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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Advance Publication

  • Effect of Humidity of Air on Hydrogen Absorption into Fe with Rust Layer Containing MgCl2 during Atmospheric Corrosion

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    DOI:10.2355/tetsutohagane.TETSU-2021-081

    An Fe plate, whose one side was electro-polished and the other was covered with the rust layer containing 25.7 g·m2 MgCl2, was used as the specimen to investigate the effect of humidity on the hydrogen absorption of the plate during atmospheric corrosion. The specimen was subjected to an electrochemical hydrogen-absorption test during which the rusted surface was exposed to the air with controlled relative humidity (RH) and atmospheric corrosion occurred on it. When the rusted surface was subjected to dry (RH 0%)–wet (RH 27%) cycle tests for 10.8 ks each, the anodic current density corresponding to the hydrogen-absorption rate was measured on the hydrogen detection surface. The maximum current density was almost independent of the cycle during the first 10 cycles, after which it decreased with an increase in the cycle, reaching almost a steady value after about 40 cycles. After 55 cycles of the dry–wet cycle test, the specimen was subjected to an electrochemical hydrogen-absorption test to obtain the relationship between the steady-state hydrogen-absorption rate and RH. Hydrogen absorption was observed at RH over about 15%, and the absorption rate increased rapidly with an increase in RH, reached a maximum at RH of about 30%, and then decreased rapidly. When RH increased beyond 40%, the absorption rate increased again, reached a maximum value at RH of 80%, and then decreased gradually. The specimen with the rust layer containing 39.8 g·m2 MgCl2 also showed two peaks in the hydrogen-absorption rate versus RH plot.
  • Hydrogen Absorption into Fe Plates with Rust Layers Containing Various MgCl2 Amounts during Atmospheric Corrosion with Controlled Humidity

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    DOI:10.2355/tetsutohagane.TETSU-2021-082

    Fe plates with rust layers containing various MgCl2 amounts were prepared as specimens. Each specimen was subjected to dry−wet repeated cycle test beyond 50 cycles, and then subjected to electrochemical hydrogen absorption test under atmospheric corrosion in the air with controlled relative humidity (RH). For an MgCl2 amount of 39.8 g·m2, a hydrogen absorption rate (iH) started to increase from an RH around 15%, steeply increased with an increase in RH up to about 30%, steeply decreased up to about 35%, gradually increased up to 65% and gradually decreased up to about 92%. A decrease in MgCl2 amount in the range between 0.514 and 39.8 g·m2 induced a decrease in iH in the whole RH range. The maximum iH at an RH around 30% increased with an increase in MgCl2 amount in the rust layer. Besides, the RH where the maximum iH was obtained beyond an RH of 40% increased with a decrease in MgCl2 amount. From theoretical relationship between RH and thickness of MgCl2 solution film on the Fe plate without rust layer, it is found that the solution film thicknesses at the RHs were about 0.18 mm, almost independent of MgCl2 amount. In addition, thicknesses of the rust layers containing 25.7 and 39.8 g·m2 MgCl2 were measured to be almost 0.18 mm each other. The trends of iH depending on MgCl2 amount were tried to be explained using nature of deliquescence for MgCl2.
  • Evaluation of Cleavage Fracture Behavior of C14 Fe2W Laves Phase by First Principle Calculation and Crystal Orientation Analysis

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    DOI:10.2355/tetsutohagane.TETSU-2021-069

    Cleavage fracture of C14 Fe2W Laves phase was investigated using crystal orientation measurement with scanning electron microscopy and first-principles calculations. Trace analysis of the orientations of cleavage planes reveals that cleavage fracture occurred at five types of crystal planes of (0001), {1100}, {1120}, {1101} and {1122}, among which the fracture at (0001) is the most preferable. The first-principle calculations of the surface energy for fracture, Young's modulus, and Poisson's ratio showed that the minimum fracture toughness value of 1.62 MPa·m1/2 was obtained at (0001). The tendency that the values of calculated fracture toughness become larger with the higher indexed planes is almost the same as the frequency of the types of cleavage planes in the trace analysis. It is concluded that the fracture toughness of C14 Fe2W Laves phase is controlled by the surface energy for fracture and Young’s modulus.
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  • Internal and External Hydrogen-related Loss of Ductility in a Ni-based Superalloy 718 and Its Temperature Dependence

