Increase In Ductile-brittle Transition Temperature Research Articles

Simulation of Transmission Electron Microscope Images of Dislocations Pinned by Obstacles

From a direct observation of dislocation-obstacle interaction utilizing in situ straining experiments in transmission electron microscope (TEM), the obstacle strength factor could be evaluated from pinning angles of dislocation cusps. We simulated this process: we produced a dislocation cusp by molecular dynamics simulation of interaction between an edge dislocation and a void or a hard precipitate in copper, and calculated the TEM image by multislice method. In two-beam conditions, cusp images showed inside-outside contrast depending on the sign of the diffracting vector and other variations with the specimen geometry. The pinning angles measured on TEM images ranged up to a few tens of degrees and were between the true angles for the two partial dislocations. Characteristics and contrast mechanisms of cusp images were discussed based on those of dislocation dipoles. (doi:10.2320/matertrans.MD201312) (Received September 3, 2013; Accepted October 2, 2013; Published November 15, 2013) Various crystal lattice defects induced by neutron irradi- ation, such as precipitates, voids, dislocation lines and loops are responsible for degradation in mechanical properties: increase in yield stress, loss of ductility, and increase in ductile-brittle transition temperature. A number of researches have been devoted for defect structural evolution and mechanical property changes under neutron irradiation for nuclear materials development. Understanding the mecha- nisms involved is necessary to construct models for estimating the lifetime of components of nuclear power plant. In an elementary process of dislocation-obstacle interac- tion, a gliding dislocation is pinned by obstacles and bows out to form arcs between the neighboring pinning points, which induces cusps on the dislocation at obstacles. The apex angle of the dislocation cusp is referred to as the pinning angle o. The dislocation breaks away by bypassing or cutting through the obstacle when the pinning angle reaches a critical value oc. Stronger obstacles have smaller critical angles. The obstacle strength factor i ¼ cosðoc=2Þ and the distance between the neighboring pinning points are the key parame- ters that relate the defect microstructure to the change in macroscopic mechanical properties. There have been re- ported a few attempts to evaluate the factor from a direct observation of dislocation-obstacle interaction utilizing in situ straining experiments in transmission electron micro- scope (TEM). 1­3) The method has not been widely applied so far, due to high technical levels required for in situ experiments. Another difficulty comes from TEM images with a limited resolution both in time and space for measuring pinning angles of radiation-induced obstacles, typically less than a few nanometers in size, at a moment of the breakaway. Alternatively, molecular dynamics (MD) simulations have been applied extensively for the dislocation- obstacle interaction. 4­7) In the present study, we performed TEM image simulation of dislocation cusps to examine whether TEM images reveal the cusp structure in the scale suitable for evaluating the obstacle strength factor. For this purpose, we stopped MD simulation of interaction between an obstacle and an edge dislocation just before the breakaway. We then calculated TEM images of dislocation cusps under various conditions using the multislice method. We compared apex angles between the cusp structure and on the calculated images, which would support the experimental evaluation of the obstacle strength factor.

Effect of helium implantation on mechanical properties and microstructure evolution of reduced-activation 9Cr–2W martensitic steel

A reduced-activation martensitic steel was implanted with helium up to 580 at. ppm by using 36 MeV α-beam between 353 and 423 K along with displacement damage up to 0.226 dpa. The implantation-induced increase in ductile–brittle transition temperature (DBTT) was estimated to be 98 K for the standard charpy V-notched (CVN) specimen implanted with 580 at. ppm He, through the conversion of small punch (SP) test results by an empirical relationship. It is clarified from comparison with neutron irradiation data that the increase in DBTT as well as implantation-induced hardening is interpreted simply in terms of displacement damage, suggesting that there is no significant effect of helium on both the irradiation hardening and the fracture toughness of the steel. No fracture mode change by the helium implantation was observed in the SP tests, showing a complete cleavage fracture mode in the lower shelf energy region.

Mechanical properties of 8Cr–2WVTa steel aged for 30 000 h

A mill production plate of a reduced activation ferritic steel was thermally aged for up to 30000 h at 400–650°C. Charpy impact tests, creep rupture tests and hardness tests were conducted. Both Vickers hardness number and creep strength decrease with aging at 650°C. The ductile–brittle transition temperature (DBTT) increases with both aging time and aging temperature. However, the DBTT does not exceed +20°C even after aging at 650°C for 30000 h. Extracted residues and extraction replicas were analyzed metallurgically. The increase in DBTT is related mainly to the precipitation of Laves phase on the prior austenite grain boundaries. The rather low DBTT after aging is caused by the fine prior austenitic grain size.

