Irradiated Steel Research Articles

Influence of high intensity ion beam irradiation on impact toughness of 12Cr1MoV steel

The impact toughness of non-treated and irradiated 12Cr1MoV steel Charpy specimens has been studied in the range of temperatures from 20 C to 600 C. It was revealed that the ion beam treatment of steel deteriorates the impact toughness by 5–40 % at the tested temperature range. The physical regularities of irradiation effect on microhardness and hardness of the treated surface were investigated. It was also found out that irradiated specimens fracture via more brittle mechanism, which is related to both more severe stress–strain state at the notch tip and gradient alteration of properties in the subsurface layer.

Simulation of Friction Stir Processing in 304L Stainless Steel

A major dilemma facing the nuclear industry is repair or replacement of stainless steel reactor components that have been exposed to neutron irradiation. When conventional fusion welding is used for weld repair, the high temperatures and thermal stresses inherent in the process enhance the growth of helium bubbles, causing intergranular cracking in the heat-affected zone (HAZ). Friction stir processing (FSP) has potential as a weld repair technique for irradiated stainless steel, because it operates at much lower temperatures than fusion welding, and is therefore less likely to cause cracking in the HAZ. Numerical simulation of the FSP process in 304L stainless steel was performed using an Eulerian finite element approach. Model input required flow stresses for the large range of strain rates and temperatures inherent in the FSP process. Temperature predictions in three locations adjacent to the stir zone were accurate to within 4% of experimentally measure values. Prediction of recrystallized grain size at a location about 6mm behind the tool center was less accurate, because the empirical model employed for the prediction did not account for grain growth that occurred after deformation in the experiment was halted.

Impact toughness of 12Cr1MoV steel. Part 2 – Influence of high intensity ion beam irradiation on energy and deformation parameters and mechanisms of fracture

The impact toughness of initial and irradiated 12Cr1MoV steel Charpy specimens has been studied in the range of temperatures from 20°C to 600°C. In addition, it was found that ion beam treatment of steel deteriorates the impact toughness at the tested temperatures by 5–42%. Moreover, it was revealed that the increase of testing temperature decreases the difference between the fracture energies of initial and modified steel specimens. Also, the physical regularities of irradiation effect on microhardness and hardness of the treated surface were investigated. It was also revealed that irradiated specimens fracture via more brittle mechanism, which is related to both more severe stress–strain state at the notch tip and gradient alteration of properties in the subsurface layer.

Microstructure and Nano-Hardness of 10 MeV Cl-Ion Irradiated T91 Steel**supported by National Natural Science Foundation of China (Nos. 11374299, 11375230, 11274309)

Hardening and elemental segregation of T91 martenstic steel irradiated by 10 MeV Cl ions to doses from 0.06 dpa to 0.83 dpa were investigated with the nanoindentation technique and transmission electron microscopy (TEM). The results demonstrated that the irradiation hardening was closely related with irradiation dose. By increasing the dose, the hardness increased rapidly at first from the initial value of 3.15 GPa before irradiation, and then tended to saturate at a value of 3.58 GPa at the highest dose of 0.83 dpa. Combined with TEM observation, the mechanism of hardening was preliminary attributed to the formation of M(Fe,Cr)23C6 carbides induced by the high energy Cl-ion irradiation.

Fatigue life predictions for irradiated stainless steels considering void swellings effects

The objective of this study is to estimate fatigue life of irradiated austenitic stainless steels types 304, 304L, and 316, which are extensively used as structural alloys in the internal elements of nuclear reactors. These reactor components are typically subjected to a long-term exposure to irradiation at elevated temperature along with repeated loadings during operation. Additionally, it is known that neutron irradiation can cause the formation and growth of microscopic defects or swellings in the materials, which may have a potential to deteriorate the mechanical properties of the materials. In this study, uniaxial fatigue models were used to predict fatigue properties based only on simple monotonic properties including ultimate tensile strength and Brinell hardness. Two existing models, the Bäumel–Seeger uniform material law and the Roessle–Fatemi hardness method, were employed and extended to include the effects of test temperature, neutron irradiation fluence, irradiation-induced helium and irradiation-induced swellings on fatigue life of austenitic stainless steels. The proposed models provided reasonable fatigue life predictions compared with the experimental data for all selected materials.

