Peculiar Features of Thermal Aging and Degradation of Rapidly Quenched Stainless Steels under High-Temperature Exposures

This article presents the results of comparative studies of mechanical properties and microstructure of nuclear fuel tubes and semifinished stainless steel items fabricated by consolidation of rapidly quenched powders and by conventional technology after high-temperature exposures at 600 and 700°C. Tensile tests of nuclear fuel tube ring specimens of stainless austenitic steel of grade AISI 316 and ferritic–martensitic steel are performed at room temperature. The microstructure and distribution of carbon and boron are analyzed by metallography and autoradiography in nuclear fuel tubes and semifinished items. Rapidly quenched powders of the considered steels are obtained by the plasma rotating electrode process. Positive influence of consolidation of rapidly quenched powders on mechanical properties after high-temperature aging is confirmed. The correlation between homogeneous distribution of carbon and boron and mechanical properties of the considered steel is determined. The effects of thermal aging and degradation of the considered steels are determined at 600°C and 700°C, respectively.     

Cumulative second-harmonic analysis of ultrasonic Lamb waves for ageing behavior study of modified-HP austenite steel

The cumulative second-harmonic analysis of ultrasonic Lamb wave has been performed to study the precipitation kinetics and microvoid initiation of ferritic Cr–Ni alloy steel during the ageing process. Ageing of ferritic Cr–Ni alloy materials have been done at 1223 K and 1173 K for different degradation time intervals and air cooled. The results show that the normalized acoustic nonlinearity of Lamb wave increases with the formation of fine precipitates at the early stage of ageing till about 1000 h and keeps as a plateau with the precipitates dynamic balance for a long-term ageing, and then decreases gradually at the final holding time with the coarsening of precipitates and initiation of microvoids. The results also show that the variation of nonlinear Lamb wave follows the same trend as that of hardness in materials. Therefore, the cumulative second-harmonic of ultrasonic Lamb waves has been found to be strongly sensitive to the precipitates behavior and microstructure evolution during the thermal ageing of ferritic Cr–Ni alloy steel.     

Crystal plasticity study on stress and strain partitioning in a measured 3D dual phase steel microstructure

Dual phase steels are advanced high strength alloys typically used for structural parts and reinforcements in car bodies. Their good combination of strength and ductility and their lean composition render them an economically competitive option for realizing multiple lightweight design options in automotive engineering. The mechanical response of dual phase steels is the result of the strain and stress partitioning among the ferritic and martensitic phases and the individual crystallographic grains and subgrains of these phases. Therefore, understanding how these microstructural features influence the global and local mechanical properties is of utmost importance for the design of improved dual phase steel grades. While multiple corresponding simulation studies have been dedicated to the investigation of dual phase steel micromechanics, numerical tools and experiment techniques for characterizing and simulating real 3D microstructures of such complex materials have been emerged only recently. Here we present a crystal plasticity simulation study based on a 3D dual phase microstructure which is obtained by EBSD tomography, also referred to as 3D EBSD (EBSD—electron backscatter diffraction). In the present case we utilized a 3D EBSD serial sectioning approach based on mechanical polishing. Moreover, sections of the 3D microstructure are used as 2D models to study the effect of this simplification on the stress and strain distribution. The simulations are conducted using a phenomenological crystal plasticity model and a spectral method approach implemented in the Düsseldorf Advanced Material Simulation Kit (DAMASK).     

Autoradiography of Light Elements in Austenite Steels tested in Liquid Metals

The multielement autoradiography has been used to investigate the redistribution of boron, carbon, oxygen, sodium and phosphorus in an austenitic chromium-nickel steel exposed to a flow of sodium. Under all experimental conditions, the steel samples became impoverished with respect to boron, carbon and phosphorus and this phenomenon became more pronounced with the increases in both the temperature from 923 to 1073 K and the time of the experiment from 500 to 8000 h. The sodium and oxygen penetration of the steel was intercrystalline by character, oxygen being approximately 10 μm ahead of sodium. The redistribution of boron and carbon in the steel samples with and without boron has been investigated after the exposure to lithium and data of the steel penetration by lithium have been obtained. Lithium penetration followed the grain boundaries of steel.     

