Brittle Transition Temperature Research Articles

Effect of low temperature on fatigue crack propagation behavior of QP980 steel and laser-welded joint

This paper aims to investigate the fatigue crack propagation behavior of quenching and partitioning 980 steel (as base metal) and its welded joint at low-temperature environments. The fatigue crack propagation tests are carried out at 25°C, −40°C and −80°C under R=0.1, 0.3 and 0.5. The results show that the fatigue crack propagation threshold value increases and fatigue crack propagation rate decrease of base metal, whereas welded joint behaved opposite results as the temperature decreases. The fatigue ductility to brittle transition temperature of base metal is lower than that of welded joint. Compared with the welded joint, the base metal has higher fatigue resistance.

Validity of Toughness Measurements From Miniature Specimens Failing in Different Fracture Modes

Abstract Using miniature compact tension (mini-C(T)) (4 mm thick, 0.16T) specimens to determine toughness in reactor pressure vessel (RPV) steels permits the ductile-to-brittle transition temperature to be derived from small amounts of material and allows more effective use of surveillance specimens. However, questions have been raised as to whether the failure mechanisms are the same in miniature and large specimens, something that must be ensured when transferring fracture results obtained in mini-C(T) specimens to larger components. This work, performed within the FRACTESUS project, presents toughness measurements and detailed fractography on both a homogeneously brittle base metal and a relatively ductile, inhomogeneous weld to assess the transferability of fracture data. The fractography shows that brittle fracture initiates within the part of specimen experiencing small-scale yielding (SSY), so long as the toughness measurement is valid. Similarly, although the precrack front asymmetry appears more marked in smaller specimens, as long as the deviation from planarity is within the American Society for Testing and Materials (ASTM) E1921 limits, the asymmetry does not affect the location of the initiation site. For materials showing a variety of fracture modes, the fracture modes observed at the initiation sites are consistent with those observed in larger specimens. Where data are available, the stress and strain conditions at the initiation sites are also found to be consistent in mini-C(T) and larger specimens. These observations support the thesis that toughness measurements made on mini-C(T) specimens reflect the same material characteristics and failure mechanisms as those made on larger specimens.

Effect of La2O3 on the properties of molybdenum rhenium alloy

Adding rhenium to molybdenum can improve room temperature processing performance and reduce the ductile brittle transition temperature. Adding lanthanum oxide to molybdenum can refine grain size and improve strength. This work analyzes the effect of adding different amounts of lanthanum oxide to molybdenum rhenium alloys on their properties. The results indicated that with the increase of lanthanum oxide, the tensile strength increased, but the room temperature elongation only increased at 0.6%; as the temperature increases, the trend of tensile strength and elongation at 900 °C and 1100 °C is similar; at 1300 °C, the strength decreases significantly but the elongation increases sharply; the corresponding changes can be explained by the fracture morphology.

Microstructural changes induced in advanced tungsten grades under high temperature neutron irradiation

Recent studies addressing the change of mechanical properties after high temperature irradiation of tungsten have reported substantial increase of the ductile to brittle transition temperature as well as increase of the hardness. In this work, we performed microstructural analysis by means of transmission electron microscopy (TEM) on various tungsten grades earlier exposed to high temperature neutron irradiation at 1200 °C up to 1 dpa. The latter corresponds to the expected irradiation temperature on the surface of a tungsten monoblock during the steady state operation in ITER. TEM was applied to five tungsten (W) materials including ITER specification pure W and precipitate strengthened W alloys. The analysis of TEM data coupled with its discussion led to a number of important conclusions regarding the damage induced at 1200 °C neutron irradiation such as: (i) weak effect of recrystallization on the accumulation of the lattice damage; (ii) promising results on the tolerance of damage accumulation in W-Y2O3 grade and (iii) internal oxidation of TiC particles which may contribute to the huge increase of the irradiation induced hardening.

Selection of EU-DEMO divertor operating conditions: water cooling thermal-hydraulic parameters and power exhaust capabilities

In the context of EUROfusion activities for the development of the DEMO reactor project, the divertor design is a major challenge. It must sustain very high heat, ion particle and neutron fluxes allowing, at the same time, the shielding of the vacuum vessel and the vacuum pumping for reducing the plasma pollution. The conceptual divertor design is based on the use of EUROFER97 for the divertor cassette body, while tungsten monoblocks bonded to CuCrZr pipes are used for plasma-facing targets. EUROFER97 was selected considering its reduced long-term activation and superior creep and swelling resistance under neutron irradiation. However, depending on the operating temperature under neutron irradiation, a pronounced shift of the Ductile to Brittle Transition Temperature (DBTT) is expected. At the same time, for the plasma-facing targets, the coolant temperature has to be identified such to allow sufficient heat removal capacity at the strike point. This study explores alternative cooling conditions for the divertor system that are able to ensure the fulfillment of functional and system requirements and to allow for divertor cassette body re-use during plant lifetime. The main aim is to identify the best water cooling thermal-hydraulic conditions avoiding material embrittlement (for EUROFER 97) and softening/hardening (for copper alloy pipes). At the same time, the goal is to reduce the inventories (enthalpy of the cooling circuit) and the radwaste at the end of divertor lifetime.