Structural characterization of nanoscale intermetallic precipitates in highly neutron irradiated reactor pressure vessel steels

Massive, thick-walled pressure vessels are permanent nuclear reactor structures that are exposed to a damaging flux of neutrons from the adjacent core. The neutrons cause embrittlement of the vessel steel that grows with dose (fluence), as manifested by an increasing ductile-to-brittle fracture transition temperature. Extending reactor life requires demonstrating that large safety margins against brittle fracture are maintained at the higher neutron fluence associated with beyond 60years of service. Here synchrotron-based x-ray diffraction and small angle x-ray scattering measurements are used to characterize highly embrittling nm-scale Mn–Ni–Si precipitates that develop in the irradiated steels at high fluence. These precipitates lead to severe embrittlement that is not accounted for in current regulatory models. Application of the complementary techniques has, for the very first time, successfully identified the crystal structures of the nanoprecipitates, while also yielding self-consistent compositions, volume fractions and size distributions.

Microstructure and fracture behavior of F82H steel under different irradiation and tensile test conditions

Specimens of martensitic steel F82H were irradiated to doses ranging from 10.7 dpa/850 appm He to 19.6 dpa/1740 appm He at temperatures between 165 and 305 °C in the second experiment of SINQ Target Irradiation Program (STIP-II). Tensile tests were conducted at different temperatures and various fracture modes were observed. Microstructural changes including irradiation-induced defect clusters, dislocation loops and helium bubbles under different irradiation conditions were investigated using transmission electron microscopy (TEM). The deformation microstructures of tensile tested specimens were carefully examined to understand the underlying deformation mechanisms. Deformation twinning was for the first time observed in irradiated martensitic steels. A change of deformation mechanism from dislocation channeling to deformation twinning was observed when the fracture mode changed from rather ductile (quasi-cleavage) to brittle (intergranular or cleavage and intergranular mixed).

Cracking behavior of thermally aged and irradiated CF-8 cast austenitic stainless steel

To assess the combined effect of thermal aging and neutron irradiation on the cracking behavior of CF-8 cast austenitic stainless steel, crack growth rate (CGR) and fracture toughness J-R curve tests were carried out on compact-tension specimens in high-purity water with low dissolved oxygen. Both unaged and thermally aged specimens were irradiated at ∼320 °C to 0.08 dpa. Thermal aging at 400 °C for 10,000 h apparently had no effect on the corrosion fatigue and stress corrosion cracking behavior in the test environment. The cracking susceptibility of CF-8 was not elevated significantly by neutron irradiation at 0.08 dpa. Transgranular cleavage-like cracking was the main fracture mode during the CGR tests, and a brittle morphology of delta ferrite was often seen on the fracture surfaces at the end of CGR tests. The fracture toughness J-R curve tests showed that both thermal aging and neutron irradiation can induce significant embrittlement. The loss of fracture toughness due to neutron irradiation was more pronounced in the unaged than aged specimens. After neutron irradiation, the fracture toughness values of the unaged and aged specimens were reduced to a similar level. G-phase precipitates were observed in the aged and irradiated specimens with or without prior aging. The similar microstructural changes resulting from thermal aging and irradiation suggest a common microstructural mechanism of inducing embrittlement in CF-8.

Microstructural Characterization of Hydrogen Irradiated Austenitic Stainless Steel

Microstructural Characterization of Hydrogen Irradiated Austenitic Stainless Steel

Surface and internal microstructure damage of He-ion-irradiated CLAM steel studied by cross-sectional transmission electron microscopy

Good understanding of blistering and embrittlement mechanism depends on good investigation of surface and internal microstructure damage of gas-ion-irradiated materials. Internal and surface microstructure of He+ ion irradiated CLAM steel were examined by cross-sectional transmission electron microscopy combining focused ion beam. Variation of helium bubble density and size distribution versus depth in CLAM steel after high dose helium irradiation at room temperature was investigated. The average size of helium bubble increased within 100–400 nm but decreased near the non-irradiated matrix with the increase of depth, while the density followed a reverse trend. The formation and growth mechanism of helium bubble is different at different irradiation depth. The formation of a zone of large bubbles under the surface is the main reason of surface blistering and flaking. Helium induced irradiation swelling and surface blistering at low temperature were also discussed.

Fatigue life enhancement by irradiation of 12Cr1MoV steel with a Zr+ ion beam. Mesoscale deformation and fracture

The structure and properties of 12Cr1MoV steel irradiated with a zirconium ion beam were studied by optical microscopy, scanning electron microscopy, and micro- and nanoindentation. It is shown that the modification covers the entire cross-section of the irradiated specimens to a depth of 1 mm. The data on irradiation-induced structural changes are used to interpret the changes in mechanical properties of the irradiated specimens under static and cyclic loading. Particular attention is given to analysis of strain estimation by the digital image correlation method.

