Application of advanced master curve approaches on WWER-440 reactor pressure vessel steels

The master curve (MC) approach used to measure the transition temperature, T₀, was standarised in the ASTM Standard Test Method E 1921 in 1997. The basic MC approach for analysis of fracture test results is intended for macroscopically homogeneous steels with a body centred cubic (ferritic) structure only. In reality, due to the manufacturing process, the steels in question are seldom fully macroscopically homogeneous.     

Static and impact crack properties of a high-strength steel welded joint

In order to gain the benefits of weldable high-strength steels in pressurized equipment applications, satisfactory toughness and crack properties of the welded joint, both in the weld metal and the heat-affected –zone (HAZ), are required. Experimental investigations of toughness and crack resistance parameters through static and impact tests of a high-strength, low-alloy steel (HSLA) with a nominal yield strength of 700 MPa and its welded joint, were performed on Charpy-sized specimens, V-notched and pre-cracked, of the parent metal, weld metal and HAZ. The selected electrode produced slight undermatching and enabled the welded joints to be manufactured without cold cracks. The impact energy and its parts responsible for crack initiation and propagation were determined by toughness evaluation. Crack sensitivity, defined as the ratio of the impact energy for V-notched and for pre-cracked specimens, enabled a comparison of the homogeneous microstructure of the parent metal and the weld metal, and of the heterogeneous microstructure of the heat-affected-zone (HAZ), which indicated a better crack toughness behaviour of the HAZ. The results obtained showed that the toughness and crack resistance of the weld metal were significantly lower than those of the parent metal and the HAZ. The fracture mechanics parameters, J_Ic integral, and plane strain fracture toughness, K_Ic, as well as J resistance curves expressed the degradation less.     

Hydrogen effect on a low carbon ferritic-bainitic pipeline steel

Hydrogen effect on an API 5L X65 low carbon ferritic-bainitic steel is investigated, by evaluating the fracture toughness parameters in air and in hydrogen environment. The hydrogen environment is manifested by in situ hydrogen charging of the X65 steel, using the electrolytic solution NS4, which simulates the electrolyte trapped between the pipeline steel and the coating in a buried pipeline. The fracture toughness results of the X65 are compared to two other pipeline steels with different microstructures, namely an X52 and an X70, possessing a banded ferritic-pearlitic and banded ferritic-mixed bainitic-pearlitic microstructure, respectively. The X65 steel exhibits significant reduction of fracture toughness parameter J₀ integral due to hydrogen charging and insignificant variation of fracture toughness parameter K_Q. Comparing the three steels, the lowest reduction of J₀ integral due to hydrogen charging, is met on the X52 and the highest in the X65.     

Radiation embrittlement modelling for reactor pressure vessel steels: II. Ductile fracture toughness prediction

Modelling for the irradiation effect on ductile fracture toughness of reactor pressure vessel steels (RPV) is performed on the basis of ductile fracture criterion proposed earlier by the authors. The irradiation effect on mechanisms controlling ductile fracture is considered from a physical viewpoint. Modelling of the irradiation effect is carried out on the critical strain for smooth cylindrical specimens and on the local critical strain for cracked specimens. On the basis of the performed studies a scheme that allows an evaluation of the upper shelf level of the K_IC(T) curve for irradiated RPV steels is proposed.     

Measurement of fracture toughness transition behaviour of Cr–Ni–Mo–V pressure vessel steel using pre-cracked Charpy specimens

