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

Tetsu-to-Hagané Advance Publication

  • Crack Initiation and Propagation Behavior of Hydrogen-induced Quasi-cleavage Fracturein X80 Pipeline Steel with Stress Concentration

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    DOI:10.2355/tetsutohagane.TETSU-2019-126

    The processes leading to hydrogen-related fracture in X80 pipeline steel with stress concentration have been investigated comprehensively through observations of fracture surfaces and subsidiary cracks, a stress analysis, crack initiation and propagation analyses and a crystallographic analysis of fracture surfaces. Fracture morphology showed quasi-cleavage (QC) fracture under various amounts of hydrogen. It was found that QC cracks initiated in the area ranging from the notch tip to 100 μm inside based on interrupted tensile tests just before fracture strength with hydrogen charging. Moreover, fracture surface topography analysis (FRASTA) revealed that QC cracks initiated at the notch tip. A finite element analysis indicated that the equivalent plastic strain was maximum at the crack initiation site at the notch tip. In addition, a backscattered electron image showed that nanovoids of 50-250 μm in diameter were present near the initiation site. Regarding the crack propagation process, field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD) and FRASTA results indicated that some microcracks in ferrite grains coalesced stepwise and propagated. Trace analyses using EBSD revealed that the QC fracture surface consisted of {011} slip planes, {001} cleavage planes and non-specific index planes. These findings indicate that QC fracture initiates at the notch tip due to the interaction between dislocations and hydrogen associated with local plastic deformation, and propagates stepwise by coalescence through vacancies, nanovoids and microcracks on various planes associated with/without plastic deformation in ferrite grains.
  • Material Modeling of Hot-Rolled Steel Sheet Considering Differential Hardening and Hole Expansion Simulation

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    DOI:10.2355/tetsutohagane.TETSU-2020-002

    The elastic-plastic deformation behavior of a 440 MPa hot-rolled steel sheet subjected to many linear stress paths is precisely measured using biaxial tensile tests with cruciform specimens (ISO 16842: 2014) and multiaxial tube expansion tests (Kuwabara and Sugawara, 2013) to determine appropriate material models for finite element analysis (FEA). It was found that the Yld2000-2d yield function (Barlat et al., 2003) correctly reproduces the contours of plastic work (CPW) and the directions of the plastic strain rates (DPSR). Differential hardening (DH) models are determined by changing the values of exponent and material parameters of the Yld2000-2d yield function as functions of reference plastic strain. Moreover, FEA of the hole expansion forming of the test material is performed. The DH model correctly predicts the minimum thickness position that matches the fracture position of the specimen in experiment.
  • Mutual Verification of Phase Fraction Analysis Techniques for Steels Comprising Deformation Induced Martensite Phases: Neutron-Diffraction-Based Rietveld Texture Analysis and Saturation Magnetization Measurement

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    DOI:10.2355/tetsutohagane.TETSU-2019-103

    The demand for a reliable and quantitative method to determine phase fractions has been increasing due to the developments of multi-phase materials, such as TRIP steels. The authors conducted a mutual verification between the two methods for phase fraction analysis, the saturation magnetization measurement and the newly developed neutron diffraction technique, neutron-diffraction-based Rietveld texture analysis (NDRTA). The chemical compositions of the current samples were Fe-18Cr-8Ni-1Mn-0.5Si (mass%) with 0, 0.1 or 0.2 mass% of C or N. The α’-martensite volume fractions analyzed by both methods showed a good linear correspondence. The analysis based on the saturation magnetization measurement required an accurate evaluation of the volume saturation magnetization of α’-martensite, which was a function of the chemical composition. The comparison with the result of NDRTA can be an effective method to calibrate the volume saturate magnetization of α’-martensite, especially in the case that a fully transformed standard sample cannot be obtained. NDRTA is also an effective method to determine the fraction of ε-martensite, which is non-magnetic and has a hexagonal close-packed (hcp) structure. Since the hcp phase tends to develop a sharp texture, the conventional X-ray diffraction method without care of texture underestimated its volume fraction. Hence, the simultaneous evaluation of volume fraction and texture by NDRTA is the optimum method to determine the fraction of ε-martensite.
  • Dehydration and Hydration Reactivity of Citrate-Added Mg(OH)2 for Thermo-chemical Energy Storage

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    DOI:10.2355/tetsutohagane.TETSU-2019-115

