Grain Boundary Segregation Of Phosphorus Research Articles

Thermal resistance of steels with increased strength properties for pressure vessels of advanced VVER reactors of various designs

The paper considers the results of structural studies and mechanical tests after a long-term thermal exposure of laboratory heats of the metallurgically improved 15Kh2NMFA steel and steel with an increased content of nickel considered as materials for the pressure vessels of advanced VVER-type reactors of various designs. It has been shown that, both for the improved 15Kh2NMFA steel and the high-nickel steel, there are no signs of grain boundary embrittlement after an segregation provoking embrittlement heat treatment. This is explained by the extremely low grain boundary segregation of phosphorus in the initial state caused by a high degree of the structure dispersity as well as by rather a low content of impurities. Besides, no changes have been found in the yield strength value for the improved 15Kh2NMFA steel, which agrees with the structure investigation results. For the high-nickel steel, a tendency towards a minor yield strength decrease by 5 to 10% and a regular reduction of the critical brittleness temperature has been revealed. A decrease in the mechanical properties has been caused by a relatively low temperature of tempering for the high-nickel steel and, accordingly, by the potential occurrence of the structure recovery during long-term thermal exposure, as evidenced by the results of an X-ray diffraction analysis. Despite the structure recovery in the high-nickel steel under the long-term thermal exposure, the main strengthening carbide phases remain stable. Due to this, the yield strength value remains at a relatively high level that exceeds the values for the modern VVER-type vessel steels, even in the case of a thermal exposure much in excess of the expected operating conditions for advanced VVER reactors. The observed decrease of critical brittleness temperature during the long-term thermal exposure contributes to an increase in the steel resistance to brittle fracture.

Relation of embrittlement to phosphorus grain-boundary segregation for an advanced Ni–Cr–Mo RPV steel

Embrittlement is an essential issue for reactor pressure vessel (RPV) steels. Grain-boundary segregation (GBS) of impurity phosphorous is one of the major reasons for non-hardening embrittlement of an RPV steel. The main aim of the present work is to establish a relation of embrittlement to phosphorous GBS for an advanced SA508-4N Ni–Cr–Mo RPV steel so that one can forecast its embrittling tendency. To obtain this type of relation, the steel samples doped with phosphorous are quenched from 890 to 1100 °C to obtain different prior austenite grain sizes (PAGSs) and then tempered at 650 °C, followed by ageing at different lower temperatures so as to achieve different phosphorous segregation levels at prior austenite grain boundaries. The characterization work is done with the use of mini-Charpy impact tests (specimen: 2.5 mm × 2.5 mm in cross section) together with Auger electron spectroscopy, scanning electron microscopy, and optical microscopy. The results indicate that there is a linear relationship between ductile-to-brittle transition temperature (DBTT/oC) and phosphorous GBS concentration (Cp/at.%) for both cases (quenching from 890 °C: PAGS = 34 μm; quenching from 1100 °C: PAGS = 112 μm). The relations can be expressed as DBTT=13.13Cp−335.70 (PAGS = 34 μm) and DBTT=6.69Cp−223.87 (PAGS = 112 μm). The rate of material embrittlement, characterized by the slope of the line, is associated with both the phosphorous segregation-induced grain-boundary (GB) embrittlement and the total GB area per unit volume. Interestingly, there is a critical phosphorous GBS concentration, below which the DBTT of the coarse-grained specimen is higher due to its higher intrinsic brittleness and above which that of the fine-grained specimen is higher at the same phosphorous GBS level due to the considerable phosphorous segregation-induced GB embrittlement along with a much larger total GB area per unit volume.

Synergistic effect of residual stress and phosphorus grain-boundary segregation on embrittlement of coarse-grained heat-affected zone in a novel Ni-Cr-Mo reactor pressure vessel steel

SA508Gr.4N steel is a novel Ni-Cr-Mo low-alloy steel for the new-generation reactor pressure vessel. Herein, a coarse-grained heat-affected zone (CGHAZ) in a P-doped SA508Gr.4N steel is simulated using Gleeble thermal cycling simulation with a peak temperature of 1320 ℃ under a heat input of 100 kJ cm−1. For the as-simulated specimen, due to the synergistic effect of high residual stress and P grain-boundary segregation, the fracture appearance changes in the order of ductile fracture to intergranular fracture, and then to cleavage fracture with decreasing impact temperature. With the residual stress relieved by annealing at 620 ℃, the specimen toughens to a certain extent even after ageing at 560 ℃, causing a decrease in ductile-to-brittle transition temperature (DBTT), although the boundary cohesion declines to a certain degree owing to the increase of P segregation. Simultaneously, the intergranular-to-cleavage fracture transition temperature (ICFTT) shifts to a lower temperature below –190 ℃, enabling the specimen to exhibit intergranular fracture even at −190 ℃. With the ageing temperature lowered to 500 ℃, the boundary segregation of P is raised to a high level, leading to a large increase in DBTT and a further decrease in ICFTT. Accordingly, the synergistic effect of residual stress and P boundary segregation affects the fracture nature of CGHAZ, thereby making fracture appearance have different characteristics. A mechanism diagram for the above effect is proposed based on the experimental results.