Investigating the ductile to brittle transition phenomenon in binary Fe-Ni systems using molecular dynamics simulation

Ductile to brittle transition (DBT) phenomenon is commonly observed in BCC Fe-based systems, which significantly deteriorates their low temperature fracture toughness. Ni is known to be an effective alloying element to improve the low temperature toughness by reducing the ductile to brittle transition temperature (DBTT). The present article investigates the role of Ni in tailoring the DBT curve of BCC Fe by employing molecular dynamics (MD) simulation. Here, we performed mode-I loading of different pre-cracked systems (pure Fe, Fe-2 wt% Ni and Fe-4 wt% Ni alloys) over a temperature range of 1–800 K to construct their DBT curves. The DBT curves are subsequently correlated with the temperature sensitivity of fracture stress, which is estimated using the concept of critical strain energy release rate (GIC). The role of Ni in reducing the fracture stress sensitivity is responsible for decreasing the DBTT as well as the slope of the DBT region. The underlying mechanism is discussed in terms of the crack blunting effect of Ni which effectively increases the GIC and hence the fracture stress, particularly at the low temperature regimes. Besides, the significantly large strain rate imposed in MD simulation is mainly responsible for the higher DBTT values when compared with the experimental results. The present paper tackles this strain rate issue by using an appropriate strain rate sensitivity factor to rescale the simulated DBTT values down to the experimental ones, resulting in a good match between the simulated and experimental DBTT.

Irradiation microstructures and hardening in commercial nuclear reactor pressure vessel steels at high extended life fluence

Nuclear reactor lifetimes up to and beyond 80 years may be limited by nano-scale Cu-Mn-Ni-Si precipitates (CRPs and MNSPs) that form under neutron irradiation in pressure vessel (RPV) steels, resulting in hardening and ductile to brittle transition temperature increases, or embrittlement. Here, Atom Probe Tomography (APT) is used to characterize precipitation in nine actual RPV steels, irradiated in the ATR-2 test reactor, to a high fluence 1.38 × 1020 n/cm2 at 292 °C and 3.68 × 1012 n/cm2 s. APT quantifies the relation between a wide range surveillance alloy Cu, Ni, Mn, and Si contents and the corresponding precipitate volume fractions, number densities, sizes, and compositions. The precipitate volume fraction (fp) is dominated by the dissolved steel Cu and Ni contents. Irradiation hardening shows the expected dependence on the √fp. The APT results also provide an opportunity to evaluate flux effects by direct comparisons with corresponding archival data from low flux surveillance irradiations. Most notably, this work establishes a strong microstructural and mechanistic foundation for the new data-driven Odette, Wells, Almirall, and Yamamoto (OWAY) model to predict high fluence RPV steel embrittlement.

Using Acoustic Emission Pulses to Assess the Ductile–Brittle Transition Temperature of Aluminum Matrix Composites at Negative Temperatures

Using Acoustic Emission Pulses to Assess the Ductile–Brittle Transition Temperature of Aluminum Matrix Composites at Negative Temperatures

Additive manufacturing of refractory metals and carbides for extreme environments: an overview

Refractory metals and their carbides possess extraordinary properties when subjected to high temperatures and extreme environments. Consequently, they can act as key material systems for advancing many sectors, including space, energy and defence. However, it has been difficult to process these materials using the conventional routes of manufacturing. Additive manufacturing (AM) has shown a lot of potential to overcome the challenges and develop new material systems with tailored properties. This review provides a fundamental understanding of the challenges in the processing of refractory metals and their carbides, including microcracking, formation of brittle oxide phases and high ductile to brittle transition temperature (DBTT). We also highlight some of the novel approaches that have been taken to improve the processability of these challenging material systems using AM. These include in-situ reactive printing, ultrasonic vibration, laser beam shaping, multi-laser deposition and substrate pre-heating with a focus on microstructural changes to improve the properties of printed parts.

