F82H Mod Research Articles

Tensile behavior of RAFM alloys after neutron irradiation of up to 16.3 dpa between 250 and 450 °C

Tensile specimen of steel EUROFER97 and other alloys on the basis of RAFM steels such, as OPTIFER and F82H alloys, and Ga3X were irradiated and post-examined during a neutron irradiation program of up to 16.3dpa between 250 and 450°C in the HFR (High Flux Reactor) in the Netherlands. These tensile results were compared with former irradiation programs, with lower neutron doses of up to 0.8 and 2.4dpa to quantify the difference in tensile strengthening. The average increase of tensile strength was in a range of 300MPa between 0.8 and 16.3dpa at temperatures of 250–300°C. This behavior can be correlated with irradiation-induced changes in the microstructure. Most of the hardening can be attributed to dislocation loops, point defects or small precipitates as observed in boron-free alloys as F82H mod. and EUROFER97. Whereas the hardening in boron-containing alloys OPTIFER alloys and Ga3X can be correlated in addition with the combination of helium bubbles. At the highest irradiation and test temperature at 450°C, all tensile data of all investigated materials were in the range of those of non-irradiated and irradiated material due to thermal aging effects.

Modeling the constitutive behavior of RAFM steels under irradiation conditions

A coupled viscoplastic deformation damage model will be presented which is modified to take into account irradiation induced hardening and its recovery due to inelastic deformation and/or high temperature annealing. The model allows the prediction of the constitutive behavior of RAFM steels under arbitrary creep-fatigue and irradiation loading conditions. It can be implemented in commercial finite element codes and thus be used for the lifetime assessment of fusion reactor components.The model is applied to describe the behavior of the RAFM steels, EUROFER97 and F82H mod, observed in post irradiation examinations of the irradiation programs ARBOR I and ARBOR II. Data from their tensile and low cycle fatigue tests were used to determine the material and temperature dependent parameters of the model and to verify its prediction capability.

IFMIF lithium target backplate design integration and thermo-mechanical analysis

In the framework of the IFMIF R&D program, a design-integration and a thermo-mechanical analysis of the Li target backplate has been carried out. The stress and displacement fields, due to the heating induced by the neutron flux, have been computed with the aim to check if the stress remains below the material's allowable values and the induced displacement field leads to an acceptable deformation of the surface where the lithium flows, so that no flux instabilities are induced in the lithium jet. The analysis has been performed for two different backplate geometries, namely a thick and a thin option, for two different thermal and mechanical constraint cases and for different materials: austenitic steel 316L, IEA F82H steel and Mod 9Cr1Mo steel (T91). The results, obtained by means of ABAQUS code, show that AISI 316L steel might be not suitable as black-plate material, conversely, both the F82H and the T91 steels satisfy ASME requirements. The thin plate solution appears preferable in order to reduce the stress field.

High temperature deformation and damage behavior of RAFM steels under low cycle fatigue loading: Experiments and modeling

Structural materials of fusion reactors are subjected to complex creep–fatigue loading and high irradiation doses. Correct modeling of their deterioration is a precondition of a sufficiently reliable lifetime prediction procedure. In the continuum mechanics approach selected for lifetime prediction of RAFM steels under creep–fatigue conditions, the inelastic strain rate modified (ISRM) damage model is coupled with a modified viscoplastic deformation model taking into account the complex non-saturating cyclic softening of RAFM steels. The resulting coupled model is a powerful prediction tool, which can be applied to arbitrary creep–fatigue loading provided that the material, temperature and possibly irradiation dose-dependent parameters of the model have been determined. Therefore, a fitting procedure has been developed for the parameters identification on the base of deformation and lifetime data from strain-controlled low cycle fatigue (LCF) tests without and with hold time as well as creep tests. The coupled deformation-damage model has been applied to F82H mod and EUROFER 97 in the reference (unirradiated) state under isothermal cyclic loading at 450, 550 and 650 °C. The comparisons between model and experiment show that the observed lifetimes in the LCF tests could be fairly well calculated even for the tests with hold time, which were not considered for the identification of the damage model parameters.

