A533B Steel Research Articles

Analysis of Ductile-to-Brittle Transition Characteristics of Reactor Pressure Vessel Steels

Ductile-to-brittle transition (DBT) characteristics of three steels for reactor pressure vessel (RPV) belt line application are analyzed from new parameters based on model functions describing the strength and toughness characteristics of the materials. In order to estimate nil-ductility temperature (NDT) from strength property, a strain rate–compensated temperature parameter based on the thermally activated deformation of materials is adopted. A measure of NDT is determined from tensile strength properties for the first time assuming an estimated notch tip strain rate at the lower shelf. It is estimated to be 110, 42, and 106 K for the Cr-Mo-V-Ni, 20MnMoNi55, and A533B steels, respectively. The measure of ductile-to-brittle transition temperature (DBTT) in steels using 41-J Charpy impact-absorbed energy on the basis of a logistic class of functions is compared and shown to be equivalent with those obtained from fitting the tanh model equation. A bi-logistic function based on the concept of separable parameters representing the fracture of ductile and brittle zones in steels within the DBTT regime was applied to model the Charpy impact energy behavior of the three steels. The bi-logistic function-fitting parameters yielded a new measure of brittleness as a DBT characteristic of steels that correlated well with other measures of transition temperature of the selected RPV steels. The parameters from the hyperbolic and logistic fitting were used to develop a model relationship suitable for the generation of a master curve based on Charpy energy in exponential form that unifies the transition temperature behavior of the selected western and eastern RPV materials. The model relationship is also found to closely predict ~5 K of the reference temperature To determined as per American Society for Testing and Materials standard E1921 of the selected RPV steels.

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.

A damage model for predicting ductile fracture with considering the dependency on stress triaxiality and Lode angle

This paper aims to present a Continuum Damage Mechanics based fracture model. Two yield surfaces, plastic and damage, are utilized to develop the model. A modified ductile fracture model coupling both stress triaxiality and Lode angle parameter is then proposed. The proposed model is applied to construct the fracture loci of four types of metal alloys: steel HY100, steel PCrNi3MoVA, aluminum 2024-T351, steel TRIP690 and steel A533B. The prediction is also involved investigation of the dependency of damage evolution on stress triaxialities for steel HY100 and steel A533B. The new model is also utilized to construct the Fracture Forming Limit Diagram (FFLD) of steel TRIP690 to validate the performance of the model. The predicted results are in good agreement with the experimental data over a wide range of stress triaxialities. Comparison between the proposed model and some fracture criteria is also provided, and the results indicate that the proposed model has significant potential to predict ductile fracture as well as FFLD at both low and high triaxialities. In order to validate the capability of the model in structural response, the proposed model has been implemented into user-defined subroutines VUMAT in the finite element program ABAQUS/Explicit. For this purpose, the explicit stress integration algorithms of the model have been explained. The model has been validated by comparing the predicted results with experimental data of steel A533B. The results show the excellent correlation between experiments and simulation for the force and damage results as well as the fracture shape.

Proširena primena sistema za predviñanje tvrdoće kod zavarivanja čelika A533B za razne niskolegirane čelike

Temper bead welding is one of effective repair welding methods in case that post weld heat treatment is not easily applied. In order to evaluate the effectiveness of temper bead welding, hardness in HAZ becomes important factor. The neural network-based hardness prediction system of HAZ in temper bead welding for A533B low-alloy steel has been constructed by the authors in the previous study. However, for HAZ hardness prediction of other steels, it is necessary to obtain hardness database for each steel which is time-cost consuming, if the same method is used. The present study has been conducted to develop the generalized hardness prediction method applicable for other steels by utilizing the hardness data-base of A533B steel assuming that the hardness in HAZ of steels after tempering have a linear relationship with LMP (Larson-Miller parameter). By using the newly proposed extended method, only a few hardness data-base for the other steels is needed to obtain. Hardness distribution in HAZ of temper bead welding for other steels was calculated by using the extended hardness prediction system. The thermal cycles used for calculation were numerically obtained by a finite element method. The experimental results have shown that the predicted hardness is in good accordance with the measured one for steels without secondary hardening. It follows that the currently proposed extended method is effective for estimating the tempering effect during temper bead welding for the steels without secondary hardening.

