Effects of Alloying Elements and Microstructures on Fracture Toughness of SA508 Gr.1A Steels

Se-Mi Hyun 고려대학교 대학원 2024 국내박사

The piping in nuclear power plants serves as a conduit for high-temperature and high-pressure steam and coolant. To ensure the safety of the piping, materials with excellent strength and J-R fracture resistance are required. The leak-before-break (LBB) concept is applied to the high-energy piping in the primary system of domestic nuclear power plants, and there are plans to extend this concept to the main steam line piping of the secondary system. The main factors influencing the assessment of LBB safety margins are yield strength and J-R fracture resistance. Therefore, to ensure a high LBB safety margin, it is necessary to use materials with excellent yield strength and J-R fracture resistance. Various factors affect J-R fracture resistance, such as strength, grain size, and precipitate size. However, further research is needed to determine which factors play a major role and how they specifically influence J-R fracture resistance. In this study, high-strength and high-toughness low alloy steel was developed by controlling alloying elements, and the effects of various heat treatment conditions on J-R fracture resistance were investigated. By reducing the carbon content and increasing the Mo and V contents up to the maximum allowable limit specified in the standards, the precipitation of coarse rod-shaped cementite decreased, and fine-grained low-temperature transformation microstructures and nano-sized VC precipitates formed. As a result, the yield strength and J-R fracture resistance increased by 117 MPa and 328 kJ/m², respectively, representing improvements of approximately 40% and 55%. Additionally, the LBB safety margin significantly improved from 1.21 to 1.51, an increase of about 25%. Although the mechanical properties improved by increasing the Mo and V contents, the influence of minor alloying elements was also evaluated to achieve further enhancements in mechanical properties. A small addition of B, distributed along the grain boundaries, formed fine-grained low-temperature transformation microstructures. Consequently, the yield strength increased by more than 50% (146 MPa), and J-R fracture resistance increased by more than 18% (103 kJ/m²), significantly improving the LBB safety margin to 1.47, an increase of approximately 22%. By controlling the alloying elements, both the yield strength and J-R fracture resistance were improved, thereby ensuring sufficient LBB safety margins. The optimal alloy composition was selected through the evaluation of the effects of alloying elements, and an actual-size prototype piping was manufactured. The microstructure was controlled by varying the heat treatment conditions to confirm the effects of grain size, precipitate size, and phase fraction on yield strength and J-R fracture resistance. As the austenitizing temperature increased, the prior austenite grain size increased, and during cooling, the transformation from austenite to ferrite was delayed, increasing the proportion of tempered bainite. Consequently, the yield strength increased by about 10%, from 396 MPa to 437 MPa. However, the formation of coarse grains caused cracks to propagate more easily, resulting in a decrease in J-R fracture resistance by approximately 20%, from 707 kJ/m² to 569 kJ/m². On the other hand, the J-R fracture resistance improved by more than 10%, from 707 kJ/m² to 787 kJ/m², even though the precipitates became coarser as the tempering time increased. This is because the decrease in yield strength due to the softening of the matrix had a greater effect on J-R fracture resistance than the coarsening of the precipitates. As the cooling rate increased, the grain size decreased and the formation of fine-grained low-temperature transformation microstructure was promoted, resulting in improved yield strength and J-R fracture resistance. However, under fast cooling conditions, the high dislocation density facilitated interactions with carbon, leading to the occurrence of dynamic strain aging (DSA). DSA increases strength but causes localized strain concentration, significantly reducing J-R fracture resistance. Microstructure analysis showed that J-R fracture resistance was primarily influenced by yield strength and grain size, while localized deformation such as DSA significantly contributed to the reduction of J-R fracture resistance. 