Dependence of impact properties on irradiation temperature in reduced-activation martensitic steels

Ductile–brittle transition (DBT) behavior of 9%Cr-2%W reduced-activation martensitic (RAM) steels has been investigated following neutron irradiation in the fast flux test facility, materials open test facility (FFTF/MOTA) at different temperatures. Both the irradiations at 663 and 733 K cause an increase in DBT temperature, while the irradiation at 663 K induces the hardening and the softening at 733 K. Microstructural observation by transmission electron microscope (TEM) revealed that small dislocation loops existed in the specimen irradiated at 663 K and no such a loop, but relatively large M 6C carbides and Laves phase were formed by the irradiation at 733 K. There appears to be a linear dependence between ΔDBTT and Δ σ Y in neutron irradiated RAM steels when irradiation induces the hardening. Irradiation embrittlement accompanied by the softening is considered to be due to reduction of cleavage fracture stress caused by the irradiation-induced recovery of the martensitic structure, namely decrease in dislocation density and formation of large precipitates.

Probability of Fracture and Life Extension Estimate of the High-Flux Isotope Reactor Vessel

The state of the vessel steel embrittlement as a result of neutron irradiation can be measured by its increase in ductile-brittle transition temperature (DBTT) for fracture, often denoted by RTNDT for carbon steel. This transition temperature can be calibrated by the drop-weight test and, sometimes, by the Charpy impact test. The life extension for the high-flux isotope reactor (HFIR) vessel is calculated by using the method of fracture mechanics that is incorporated with the effect of the DBTT change. The failure probability of the HFIR vessel is limited as the life of the vessel by the reactor core melt probability of 10−4. The operating safety of the reactor is ensured by periodic hydrostatic pressure test (hydrotest). The hydrotest is performed in order to determine a safe vessel static pressure. The fracture probability as a result of the hydrostatic pressure test is calculated and is used to determine the life of the vessel. Failure to perform hydrotest imposes the limit on the life of the vessel. The conventional method of fracture probability calculations such as that used by the NRC-sponsored PRAISE CODE and the FAVOR CODE developed in this Laboratory are based on the Monte Carlo simulation. Heavy computations are required. An alternative method of fracture probability calculation by direct probability integration is developed in this paper. The present approach offers simple and expedient ways to obtain numerical results without losing any generality. This approach provides a clear analytical expression on the physical random variables to be integrated, yet requires much less computation time. In this paper, numerical results on 1) the probability of vessel fracture, 2) the hydrotest time interval, and 3) the hydrotest pressure as a result of the DBTT increase are obtained. Limiting the probabilities of the vessel fracture as a result of hydrotest to 10−4 implies that the reactor vessel life can be extended up to 50 EFPY (100 MW) with the minimum vessel operating temperature equal to 85°F.

Embrittlement of CrMo steels after low fluence irradiation in HFIR

Subsize Charpy impact specimens of 9Cr1MoVNb (modified 9Cr1Mo) and 12Cr1MoVW (Sandvik HT9) steels and 12Cr1MoVW with 2% Ni (12Cr1MoVW2Ni) were irradiated in the High Flux Isotope Reactor (HFIR) at 300 and 400°C to damage levels up to 2.5 dpa. The objective was to study the effect of the simultaneous formation of displacement damage and transmutation helium on impact toughness. Displacement damage was produced by fast neutrons, and helium was formed by the reaction of 58Ni with thermal neutrons in the mixed-neutron spectrum of HFIR. Despite the low fluence relative to previous irradiations of these steels, significant increases in the ductile-brittle transition temperature (DBTT) occurred. The 12Cr1MoVW2Ni steel irradiated at 400°C had the largest increase in DBTT and displayed indications of intergranular fracture. A mechanism is proposed to explain how helium can affect the fracture behavior of this latter steel in the present tests, and how it affected all three steels in previous experiments, where the steels were irradiated to higher fluences.