The potential application of small pre-cracked Charpy specimens for the prediction of the fracture toughness of the 1T-thickness specimens and the reference temperature T₀ has been examined. Transition fracture behaviour of plane sided, side-grooved and 1T SENB specimens, respectively, was investigated over a wide temperature range. The fracture toughness regions with various fracture initiation mechanisms were defined and ductile to brittle transition temperatures denoted. The fracture toughness transition region of small pre-cracked specimens was shifted to lower temperatures as compared with that of 1T SENB specimens. The fracture toughness data of small pre-cracked specimens have been size corrected (weakest link) to 1T thickness and used to establish the reference temperature T₀ and K_Jc(mean) fracture toughness vs. temperature curve. The calculated temperature T₀ has been in consistence with that of the 1T SENB specimen. However, some corrected fracture toughness data lay outside the scatter band of 1T thickness specimens and the shape of the K_Jc(mean) curve has been quite different from the K_Jc(med)(1T) curve. It was found out that the original measured fracture toughness results of corrected data points lying outside the scatter band violated the validity condition b₀R_p0.2/J_c≥30. Bearing in mind the work of Koppenhoefer and Dodds Jr. (Engng Fract Mechanics (1997);58:249–270), and the most recent analysis of Ruggieri et al. (Engng Fract Mechanics (1998);60:19–36), the fracture data of small pre-cracked specimens having the validity parameter lower than 50 have been first constraint adjusted using the cleavage fracture toughness scaling model of Dodds and coworkers (J Testing Evaluation (1991);19:123–134; Int J Fracture (1995); 74:131–161; Engng Fract Mechanics (1997);58:249–270), and only then size corrected. The K_Jc(mean) curve of such treated data was identical with K_Jc(med)(1T).     

Weibull statistical models of KIc/KIa fracture toughness databases for pressure vessel steels with an application to pressurized thermal shock assessments of nuclear reactor pressure vessels

This paper presents new statistical representations of recently extended fracture toughness K_Ic and K_Ia databases for pressure vessel steels. These models were developed by the Heavy Section Steel Technology program at Oak Ridge National Laboratory in support of the current effort by the U.S. Nuclear Regulatory Commission to update its regulatory guidance for pressurized-thermal-shock (PTS) transients in nuclear reactor pressure vessels. The Weibull distribution, with two of its parameters calculated by the Method of Moments point-estimation technique, forms the basis for the new statistical models. An application of the new K_Ic/K_Ia models, as implemented in the favor probabilistic fracture mechanics computer program, is also presented for three PTS transients.     

Material characterization by J-R curves of a 20MnMoNi55 forging

J-R curves have been determined for one specific forging of the reactor pressure vessel steel 20MnMoNi55 to characterize its fracture resistance in the upper shelf toughness regime. The multiple specimen unloading (MSU), a direct current potential drop (DCPD), and the single specimen partial unloading compliance (SSPUC) methods have been applied to test different CT-specimen geometries at temperatures between 25 and 300°C.The J–R curves are temperature dependent showing a minimum of slope and absolute values at 200°C. The critical J-values for onset of stable crack growth in the scatter of the results are independent of temperature and geometry: J_i ≈ 180 kJ/m². The values of the technical initiation toughness J_IC according to ASTM E813⁷ and according to Loss et al. (1979),⁸ the tearing modulus T, and the instability parameter J₅₀ show corresponding temperature trends as the J-R curves with a minimum at about 200°C.The material data have been used to evaluate the ductile failure initiation and tearing instability of several structural tests. The quality of agreement between calculable predictions and experiments is shown to depend on the input material J-R curve and evaluation of the loading which requires detailed knowledge about the material stress-strain behaviour and the relevant crack tip constraint.     

Small punch tests to estimate the mechanical properties of steels for steam power plant: II. Fracture toughness

Accurate measurements of the fracture appearance transition temperature and fracture toughness K_IC are greatly desired for engineering in steam power plant, especially from the standpoint of life extension. The current toughness of materials at critical locations is a key characteristic necessary for the remaining life analysis. For this purpose, small punch test techniques using miniature-sized specimens have been investigated on header components. Linear relationships, obtained between mechanical characteristics determined from small punch tests and from Charpy tests, have been used to estimate the fracture appearance transition temperature. Energy based and fracture stress formulations have been applied to estimate the fracture toughness in the upper- and lower-shelf regimes on a class of low alloy steels.     

Quantification of ductile crack growth in 18G2A steel at different constraint levels

A constraint theory in fracture mechanics is used to analyze the test data of 18G2A steels using single edge-notched bend (SENB) specimens with various crack depth to specimen width ratios (a/W). A bending correction factor is included in the two-parameter (J–A₂) asymptotic solution to improve the theoretical prediction of the stress field for deep cracks under large-scale yielding condition, where J is the J-integral and A₂ is the constraint parameter, which depends on the in-plane geometry of the cracked body (a/W). As a result, the valid region for a traditional J-controlled crack growth is extended, and the ASTM specimen size requirements for fracture toughness testing can be relaxed. In addition, it is shown that the functional dependence of J–R curves on A₂ for 18G2A steels is established with test data; and the predicted J–R curves agree very well with the experimental curves. This ensures the transferability of laboratory test data to an actual structure provided the constraint level (A₂) of the cracked structure is known or determined. This procedure allows an appropriate J–R curve with the same constraint level to be constructed and used in flaw stability analysis of any cracked body.     

