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.     

Scatter of a ferritic steel in the transition region analyzed by Charpy tests and dynamic tensile tests

Several series of static and dynamic experiments with a German reactor pressure vessel steel have been performed at different temperatures. Besides results of standardized compact tension tests and static and dynamic tensile tests, results of instrumented Charpy-V tests and of dynamically loaded notched round bars are presented. In addition to the evaluation of J values these tests were analyzed numerically by means of a micromechanical damage model in order to finally determine the Weibull parameters of the Beremin model. However, not all of the results of the different series of tests are consistent, suggesting the need for modifications of the evaluation or the model to enable the application to arbitrary temperatures and loading conditions.     

Depinning transition of a dislocation line in ferritic oxide strengthened steels

The dynamics of an edge dislocation in a medium with random oxide dispersoid particles acting as pinning centres is analysed. The dislocation line undergoes a depinning transition, where the order parameter is the dislocation line velocity v, which increases from zero for driving external resolved shear stresses τ beyond to a threshold value τ_c, known as the critical resolved shear stress. The critical stress is obtained by means of statistical analysis of the motion of a single dislocation in its glide plane, using overdamped, discrete dislocation dynamics simulations.     

Damage region formation in ferritic steels induced by light water reactor neutron spectrum

A new theoretical model for damage region formation is proposed. The model is based on numerical solution of the Boltzmann transport equation for knocked-on atoms. A key point of this model is the selfconsistent determination of subcascade overlapping energy E_over (the threshold energy for distinguished damage region formation). Damage region density and size distributions in ferritic steels (Fe–0.2 wt% Cu and Fe–0.2 wt% Cu–0.3 wt% Si) under neutron irradiation in light water reactor spectrum were calculated.     

Effect of loading rate upon fracture behavior of ferritic steel under large scaled yielding

Dynamic loading to ferromagnetic materials and large scaled yielding result in peak or valley and non-linear curve, respectively, on the Direct Current Potential Drop (DCPD) versus Crack Opening Displacement (COD) plots, which make it difficult to determine the crack initiation point. In this work high intensity of current up to 100 A was applied to the specimens of SA106Gr.C ferritic steel and the crack growth behavior was directly monitored by a high speed camera to obtain the crack initiation point. The effects of loading rate up to 1200 mm min⁻¹ upon the fracture resistance were explored. As the results, it has been shown that, although no substantial difference was seen in the load–COD plots, the crack initiation and then J_i and J–R curve were quite sensitive to the loading rate. That is, under the loading rate of 300 mm min⁻¹ the material showed the worst fracture resistance than under static loading and even under the higher loading rates of 600 and 1200 mm min⁻¹. Also applying the high speed camera and high current source have been proved to be an effective way to find out the accurate crack initiation point and to compensate the pulse of DCPD due to the ferromagnetic effect.     

Influences of the dynamic strain aging on the J–R fracture characteristics of the ferritic steels for reactor coolant piping system

The leak-before-break (LBB) design of the piping system for nuclear power plants has been based on the premise that the leakage due to the through-wall crack can be detected by using leak detection systems before a catastrophic break. The piping materials are required to have excellent J–R fracture characteristics. However, where ferritic steels for reactor coolant piping systems operate at the temperatures where dynamic strain aging (DSA) could occur, the fracture resistance could be reduced with the influence of DSA under dynamic loading. Therefore, in order to apply the LBB design concept to the piping system under seismic loading, both static and dynamic J–R characteristics must be evaluated.Materials used in this study are SA516 Gr.70 for the elbow pipe and SA508 Cl.1a for the main pipe and their welding joints. The crack extension during the dynamic and the static J–R tests was measured by the direct current potential drop (DCPD) and the compliance method, respectively. This paper describes the influences of the dynamic strain aging on the J–R fracture characteristics with the loading rate of the pipe materials and their welding joints.     

Tensile properties of ferritic/martensitic steels irradiated in STIP-I

Specimens of ferritic/martensitic (FM) steels T91, F82H, Optimax-A and the electron beam weld (EBW) of F82H were irradiated in the Swiss spallation neutron source (SINQ) Target-3 in a temperature range of 90-370 °C to displacement doses between 3 and 12 dpa. Tensile tests were performed at room temperature and the irradiation temperatures. The tensile test results demonstrated that the irradiation hardening increased with dose up to about 10 dpa. Meanwhile, the uniform elongation decreased to less than 1%, while the total elongation remained greater than 5%, except for an F82H specimen of 9.8 dpa tested at room temperature, which failed in elastic deformation regime. At higher doses of 11-12 dpa, the ductility of some specimens recovered, which could be due to the annealing effect of a short period of high temperature excursion. The results do not show significant differences in tensile properties for the different FM steels in the present irradiation conditions.     

