Analysis of warm prestressing effect on fracture toughness of reactor pressure vessel steels

We have performed finite element calculation of precracked Charpy-like three-point bending specimen, in order to analyze the effect of warm prestressing on fracture toughness value of reactor pressure vessel steels. Two different hardening laws were applied in the calculation. J-integral was determined in both cases and comparative analysis was made.     

Control of ductile to brittle transition temperature in ferritic pressure vessel steel

Abstract

Controlled amounts of cold work are shown to cause a minimum in the ductile to brittle transition temperature (DBTT) in a ferritic steel at a critical level of ～1·5%. Mechanical property assessments show that the hardness values exhibit the same trend. A theory is advanced for explanation of these effects, based on work hardening and Cottrell–Bilby locking models. Consideration is given to an alternative Ashby–Embury model, but it is concluded that the former approach is most successful in predicting the observed DBTT shift behaviour. Although independent of fracture surface type, the degree of plastic deformation shows some dependency on the grain boundary character. This leads to the conclusion that the matrix yield strength is the primary factor in determining the DBTT in these steels. Discussion focuses on methods for exploiting the effect to give higher toughness steels utilising knowledge of how to control matrix hardening and cleavage fracture strength.     

Dislocation structure and impact toughness of reactor pressure vessel steel 15Kh2NMFA upon ultrasonic treatment

The paper addresses the influence of high-temperature ultrasonic treatment on the structure and impact toughness of steel 15Kh2NMFA. The observed increase of impact toughness and decrease of ductile-brittle transition temperature for the steel upon ultrasonic treatment of various durations are due to the formation of a uniform defect structure.     

Analysis of impact properties of A533 steel for nuclear reactor pressure vessel by instrumented Charpy test

Instrumented Charpy impact test can become more useful by connecting the digital memory and microcomputer, where yield load (P_y), maximum load (P_m), premaximum load energy (E_i) and post-maximum load one (E_p) are automatically and rapidly analyzed. In such analysis, cyclic oscillations in a load signal are corrected and smoothened by using a method of moving averages (the nonrecursive low pass digital filter). Dynamic J_1c value (J_1d) of A533 steel can be measured in a fatigue precracked type Charpy specimen, provided that a true deflection of specimen and a true crack initiation point in the load-deflection curve are known. For this purpose, elastic compliance values of the testing machine and the specimen are measured dynamically by the elastic low blow test to correct the apparent deflection. Crack initiation point, on the other hand, is detected by the plastic low blow test. It is shown that the crack initiation in the fatigue pre-cracked specimen of this material occurs prior to the maximum load, and that the relation between crack initiation energy (E_Δα) and pre-maximum load energy (E_i) is $\text{E}{}_{\mathrm{\Delta \alpha }}\text{E}{}_{\mathrm{i}}\text{? 0.8}$.     

Low-temperature fracture mechanisms in a spheroidised reactor pressure vessel steel

The micromechanisms of fracture of a spheroidised A533B reactor pressure vessel steel over the temperature range of −190°C to + 60°C were investigated by performing uniaxial tensile tests on double-notched cylindrical specimens. Failure was by quasi-cleavage at temperatures between −190°C and −145°C. Quasi-cleavage fracture surfaces are characterised by clusters of planar facets that are separated from other facets either by large voids or by clusters of microvoids. At temperatures between −145°C and −25°C failure was by mixed microvoid coalescence and cleavage while complete microvoid coalescence was observed at temperatures higher than −25°C. Over the whole temperature range studied, fracture nucleation was either from large single voids or localised regions of microvoids.     

Assessment of the master curve approach on three reactor pressure vessel steels

This study aims at assessing the applicability of the Master Curve procedure to the measurement of the reference temperature for three well-characterised reactor pressure vessel steels (22NiMoCr37, JSPS, JRQ). The following aspects of the methodology were investigated, using statistical tools such as the Generalised Maximum Likelihood (GML) and Monte Carlo methods: independence of T _o from test temperature and specimen type (configuration/dimensions); formula given in ASTM E1921 for evaluating the standard deviation of the reference temperature, and possibile expressions for estimating the standard deviation of the other Weibull parameters (m and K _min); proposed relationships for estimating the median toughness and standard deviation for T _o(_To) in the case of multi-temperature Master Curve analysis. In reference to the three Reactor Pressure Vessel Steels (RPVS) investigated, the independence of T _o was assessed from the test temperature (within the range prescribed by the following revision of the ASTM standard, T _o±50 °C) and, as far as C(T) specimens are concerned, from the sample dimensions; the well-known 10÷15 °C difference was however found between PCCv and C(T) geometries. Furthermore, using the Monte Carlo method, we assessed the relationship proposed by the ASTM standard for estimating the standard deviation of the reference temperature, which results fairly conservative; an alternative analytical function has been proposed. Clear trends have also been identified for the standard deviation of m (with respect to the number of valid data r) and K _min (with respect to the median toughness). Finally, the proposed relationships for estimating K _Jc,med and _To for the multi-temperature analysis have been validated by comparison with the results of the Monte Carlo method: an excellent agreement was found in terms of _To (better than 0.2 °C).     

