Low-alloy Pressure Vessel Steel Research Articles

Development of microstructure and residual stress in electron beam welds in low alloy pressure vessel steels

Reduced-pressure electron beam (EB) plate butt welds were manufactured in two low-alloy pressure-vessel steels, SA508 Gr 3 Cl 1 and SA508 Gr 2, at two thicknesses in both steels, 30 mm and 130 mm. Transient temperatures during welding were recorded using thermocouple arrays. Residual stresses in the as-welded condition and after post-weld heat treatment were measured using diverse methods: neutron diffraction and the contour method at 30 mm thickness; and deep hole drilling and the contour method at 130 mm. Incremental centre hole drilling measurements were performed at 130 mm thickness to better understand near-surface stresses. Weld and heat-affected zone microstructures and microconstituents were evaluated using a combination of hardness mapping, optical microscopy and electron microscopy. The as-welded residual stresses exhibit the characteristic M-shaped distribution for hardenable steels, reaching 500–600 MPa in tension in both steels at both thicknesses. However, the modest changes to the chemical composition and the change in plate thickness both significantly influenced microstructures, mechanical properties and residual stress distributions. These sensitivities underline the need for physically faithful models. This extensive characterisation study enables the development and validation of models that predict the development of microstructures and residual stresses in EB welds in low alloy pressure vessel steels.

Mechanical and metallurgical considerations on the effects of ppb-level chloride on stress corrosion cracking of low alloy steels in high-temperature water

This paper studies the effects of very low-level chloride (< 5 ppb) on the stress corrosion cracking (SCC) growth of low-alloy pressure vessel steel in oxygenated high-temperature water. Chloride levels as low as 3 ppb increase SCC susceptibility. Besides the chemical reason, this paper provides perspectives on the underlying mechanical and metallurgical contributions. A substantial variation was observed from three high-sulphur steels. Sulphur is not the only contributor to SCC. The synergy of material, environmental, and mechanical parameters controlled the crack-tip strain rate and crevice chemistry that govern the SCC response in the high-purity high-temperature water environments at intermediate K.

Evaluation of Quasi-Static and Dynamic Fracture Toughness on the Low-Alloy Reactor Pressure Vessel Steel JRQ in the Transition Region

Three point bending and impact tests with sub-sized Charpy specimens were performed on the JRQ reference steel for reactor pressure vessels. Quasi-static and dynamic fracture toughness data were calculated and the fracture behavior in the ductile to brittle transition region was evaluated within the frame of the master curve method (ASTM E1921). Specimens with shallow and deep cracks were studied and the respective influence of crack length and loading rate on the reference transition temperature was determined. The force-time curves of specimens with shallow cracks presented significantly smaller oscillations with respect to the absolute force, making the fracture toughness evaluation more accurate.

Contribution of research to the understanding and mitigation of environmentally assisted cracking in structural components of light water reactors