Effect of specimen size on the ductile-brittle transition behavior and the fracture sequence of 9Cr-W steels

The effect of specimen size on the ductile-brittle transition behavior and the fracture sequence were investigated by means of Charpy absorbed energy measurement and fractography, using the full size, the half size and the one-third size V-notch specimens of 9Cr-W steels. The steels used are reduced-activation ferritic steels for fusion reactor structures. Attempts were made to correlate the impact data between the different specimen sizes by using normalizing parameters, such as nominal fracture area and nominal fracture volume for the upper shelf energy and ligament size for the ductile-brittle transition temperature. Fractography showed a similar fracture sequence for the three different sizes of the specimens.     

Carbon and nitrogen transfer in Fe-9 Cr-Mo ferritic steels exposed to a sodium environment

Kinetics of carburization/decarburization of five commercial and two high-purity Fe-9 Cr-1 to 2.5 Mo feiritic steels have been studied in a sodium environment at temperatures between 773 and 973 K. Carbon concentration-distance profiles were obtained as a function of sodium-exposure time, temperature, and carbon in sodium and the carburization/ decarburization rate constants were evaluated. The results show that the Fe-9 Cr-Mo steels are more resistant to carbon transfer than the low-alloy Fe-$\text{2}\text{1}\text{4}$ Cr-1 Mo steel. The conditions of temperature and carbon concentration in sodium for carburization or decarburization of Fe—9 Cr—Mo steels are quite similar to those for stainless steels. However, the extent of carbon transfer in Fe-9 Cr—Mo steels is lower than that of the stainless steels. The composition and carbide structure of the steel had a significant effect on the carburization/decarburization behavior. Fe-9 Cr-Mo steels that decarburize to very low carbon concentrations either contain M₂X phase or have M₆C as the only stable carbide.     

Deuterium permeation behavior of erbium oxide coating on austenitic,ferritic, and ferritic/martensitic steels

Tritium permeation barrier is required in fusion blanket for reduction of loss of fuel and health hazard. In this study, deuterium permeation experiments have been performed on four kinds of steels and erbium oxide coatings fabricated by a filtered arc deposition method. The permeation flux of uncoated samples shows diffusion-limited regime in the temperature range 573–723 K and the permeability is corresponding to literature data. The coated samples deposited at room temperature have been tested at 773 K. It is found that the coatings suppress the deuterium permeation to a close level in spite of different types of steel substrates. In addition, the exponent of the driving pressure slightly changes compared to the uncoated sample. However, the permeation regime is still near diffusion limited.     

Determination of the dynamic fracture toughness using a new stress pulse loading method

This paper presents a method for the determination of the dynamic fracture toughness K_Id of metallic materials at loading rates K_I of about . The method is derived from the known split Hopkinson pressure bar technique and uses a well-defined stress pulse for the loading of a fatigue precracked specimen. The interpretation of the experimental data is strictly based on a numerical analysis of the specimen under the given dynamic loading conditions. It is shown, that a conventional quasi-static approach would yield incorrect fracture toughness values. The results for some steels confirm, that the fracture toughness decreases with increasing loading rate. Therefore, in some sense the fracture toughness versus temperature curve determined with the presented stress pulse method can be regarded as lower bound curve.     

Embrittlement of irradiated ferritic/martensitic steels in the absence of irradiation hardening

Irradiation damage caused by neutron irradiation below 425-450 °C of 9-12% Cr ferritic/martensitic steels produces microstructural defects that cause an increase in yield stress. This irradiation hardening causes embrittlement observed in a Charpy impact test as an increase in the ductile-brittle transition temperature. Little or no change in strength is observed in steels irradiated above 425-450 °C. Therefore, the general conclusion has been that no embrittlement occurs above these temperatures. In a recent study, significant embrittlement was observed in F82H steel irradiated at 500 °C to 5 and 20 dpa without any change in strength. Earlier studies on several conventional steels also showed embrittlement effects above the irradiation-hardening temperature regime. Indications are that this embrittlement is caused by irradiation-accelerated or irradiation-induced precipitation. Observations of embrittlement in the absence of irradiation hardening that were previously reported in the literature have been examined and analyzed with computational thermodynamics calculations to illuminate and understand the effect.     

Corrosion of various ferritic steels in an isothermal sodium loop system

Ferritic chromium-molybdenum steels with chromium contents of 1 wt% up to 12 wt% have been exposed for 8370 h to flowing sodium at 550°C. The oxygen content of the sodium was 6–7 ppm by weight. Weight measurements, carbon analyses and metallographic examinations were carried out. The low chromium steels show weight loss and decarburisation. The high chromium steels show weight gain and carburisation. The crossover point is at about 5 wt% Cr. The composition at the utmost surface (<10 μm) of the various steels tends to about 8 wt% chromium, about 2 wt% nickel and 0.02–0.09 wt% carbon. Sodium chromite crystals were present on the steels with a chromium content of 5 wt% or more. At the exposed surface of the $\text{2}\text{1}\text{4}$ wt% chromium steel sodium chromite crystals were found locally.     