JLF-1钢高温循环变形后硬度与微观结构的数值关系

高温循环变形是结构材料性能降级的主要原因之一.用透射电子显微镜对低活化铁素体/马氏体钢——JLF-1钢低周疲劳样品的微观结构进行了分析,并测试了循环变形前后此钢显微维氏硬度的变化.为了掌握JLF-1钢性能在高温循环变形中的变化机理,依据位错理论,用最小二乘法对高温循环变形后的显微维氏硬度与微观结构进行了回归计算,得到了此钢显微维氏硬度与板条尺寸、位错胞尺寸、位错密度的数值关系. 关键词： 低活化铁素体/马氏体钢 循环变形 微观结构 显微维氏硬度     

Research on splitting phenomenon of isochronal martensitic transformation in T91 ferritic steel

T91 steel is a representative type of ferritic heat-resistant steel currently used in power plant components, and is a potential candidate for structural steel in nuclear reactors. The isochronal martensitic transformation behaviors during continuous cooling after austenitization in T91 ferritic steel were systematically investigated by high-resolution dilatometry and microstructure observation. The splitting phenomenon of martensitic transformation is accompanied with the precipitation of needle-like M₃C particles, which is suppressed by rapid cooling after austenitization. The appearance of this splitting is ascribed to the concentration gradient caused by the consumption of alloy element in process of the formation of M₃C. This concentration gradient results in the appearance of wide martensitic laths ahead of the generation of normally narrow laths. These two types of martensitic laths possess different M _s (martensitic start transformation) temperatures, which are attributed to the splitting transformation phenomenon.     

Microstructure,Mechanical Properties and Fracture of EP-823 Ferritic/Martensitic Steel After High-Temperature Thermomechanical Treatment

Russian Physics Journal - The paper investigates the microstructure and mechanical properties of the EP-823 ferritic/martensitic steel containing 12 wt.% chromium after the high-temperature...     

Microstructure evolution of T91 steel after heavy ion irradiation at 550 ℃

Fe-Cr ferritic/martensitic (F/M) steels have been proposed as one of the candidate materials for the Generation IV nuclear technologies. In this study, a widely-used ferritic/martensitic steel, T91 steel, was irradiated by 196-MeV Kr⁺ ions at 550 ℃. To reveal the irradiation mechanism, the microstructure evolution of irradiated T91 steel was studied in details by transmission electron microscope (TEM). With increasing dose, the defects gradually changed from black dots to dislocation loops, and further to form dislocation walls near grain boundaries due to the production of a large number of dislocations. When many dislocation loops of primary a₀/2<111> type with high migration interacted with other defects or carbon atoms, it led to the production of dislocation segments and other dislocation loops of a₀<100> type. Lots of defects accumulated near grain boundaries in the irradiated area, especially in the high-dose area. The grain boundaries of martensite laths acted as important sinks of irradiation defects in T91. Elevated temperature facilitated the migration of defects, leading to the accumulation of defects near the grain boundaries of martensite laths.     

Cavity swelling of RAFM steel under 792 MeV Ar-ions irradiation at 773 K

China reduced-activation ferritic/martensitic steel is irradiated at 773 K with 792 MeV Ar-ions to fluences of 2.3×10²⁰ and 4.6× 10²⁰ ions/m², respectively. The variation of the microstructures of the Reduced-activation ferritic/martensitic (RAFM) steel samples with the Ar-ion penetration depth is investigated using a transmission electron microscope (TEM). From analyses of the microstructure changes along with the Ar-ions penetrating depth, it is found that high-density cavities form in the peak damage region. The average size and the number density of the cavities depend strongly on the damage level and Ar-atom concentration. Swelling due to the formation of cavities increases significantly with an increased damage level, and the existence of deposited Ar-atoms also enhances the growth of the average size of the cavities. The effect of atom displacements and Ar-atoms on the swelling of the RAFM steel under high energy Ar-ion irradiation is discussed briefly.     

Effect of hydrogen and helium on the properties of nuclear reactor materials

The effect of hydrogen and helium on the physicochemical properties of ferritic steel has been investigated. Nondestructive testing was used: acoustic methods, nanoindentation, and the eddy-current method. A nonlinear dependence of the nanoindentation parameters and acoustic emission signals on the helium implantation dose is shown. It is established that helium implantation significantly increases the speed of sound, whereas hydrogen only slightly affects this parameter.     