Structural and mechanical properties of welded joints of reduced activation martensitic steels

Gas tungsten arc welding and electron beam welding methods were used to realise welding pools on plates of reduced activation martensitic steels. Structural and mechanical features of these simulated joints have been investigated in as-welded and post-welding heat-treated conditions. The research allowed to assess how each welding technique affects the original mechanical properties of materials and to find suitable post-welding heat treatments. This paper reports results from experimental activities on BATMAN II and F82H mod. steels carried out in the frame of the European Blanket Project – Structural Materials Program.

Thermo-mechanical fatigue behavior of reduced activation ferrite/martensite stainless steels

The thermo-mechanical cycling fatigue (TMCF) behavior of reduced activation ferrite/martensite stainless steels is examined. The test rig consists of a stiff load frame, which is directly heated by the digitally controlled ohmic heating device. Cylindrical specimens are used with a wall thickness of 0.4 mm. Variable strain rates are applied at TMCF test mode, due to the constant heating rate of 5.8 K/s and variable temperature changes. TMCF results of as received EUROFER 97 in the temperature range between 100 and 500–600 °C show a reduction in life time (a factor of 2) compared to F82H mod. and OPTIFER IV. TMCF-experiments with hold times of 100 and 1000 s show dramatic reduction in life time for all three materials.

Mechanical and microstructural behaviour of isothermally and thermally fatigued ferritic/martensitic steels

Isothermal low cycle fatigue (LCF) and thermal low cycle fatigue results in the temperature range between room temperature and 823 K of the quenched and tempered reduced activation ferritic/martensitic steels F82H mod. and EUROFER 97 are reported. Under these test conditions both steels show, after the first few cycles and for both types of tests, a pronounced cyclic softening up to failure. The softening during LCF tests described by a simple empirical relationship is dependent on temperature but independent of the total strain amplitude of the tests. From the analysis of the hysteresis loops and corroborated by electron microscopy observations it can be concluded that the cyclic softening is produced by the softening observed in the internal stress as a consequence of the evolution of the microstructure. During cycling, the martensitic lath structure with high dislocation density and carbides along the lath interfaces evolves to a softer dislocation subgrain structure. This conclusion could be correlated with transmission electron microscopy observations.

Comparison of the low cycle fatigue behaviour of F82H mod. and Eurofer 97 in water coolant

A comparative study has been done about the susceptibility to environmentally induced damage of the reduced activation martensitic steels F82H mod and Eurofer 97. Both alloys were found to suffer premature fracture compared to air behaviour, during conventional fully reversed load controlled low cycle fatigue tests, run in high temperature aqueous environment with the alkaline chemistry specified for the lithium lead blanket water coolant. The different steel low cycle fatigue responses in water, especially, the scatter of fatigue lives and fracture modes observed from plate-to-plate and from specimen-to-specimen in F82H and Eurofer, respectively, appeared to coincide with the presence of different inclusion types, size and density. Based on electrochemical and fractographic indexes, the results were discussed with the support of the hydrogen decohesion model, in terms of mutually competitive trap effects on hydrogen diffusion and partitioning among the susceptible cracking sites.

Thermal and mechanical behaviour of the reduced-activation-ferritic-martensitic steel EUROFER

Reduced activation ferritic/martensitic (RAFM) steels are being considered for structural application in potential fusion energy systems. Based on the substantial experience with RAFM developmental steels of OPTIFER type, an industrial 3.5 tons batch of a 9CrWVTa-RAFM steel, called EUROFER 97 had been specified and ordered. A characterisation programme has been launched to determine the relevant mechanical and physical-metallurgical properties in order to qualify the steel for fusion application. The hardening, tempering and transformation behaviour of EUROFER is in good agreement with that of other RAFM-steels like OPTIFER and the Japanese industrial scale heat F82H mod. Tensile tests, performed between RT and 750 °C, show comparable strength and ductility values that are not strongly affected by different heat treatments and ageing at 580 and 600 °C up to 3300 h. Impact bending tests indicate a superior ductile to brittle transition temperature (DBTT) of EUROFER in the as-received condition compared with that of F82H mod. Creep tests between 450 and 650 °C up to test times of 15 000 h reveal a creep strength similar to other RAFM steels like OPTIFER and F82H mod. EUROFER shows a good low-cycle fatigue behaviour with longer lifetimes than F82H mod. The deformation and softening behaviour is similar.