ABSTRACTEnvironmentally assisted cracking (EAC) is a potential threat to the safety and integrity of water-wetted components in operating water-cooled nuclear power plants. Two forms of EAC are commonly distinguished, depending on the form of loading contributing to damage: stress corrosion cracking (SCC) and corrosion fatigue. A number of instances of in-service degradation due to EAC have occurred in operating plants worldwide, often leading to unplanned plant outages. Understanding the causes of EAC is essential to minimise the loss of plant availability due to its occurrence and to avoid the possibility of catastrophic failure, for example, if a crack grew to a critical size in a major pressure boundary component. This paper will describe some examples of these phenomena in the main materials of construction of pressure boundary and other critical components in pressurised and boiling water reactors (BWRs). Over the last several decades, substantial research programmes have been carried out in a number of laboratories worldwide, aimed at further understanding of the processes leading to EAC to manage occurrences in plant and minimise future failures. Selected areas of research on EAC in light water reactor environments are discussed. Corrosion fatigue in low-alloy pressure vessel steels was the subject of considerable attention in the 1980s and early 1990s because of its potential threat to pressure vessel integrity and the publication of data, suggesting that there is a major influence of environment on fatigue crack growth in some laboratory tests. The author’s research provided insight into the conditions under which the major environmental effects occur and contributed to the development of an ASME Code Case for pressurised water reactor (PWR) conditions which provided a means of screening based on steel sulphur content and loading conditions. More recently, the research focus in this area has moved to austenitic stainless steels, again providing support to Code Case development and furthering mechanistic understanding. A recent review of knowledge gaps for EPRI provides a basis for future research on environmentally assisted fatigue and will inform the development of new assessment methodologies. A key area of the current study concerns differences in loading conditions between specimens in laboratory tests and plant components subject to transient loading. In the case of SCC, stainless steels have shown the greatest propensity to cracking in BWRs, while Alloy 600 has been a major cause of in-service failures in PWRs, both on the primary side, as recognised by Coriou in the early 1960s, and in secondary environments where a number of different corrosion-related failure processes have been identified. High-strength alloys, such as Alloy X-750 used for fastener applications, have also caused failures in both reactor types. For austenitic materials, SCC susceptibility is enhanced by irradiation, resulting in failures in core internals components. Ferritic stainless steels also undergo SCC under some specific circumstances but are generally more resistant than the lower chromium austenitic materials.

Effect of long-term thermal aging on magnetic hysteresis for low-alloy pressure vessel steel

Long-term thermal aging experiments have been performed at 290 and 550°C up to ∼ 10000 h for low-alloy pressure vessel steels with low and high Ni contents, in order to elucidate effects of thermal embrittlement on a structure-sensitive property of minor B-H loops. While a minor-loop property obtained from a power-law scaling of minor-loop parameters exhibits a small but steady increase after aging at 290°C, that exhibits a decreasing trend after aging at 550°C. The observations suggest that the evolution of nanometer precipitates plays a crucial role for a magnetic property change at 290°C, while relaxation of lattice strain in a matrix associated with precipitation may dominate at 550°C.

Investigation of effects of long-term thermal aging on magnetization process in low-alloy pressure vessel steels using first-order-reversal-curves

We have investigated effects of long-term thermal aging at 550°C up to 10000 h on major-loop coercivity, hysteresis scaling of minor loops, and first-order reversal curves (FORCs) for low-alloy pressure vessel steels with low and high Ni contents. While major-loop coercivity and minor-loop coefficient of the scaling exhibit a gradual decrease with aging for high-Ni steel, those for low-Ni one are very weakly dependent on aging time. On the other hand, we found that FORC distribution becomes steep along both axes of interaction and switching fields and the peak shifts toward a lower switching field for both steels. Considering that there is no significant development of nanoscale precipitates during the aging as revealed with small-angle neutron scattering experiments, a relaxation of lattice strain in a matrix, possibly associated with diffusion of Ni atoms, may dominate magnetic properties at 550°C.

Partitioning and segregation of trace element Sn in a low-alloy steel

The partitioning of the trace element Sn into Cu-rich precipitates in a low-alloy pressure-vessel steel has been characterized using the three-dimensional atom probe (3DAP) technique. This investigation has revealed for the first time that the trace element Sn, present at only 0.007 at.% in the steel, partitions strongly to both small spherical precipitates (<4 nm in diameter) and to large non-spherical precipitates (largest dimension 10–50 nm) during thermal ageing. Sn was also seen to segregate strongly to the precipitate/matrix interface of a large Cu precipitate and particularly in the region where a dislocation appears to intersect the precipitate. The strong attraction of large solute atoms to special sites probably drives the interfacial segregation of Sn. This is consistent with the observation of stronger segregation of Sn to the interface of large precipitates than to the coherent interface of smaller precipitates.