Proširena primena sistema za predviđanje tvrdoće kod zavarivanja čelika A533B za razne niskolegirane čelike (nastavak iz prethodnog broja - 2. deo)

Proširena primena sistema za predviđanje tvrdoće kod zavarivanja čelika A533B za razne niskolegirane čelike (nastavak iz prethodnog broja - 2. deo)

An assessment of impact toughness of the pressure vessel steel A533-B and its 308 austenitic stainless steel weld metal

The steel A533-B is a low alloy steel used as a structural material for the construction of light water nuclear reactor pressure vessels. It is normally cladded with the 308 austenitic stainless steel weld metal overlay for protection against extension of surface flaws generating during operation. In this investigation the impact toughness of the A533-B steel and its 308 weld metal were evaluated. The microstructure of the A533-B steel consisted of bainitic structure while that of the 308 weld metal comprised austenite with ferrite phase of about 5%. The impact toughness was assessed using instrumented impact testing machine at temperatures between -196 and 300 C. The determined impact toughness parameters involved the total impact fracture energy, the energy expended in the fracture crack initiation and crack propagation, the ductile to brittle transition temperature as well as the dynamic yield strength. The A533-B steel exhibited distinct ductile to brittle transition behavior with superior resistance to ductile fracture at high test temperatures. In contrast, the 308 weld metal did not display such transition. However, it showed low energy ductile fracture performance with evident resistance to brittle fracture even at low test temperatures. The findings were substantiated with the load-time traces derived from the instrumented impact tests as well as the fracture surface morphology. The results were discussed in relation to the difference in microstructure and flow properties of the examined steels.

Effect of Biaxiality on Engineering Critical Assessments

Common practice for the integrity assessment of structures, follows the use of Fitness for Service procedures, such as BS 7910/R6. There are many common structures (e.g. pipes) that are under biaxial loading. Even though the load biaxiality has been proven experimentally to have an effect on both the fracture toughness and the limit load of a component, the influence of biaxial stresses is not always directly addressed in the procedures. In order to address the effect of biaxiality on engineering critical assessments, BS 7910 and R6 fitness for service procedures are applied to four biaxially loaded wide plate tests previously conducted on A533B steel. The plates had been loaded in biaxial tension, with different biaxiality ratios, while the temperatures of the plates tested correspond to lower transition fracture toughness. The assessments include failure assessment lines created using Option 1, which requires the knowledge of the yield stress and tensile strength, Option 3 which is a material, structure and load dependent option, as well as a constraint modified Option 3 failure assessment line. To obtain Option 3 failure assessment lines, finite element analyses are conducted for the specimens. The tests are first assessed with the proximity to plastic collapse (Lr) and fracture (Kr) being calculated with the use of the main fracture clauses of BS 7910/R6, which do not take into account the effects of biaxiality. The conventional assessment is then followed by a more detailed assessment using the values calculated from Finite Element Analyses (FEA).

Extended Application of Hardness Prediction System for Temper Bead Welding of A533B Steel to Various Low-Alloy Steels

Temper bead welding is one of effective repair welding methods in case that post weld heat treatment is not easily applied. In order to evaluate the effectiveness of temper bead welding, hardness in HAZ becomes important factor. The neural network-based hardness prediction system of HAZ in temper bead welding for A533B low-alloy steel has been constructed by the authors in the previous study. However, for HAZ hardness prediction of other steels, it is necessary to obtain hardness database for each steel which is time-cost consuming, if the same method is used. The present study has been conducted to develop the generalized hardness prediction method applicable for other steels by utilizing the hardness data-base of A533B steel assuming that the hardness in HAZ of steels after tempering have a linear relationship with LMP (Larson-Miller parameter). On using the newly proposed extended method, only a few hardness data-base for the other steels is needed to obtain. Hardness distribution in HAZ of temper bead welding for other steels was calculated by using the extended hardness prediction system. The thermal cycles used for calculation were numerically obtained by a finite element method. The experimental results have shown that the predicted hardness is in good accordance with the measured one for steels without secondary hardening. It follows that the currently proposed extended method is effective for estimating the tempering effect during temper bead welding for the steels without secondary hardening.

