental results.FEA results indicate that the peak temperature increases and the cooling rate decreases,as the layer gets further from the substrate.Additionally,the temperature and the cooling rate resulting from the Gaussian laser model were higher at the midline of the samples and lower around the edges,whereas those of the ring-shaped laser model were consistent with both at the center and around the edges.

Metallurgical characterization of Ti-bearing 9Cr low activation martensitic steel

Two heats of low activation martensitic （LAM） steels with Ti and Ta （denominated as 9Cr-Ti and 9Cr-Ta）, respectively, developed as candidate structure materials for nuclear reactor were characterized. This paper was focused on the effect of titanium on the microstructures and mechanical properties of 9Cr LAM steel in as-received condition （normalized at 950 ℃ for 30 min with water quenching plus tempered at 780 ℃ for 90 min with air cooling）. Chemical analysis and microstructure observation were conducted on 9Cr-Ti and 9Cr-Ta with optical microscopy, X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. Impact properties and tensile strengths were measured with Charpy impact experiments and tensile tests. The results indicated that 9Cr-Ti and 9Cr-Ta were fully martensitic steels in as-received condition. MX type and M23C6 type precipitates were observed distributing along boundaries of prior austenite grains and martensite laths in 9Cr-Ti.The addition of titanium accelerated the precipitation of TiC and TiN, and produced much finer grains in 9Cr-Ti than 9Cr-Ta at the same normalization temperature. Mechanical properties tests showed the ductile brittle transition temperatures of 9Cr- Ti and 9Cr-Ta were about -90℃ and -85℃, respectively. The ultimate tensile strengths at room temperature and 600℃,were 680 MPa and 365 MPa for 9Cr-Ti, and 660 MPa and 335 MPa for 9Cr-Ta, respectively. The favorite impact toughness and tensile properties of 9Cr-Ti could be attributed to the fine grains in as-received condition.

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 14.8µm 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℃.

Experimental and theoretical analysis of equilibrium segregation and radiation-induced segregation of Cr at grain boundariesin a reduced activation ferritic/martensitic(RAFM)steel

Helium ion irradiation at 350℃was performed to study equilibrium segregation and radiation-induced segregation(RIS)of Cr at grain boundaries in reduced activation ferritic/martensitic steels.Cr concentration at grain boundary was measured by scanning transmission electron microscopy with an energy-dispersive spectrometer.The measured Cr concentration at grain boundaries in heat treated zone was 11.7 and 12.8 wt.%in irradiated zone,respectively,which matched well to the calculated results from Mclean and modified Perk model.Equilibrium segregation and RIS of Cr mechanisms were theoretically analysed.The analysis indicates that as temperature rises,equilibrium Cr segregation decreases monotoni-cally,while RIS of Cr has a bell-shape profile,which increases first and then decreases.It is also shown that at low and high temperatures,equilibrium segregation of Cr is higher than that of RIS;at intermediate temperatures,equilibrium Cr segregation is lower than RIS.

Morphology of Oxide Scale and Oxidation Kinetics of Low Carbon Steel

The oxidation kinetics and composition of oxide scales on low carbon steel （SPHC） were studied during i- sothermal oxidation. Thermogravimetric analyzer （TGA） was used to simulate isothermal oxidation process of SPHC for 240 min under air condition, and the temperature range was from 500 to 900 ℃. Scanning electron microscope （SEM） was used to observe cross-sectional scale morphology and analyze composition distribution of oxide scales. The morphology of oxide scale was classical three-layer structure. Fe2 03 developed as whiskers at the outermost lay- er, and interlayer was perforated-plate Fe3 04 while innermost layer was pyramidal FeO. From the oxidation curves, the oxidation mass gain per unit area with time was of parabolic relation and oxidation rate slowed down. On the ba- sis of experimental data, the isothermal oxidation kinetics model was derived and oxidation activation energy of SPHC steel was 127. 416 kJ/mol calculated from kinetics data.

Hot Deformation Behavior of a Novel Ni-Cr-Mo-B Ultra-heavy Plate Steel by Hot Compression Test

Hot deformation behavior of a novel Ni-Cr-Mo-B heavy plate steel was studied by hot compression tests,which were conducted on a Gleeble-3800thermo-mechanical simulator corresponding to the temperature range of850-1 150℃ with the strain rates of 0.01-10s-1 and the true strain of 0.8.The results suggest that the majority of flow curves exhibit a typical dynamic recrystallization（DRX）behavior with an apparent single peak stress followed by agradual fall towards a steady-state stress.Important characteristic parameters of flow behavior as critical stress/strain for initiation of DRX and peak and steady-state stress/strain were derived from curves of strain hardening rate versus stress and stress versus strain,respectively.Material constants of the investigated steel were determined based on Arrhenius-type constitutive equation,and then the peak stress was predicted by the equation with the hot deformation activation energy of 379 139J/mol,and the predicted values agree well with the experimental values.Furthermore,the effect of Zener-Hollomon parameter on the characteristic points of flow curves was studied using the power law relation,and the ratio of critical stress and strain to peak stress and strain were found to be 0.91and0.46,respectively.

Improvement of Impact Toughness and Creep Properties in Reduced Activation Ferritic Steels by Consumable Electrode Remelting

The effect of smelting processes on mechanical properties and microstructure of reduced activation ferritic steels was studied.Creep properties and impact toughness of reduced activation ferritic steels were obviously improved by vacuum induction melting followed by consumable electrode remelting process in comparison with the conventional vacuum induction melting process.The difference of impact toughness and creep properties between both steels mainly depended on the aspect ratio and mean size of precipitates.Decreasing the aspect ratio of carbides makes development of a shear band more difficult , which could increase impact energy and creep resistance.

Effect of TaC Particles Dissolution on Grain Coarsening in Reduced Activation Steels

The effect of TaC on grains and packets coarsening in the reduced activation ferritic/martensitic （RAFM） steels was investigated. It was found that the combined effect of the austenitizing temperature and heating rate resulted in the heterogeneous austenite grain growth. When the austenitizing temperature was raised above 1 423 K, the TaC particles disappeared, and the homogenous grains abruptly increased. The thermodynamic model for dissolution of TaC particles during austenitizing was applied to interpret the results.

Effect of alloying on the properties of 9Cr low activation martensitic steels

Two types of 9Cr low activation martensitic steels （named 9Cr-1 and 9Cr-2） were developed in University of Science and Technology Beijing. 9Cr-1 and 9Cr-2 were produced by vacuum induction melting method, and examinations of the microstruc- tures were carried out with X-ray diffraction analysis, optical microscopy, scanning electron microscopy and transmission electron microscopy. The ultimate tensile strength and yield tensile strength were evaluated with tensile tests. The impact properties were characterized with Charpy impact experiments. The results indi- cated that 9Cr-1 and 9Cr-2 on as-received condition （95 ~C/30 min/water quenching plus 780 ~C/90 min/air cooling） were flflly martensitic steels free of ^-ferrite. The ul- timate tensile strength of 9Cr-1 and 9Cr-2 were 695 MPa and 680 MPa, respectively. However, 9Cr-2 showed a fine grain size of 4.8 pm, and its value of ductile-brittle transition temperature （DBTT） was -90 ~C under as-received condition. The additions of vanadium, titanium and boron accelerated the formation of MX precipitates and resulted in fine grains and precipitates. The fine grains effectively reduced the value of DBTT from -60 ℃ to -90 ℃ with identical upper shelf energy （USE）. The decrease in silicon concentration of 9Cr-2 induced a slight reduction in ultimate tensile strength from 695 MPa to 680 MPa.