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

Tetsu-to-Hagané Vol. 104 (2018), No. 7

  • Influence of Transformation Pseudoelasticity and Accumulated Plastic Strain on Low Cycle Fatigue Characteristics of Fe-30Mn-4Si-2Al Alloy

    pp. 393-399

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    DOI:10.2355/tetsutohagane.TETSU-2017-086

    Fe-30Mn-4Si-2Al alloy (mass%) was reported to show excellent low cycle fatigue properties. We investigated fatigue characteristics of the Fe-30Mn-4Si-2Al alloy as a function of accumulative plastic strains, comparing with the low cycle fatigue test results of Fe-28Mn-6Si-Cr-0.5Nb C alloy and SUS304 steel. The obtained results are shown below. The fatigue life of Fe-30Mn-4Si-2Al alloy is the longest in all the strain ranges as compared with Fe-28Mn-6Si-5Cr-0.5NbC alloy and the SUS 304 steel. In particular, it has a long life in test of high strain amplitude. The εpa - Nf characteristics of Fe-30Mn-4Si-2Al alloy show a straight relationship (εpa = Cp/NfKp). The result that the Manson-Coffin rule holds was obtained. In addition, Cp = 5.62, Kp = 0.72, which is an extremely high value. The fatigue damage value D obtained from the Manson-Coffin equation of Fe-30Mn-4Si-2A alloy was almost 1, similar to Fe-28Mn-6Si-5Cr-0.5NbC alloy or SUS 304 steel. However, the relationship between the accumulative plastic strain λp and fatigue life N is much higher than the limit λp of Fe-28Mn-6Si-5Cr-0.5NbC alloy obtained in the previous report. In particular, the results of εta = 2.0% and 1.4% were 20 times the limit λp. It was found that the excellent low cycle fatigue life of Fe-30Mn-4Si-2Al alloy is caused by the much slower accumulation of plastic strain and the extremely high values of Cp and Kp. The above results show that the repetitive motion of partial dislocation progresses slowly as ε martensite repeats normal and reverse transformation and the developmental process of repeated deformed tissue and fatigue crack propagates in a zigzag along γ/ε interface. As a result, it agrees with previously reported fact that crack growth is suppressed.
  • Two-dimensional Observation of Surface Layer Formed on Chemically-treated Aluminum Alloy Using Laser-induced Breakdown Plasma Optical Emission Spectrometry with Scanning Laser Beam

    pp. 358-362

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    DOI:10.2355/tetsutohagane.TETSU-2018-007

    Laser-beam scanning – laser-induced breakdown plasma optical emission spectrometry was applied to obtain the elemental distribution in three-dimensional direction for different surface states corresponding to chemically-pretreatments of an Al-alloy sample. The distribution of magnesium oxide inclusion particles could be estimated from the intensity ratio of Mg/Al each for the irradiation points, indicating that chemical-etching and desmutting treatments made the magnesia particles to be reduced both in the lateral and in-depth directions. The resolution of our measurement was within 200 μm in the lateral position and ca. 15 μm in the depth direction, which were mainly determined by the crater size of the laser shot. The objective of this paper is to suggest an analytical technique suitable for evaluating the spatial distribution of inclusion or contaminant particles on the surface rapidly during the chemical surface pretreatment of Al-alloy materials.
  • Effect of Cr Content on Softening during Aging in High Cr Ferritic Heat Resistant Steel

    pp. 377-384

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    DOI:10.2355/tetsutohagane.TETSU-2017-102

    In order to clarify the effect of Cr content on creep strength in high Cr ferritic steels for turbine rotors, a series of experiments is carried out using the steels in the tampered state and after simple aging at 650°C. In the tempered state, the Vickers hardness of 10.5 Cr steel is higher than that of 9 Cr steel, but it decreases remarkably in 10.5 Cr steel and becomes almost the same value in both steels at after aging for 6700 h. In the tempered state, 10.5 Cr steel shows large number density and small average particle diameter of the fine Laves phase compared with 9 Cr steel, and this condition continues even after aging for 1000 h. However, M23C6 carbide and the Laves phase grow with increasing aging time and their average particle sizes in 10.5 Cr steel become greater than those in 9 Cr steel after aging for over several thousand hours. In 10.5 Cr steel, the amounts of carbonitrides such as NbX, Cr2X, and VX decrease after 3000 to 6000 h due to precipitation of the Z phase, whereas in 9 Cr steel, carbonitrides are still present beyond 6000 h due to retardation of the Z phase precipitation.
  • Accurate Online Measurement of Iron Concentration in Galvannealed Coating Layers by Shift of X-ray Diffraction Peak

    pp. 363-368

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    DOI:10.2355/tetsutohagane.TETSU-2018-019