The Cause of Intergranular (IG) Fracture by Thermal Embrittlement in SA508 of Reactor Pressure Vessel (RPV) Steel

Intergranular(IG) fracture due to thermal treatment has been reported in a reactor pressure vessel(RPV) steel of Russian light water reactor in last decade. This is attributed to grain boundary segregation of phosphorus (P) or precipitation of carbide, etc.. This is a finding a difference in microstructure before and after IG cracking; this cannot explain the cause of the IG embrittlement. This old paradigm follows only correlation. Recently, a mechanism in which IG embrittlement occurs due to a decrease in entropy of a material has been reported at a temperature where atomic diffusion is possible. It is anticipated that new paradigm can explain the IG embrittlement of RPV based on a causal relationship. Thus, the thermal treatment at 350-420 oC was applied to RPV steel of SA508 and IG cracking was confirmed. DSC analysis was applied to confirm whether a decrease in entropy due to a short range ordering reaction occurs in SA508. It was possible to quantify the entropy change(⊿S= Q/T) through DSC measurement. A lattice changes due to thermal treatment were confirmed using XRD analysis in aged specimens. The results showed that lattice contraction by aging causes a reduction of fracture toughness. The internal stress formed inside the material due to entropy reduction can be calculated by multiplying the exothermic energy per unit mass by the density. This relationship is expressed by a equation of stress(σ) = exothermic heat(⊿Q) x density(ρ).

Chloride stress corrosion cracking of a non-standard, ‘Borderline’ Chromium-Manganese stainless steel – Problems of counterfeits and substandard materials

Nonstandard chromium-manganese (stainless) steels are gaining popularity nowadays, but data for them are inadequate as the majority of literature is on the ‘pedigrees’ such as the standard 200 series and 300 series steels. This paper begins with a brief overview of the counterfeit problems of such steels. It then presents the slow-strain-rate test results of a chromium-manganese ‘borderline’ stainless steel in room-temperature air, 60 °C NaCl and 60 °C distilled water (the lower-bound temperature for chloride stress corrosion cracking in austenitic stainless steels). At low strain rates (10-6 s−1), grain-boundary segregation of phosphorus and sulphur (especially the latter) can cause intergranular cracking in air and in 60 °C distilled water. In 60 °C NaCl, manganese sulphides may serve as the initiation sites of transgranular cracking. Many producers of nonstandard chromium-manganese (stainless) steels lack impurity control technologies, and so users must be vigilant of this issue. The fracture mode of the ‘borderline’ stainless steel is strain-rate dependent, changing from being mainly intergranular to being transgranular and dimpled as the strain rate goes up. The reason behind this is unclear, but a plausible suggestion is given in this paper.

Role of applied stress on the changes of precipitation and phosphorus segregation behavior in a TP347H austenitic stainless steel

The effect of applied stress on the precipitation (Nb-rich MC and Nb-rich M2P) behavior in the matrix and its role in the phosphorus segregation at the grain boundary (GB) are investigated in a TP347H austenitic stainless steel. The two parts (gage and grip) in the rupture test specimens under the applied stress of 80 MPa at 750 °C are investigated using TEM, SEM, and EPMA. Continuous precipitation and saturation behaviors of MC and M2P were observed in the grip part. Excess precipitation behavior of MC in the early stage of rupture test and the retarded precipitation of M2P were confirmed in the gage part. The retarded precipitation and the low volume fraction of M2P promoted phosphorus segregation at the GB. The correlation between the matrix precipitation and GB phosphorus segregation under the applied stress is further discussed.