Role of dynamic strain aging in corrosion fatigue of low-alloy pressure vessel steel in high temperature water

Low cycle fatigue (LCF) behavior of an A533B-type low-alloy pressure vessel steel was investigated in 200 °C and 288 °C water. Major attention was paid to the role of dynamic strain aging (DSA) on corrosion fatigue behavior of the steel. It was found that DSA played a noticeable role in the present corrosion fatigue process, manifested by the strain-rate and temperature dependent cyclic stress and plastic strain amplitude response. DSA was found to occur at a low strain rate in 200 °C water or at a high strain rate in 288 °C water regardless of dissolved oxygen concentration (DO) in water. In low-DO water, DSA improved the LCF resistance of the steel to some extent, but such an effect was screened by the environmental effects in high-DO water. The interactions between DSA and surrounding environmental factors as well as their possible influence on environmentally assisted cracking and fatigue resistance of the steel in high temperature water are discussed.

Effects of loading factors on environmental fatigue behavior of low-alloy pressure vessel steels in simulated BWR water

Low cycle fatigue resistance of low-alloy pressure vessel steels was investigated in simulated boiling water reactor (BWR) water. Much attention was paid to the effects of loading factors on fatigue life and environmentally assisted cracking (EAC) behavior, in which strain rate, strain waveform and strain amplitude were taken into account. The fatigue resistance and EAC behavior of the steels in simulated BWR water were found to be closely dependent on the strain rate, strain waveform and strain amplitude applied. The above fatigue behavior may be attributed to loading-factor-induced change in dominant EAC processes in high temperature water environments. Related EAC mechanisms are also discussed.

Influence of statistical and constraint loss size effects on cleavage fracture toughness in the transition—A single variable experiment and database

This paper, which is the first of two parts, describes a comprehensive experimental study to systematically measure constraint loss and statistical size effects on fracture toughness of a typical low alloy pressure vessel steel in the brittle-to-ductile transition. Both of these size effects lead to higher toughness in smaller specimens. Previous studies primarily involved specimens that differed in size, but with self-similar standard geometric configurations, making it difficult to separate constraint loss from statistical size effects. The focus here is on a single-variable database developed for a homogeneous plate section of the decommissioned Shoreham Reactor vessel steel, tested at the 3/4-plate thickness position. Tests were carried out at a common set of conditions and covered a full matrix of specimens with six thicknesses ( B) ranging from 7.9 to 254 mm and four widths ( W) ranging from 6.4 to 50.8 mm. The variations in B were designed to provide readily observable statistical sampling effects, while variations in W result in fracture at deformation conditions ranging from small scale yielding to those involving extensive loss of constraint. A detailed analysis of this database is presented in a companion paper, where physically based models are used to quantify the relative contributions of constraint loss and statistical size effects. The models also provide a general approach to adjusting measured toughness for both testing and applications. The B– W database and analysis also provides a comprehensive independent assessment of some key aspects of the master curve method in the ASTM E 1921 Standard discussed in detail elsewhere. In this paper, we present an empirical analysis that shows a high constraint toughness scaling with B that is reasonably consistent with ASTM E 1921, as well as effects of loss of constraint that occur at much lower deformation levels than reflected in the data censoring limit in ASTM E 1921.

Strain-amplitude dependent fatigue resistance of low-alloy pressure vessel steels in high-temperature water

Low cycle fatigue resistance of low-alloy pressure vessel steels was investigated in 561 K air and water over a wide strain amplitude range. It was found that fatigue resistance of the steels was enhanced in high-temperature water relative to high-temperature air under the low strain amplitude conditions ( 2 × 104 cycles), while it was remarkably degraded in high-temperature water under the higher strain amplitude conditions. Fatigue cracking and fractographic features suggested that effects of hydrogen be involved in the present corrosion fatigue process in high-temperature water. Possible environmentally assisted cracking mechanisms are discussed.