    The authors have developed a new online measurement system for the Fe concentration in galvannealed (GA) coating layers based on the proportionality between the X-ray diffraction (XRD) peak angle of the δ1 phase and the Fe concentration. The online system was realized by high speed measurement of the XRD peak angle by a one-dimensional detector and use of a correction algorithm for the angular error caused by the displacement of the GA strip surface from the measurement position. The XRD profile and displacement were simultaneously measured by the one-dimensional detector and a laser displacement meter, respectively. The high accuracy of this new online system was demonstrated in a continuous galvanizing line with GA steel strips of Si-added and Si-free base steels. The developed system showed sufficiently high accuracy even with the Si-added base steel, with which adequate accuracy could not be obtained with conventional online systems, based on the diffracted intensity of the Γ phase, due to the suppression of Γ phase formation. Since the δ1 phase is the major phase of the GA layers, the main advantage of the developed system is that its accuracy is less sensitive to the composition of the base steel. The developed system can be applied to other online measurement applications in the steel industry for various purposes such as measurement of phase transition, orientation, crystallinity, strain and other features.
  • Friction Stir Welding of Medium Carbon Steel with Laser-Preheating

    pp. 369-376

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    DOI:10.2355/tetsutohagane.TETSU-2018-001

    Friction Stir Welding (FSW) has expanded to many metallic materials with higher melting points or much higher strength than the aluminum alloys. If the tool travels too quickly along the welding seam during the welding process or if the melting point of the workpiece is high, the frictional heat generated between the tool and the workpiece may not be sufficient to cause material flow. Insufficient heat input results in the formation of groove or tunnel-shaped defects in the stir zone and also severe wear or breaking of the FSW tool. To solve these problems, a higher heat input is required to soften the materials. Therefore, several preheating methods have been adopted to increase the heat input. In this study, a fiber laser was used as the preheating source during the FSW. In this experiment, the effect of the laser-preheating on the defect formation and tool rotational torque during the FSW was investigated. Additionally, a difference in the material flow during the conventional FSW and laser-preheating FSW was observed by two pairs of x-ray transmission real-time imaging systems. As a result, it was found that the laser preheating reduced the defect formation and the tool rotational torque during the FSW. Furthermore, laser beam irradiation on the retreating side (RS) was the most effective in reducing the defect formation. On the other hand, the irradiation on the advancing side (AS) was the most effective in reducing the tool rotational torque.
  • Evaluation of Coke Degradation Effect on Flow Characteristics in Packed Bed Using 3D Scanning for Rotational Mechanical Strength Test and Solid-liquid-gas Three-phase Dynamic Model Analysis

    pp. 347-357

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    DOI:10.2355/tetsutohagane.TETSU-2018-005

    A 3D scanning technique was applied for understanding coke shapes obtained by a rotational strength test, and a numerical dynamic analysis based on the multi-sphere type discrete element method was carried out to clarify the influence of coke degradation on the packed structure. We constructed a trickle flow simulation of molten slag via the smoothed particle hydrodynamics model, and the liquid-gas permeability characteristics exhibited by the coke shapes in the lower part of the blast furnace are discussed accordingly. Coke diameter decreased due to collisions between particles, via the progress of surface- and volume-destruction, and that the particles subsequently became sphere-like in shape. Static holdup of molten slag showed a decreasing tendency with the coke degradation progress, as the void shape and holdup site became spatially uniform as sphericity increased. In the case of packed bed formed by the initial low sphericity or large-sized cokes, the size of the air gap was maintained, although the flow path was non-uniform. Therefore, even if the large amount of holdup did not block the gaseous main flow, in the case of lower coke strength, the sphericity increased due to the deformation progress, and void uniformity could be retained. However, the existence of many narrow void regions remarkably decreased the gas permeability.
  • Effect of Chromium Addition on the Coarsening of Cu-particle in Ferritic Stainless Steels

    pp. 385-392

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    DOI:10.2355/tetsutohagane.TETSU-2018-003

    Coarsening of Cu-particles in 11.4%Cr-1.48%Cu (11Cr steel) and 23.7%Cr-1.48%Cu (24Cr steel) ferritic stainless steels were investigated mainly using transmission electron microscopy and energy dispersive X-ray spectrometry. The samples were solution-treated at 1250°C and then isothermally aged at 800°C for up to 604.8 ks. Precipitation hardening in steels occurred due to precipitation of Cu-particles during aging. Decrease of hardness of 24Cr steel associated with coarsening of Cu-particles occurred slower than that of 11Cr steel during aging. The normalized standard deviation of the size distribution of Cu-particles increased and reached saturation at a certain value during aging. The kinetic analysis for coarsening of Cu-particles showed that the interfacial energy of Cu-particles/matrix of 24Cr steel was lower than that of 11Cr steel, while diffusion coefficient of the steels did not depend on chromium concentration. The segregation of chromium formed at the interface of Cu-particles/matrix may decrease interfacial energy of Cu-particles resulting in retardation of coarsening of Cu particles.

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