Bong Sang Lee

J-R fracture properties of SA508-1a ferritic steels and SA312-TP347 austenitic steels for pressurized water reactor's (PWR) primary coolant piping

Abstract J-R fracture resistance of SA508-1a and SA312-TP347 steels, which are both rather peculiar as PWR primary coolant piping materials, were evaluated for application of leak-before-break methodology to the design basis of nuclear power plant piping. Archive materials from various heats of both steel pipes showed apparent heat-to-heat variations in ductile fracture resistance at the operating temperature 316°C. The SA508-1a ferritic steels showed relatively good J-R curve properties although they varied with the microstructures depending on the manufacturing process and chemical compositions. On the other hand, ductile crack growth resistance of SA312-TP347 austenitic stainless steel was unexpectedly poor when carbon content is moderately high. It was found that coarse niobium carbides deteriorated the ductile fracture resistance, so that more rigorous specification is needed in carbon and niobium contents to improve fracture properties of Type 347 stainless steels.

Evaluation of the Cleavage Fracture Toughness of RPV Steels Using Small Bend Specimens

The cleavage fracture toughness of several reactor pressure vessel (RPV) steels was measured by the ASTM E 1921 master curve method using small bend (pre-cracked Charpy V-notched) specimens. The applicability of the method was experimentally evaluated by using different types of specimens and materials. Micro-structural and fracto-graphical observations were performed on cleavage fracture surfaces and polished-and-etched specimens as well. The measured cleavage fracture toughness was strongly related to the distance from the crack tip to the cleavage initiation site, cleavage initiation distance (CID), which was slightly beyond the local peak stress region. Based on the microscopic observations, the cleavage initiation site was not influenced by the grain size of the material. Metallurgical particles, such as inclusions and/or carbides, may play an important role in the cleavage fracture of low alloy RPV steels in the transition range.

Weibull Statistics as a Basis for Assessment of Ductile-Brittle Transition Behavior

The objective of this paper is to investigate failure characteristics of SA508 carbon steel in ductile-brittle transition temperature region. To achieve this goal, a series of finite element analyses as well as fracture toughness tests are performed for pre-cracked V-notch specimens. An assessment of failure probabilities is, then, carried out employing Weibull stress models with different rank probability options. Finally, a prototype of toughness scale diagram is derived through comparison of estimated fracture toughness data with those for compact tension specimens. The present results can be utilized to found a basis of realistic integrity evaluation on major nuclear components containing defect.

Fatigue Crack Propagation Behavior Near Fusion Line between SA508 Steel and Ni-Based Buttering Metal

Fatigue crack propagation behavior near the fusion line between SA508 ferritic steel and Ni-based buttering metal was studied to assess the integrity of dissimilar metal welded zone in reactor pressure vessels. Ni-based filler metal has been used as a buttering or filler metal to weld the ferritic steel to the Ni-alloy or austenitic stainless steel. The J integral value and stress field at the crack tip in a simulated small-CT welded specimen model was calculated by using the commercial FE calculation code to anticipate the effect of the yield strength differences between dissimilar metals. If the Ni-based buttering metal has lower yield strength, which means the decrease of material constraint by the weld metal, the J integral value of the crack tip in the base metal near the fusion line was calculated higher than that of the base metal. The fatigue crack propagation behavior near the fusion line was measured by using the small-CT welded specimens of 5 mm thickness. The relationships between da/dN and )K were measured in the base metal and the HAZ near fusion line. The yield strength of the weld metal including microstructure at the joint can be considered more effective than the material constraint on explaining the behavior of fatigue crack propagation near the fusion line.

Effects of Alloying Elements on Fracture Properties of Niobium Stabilized Austenitic Steels at Elevated Temperature

The present work is a further investigation into the effects of the carbon (C), nitrogen (N) and niobium (Nb) contents on then fracture properties of the Type 347 stainless steels at 316oC. 9 heats of systematically designed alloys were examined. Through SEM-EDS, TEM and XRD analyses, two kinds of precipitates, Nb(C,N), CrNbN were identified in the Type 347 steels with a high ratio of wt% N to wt% C, on the other hand only Nb(C,N)s were found in the Type 347 steels with a low ratio of wt% N to wt% C. The tearing moduli were decreased in the range of 52~60% as the carbon content increased from 0.03wt% to 0.05wt%. The tearing moduli were lowered by 52~59% in the alloys with a high nitrogen. It was deduced from the microstructure analysis results that the coarse Nb-rich precipitates control the fracture resistance of the Type 347 as they act as the potential sites for the nucleation of micro-voids.

