Influence of hot rolling and high speed hydrostatic extrusion on the microstructure and mechanical properties of an ODS RAF steel

An argon gas atomized, pre-alloyed Fe-14Cr-2W-0.3Ti (wt.%) reduced activation ferritic (RAF) steel powder was mechanically alloyed with 0.3wt.% Y₂O₃ nano-particles in an attritor ball mill and consolidated by hot isostatic pressing at 1150 °C under a pressure of 200 MPa for 3 h. In the aim to improve its mechanical properties the ODS steel was then submitted to a thermo-mechanical treatment (TMT): hot rolling (HR) at 850 °C or high speed hydrostatic extrusion (HSHE) at 900 °C, followed by heat treatment (HT).Transmission electron microscopy (TEM) observations of the ODS alloys after TMT and heat treatment revealed the presence of elongated grains in the longitudinal direction, with an average width of 8 μm and an average length of 75 μm, and equiaxed grains, a few microns in diameter, in the transverse direction. Two populations of oxide particles were observed by TEM: large Ti-Al-O particles, up to 250 nm in diameter, usually located at the grain boundaries and small Y-Ti-O nanoclusters, about 2.5 nm in diameter, uniformly distributed in the matrix. Charpy impact tests revealed that the HSHE material exhibits a larger upper shelf energy (5.8 J) than the HR material (2.9 J). The ductile-to-brittle transition temperature of both alloys is relatively high, in the range of 55-72 °C. Tensile mechanical properties of both ODS alloys were found satisfactory over the full range of investigated temperatures (23-750 °C). The HSHE material exhibits better tensile strength and ductility than the HR material. These results indicate that HSHE can be considered as a promising TMT method for improving the mechanical properties of ODS RAF steels.     

Tensile properties and deformation mechanisms of a 14Cr ODS ferritic steel

The search for a new cladding material is part of the research studies carried out at CEA to develop a sodium-cooled fast reactor meeting the expectations of the Generation IV International Forum. In this study, the tensile properties of a ferritic oxide dispersion strengthened steel produced by hot extrusion at CEA have been evaluated. They prove the studied alloy to be as resistant as and more ductile than the other nano-reinforced alloys of literature. The effects of the strain rate and temperature on the total plastic strain of the material remind of diffusion phenomena. Intergranular damage and intergranular decohesion are clearly highlighted.     

Influence of HIP pressure on tensile properties of a 14Cr ODS ferritic steel

An oxide dispersion strengthened ferritic steel with a nominal composition of Fe–14Cr–2W–0.3Ti–0.3Y₂O₃ (in wt.%) was consolidated by hot isostatic pressing at 1150 °C under various pressures in the range of 185–300 MPa for 3 h. The microstructure, microhardness and high temperature tensile properties of the steel were investigated. With increasing compaction pressure the density of specimens also increased, however OM and SEM observations revealed residual porosity in all tested specimens and similar ferritic microstructure with bimodal-like grains and numerous of large oxide particles, located at the grain boundaries. Mechanical testing revealed that compaction pressure has negligible influence on the hardness and tensile strength of the ODS steel, however improves the material ductility.     

Effect of thermo-mechanical processing on microstructure and creep properties of the foils of alloy 617

The effect of rolling and annealing on the microstructure and high temperature creep properties of alloy 617 were investigated. Two types of foil specimens with different thickness reductions were prepared by thermo-mechanical processing. Recrystallization and grain growth were readily observed at specimens annealed at 950 and 1100 °C. The uniform coarse grains increase resistance against creep deformation. The grain size effect in creep deformation was dominant up to 900 °C, while dynamic recrystallization effect became dominant at 1000 °C. Dynamic recrystallization was observed in all the creep deformed foils, even though some specimens had already been (statically) recrystallized during annealing. Steady state creep rates decreased with increasing annealing temperature in the less rolled foils. The apparent activation energy Q_app for the creep deformation increased from 271 to 361 kJ/mol as the annealing temperature increased from 950 to 1100 °C.     