Some material toughness properties of a series of turbine casing bolts removed from service after over 100 000 hours

The present study was aimed at examining the effects of temperature on both the Charpy and slow strain rate fracture toughness properties of a series of large steam turbine casing bolts which had been subjected to temperatures of around 500°C for some 122 000 h. Also, the various toughness properties were assessed in terms of the existing fracture toughness–Charpy correlations.The present CrMoV steel bolts had suffered varying degrees of Reverse Temper Embrittlement (RTE) during service and exhibited strong fracture toughness–temperature trends. Although no single fracture toughness–Charpy correlation effectively described the full toughness–temperature transition curves, it was evident that, for embrittled and partially embrittled bolts, the Wallin correlation best decribed the lower temperature portion of the curves. At the upper temperature region all embrittlement conditions exhibited reasonable agreement with the upper bound correlation of Barsom and Wolfe.Finally, it was shown that the fracture toughness transition temperatures, T_K, and the Charpy Fracture Appearance Transition Temperatures (FATT) exhibited good agreement with the fracture toughness transition temperature being equal to the temperature at which the fracture toughness value was 3000 N/MM^3/2.     

Simulation of JR-curves for reactor pressure vessels steels on the basis of a ductile fracture model

A procedure for the prediction of J_R-curves for reactor pressure vessels (RPVs) steels in the initial and embrittled states is presented. Prediction of the J_R-curves is performed over the ductile fracture temperature range on the basis of a ductile fracture model. A procedure for the determination of ductile fracture model parameters based on tests of smooth and notched cylindrical specimens is proposed. The stress and strain fields near the stationary and growing crack tip are analyzed by FEM. Approximate analytical solutions for stress and strain fields near a growing crack tip are proposed. Comparison of the predicted J_R-curves and experimental 2T–CT data for the initial and embrittled RPV 2Cr–Ni–Mo–V steels for WWER-1000 is performed.     

Development of Prometey local approach and analysis of physical and mechanical aspects of brittle fracture of RPV steels

In the present paper, the probabilistic model for brittle fracture toughness prediction proposed by the authors earlier and known as the Prometey model, is formulated with two stochastic parameters—the critical stress for microcrack nucleation σ_d and the critical stress for microcrack propagation S_C The stochastic dependence of the critical stress for microcrack propagation S_C is experimentally studied for an RPV steel. The Prometey model with two stochastic parameters is applied to model the effect of irradiation on the fracture toughness transition curve and to predict the loss of constraint associated with shallow cracks.     

On the ΔT41 criterion for irradiation shift

Current methodologies for assessing the effects of neutron irradiation on the ductile to brittle transition in reactor pressure vessel steels include the use of the Charpy-based ΔT₄₁ irradiation shift criterion. This criterion is commonly used in the assessment of irradiation shift for setting limits to the pressure/temperature operating diagram and in defect assessment methodologies. In both of these cases the Charpy-based temperature shift is assumed to produce an equivalent shift in the fracture toughness/temperature curve. Based on the known behaviour of steels, this study shows that this assumption may not be justified when it is applied to static fracture toughness measurements. The results of this analysis indicate that the Charpy-based shift will underestimate the shift in fracture toughness by an amount proportional to the hardening produced by irradiation. Application of the analysis to the USA database on plate and forging materials shows that the experimentally measured differences between Charpy-based and fracture toughness-bassed temperature shifts are consistent with the anticipated effects of irradiation hardening on fracture behaviour. A yield stress augmented Charpy-based shift criterion improves the equivalence in shift values as determined by the Charpy-based and fracture toughness-based approaches. The implications for the use of the current ΔT₄₁ criterion in safety assessments are discussed.     