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    DOI:10.2355/tetsutohagane.TETSU-2021-056

    Toward a better understanding of the hydrogen embrittlement characteristics in nickel-based superalloy 718, tensile tests were performed under hydrogen pre-charged states (internal hydrogen) as well as in hydrogen gas environment (external hydrogen) at various temperatures ranging from −196 to 300°C. Under the internal hydrogen conditions, hydrogen-induced loss of ductility was maximized at around 25°C, while it was recovered with increasing/decreasing test temperature and almost fully mitigated particularly at −196°C. On the other hand, under the external hydrogen conditions, deleterious impact of hydrogen on the ductility monotonically increased with temperature elevation. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analyses on post-mortem samples revealed that the microstructural initiation sites of hydrogen-induced micro-cracks in internal hydrogen states were annealing twin boundaries or crystallographic slip planes (i.e., {111} planes) at −40~300°C wherein the loss of ductility was substantial, albeit intergranular fracture prevailed at −196°C, accompanying minimum embrittlement effect. Meanwhile, in the case of external hydrogen states, the fracture modes were transitioned from intergranular to slip plane cracking with increasing temperature in response to the augmentation of embrittlement magnitude. The rationales of these multiple hydrogen-related failure modes and their roles on macroscale material performance are discussed on the basis of hitherto-known, unique deformation mechanisms driving the plasticity in this alloy in addition to the hydrogen diffusion rate/pathways which are strongly dependent on temperature.
  • Development of Auto-Searching Method of Brittle Fracture Initiation Point Based on River-Pattern and Tear Ridge

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    DOI:10.2355/tetsutohagane.TETSU-2021-025

    A method to automatically search brittle fracture initiation point is proposed. The method calculates flow paths from each edge of an image to opposite side along flow potential. The potential is derived from the direction of river-pattern and tear ridge. The most concentrated position of flow paths is determined as a fracture initiation point. The method achieves 99% precision and high speed analysis for low magnification images of 206 thick plate samples. Furthermore, discriminating ductile fracture by machine learning and excluding noise potential obtained from it, and considering local feature of river-pattern that spread radially near a fracture initiation point, the method precision is improved for high magnification images. Using fractal feature of brittle fracture and repeatedly applying the method, a fracture initiation point is determined within the size of a single grain level.
  • Carbon Enrichment of Austenite during Ferrite-bainite Transformation in Low-alloy-steel

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    DOI:10.2355/tetsutohagane.TETSU-2021-045

    The bainitic transformation kinetics and carbon enrichment of austenite during isothermal holding at 723–923 K were investigated for an Fe-0.1mass%C-0.5mass%Si-2.0mass%Mn alloy. The transformation progressed rapidly until approximately 50 s, after which transformation stasis was observed at 823 K. The carbon concentration of austenite increased as the transformation proceeded, and showed an almost constant value during stasis. It reached approximately 0.45-0.50% at 823 K, which corresponds to the carbon concentration at the T0’ composition with an additional strain energy of 100 J/mol associated with the transformation. After stasis, a slight increase in the ferrite or bainitic ferrite fraction was observed. The carbon concentration of austenite also increased and reached approximately 0.60%, clearly exceeding the carbon concentration at the T0 composition. These results imply that at the first stage, the bainite transformation occurs and shows the incomplete transformation, following which at the second stage, diffusional ferrite transformation proceeds. The additional strain energy associated with the transformation calculated from the carbon concentration at stasis due to the incomplete bainite transformation tends to decrease as the holding temperature increases. This indicates that strain relaxation due to the transformation occurred at higher holding temperatures.
  • Crystal Plasticity Analysis of the Differential Hardening Behavior under Proportional Loading Path of the Steel Sheet

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    DOI:10.2355/tetsutohagane.TETSU-2021-066