Corrosion Fatigue Behavior of Low-Alloy Pressure Vessel Steels in High Temperature Water Under Multi-Factor Conditions

The corrosion fatigue resistance of pressure vessel steels in simulated light water reactor coolant water is greatly influenced by the mechanical factor such as strain rate, environmental factor such as temperature and dissolved oxygen (DO) concentration in water, and material factor such as sulfur content in steels. In most of previous work, the mechanical or environmental conditions were usually fixed as a constant throughout an individual test. However, these factors may change frequently during actual operations in power plants. So, it is of great interest to investigate the effects of change of the above factors on fatigue resistance of components materials in service environments and to develop appropriate methods for evaluating the environmental effects on fatigue damage. The present work was to investigate the low cycle fatigue (LCF) behavior of low-alloy pressure vessel steels in high temperature water. Special attention was paid on the influence of strain rate change in an individual LCF test on fatigue resistance of the steels. The alternate sequence of strain rate in a test was also considered. Moreover, the influence of DO, temperature and sulfur content in the steels was investigated. The detailed cracking and fractographic features were examined to assist the understanding of underlying corrosion fatigue mechanisms.

Cyclic cracking behavior of low-alloy pressure vessel steel in simulated BWR water

Fatigue crack initiation and growth behavior of low-alloy pressure vessel steel A533B has been investigated in simulated BWR water. Two distinguished crack morphologies were observed, closely depending on the cyclic strain rate. At a high strain rate, fatigue cracks grew in a tortuous manner. Final failure pre-dominantly resulted from the continuous growth of a single fatigue crack. A rough fracture surface was obtained on which hydrogen-induced cracking features were dominant. At a low strain rate, however, fatigue cracks developed in an entirely straight manner. Final failure was usually caused by the joint development of multi-site fatigue cracks. A flat fracture surface was obtained on which slight crack arrest rather than typical hydrogen-induced cracking feature was found. In addition, apparent acceleration in fatigue crack growth rate was observed in the temperature and strain-rate regions of dynamic strain aging (DSA). Related environmentally assisted cracking (EAC) mechanisms are also discussed.

Hydrogen-involved tensile and cyclic deformation behavior of low-alloy pressure vessel steel

The temperature- and strain-rate-dependent tensile behavior of hydrogen-charged low-alloy pressure vessel steel ASTM A508 C1.3 has been investigated. The fatigue crack initiation and propagation behavior of the steel in high-temperature water environments has also been evaluated. It was found that hydrogen played significant roles in both tensile and cyclic deformation processes, especially in the temperature and strain-rate region of dynamic strain aging (DSA). The presence of hydrogen resulted in a distinct softening in tensile strength and a certain loss in tensile ductility in the DSA region. Remarkable degradation in fatigue crack initiation and propagation resistance in high-temperature water environments was observed in the DSA strain-rate region. Typical hydrogen-induced cracking features also appeared on the corresponding fatigue fracture surfaces. The interactions between hydrogen and DSA in tensile and cyclic deformation processes are discussed as well as their combined effects on the environmentally assisted cracking (EAC) mechanism of pressure vessel steels in high-temperature water environments.

Analysis of the Transferability of Micromechanical Parameters of Damage of Steel under the Conditions of Ductile-Fracture Initiation

For the description of the mechanical behavior of a cracked material, we use a macromechanical model of fracture based on the well-known criterion of plastic yield for porous materials. This model is realized as a computational scheme with the help of the finite-element method. The critical values of the micromechanical parameters of damage are experimentally measured and numerically evaluated for smooth specimens with an aim to predict the onset of plastic yield in a compact cracked specimen. For a low-alloy pressure-vessel steel considered as an example, we show that the proposed micromechanical model can be used for the prediction of the onset of fracture processes in both smooth and compact specimens.

Fatigue Crack Growth Testing of Sub-Clad Defects

Fatigue crack growth tests were performed on four-point bend specimens with cracklike defects intentionally placed in A302B low-alloy pressure vessel steel clad with 308/309L weld-deposited stainless steel. The defects were placed in the base metal under the cladding by machining a cavity from the side opposite the cladding, electric-discharge machining a very sharp flaw, fatigue precracking the flaw, and then filling up the cavity by a weld repair process. The specimens were stress relieved before fatigue testing. The specimens were fatigue cycled at positive load ratios until the defects broke through to the surface. The specimens were then fractured at liquid nitrogen temperatures to reveal the fracture surfaces. Seven different sub-clad flaw specimens were tested in room temperature air and each test provides a record of cycles to defect breakthrough. Changes in defect size and shape as a function of applied load cycles were obtained by beach-marking the crack at various stages of the load history. The results provide a set of embedded defect data which can be used for qualifying fatigue crack growth analysis procedures such as those in Section XI of the ASME Boiler and Pressure Vessel Code. A comparison between calculated and measured values shows that the ASME B&amp;PV Section XI fatigue crack growth procedures conservatively predict cycles to defect breakthrough for small sub-clad defects.