Grain-boundary phosphorus segregation in highly neutron-irradiated reactor pressure vessel steels and its effect on irradiation embrittlement

Reactor pressure vessel (RPV) steels for pressurized water reactors (PWRs) with bulk P contents ranging from 0.007 to 0.012wt.% were subjected to neutron irradiation at fluences ranging from 0.3 to 1.2 × 1020 n/cm2 (E > 1 MeV) in PWRs or a materials testing reactor (MTR). Grain-boundary P segregation, which was analyzed using Auger electron spectroscopy (AES) on intergranular facets, increased with increasing neutron fluence. A rate theory model based on four diffusion-reaction equations for substitutional P atoms, octahedral interstitial P atoms, vacancies, and self-interstitial atoms was also used to simulate the increase in grain-boundary P segregation for RPV steels with a bulk P content up to 0.020wt.%, using parameters optimized by the present AES data. The increase in grain-boundary P segregation in RPV steel with a bulk P content of 0.015wt.%, which is the maximum P concentration in RPV steels used in Japanese nuclear power plants intended for restart, was estimated to be less than 0.1 in monolayer coverage at 1 × 1020 n/cm2 (E > 1 MeV). A comparison of the PWR data with the MTR data, including that from the literature, showed that neutron flux had no effect upon grain-boundary P segregation for A533B steels. The relationships of the ductile-brittle transition temperature (DBTT) shifts to grain-boundary P segregation and to yield strength were also discussed. A linear relationship between the yield strength and the DBTT shift with a slope of 0.63 was obtained for RPV steels with a bulk P content up to 0.026wt.%, which is higher than that of most U.S. A533B steels. It is concluded that the intergranular embrittlement is unlikely to occur for RPV steels irradiated in PWRs.

Complex study of grain boundary segregation in long-term irradiated reactor pressure vessel steels

Grain boundary (GB) embrittlement of reactor pressure vessel steel occurs at the long-term operation through the radiation-enhanced segregation of impurities. All the known techniques of studying the GB segregation have their advantages and disadvantages, and complete information can be obtained only through a comprehensive analysis by different methods.In this paper the level of GB segregation in VVER-1000 RPV steels with high nickel content was assessed by combined methods of Auger electron spectroscopy (by the P monolayer coverage and atomic concentration of other elements); atom probe tomography (by the Gibbsian interfacial excess) and fractographic analysis (by the maximum fraction of brittle intergranular fracture).The results obtained by different methods are in good agreement and show that phosphorus GB segregation increases with the increase of fast neutron fluence, GB concentration of Ni and Mn, the bulk concentration of Ni and Mn, and correlates with the fraction of brittle intergranular fracture.

Competitive grain boundary segregation of phosphorus and carbon governs delamination crack in a ferritic steel

The present study aims at unveiling the influence of trace amounts of phosphorus on the macro-scale delamination in a ferritic steel. Two different steels with trace amounts of phosphorus were examined to reveal the cause: One was made by long-time high temperature holding to maximize the phosphorus segregation, and the other one was made by short-time high temperature holding to minimize the phosphorus segregation. The atom probe results at the grain boundaries ahead of the delamination-related cracks provide a strong evidence that phosphorus-enrichment and carbon-depletion for the long-time high temperature holding induces a larger degree of delamination as compared to the short-time high temperature holding. Reasons for the different segregation tendency were discussed by correlating the isothermal holding time at high temperature and the competitive segregation between phosphorus and carbon.

Mechanical properties and microstructure of long term thermal aged WWER 440 RPV steel

The integrity assessment of the Reactor Pressure Vessel (RPV) is essential for the safe and Long Term Operation (LTO) of a Nuclear Power Plant (NPP). Hardening and embrittlement of RPV caused by neutron irradiation and thermal ageing are main reasons for mechanical properties degradation during the operation of an NPP. The thermal ageing-induced degradation of RPV steels becomes more significant with extended operational lives of NPPs. Consequently, the evaluation of thermal ageing effects is important for the structural integrity assessments required for the lifetime extension of NPPs.As a part of NRG's research programme on Structural Materials for safe-LTO of Light Water Reactor (LWR) RPVs, WWER-440 surveillance specimens, which have been thermal aged for 27 years (∼200,000 h) at 290 °C in a surveillance channel of Armenian-NPP, are investigated. Results from the mechanical and microstructural examination of these thermal aged specimens are presented in this article.The results indicate the absence of significant long term thermal ageing effect of 15Cr2MoV-A steel. No age hardening was detected in aged tensile specimens compared with the as-received condition. There is no difference between the impact properties of as-received and thermal aged weld metals. The upper shelf energy of the aged steel remains the same as for the as-received material at a rather high level of about 120 J. The T41 value did not change and was found to be about 10 °C. The microstructure of thermal aged weld, consisting carbides, carbonitrides and manganese-silicon inclusions, did not change significantly compared to as-received state. Grain-boundary segregation of phosphorus in long term aged weld is not significant either which has been confirmed by the absence of intergranular fracture increase in the weld. Negligible hardening and embrittlement observed after such long term thermal ageing is attributed to the optimum chemical composition of 15Cr2MoV-A for high thermal stability.