Kinetics of isochronal austenization in modified high Cr ferritic heat-resistant steel

Employment of high Cr ferritic steels as a main structural material is considered as a way to achieve economical competitiveness of main steam pipe and nuclear reactors in power plants. Differential dilatometry and microstructure observation were employed to investigate the isochronal austenitic transformation of the modified high Cr ferritic steel. The kinetics of the isochronal austenitic transformation were described by a phase-transformation model involving site saturation (pre-existing nuclei), diffusion-controlled growth, and incorporating an impingement correction. The experimental results and kinetic analysis indicate that an increase of the heating rate promotes the diffusion-controlled austenitic transformation. The dissolving degree of precipitates during the austenization process affects the activation energy for diffusion and the undissolved precipitates lead to an increase of the onset temperature of the subsequent martensite transformation upon cooling.     

Investigation of laminar plasma remelting/cladding processing

Investigation of remelting and cladding processing with laminar plasma jets on several metals has been conducted looking for possible development of a new surface modification technique. The remelting tests illustrated that the new method could evidently improve the material microstructure and properties of cast iron. The cladding was done with Al₂O₃ ceramic powder on stainless steel. The energy dispersive spectra (EDS) analysis was used to determine the distribution of the major cladding element in the plasma-processed layers, for which the microstructure observations and hardness measurements were also performed.     

Investigation of boron distribution in semiconductors and Mo---Re alloys by SSNTD method

Alpha particle track detection technique with cellulose nitrate film for determination of boron contents in solids was developed. The technique was applied to control non-uniformity of boron distribution on the surface of Cd_xHg_1−xTe, to study boron behavior in polycrystalline beta-silicon carbide, to determine local boron concentration and its micro distribution in monocrystalline silicon carbide, silicon, “mozaic” silicon structures and the Mo---Re alloys.     

Study of Diffusion Bonding of 45 Steel through the Compacted Nickel Powder Layer

The microstructure of the transition zone and powder spacer, the concentration distribution of chemical elements over the width of the diffusion-bonded joint, and microhardness of 45 steel–compacted Ni powder spacer–45 steel layered composites formed by diffusion bonding have been investigated. It has been shown that the relative spacer thickness χ < 0.06 is optimal for obtaining a high-quality joint has been formed under a compacting pressure of 500 MPa. The solid-state diffusion bonding is accompanied by sintering the nickel powder spacer and the formation of the transition zone between the spacer and steel. The transition zone consists of solid solution of nickel in the α-Fe phase and ordered solid solution of iron in nickel (FeNi₃).     

On Quantitative Autoradiography of Semiconductor Basic Material

It will be described methods of the quantitative evaluation of elements interesting in semiconductor technology and their distribution in silicon using autoradiographic techniques. The local concentration of the element phosphorus in dependence of their local distribution is determined with silver halide films. As standard samples silicon disks phosphorus diffused are used.

The distribution of the element boron and their local concentration is determined on the way of neutron induced autoradiography (NIAR). In this technique boron implanted or diffused silicon disks are used as standard samples. Different possibilities of the quantitative evaluation of autoradiograms will be considered and compared.     

Evaluation of particle size distributions of precipitates in a 9% chromium steel using energy filtered transmission electron microscopy

Energy filtered transmission electron microscopy (EFTEM) has been used to evaluate particle size distributions of different phases in the creep resistant 9% chromium steel P92, aged at 600 or 650°C for up to 26,000 h. Precipitates of type VN, M₂₃C₆ and Laves phase were visualised by forming so-called jump ratio images of the elements vanadium, chromium and tungsten. Digital image analysis was used to measure the size of the precipitates at different ageing times. As a complement to EFTEM, atom probe field ion microscopy (APFIM) has been used to monitor the composition of the ferritic matrix. This indirectly gives information about precipitated volume fraction of different phases. In the case of VN precipitates the volume fraction was also determined from EFTEM images and the result was in good agreement with the data obtained with APFIM. The high-temperature stability of the secondary phases present in steel P92 and their relation to creep properties are discussed. To further study the applicability of EFTEM, steel P122 (with a composition similar to steel P92, but with an addition of Cu) was investigated and a Cu-rich phase was found.     