Development and characterisation of a new ODS ferritic steel for fusion reactor application

This paper describes the microstructure, tensile properties and Charpy impact resistance of a reduced activation oxide dispersion strengthened ferritic steel Fe-14Cr-2W-0.3Ti-0.3Y₂O₃ produced by mechanical alloying of a pre-alloyed, gas atomised steel powder with Y₂O₃ particles, compaction by hot extrusion at 1100 °C, hot rolling at 700 °C and heat treatment at 1050 °C for 1 h. At room temperature the material exhibits a high ultimate tensile strength of about 1420 MPa and high yield strength of about 1340 MPa in the transverse direction. In the longitudinal direction the values are about 10% lower, due to the anisotropy of the microstructure (elongated grains in the rolling direction). At 750 °C the material still exhibits relatively high yield strengths of about 325 MPa and 305 MPa in the longitudinal and transverse directions, respectively. The material exhibits reasonable uniform and total elongation values over the temperature range 23-750 °C, in both transverse and longitudinal directions. The material exhibits weak Charpy impact properties in the transverse direction. Charpy impact properties are slightly better in the longitudinal direction, with upper shelf energy of about 4.2 J and a ductile-to-brittle transition temperature of about 8.8 °C.     

低活化马氏体钢的微观结构与力学性能

介绍了作为聚变反应堆候选结构材料的低活化马氏体钢的基本设计思路,初步确定了材料的化学成分和热处理工艺,研究了材料的冶金特性、微观组织和力学性能.同时,对比了添加少量钇和硅对材料性能的影响,发现添加硅可以提高材料强度,同时能保证材料具有足够的塑性和韧性;钇的添加对改善材料的塑性很有帮助,但是会使材料强度降低.     

A simple approach for the modeling of an ODS steel mechanical behavior in pilgering conditions

The optimization of the forming of ODS tubes is linked to the choice of an appropriated constitutive model for modeling the metal forming process. In the framework of a unified plastic constitutive theory, the strain-controlled cyclic characteristics of a ferritic ODS steel were analyzed and modeled with two different tests. The first test is a classical tension–compression test, and leads to cyclic softening at low to intermediate strain amplitudes. The second test consists in alternated uniaxial compressions along two perpendicular axes, and is selected based on the similarities with the loading path induced by the Fe–14Cr–1W–Ti ODS cladding tube pilgering process. This second test exhibits cyclic hardening at all tested strain amplitudes. Since variable strain amplitudes prevail in pilgering conditions, the parameters of the considered constitutive law were identified based on a loading sequence including strain amplitude changes. A proposed semi automated inverse analysis methodology is shown to efficiently provide optimal sets of parameters for the considered loading sequences. When compared to classical approaches, the model involves a reduced number of parameters, while keeping a good ability to capture stress changes induced by strain amplitude changes. Furthermore, the methodology only requires one test, which is an advantage when the amount of available material is limited. As two distinct sets of parameters were identified for the two considered tests, it is recommended to consider the loading path when modeling cold forming of the ODS steel.     

Neutron/proton irradiation and He effects on the microstructure and mechanical properties of ferritic/martensitic steels T91 and EM10

In the European EUROTRANS/DEMETRA program, the synergistic effect of radiation damage and helium on microstructure and mechanical properties of two 9Cr 1Mo ferritic/martensitic (FM) steels T91 and EM10 was evaluated after irradiation in SINQ targets. In addition, the helium induced effect was investigated using helium implanted specimens. The results demonstrate that helium can induce significant embrittlement effect in FM steels as shown by the tremendous increase in ductile-to-brittle transition temperature, the great reduction in ductility and fracture toughness at >∼15 dpa and 1000 appm He and the occurrence of intergranular fracture mode. Further, high-density helium bubbles can produce pronounced hardening effect.     

Effect of Cr on the mechanical properties and microstructure of Fe-Cr model alloys after n-irradiation

High-chromium ferritic-martensitic steels are candidate structural materials for high-temperature applications in fusion reactors and accelerator driven systems (ADS). Cr concentration has been shown to be a key parameter which needs to be optimized in order to guarantee the best corrosion and swelling resistance, together with the minimum embrittlement. The behavior of Fe-Cr model alloys with different Cr concentrations (0, 2.5, 5, 9 and 12 wt%Cr) has been studied. Tensile tests have been performed in order to characterize the flow properties in the temperature range from −160 °C to 300 °C. The trend of the yield strength with temperature shows that the strain hardening is the same for all materials at low temperatures, even though they have different microstructures. The same materials have been neutron-irradiated at 300 °C in the BR2 reactor of SCK·CEN, up to three different doses (0.06, 0.6 and 1.5 dpa). The results obtained so far indicate that even at these low doses, the Cr content affects the hardening behavior of Fe-Cr binary alloys. Using the Orowan mechanism, the TEM observed microstructure provides an explanation of the obtained hardening but only at the very low dose, 0.06 dpa. At higher doses, other hardening mechanisms are needed.     

