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.

Overview of the Research and Development for Reduced Activation Ferritic/Martensitic Steel CLF-1

Recent accomplishment by the SWIP for the reduced activation ferritic/martensitic steel CLF-1 development has been reviewed. It＇s found that CLF- 1 steel has better room temperature tensile properties than Eurofer97 steel and has a fully martensitic microstructure.

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.

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.

Designing a high Si reduced activation ferritic/martensitic steel for nuclear power generation by using Calphad method

A high Si reduced activation ferritic/martensitic(RAFM) steel for nuclear structure application is successfully designed by using Calphad method. The main designed chemical composition is C 0.18–0.22%, Cr10.0–10.5%, W 1.0–1.5%, Si 1.0–1.3%, V+Ta 0.30–0.45%, and Fe in balance. High Si design brings excellent corrosion resistance, while low activation is advantageous in the nuclear waste processing. The experimental results indicate that the newly designed high Si RAFM steel had full martensitic structure and uniformly distributed fine second phase particles, and exhibited excellent mechanical properties and corrosion resistance. Compared to the P91 steel, this new RAFM steel designed by Calphad method is expected to be a promising candidate used in nuclear power generation, which also provides a new and effective approach to the development of RAFM steel for nuclear application.

Hot Deformation Behavior of a New Nuclear Use Reduced Activation Ferritic/Martensitic Steel

The hot deformation behavior and workability of a new reduced activation ferritic/martensitic steel named SIMP steel for accelerator-driven system were studied. The flow curve and its microstructure were studied at 900-1200 ℃ and strain rate range of 0.001-10 s^-1. The results showed that the deformation behavior of the SIMP steel during hot compression could be manifested by the Zener-Hollomon parameter in an exponent-type equation. Based on the obtained constitutive equation, the calculated flow stresses were in agreement with the experimentally measured ones, and the average activity energies Qdrv and QHw for the initiation of dynamic recrystallization and the peak strain were calculated to be 476.1 kJ/mol and 462.7 kJ/mol, respectively. Furthermore, based on the processing maps and microstructure evolution, the optimum processing condition for the SIMP steel was determined to be 1050-1200 ℃/0.001-0.1s^-1.

Microstructure Stability of V and Ta Microalloyed 12%Cr Reduced Activation Ferrite/Martensite Steel during Long-term Aging at 650℃

In view of developing novel alloys for applications in supercritical water-cooled reactor fuel cladding and in-core components, a 12%Cr reduced activation ferrite/martensite(RAFM) steel with good corrosion resistance and irradiation performance was developed. V and Ta were added to form fine MX type carbonitrides and enhance the high temperature creep rupture strength. Microstructure stability of the steel during long-term aging at 650 C was studied experimentally combined with the simulation of ThermoCalc and DICTRA software. The results show that the precipitates in the steel during long-term aging contain M23C6, MX and Laves phase. M23C6 carbides play a major role in the stabilization of the tempered martensite lath structure by exerting a large Zener pinning force as compared with MX and Laves phase.Adding V and Ta in the steel can not only promote MX precipitation, but also refine M23C6 carbides and thus improve the thermal stability of lath/subgrains, which is beneficial to the improvement of high temperature microstructure stability of the 12%Cr RAFM steel.

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.

Low-temperature mechanical and magnetic properties of the reduced activation martensitic steel

Mechanical and magnetic properties as well as their relationship in the reduced activation martensitic （RAM） steel were investigated in the temperature range from -90℃ to 20℃. Charpy impact tests show that the ductile-to-brittle transition temperature （DBTT） of the RAM steel is about -60℃. Low-temperature tensile tests show that the yield strength, ultimate tensile strength and total elongation values increase as temperature decreases, indicating that the strength and plasticity below the DBTT are higher than those above the DBTT. The coercive field （Hc） in the scale of logarithm decreases linearly with the increasing temperature and the absolute value of the slope of InHc versus temperature above the DBTT is obviously larger than that below the DBTT, also confirmed in the T91 steel. The results indicate that the non-destructive magnetic measurement is a promising candidate method for the DBTT detection of ferromagnetic steels.

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.

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.