Nonlinear Impact Damage Evolution of Charpy Type and Analysis of Its Key Influencing Factors

The current research of Charpy impact mainly focuses on obtaining the ductile brittle transition temperature of materials by experiments. Compared with experiments, numerical simulation can study many problems with harsh conditions. However, there are still few studies on the influence of geometric factors such as side grooves. In this paper, the geometry of standard Charpy impact test is designed. Specimens with different widths and side grooves are tested. The finite element model of Charpy impact was established by ABAQUS software. Use test results and simulation results to verify each other. The effects of sample width, side groove depth and side groove bottom fillet on the impact fracture resistance of the sample were studied. The results show that the specimen width is positively correlated with the impact toughness of the specimen. The side groove greatly reduces the impact toughness of the material; the toughness of side groove decreases with the increase of depth; the fracture toughness of side groove decreases with the increase of fillet at the bottom of side groove. The proportion of toughness energy to impact energy of samples was analyzed. The results show that the toughness energy accounts for about 70% of the impact energy of the sample, which has little to do with the geometric characteristics of the sample. This study presents a reliable method for studying Charpy impact tests. The influence of geometric parameters is obtained, which provides a reference method for the study of impact toughness of high toughness materials.

Effect of Texture on the Ductile–Brittle Transition Range and Fracture Mechanisms of the Ultrafine-Grained Two-Phase Ti-6Al-4V Titanium Alloy

In this work, the technique of equal-channel angular pressing (ECAP) that enables producing bulk billets was used to form a UFG structure in Ti-6Al-4V alloy. A subsequent warm upsetting simulates die forging and the production of a part. We studied the evolution of the UFG alloy’s crystallographic texture in the process of deformation during the production of a semi-product and/or a part, as well as its effect on the ductile–brittle transition region in the temperature range from −196 °C to 500 °C and the material’s fracture mechanisms. To test Charpy impact strength, standard samples of square cross-section with a V-shape notch were used (KCV). It was found that the impact toughness anisotropy is caused by textural effects and has a pronounced character at temperatures in the ductile–brittle transition range. Up to 100 °C the KCV values are close in the specimens processed by ECAP and ECAP+upsetting (along and perpendicularly to the upsetting axis—along the Z-axis and along the Y-axis, respectively), while a large difference is observed at test temperatures of 200 °C and higher. At a temperature of 500 °C, the impact toughness of the UFG Ti-6Al-4V alloy after ECAP reaches a level of that after ECAP+upsetting in the fracture direction along the Z-axis (1.60 and 1.77 MJ/m2, respectively). Additionally, an additional ECAP upsetting after ECAP decreases the ductile–brittle transition temperature of the UFG Ti-6Al-4V alloy, which increases the temperature margin of the toughness of the structural material and reduces the risk of the catastrophic failure of a product. The fractographic analysis of the fracture surface of the specimens after Charpy tests in a wide temperature range revealed the features of crack propagation depending on the type of the alloy’s microstructure and texture in the fracture direction.

Fracture behavior of S355J2W steel and weld at low temperature

AbstractThe weather‐resistant steel S355J2W shows potential for use in the bogies of high‐speed trains operating in extremely cold regions. In this study, the mechanical properties and fracture behavior of S355J2W steel and its MAG welding joints were investigated using low‐temperature uniaxial tensile tests, Charpy impact tests, and three‐point bending tests (TPB), which play an important role in the safety and reliability of train operation. The ductility and toughness indices decreased as the temperature decreased from room temperature to −114°C, and the weld is more dangerous and prone to fracture than the base material area. The бs/бb values of WM and HAZ samples slightly increased by 3.7% and 3.8% from 20 to −114°C, respectively. And the ductile–brittle transition temperature (DBTT) of BM, WM, and HAZ were −75.6, −46.1, and −56.6°C, respectively. The fracture micro‐mechanisms were evaluated by analyzing the grain misorientation along the residual cracks using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The high‐angle grain boundaries (GBs) in the base metal were observed to greatly contribute to stopping or deflecting the crack propagation, thereby improving the low‐temperature toughness. In contrast, the cracks easily penetrate the low‐angle GBs of the weld (which made up 38% of the grain boundaries of the welds), thereby decreasing the lower temperature fracture toughness of the weld. The dislocations of S355J2W steel and the weld metal were investigated by TEM analysis.