First results on the characterisation of the reduced-activation-ferritic-martensitic steel EUROFER

Reduced activation ferritic/martensitic (RAFM) steels are being considered for structural application in potential fusion energy systems. Based on the substantial experience with RAFM developmental steels of OPTIFER type, an industrial 3.5 tons batch of a 9CrWVTa-RAFM steel, called EUROFER 97 had been specified and ordered. A characterisation programme has now been launched to determine the relevant mechanical and physical-metallurgical properties in order to qualify it for fusion application. The hardening, tempering and transformation behaviour of EUROFER is in good agreement with that of other RAFM-steels like OPTIFER and the Japanese industrial scale heat F82H mod. Tensile tests, performed between RT and 750 °C, show comparable strength and ductility values that are not strongly affected by ageing at 580 and 600 °C up to 3300 h. Impact bending tests indicate a superior ductile to brittle transition temperature (DBTT) of EUROFER in the as-received condition compared to that of F82H mod. First results of creep tests between 450 and 650 °C up to test times of 4000 h reveal a creep strength similar to OPTIFER and F82H mod. The first results of the investigations show that it is possible to transfer the good mechanical and structural properties of precursor laboratory melts to an industrial scale melt.

Behaviour of F82H mod. stainless steel in lead–bismuth under temperature gradient

Austenitic steels can be used in a hybrid system in contact with liquid lead–bismuth eutectic if the region of operating temperatures is not beyond 400°C. For higher temperatures, martensitic steels are recommended. However, at long times, the interaction between the structural material and the eutectic leads to the dissolution of some elements of the steel (Ni, Cr and Fe, mainly) in the liquid metal. In a non-isothermal lead–bismuth loop, the material dissolution takes place at the hot leg of the circuit and, due to the mass transfer, deposition occurs at the cold leg. One of the possible ways to improve the performance of structural materials in lead–bismuth is the creation of an oxide layer. Tests have been performed in a small natural convection loop built of austenitic steel (316L) that has been operating for 3000 h. This loop contains a test area in which several samples of F82Hmod. martensitic steel have been tested at different times. A gas with an oxygen content of 10 ppm was bubbled in the hot area of the circuit during the operation time. The obtained results show that an oxide layer is formed on the samples introduced in the loop at the beginning of the operation and this layer increases with time. However, the samples introduced at different times during the loop operation, are not protected by oxide layers and present material dissolution in some cases.

Tensile and impact behavior of the reduced-activation steels OPTIFER and F82H mod

Tensile and charpy impact tests were carried out on some OPTIFER steel grades and F82H mod. The steels show little difference in tensile properties, but pronounced differences in charpy impact properties. Combinations of low ductile–brittle transition temperature (DBTT) and high yield strength are favored for OPTIFER-IV. After aging at 600°C and higher, F82H mod steel embrittles due to precipitation of Laves phase (Cr, Fe)2W, whereas OPTIFER-IV is resistant to aging.

Impurity effects on reduced-activation ferritic steels developed for fusion applications

Reduced-activation steels are being developed for fusion applications by restricting alloying elements that produce long-lived radioactive isotopes when irradiated in the fusion neutron environment. Another source of long-lived isotopes is the impurities in the steel. To examine this, three heats of reduced-activation martensitic steel were analyzed by inductively coupled plasma mass spectrometry for low-level impurities that compromise the reduced-activation characteristics: a 5-ton heat of modified F82H (F82H-Mod) for which an effort was made during production to reduce detrimental impurities, a 1-ton heat of JLF-1, and an 18-kg heat of ORNL 9Cr–2WVTa. Specimens from commercial heats of modified 9Cr–1Mo and Sandvik HT9 were also analyzed. The objective was to determine the difference in the impurity levels in the F82H-Mod and steels for which less effort was used to ensure purity. Silver, molybdenum, and niobium were found to be the tramp impurities of most importance. The F82H-Mod had the lowest levels, but in some cases the levels were not much different from the other heats. The impurity levels in the F82H-Mod produced with present technology did not achieve the low-activation limits for either shallow land burial or recycling. The results indicate the progress that has been made and what still must be done before the reduced-activation criteria can be achieved.

