Ferritic Martensitic Steel T91 Research Articles

Magnetic pulse welding of thin walled ferritic martensitic steel T91 tube to T91 rod

Magnetic pulse welding of thin walled ferritic martensitic steel T91 tube to T91 rod

Solute segregation and precipitation across damage rates in dual-ion–irradiated T91 steel

Dual-ion irradiations using 5.0 MeV defocused Fe2+ ions and co-injected energy degraded 2.00 to 2.85 MeV He2+ ions were conducted on a Fe9CrMo ferritic-martensitic steel T91 to 17 dpa at a damage rate range of 5 × 10−5 dpa/s to 3 × 10−3 dpa/s at 445°C, followed by characterization of the microstructure using conventional and scanning transmission electron microscopy. Radiation induced Ni/Si clusters and radiation induced segregation were quantified using energy dispersive X-ray spectroscopy at each condition and were compared with the same material irradiated in the BOR-60 reactor and in the as-received condition. No significant Cr segregation was found at lath boundaries after dual-ion irradiation, while Ni and Si enrichments both decreased with increasing damage rate leading to a sharp decrease in the density of Ni/Si clusters with damage rate. Increased point defect recombination at higher ion damage rates likely reduced the Ni/Si cluster density compared with BOR-60. Although the overall vacancy concentration and diffusion are enhanced by the irradiation damage rate, the lack of time for thermal diffusion and ballistic displacements of solutes are significant limiting factors for Ni/Si cluster formation. This work demonstrates the effect of irradiation damage rate on elemental segregation and clustering when using ion irradiation to simulate reactor irradiation.

Early-stage microstructural evolution and phase stability in neutron-irradiated ferritic-martensitic steel T91

A Fe-9Cr ferritic martensitic (F/M) steel T91 was neutron-irradiated in the Advanced Test Reactor up to 3.96 dpa in two temperature ranges, 466 °C to 534 °C and 571 °C to 632 °C. The microstructure evolution including dislocation loops, precipitation, segregation of elements and phase stability were studied using analytical scanning-transmission electron microscopy and atom probe tomography. The hardening induced by irradiation was measured by nanoindentation. Ni/Si/Mn clusters were identified in all conditions except the one irradiated around 600 °C to 3.23 dpa. The compositions of Ni/Si/Mn clusters were found to be converging to G phase stoichiometrically with increasing dose, with Mn partially substituted by Cu. Significant coarsening of this phase was observed in high temperature cases, with total dissolution of intragranular G phase after prolonged irradiation. A similar trend was also identified for dislocation loops. The results obtained in this experiment provide evidences that the absence of dislocation loops in some specimens irradiated to high dose level under high irradiation temperature range (typically above 500 °C) is not due to the suppression of nucleation and growth by high point defect recombination rate, but rather fast coalescence of dislocation loops. Hardness measurements show different dose dependences for two temperature ranges. For specimen irradiated around 600 °C significant hardening before 0.5 dpa followed by softening process was observed, while in lower temperature range (450 °C ~ 500 °C) the normal hardening pattern with increasing dose was observed.

A new approach to thermo-mechanical treatment of steel T91 by multiple upsetting-extrusion in a ferritic range

The microstructure and mechanical properties of the ferritic-martensitic steel T91 after severe plastic deformation (SPD) and subsequent thermal treatment are investigated. SPD was realized by the method of multiple “upsetting-extrusion” (MUE) in temperature range of stability of ferritic phase (750-575 °C), thermal treatment of SPD specimens was carried out at temperatures 550–730 °C during 1–100 h. It is revealed that decrease of temperature of “upsetting-extrusion” and increase of cycle number to 5 induces the decrease of mean grain size to 145 nm and increase of degree of microstructure homogeneity. At the same time microhardness increases from 1800 MPa to 2850 MPa. Composition, shape, size and density of carbide precipitates in steel after SPD and thermal treatment are determined. Obtained submicron grain ferritic microstructure with high value of microhardness is stable at exposure temperature of 550 °C. It is shown that temperature variation of thermal treatment induces the considerable change in grain size and mechanical characteristics.

EFFECT OF ARGON ION IRRADIATION ON HARDENING AND MICROSTRUCTURE OF FERRITIC-MARTENSITIC STEEL T91

Ferritic-martensitic steel T91 was irradiated with 1.4 MeV Ar+ ions to doses from 0.5 to 7 displacements per atom (dpa) at room temperature. Microstructure characterization was performed using transmission electron microscopy (TEM). TEM observations indicated that black dots and dislocation loops dominated the damage microstructure after ion irradiation. The nanoindentation technique was used to find out the changes in the hardness values with irradiation dose. Nanoindentation tests were performed at room temperature in continuous stiffness measurement mode before and after the irradiation. Nix-Gao model was used to extract the bulk-equivalent nanohardness of specimens. Pile-up of material around the indentation site that affects the calculated contact area was taken into account for correct interpretation of the mechanical properties of the irradiated material. It is shown that ion irradiation leads to hardening by 21% (∆H ~ 0.7 GPa) which reaches quasi saturation at doses ≥1 dpa.