Thermodynamic effect of elastic stress on grain boundary segregation of phosphorus in a low alloy steel

Grain boundary (GB) segregation of P in 2.25Cr1Mo steel induced by elastic stress shows that the P equilibrium concentration, after reaching the non-equilibrium concentration maximum at critical time, returns to its initial thermal equilibrium level. This finding confirms the interesting phenomenon that the effect of elastic stress on GB segregation of P is significant in kinetics while slight in thermodynamics. Through extending the “pressure” in classical theory of chemical potential to the “elastic stress”, the thermodynamic effect of elastic stress on GB segregation is studied, and the relationship between elastic stress and segregation Gibbs energy is formulated. The formulas reveal that the difference in the segregation Gibbs energy between the elastically-stressed and non-stressed states depends on the excess molar volume of GB segregation and the magnitude of elastic stress. Model calculations in segregation Gibbs energy confirm that the effect of elastic stress on the thermodynamics of equilibrium GB segregation is slight, and the theoretical analyses considerably agree with the experimental results. The confirmation indicates that the nature of the thermodynamic effect is well captured.

Segregation mechanism of phosphorus in Ti-stabilized interstitial-free steel

The grain boundary segregation of phosphorus in an interstitial-free (IF) steel aged for different times at 600 and 720°C was investigated using Auger electron spectroscopy. Results revealed that a significant degree of phosphorus segregation occurred during annealing for Ti-stabilized IF steel with 0.045wt% bulk content of phosphorus. In addition, the segregation degree increased with increased holding time. The segregation behavior of phosphorus in IF steel was analyzed based on the theory of grain boundary segregation, along with the present experimental data and previous results. The phosphorus segregation had equilibrium segregation characteristics, and the bulk concentration of phosphorus and holding temperature strongly influenced the final grain boundary segregation concentration of phosphorus in IF steel during annealing. Furthermore, the segregation level of phosphorus substantially decreased with increased annealing temperature or decreased bulk concentration of phosphorus. Results of theoretical calculation were consistent with the experimental ones, indicating that the grain boundary segregation behavior of phosphorus in IF steel during annealing can be qualitatively and quantitatively explained by the theory of equilibrium segregation.

Non-equilibrium Grain-boundary Segregation of Phosphorus in an Fe-17Cr Alloy

In order to confirm the segregation characteristic of phosphorus in an Fe-17Cr alloy at grain boundary, the phosphorus segregation was obtained by Auger electron spectroscopy. The results show that a maximum phosphorus segregation appeared at 450 °C for all specimens aged for 30 min at 350, 400, 450, 500, and 600 °C after being solution-treated at 1000 °C for 1 h. These results were analyzed by the characteristics of segregation peak temperature of non-equilibrium segregation.

Non-equilibrium phosphorus grain boundary segregation and its effect on embrittlement in a niobium-stabilized interstitial-free steel

Grain boundary segregation of phosphorus in an Nb-stabilized and P-strengthened interstitial-free steel was investigated using Auger electron spectroscopy after the steel specimens were quenched from 880°C and aged at 680°C for different times. The segregation initially increased with increasing aging time until reaching a maximum value and then decreased gradually to its equilibrium value with further increasing aging time, demonstrating a non-equilibrium segregation kinetic characteristic with a critical time of around 30min. The ductile-to-brittle transition temperatures (DBTTs) of the aged specimens were determined through fracture appearance measurements. There was a linear relationship between DBTT and phosphorus boundary concentration (Cp, at%): DBTT(°C)=4.39Cp−64.06, suggesting that a P-strengthened IF steel should avoid slow cooling or staying in the intermediate temperature range so as to avoid apparent phosphorus grain boundary segregation and in turn to avoid embrittling.

Effects of Thermal History and Microstructure on Segregation of Phosphorus and Alloying Elements in the Heat-Affected Zone of a Low Alloy Steel

The grain boundary segregation of phosphorus and alloying elements in the heat-affected zone (HAZ) of a low alloy steel was studied quantitatively with atom probe tomography. Non-equilibrium segregation mainly occurred during welding and subsequent fast cooling, leading to remarkable segregation of P, C, Mn, and Mo. The segregation of these four types of solutes showed similar microstructure-dependence at this stage, in which the segregation levels are higher in coarse-grained HAZ and intercritically reheated coarse-grained HAZ than in fine-grained HAZ. After simulated aging, P and Mn showed further enrichment at grain boundaries through equilibrium segregation, while desegregation was observed for C and Mo. In addition, it seems that precipitation of Mo at dislocations was greatly promoted during aging, which probably also contributed to the increase of P and Mn at grain boundaries.