Effect of direct and reheated quenching on microstructure and mechanical properties of CLAM steels

Direct and reheated quenching on microstructural mechanical properties of China Low Activation Martensitic (CLAM) steel was investigated. Three direct quenching and tempering (DQ&T) CLAM steels were rolled at the same finish rolling temperature with different quenching rate, and their microstructures and mechanical properties were compared with those of a reheated quenching and tempering (RQ&T) CLAM steel. DQ&T process was proven to strengthen the tensile property but lower the toughness compared to RQ&T process due to the finer austenite grain size formed under RQ&T process. Tensile property first increased with quenching rate and then decreased. The reason for this was discussed.     

High-temperature mechanical properties improvement on modified 9Cr-1Mo martensitic steel through thermomechanical treatments

In the framework of the development of generation IV nuclear reactors and fusion nuclear reactors, materials with an improved high temperature (≅650 °C) mechanical strength are required for specific components. The 9-12%Cr martensitic steels are candidate for these applications. Thermomechanical treatments including normalisation at elevated temperature (1150 °C), followed by warm-rolling in metastable austenitic phase and tempering, have been applied on the commercial Grade 91 martensitic steel in order to refine its microstructure and to improve its precipitation state. The temperature of the warm-rolling was set at 600 °C, and those of the tempering heat-treatment at 650 °C and 700 °C thanks to MatCalc software calculations. Microstructural observations proved that the warm-rolling and the following tempering heat-treatment lead to a finer martensitic microstructure pinned with numerous small carbide and nitride particles. The hardness values of thermomechanically treated Grade 91 steel are higher than those of the as-received Grade 91. It is also shown that the yield stress and the ductility of the thermomechanically treated Grade 91 steel are significantly improved compared to the as-received material. Preliminary creep results showed that these thermomechanical treatments improve the creep lifetime by at least a factor 14.     

Effect of sodium environment on the low cycle fatigue properties of modified 9Cr-1Mo ferritic martensitic steel

Modified 9Cr-1Mo ferritic steel is the material of current interest for the steam generator components of liquid metal cooled fast breeder reactors (LMFBRs). The steam generator has been designed to operate for 30-40 years. It is important to accurately determine the life of the components in the actual environment in order to consider the extension of life beyond the design life. With this objective in view, a programme has been initiated at our laboratory to evaluate the effects of flowing sodium on the LCF behaviour of modified 9Cr-1Mo steel. LCF tests conducted in flowing sodium environment at 823 K and 873 K exhibited cyclic softening behaviour both in air and sodium environments. The fatigue lives are significantly improved in sodium environment when compared to the data obtained in air environment under identical testing conditions. The lack of oxidation in sodium environment is considered to be responsible for the delayed crack initiation and consequent increase in fatigue life. Comparison of experimental lifetimes with RCC-MR design code predictions indicated that the design curve based on air tests is too conservative.     

Effect of biaxial loading on the fracture behaviour of a ferritic steel component

The effect of biaxial loading on the ductile behaviour of a through-wall crack in a ferritic steel structure under contained yield is of particular interest to the structural integrity argument for reactor pressure vessels. This results from the fact that there are many instances in practice (for example a crack in a circumferential weld), where a significant applied stress is present in the direction parallel to the crack as well as in the perpendicular direction. Two large plate ductile tearing tests have been performed on centre through-crack specimens (75 mm by 2 m by 2 m) manufactured from a ferritic steel. The first test specimen was loaded in uniaxial tension and the second test specimen was loaded biaxially. This paper presents experimental details and results of the two plate tests and describes the analysis work undertaken to interpret the experiments satisfactorily.     

Characterization of recrystallization of 12Cr and 15Cr ODS ferritic steels

The recrystallization behavior of 12Cr and 15Cr oxide dispersion-strengthened (ODS) ferritic steels, which are the promising candidate materials for long-life core materials of the advanced fast breeder reactors, was investigated in terms of an intermediate softening heat treatment. It was clarified that keeping recovery structure at the intermediate heat treatment is indispensable for producing recrystallized structure at the final heat treatment. Prevention of repeating recrystallization is owing to the stable {100} 〈110〉 texture formation with less stored strain energy by the cold-rolling of the recrystallized structure. The two-step softening process was proposed to suppress the recrystallization and obtain adequate hardness reduction at the intermediate heat treatment. This process is effective for producing a stable recrystallized structure at the final heat treatment of the manufacturing process of ODS ferritic steel cladding.     