13 different heat treatments were applied to commercial materials and prototypes to obtain more than 30 J-R fracture resistance data. This was done to quantify the correlation between microstructural factors and J-R fracture resistance and to derive a model for predicting J-R fracture resistance. Considering the grain size and yield strength, the number of grains within the plastic zone (N), a key parameter influencing J-R fracture resistance, was derived. It was confirmed that the N value is linearly proportional to the J-R fracture resistance. Additionally, when predicting J-R fracture resistance, it is necessary to also consider the work hardening that occurs due to material deformation during the J-R fracture resistance test. Finally, a J-R fracture resistance prediction model that considers effective grain size, plastic zone size, yield strength, and tensile strength was proposed. Using this model to analyze more than 30 data, it was found that the coefficient of determination (R²) was 0.81, indicating a relatively accurate fit. 원자력발전소의 배관은 고온 및 고압의 증기와 냉각재가 이동하는 통로로, 배관의 높은 안전성을 확보하기 위해서는 우수한 강도와 파괴저항성을 가진 소재의 적용이 필요하다. 국내 원자력발전소의 1차 계통 고에너지 배관에는 파단전누설(LBB; leak before break) 개념이 적용되고 있으며, 2차 계통의 주증기배관까지 LBB 개념을 확장하여 적용하려고 한다. LBB 안전여유도 평가에 영향을 미치는 주요 인자는 항복강도와 J-R 파괴저항성이기 때문에 높은 LBB 안전여유도를 확보하기 위해서는 높은 항복강도와 J-R 파괴저항성을 가진 소재를 배관에 적용하여야 한다. J-R 파괴저항성에 영향을 미치는 인자에는 강도, 결정립 및 석출물 크기 등 여러가지 인자들이 알려져 있으나, 어떤 인자가 주요하게 작용하며 어떻게 작용하는지에 대한 추가 연구가 필요하다. 이에 본 연구에서는 합금원소 조절을 통해 고강도와 고인성을 가지는 배관용 저합금강을 개발하고, 다양한 열처리 조건 변화가 J-R 파괴저항성에 미치는 영향을 고찰하였다. C 함량을 낮추고 Mo 및 V 함량을 규격의 한계 범위까지 첨가함에 따라 조대한 막대 모양의 세멘타이트 석출은 감소하고 미세한 저온 변태상과 나노 크기의 VC 석출물이 형성되었다. 이로 인해 항복강도 및 J-R 파괴저항성은 각각 117 MPa, 328 kJ/m² 정도 증가해 약 40%, 55% 이상 증가하였고, LBB 안전여유도는 1.21에서 1.51로 약 25% 크게 향상되었다. Mo, V 함량의 증가로 기계적 특성을 모두 향상시킬 수 있었으나, 추가적인 기계적 특성의 향상을 위해 미소합금원소의 영향도 평가하였다. 소량의 B 첨가는 결정립계에 분포해 미세한 저온변태상을 형성하여 항복강도는 146 MPa만큼 50% 이상, J-R 파괴저항성은 103 kJ/m²만큼 18% 이상 증가하여 LBB 안전여유도가 1.47로 계산되어 약 22% 크게 향상되었다. 이와 같이 합금원소 조절을 통해 항복강도와 J-R 파괴저항성을 모두 향상시켜 충분한 LBB 안전여유도를 확보할 수 있었다. 합금원소의 영향 평가를 통해 최적의 합금 조성을 선정하고 실규격 배관 시제품을 제작하였다. 항복강도 및 J-R 파괴저항성에 영향을 미치는 결정립 및 석출물 크기, 그리고 상분율의 영향을 확인하기 위해 열처리 조건을 변화시켜 미세조직을 제어하였다. 오스테나이트화 온도가 증가할수록 구 오스테나이트 결정립 크기가 커지며, 냉각 시 오스테나이트에서 페라이트로의 변태가 지연되어 템퍼드 베이나이트의 분율이 증가하였다. 그 결과, 항복강도는 396 MPa에서 437 MPa로 약 10% 향상되었으나, 조대한 결정립의 형성으로 인해 균열이 쉽게 진전되어 J-R 파괴저항성은 707 kJ/m²에서 569 kJ/m²로 약 20% 감소하였다. 반면, 템퍼링 유지시간이 증가하는 경우, 석출물이 조대화되었음에도 불구하고 J-R 파괴저항성은 707 kJ/m²에서 787 kJ/m²로 10% 이상 향상되었다. 이는 석출물의 조대화보다 매트릭스의 연화로 인한 항복강도의 감소가 J-R 파괴저항성에 더 큰 영향을 주었기 때문이다. 냉각속도가 증가하면 결정립 크기는 감소하고 저온 변태상의 형성이 촉진되어 항복강도와 J-R 파괴저항성이 향상되었다. 그러나 빠른 냉각속도 조건에서는 높은 전위 밀도로 인해 C와의 상호작용이 쉽게 나타나 동적변형시효(DSA; dynamic strain aging)현상이 유발되었다. 동적변형시효 현상은 강도를 증가시키기만, 국부적인 변형률 집중 현상을 발생시켜 J-R 파괴저항성을 크게 감소시킨다. 미세조직 분석 결과, J-R 파괴저항성은 주로 항복강도와 결정립 크기에 영향을 받았으며, DSA와 같은 국부적인 변형은 J-R 파괴저항성을 감소시키는 주요 요인으로 작용하였다. 미세조직적 요인들과 J-R 파괴저항성 간의 상관관계를 수치화하고 J-R 파괴저항성을 예측하는 모델을 도출하기 위해, 상용소재 및 시제품에 13조건의 열처리를 적용하여 확보한 30개 이상의 J-R 파괴저항성 데이터를 분석하였다. 결정립 크기와 재료의 항복강도를 고려하여 J-R 파괴저항성에 주요하게 영향을 미치는 인자인 소성영역 내 포함된 결정립의 수(N)를 도출하였으며, N값과 J-R 파괴저항성 결과가 선형적으로 비례함을 확인하였다. 또한, J-R 파괴저항성을 예측할 때는 J-R 파괴저항성 실험 시 발생하는 재료의 변형에 따른 가공경화도 함께 고려해야 한다. 이를 통해 유효결정립 크기, 소성영역 크기, 항복강도 및 인장강도를 모두 고려한 J-R 파괴저항성 예측 모델을 제시하였다. 이 모델을 이용하여 30개 이상의 데이터를 분석한 결과, 결정계수 값(R²)이 0.81로 비교적 정확하게 일치함을 확인하였다.