True stress–strain curve acquisition for irradiated stainless steel including the range exceeding necking strain

True stress–strain curves were obtained for irradiated 316L stainless steel by a tensile test and by a curve estimation procedure. In the tensile test, the digital image correlation technique together with iterative finite element analysis was applied in order to identify curves for strain larger than the necking strain. The true stress–strain curves were successfully obtained for the strain of more than 0.4 whereas the necking strain was about 0.2 in the minimum case. The obtained true stress–strain curves were approximated well with the Swift-type equation including the post-necking strain even if the exponential constant n was fixed to 0.5. Then, the true stress–strain curves were estimated by a curve estimation procedure, which was referred to as the K-fit method. Material properties required for the K-fit method were the yield and ultimate strengths or only the yield strength. Some modifications were made for the K-fit method in order to improve estimation accuracy for irradiated stainless steels.

Effect of the bainitic and martensitic microstructures on the hardening and embrittlement under neutron irradiation of a reactor pressure vessel steel

The hardening and the embrittlement under neutron irradiation of an A508 type RPV steel considering three different microstructures (bainite, bainite-martensite and martensite)have been investigated These microstructures were obtained by quenching after autenitization at 1100 °C. The irradiation induced hardening appears to depend on microstructure and is correlated to the yield stress before irradiation. The irradiation induced embrittlement shows a more complex dependence. Martensite bearing microstructures are more sensitive to non hardening embrittlement than pure bainite. This enhanced sensitivity is associated with the development of intergranular brittle facture after irradiation; the pure martensite being more affected than the bainite-martensite. It is of interest to note that this mixed microstructure appears to be more embrittled than the pure bainitic or martensitic phases in terms of temperature transition shift. This behaviour which could emerge from the synergy of the embrittlement mechanisms of the two phases needs further investigations. However, the role of microstructure on brittle intergranular fracture development appears to be qualitatively similar under neutron irradiation and thermal ageing.

Ab Initio Nonequilibrium Thermodynamic and Transport Properties of Ultrafast Laser Irradiated 316L Stainless Steel

We present calculations of transient behavior of thermodynamic and transport coefficients on the time scale of electron–phonon relaxation upon ultrashort laser excitation of ferrous alloys. Their role defining energy deposition and primary microscopic material response to the laser irradiation is outlined. Nonequilibrium thermodynamic properties of 316L stainless steel are determined from first-principles calculations. Taking into account the complexity of multimetallic materials, the density functional theory is first applied to describe the electronic density of states of an alloy stainless steel matrix as a function of electronic heating. An increase of the localization degree of the charge density was found to be responsible for the modification of the electronic structure upon electronic heating, with consequences on chemical potential, electronic capacity, and pressure. It is shown that the electronic temperature dependence of stainless steel thermodynamic properties are consistent with the behavior...

Warm PreStress effect on highly irradiated reactor pressure vessel steel

This study investigates the Warm Prestress (WPS) effect on 16MND5 (A508 Cl3) RPV steel, irradiated up to a fluence of 13·1023n.m-2 (E>1MeV) at a temperature of 288°C, corresponding to more than 60years of operations in a French Pressurized Water Reactor (PWR). Mechanical properties, including tensile tests with different strain rates and tension–compression tests on notched specimens, have been characterized at unirradiated and irradiated states and used to calibrate constitutive equations to describe the mechanical behavior as a function of temperature and fluence. Irradiation embrittlement has been determined based on Charpy V-notch impact tests and isothermal quasi-static toughness tests. Assessment of WPS effect has been done through various types of thermomechanical loadings performed on CT(0.5T) specimens. All tests have confirmed the non-failure during the thermo-mechanical transients. Experimental data obtained in this study have been compared to both engineering-based models and to a local approach (Beremin) model for cleavage fracture. It is shown that both types of modeling give good predictions for the effective toughness after warm prestressing.

Characterization of the absorptance of laser irradiated steel sheets

This work presents an experimental-numerical methodology aimed at deriving the absorptance of both a low output power CO2 laser beam and a Yb fiber laser beam applied to an AISI 304 stainless steel cold-rolled sheet under two different conditions of the sample surface: without and with a spray graphite coating. The absorptance values were obtained by minimizing the error between temperature evolution measurements at various locations of the irradiated sheet and the corresponding finite element predictions. The values obtained for the CO2 laser were a = 0.20 for the stainless steel and a = 0.41 for the graphite coating. For the Yb fiber laser, the values were a = 0.40 for the stainless steel and a = 0.64 for the graphite coating. Degradation of the graphite layer was observed when the resistance temperature of the graphite coating was exceeded.