The Cold-Brittleness Regularities of Low-Activation Ferritic-Martensitic Steel EK-181

The behavior of the EK-181 low-activation ferritic-martensitic reactor steel (Fe–12Cr–2W–V–Ta–B) in the states with different levels of strength and plastic properties after traditional heat treatment (THT) and after high-temperature thermomechanical treatment (HTMT) in the temperature range from −196 to 25 °C, including the range of its cold brittleness (ductile–brittle transition temperature, DBTT) is studied. The investigations are carried out using non-destructive acoustic methods (internal friction, elasticity) and transmission and scanning electron microscopy methods. It is found that the curves of temperature dependence of internal friction (the vibration decrement) of EK-181 steel after THT and HTMT are similar to those of its impact strength. Below the ductile–brittle transition temperature, it is characterized by a low level of dislocation internal friction. The temperature dependence curves of the steel elastic modulus increase monotonically with the decreasing temperature. In this case, the value of Young’s modulus is structure-sensitive. A modification of the microstructure of EK-181 steel as a result of HTMT causes its elastic modulus to increase, compared to that after THT, over the entire temperature range under study. The electron microscopic studies of the steel microstructure evolution near the fracture surface of the impact samples (in the region of dynamic crack propagation) in the temperature range from −196 to 100 °C reveal the traces of plastic deformation (increased dislocation density, fragmentation of the martensitic structure) at all of the temperatures under study, including those below the cold brittleness threshold of EK-181 steel.

Testing the impact energy and determination of the brittleness temperature of the welded joint of steel A516 Gr.70

Most problems with steel structures occur in the welded joint or its immediate vicinity. For this reason, special attention is paid to testing the properties of the welded joint. Fractures of welded steel structures in operating conditions have led to the need to study the influence of temperature on the behavior of steel and welded joints. For the purpose of these tests, the critical test conditions (temperature) that cause brittle fracture are defined. The paper will present the results of experimental tests of the impact energy of the welded joint of steel A516 Gr.70. Impact energy testing at different temperatures was done with the aim of determining the brittleness transition temperature of the base metal and components of the welded joint. Also, these tests should give an assessment of the influence of temperature on the ratio of the crack formation energy Wiu, and the crack propagation energy Wa in the total fracture energy Wt. This test procedure determines the tendency to brittle fracture, that is, the tendency to increase brittleness during exploitation. The obtained results are most often used for ongoing quality control and evaluation of material homogeneity.

Studies of the formation and distribution of cracks and various defects on the heated tungsten plate surface during pulsed plasma flux impact

In this work, cracking of the surface of a tungsten plate was studied using a PW-7 pulsed plasma accelerator. The surface of the tungsten plate was exposed at an energy density and plasma flux duration of 1.1 MJ/m2 and 200 μs, respectively. Before each shot, the base of the tungsten plate was heated to a temperature of 823 K. Heating of the base of the tungsten plate above the ductile–brittle transition temperature made it possible to control the temperature gradient on its surface under pulsed plasma exposure. In other words, the temperature gradient of the tungsten plate before and after exposure to the plasma flux decreased. Compared to the data at room temperature, the results of heating of the tungsten plate to the tungsten ductile–brittle transition temperature of 823 K showed a reduction in crack width to several hundred nanometers under the influence of plasma. It should be noted that heating of the plate did not completely eliminate cracking of the surface of the tungsten plate. However, this significantly minimizes the emission of dust from the surface of the tungsten plate under the influence of the plasma flux (barely no dust was observed).

Dose and dose rate dependence of precipitation in a series of surveillance RPV steels under ion and neutron irradiation

Life extension of light water reactors requires robust models to predict the embrittlement of reactor pressure vessel (RPV) steels under long-term neutron irradiation. Embrittlement is due to nanoscale precipitation hardening by phases containing Cu, Mn, Ni, and Si, resulting in increases of the ductile to brittle transition temperature. Embrittlement data for low flux, high fluence extended life conditions, up to 80 years or more, are largely not available. Thus, higher flux, accelerated test reactor irradiations have been used to help develop models to predict low flux embrittlement at high fluence. Here flux, or dose rate, is expressed in units of displacements per atom per second (dpa/s). The dose rate can significantly influence precipitate evolution in a way that depends on fluence (dpa), temperature, and alloy composition, as well as flux itself. Here, we explore precipitation in surveillance steels with varying compositions at high fluxes (10−5–10−6 dpa/s) produced using 70 MeV Fe2+ ion irradiations. Atom probe tomography was used to characterize the dose dependence of precipitation at very high ion irradiation dose rates, and to evaluate corresponding similarities or differences compared to neutron irradiated steels at a much lower dose rate of ≈ 5 × 10−9 dpa/s. The data show that, on average, the ion irradiation precipitate volume fractions increase by factors of ∼ 1.5 to 2 between 0.3 dpa and 1.2 dpa. Limited data show that the major effect of neutron versus ion irradiation is the former has lower precipitate number densities and larger sizes. Otherwise, the precipitates are remarkably similar in composition and volume fraction.

