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

Tetsu-to-Hagané Vol. 108 (2022), No. 3

  • Viscosity of Na–Si–O–N–F Melts: Mixing Effect of Oxygen, Nitrogen and Fluorine

    pp. 175-190

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    DOI:10.2355/tetsutohagane.TETSU-2021-108

    Fluorine and nitrogen are important elements of metallurgical slags and fluxes. Studies on their viscosity have often focused on the additive effect of fluoride and nitride compounds (e.g., CaF2 and Si3N4), whereas the influence of anionic composition (i.e., oxygen, fluorine and nitrogen concentrations) with a fixed cationic composition remains unclear. The present study reports the scarcely quantified viscosity variations due to changes in the anionic composition of a simple sodium silicate system by rotating crucible method under a controlled atmosphere. 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to characterize the structural changes against varying nitrogen and fluorine concentrations in the quenched glassy sample. The observed change in the local silicon structure was consistent with the expected variation from the conventional structural roles of nitrogen and fluorine in silicate glasses: nitrogen atoms tend to bond with silicon atoms, whereas fluorine atoms prefer to exist in surrounding sodium cations. Moreover, nitrogen tends to increase the viscosity, whereas fluorine strongly decreases the viscosity of the sodium silicate melts even with the enhancing effect of the latter on the polymerization of silicate anions. The viscosity of silicate melts has been commonly related to the overall polymerization degree of the liquid. However, the viscosity of fluorine-containing silicates cannot be explained by this conventional scenario. Fluorine ions tend to loosely bond with sodium cations. These sodium–fluorine complexes played a strong lubricant role in the network liquids.
  • Comparison of Fatigue Crack Propagation Behavior of Additive Manufactured Zero Thermal Expansion Alloy with Forged and Casted Materials

    pp. 191-198

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    DOI:10.2355/tetsutohagane.TETSU-2021-084

    Fatigue crack propagation (FCP) behavior was studied on three kinds of zero thermal expansion (ZTE) alloy. The specimens manufactured by three different processes, i.e., casting, forging and laser additive manufacturing: selective laser melting (SLM), were employed. The FCP rates in casted alloy were almost same with SS400 which was used for comparison. In SLM product, FCP rates were higher than casted alloy and slightly higher than forged alloy especially in low ∆K region. The characterization of fracture surfaces was conducted by scanning electron microscope. Through the observation, rough crack surface was observed in casted alloy, on the other hand, small marks along the crack propagate direction were observed in SLM specimen. In order to consider the effective stress intensity factor range, the crack opening load was measured to estimate FCP behavior. In evaluation by the effective stress intensity factor range, FCP rates of the specimens with three kinds of manufacturing processes were similar. From these results, it is concluded that the ZTE alloy, manufactured by SLM showed enough performance in FCP examination compared with forged alloy. The difference in the results between the two products is attributed to the difference in the fracture surface due to the different microstructure.
  • Observation and Quantification of Crack Initiation from TiN Particle in a Ferrite-pearlite Steel

    pp. 199-210

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    DOI:10.2355/tetsutohagane.TETSU-2021-091

    This article shows a microscopic observation of the cracking of TiN particle and its quantification in ferrite-pearlite steels as a model case of the crack initiation in inclusions. Three model steels containing different amount of TiN particles were produced by vacuum melting, hot rolling, and heat treatment. The microstructures of model steels were evaluated by optical microscopy, image analysis, and EBSD analysis. From the results of Charpy impact test and fractography, it was confirmed that TiN particles behave as initiation sites of cleavage fracture in two of the three model steels. Tensile tests using circumferential notched round bar specimens were interrupted at a certain stroke condition to obtain deformed specimens with distributed strain and stress. By measuring TiN particles’ diameter and cracking under various strain and stress conditions on the deformed specimens, we made a dataset in which individual TiN particles’ cracking were associated with various information, i.e., TiN particles’ size, strain, and stress condition. Through the statistical analysis of the dataset, the probability of TiN particles’ crack initiation was formulated as a function of TiN particles’ diameter, macroscopic strain and stress.
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  • Mn Depletion Behavior at Oxide/matrix Interface in Low Oxygen Weld Metal of Low Carbon Steel

    pp. 211-223

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    DOI:10.2355/tetsutohagane.TETSU-2021-087

    The structure of the oxide as a nucleus for intragranular ferrite formation in the low O weld metal and the formation mechanism of Mn depleted zone (MDZ) at the oxide/matrix interface were studied using liquid-tin quenched specimens at high-temperature conditions during laser welding of low carbon Ti added steel. At a high temperature of 1720 K, MDZ is formed around the complex oxides of (Ti,Mn)3O5, (Ti,Mn)2O3, and liquid phase oxides (containing Si, Mn, Ti, and S). The width of the MDZ increases with cooling, and at low temperatures (1275 K) MDZ is formed around the complex oxide consisting mainly of (Ti,Mn)2O3 with MnS and Si-Mn oxides. These MDZs are formed all around the complex oxides, regardless of the kind of oxide. The formation of MDZs is considered to promote the ferrite transformation around the oxides. The equilibrium Mn concentration in each of the oxide phases increases during the cooling process and the thermodynamically stable phase changes from (Ti,Mn)3O5 with a low equilibrium Mn concentration to (Ti,Mn)2O3 with a high equilibrium Mn concentration, which drives the diffusion of Mn from the matrix phase to the oxide. In this process, MDZs are formed all around the complex oxides.
  • Formation of Vacancy-type Defects in Cold-drawn Pearlitic Steel during Tensile Deformation with Hydrogen Charging

    pp. 224-232

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    DOI:10.2355/tetsutohagane.TETSU-2021-101

    The effects of hydrogen charge on the formation of vacancy-type defects in cold-drawn pearlitic steel were investigated by using positron lifetime spectroscopy, and the correlation between vacancy-type defects and susceptibility to hydrogen embrittlement was discussed. Pearlitic steels aged at 300°C and 450°C after cold-drawing were used. Both tensile deformed and subsequently fractured samples were prepared by slow strain rate tests (SSRT) with and without cathodic hydrogen charge. Average positron lifetime increased with increasing tensile strain both in the case of the 300°C-aged and 450°C-aged steels. There is no remarkable effect of hydrogen charge on the average positron lifetime of both steels. The results mean applying tensile strain increases the amount of lattice defects such as dislocation or vacancy, although hydrogen has little effect. On the contrary, positron lifetime of vacancy cluster component was increased by hydrogen-charged SSRT, and an increase in hydrogen concentration in both steels promoted vacancy clustering furthermore. The 450°C-aged steel showed more remarkable vacancy clustering than the 300°C-aged steel, implying that the lamellar structure in the 300°C-aged steel prevented vacancy clustering. Fracture strains after SSRT were decreased by hydrogen charge due to hydrogen embrittlement, and an increase in hydrogen concentration decreased the fracture strain in both steels. Fracture strains of all tested samples with and without hydrogen charge showed a strong dependence on the degree of vacancy clustering. That means vacancy clustering has a big effect on fracturing of steels, and a detrimental effect of hydrogen is attributable to the promotion of the vacancy clustering.
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