The effects of heat treatments on the microstructure and mechanical properties of the Ti–2.19Al–2.35Zr alloy

Ti–2.19Al–2.35Zr alloy is one of the candidate materials for the steam generator tubing of an integrated reactor, System Modular Integrated Advanced ReacTor (SMART) being developed in Korea. In this study, the effects of heat treatments on the mechanical properties of Ti–2.19Al–2.35Zr alloy were evaluated. Mechanical tests were implemented to examine the effects of an annealing, cooling rate and re-annealing temperature/time on the mechanical properties of the alloy. The annealing temperatures ranged from 600 to 1050 °C and the cooling rates were controlled by introducing a water-quenching (WQ), air-cooling (AC), and furnace-cooling (FC). As for the re-annealing heat treatment, after a β water quenching, the re-annealing temperature was selected as 800 °C for the α-phase heat treatment and 940 °C for the α + β-phase heat treatment with various time intervals (1, 10 and 24 h). The results showed that an increase of the annealing temperature to above the β-region temperature induced an increase of the tensile strength and a decrease of the elongation in the 25 and 300 °C tests. A decrease of the cooling rates from water-quenching to a furnace-cooling revealed a decrease of the tensile strength and an increase of the elongation. Also an increase of the re-annealing time with different phase regimes exhibited a decrease of the strength and an increase of the elongation. These tendencies were more dominant in the 300 °C test rather than the room temperature test from the characteristics of the microstructures which were affected by the heat treatments.     

The influence of helium on the high temperature mechanical properties of an austenitic stainless steel

Foil specimens of D1N 1.4970 austenitic stainless steel were cyclotron implanted to uniform helium concentrations up to 150 ppm. Subsequently, tensile and creep tests at different stress levels and temperatures between 600 and 800°C were performed and compared with the results from He-free control specimens. Whereas the latter exhibit transgranular ductile fracture, the implanted material breaks by intergranular failure with a corresponding reduction of the elongation to fracture values by factors between 3 and 6. These results and the TEM and SEM investigations of the microstructure are discussed by means of an embrittlement model which is based on the stress assisted nucleation and growth of helium filled grain boundary cavities.     

Effect of V and Nb on precipitation behavior and mechanical properties of high Cr steel

Employment of high Cr steel as a main structural material is considered as a way to achieve economical competitiveness of fast breeder reactors. V and Nb are believed to improve the high temperature strength of high Cr steels by precipitating as carbides and/or nitrides, namely fine MX particles. However, the long term efficiency and stability of such precipitation strengthening mechanisms provided by the fine MX particles have not been clarified yet. A series of trial products controlling V and Nb contents is produced and mechanical tests are conducted to investigate the effect of these elements on the mechanical properties and the long term stability of the MX strengthening mechanism. Before and after a long term aging process, metallurgical examinations and quantitative analyses are conducted to investigate the effect of these elements on microstructure evolutions. Based on these results, the long term efficiency and stability of the strengthening mechanisms provided by the fine MX particles are discussed. Higher strength and lower ductility are obtained with the increases of V and Nb contents, although the influence of Nb content tends to be saturated at about 0.01 mass%. MX does not grow and any new precipitates cannot be observed after aging at 873 K for 6000 h. Therefore, it is expected that MX is stable after aging at 823 K for approximately 167,000 h based on Larson-Miller parameter.     

CLAM钢的钨极氩弧焊及焊接后的结构与性能

利用钨极氩弧焊(TIG)焊接方法对中国自行研制的低活化铁素体/马氏体(CLAM)钢进行焊接试验,分别采用了Y型坡口和双Y型坡口两种焊接工艺。对焊接件各部分的硬度和显微结构进行测试分析,利用透射电镜观测材料的微观形貌并进行析出物相分析,以此对CLAM钢的焊接性能进行初步评价。观察发现,焊缝区没有产生缩孔和裂纹;焊缝区和母材的微观形貌主要为板条马氏体,板条宽度约为600 nm,其析出物均为20~200 nm的M23C6型碳化物;经过焊后热处理,焊缝区、热影响区和母材的硬度之间的差别减小。     

Effect of neutron irradiation on the mechanical properties of an alpha-titanium alloy

Translated from Atomnaya Énergiya, Vol. 69, No. 6, pp. 382–386, December, 1990.