Charpy Transition Temperature Research Articles

Development of new embrittlement trend curve based on Japanese surveillance and atom probe tomography data

• A new embrittlement trend curve based on Japanese surveillance data was developed. • A simple analytical solution of the embrittlement trend curve (ETC) can be obtained from the original differential equation. • The new ETC can predict microscopic and macroscopic changes caused by neutron irradiation. We have developed a new embrittlement trend curve (ETC) for neutron-irradiated reactor pressure vessel(RPV) steels based on Japanese surveillance and material test reactor data. Our model is based on experimental data obtained by atom probe tomography (APT) and Charpy V-notch impact tests. The square root of the volume fraction ( V f ) of the solute atom cluster (SC) formed by neutron irradiation, which is determined by APT, is correlated to the Charpy transition temperature shift ( ΔT 41J ). In addition to the ΔT 41J and V f data, other APT data (the average volume of solute clusters (SC), the number density of SC, and the matrix Cu content in RPV steels) were also used to calibrate the coefficients in the new ETC model. The new ETC model has a simple functional form and, unlike the current version of JEAC4201, does not require numerical time integration. The new ETC model has similar ΔT 41J prediction performance to the current JEAC4201 model. Furthermore, the APT data are also predicted well, which suggests a good physical basis for the new ETC.

Empirical estimation of uncertainties of Charpy impact testing transition temperatures for an RPV steel

The aim of the present work is to derive empirically a simple expression of the uncertainties on Charpy transition temperatures (TT), which depends only on the test temperatures and the number of tests at each of these temperatures. Calculations of the TT uncertainties of a RPV steel are performed based on an empirical representation of Charpy test results and a Monte Carlo procedure to generate large numbers of data sets for different test matrices. Applying usual procedures to determine the TT to the generated data sets gives access to the distributions of these quantities from which the uncertainties are calculated.An analytical expression for these uncertainties is proposed.

Delamination toughening in a low carbon microalloyed steel plate rolled in the dual-phase region

It is still a big challenge to obtain excellent low-temperature toughness for bulk steel materials. Delamination is an effective method to improve low-temperature toughness. In the present study, delamination toughening in a low carbon microalloyed steel plate with elongated and ultrafine-grained microstructure rolled in the dual-phase region has been investigated in detail. When toughness was measured along normal direction, the steel plate had a high upper shelf energy and no delamination occurred in the upper shelf region. A large delaminated crack parallel to rolling plane started to appear and changed the propagation path of main crack when testing temperature was lower than −60 °C. We find this kind of delamination induces a second upper shelf in the Charpy transition–temperature curve. The second upper shelf, reaching up to 300 J in the temperature range of −60 °C to −140 °C, results in excellent low-temperature toughness for the steel plate, and the ductile-brittle transition temperature is lowered to −157 °C. The developed steel plate also has high low-temperature toughness measured along transverse direction due to delamination. The effect factors on upper shelf energy, delamination mechanism and delamination toughening are discussed.

Effect of anisotropic microstructure of ODS steels on small punch test results

Oxide dispersed strengthened (ODS) steels can exhibit a strongly anisotropic microstructure leading to anisotropic mechanical properties. The ductile to brittle transition temperature in the small punch (SP) test is therefore dependent on the specimen orientation. Three ODS steels with 13-14 mass percent Cr, manufactured through hot extrusion and hot rolling respectively, were investigated by means of SPT in different orientations. Existing microstructural data (EBSD) are used to discuss the anisotropic fracture behavior observed in the SPT. In addition, the SPT results are compared with those from existing fracture mechanics tests based on sub-sized C(T) samples. The applicability of the empirical conversion of SPT based transition temperatures into Charpy transition temperatures – well established for isotropic homogeneous metals – is investigated for materials with anisotropic microstructure.

