Ion Selective Permeability of Rust Layers on Low Alloy Steels under the Chloride Ion Existence

Noda, Kazuhiko,Nishimura, Toshiyasu,Masuda, Hiroyuki,Kodama, Toshiaki 한국부식방식학회 2002 Corrosion Science and Technology Vol.31 No.1

The ion selective permeability of the rust on carbon steel and Fe-Co and Fe-Ni low alloy steel was investigated by measurement of membrane potential to evaluate the characteristics of the rust layer on low alloy steels. These steels were selected as testing materials because they were thought to be basic alloys of weathering steels for the use in coastal environments, A rust film was formed on two kinds of low alloy steels (Fe-Co and Fe-Ni steel) by corrosion cycle tests that imitated the atmospheric corrosion environment in the seashore. In the case of the carbon steel, the rust layer on it had anion selective permeability. On the other hand, in spite of the rust layer on the carbon steel was anion selective, it was found out that the permeability changed to the cation selective by the result of determining the membrane potential of the rust on Fe -Co low alloy steel. The addition of a few Co was effective for decreasing the corrosion rate, because the rust layer of the cation selective was obstructed the invasion of chloride. The addition of Ni was known to be effective in decreasing corrosion rate of steel, despite their anion selective nature in rust layer. Alternative mechanism other than ion selectivity is required for the explanation of inhibition by Ni. The tendency of the membrane potential measurement of rust layers on all samples formed by corrosion cycle test was same as the case of rust films formed by exposure tests.

논문 : 압력용기용 Ni-Mo-Cr계 고강도 저합금강의 합금원소 함량 변화에 따른 미세조직학적 특성변화의 열역학 계산 및 평가

박상규 ( Sang Gyu Park ),김민철 ( Min Chul Kim ),이봉상 ( Bong Sang Lee ),위당문 ( Dang Moon Wee ) 대한금속재료학회 ( 구 대한금속학회 ) 2008 대한금속·재료학회지 Vol.46 No.12

An effective way of increasing the strength and fracture toughness of reactor pressure vessel steels is to change the material specification from that of Mn-Mo-Ni low alloy steel(SA508 Gr.3) to Ni-Mo-Cr low alloy steel(SA508 Gr.4N). In this study, we evaluate the effects of alloying elements on the microstructural characteristics of Ni-Mo-Cr low alloy steel. The changes in the stable phase of the SA508 Gr.4N low alloy steel with alloying elements were evaluated by means of a thermodynamic calculation conducted with the software ThermoCalc. The changes were then compared with the observed microstructural results. The calculation of Ni-Mo-Cr low alloy steels confirms that the ferrite formation temperature decreases as the Ni content increases because of the austenite stabilization effect. Consequently, in the microscopic observation, the lath martensitic structure becomes finer as the Ni content increases. However, Ni does not affect the carbide phases such as M23C6 and M7C3. When the Cr content decreases, the carbide phases become unstable and carbide coarsening can be observed. With an increase in the Mo content, the M2C phase becomes stable instead of the M7C3 phase. This behavior is also observed in TEM. From the calculation results and the observation results of the microstructure, the thermodynamic calculation can be used to predict the precipitation behavior.

DEPENDENCY OF SINGLE-PHASE FAC OF CARBON AND LOW-ALLOY STEELS FOR NPP SYSTEM PIPING ON PH, ORIFICE DISTANCE AND MATERIAL

Moon, Jeong-Ho,Chung, Hung-Ho,Sung, Ki-Woung,Kim, Uh-Chul,Rho, Jae-Seong Korean Nuclear Society 2005 Nuclear Engineering and Technology Vol.37 No.4

To investigate the flow-accelerated corrosion (FAC) dependency of carbon steel (A106 Gr. B) and low-alloy steels (1Cr-1/2Mo, 21/4Cr-1Mo) on pH, orifice distance, and material, experiments were carried out. These experiments were performed using a flow velocity of 4 m/sec (partly 9 m/sec) at pH $8.0\~10.0$ in an oxygen-free aqueous solution re-circulated in an Erosion-Corrosion Test Loop at $130^{\circ}\;{\ldots}$ for 500 hours. The weight loss of the carbon steel specimens appeared to be positively dependent on the flow velocity. That of the carbon and low-alloy steel specimens also showed to be distinguishably dependent on the pH. At pH levels of $8.0\~9.5$ it decreased, but increased from 9.5 to 10.0. Utility water chemistry personnel should carefully consider this kind of pH dependency to control the water system pH to mitigate FAC of the piping system material. The weight loss of the specimens located further from the orifice in the distance range of $6.8\~27.2$ mm was shown to be greater, except for 21/4Cr-1Mo, which showed no orifice distance dependency. Low alloy steel specimens exhibited a factor of two times better resistance to FAC than that of the carbon steel. Based on this kind of FAC dependency of the carbon and low-alloy steels on the orifice distance and material, we conclude that it is necessary to alternate the composition of the secondary piping system material of NPPs, using low-alloy steels, such as 21/4Cr-1Mo, particularly when the system piping has to be replaced.