Microstructure and pitting behavior of the dissimilar metal weld of 309L cladding and low alloy steel A533B

The microstructure and pitting behavior of a dissimilar metal weld (DMW) of 309L and low alloy steel (LAS) were investigated with various surface analysis techniques and potentiodynamic polarization. The heat affected zone (HAZ) consists of four zones i.e. are heavily etched zone, lightly etched zone, coarse bainite zone and fine bainite zone, respectively. Cr depletion takes place in some δ ferrite/γ boundaries in 309L. Pits prefer to initiate along the δ ferrite/γ boundaries and mainly grows in γ phase, leaving a δ ferrite skeleton.

Bifurcation analysis of fracture mode by simulated and experimental ductile fracture progress based on the proposed crack opening criterion

In the previous papers (Kobayashi, 2017a, b), the crack opening criterion was deduced from the proposed micro-crack evolution equation. To solve the discrepancies between the estimated and experimental strengths of ductile materials by considering the plastic/damage energy dissipation according to ASTM standard E1820-01, an improved version of the proposed crack opening criterion was proposed and successively applied to the ductile materials such as A533B steel, Al2024-T4 and SiAlon. With regard to the fracture progress, it is well known that the chaotic behavior of crack propagation seems to be caused by the bifurcation of the fracture mode. In this paper, the simulation of ductile fracture progress in notched FCC single crystal specimens characterized by different crystal orientations under the uniaxial tension is performed using the crystal plasticity finite element model (CPFE) to consider specifically the dependence of ductile fracture progress on the crystal orientation. The model was also integrated with a proposed micro-/macro-crack nucleation criteria deduced by two different stationary discontinuity bands characterized by the vanishing velocity condition. The simulated results were compared with the experimental ones. Both bifurcation mechanisms of the fracture mode of the simulated and experimental results were then studied using the proposed crack opening criterion based on the bifurcation structure characterized by two different stationary discontinuity bands. It can be concluded that the proposed crack opening criterion and its improved application including the proposed diagram contribute to study fracture physical mechanisms.

Ductile to brittle fracture analysis based on improved application of proposed crack opening criterion deduced by micro-crack evolution equation

In the previous paper (Kobayashi, 2017), the new crack opening criterion deduced by a micro-crack evolution equation was proposed and verified by examination of the fracture mechanisms of metallic and ceramic materials using those well-known data, i.e., fracture toughness and so forth, based on the stresses, strains and strain gradients substituted by the stress intensity factor (SIF). However, discrepancies between experimental and estimated results in A533B steel, Al2024-T4 and SiAlon cases were caused by the plastic/damage energy dissipation. In the present paper, the improved application of the crack opening criterion to be able to apply ductile materials such as A533B steel, Al2024-T4 and SiAlon by considering the plastic/damage energy dissipation based on ASTM standard E1820-01 is proposed and the plastic/damage process zone area at the crack-tip caused by the crack growth is estimated by the crack opening criterion; even though SiAlon is ceramic material, the fracture progress of A533B steel, Al2024-T4 and SiAlon are characterized by stable crack growth (ductile fracture) in the limited range of the crack extension. Moreover, significance of the theoretically estimated yielding stress and/or fracture toughness under the predicted fracture mode is pointed out to identify the phenomenological fracture mode experimentally observed by comparison of the experimental ones; the proposed criterion of micro-crack nucleation for the ductile fracture (Kobayashi, 2012) derived by the vanishing velocity condition is examined by the improved application of the crack opening criterion in the physical point of view. Also the fracture progress of an elastic-perfect plastic material is schematically interpreted by the improved application of the crack opening criterion.

T-scaling method for stress distribution scaling under small-scale yielding and its application to the prediction of fracture toughness temperature dependence

In this paper, a new method for scaling the crack-tip stress distribution under small-scale yielding conditions is proposed and named the T-scaling method. This method enables identification of the different stress distributions for materials with different tensile properties but identical load in terms of K or J. Then, by assuming that the temperature dependence of a material is represented by the stress-strain relationship temperature dependence, a method to predict the fracture load at an arbitrary temperature from the already known fracture load at a reference temperature is proposed. This method is combined with the T-scaling method and the knowledge that the “fracture stress for slip induced cleavage fracture is temperature independent.” Once the fracture load is assumed, the fracture toughness Jc at the temperature under consideration can be evaluated by running elastic-plastic finite element analysis. Finally, the above-mentioned framework for predicting the Jc temperature dependence of a material in the ductile-to-brittle temperature region was validated for 0.55% carbon steel JIS S55C and reactor pressure vessel steel A533B. The proposed framework seems to be able to solve the problem faced by the master curve in the relatively high temperature region by requiring only tensile tests.