Microstructural Investigation of LME Crack Initiated in Ferritic/Martensitic Steel T91 Loaded in Liquid Lead-Bismuth Eutectic at 300 °C.

Liquid lead-bismuth eutectic (LBE) is one of the candidate materials for advanced nuclear systems. The structural materials used in contact with LBE are selected according to the resistance to liquid metal corrosion, irradiation embrittlement, and compatibility with the coolant. However, simultaneous presence of mechanical strain and LBE environment can induce liquid metal embrittlement (LME) in these materials. In this study, a specimen from candidate ferritic-martensitic steel T91 was tested by Constant Extension Rate Tensile (CERT) test exposed to PbBi environment with oxygen concentration 6 × 10−6 wt % at 300 °C up to rupture. Post-test examination using scanning electron microscopy (SEM) showed a deep crack indicating features of LME in the plastic strained region of the tested specimen. Further investigations focused on characterization of the fracture path and microstructure determination using focused ion beam (FIB) and energy dispersive X-ray spectrometry/electron backscatter diffraction (EDX/EBSD). This observation revealed that the dominant LME failure mode of the observed crack is translath or transgranular and the crack stopped at the high-angle grain boundary. The role of oxides in the crack initiation is discussed.

Effect of Applied Stress on T91 Steel Performance in Liquid Lead at 400 °C.

The environmental performance of structural materials (e.g., corrosion and environmental assisted cracking) is one of the critical issues being addressed in the development of Generation IV fast reactors. This work aims to support the study of the performance of the ferritic-martensitic steel T91 in liquid lead, under close to operation conditions, in order to assess its suitability for an application in lead fast reactors. T91 steel was tested in air and liquid lead at 400 °C using static and slow loading regimes. Applied stresses were chosen to be slightly above the yield strength in order to evaluate the threshold stress and strain for crack initiation. Three-point bending static exposure and constant extension tests of tapered specimens were performed. Post-test surfaces and cross sections of specimens were observed using scanning electron microscopy techniques in order to detect cracks and to analyze oxide layers. The effect of strain rate of the oxide layer cracking was observed. In conclusion, T91 was not susceptible to liquid metal embrittlement, a special case of environmentally assisted cracking under the testing conditions. The cracking conditions are discussed based on previous experience.

Coatings on Steels T91 and 316L in Lead-Bismuth Eutectic Environment

The work is aimed on the coatings used as a barrier against Heavy Liquid Metal (HLM) influence and coated material behaviour under mechanical loading. The comparison of two kinds of coatings deposited on two different steels is examined: ferritic-martensitic steel T91 coated with AlTiN and austenitic steel 316L coated with Al2O3. Both samples underwent tensile tests with constant extension rate in the HLM autoclave-cell for material testing. The testing environment in the cell was liquid PbBi eutectic at the temperature of 550 °C. The examination by Scanning electron microscopy and Energy-dispersive X-ray spectroscopy followed after exposure. The yield strength of the steel was exceeded in both cases, cracks and adhesion were studied in depth to characterize their resistance to the simultaneous effect of load and environment. Both coatings offered protection against corrosion; however, localized cracking gave rise to the damage of the substrate. These coatings provided a corrosion barrier, which once damaged is not able of self-healing.

CHARACTERISATION OF OXIDES BY ADVANCED TECHNIQUES

For the safe development of GenIV nuclear reactors, it is necessary to study the compatibility of structural materials with new coolants. The current work describes the behavior of the ferritic-martensitic steel T91 in a Heavy Liquid Metal environment. Specimens were pre-stressed up to yield strength and subsequently exposed to lead-bismuth eutectic (LBE) in static conditions for 2000 hours. The aim was to identify the susceptibility to crack initiation in the selected experimental conditions. In a reference position of the sample the examination of the metal-LBE interface was carried out by means of SEM equipped with EDX. On the interface, the formation of oxide scales was observed without trace of crack initiation. The oxide was characterized by a two layers structure. From the sample, a TEM lamella was produced by FIB and subsequently the lamella was analyzed in HRTEM. The individual oxide layers were identified and characterized by SAED, EELS and EDS techniques. For a visualization of the interface between matrix and oxide STEM-HAADF and EFTEM techniques were used.