Application of SSNTDs for study of boron distribution in alloys

A study of boron behavior and its distribution in boron doped iron-, nickel- and aluminum-base alloys subjected to various treatment conditions has been performed. For this purpose a technique based on the detection of ¹⁰B(n,)⁷Li reaction products with track etch detectors has been applied. High boron enrichment of grain boundaries of both iron base and ingot nickel base alloys has been observed. After hot and then cold rolling of the latter specimens boron rearrangement along with the proper deformation of the structure took place. Striation of boron in a hot rolled aluminum alloy along the rolling direction has been observed.     

Microstructure and mechanical properties of nano-Y2O3 dispersed ferritic steel synthesized by mechanical alloying and consolidated by pulse plasma sintering

Ferritic steel with compositions 83.0Fe–13.5Cr–2.0Al–0.5Ti (alloy A), 79.0Fe–17.5Cr–2.0Al–0.5Ti (alloy B), 75.0Fe–21.5Cr–2.0Al–0.5Ti (alloy C) and 71.0Fe–25.5Cr–2.0Al–0.5Ti (alloy D) (all in wt%) each with a 1.0?wt% nano-Y₂O₃ dispersion were synthesized by mechanical alloying and consolidated by pulse plasma sintering at 600, 800 and 1000°C using a 75-MPa uniaxial pressure applied for 5?min and a 70-kA pulse current at 3?Hz pulse frequency. X-ray diffraction, scanning and transmission electron microscopy and energy disperse spectroscopy techniques have been used to characterize the microstructural and phase evolution of all the alloys at different stages of mechano-chemical synthesis and consolidation. Mechanical properties in terms of hardness, compressive strength, yield strength and Young's modulus were determined using a micro/nano-indenter and universal testing machine. All ferritic alloys recorded very high levels of compressive strength (850–2850?MPa), yield strength (500–1556?MPa), Young's modulus (175–250?GPa) and nanoindentation hardness (9.5–15.5?GPa), with up to 1–1.5 times greater strength than other oxide dispersion-strengthened ferritic steels (<1200?MPa). These extraordinary levels of mechanical properties can be attributed to the typical microstructure of uniform dispersion of 10–20-nm Y₂Ti₂O₇ or Y₂O₃ particles in a high-alloy ferritic matrix.     

硼膜制备工艺、微观结构及其在硼化镁超导约瑟夫森结中的应用

MgB_2材料具备临界转变温度较高、相干长度大、临界电流和临界磁场高等优点,被认为有替代Nb基超导材料的潜力.研究了不同温度下以化学气相沉积法制备的硼(B)薄膜的微观结构.实验结果表明:较低温度沉积的B先驱薄膜为无定形B膜,可以与Mg蒸气反应生成MgB_2超导薄膜;当沉积温度高于550?C时,所得硼薄膜为晶型薄膜;以晶型硼薄膜为先驱膜在镁蒸气中退火,不能生成硼化镁超导薄膜.利用晶型B膜的这一特点,成功制备了以晶型硼薄膜为介质层的硼化镁超导约瑟夫森结.     

The role of nitrogen in the preferential chromium segregation on the ferritic stainless steel (1 1 1) surface

The temperature dependence on the segregation behavior of the ferritic stainless steel single crystal (1 1 1) surface morphology has been examined by scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). AES clearly showed the surface segregations of chromium and nitrogen upon annealing. Nanoscale triangular chromium nitride clusters were formed around 650 °C and were regularly aligned in a hexagonal configuration. In contrast, for the ferritic stainless steel (1 1 1) surface with low-nitrogen content, chromium and carbon were found to segregate on the surface upon annealing and Auger spectra of carbon displayed the characteristic carbide peak. For the low-nitrogen surface, LEED identified a facetted surface with (2 × 2) superstructure at 650 °C. High-resolution STM identified a chromium carbide film with segregated carbon atoms randomly located on the surface. The facetted (2 × 2) superstructure changed into a (3 × 3) superstructure with no faceting upon annealing at 750 °C. Also, segregated sulfur seems to contribute to the reconstruction or interfacial relaxation between the ferritic stainless steel (1 1 1) substrate and chromium carbide film.