Effect of vanadium and titanium on mechanical properties of chromium-tungsten steels

Increasing the vanadium content from 0.1 to 0.50% in a 2.25Cr-2W-0.1C steel (all concentrations are in wt%) increased the yield stress 20% and resulted in a higher ductile-brittle transition temperature (DBTT). When vanadium was increased to 0.5%, a further slight increase in strength occurred with a large increase in DBTT. Thus, optimum strength and impact toughness is achieved at an intermediate vanadium concentration. With the addition of 0.02% Ti to 2.25Cr-0.25V-0.1C, 2.25Cr-2W-0.1C, and 2.25Cr-2W-0.25V-0.1C steels, yield stress decreased 10 to 30%. An increase in impact toughness accompanied the strength loss. The toughness may have been affected by a decrease in the prior austenite grain size. There was little difference in the DBTT of the Ti-modified steels tempered at 700 or 750°C. The use of a Ti-modified steel tempered at 700°C might offset the strength advantage of steels without titanium, which have to be tempered at the higher temperature.

Effect of cobalt on impact toughness of steels

The addition of Co to 18%Ni maraging steels produces an increase in hardness due to a synergistic effect. A similar effect is reported in this paper for maraging steels based on Fe–5Mn–6Ni. However, Co is thought to have an effect on the ductile-brittle transition temperature (DBTT) of mar aging steels. The addition of 9%Co to the base alloy plus 2·5%Mo increases the DBTT to 120°C in unaged material. Similar to reports by other workers on Fe–Co alloys, it is considered that this increase in DBTT is due to Co restricting cross-slip. In contrast, Mo is thought to lower the DBTT by increasing cross-slip. The reason for 2·5%Mo changing the mode of brittle fracture from intergranular to transgranular cleavage in the base alloy is discussed. Mention is also made of the effect of Ni on the DBTT in lath martensite structures. In classical 18%Ni maraging steel, the DBTT of the Fe–18Ni base is probably so low that the addition of Co does not seriously embrittle the steel.MST/1828

Impact behavior of reduced-activation ferritic steels irradiated in the Fast Flux Test Facility

Reduced-activation Cr-W steels with chromium concentrations varying from 2.25 to 12 wt% were irradiated at 365°C to ~ 7 dpa in the Fast Flux Test Facility. Irradiation caused a loss of toughness for all steels, as measured in Charpy impact tests; increases in the ductile-brittle transition temperature (DBTT) and decreases in the upper-shelf energy were observed after irradiation. Two steels with 9% Cr showed the greatest resistance to the effects of irradiation: the DBTT of a 9% Cr steel with 2% W and 0.25% V increased by 68°C during irradiation; this same composition with an addition of 0.07% Ta developed an increase in DBTT of only 4°C. These shifts in DBTT compared to increases of over 100°C for the rest of the steels. Observations on the steels with 2.25% Cr indicated that a steel containing a combination of 2% W and 0.25% V was less severely affected by irradiation than steels with vanadium and no tungsten, tungsten and no vanadium, or with 1% W and 0.25% V.

Impact behavior of 9Cr-1MoVNb and 12Cr-1MoVW steels irradiated in HFIR

Charpy impact specimens of 9Cr-1MoVNb and 12Cr-1MoVW steels were irradiated in the High Flux Isotope Reactor (HFIR) at 300 and 400°C to displacement-damage levels as high as 42 dpa. For both steels, irradiation caused large increases in the ductile-brittle transition temperature (DBTT) and a decrease in the upper-shelf energy (USE). The increase in DBTT and the decrease in USE were greater at 400°C than at 300°C. At 400°C, irradiation to ~ 40 dpa produced shifts in DBTT of 204 and 242°C for the 9Cr-1MoVNb and 12Cr-1MoVW steels, respectively. These are the largest shifts ever observed for these steels and are attributed to the high helium concentration generated during irradiation in HFIR.

Comparison of effects of aging on fracture of 9Cr–1Mo and 2·25Cr–1Mo steel Part 1 Quenched and tempered material

A 9Cr–1Mo (wt- %) steel and a 2·25Cr–1Mo steel have been investigated in the quenched and tempered condition and after subsequent aging at 550°C for either 1000 or 5000 h. Transmission electron microscopy (TEM) was used to characterise the microstructures produced by each heat treatment. The ductility and fracture resistance were measured using ambient temperature tensile tests and Charpy tests at various temperatures, respectively. TEM was also used to monitor void nucleation during tensile deformation. Differences in the ductility of the steels before aging and their different responses to aging, which decreases the ductility of the 9Cr–1Mo steel while leaving that of the 2·25Cr–1Mo steel unaffected, are attributed to the different void nucleation characteristics of the precipitates in each steel. Differences in ductile–brittle transition temperature (DBTT) before aging are discussed in terms of differences in the lower shelf fracture mode of each steel and the possibility of crack nucleation at carbide clusters. Segregation of P during aging is considered to be responsible for the aging induced increase in DBTT in each steel, but it is suggested that interaction between P and the carbides in the 9Cr–1Mo steel might modify the segregation characteristics in that material.MST/1232a