Fracture toughness characterisation in the ductile-to-brittle transition and upper shelf regimes using pre-cracked Charpy single-edge bend specimens

Fracture toughness data of pre-cracked Charpy single-edge bend, SE(B), specimens are compared with those of standard compact, C(T), specimens in the upper shelf and ductile-to-brittle transition regimes. Charpy sized SE(B) specimens provide ductile fracture toughness data, which are compatible with those of standard C(T) specimens. Statistical methods such as the exponential curve fitting method (ECF), the engineering lower bound toughness method (ELB), and the Master Curve method (MC) are used to provide meaningful lower bound cleavage fracture toughness estimates from the toughness scatter of the Charpy sized SE(B) specimens in the ductile-to-brittle transition regime. In this regime, according to the ELB and MC methods, SE(B) specimens provide cleavage toughness data, which tend to be non-conservative compared to those of standard C(T) specimens. However, analyses based on the exponential curve fitting method show good agreement between the fracture toughness estimates for the C(T) and Charpy size SE(B) specimens. At the lower bound toughness level (5% cleavage failure probability), corresponding to J=100 N/mm, the ductile-to-brittle transition curves of SE(B) specimens are reduced by 5–8 °C compared to those of standard C(T) specimens according to the MC-method. A constraint correction function for SE(B) specimens is presented that can be used to make cleavage toughness data of SE(B) specimens compatible with those of standard C(T) specimens.     

Studies of fracture processes in Cr–Mo–V ferritic steel with various types of microstructures

In this paper, the authors report on analysis of the influence of microstructure on ductile and cleavage fracture mechanisms. The question investigated was whether microstructure observations alone can provide sufficient information to predict the possible fracture mechanism or change in fracture mechanism. Four different microstructures of ferritic steel were tested after four different heat treatments. The microstructures examined were ferritic, ferritic–pearlitic, ferritic–bainitic, and tempered martensitic types. It was concluded that the ratio (S_C/S₀) of the area covered by carbides to the total area of a ferritic grain (measured by taking into account large carbides) is the only possible quantitative measure that can be used to predict cleavage fracture.     

Improved safety margins for Belgian nuclear power plants by the application of the Master Curve approach to RPV surveillance materials

In the context of existing regulatory codes, the integrity assessment of the pressure vessel of a nuclear power plant (NPP) is based on the empirical assumption that the fracture toughness of the surveillance materials, expressed in terms of a lower bound curve indexed by a reference temperature RT_NDT, undergoes a shift under irradiation by an amount equal to the increment of the T_41 J index temperature measured from surveillance Charpy tests. Nowadays, an alternative route exists, based on: reconstitution of previously tested specimens; execution of fracture toughness tests in the irradiated condition; Master Curve analysis of the results obtained and finally determination of an alternative toughness-based reference temperature (_RTT0)$({\mathrm{RT}}_{T_0})$, which can be used to index the lower bound K_Ic curve. As we demonstrate in this paper for several surveillance materials extracted from Belgian power plants, this “advanced” approach can provide NPP owners and plant engineers an additional safety margin with respect to the operating limits of the reactor and the pressurized thermal shock (PTS) screening criteria. These additional safety margins have been found particularly significant for older plants, for which the actual fracture toughness in the unirradiated condition is often underestimated by the approach based on RT_NDT.     

Fracture toughness of structural components: influence of constraint

In this paper, in- and out-of-plane constraint are considered in the process of fracture toughness determination. A three parameter approach—J_IC, Q and T_z is utilized. The plane strain fracture toughness J_IC must be determined experimentally according to valid standards. It is the only quantity measured experimentally except for the classical uniaxial stress-strain curve determined for the material tested. Two additional parameters should be computed numerically using 2D and 3D FE analysis for the structural member which is actually analyzed. The T_z function is the generalized Poisson's ratio for elastic–plastic materials. The general model proposed allows for fracture toughness determination in two cases—when one fracture mechanism dominates the fracture process or two or more mechanisms are active simultaneously. To apply the model the fracture toughness for the particular fracture mechanisms should be known. In the paper simple models to determine these quantities are proposed both for cleavage and ductile fracture. In the latter case a distinction is made between fracture mechanisms along the shear lips and in the central part of the specimen.