Effect of Alloying Elements on the Corrosion Behavior of Low Alloy Steels

레디엠풍 성균관대학교 일반대학원 2008 국내석사

Alloying effect of antimony (Sb) and chromium (Cr) on the corrosion behavior of low alloy steel for flue gas desulfurization (FGD) systems and for acid rain corrosion resistance, respectively, were studied based on the electrochemical measurements (potentiodynamic polarization test, potentiostatic test, electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR)) and chemical measurement (weight loss test), together with surface analysis techniques (scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS)). The results of the measurements in modified green death solution(pH -0.3, 60oC) revealed that Sb addition (0.05 or 0.10 wt.%) improved the corrosion rate of blank steel due to the formation of a highly protective Sb2O5 containing oxide film on the surface of the Sb-containing steels. Moreover, the addition of 0.10% Sb stimulated the development of high corrosion inhibiting, Cu-containing compounds which further inhibited the anodic and cathodic reactions. The results of the measurements in mild acid-chloride solution (200 ppm Cl-, pH 4) revealed that Cr addition (0.1or 0.3 wt.%) had a beneficial effect to blank steel since Cr promoted the formation of the Cu compounds on the rust layer surface of steels. However, the over-alloying circumstance of 0.5% Cr addition caused the negative effect of Cr addition by the mechanism of the hydrolysis of metal chlorides. That yielded to the localized pH decrease in the concentration cell, promoted the autocatalytic process and accelerated the propagation of the localized corrosion. The localized corrosion occurred on the steel surfaces was observed by means of SEM images.

Effect of Alloying Elements on the Corrosion Behavior of Low Alloy Steels

Le Diem Phuong 성균관대학교 2007 국내석사

Alloying effect of antimony (Sb) and chromium (Cr) on the corrosion behavior of low alloy steel for flue gas desulfurization (FGD) systems and for acid rain corrosion resistance, respectively, were studied based on the electrochemical measurements (potentiodynamic polarization test, potentiostatic test, electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR)) and chemical measurement (weight loss test), together with surface analysis techniques (scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS)). The results of the measurements in modified green death solution (pH -0.3, 60oC) revealed that Sb addition (0.05 or 0.10 wt.%) improved the corrosion rate of blank steel due to the formation of a highly protective Sb2O5 containing oxide film on the surface of the Sb-containing steels. Moreover, the addition of 0.10% Sb stimulated the development of high corrosion inhibiting, Cu-containing compounds which further inhibited the anodic and cathodic reactions. The results of the measurements in mild acid-chloride solution (200 ppm Cl-, pH 4) revealed that Cr addition (0.1or 0.3 wt.%) had a beneficial effect to blank steel since Cr promoted the formation of the Cu compounds on the rust layer surface of steels. However, the over-alloying circumstance of 0.5% Cr addition caused the negative effect of Cr addition by the mechanism of the hydrolysis of metal chlorides. That yielded to the localized pH decrease in the concentration cell, promoted the autocatalytic process and accelerated the propagation of the localized corrosion. The localized corrosion occurred on the steel surfaces was observed by means of SEM images.