Influence of thermal aging on tensile and impact bending properties of the steel grades OPTIFER and F82H mod.

The low activation martensitic chromium steels OPTIFER Ia, Ib, II, III and IV and F82H mod. were subjected to aging by annealing between 550°C and 650°C for up to 10,000 h. In general, this caused a decrease in tensile strength by up to 10%. The decrease in total elongation depended on the thermal treatment of the material. Impact bending strength was hardly affected by aging. With one exception, all tested steels turned out to be sufficiently resistant to aging.

Thermal fatigue behavior of low activation ferrite–martensite steels

Components like test blankets of the thermonuclear fusion reactor ITER or like DEMO-blankets are subjected during service to alternating thermal and mechanical stresses as a consequence of the pulsed reactor operation. Of particular concern is the fatigue endurance of reduced activation ferrite-martensite steels like the Japanese F82H mod. and the German OPTIFER IV, compared to martensitic steel for the next European torus (MANET II), as reference material, under cyclic strains and stresses produced by typical temperature changes. Into self developed thermal cycling fatigue test rigs variable strain rates are applied to these materials, due to the constant heating rate and variable temperature changes. Thermal fatigue behavior of ferrite-martensite MANET II, including dwell times, will be reported. On the other hand F82H mod. as well as OPTIFER IV samples are thermal cyclically tested in different as received conditions and the the results are compared to those of MANET II samples after the three-step reference annealing.

Dynamic strain ageing evidences during low cycle fatigue deformation in ferritic–martensitic stainless steels

The influence of dynamic strain ageing (DSA) on the strain cyclic behaviour of ferrite–martensite stainless steels was investigated at temperatures ranging from room temperature to 823 K. For fully annealed AISI 420 initial hardening followed by a saturation stage was observed at each test temperature. This steel was found to be susceptible to DSA as evidenced by the temperature independent stress saturation observed between 523 and 723 K. Normalized and tempered MANET II and F82H mod. softens during cyclic loading at all temperatures. In this steel DSA manifestations were observed on plotting the peak tensile stress difference between hysteresis loops obtained at different strain rates. Strongly abnormal behaviour with higher peak tensile stresses corresponding to slower strain rates was observed in the temperature range between 500 and 700 K. It is proposed that DSA mechanisms caused by the drag of solution carbon atoms is responsible for this unusual behaviour.

Microstructure Evolution of Selected Ferriticmartensitic Steels under Dual-Beam Irradiation

AbstractWe present experimental results on the microstructure evolution of dual-beam irradiated (300 keV heavy ions plus 15 keV helium ions) ferritic-martensitic steels (Manet, DIN 1.4926, F82H mod). The helium bubble morphology as well as microchemistry of the alloys are investigated by means of transmission electron microscopy and field-ion microscopy with atom probe. The alloys were irradiated to fluences up to 50 dpa and implanted with helium up to a concentration of 1 at.% at the temperatures of 723 K and 773 K. The damage and implantation rates variied from 2.5 · 10−3 dpa/s to 2.5·10−2 dpals and from 0.5 appmls to 5 appm/s, respectively. Size and number density of helium bubbles is found to be rate dependent. Smaller implantation rates produce larger helium bubbles and smaller bubble number densities. Regions of local enrichment of alloy elements, typically 5 nm in size, containing chromium (up to 40 at.%), silicon, and nickel are detected. Number densities of helium bubbles and of regions of chromium enrichments are comparable and lie between 1023/m3 and 1024/m3. Possible extrapolation of the present ion irradiations to spallation source and fusion reactor conditions is shortly adressed.