    This research focuses on dehydration / hydration of magnesium hydroxide as a chemical heat storage material. Previous studies have reported that the use of additives in magnesium hydroxide improved the dehydration / hydration reactivity. However, additives used in previous studies have had problems in terms of environmental impact and cost. Therefore, the purpose of this study is to search for safe and inexpensive additives. We have selected citrate compounds as an inexpensive and safe additive. The effect of the additive was verified by measuring the dehydration / hydration reaction of magnesium hydroxide using a thermogravimetric instrument. Furthermore, XRD was used for sample characterization. As a result, the most improved reactivity was confirmed in the sample using sodium citrate as an additive. SC5 (molar ratio, magnesium hydroxide : sodium citrate dihydrate = 100 : 5) decreased the dehydration peak temperature by about 31ºC compared to pure magnesium hydroxide. Sodium citrate dihydrate was found to undergo thermal degradation during sample heating. Then, when the repeated reaction test was implemented, the improvement of the dehydration rate after the 2nd time was confirmed. These results indicate that the product of thermal decomposition of sodium citrate dihydrate is effective as an additive.
  • Effect of Environmental Factors on Hydrogen Absorption into Steel Sheet under a Wet-dry Cyclic Corrosion Condition

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    DOI:10.2355/tetsutohagane.TETSU-2019-134

    Effect of temperature and chloride deposition on hydrogen absorption into steel was evaluated under wet-dry cyclic corrosion conditions by using a temperature compensated hydrogen absorption monitoring system which is based on electrochemical hydrogen permeation method. Peaks of hydrogen permeation current were detected during the wetting and drying periods in the wet-dry cyclic corrosion conditions. Hydrogen absorption was increased with increasing temperature and chloride deposition. It was suggested that the hydrogen absorption behavior under the wet-dry cyclic corrosion conditions is related to the change in solution chemistry during the wetting and drying periods where the increase of chloride ion concentration and the decrease in pH due to hydrolysis reaction of Fe3+ occurred. Based on these results, the amount of absorbed hydrogen map effected by temperature and chloride deposition in atmospheric corrosion environment was described.
  • Experimental Evaluation of Texture Change during Grain Growth in Electrical Steel Sheets and Its Prediction by Phase Field Simulation

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    DOI:10.2355/tetsutohagane.TETSU-2019-117

    Electrical steel sheets require an increase in grain diameter in order to reduce iron loss. Texture changes during grain growth also affect iron loss. Therefore, it is important for the improvement in magnetic properties to control texture changes during grain growth. Especially, the texture prediction from the initial recrystallized structure is industrially useful. Our goal is the texture prediction by phase field simulation method. In this study, we first investigated experimentally the texture change during grain growth in Fe-0.5%Si and Fe-3.3%Si steels to get the systematic knowledge and the mechanism behind. Then, experimental results were compared with the predicted ones obtained by exploiting the multi-phase field (MPF) simulation.In the experimental results, in Fe-0.5%Si alloy, {111}<112> component further developed during grain growth. While in the case of Fe-3.3%Si alloy, {411}<148> component significantly developed by consuming {111}<112> component during grain growth. In both cases, the mechanism for the texture change during grain growth could be commonly explained by size advantage. The MPF simulation for both cases succeeded in reproducing the experimental results in terms of the texture changes during grain growth. However, the simulated texture changes were slightly smaller than that of experiment, presumably due to the difference in dimension; i.e. two dimension in MPF simulation and three dimension in experiment. Thus, the validity of the prediction of texture change exploiting MPF simulation was verified.
  • Dependence of Carbon Concentration and Alloying Elements on the Stability of Iron Carbides

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    DOI:10.2355/tetsutohagane.TETSU-2019-096

    The precipitation of iron carbides is a crucial factor that determines the properties of tempered martensite. However, the effect of alloying elements on the carbon concentration of ε carbide has not yet been clarified. In this work, we studied the effect of alloying elements on the carbon concentration of ε carbide using first-principles calculations and a three-dimensional atom probe. The first-principles calculations showed that ε carbide with a lower carbon concentration tends to form by the inclusion of Si. The carbon concentration in ε carbide measured by the three-dimensional atom probe was consistent with the first-principles calculations.
  • Thermal Stability of Resistance to Propagation of Mechanically Small Fatigue-cracks in a Fe-N Binary Ferritic Steel

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    DOI:10.2355/tetsutohagane.TETSU-2019-091

    We investigated the effect of solute nitrogen on threshold stress intensity factor range, ΔKth, of the growth of small cracks using a water-quenched Fe-0.011N (wt.%) binary alloy, in terms of strain-age hardening. Fatigue tests were carried out for micro-notched specimens at 20°C and 160°C at a frequency of 30 Hz with a stress ratio of –1. The nitrogen effect on ΔKth at room temperature was significant, but smaller than the carbon effect. On the other hand, the thermal stability of the strain aging effect on ΔKth was higher in the Fe-0.011N steel than in Fe-C steels containing supersaturated carbon, because the nitrogen solubility above room temperature is higher than the carbon solubility in ferritic steels.
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