건축구조용 내화내진 저합금 H형강의 열노출에 따른 미세조직 변화 및 기계적 특성 고찰

심정호 창원대학교 2021 국내석사

Recently, there is a tendency for the buildings to have been constructed in the form of high-rise and multi-purposes. Besides, to secure the safety design of structures from natural disasters such as earthquakes and fire, the building standards have become more stringent. Earthquakes cause serious damage to the buildings, and in many cases, earthquake events are followed by fire. This post-earthquake fire may bring more damage than the earthquake by itself. Thus, many attentions to the development of high performance in low carbon steels, which have resistance to sudden damage, have been paid. To perform seismic resistance, low yield ratio (YR, YS/TS) is required, reflecting their capability to accommodate deformation to fracture. Meanwhile, the fire-resistant performance is required to prevent fire accident from the earthquake. Therefore, the yield strength of the steel at 600°C should exceed two-thirds of the yield strength at room temperature((YS600℃)/(YSRT)) to guarantee the stability against thermal exposure. There have been few studies about development of steels with properties for both seismic and fire-resistance. Previous studies have focused on the development of fire-resistant steels to meet the regulations on the fire-proof structures. Alloy elements such as Mo, Cr and Nb have been added for high-temperature strength. Especially Mo content can increase the yield strength at 600°C about 137MPa through its solid solution strengthening. However, low-Mo alloys are preferred because Mo is relatively expensive. Meanwhile, bainitic strengthening can guarantee the high-temperature strength. The bainitic microstructure could be achieved by increasing cooling rate after hot-rolling or increasing a hardenability elements such as Nb, Mo. Additionally, their additions contribute to the formation of a large quantity of fine MX precipitates, which leads to precipitation hardening in the low-Mo steels. To examine the feasibility of fire and seismic resistance, it is very important to understand which metallurgical factors are more favorable to enhance the seismic and fire resistance, and to know how to tailor microstructures for both seismic and fire-resistant aplications. Recently, Kim(J.Y. Kim, Master Thesis, Changwon National University, 2018) studied the metallurgical factors affecting the mechanical properties of the low-Mo steels. However, it remains further studies to rationalize strengthening effects in more detail. In the present study, low carbon ferritic and bainitic steels with different contents of Mo, Ti, and Nb were designed. And, low cycle fatigue tests, as well as tensile tests after thermal exposure at 600°C, were conducted to investigate which microstructure combination is favorable to satisfy fire and seismic resistant characteristics. For the fire resistance, in spite of superior fire resistance of bainitic microstructure to ferritic microstructure, YR increased and (YS600℃)/(YSRT) decreased after thermal exposure for 200hr. This mechanical degradation resulted from a significant increase in yield strength at room temperature, which can be attributed to additional precipitation of fine MX particles during thermal exposure. For the seismic resistance, low cycle fatigue(LCF), bainitic microstructure showed superior LCF properties to C45 conventional steel, which consists of tempered martensitic microstructure. Meanwhile, for the aplication in welding, LCF tests on cross-welds produced by shielded metal arc welding(SMAW) and flux cored arc welding(FCAW) processes were conducted. As a result, the SMA weld exhibited a better fatigue life than the FCA weld due to micron sized-inclusions. The interface of inclusions provided a preferential path for crack propagation. To rationalize the superior fire and seismic resistance of bainitic microstructure, the correlation with microstructure evolution and mechanical properties was discussed in terms of thermal exposure.