Effects of austenitisation heat treatment on the fracture resistance and temper embrittlement of MnMoNi steels

The resistance to ductile and brittle fracture of four experimental melts of MnMoNi steel containing varying levels of sulphur, copper and phosphorus has been examined as a function of austenitisation heat treatment, with and without subsequent ageing at 500°C following tempering at 650°C. Fracture resistance was assessed by Charpy impact tests, fracture modes were studied using the scanning electron microscope, grain boundary segregation was quantified from Auger spectroscopy, and boron distribution determined by boron autoradiography. The results indicate that austenitisation heat treatment strongly influences the ductile-brittle transition temperature (DBTT) and upper shelf fracture energy (USE) in the quenched and tempered condition. The subsequent susceptibility to temper embrittlement is also markedly affected, high austenitisation temperatures being detrimental in all respects. Phosphorus segregation has been shown to occur during air cooling from tempering and during isothermal ageing, the degree of segregation increasing with austenitisation temperature, resulting in an increase in DBTT and a reduction in USE. Changes in DBTT and USE on isothermal ageing have been attributed to phosphorus segregation in all four composition melts. Microstructures susceptible to embrittlement have also shown enhanced levels of boron or boron-containing particles at prior austenite grain boundaries.

The calculation of transition temperature changes in steels due to temper embrittlement

A study was made of quenched and tempered laboratory heats of 3.5 Ni-1.7 Cr steels doped with P, Sn, or Si in which the hardness, the grain size, and the extent of intergranular segregation of the metalloid dopents were varied. The segregation was accomplished by aging treatments of various times at 480 °C. It was found that the increases in ductile-brittle transition temperature (ΔTT) were determined completely by the above three variables and that they can be combined in an “embrittlement equation” that permits the calculation of theΔTT that will occur from a particular degree of segregation in a steel of a given hardness and grain size. The embrittlement equation contains coefficients which must be determined empirically for each type of steel and embrittling element; however, the equation is derived by a straight-forward Taylor series expansion of theTT as a function of the three variables. Each term of the resulting equation can be justified from physical considerations. The embrittlement equation can serve as a rational basis for analysis of a given type of steel or for comparisons between types. In particular, it allows one to account for variations in hardness and grain size, and to determine the benefits to be gained from control of these two factors.

The Influence of Impurity-Defect Interactions on Radiation Hardening and Embrittlement

It has been established that copper and other trace elements have a detrimental effect on the notch-impact properties of ferritic pressure vessel steels irradiated at reactor operating temperatures of about 288°C (550°F). The underlying cause, however, of this trace element effect has not been established. But based on radiation damage work on vanadium containing oxygen and iron containing nitrogen, a possible mechanism appears to involve stabilization of radiation-produced defects as a result of migration of impurity atoms to the defects where they become trapped. The evidence for this impurity-defect interaction in vanadium and iron is described, and its effect on radiation hardening and embrittlement is discussed. The influence of dislocation channeling in promoting the early onset of plastic instability is also discussed, and the possible relevance of dislocation channeling is indicated with regard to the radiation-produced increase in ductile-brittle transition temperature and decrease in ductile shelf energy.

Control of Irradiation Embrittlement in Molybdenum

IRON, steel, and other body-centred cubic metals show an increase in ductile-brittle transition temperature as a result of neutron irradiation. In an attempt to elucidate the mechanism of this embrittlement for EN2 steel, Hull and Mogford1 studied the effect of neutron irradiation on the parameters σi and k y in the Petch relation: where σy is the yield stress and 2d the grain diameter. They found that σi, the lattice friction stress, increased, but k y which is a measure of the strength of dislocation locking, remained constant. They interpreted this to mean that the embrittlement was caused by point defects or groups of defects which impeded the movement of slip dislocations in their glide planes. This simple phenomenological description of irradiation embrittlement enables changes in ductile-brittle transition temperature caused by irradiation to be predicted from Cottrell's brittle fracture criterion2: once σi has been measured as a function of integrated neutron flux.