Void-swelling in irons and ferritic steels: I. Mechanisms of swelling suppression

Point defect and dislocation behaviour in α-iron and ferritic steels relevant to the understanding of their void-swelling resistance during elevated temperature irradiation is summarized. The key role played by both interstitial and substitutional solutes in inducing enhanced mutual recombination of point defects through trapping processes and also in lowering dislocation mobility and bias for preferential self-interstitial capture in these materials, and thereby controlling their void-swelling response, is emphasized.     

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 behaviour of WB 35 piping material under dynamic loading in the high speed tensile test

For the determination of the strength-, deformation- and fracture behaviour of the material 17 MnMoV 6 4 (WB 35) which is used for piping components, tensile tests were carried out at different loading rates (monotonic and impact-type) on smooth and notched pipe strip specimens over a temperature range extending from − 30°C to 250°C.For the conduct of the tests a hydraulic high speed tensile machine having a free motion device was used; the velocity of impact was preset at ca. 7 m/s.With impact-type (dynamically) loaded specimens in general higher strength and deformation values were obtained than with monotonic (statically) loaded ones. In all of the specimens having low deformation values which were investigated microfractographically, ductile portions were found adjacent to the notch on the fracture surface.     

Metallography studies and hardness measurements on ferritic/martensitic steels irradiated in STIP

In this work metallography investigations and microhardness measurements have been performed on 15 ferritic/martensitic (FM) steels and 6 weld metals irradiated in the SINQ Target Irradiation Program (STIP). The results demonstrate that all the steels have quite similar martensite lath structures. However, the sizes of the prior austenite grain (PAG) of these steels are quite different and vary from 10 to 86 μm. The microstructure in the fusion zones (FZ) of electron-beam welds (EBWs) of 5 steels (T91, EM10, MANET-II, F82H and Optifer-IX) is similar in respect to the martensite lath structure and PAG size. The FZ of the inert-gas-tungsten weld (TIGW) of the T91 steel shows a duplex structure of large ferrite gains and martensite laths. The microhardness measurements indicate that the normalized and tempered FM steels have rather close hardness values. The unusual high hardness values of the EBW and TIGW of the T91 steel were detected, which suggests that these materials are without proper tempering or post-welding heat treatment.     

Leak-before-break behaviour of austenitic and ferritic pipes containing circumferential defects

In recent years several research projects have been carried out at MPA Stuttgart to investigate the leak-before-break (LBB) behaviour of pressure-bearing components which are relevant to plant safety. In these investigations the test pipes have for the most part been made of ferritic material. International research programmes such as, for example, the Degraded Piping Programme (Wilkowski et al., 1986 and Wilkowski et al., 1989. Degraded Piping Program, Phase II. Report NUREG/CR-4082, vol. 4, Sept. 1986, and vol. 8, March 1989, Battelle, Columbus, Ohio, USA) or the IPIRG-Program (Schmidt et al., 1991. The International Piping Integrity Research Group (IPIRG), Program—An Overview. SMiRT 11 Proceedings, Paper G23/1, Tokyo, Japan, August 1991) have also dealt with pipes made of austenitic materials. However, they were fabricated of not stabilized quality. To take into account the material of comparable components of German nuclear power plants, the experiments reported in the following are focussed on pipes made of Ti- and Nb-stabilized austenitic material. The results presented below relate to pipes containing circumferential defects subjected to internal pressure and external bending loading. As regards the ferritic components an overview of the experimentally determined results is presented. The predictive capability of engineering calculational methods are presented by way of example. The current programme of investigations is presented together with the testing techniques and the initial results.     

Microstructural investigation of the stability under irradiation of oxide dispersion strengthened ferritic steels

Some fuel pin cladding made from a ferritic steel reinforced by titanium and yttrium oxides were irradiated in the French experimental reactor Phénix. Microstructural examination of this alloy indicates that oxides undergo dissolution under irradiation. This irradiation shows the influence of dose and, in a smaller part, of temperature. In order to better understand the mechanisms of dissolution, three ferritic steels reinforced by Y₂O₃ or MgO were irradiated with different charged particles. Inelastic interactions induced by 1 MeV He ion irradiation do not lead to any modification, neither in their chemical composition, nor in their spatial and size distribution. In contrast, isolated Frenkel pairs created by electron irradiation lead to significant oxide dissolution with a radius decrease proportional to the dose. Moreover, the comparison between irradiation with ions (displacements cascades) and electrons (Frenkel pairs only) shows the importance of free point defects in the dissolution phenomena.