Corrosion and fracture characteristics of low-alloy steels for power plants

Kim, Seonhong Sungkyunkwan university 2018 국내박사

The alloying effect of Cu for a flue gas desulfurization materials was investigated using the electrochemical methods in the modified green death solution and surface analyses. The test results demonstrated that the densely formed rust layer with high metallic Cu content improves the corrosion resistance of Cu-containing steel in the FGD environment. Because the free standing Cu2S precipitates had the insoluble characteristic in highly acidic solution, it produced the relatively porous Cu-enriched layer on the high Cu-containing steels surface. In addition, the degradation mechanism of the Cu-contianing steel in the green death solution was investigated. After 30 min of immersion, the steel surface was covered with a Cu-enriched film. Improvement of the film properties and the corrosion resistance were realized for the immersion time up to 6 h due to the development of the Cu-enriched layer. However, the Cu particle was formed in the Cu-enriched layer for the immersion time beyond 6 h. Since the formation of the Cu particle generated a Cu-depletion region, micro-galvanic corrosion between the Cu particle and the Cu-depletion region leads to the localized film breakdown on the surface film. The localized film breakdown, which decreased the corrosion properties of the Cu-containing steel, was accelerated by the continuous formation of Cu particles in the rust layer. The inhibition effect of the molybdate on the microbiologically influenced corrosion (MIC) behaviour of the cooling water pipeline under the incubation of the iron oxidizing bacteria (IOB) condition was examined. The decreased Fe2+ ions activity caused by the IOB metabolism increased the metal dissolution reaction and decreased the anodic inhibition reaction of the phosphate. As the molybdates injected into the IOB incubation environment, the growth of the IOB cells was suppressed and the deterioration of the protective film on the steel surface decreased. These results are due to the formation of the molybdenum oxides on the steel surface under the IOB incubation condition. In addition, the molybdate addition was effective on the corrosion behaviour of the steel with the previously formed IOB biofilm as well as the fresh steel under the IOB incubation condition. To identify the effects of the stress concentration at the pit on the fracture of the district heating (DH) pipeline, the stress increase with the wall thickness reduction was formulated. The formula includes the stress intensity factor with the propagation of pitting corrosion. As the result of the application of the DH pipeline condition to ASME B31.3 code formula and the corrosion test, the interplay with only uniform corrosion and the stress due to the internal pressure can not be an immediate reason for the DH pipeline. Furthermore, the thickness loss due to both uniform and localized corrosion exceeded before KI reaches the fracture toughness (63 MPa√m) of the pipeline steel (ASTM A106 Gr.B) under all of the studied conditions. The stress concentration at the pit was not sufficient to cause the pipeline fracture except that other factors affect the fracture of pipeline.

Corrosion behavior of low alloying steels contained alloying elements in synthetic potable water

심재주 Graduate School of Sungkyunkwan University 2003 국내박사

The corrosion behavior of newly alloyed steel was investigated by the potentiodynamic (PD) test, electrochemical impedance spectroscopy (EIS) and galvanostatic test in synthetic potable water. PD test results showed that all specimens had active corrosion behavior, and corrosion rate tended to decrease as the result of adding alloying elements. The EIS measurement was taken to find the polarization resistance (R_(p)) of the rust layer. The R_(p) of newly alloyed steel was much larger than that of carbon steel. Furthermore, more alloying elements lid to a remarkable increase in the R_(p) values. The chemical state of alloying elements in the rust of newly alloyed steels was analyzed by electron probe microanalysis (EPMA) and x-ray photoelectron spectroscopy (XPS). EPMA and XPS results showed that alloying elements existed as protective compounds in the rust layer. The corrosion of the newly alloyed steel was suppressed by the insoluble alloy metal compound formed near the surface.

Alloying Effect of Molybdenum and Niobium on the Chromium-Bearing Low Carbon Steel for Water Ballast Tank

신영웅 성균관대학교 일반대학원 2016 국내석사

This study examines the effect of the Cr, Mo and Nb alloying on the corrosion properties of low-alloy steel for water ballast tank using electrochemical tests for 15 days in artificial seawater and surface analyses. The weight loss test, EIS, and LPR tests reveals that Cr+Mo+Nb bearing steel represented outstanding corrosion resistance. Through the SEM analysis for the surface of specimens after 15 days immersion in artificial seawater, it is indicated that Cr+Mo+Nb bearing steel formed the smoothest rust layer with the least cracks among the tested specimens. EPMA analysis for cross section of rust layer confirmed that Nb elements of Cr, Cr+Nb, and Cr+Mo+Nb bearing steels were enriched at the rust layer. However, Mo element was enriched at the rust layer of Cr+Mo+Nb bearing steel while distributed constantly through rust layer and substrate of Cr+Mo bearing steel. This is because of small grain size of Cr+Mo+Nb bearing steel resulting in formation of more active sites where dissolution and precipitation of alloying elements occurred. From this reason, Cr, Mo, and Nb elements were concentrated at the rust layer of Cr+Mo+Nb bearing steel improving the corrosion resistance.