Mechanism of dislocation channel-induced irradiation assisted stress corrosion crack initiation in austenitic stainless steel

The mechanism by which dislocation channeling induces irradiation assisted stress corrosion cracking was determined using Fe–13Cr15Ni austenitic stainless steel irradiated with protons to a dose of 5dpa and strained at high temperature in both argon and simulated boiling water reactor normal water chemistry environments. Straining induced dislocation channels that were characterized by digital image correlation and confocal microscopy. Dislocation channels were found to be either continuous across the boundary, discontinuous, or discontinuous with slip in the boundary. Discontinuous channels were found to contain the least amount of strain but have the highest propensity for initiating cracks. Discontinuous dislocation channel–grain boundary intersections were shown to have the highest local stress. TEM in-situ straining of irradiated steels and atomistic simulation of dislocation–grain boundary interaction provided supporting evidence that channels that were unable to transfer strain underwent cracking. The inability of channels to relieve stress, by either slip in the adjacent grain or in the grain boundary, resulted in high local stresses and increased susceptibility to stress corrosion cracking initiation.

The effect of electron irradiation on the tribological property of perfluoropolyether grease in vacuum

Abstract A perfluoropolyether (PFPE) grease was exposed to 110 keV electron beam for fluence of 1 × 1015, 5 × 1015 and 1 × 1016 e/cm2 in vacuum. The friction and wear behaviour of the electron irradiated grease-lubricated 2Cr13 steel in vacuum was investigated by using a pin-on-disk type tribometer. The morphologies of the worn surfaces for the irradiated grease-lubricated 2Cr13 steel were observed by a scanning electron microscope (SEM). Gaseous products from the PFPE grease during the electron irradiation were monitored with a quadrupole mass spectrometer (QMS). The compositions and chemical states of grease samples before and after electron irradiation were analysed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Results show that the irradiated grease-lubricated 2Cr13 steel sliding against the same material in vacuum presents a high friction and a high wear. The QMS, XPS and FTIR analyses reveal that COF2 gas, low molecular weight perfluoroethers and acid fluoride groups are formed due to decomposition when the PFPE grease is exposed to 110 keV electron beam in vacuum. Moreover, the base oil content of the PFPE grease decreases with increasing the electron irradiation fluence, leading to the loss of lubricating ability of the PFPE grease.

A study on the microstructure and mechanical property of proton irradiated A508-3 steel

Transmission electron microscopy and the nanoindentation technique were employed to study the dislocation loops and hardening induced in proton irradiated A508-3 steel. The A508-3 steel specimens were irradiated to the dose of 0.054, 0.163, 0.271dpa at room temperature (RT), 0.163 pa at 250°C and 0.163, 0.271dpa at 290°C. The effect of dose and temperature on the dislocation loops and irradiation hardening was investigated. The results indicated that the dislocation loops were formed in proton irradiated A508-3 steel. The size and number density generally increased with increasing dose at RT. When the irradiation temperature changed from RT to 290°C, the loop size increased and the loop number density decreased. The irradiation hardening increased with dose. The effect of temperature on the irradiation induced hardening was discussed.

Microstructural stability of a self-ion irradiated lanthana-bearing nanostructured ferritic steel

Thermally stable nanofeatures with high number density are expected to impart excellent high temperature strength and irradiation stability in nanostructured ferritic steels (NFSs) which have potential applications in advanced nuclear reactors. A lanthana-bearing NFS (14LMT) developed via mechanical alloying and spark plasma sintering was used in this study. The sintered samples were irradiated by Fe2+ ions to 10, 50 and 100dpa at 30°C and 500°C. Microstructural and mechanical characteristics of the irradiated samples were studied using different microscopy techniques and nanoindentation, respectively. Overall morphology and number density of the nanofeatures remained unchanged after irradiation. Average radius of nanofeatures in the irradiated sample (100dpa at 500°C) was slightly reduced. A notable level of irradiation hardening and enhanced dislocation activity occurred after ion irradiation except at 30°C and ⩾50dpa. Other microstructural features like grain boundaries and high density of dislocations also provided defect sinks to assist in defect removal.