A calculation-experimental study of crack-tip opening displacement and residual stresses upon warm prestressing

For a reactor pressure vessel steel 15Kh2MFA(III) experiments and calculations have been carried out to study the factors that have an influence on the increase of the lower-shelf fracture toughness in the temperature dependence diagrams upon warm prestressing. Stereoscopic fractography and numerical investigation have demonstrated that after the warm prestressing the crack tip remains blunt. This reduces the stress singularity during subsequent loading and raises the material fracture toughness. The paper gives the calculated data on residual stresses and crack-tip opening displacement during warm prestressing and upon relief. The calculated results are shown to agree well with the known analytical relations and experimental data.     

A weibull analysis of fatigue-crack propagation data from a nuclear pressure vessel steel

Techniques of curve fitting, based upon an equation in the form of the four parameter Weibull Survivorship Function, are presented for the reduction of fatigue-crack propagation rate data obtained from a nuclear pressure vessel steel. The importance of data in the near threshold and the high crack propagation rate regions of the $\text{D}\text{A/}\text{D}\text{N}$vs ΔK, curve, as it relates to the curve fit,i s demonstrated with respect to a change in the four parameters of the Weibull function.     

Fem prediction of the influence of warm prestressing on fracture toughness of heat-resistant steel

We present a procedure of prediction of the influence of warm prestressing combined with cycling on the brittle strength of steel 15Kh2MFA. Using a finite-element method, the effect of the combined warm prestressing on the stress-strain state at a fatigue crack tip is studied in an elastic-plastic statement. Electron microscopic observations of fracture surfaces have revealed that fracture is initiated at some distance from the fatigue crack front. Based on the pattern of influence of the plastic prestrain level on the cleavage stress of steel 15Kh2MFA and the experimental CID value, a method is put forward for finite-element modeling of the stress-strain state at a crack tip during the specimen fracture. Using the results of the finite-element modeling, the relevant curves have been plotted and an approximating formula has been proposed to represent the influence of the combined warm prestress level on the fracture toughness of steel 15Kh2MFA.     

Correlation between Charpy impact energy and J fracture toughness for thermally embrittled reactor pressure vessel steel

Abstract

The Charpy impact energies of a reactor pressure vessel steel in the as received and several thermally embrittled conditions have been tentatively correlated to three J fracture toughness parameters derived under quasi-static loading regimen. Very good correlation has been achieved over the whole fracture resistance range of the structural steel, as obtained by the application of special heat treatments. It has been established that the parameters controlling the impact energy absorption capacity of the materials are the equivalent grain size of dual phase (ferrite/bainite) annealed microstructures and the bainite packet size of single phase quenched and tempered materials. The dependence of the Charpy impact energy of precracked, side grooved bend bars on the representative grain size of the microstructures tested has been disclosed as a Hall-Petch relationship.     

Effects of dynamic strain aging on mechanical properties of SA508 class 3 reactor pressure vessel steel

An as-received reactor pressure vessel (RPV) steel SA508 class 3 (SA508 Cl.3) has been subjected to uniaxial tension tests in the strain-rate range of 6.67 × 10⁻⁵ s⁻¹ to 1.2 × 10⁻² s⁻¹ and the temperature range of 298 K to 673 K to investigate the effects of temperature and strain rate on its mechanical properties. It was found that the region of dynamic strain aging (DSA) was in the temperature range of 523–623 K at a strain rate of 1.2 × 10⁻³ s⁻¹, 473–573 K at 1.2 × 10⁻⁴ s⁻¹, and 473–573 K at 6.67 × 10⁻⁵ s⁻¹, respectively. Serrated stress–strain behaviors, predominately consisting of type A, B, and C, have been observed in these temperatures and strain-rate ranges. The solutes responsible for DSA have been identified to be carbon and nitrogen, and nitrogen atoms play a more important role. The relative DSA mechanisms for this RPV steel are discussed.     