Girth Welding of Internally Clad API 5L Grade X65 Pipes using Low Alloy Steel Filler Metal

Alvarez, Alejandro ProQuest Dissertations & Theses The Ohio State Uni 2021 해외박사(DDOD)

소속기관이 구독 중이 아닌 경우 오후 4시부터 익일 오전 9시까지 원문보기가 가능합니다.

This study focuses on the development of low alloy steel (LAS) girth welds on internally clad API 5L Grade X65 steel by investigating the metallurgical phenomena of welds made using a high melting temperature consumable over a low melting temperature substrate.The metallurgical phenomena of welds made using a low melting temperature consumable over a high melting temperature substrate have been widely reported in literature. The solidification behavior of alloy 625 overlays on high-strength steel (HSS) has been reported in works pertaining to the oil and gas, petrochemical, and power generation industries. Extensive investigations have been conducted analyzing microstructural and compositional gradients along the fusion boundary and transition zone that degrade the mechanical properties of such welds. Alloy 625 girth welds on internally clad HSS have also become a topic of continued discussion as premature failures have been associated to the fusion boundary between the Ni-based alloy weld metal and HSS pipe.The oil and gas industry is investigating the potential replacement of alloy 625 girth welds with LAS girth welds. LAS girth welds could possibly reduce susceptibility topremature failures while also reducing pipeline manufacturing and installation expenses. Reel pipelay is a method of installing pipelines to the ocean floor from giant reels mounted on an offshore vessel. Reel pipelay is known to increase installation rates and reduce manufacturing expenses since welding and inspection is performed onshore. DNV-OS-F101, however, states that girth welds intended for reel pipelay applications shall overmatch the base metal yield strength by 100 MPa. Careful consideration is also required during girth welding to ensure that the corrosion resistant properties of the internally clad layers remain intact. Such girth welds have been challenging to develop due to poor weldability. Dilution from the low melting temperature substrate into the high melting temperature girth weld metal increases susceptibility to solidification cracking, liquation cracking and shrinkage porosity.Industry has not yet developed single U-groove LAS girth welds on internally clad HSS pipes due to a lack of understanding of the metallurgical phenomena in such welds. This study presents a methodology used for determining material compatibility to alleviate weldability concerns. Thermodynamic computational modeling is used to analyze the solidification behavior, partitioning characteristics, and phase transformations. Button melting, SS-DTA, and dilatometry is used to analyze the microstructural evolution in ER80S-G as a function of dilution from FM-686. Such testing methods help identify critical transformation temperatures such as AC1, AC3, Ms and Mf and, mechanisms that could be used to alleviate cracking susceptibility.Upon identifying the material compatibility between ER80S-G and FM-686, this study presents a procedure optimization approach to develop defect-free LAS girth welds. LAS girth welds are developed utilizing a combination of arc welding processes (i.e., CMT and GMAW-P) to minimize dilution from the Ni-based alloy substrate. A controlled sequence for depositing weld passes is also applied to further decrease dilution and create a temper bead welding effect to help reduce hardness.Lastly, this study subjects the newly developed LAS girth welds to metallurgical characterization and mechanical testing to develop a correlation between microstructure and mechanical properties. Testing is first conducted in accordance with DNV-OS-F101 to determine strength properties. Afterwards, hardness mapping and customized tensile testing utilizing Digital Image Correlation (DIC) is performed to determine the local mechanical properties of regions that contain both compositional and microstructural gradients.This study presents the successful development of LAS girth welds on internally clad API 5L Grade X65 steel. Girth welds exhibit a 200 MPa overmatching strength but failed to meet bend and hardness requirements stated by DNV. 1 of 3 bend specimens contained a crack longer than 3-mm that propagated from a pore. Per DNV, re-testing is required. Hardness is also above the 250 HV10 maximum requirement. Further testing is required to determine if PWHT could reduce hardness below the desired requirement.