球形压头半径对离子辐照后RAFM钢硬度及力学性能提取的影响

纳米压痕仪中压头形状直接决定压头载荷-深度曲线,进而影响宏观力学性能的提取,但压头半径对辐照后材料硬度及力学性能提取的影响依然缺乏深入的研究。本文研究了不同半径球形压头(0.3、1.0和10.0μm)对辐照后低活化铁素体/马氏体(RAFM)钢硬度和力学性能提取的影响。不管压头半径如何改变,所有辐照样品均出现反尺寸效应,纳米压痕仪接触面积测量误差是导致该现象产生的主要原因。此外,离子辐照后所有样品均出现辐照硬化,但随着辐照剂量的增加,硬化现象逐渐减弱,直至出现辐照软化。相同载荷与辐照剂量条件下,低活化铁素体/马氏体钢硬度随压头半径的增加而逐渐降低。不管辐照剂量如何改变,随着压头半径的增加,提取出的弹性模量逐渐增加而屈服强度逐渐减小。相同压头半径时,弹性模量和屈服强度随辐照剂量的增加均呈现先增加后减小的趋势。对比未辐照样品,压头半径增加至10.0μm时,所提取出的屈服强度和弹性模量更接近于拉伸实验所获得的数值。以上结果表明:球形压头可用于提取离子辐照后低活化铁素体/马氏体的宏观力学性能,并且半径越大预测结果越接近拉伸实验所获得的数值。

Mechanical property and irradiation damage of China Low Activation Martensitic(CLAM) steel

China Low Activation Martensitic （CLAM） steel is being studied to develop the structural materials for a fusion reactor, which has been designed based on the well-known 9Crl.5WVTa steel. The effect of tempering temperature on hardness and micro- structure of CLAM steel was studied. The strength of CLAM steel increased by adding silicon, and the ductility remained con- stant. Conversely, while CLAM steel maintained good ductility with the addition of yttrium, its tensile strengths were greatly degraded. Behaviors under electron irradiation of CLAM steel were examined using the high voltage electron microscope. Electron irradiation at 450℃ formed many voids in CLAM steel with basic composition, whereas CLAM with silicon steel did not change the microstructure significantly.

不同方法制备的ODSRAFM钢的结构和性能比较

比较了机械合金化法、溶胶-凝胶-等离子体烧结法和真空熔炼法三种方法制备的氧化物弥散强化低活化铁素体/马氏体钢的结构和性能。机械合金化方法和溶胶-凝胶-等离子体烧结法制备得到的材料氧化物粒子达到纳米级;真空熔炼方法可制备出微米级大小的氧化物弥散强化材料。上述方法制备的材料强度均得到改进。

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.

喷雾辅助FSW焊接RAFM钢数值模拟与性能

低活化铁素体/马氏体(reduced activation ferritic/martensitic,RAFM)钢搅拌摩擦焊(friction stir welding,FSW)接头中的高温δ铁素体是影响其冲击韧性的主要因素.通过喷雾冷却,降低焊接峰值温度并对接头进行快速降温,从而达到抑制δ铁素体生成的目的.采用Fluent流体软件对RAFM钢FSW在不同喷雾冷却工况下的温度场进行模拟研究,综合模拟结果进行试验验证.结果表明,液氮辅助FSW(FSW+LN)可有效降低焊接接头的峰值温度并加速焊后的降温速率.FSW+LN焊接接头冲击韧性由常规FSW接头的冲击吸收能量23 J提升至33 J,达到与母材等韧匹配,硬度变化趋势与常规FSW接头基本一致,焊接接头硬度远高于母材.