Texture, Grain Boundary Characterization and Segregation of Phosphorus in an Annealed Interstitial Free Steel

The {111} // ND plane texture, grain boundary characterization and grain boundary segregation of phosphorus are investigated for the phosphorus added high strength interstitial free (IF) steel annealed at 810 °C for 60 s to 180 s. The results show that the maximum volume fraction of {111} // ND plane texture is about 85% and the grain boundary segregation peak of phosphorus is about 14 at. % for the steel annealed at 810 °C for 120 s. The Σ3 and other low-ΣCSL (coincident site lattice) boundaries are lowest for the steel annealed for 120 s. Segregation of phosphorus is also found at low angle grain boundaries.

Grain Boundary Fracture Behavior and Segregation of P under Low Tensile Stresses in Steel 2.25Cr1Mo and 12Cr1MoV

Experimental investigation about the grain boundary fracture behavior and segregation behavior of phosphorus under low tensile stresses in steel 2.25Cr1Mo and 12Cr1MoV was carried out in this paper. AES (Auger electron spectroscopy) experiments and dynamic analyses on the non-equilibrium grain-boundary segregation (NGS) of phosphorus and the SEM photos of grain boundary fracture in Auger specimens of both steels were obtained. The variation of phosphorus segregation level in grain boundary at different aging time was studied. Results show that the non-equilibrium segregation of phosphorus occurred at the grain boundaries in the two steels while subjected to a tensile stress of 30MPa and held at 500°C. The corresponding critical time was about 1 hour for steel 2.25Cr1Mo and 3.5hour for steel 12Cr1MoV respectively. SEM photos of grain boundary fracture in Auger specimens of the test steels show that the grain boundary fracture rate increased with increasing concentration of phosphorus, and that the fracture toughness of steel 12Cr1MoV is much lower than the one of steel 2.25Cr1Mo.

Effect of non-equilibrium grain-boundary segregation of phosphorus on the intermediate-temperature ductility minimum of an Fe–17Cr alloy

The ductility of an Fe–17Cr alloy was examined experimentally in the range of 350–700°C. The results showed that the ductility of the alloy, evaluated by the reduction in area, had a minimum at an intermediate temperature of approximately 450°C. Auger electron spectroscopy demonstrated that the ductility minimum corresponded to a maximum in grain boundary segregation of phosphorus, which could be elucidated by the theory of non-equilibrium grain-boundary segregation. Thus, the intermediate temperature ductility minimum for the Fe–17Cr alloy could be explained on the basis of non-equilibrium grain-boundary segregation of phosphorus.

Introduction of vacancy drag effect to first-principles-based rate theory model for irradiation-induced grain-boundary phosphorus segregation

In order to examine the vacancy drag effect on phosphorus segregation at grain-boundaries, we estimated partial diffusion coefficients by kinetic Monte Carlo simulations considering atomic transitions for all the binding states of a phosphorus atom and a vacancy, and incorporated them into a rate theory model to simulate grain-boundary phosphorus coverage in neutron-irradiated reactor pressure vessel steels. As a result, it was confirmed that phosphorus atoms are transported to grain-boundaries by the vacancy drag effect. We found that the vacancy drag effect occurs even when only the first-nearest-neighbor binding states are considered, though it does not occur if the partial diffusion coefficients are obtained through the expression derived by theoretical analysis [Ebihara, et al., J. Nucl. Mater. 414 (2011) 328–335]. In addition, we found that grain-boundary phosphorus segregation is suppressed by the vacancy mechanism of phosphorus transport regardless of the vacancy drag effect.

Intergranular embrittlement of iron by phosphorus segregation: an atomistic simulation

The intergranular embrittlement in bcc iron by the grain boundary (GB) segregation of phosphorus is investigated using an atomistic simulation. The inhibition of the nucleation of dislocations near the crack tip is found to be the governing mechanism of the intergranular embrittlement in phosphorus-containing iron, in contrast to the conventional reasoning that focuses on the GB decohesion. The correlation between the nucleation of dislocations and dislocation transfer across a GB (GB strengthening) is discussed. Experimental evidence and supplementary simulation results that support the new finding in terms of the GB strengthening are also demonstrated.