탈황설비환경에서 Cu , Sb가 첨가된 저합금강과 409L 스테인리스강의 부식 거동 비교 연구

박선아 ( Sun-ah Park ),신수빈 ( Su-bin Shin ),김정구 ( Jung-gu Kim ) 대한금속재료학회(구 대한금속학회) 2016 대한금속·재료학회지 Vol.54 No.7

The corrosion behavior of low alloy steel containing Cu, Sb and 409L stainless steel was investigated for application in the low-temperature section of a flue gas desulfurization (FGD) system. The electrochemical properties were evaluated by potentiodynamic polarization testing and electrochemical impedance spectroscopy (EIS) in 16.9 vol% H2SO4 + 0.35 vol% HCl at 60 ℃. The inclusions in these steels were identified by electron probe microanalyzer (EPMA). The corrosion products of the steels were analyzed using scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The corrosion rate of the low alloy steel containing Cu, Sb was about 100 times lower than that of 409L stainless steel. For stainless steel without passivation, active corrosion behavior was shown. In contrast, in the low alloy steel, the Cu, Sb compounds accumulated on the surface improved the corrosion resistance by suppressing the anodic dissolution reaction. †(Received July 30, 2015; Accepted February 22, 2016)

Cyclic loading behavior of high-strength steel framed-tube structures with replaceable shear links constructed using Q355 structural steel

Yan Guo,Ming Lian,Hao Zhang,Qianqian Cheng 국제구조공학회 2022 Steel and Composite Structures, An International J Vol.42 No.6

The rotation capacities of the plastic hinges located at beam-ends are significantly reduced in traditional steel framed-tube structures (SFTSs) because of the small span-to-depth ratios of the deep beams, leading to the low ductility and energy dissipation capacities of the SFTSs. High-strength steel framed-tube structures with replaceable shear links (HSSFTSRSLs) are proposed to address this issue. A replaceable shear link is located at the mid-span of a deep spandrel beam to act as a ductile fuse to dissipate the seismic energy in HSSFTS-RSLs. A 2/3-scaled HSSFTS-RSL specimen with a shear link fabricated of high-strength low-alloy Q355 structural steel was created, and a cyclic loading test was performed to study the hysteresis behaviors of this specimen. The test results were compared to the specimens with soft steel shear links in previous studies to investigate the feasibility of using high-strength low-alloy steel for shear links in HSSFTS-RSLs. The effects of link web stiffener spaces on the cyclic performance of the HSSFTS-RSLs with Q355 steel shear links were investigated based on the nonlinear numerical analysis. The test results indicate that the specimen with a Q355 steel shear link exhibited a reliable and stable seismic performance. If the maximum interstory drift of HSSFTS-RSL is designed lower than 2% under earthquakes, the HSSFTS-RSLs with Q355 steel shear links can have similar seismic performance to the structures with soft steel shear links, even though these shear links have similar shear and flexural strength. For the Q355 steel shear links with web height-tothickness ratios higher than 30.7 in HSSFTS-RSLs, it is suggested that the maximum intermediate web stiffener space is decreased by 15% from the allowable space for the shear link in AISC341-16 due to the analytical results.

플랜트 엔지니어링을 위한 BCC-Fe 기반 저합금강의 기계적 및 열팽창 특성 합금 효과: Ab Initio 계산

김명재,곽종욱,김지웅,김경남 한국재료학회 2023 한국재료학회지 Vol.33 No.10

High-strength low-alloy steel is one of the widely used materials in onshore and offshore plant engineering. We investigated the alloying effect of solute atoms in α-Fe based alloy using ab initio calculations. Empirical equations were used to establish the effect of alloying on the Vicker’s hardness, screw energy coefficient, and edge dislocation energy coefficient of the steel. Screw and edge energy coefficients were improved by the addition of V and Cr solute atoms. In addition, the addition of trace quantities of V, Cr, and Mn enhanced abrasion resistance. Solute atoms and contents with excellent mechanical properties were selected and their thermal conductivity and thermal expansion behavior were investigated. The addition of Cr atom is expected to form alloys with low thermal conductivity and thermal expansion coefficient. This study provides a better understanding of the state-of-the-art research in low-alloy steel and can be used to guide researchers to explore and develop α-Fe based alloys with improved properties, that can be fabricated in smart and cost-effective manners.

Hot Deformation Behaviors and Process Parameters Optimization of Low-Density High-Strength Fe–Mn–Al–C Alloy Steel

Peng Wan,Huixiang Yu,Feng Li,Pengfei Gao,Lei Zhang,Zhengzhi Zhao 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.10