Application of neural network-based hardness prediction method to HAZ of A533B steel produced by laser temper bead welding

Temper bead welding is one of the effective repair welding methods instead of post weld heat treatment. With the development and popularization of the laser welding, the advanced laser temper bead welding repair technique has been developed recently. However, it is always laborious works to determine the optimum welding condition for laser temper bead welding. Therefore, in the present study, the hardness prediction system for laser temper bead welding has been constructed using a neural network. Thus, the appropriate welding conditions can be selected before the actual repair welding. Firstly, the high cooling rate in heat affect zone (HAZ) of laser temper bead welding has been measured. Secondly, the hardness database are prepared by the experiment. Thirdly, on the basis of experimentally obtained database, the neural network-based hardness prediction system for laser temper bead welding has been constructed. With it, the hardness distribution in HAZ of laser temper bead welding was calculated based on the thermal cycles numerically obtained by FEM. The predicted hardness was in good accordance with the experimental results. It follows that the new prediction system is effective for estimating the tempering effect during laser temper bead welding.

ニューラルネットワークを用いた多層溶接HAZ硬さ予測システムの適用鋼種拡張手法

Temper bead welding is one of effective repair welding methods in case that post weld heat treatment is not easily applied. In order to evaluate the effectiveness of temper bead welding, hardness in HAZ becomes important factor. The neural network-based hardness prediction system of HAZ in temper bead welding for A533B low-alloy steel has been constructed by the authors in the previous study. However, for HAZ hardness prediction of other steels, it is necessary to obtain hardness database for each steel which is time-cost consuming, if the same method is used. The present study has been conducted to develop the generalized hardness prediction method applicable for other steels by utilizing the hardness data-base of A533B steel assuming that the hardness in HAZ of steels after tempering have a linear relationship with LMP (Larson-Miller parameter). On using the newly proposed extended method, only a few hardness data-base for the other steels is needed to obtain. Hardness distribution in HAZ of temper bead welding for other steels was calculated by using the extended hardness prediction system. The thermal cycles used for calculation were numerically obtained by a finite element method. The experimental results have shown that the predicted hardness is in good accordance with the measured one for steels without secondary hardening. It follows that the currently proposed extended method is effective for estimating the tempering effect during temper bead welding for the steels without secondary hardening.

Prediction of fracture toughness in ductile-to-brittle transition region using combined CDM and Beremin models

Scatter of fracture toughness in the ductile-to-brittle transition (DTB) region for ferritic steels may show a dependence on the specimen geometry constraint. Consequently, this can be a source of uncertainty in transferring fracture data from one test geometry to other configurations. In this work, the modified Beremin model (MBM) for cleavage and the Bonora damage model (BDM) for ductile tearing, were combined to account for the two failure mechanisms competing in the DTB and to predict fracture toughness scatter band with temperature. The model was validated predicting the scatter in the fracture toughness data of A533B steel for SEB specimens with shallow and deep crack. Present model predictions were compared with master curve results. For the MBM, a more physical definition for the process zone was introduced and the temperature dependence of the reference stress σu was described by the sum of athermal and thermally activated stress contributions. Results indicate that ductile crack growth occurring before fracture, which is predicted by the ductile damage model, has a relevant effect on the computed probability of brittle fracture. The proposed combined model is able to accurately account for the loss of constraint occurring in different specimen geometries confirming the geometry transferability of model parameters.

Hardening and microstructural evolution of A533b steels irradiated with Fe ions and electrons

Radiation hardening and embrittlement of A533B steels is heavily dependent on the Cu content. In this study, to investigate the effect of copper on the microstructural evolution of these materials, A533B steels with different Cu levels were irradiated with 2.4 MeV Fe ions and 1.0 MeV electrons. Ion irradiation was performed from room temperature (RT) to 350 °C with doses up to 1 dpa. At RT and 290 °C, low dose (<0.1 dpa) hardening trend corresponded with ΔH ∝ (dpa)n, with n initially approximately 0.5 and consistent with a barrier hardening mechanism, but saturating at ≈0.1 dpa. At higher dose levels, the radiation-induced hardening exhibited a strong Cu content dependence at 290 °C, but not at 350 °C. Electron irradiation using high-voltage electron microscopy revealed the growth of interstitial-type dislocation loops and enrichment of Ni, Mn, and Si in the vicinities of pre-existing dislocations at doses for which the radiation-induced hardness due to ion irradiation was prominent.