Fracture Toughness Evaluation of Reactor Pressure Vessel Steels by Master Curve Method Using Miniature Compact Tension Specimens

We conducted fracture toughness testing on five types of commercially manufactured steel with different ductile-to-brittle transition temperatures. This was performed using specimens of different sizes and shapes, including the precracked Charpy-type (PCCv), 0.4T-CT, 1T-CT, and miniature compact tension specimens (0.16T-CT). Our objective was to investigate the applicability of 0.16T-CT specimens to fracture toughness evaluation by the master curve method for reactor pressure vessel (RPV) steels. The reference temperature (To) values determined from the 0.16T-CT specimens were overall in good agreement with those determined from the 1T-CT specimens. The scatter of the 1T-equivalent fracture toughness values obtained from the 0.16T-CT specimens was equivalent to that obtained from the other larger specimens. Furthermore, we examined the loading rate effect on To for the 0.16T-CT specimens within the quasi-static loading range prescribed by ASTM E1921. The higher loading rate gave rise to a slightly higher To, and this dependency was almost the same for the larger specimens. We suggested an optimum test temperature on the basis of the Charpy transition temperature for determining To using the 0.16T-CT specimens.

Metallurgical Design and Performance of High-Frequency Electric Resistance Welded Linepipe With High-Quality Weld Seam Suitable for Extra-Low-Temperature Services

To clarify the effect of inclusions on the Charpy impact properties, the 2 mm V-notched Charpy properties of X60–X80-grades steel were numerically simulated using the finite element method code abaqus. The yield strength and the tensile strength of the steel were 562 MPa and 644 MPa, respectively. The striker's velocity and the temperature dependency of the stress–strain curve were taken into account. To estimate the effect of nonmetallic inclusions, a 200 μm long virtual inclusion with a 1 μm edge radius was situated at the maximum point of the stress triaxiality. Four types of microcrack initiation were determined: (a) ductile void generation in the matrix, (b) cleavage crack generation in the matrix, (c) void generation by inclusion fracture, and (d) void generation by matrix–inclusion interface debonding. Without inclusions, a ductile microvoid was generated when the striker stroke was 3.3 mm, independent of the temperature. With inclusions, an inclusion fracture occurred when the striker stroke was 0.6 mm at room temperature. The striker stroke decreased as the temperature decreased. Based on the above numerical estimation results, high-frequency electric resistance welded (HFW) linepipe with high-quality weld seam MightySeam® has been developed. Controlling the morphology and distribution of oxides generated during the welding process by means of temperature and deformation distribution control is the key factor for improving the low-temperature toughness. The Charpy transition temperature of the developed HFW pipe was much lower than −45 °C. Based on the low-temperature hydrostatic burst test with a notched weld seam at −20 °C, the MightySeam® weld provides a fracture performance that is the same as UOE double submerged arc welded pipe. The pipe has been used in actual, highly demanding, and severe environments.

On the use of Charpy transition temperature as reference temperature for the choice of a pipe steel

Transition temperature is not intrinsic to material but depends on specimens and mode of loading used for tests. Here, the linear dependence of transition temperature with constraint is shown. Constraint is evaluated by the effective T stress which is the value of the stress difference distribution for the effective distance provided by the Volumetric method.Application of this approach gives the best choice of the reference transition temperature by reducing conservatism when comparing with intrinsic transition temperature of the studied structure.

Irradiation embrittlement of reactor pressure vessel steel at very high neutron fluence

For the prediction of radiation embrittlement of RPV materials beyond the NPP design time the analysis of research data and extended surveillance data up to a fluence ∼23×1020cm−2 (E>0.5MeV) has been carried out. The experimental data used for the analysis are extracted from the International Database of RPV materials. Key irradiation embrittlement mechanisms, direct matrix damage, precipitation and element segregation have been considered. The essential part of the analysis concerns the assessment of irradiation embrittlement of WWER-440 steel irradiated with very high neutron fluence. The analysis of several surveillance sets irradiated at a fluence up to 23×1020cm−2 (E>0.5MeV) has been performed. The effect of the main influencing chemical elements phosphorus and copper has been verified up to a fluence of 4.6×1020cm−2 (E>0.5MeV). The data are indicating good radiation stability, in terms of the Charpy transition temperature shift and yield strength increase for steels with relatively low concentrations of copper and phosphorus. The linear dependence between ΔTk and ΔRp0.2 can be an evidence of strengthening mechanisms of irradiation embrittlement and absence of non-hardening embrittlement even at very high neutron fluence.