Effect of grain size and grain boundary on ductile to brittle transition behavior of Fe-6.5wt.%Si alloy under miniaturized three-point bending tests

This research investigates the influence of grain size and grain boundary on the ductile to brittle transition (DBT) behavior using three-point bending tests. With increasing grain size from 91 μm to 255 μm, the ductile to brittle transition temperature (DBTT) increases by approximately 80 °C, accompanied by a corresponding 0.196 eV increase in DBT activation energy. An evident linear reciprocal relationship exists between DBTT and grain boundary ratio. The Hall-Petch equation is employed to determine the quantitative relationship between the DBTT and grain size, offering novel guidance for determining the rolling temperature of Fe-6.5wt.%Si alloy.

The effect of phosphorus on precipitation in irradiated reactor pressure vessel (RPV) steels

Embrittlement of light water reactor pressure vessel (RPV) steels by fast neutron irradiation may limit extended nuclear plant life. Embrittlement, which is manifested as increases in various indexes of a ductile to brittle transition temperatures (ΔT), is primarily due to hardening by nanoscale precipitates containing Cu, Ni, Mn, and Si, which form under irradiation. In addition to these elements, P has also been found to play a role in embrittlement. While only slightly enriched in the precipitates, hardening and embrittlement increase with trace P concentrations in low-Cu steels. Here, we characterize the individual and synergistic irradiation precipitation and hardening mechanisms in a series of RPV steels containing no to low-Cu and with systematic variations in Ni and P. The steels were irradiated to a fluence of ∼ 1.38×1020 n/cm2 at ∼ 292 °C in the UCSB ATR-2 experiment. In nominally Cu-free medium and high-Ni RPV steels, atom probe tomography shows that P and Ni promote precipitation of P-Mn-Si-Ni and Mn-Si-Ni precipitates, respectively. The precipitate microstructure correlates with the observed irradiation hardening.

Effect of B and N Content and Austenitization Temperature on the Tensile and Impact Properties of Modified 9Cr-1Mo Steels

The present study investigates the relative effect of B and N concentrations and the austenitization temperature on the microstructure and mechanical properties (tensile and Charpy impact) of modified 9Cr-1Mo (P91) steels. Initially, a B-free P91 steel (with 500 ppm N) and four different B-containing steels (25–100 ppm) with varying N concentrations (20–108 ppm) were hot-rolled, normalized from different austenitization temperatures (1000–1100 °C/1 h) and finally tempered at 760 °C for 1 h. A Charpy impact test shows that the ductile–brittle transition temperature (DBTT) of all the B-added steels decreases with an increase in the austenitization temperature, where the 100 ppm B steel offers the lowest DBTT (−85 °C). Similarly, the strength increases with the increase in the austenitization temperature (1100 °C), with a slight drop in ductility. The influence of precipitates on the microstructure and mechanical properties is explained considering the B enrichment at the precipitates and the thermodynamic stability of the precipitates. The 100 ppm B steel (containing the maximum B and minimum N), normalized from 1100 °C austenitization, shows the best combination of tensile and Charpy impact properties, owing to the effective dissolution of coarse M23C6 and MX precipitates during the normalization treatment and the formation of fine B-rich (Fe,Cr)23(B,C)6 precipitates during the subsequent tempering.

Effect of V, Nb, and Ti microalloying on low–temperature impact fracture behavior of non–quenched and tempered forged steel

In this study, three kinds of non-quenched and tempered forged steels with V, V–Nb and V–Nb–Ti microalloying elements were designed. The microstructure and low–temperature impact fracture behavior of the three steels were comprehensively compared and analyzed. The results showed that the addition of Nb and Ti elements can significantly refine the grains and form precipitated particles to improve the strength of the steel. The fracture behavior of the three steels types at a low temperature of −28 °C was dominated by brittle cleavage fractures. V–Nb–Ti steel exhibited the worst low–temperature impact toughness. The SEM analysis result showed that coarse (V, Nb, Ti) (C, N) particles present in the steel acted as local cleavage initiation sites. No cleavage fracture caused by (V, Nb) (C, N) particles occurred in the V–Nb steel. The results showed that the high impact energy (70.4 J) at room temperature and low ductile–brittle transition temperature (DBTT, −20 °C) of the V–Nb steel were mainly attributable to the grain refinement of Nb. The micron–sized coarse (V, Nb, Ti) (C, N) particles were the main factor responsible for the low impact energy (35.5 J) at room temperature and high DBTT (−7 °C) of V–Nb–Ti steel and can act as the local cleavage initiation sites to dominate the cleavage fracture at low temperature and decrease the impact toughness.