Joint evaluation of fracture results from distinct test conditions, implying loading, specimen size and geometry

The reliability of the statistical assessment of test results is strong influenced by the reduced data size emanating from the low number of specimens included in each of the groups, characterized by test type, specimen geometry and size implied in the material sample being tested. Due to the traditional economical and time limitations, the available experimental results owned by a research group lack of parameter diversity so that they must be complemented by foreign results reported elsewhere in order to verify a model, a methodology or reinforced probabilistic conclusions. Generally, difficulties arise in assessing the parameters involved in the phenomenon due to the diversity of samples and test conditions supported by few data results. In this paper, a methodology based on a generalized local model, denoted GLM, is proposed to overcome this limitation allowing a joint evaluation diversified tests as those resulting from different loading conditions and specimen shape and size. It implies the primary failure cumulative distribution function, PFCDF, as the failure characteristic of the material to be derived from experimental results obtained from distinct test programs. In this work, the local methodology is applied for probabilistic assessment of cleavage fracture toughness data of steel A533B lent from an external experimental program carried out by Rathbun et al. (2006), aiming at analyzing the constraint influence on the cleavage fracture once the scale effect is recognized

Correlations between Charpy V-notch impact energy and fracture toughness of nuclear reactor pressure vessel (RPV) steels

Correlations between Charpy V-notch (CVN) impact energy and fracture toughness are of great practical significance for reactor pressure vessel (RPV) structural integrity assessment. In this paper, correlations between the two for three commonly used RPV steels, namely Chinese A508-3 steel, USA A533B steel, Euro 20MnMoNi55 steel, are investigated with both a direct conversion and the Master Curve method. It is found that fracture toughness can be well predicted from CVN impact energy through reference temperature T0 of the Master Curve method. Five predicted T0 values from different correlations are close to the T0 from actual test. The mean value of the five T0 values provides a good estimate for the fracture toughness in the transition region.

Investigation of non-equilibrium grain boundary segregation on CGHAZ in reactor pressure vessel steel

In order to explore the reasons for coarse grained embrittlement, post-weld heat treatment (PWHT) with different times was conducted on the coarse grained heat affected zone (CGHAZ) of A533B steel. The results reveal that the impact toughness of the CGHAZ at room temperature decreases with the prolongation of PWHT time from 0.5 to 4 h. As the PWHT time increases to 4 h, the fracture feature of the CGHAZ presents intergranular cleavage ruptures, and the impact toughness exhibits the minimum value. After PWHT for 4 h, the area of the intergranular cleavage fractures decreases, and the impact toughness gradually increases. Further research reveals that there is phosphorus grain boundary segregation in the CGHAZ. The theory of non-equilibrium grain boundary segregation (NGS) is proposed to explain the phenomenon. It is found that the PWHT time with minimum impact toughness is consistent with critical time of phosphorus NGS.

Microcracking susceptibility in dissimilar multipass welds of Ni-base alloy 690 and low-alloy steel

Microcracking susceptibility in the dissimilar multipass weld metal of alloy 690 and low-alloy steel A533B was evaluated, and the effect of dilution on hot cracking (ductility-dip and liquation cracking) behaviour was investigated. In order to simulate the dissimilar multipass weld metal of alloy 690 to A533B steel, the A533B plate was welded under various dilution ratios using alloy 690 filler metal with different contents of P and S. Several weld metals, which had different alloy compositions at the fixed (P+S) content, were manufactured, and then ductility-dip and liquation cracking susceptibilities of the reheated weld metals were evaluated by the spot-Varestraint test. Ductility-dip cracking susceptibility heightened as the dilution ratio was increased even when the amounts of P and S were fixed. The increased dilution ratio (contamination of Fe into the weld metal) should reduce the tortuous character of the grain boundary (GB) due to inhibiting the constitutional supercooling (the instability of the solidification boundary), as well as enhance the GB embrittlement due to promoting the GB segregation of P and S. Furthermore, the liquation cracking susceptibility slightly heightened with an increase in the dilution ratio at the fixed (P+S) content. The increased liquation cracking susceptibility would be attributed to the enhancement of solidification segregation of P and S with increasing the dilution ratio.