Correlation between Charpy Transition Temperature and Brittle Crack Arrest Temperature Considering Texture

The correlation between ESSO test results and Charpy impact properties has been empirically proposed to qualify brittle crack arrest toughness, Kca of structural steels. The brittle crack arrest temperature TK6000 at Kca=6000N/mm3/2, conventionally used as a measure of crack arrest property, is interrelated generally with the Charpy transition temperature vTrs. However, ESSO test results occasionally deviates from the TK6000-vTrs correlation in some TMCP steels. Considering a thickness effect in ESSO test results, TK6000 was converted to a reference temperature 25TK6000 for 25mm thickness. Nevertheless, a scatter is found in 25TK6000-vTrs correlation.This study focuses on the effect of texture intensity on crack arrest properties. It is found that steel plates with texture show lower 25TK6000 than other steels without texture, even if the transition temperatures vTrs of these steels are on the same level. The role of the (211) texture is more significant than the (100) texture. A new parameter, vTrs (°C) -12X(100) -22X(211), is proposed in this study to correlate the crack arrest temperature 25TK6000 with the material properties of structural steels, where X(100) : pole density of (100) parallel to surface of steel plate, X(211): pole density of (211) parallel to surface of steel plate.

Strength and toughness tradeoffs for an ultrafine-grain size ferrite/cementite steel produced by warm-rolling and annealing

Abstract For an ultrafine grain ferrite/cementite (UGF/C) steel, the Charpy impact energy was measured at temperatures from 373 K to 4.2 K, and tensile tests were carried out at temperatures between 323 K and 77 K. For the steel with annealed microstructure, the ductile-to-brittle transition appearance temperature ( DBTT ) was lower than the Charpy transition temperature ( CTT ). With increasing annealing time at 873 K, the DBTT and the CTT increased, and the DBTT approached the CTT. The DBTT decreased with decreasing effective grain size. The effective grain size correlated to the grain size of the larger grain size peak in the distribution of grains with {1 0 0} planes. The annealed microstructures had higher yield strength for equivalent toughness (including upper shelf energy, DBTT and CTT ) compared to the conventional ferrite/pearlite steel.

Prediction of yield stress and Charpy transition temperature in highly neutron irradiated ferritic steels

Recent predictions have been made of metallurgical properties of low-activation ferritic/martensitic steels alloys at the high irradiation levels (displacements per atom or dpa) needed for a fusion power plant as based on measurements at low irradiation levels where more data are available. These predictions have been published for the yield stress and for the Charpy ductile to brittle transition temperature shift. The neural network model predictions use training data up to a certain dpa level to predict metallurgical properties above this level. This ‘extrapolation’ mode of neural networks is explored in some detail. Our studies revealed an increasing accuracy of predictions as the test dpa level is increased for both yield stress and Charpy shift predictions. This result suggests that a model exists for these metallurgical properties as a function of dpa level which becomes more accurate as the available irradiation range in the training data is increased. The explanation suggested is that the metallurgical annealing, which occurs as the irradiation level is increased, simplifies the microstructure and makes prediction more reliable.

Embrittlement Correlation Method for the Japanese Reactor Pressure Vessel Materials

Abstract A new embrittlement correlation method developed for the Japanese reactor pressure vessel (RPV) steels is presented. The Central Research Institute of Electric Power Industry and the Japanese electric utilities conducted a project to develop a new embrittlement correlation method for the Japanese RPV steels based on the understandings on the mechanisms of the RPV embrittlement. In addition to the information from the literatures, we generated new information by characterizing the microstructural changes in the surveillance materials of the Japanese commercial reactors. We found that in low Cu materials, solute atom clusters containing little or no Cu atoms are formed at relatively low fluence of 3×1019 n/cm2, E>1 MeV. The volume fraction of the solute atom clusters has a good correlation with the Charpy transition temperature shift regardless of the Cu content. We also found that the microstructure of the boiling water reactor surveillance material is different from that of the archive material irradiated in material testing reactor. The understandings on the RPV embrittlement mechanisms were formulated using a set of rate equations, and the coefficients of the equations were optimized using the ΔRTNDT values of the Japanese surveillance database. This method considers the effect of neutron flux. Only one set of coefficients was developed, and they are independent of the product form. Predictions of the new embrittlement correlation method were compared with those of the recent U.S. correlation method as well as the U.S. surveillance data. The comparison shows the characteristics of the present method.