Investigation on early formation and evolution of oxide scales on ferritic–martensitic steels in supercritical water

Early oxidation characteristics of ferritic-martensitic steel T91 in supercritical water (SCW) from 1 h to 120 h were investigated. Relative to austenitic steels, occurrence of each oxidation stage for T91 was delayed. Both inner Cr-rich layer and outer magnetite layer formed even after a 1h-exposure at 540 °C, implying their almost simultaneous formations. Several assembled-pore lines were observed in the inner layer. Abundant pores accumulated at the boundary between the inner/outer oxide layers in steam, while mainly along the inner/diffusion layer interface in SCW, due to lower difference between iron diffusion through the inner and outer layers for the later.

Analysis of the Coating for Heavy Liquid Metal Applications

Abstract This paper deals with the behavior of the ferritic-martensitic steel T91 samples covered with an AlTiN black coating, which is mainly used for high temperature applications (> 800 ° C) because of its resistance to oxidation. The coating was applied by use of a combination of High Power Impulse Magnetron Sputtering (HiPIMS) and Direct Current Magnetron Sputtering (DCMS). Samples were subjected to a tensile test in a static tank CALLISTO. The environment was liquid PbBi eutectic at a temperature of 550° C. Two types of samples, with a notch in the middle and without a notch, were tested. After exposure, the samples were subjected to morphological and chemical analyzes on SEM and EDX. Although the coating cracked over the entire length of the sample, high adhesion of the layer was demonstrated, the coating was delaminated locally only in the notch.

Analysis of resistance of T91 steel under load in liquid PbBi

Abstract This paper describes behaviour of the ferritic-martensitic steel T91 after long-term exposure in liquid eutectic PbBi. Small specimens of the steel were pre-loaded according to the ISO Standard No. 7539-2 and subsequently exposed to PbBi. The experiment was carried out in a flowing and static liquid metal medium at 350°C. The experimental time was selected for the flowing environment at 1000 hours and for static environment at 2000 hours. After exposure, the specimens were analysed by use SEM and EDX. Surface changes of the specimens from different experimental conditions were compared and discussed. The initiation of the cracks wasn´t observed in the monitored expositions.

Behavior of the Steel T91 under Multi Axial Loading in Contact with Liquid and Solid Pb

In this work, the conditions for the occurrence of Liquid Metal Embrittlement (LME) in the ferritic-martensitic steel T91 in contact with lead, Pb, were examined. Slow tensile tests with notched specimens revealed that in a temperature range close to the melting point of Pb, the steel is sensitive to LME (350–400 °C) and to Solid Metal Induced Embrittlement (SMIE) (300 °C). The cracking was stimulated by wetting (using a chemical flux) and the notch effect. It was found that the multi axial stresses state and the high level of plastic strain in front of the notch were the key factors triggering crack initiation.

Critical evaluation of the small punch test as a screening procedure for mechanical properties

Within a collective exercise, a systematic and statistically based analysis of the repeatability and device dependence of small punch test results was performed. An unirradiated ferritic-martensitic steel T91 was selected for this. The test results allowed an evaluation of accuracy and reliability of material properties extracted from load–displacement-based and energy-based parameters. In a second step, neutron irradiated T91 with doses of 2.3 and 4 dpa was investigated in the temperature range −165 °C to 300 °C. The effects of test temperature and irradiation on the load–displacement curves and on the derived parameters were analysed. It was found that the small punch test is well suited for estimation of neutron embrittlement in cases of small amounts of available material or high activity. The preferred procedure for that purpose was specified.

Approach for Determining Fracture Mechanical Properties from Tests on Small Size Specimens at Room Temperature

An approach based on finite-element (FE) simulations of the crack propagation process is used to determine fracture mechanical properties on sub-size specimens. The objective is to establish small size testing technology for the determination of fracture mechanical properties. The applicability of the evaluation procedure is verified on the ferritic-martensitic steel T91. In this approach the fracture process is described by using a cohesive zone model (CZM). The CZM model needs two parameters to be identified on sub-size specimens.The geometry of sub-size three point bending specimen is 27x3x4 mm3 with side grooves. To identify both parameters another experimental test on notched sub-size tensile specimens is required. After parameter identification on sub-size specimens simulation of a standard-size specimen is performed using the same cohesive zone parameters to predict the J-R curve of the standard-size specimen.