Fe-Cr-W合金内富Cr团簇形核析出动力学的原子动力学蒙特卡罗模拟

低活化铁素体马氏体钢被认为是第四代堆及聚变反应堆候选结构材料之一。低活化铁素体马氏体钢以Fe-Cr为基体,W是其中重要的溶质元素。低温辐照脆化行为是限制低活化铁素体马氏体钢服役的重要问题之一。服役过程中析出的团簇因阻碍位错运动而引起材料硬化脆化。深入理解服役过程中团簇析出行为有助于认识低活化铁素体马氏体钢的低温辐照脆化问题。基于优化后的自主开发程序MIET_AKMC,利用原子动力学蒙特卡罗(atomic kinetic Monte Carlo, AKMC)方法研究了Fe-Cr(8%,10%,16%,20%)-W(1%,2%)合金在热老化条件下富Cr团簇形核析出动力学行为。结果表明,热老化后Cr原子析出形成富Cr的团簇,W仍保持固溶状态。Cr团簇析出分为形核、长大、粗化3个阶段。在团簇开始粗化时,团簇平均半径约(0.57±0.03) nm,且该值不受温度和Cr含量的影响。W会延迟团簇析出动力学过程,尤其是当Cr含量为10%时,延迟效果最明显,这是因为Cr含量为10%时,体系SRO参数绝对值达最大;W延迟效果得益于W与空位间较强的结合能(0.14 eV,空位与Cr结合能为0.06 eV)。从模拟结果可知,除已知的固溶强化外,W元素加入会通过延迟富Cr团簇析出从而有益于Fe-Cr钢性能,因为富Cr团簇会引起Fe-Cr合金硬化、脆化。

热处理对电弧熔丝增材制造核电用铁素体/马氏体钢微观组织与力学性能的影响

采用电弧熔丝增材制造(wire and arc additive manufacturing,WAAM)技术制备了低活化铁素体/马氏体钢(reduced activation ferrite/martensite steel,RAFM钢),通过光学显微镜、扫描电子显微镜和透射电子显微镜等观察微观组织变化,通过拉伸试验进行力学性能测试,研究了热处理工艺对其微观组织和力学性能的影响.结果表明,打印态的RAFM钢微观组织为铁素体+回火马氏体的双相组织,平均晶粒尺寸约为1.51μm.经过热处理,RAFM钢的晶粒尺寸没有明显增长(1.84μm),并在微观组织中保留了高密度位错.此外,热处理后高数密度TiO2二次相纳米颗粒在基体中析出,并弥散分布在基体中,其尺寸在5~10 nm.热处理后的RAFM钢抗拉强度显著提高,断后伸长率略微下降,其室温抗拉强度为1080 MPa,在650℃下测试的抗拉强度仍可达285 MPa.创新点:(1)系统研究热处理对电弧熔丝增材制造低活化铁素体马氏体钢的相组成、晶粒尺寸、析出相的影响.(2)阐明热处理对电弧熔丝增材制造低活化铁素体马氏体钢力学性能的强化机制.

低活化铁素体/马氏体钢搅拌摩擦焊接头的高温蠕变性能

对5 mm厚度的低活化铁素体/马氏体钢(Reduced activation ferritic/martensitic steel,RAFM)进行了搅拌摩擦焊接,通过焊接接头的高温蠕变实验,研究低活化钢搅拌摩擦焊接头的高温蠕变性能与失效机理。结果表明,低活化钢搅拌摩擦焊接头高温力学性能良好。合金元素W和Ta含量高的RAFM钢的接头高温蠕变寿命更长,原因是Ta和W在高温环境中形成了更多的析出相,包括MX相和Laves相,对位错滑移的阻碍作用更强。650℃的蠕变寿命远远短于600℃的原因可能是,高温使得位错非常容易发生滑移,在Laves相还没有长大、MX相对位错滑移的阻碍作用不够强的情况下,在某些部位形成了密集的蠕变孔洞,造成有效横截面积急剧减少,应力集中严重,引发裂纹导致断裂。

低活化铁素体/马氏体钢厚板光纤激光焊接接头组织及力学性能分析

针对核聚变反应堆试验包层模块(TBM)中使用的CLF-1低活化铁素体/马氏体钢进行焊接试验,采用15 k W光纤激光,实现了17.5 mm厚CLF-1钢的穿透焊接,得到了正反表面成形良好、无明显缺陷的焊接接头,并对接头显微组织及力学性能进行了分析研究.结果表明,焊缝区主要为粗大的板条马氏体;熔合线附近热影响区为细小的板条马氏体和少量贝氏体;不完全淬火区为经焊接热循环作用下二次回火的回火索氏体及马氏体双相组织;接头室温及550℃高温抗拉强度较高,均断裂于母材;焊缝显微硬度高于母材,且热影响区无明显软化;接头冲击韧性良好.接头综合力学性能良好.