Microstructural Characterization of RPV Materials Irradiated to High Fluences at High Flux

Abstract Understanding the embrittlement of reactor pressure vessel (RPV) steels at high fluence region is very important for the long term operation of nuclear power plants. In this study, extensive microstructural analyses were performed on the RPV steels irradiated to very high fluences beyond 1020n/cm2, E>1 MeVat high fluxes under the Pressurized Thermal Shock and Nuclear Power Plant Integrity Management projects in Japan. Three dimensional atom probe analyses were performed to characterize the solute atom cluster formation in these materials. The effects of fluence, flux, and chemical compositions on the characteristics of clusters were analyzed. The formation of dislocation loops was identified in the transmission electron microscopy analyses of high and low Cu steels, and the changes in loop size and number density with fluence were studied. P segregation on grain boundaries was also studied by surface analyses as well as grain boundary chemical analyses. We found that nonhardening embrittlement due to grain boundary fracture is not a major contributor to the embrittlement in these materials and irradiation conditions. The correlation of the microstructural changes and the Charpy transition temperature shifts was studied. The volume fraction of solute atom clusters has an excellent correlation with the transition temperature shifts. The Orowan model calculations of the contributions of dislocation loops to the transition temperature shifts show that in low Cu materials, dislocation loops may be a major contributor, but in Cu containing materials its contribution is weak. Root-sum-square of the contributions of solute atom clusters and dislocation loops seems to be a reasonable model to describe the total ΔRTNDT.

Investigation of Beltline Welding Seam of the Greifswald WWER-440 Unit 1 Reactor Pressure Vessel

Abstract The investigation of reactor pressure vessel (RPV) materials from decommissioned nuclear power plants (NPP) offers the unique opportunity to scrutinize the irradiation behavior under real conditions. The paper describes the investigation of trepans taken from the decommissioned WWER-440 RPVs of the Greifswald NPP. The key part of the testing is aimed at the determination of the reference temperature T0 following the ASTM Test Standard E1921 to determine the fracture toughness of the RPV steel in different thickness locations. In a first step, the trepan taken from the RPV Greifswald Unit 1 containing the multilayer welding seam located in the beltline region was investigated. This welding seam represents the irradiated, recovery annealed, and reirradiated condition. It is shown that the Master Curve approach as adopted in ASTM E1921 is applicable to the investigated original WWER-440 weld metal. The evaluated T0 varies through the thickness of the welding seam. After an initial increase of T0 from 10°C at the inner surface to 49°C at 22 mm distance from it, T0 decreases to −32°C at a distance of 70 mm, finally increasing again to 61°C near the outer RPV wall. The lowest T0 value was measured in the root region of the welding seam representing a uniform fine grain ferritic structure. The highest T0 of the weld seam was not measured at the inner wall surface. This is important for the assessment of ductile-to-brittle temperatures measured on subsize Charpy specimens made of weld metal compact samples removed from the inner RPV wall. Our findings imply that these samples do not represent the most conservative condition. Nevertheless, the Charpy transition temperature, TT41J, estimated with results of subsize specimens after the recovery annealing, was confirmed by the testing of standard Charpy V-notch specimens.

Weld Material Investigations of a WWER-440 Reactor Pressure Vessel: Results From the First Trepan Taken From the Former Greifswald NPP

Between 1973 and 1990 four units of the Russian nuclear power plants type WWER-440/230 were operated in Greifswald (former East Germany). Material probes from the pressure vessels were gained in the frame of the ongoing decommissioning procedure. The investigations of this material started with material from the circumferential core weld of unit 1. First, this paper presents results of the reactor pressure vessel (RPV) fluence calculations depending on different loading schemes and on the axial weld position based on the Monte Carlo code TRAMO. The results show that the use of the dummy assemblies reduces the flux by a factor of 2–5 depending on the azimuthal position. The circumferential core weld (SN0.1.4) received a fluence of 2.4×1019 neutrons/cm2 at the inner surface; it decreases to 0.8×1019 neutrons/cm2 at the outer surface. The material investigations were done using a trepan from the circumferential core weld. The reference temperature T0 was calculated with the measured fracture toughness values, KJc, at brittle failure of the specimen. The KJc values show a remarkable scatter. The highest T0 was about 50°C at a distance of 22 mm from the inner surface of the weld. The Charpy transition temperature TT41J estimated with results of subsized specimens after the recovery annealing was confirmed by the testing of standard Charpy V-notch specimens. The VERLIFE procedure prepared for the integrity assessment of WWER RPV was applied on the measured results. The VERLIFE lower bound curve indexed with the Structural Integrity Assessment Procedures for European Industry (SINTAP) reference temperature, RTT0SINTAP, envelops the KJc values. Therefore for a conservative integrity assessment the fracture toughness curve indexed with a RT representing the brittle fraction of a data set of measured KJc values has to be applied.