Stress corrosion cracking of pressure vessel steels in high-temperature caustic aluminate solutions

Stress corrosion cracking (SCC) behavior of three kinds of low alloy pressure vessel steels in high-temperature (200 °C to 300 °C) caustic aluminate (AIO-2) solutions has been studied by slow strain rate tests (SSRT). The results indicate that these pressure vessel steels are susceptible to SCC in caustic aluminate solution and that the SCC susceptibility increases with increasing temperature between 200 °C to 300 °C. Sulfide content and stringered sulfide inclusions severely and anisotrop-ically affect the caustic SCC of these low alloy steels. The inclusions in the rare-earth-treated steel are predominantly globular rare-earth sulfides or oxysulfides, resulting in improved transverse prop-erties. The effect of inclusions on SCC behavior correlates with the projected area of inclusions perunit volume at the crack tip,A v , on the plane perpendicular to the tensile direction. The susceptibility to SCC increases with increasingA v .

Life Prediction of Notched Components

In this study, keyhole and smooth specimens, made from a low alloy pressure vessel steel (ASTM A-533 grade B), were subjected to monoaxial fatigue tests. The results show the influence of the stress concentration factor, Kt, on the number of cycles to failure, Nf. Total strain energy per cycle, ΔWt = ΔWp + ΔWe, was proved to be a good parameter for predicting the life of notched components. Elasto-plastic FEM analysis, utilizing the cyclic and monotonic curve of the material, showed close agreement with the experimental values.

Intergranular segregation in pressure-vessel steels

The degradation of low alloy pressure vessel steels in the presence of neutron irradiation is of fundamental importance in the nuclear industry. Numerous studies have been performed to evaluate the irradiation-induced changes in mechanical properties, and to a lesser extent, the changes in microstructure, of these pressure vessel materials. Irradiation results in an increase in the ductile to brittle transition temperature and a decrease in the upper shelf energy of the steels. The exact contribution of intergranular segregation to the concomitant degradation in mechanical properties is, however, unclear. To understand the microstructural changes caused by neutron irradiation and to clarify this contribution, it is important to fully characterize the material prior to irradiation. In this initial investigation, the techniques of analytical electron microscopy (AEM), high spatial resolution field emission gun scanning transmission electron microscopy (FEG-STEM) and energy dispersive x-ray spectroscopy (EDS) microanalysis have been employed to evaluate the microstructure of a low alloy steel as a first step in addressing this issue.

Effect of Dissolved Oxygen Concentration on Fatigue Crack Growth Behavior of A533B Steel in High Temperature Water.

Effect of dissolved oxygen concentration on fatigue crack growth behavior of a low alloy pressure vessel steel A533B CI. 1 in high temperature water was investigated. Fatigue tests were conducted at 561 K in water, of which DO concentration ranged from 1 to 8000 ppb including of BWR coolant condition. In a range of DO concentration below 1000 ppb fatigue crack growth rates (da/dN) showed independence on DO concentration, while above 1000 ppb striking DO dependence was observed, i.e. da/dN increased with increasing DO concentration. Corrosion products formed on the fracture surfaces were analyzed by an X-ray diffractometer. Magnetite was formed at all the levels of DO concentration, while hematite above 200 ppb and its X-ray diffraction intensity increased with increasing DO concentration.Effect of DO concentration on corrosion potential of the material was also investigated and discussed with the environmentally assisted cracking by DO concentration of fatigue crack growth rate in high temperature water from the viewpoint of electrochemistry.