The hot deformation behavior of low-density high-strength Fe–Mn–Al–C alloy steel at T = 900-1150 °C and ̇ = 0.01-10 s−1was studied by the Gleeble-3500 thermo-mechanical simulator. The rheological stress curve characteristics of the steel wereanalyzed through experimental data, and a physical constitutive model considering strain coupling was established. At thesame time, the finite element software DEFORM was used to calculate the critical damage value of the steel, and the influenceof T and ̇ on the maximum damage value was considered. By introducing the dimensionless parameter Zener–Hollomon, thecritical damage model was established. Finally, the workability of the steel was evaluated by using the intuitive processingmap technology. The results indicated that Fe–Mn–Al–C alloy steel is a positive strain rate-sensitive and a negative temperature-sensitive material, and the constitutive model considering physical parameters can well predict the rheological stress ofthe steel during hot deformation (R = 0.997). The critical damage factor of Fe–Mn–Al–C alloy steel varies with the changeof T and ̇ , and the range is 0.359-0.535. At the same time, the critical damage factor is more sensitive to ̇ . At a constantT, the damage factor decreases with the increase of ̇ . Based on the Prasad instability criterion, the dynamic material modelprocessing map and the microstructure verification after thermal compression, the rheological instability characteristics ofthe steel are mainly mechanical instability and local plastic flow, and the stable deformation area is mainly characterized bydynamic recrystallization. The optimal hot working process window of the steel is 975-1050 °C/0.01-0.032 s−1.

Surface modification and induced ultra high surface hardness by nitrogen ion implantation of low alloy steel

A.O. OLOFINJANA,Z. CHEN,J.M. BELL 한국트라이볼로지학회 2002 한국트라이볼로지학회 학술대회 Vol.2002 No.10

A surface hardenable low alloy carbon steel was implanted with medium energy (20 - 50KeV) N₂? ions to produced a modified hardened surface. The implantation conditions were varied and are given in several doses. The surface hardness of treated and untreated steels were measured using depth sensing ultra micro indentation system (UMIS). It is shown that the hardness of nitrogen ion implanted steels varied from 20 to 50㎬ depending on the implantation conditions and the doses of implantation. The structure of the modified surfaces was examined by X-ray photoelectron spectroscopy (XPS). It was found that the high hardness on the implanted surfaces was as a result of formation of non-equilibrium nitrides. High-resolution XPS studies indicated that the nitride formers were essentially C and Si from the alloy steel. The result suggests that the ion implantation provided the conditions for a preferential formation of C and Si nitrides. The combination of evidences from nano-indentation and XPS, provided a strong evidence for the existence of sp³ type of bonding in a suspected (C,si)xNy stoichiometry. The formation of ultra hard surface from relatively cheap low alloy steel has significant implication for wear resistance implanted low alloy steels.

Effect of Intercritical Heat Treatment on J-R Fracture Resistance of SA508 Gr.1A Low-Alloy Steels

Se‑Mi Hyun,Seokmin Hong,Min‑Chul Kim,Jongmin Kim,Seok Su Sohn 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.12

SA508 Gr.1A low-alloy steel is used for main steam line piping in nuclear power plants. The yield strength and J-R fractureresistance of the piping material must be high in order to apply the leak-before-break concept. In this study, intercritical heattreatment (IHT) was applied to two SA508 Gr.1A low-alloy steels having different chemical compositions, namely, samplesC and P; the latter containing Mo and V. Their microstructures, tensile properties, impact properties, and J-R fractureresistances were evaluated, and the effects of IHT on the mechanical properties of the steels were analyzed. After IHT, finegrains formed at grain boundaries and coarse cementite decomposed, which greatly improved toughness without reducingthe strength. Further, a model to predict the J-R fracture resistance (JIc) of the SA508 Gr.1A low-alloy steel was developedby considering the microstructural and mechanical factors that affect the J-R fracture resistance. According to the JIc resultsof 12 kinds of SA508 Gr.1A low alloy steel, subjected to different heat treatments, the JIc was linearly proportional to thenumber of effective grains (N) contained in the plastic zone, which showed that N is the main factor affecting JIc. Furthermore,the yield and tensile strengths were considered to compensate for the strength difference owing to the different chemicalcompositions. The JIc prediction model was derived considering the effective grain size, plastic zone size, and tensile properties. The predicted JIc values agreed well with the test JIc values.

Effects of microalloying element addition on mechanical properties of SA508 Gr.1A low-alloy steels

현세미,김민철,홍석민,김종민,손석수 한국원자력학회 2024 Nuclear Engineering and Technology Vol.56 No.9

SA508 Gr.1A low-alloy steel is being considered as a candidate material for main steam line piping in nuclear power plants. Therefore, improving its strength and toughness is essential for enhancing the leak-before-break (LBB) margin. In this study, six types of model alloys were fabricated by varying the contents of microalloying elements (C, Cu, B, Ti, and Nb) to enhance the mechanical properties of the specimens. The addition of a few ppm of B led to the formation of a fine-grained low-temperature transformation microstructure, resulting in the highest strength among the model alloys. However, the addition of Nb and Ti increased the formation of coarse ferrite, significantly decreasing the strength of the alloys. Reducing the C content while adding a small amount of B simultaneously maintained strength and enhanced toughness. Furthermore, the LBB margins of model alloys and commercial steel were evaluated to validate the influence of varying microalloying content. The model alloys exhibited a substantial increase in yield strength and fracture resistance, resulting in a more than 10% increase in the LBB margin. Notably, the LBB margin of the alloy with 15 ppm B was 1.39, approximately 25% higher than that of commercial steels.