Prediction of the Charpy transition temperature in highly irradiated ferritic steels

A prediction has been made of the Charpy ductile–brittle transition temperature at high irradiation levels from a dataset of 459 low-activation ferritic/martensitic steels. It follows a similar study of the yield stress of some 1811 similar alloys (Windsor et al 2008 Modelling Simul. Mater. Sci. Eng. 16 025005). Neural networks have previously been used by Cottrell et al (2007 J. Nucl. Mater. 367–370 603–9) to model the Charpy ductile–brittle transition temperature of these alloys. The same dataset has been used in this study, but has been divided into a training set containing the majority of the dataset with low irradiation levels, and a test set which contains just those alloys which have been irradiated above a given level. For example, some 7.2% of the dataset was irradiated above 20 dpa. Good predictions were made using two different codes and methods. As with the yield stress prediction, the target-driven components method, where linear combinations of the atomic inputs are chosen to reduce the test residual, achieved the best results.

Information Fusion Embrittlement Models for U.S. Power Reactor Pressure Vessel Steels

Abstract A new approach of utilizing information fusion technique is developed to predict the radiation embrittlement of reactor pressure vessel steels. The Charpy transition temperature shift data contained in the Power Reactor Embrittlement Database is used in this study. Six parameters—Cu, Ni, P, neutron fluence, irradiation time, and irradiation temperature—are used in the embrittlement prediction models. The results indicate that this new embrittlement predictor achieved reductions of about 49.5 % and 52 % in the uncertainties for plate and weld data, respectively, for pressurized water reactor and boiling water reactor data, compared with the Nuclear Regulatory Commission Regulatory Guide 1.99, Rev. 2. The implications of dose-rate effect and irradiation temperature effects for the development of radiation embrittlement models are also discussed.

Neural network analysis of Charpy transition temperature of irradiated low-activation martensitic steels

We have constructed a Bayesian neural network model that predicts the change, due to neutron irradiation, of the Charpy ductile-brittle transition temperature (ΔDBTT) of low-activation martensitic steels given a set of multi-dimensional published data with doses <100 displacements per atom (dpa). Results show the high significance of irradiation temperature and (dpa) 1/2 in determining ΔDBTT. Sparse data regions were identified by the size of the modelling uncertainties, indicating areas where further experimental data are needed. The method has promise for selecting and ranking experiments on future irradiation materials test facilities.

Comparison between Charpy Transition Temperature T&lt;sub&gt;CVN&lt;/sub&gt; and Reference Temperature T&lt;sub&gt;°&lt;/sub&gt; for 10Ni3CrMoV Steel

Charpy transition temperature TCVN and reference temperature To for 10Ni3CrMoV steel were determined using two different experimental techniques such as Charpy V-notch impact test technique and reference temperature To test technique. It was found that two methods provided different test results. The median master curve with upper and lower tolerance bounds was got from the test. The upper tolerance bound is often used for material design and application. At the same time the TCVN and To results were discussed for two kinds of heat treatment which are QT(Quenching and Temper) and QLT(Quenching, anneal and temper). JC (med ) K values calculated were 101 and 105MPam1/2 for the QT and QLT steels, respectively. These indicated that the QLT steels have the higher JC (med ) K , the lower reference temperature and lower energy (or fracture appearance) transition temperature(ETT50 or FATT50) than the QT steels. This was mainly related with the different microstructures of two kinds of heat treatment.

The development of radiation embrittlement models for US power reactor pressure vessel steels

A new approach of utilizing information fusion technique is developed to predict the radiation embrittlement of reactor pressure vessel steels. The Charpy transition temperature shift data contained in the Power Reactor Embrittlement Database is used in this study. Six parameters-Cu, Ni, P, neutron fluence, irradiation time, and irradiation temperature – are used in the embrittlement prediction models. The results indicate that this new embrittlement predictor achieved reductions of about 49.5% and 52% in the uncertainties for plate and weld data, respectively, for pressurized water reactor and boiling water reactor data, compared with the Nuclear Regulatory Commission Regulatory Guide 1.99, Rev. 2. The implications of dose-rate effect and irradiation temperature effects for the development of radiation embrittlement models are also discussed.