Liquid droplet impingement erosion mechanism of low-alloy steels in the secondary side of pressurized water reactors

남원창 서울대학교 대학원 2015 국내박사

Reliable operation of nuclear power plants (NPPs) is necessary for producing stable electricity and reducing the carbon emission. In this post-Fukushima era where the safety enhancement of NPPs has emerged as the most mandates, aging management of operating NPPs emerges as an imminent challenge. There are several material degradation issues relating to pressurized water reactors (PWRs) that constitute most of operated fleets in Korea. In order to ensure the safety of NPPs throughout a long-term operating period, the integrity of the structural materials within the NPPs must be secured. Past studies on the material degradation issues have been focusing on the safety-critical problems which may arise with the structural materials constituting the primary pressure boundary (i.e. primary circuit) of PWRs. In contrast, issues with the secondary side of PWRs have received relatively insufficient attention. However, ruptures of piping systems in the secondary side of PWRs caused by the pipe wall-thinning phenomenon have persisted safety impact both within and outside Korea including losses of life and costly damages. Flow-accelerated corrosion (FAC) and liquid droplet impingement erosion (LDIE) have been identified as key causes of pipe wall-thinning phenomenon. While significant understanding has been made for the former, little research has been conducted on the latter. Interestingly, ruptures caused by LDIE have only recently been reported and no systematic research has been conducted on mechanisms or remedies. For long-term operation, it is thus essential to understand on the damage mechanism of LDIE. Difficulties in carrying out investigations arise from the fact that it is difficult to (i) simulate at a lab scale a phenomenon slowly develops in a time scale of over several decades; and (ii) identify the detailed micro-processes for such phenomenon due to the complex interactions among several key variables. In this thesis, an accelerated test method has been developed and fundamental mechanisms as well as damage rates are examined for LDIE in simulated physico-chemical environments that are relevant to the secondary side of PWRs. In order to accelerate the LDIE phenomenon, experiments have been conducted under reducing conditions with; (i) the pH level is adjusted to a level lower than those of the secondary side of PWRs; and (ii) the velocity and size of water droplets are adjusted to levels higher than those of the secondary side of PWRs. LDIE experiments by controlling the pH level and velocity and size of water droplets under water chemistry conditions typical of actual plant have been made as a unique attempt to understand mechanisms. A unique high temperature test apparatus has been developed in this thesis starting from the ASTM G-73-10 standard testing method which requires the control the velocity and size of water droplets. Water droplets having uniform density and size are sprayed through a nozzle and impinged upon specimens that are attached to the rim of rotating disc. Water droplets are produced by using chemically controlled water as a function of DO/DH concentrations, pH level and time. Two types of typical low-alloy steels for PWRs have been studied, including A106 Gr.B (UNS K03006) and A335 P22 (UNS K21590). The latter with higher Cr contents has lower general corrosion rate than the former. High sulfur grade low-alloy steel, SUM24L (UNS G12144), is also tested to examine the effect of MnS inclusions. All tested materials have microstructure consisted of relatively equiaxed ferrite subgrains and lamellar pearlite regions. Then, LDIE tests were conducted on test coupons by using the specially developed apparatus. After measuring the accumulated mass-loss over testing time, it has been observed for all materials that: (i) there exists a transition point at which the damage rate thereafter increase significantly; (ii) prior to the transition point, damages tend to be initiated from a particular region of the surface of the specimen; and (iii) after the transition period, the test specimen, while under the same experimental conditions, displayed a higher damage rate than the rate displayed prior to the transition point. By examining microstructure and nanostructure of damaged coupons, it has been observed that for all tested materials early damages tend to be mainly inflicted within the pearlite structure whereas the ferrite structure remained relatively intact. Microscopic observations of damaged area revealed that the LDIE phenomenon occurred in the following sequence: the selective dissolution of ferrite layers within the pearlite structure leaving cementite plate behind. The selective dissolution of ferrite has been previously reported to have been caused by a galvanic effect arising from the electrochemical disparity between alternating layers of cementite and ferrite, acting as the cathode and anode, respectively. Subsequent erosion by impinged water droplets of the cementite layers within such pearlite structure leads to the detachment of the top surface layer and reveal underlying layer with high roughness. Water droplet impingement on the roughened surface thereafter significantly increase momentum transfer and the erosion rate. Similar behavior was observed with A335 P22. Microscopic observations of A335 P22 displayed the same phenomenon where damages were mainly inflicted on the pearlite structure and the ferrite structure remained relatively stable until a certain point in time, as well as the same sequence of events observed on A106 Gr.B. Therefore, this research proposes a damage mechanism for liquid droplet impingement erosion (LDIE), an identified cause of the pipe wall-thinning phenomenon in the secondary side of PWRs, which is explained by the first step of corrosion-induced roughing of surface layer and the second step of erosion-induced material removal. The transition from the first to the second step occurred when the surface grains are removed. Based on the results of this research, a bi-linear prediction model has been proposed, as functions of time, pH, and water droplet momentum to delineate the corrosion-induced period and the erosion-dominating period. The model was applied to predict one known case of a field failure to fail that the developed model over-predict the damage rate and under-predict the failure time by about 30%, indicating a fair agreement.