Local approach to fracture-based prediction of reactor pressure vessel lifetime

New version of the local approach to fracture is presented. Within the framework of this approach a new methodology is developed, which provides prediction of radiation life time of a reactor pressure vessel not by ultimate shift of the Charpy critical temperature, ΔT_K, or by reference temperature, ΔT₀, but by the condition of brittle fracture initiation of irradiated metal ahead of a crack tip in reactor pressure vessels. __________ Translated from Problemy Prochnosti, No. 1, pp. 53–60, January–February, 2009.     

Influence of combined preliminary loading on the brittle strength of refractory steel

We study the influence of combined preliminary loading (tensile straining combined with a low-amplitude cyclic component) on the tensile strength, crack-tip opening displacement, and brittle-fracture resistance of 15Kh2MFA refractory steel after thermal treatment simulating the process of embrittlement of the reactor vessel at the end of its service life. The tensile strength is determined under the conditions of uniaxial tension for cylindrical specimens with diameters of the working part of 5 and 8 mm in liquid nitrogen. The influence of combined thermomechanical loading on the brittle strength and kinetics of crack-tip opening displacement is investigated under the conditions of eccentric tension for compact specimens 19 mm in thickness. The basic regularities of the influence of temperature and the level and amplitude of the cyclic component of overloading on the tensile strength and critical stress intensity factor of steel are established. For a temperature of preloading of 623°K, which is much higher than the temperature of brittleness of steel (T _br = 390°K), the combined preliminary thermomechanical loading increases the critical stress intensity factor K _f of steel by up to 30% as compared with its value under static loading. At the same time, for a temperature of preloading of 423°K (close to the temperature of brittleness), the procedure of combined preliminary thermomechanical loading decreases the value of K _f for the analyzed steel as compared with the case of static loading. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 100–106, May–June, 2006.     

Effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens

The effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens were studied by experiments and FEM calculations. The results show that the local stress concentration around the voids promotes the cleavage initiation and decreases the notch toughness and cleavage fracture stress. The fibrous cracks ahead of notch tips caused by the ductile tearing in the WPS obviously raise the normal stress in front of their tips and decrease fracture load and notch toughness. When the beneficial effects of WPS on improving apparent fracture toughness for specimens or structures are used, the loads in WPS need to be limited so that no obvious void damage and ductile tearing are produced in front of defects.     

On the information about fracture characteristics obtained from instrumented impact test of A533 steel for reactor pressure vessel

Analysis of the information obtained from instrumented impact test and of its correlations with static and dynamic J integral values and with various tensile properties is made in this study.     

Lifetime analysis of wwer reactor pressure vessel internals concerning material degradation

Reactor internals are subjected to three principal operation effects: neutron and gamma irradiation, static and dynamic mechanical stresses and coolant chemistry. In this study, we investigate the effect of these mechanisms on WWER 440 reactor vessel internals and present lifetime analysis, in order to extend the operational lifetime of reactor vessel internals.     

Effects of strength mis-matching on the fracture behavior of nuclear pressure steel A508-III welded joint

In this paper, according to the nuclear pressure steel A508-III, the effect of strength mis-matching on the fracture behavior was analyzed by fracture mechanics test and the crack tip stress field of three-point bend specimen was analyzed by using finite element analysis method (FEM). The fracture of heat-affected zone (HAZ) was emphasized especially. The results of FEM show that if the under-matching weld was used, the opening stress and stress triaxiality in the vicinity of crack tip would increase for weld-crack specimen, and would reduce for HAZ-crack specimen. This tendency was confirmed by the test results.     

Probabilistic analysis of mechanical properties of WWER-1000 reactor pressure vessel materials

The variance of strength and plastic characteristics of 15Kh2NMFA steel and its welds is assessed by the results of statistical analysis of material mechanical properties for nineteen WWER-1000 reactor pressure vessels manufactured by Izhorskii and Atommash factories. The values determined with a 95% probability are well within the limits specified by the applicable regulations. The parameters which are needed for the statistical brittle-strength analysis of a reactor pressure vessel are presented with the same veracity. __________ Translated from Problemy Prochnosti, No. 2, pp. 15–28, March–April, 2006.