LIMETS 3, a novel system for high strain fatigue testing in lead–bismuth eutectic

Lead Bismuth Eutectic (LBE) is known to induce under some particular conditions, an embrittlement which can lead to a strong reduction of ductility during tensile elongation and to a reduction of the fatigue endurance. In order to provide the specific fatigue data necessary for the design of the LBE cooled MYRRHA reactor, a new fatigue testing equipment has been developed at SCK•CEN. The LIMETS 3 high strain fatigue set-up has the following main characteristics: it is equipped with a mechanical extensometer placed on the gauge length and capable of working immersed in stagnant LBE at temperatures between 200 and 500°C. It is connected to a gas conditioning system using a mixture of Ar and hydrogen or oxygen, to control the oxygen concentration of the liquid metal. The oxygen concentration is measured by a potentiometric oxygen sensor composed of yttria stabilized zirconia as a solid electrolyte and Bi/Bi2O3 as a reference electrode.The first results obtained in LBE at 300, 350 and 400°C with the ferritic–martensitic steel T91 and the austenitic steel 316L are presented and compared with results obtained at room temperature, in air, using LIMETS 3 and with recently published results.

Material Selection for Lead Cooled Fast Reactors

Materials are one of the challenging issues for all future GENIV reactors. Several EU funded projects, e.g. ELSY and LEADER, aimed to design Pb cooled nuclear systems including the European lead cooled fast reactor ELFR and the demonstrator ALFRED. The selection of the structural materials determines the design and the design has to consider the properties and the availability of the materials. Beside general requirements for material properties that are common for all fast reactor types, specific issues arise from coolant compatibility. The high solubility of steel alloying elements in liquid Pb at reactor relevant temperatures is clearly detrimental. Therefore, all steels that are considered as structural materials have to be protected by dissolution barriers. The common barriers for all steels under consideration are oxide scales that form in situ during operation. However, increasing the temperature above 500 °C will result either in dissolution attack or in enhanced oxidation. For higher temperatures additional barriers like alumina forming surface alloys are discussed and investigated. Mechanical loads like creep stress and fretting will act on the steels. These mechanical loads will interact with the coolant and can enhance negative effects.For ALFRED austenitic steels (316 and 15-15Ti) are selected for most in core components. The 15-15Ti is the choice for the fuel cladding of ALFRED and the ferritic martensitic steel T91 was selected as fuel clad material of the ELFR. T91 is in both designs the material to be used for the heat exchanger. Surface alloying with alumina forming alloys is considered to assure material functionality at higher temperatures and is therefore selected for fuel cladding of the ELFR and the heat exchanger tubes.

SANS and TEM of ferritic–martensitic steel T91 irradiated in FFTF up to 184dpa at 413°C

Ferritic–martensitic steel T91 was previously irradiated in the Materials Open Test Assembly (MOTA) program of the Fast Flux Test Reactor Facility (FFTF) at 413 °C up to 184 dpa. The microstructure was analyzed by small angle neutron scattering (SANS) and transmission electron microscopy (TEM). Both SANS and TEM revealed a large fraction of voids with an average size of 29–32 nm leading to a calculated void swelling of 1.2–1.6% based on the volume fraction of the voids in the sample. SANS gave no indication of second phase particles having formed under irradiation in the material. Using TEM, one zone was found where a few G-phase particles were analyzed. Quantities were however too low to state reliable particle densities. No alpha prime (α′) or Laves phase were observed in any of the investigated zones.

Fretting corrosion in liquid lead of structural steels for lead-cooled nuclear systems: Preliminary study of the influence of temperature and time

Structural materials development is a key issue for the realization of advanced lead cooled nuclear systems. One specific challenge for fuel claddings and heat exchanger tubes is the so called fretting corrosion by flow induced vibrations. To address this matter, the ferritic–martensitic steel T91 and the austenitic steel 1.4970 were submitted to fretting corrosion test in liquid lead. Preliminary indications concerning the role of temperature and time/number of cycles in the fretting corrosion process, were achieved performing fretting corrosion tests at 450/500°C and for 150/600h (5.4×106/2.16×107cycles). The interaction between fretting (mechanical wear) and corrosion processes (oxidation/dissolution) in liquid lead was also investigated.The experiments revealed that protective oxide scales that formed on steels surfaces are removed by the fretting action. Thus, dissolution attack can take place. Besides, in the fretted areas, the formation of interposing layers was noticed. Such scales, created by debris retention and compaction, consist of an outer layer of oxidized debris and an inner layer of metallic debris. Due to the high Ni content, fretting corrosion affects more significantly the 1.4970 than the T91. Moreover after the 600h test, fatigue cracking was observed on the 1.4970. The volume of material removed from the specimens by fretting corrosion, increased with temperature and time respectively number of cycles.

Creep-to-rupture of 9%Cr steel T91 in air and oxygen-controlled lead at 650 °C

This article reports results of uniaxial creep-to-rupture experiments at static loads ranging from 100 to 200 MPa on ferritic–martensitic steel T91 in stagnant lead at 650 °C and oxygen concentration c o in a narrow range around 10 −6 mass%. Respective experiments in stagnant air have been performed for comparison. The steel showed almost no difference in creep performance in oxygen-controlled lead and air at 650 °C. No dissolution attack and no lead penetration were found on the steel.