Novel Technologies for Mitigation of Flow Accelerated Corrosion of the Secondary Side of Pressurized Water Reactors

Kim, Seunghyun Graduate School of UNIST 2019 국내박사

Secondary side of pressurized water reactors undergo severe erosion-corrosion (E-C) under high-temperature flowing water, and one of the most damaging phenomenon is flow accelerated corrosion (FAC). Due to FAC, many failures of the secondary system occurred which threatened the safety and the integrity of nuclear power plants. In this study, novel technologies, which include development of FAC resistive coatings and alloys, are proposed and their performance was evaluated in various FAC conditions. As the resistive coatings, Ni-P/TiO2 nanocomposite and Fe-based amorphous metallic coatings (AMCs) were deposited on carbon steel substrate. Since they are known to possess outstanding corrosion and erosion resistance. In case of FAC resistive alloys, low alloy steels with different Cr and Mo contents were developed since Mo could be substituted by higher Cr contents according to Ducreux’s equation. Also, Nakamura’s crack susceptibility model and manufacturing costs of the alloys were considered. As a product, FAC resistive alloys (FRAs) are manufactured via vacuum arc melting method. And, the performance of the FAC resistive coatings and the alloys have been experimentally demonstrated by employing various FAC instruments. At first, their electrochemical properties under seawater have been evaluated using linear sweep voltammetry and electrochemical impedance spectroscopy. Ni-P/TiO2 coating shows excellence corrosion resistance due to the anodic protection for Ni-P matrix by spatially dispersed TiO2 nanoparticles and the stability of NiO. However, Fe-based AMC shows susceptibility in seawater corrosion due to intrinsic surface morphology, defects, and surface area. In case of FRAs, the corrosion behavior in seawater condition has similar trend indicating that Mo could be substituted by high Cr. Electrochemical behavior of Ni-P and Ni-P/TiO2 coatings under simulated secondary water chemistry is also evaluated. Compared to Ni-P coating, Ni-P/TiO2 coating is less activated where FAC is favored i.e., 150 oC. However, the effect of spatially dispersed TiO2 nanoparticle is vanished at 125, 175 and 200 oC but still the coating can suppress the corrosion. To evaluate FAC resistive performance, secondary water chemistry control system and an autoclave system is prepared. At temperature range 125 to 200 oC, FAC simulation performance of the coatings and the alloys have been tested under deaerated water with pH 9.3, which is controlled by ethanolamine. In case of the coatings, Ni-P/TiO2 coating effectively suppress the corrosion by the galvanic coupling compared to Ni-P. Fe-based AMC also shows remarkable corrosion resistance since FAC is a complex phenomenon of corrosion and erosion. Instead of weight loss, slight weight gain is observable for Fe-based AMC. In case of FRAs, the oxide morphology is two-layer structure. The outer oxide layer is composed of Cr-rich amorphous oxide and the inner oxide is Cr-substituted magnetite according to their crystal structure. The corrosion rates of the alloys are in good agreement with Ducreux’s model. Finally, FAC tests at 5.7 m/s of flow were carried in the test facilities. To test FAC performance of the coatings, coated 90o elbows were prepared and their thickness is measured by ultrasonic thickness technique. In case of A106 Gr.B carbon steel, significant thickness reduction is observable at intrados due to localized acceleration of flow velocity while A335 P22 keeps its thickness for 21 days of immersion. Ni-P/TiO2 coating also shows significant thickness reduction at intrados and surface and cross section morphologies confirms that the coating is detached. However, Fe-based AMC does not show thickness reduction but slight increased thickness is observable due to the formation of thick oxide. Thus, it can be concluded that: Ni-P/TiO2 coating is good corrosion resistive coating under corrosive condition but not effective under erosion condition; Fe-based AMC is susceptible to seawater corrosion but highly effective under erosion-corrosion condition; Mo-free FRA is available option for the substitution of commercial low alloy steel.