Effects of Chemical Composition and Welding Process on Fracture Resistance of Type 347 Stainless Steel Weld

The present study is a systematic investigation of the effects of microstructural changes, which have originated from the variations of filler metals and welding processes, on the J-R properties of simulated welds. Two AISI Type 347 weld metals, with different carbon contents, deposited by a GTAW process and two AISI Type 347 weld metals, with different carbon contents, deposited by a SMAW process were used in this study. The J-R tests were conducted at 316oC (600oF). The welds deposited by the GTAW process showed higher fracture resistances when compared to the welds deposited by the SMAW process. The J-R fracture resistance of the Type 347-GTAW processed weld with high carbon content was remarkably low when compared to the weld with low carbon. The J-R fracture resistances were decreased by coarse Nb(C, N) precipitates in the Type 347 weld deposited by the GTAW process. In the case of the SMAW welds, mainly coarse Ti-rich particles which had originated from the shielding of the welding rods deteriorated the fracture resistances.

Evaluation of Critical Fracture Stress in Low Alloy Steels by Finite Element Analysis of Small Punch Test

The critical fracture stresses (σ* f(sp)) in various low alloy steels for a reactor pressure vessel(RPV) were evaluated by a small punch test (SP test) and a finite element analysis (FEA) in the cleavage temperature region(-150~-196 °C). The load-displacement curves and distances from the center to the fracture surface (Df) of the FEA results are in good agreement with the experimental results. The maximum principal stresses (SP fracture stresses, σf(SP)) were determined from the FE analysis, when the maximum load was applied to the SP test. The SP critical fracture stress, σ* f(sp) in various reactor pressure vessel (RPV) steels was found to have a linear relationship with the values obtained from the precracked specimens (σ* f(PCVN)). The σ* f(sp) shows a lower value than σ* f(PCVN) because the SP specimen had a lower triaxial stress condition. However, this result indicates that a small punch test could be a useful method to evaluate the cleavage fracture behavior of low alloy steels.

Development of Toughness Scale Diagram Considering Temperature Dependency

Fracture toughness data from the cleavage resistance test of structural steels often show a large scatter. Geometry dependency as well as the scatter makes it difficult to evaluate appropriate fracture integrity of cracked components. To address these restrictions, several stochastic models have been proposed by Beremin group, Mudry and other researchers while each of them employs specific estimation scheme and micro-mechanical parameters. The purpose of this paper is to investigate applicability of the Weibull stress model in transition temperature regime and to quantify constraint effect among different-sized CT and PCVN specimens. The constituting parameters m and σu are determined at three temperatures by maximum likelihood estimate (MLE) technique in use of FE analysis results and experimental data of PCVN specimens. Also, failure probabilities of PCVN and CT specimens are calculated from the Weibull parameters, which are used for derivation of a prototype of toughness scale diagram. The diagram provides a technical basis to resolve transferability issue in the same material under different temperatures and constraint conditions.

Effects of Notch Tip Geometry on the Cleavage Fracture of RPV Steel

A series of fracture tests and elastic-plastic FEM analysis were carried out for three-point bend specimens with different notch root radius in the cleavage temperature region of a RPV low alloy steel. The cleavage initiation distance (CID) from the notch tip was measured by SEM photographs on the fracture surface of each specimen. The local cleavage fracture stress σf * was defined as the stress σyy at the cleavage initiation site. The σf * increased with decreasing notch root radius. The CID’s were larger in specimens with bigger root radius. This implies that a larger volume of material and possibly bigger microcracks should be involved in the cleavage fracture process of those specimens. The σf * is considered to have a certain relationship with the level of stress-strain concentration in different geometry specimens.

Empirical Correlation between Parameters from Small Punch and Tensile Curves of Mn-Mo-Ni Low Alloy Steels by Using Test and FE Analysis

A load-displacement curve from a small punch test includes several useful information that is related to standard test properties such as the tensile property, fracture toughness and ductilebrittle transition temperature. In this study, the empirical relationship between the material property factors in SP curves and the tensile curves has been investigated by comparing test results and finite element analysis results. SP and tensile tests and finite element analyses were performed for several Mn-Mo-Ni low alloy steels with different manufacturing processes. It was found that the yield loads (Py) in the SP curves, if they were adequately defined, were linearly related to the yield strength (σ0). The yield loads defined from the intersection point of two lines tangent to the elastic bending region and plastic bending region showed a better relation with the yield strength than those from the offset line. The slope of the SP curves from the simulation results had a close correlation with the hardening coefficient and strength constant as well.