    To enhance the accuracy of sheet forming simulation, applying a material model based on a physical understanding that enables the description of material behavior under multi-axis stress is beneficial. To achieve this, it is necessary to clarify the work hardening behavior of the material under multi-axis stress and its mechanism. It is especially known that steel sheets for deep drawing with an increased r value have different degrees of work hardening under uniaxial and biaxial stresses, which is called anisotropic work hardening. Anisotropic work hardening is considered to be brought about mainly by a texture or dislocation cell structure, but details are unknown. This study thus discusses the physical mechanism using the crystal plasticity finite element method.The crystal plasticity finite element method was executed with the model that Hoc et al. developed by modeling the accumulation of dislocation. In the analysis, the anisotropic work hardening was reproduced where the equal plastic work surface stuck out around the equal biaxial stress. It is presumed that the anisotropic work hardening occurred because the equal biaxial stress had more slip systems than the uniaxial stress, and eventually had more latent hardening. It was confirmed by changing the crystal orientation virtually that anisotropic work hardening behavior depends strongly on texture. From this, it is concluded that ferrite steel materials have different numbers of active slip systems depending on the texture, and the amount of latent hardening varies accordingly, resulting in anisotropic work hardening.
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  • Mass Gain Rates of Various Steels During Atmospheric Corrosion Under Cyclic Conditions of Dry and Controlled-humidity Air

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    DOI:10.2355/tetsutohagane.TETSU-2021-062

    Mass gain rates of various steels during atmospheric corrosion under cyclic conditions of dry and controlled-humidity air were investigated. Fe, SBHS500 carbon steel and SMA490AW weathering steel were selected for the test materials. A droplet of MgCl2 solution was set on the plate steel specimen. The specimen was exposed in the humidity-controlled air for about 82.8 ks and in the dry air for 3.6 ks. After the process, an area as well as a volume of the corroded part, surface appearance and a mass of the specimen were recorded. The process was repeated over 5000 ks of an accumulated wet time. Selected values of relative humidity were 75, 43 and 33%. For the Fe specimen, the condition of RH75% induced Region(i), (ii) and (iii), where Region(i) is the period in which a corroded area was independent of time and a mass of the specimen linearly increased with time, Region(ii) is the period in which an area and a mass increased more rapidly, and Region(iii) is the period in which increments of an area and a mass relatively decrease. The conditions of RH43% induced only the Region(i) and RH33% provided Region(ii) in addition to (i). A mass gain rate in Region(i) was larger under RH75% than those under RH43 and 33%. For the condition of RH75%, any of the three specimens showed the three regions, and a mass gain rate in Region(i) was a maximum for SBHS500 steel and a minimum for Fe.
  • Micro-electrochemical Properties and Pitting Corrosion Resistance of Microstructures of Carbon Steels

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    DOI:10.2355/tetsutohagane.TETSU-2021-032

    This paper presents an introduction of the relationship between the electrochemical properties of microstructures and pitting corrosion resistance of carbon steels in chloride-containing near-neutral pH environments. Recent investigations by micro-scale electrochemical measurements have been demonstrated that the pitting corrosion resistance of typical microstructures was ordered as follows: (high) as-quenched martensite > primary ferrite > pearlite (low). In the case of pearlite, it has been reported that pits proceeded along the lamellar structure consisted of Fe3C and ferrite. On the other hand, in the case of martensite, according to the studies based on the first-principles calculations, it has been proposed that the superior corrosion resistance was related with the electronic interaction between Fe and interstitial C. It was reported that the electronic density of states of Fe around the Fermi level decreased by the presence of interstitial C.
  • Role of CaS Inclusions in Pitting Initiation of Carbon Steel: Triggering Steel Depassivation

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    DOI:10.2355/tetsutohagane.TETSU-2021-009

    tting occurred after the wet-dry corrosion test, and calcium and sulfur were detected near the center of the pit. From the results of the microscale polarization measurements, the pitting initiation sites for the SBHS500 steel were determined to be the CaS inclusions. No pitting was observed at the microscale electrode area without inclusions. In a boric-borate buffer solution containing 10 mM NaCl, the depassivation pH at the microscale electrode area without inclusions was 6.0. The depassivation at the microscale electrode area with the CaS inclusions occurred at approximately pH 6.6. The CaS inclusions in the SBHS500 steel were found to be a trigger of the depassivation of the steel matrix surrounding the inclusions.

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