Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 101 (2015), No. 7

  • Development of a Remote Defect Imaging System with the Scanning Laser Source Technique

    pp. 351-357

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    DOI:10.2355/tetsutohagane.101.351

    Laser ultrasonics have been studying for many years as a promising technique for evaluating industrial materials. The non-contact technique, however, still have some problems in practical use for large structures such as pipes, tanks, bridges, etc. Authors have adopted the scanning laser source technique for imaging defects in a plate-like structure to solve one of the problems that elastic wave cannot be measured stably with laser interferometory due to the unstable detection of scattering light at rough and inclined surfaces of existing structures. In this study, a remote experimental system of the defect imaging technique with the scanning laser source, which does not require cables between receiving transducers and experimental equipments, was developed. In the experimental system, laser emission signal detected by a photo-detector was used as trigger signal that requires quick response for accurate measurements. The other data that does not require such quick responses were transffered with local area network (LAN) communications. Using the remote defect imaging system, we confirmed that defect images can be obtained clearly as the conventional cablewired experimental system was used. Moreover, we obtained defect images at the distances of 2.6 m and 7.6 m between the plate specimen and laser equipment.
  • Convenient Debonding Strength Evaluation for Spray Coating Based on Intensity of Singular Stress

    pp. 358-364

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    DOI:10.2355/tetsutohagane.101.358

    Hearth rolls are used in continuous annealing furnace to produce thin steel sheet. The roll surface is usually coated by using thermal spraying, which has high adhesive strength and wears resistance. However, in the ceramics coating, thermal stress caused during heating and cooling process in the furnace may lead to debonding due to the low toughness of ceramics. In order to improve the heat resistance of the thermally sprayed coating, it is essential to evaluate the debonding strength. Generally, heat resistance of thermal spray coating is evaluated by thermal shock test prescribed by JIS H8304 although few research is available in terms of singular stress at the end of the interface for JIS specimen under thermal shock. This paper focuses on the intensity of the singular stress at the end of interface. Then, the most suitable conditions are discussed with varying the coating material and the coating thickness.
  • Numerical Analysis of Thermal Expansion of Rolls with Equivalent Heat Transfer Model on the Circumference

    pp. 365-371

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    DOI:10.2355/tetsutohagane.101.365

    In hot strip rolling, the work roll shift method has been widely used to disperse thermal crown and wear of work roll in axial direction. This paper provides a strategic control method for the work roll profile surpassing the conventional work roll shifting method.
    A numerical simulation model which enables prediction of thermal crown and wear of work roll with high accuracy was developed.
    Focusing on the problems of the conventional shifting method, a new shifting method was proposed.
    In the conventional shifting method, thermal crown within the contact area is calculated with a cyclic fixed stroke and step. As the stroke increases, the average value of the thermal crown in rolling campaign decreases without concentration of heat input from the strip to the axial center of the work roll. On the other hand, as the stroke increases, variance of the thermal crown in rolling campaign increases.
    Although the work roll shift method is effective for the dispersion of thermal crown and wear of work roll, the thickness profile of the strip is affected by the positional relationship between the work roll and strip.
    For further improvement of the work roll shift method, the need for a flexible shift method which takes into account the positional relationship between work rolls throughout the whole rolling campaign is suggested.
  • Effect of Graphite and Eutectic Carbides on the Seizure and Wear Characteristic in Wear-resistant Cast Iron

    pp. 372-377

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    DOI:10.2355/tetsutohagane.101.372

    Seizure tests were carried out on Ni-Cr-Mo abrasion resistant cast irons containing different amounts of graphite and eutectic carbides. Specimens were prepared with changing Cr and Ni contents based on 2.3% C-1.5% Si- 0.6 Mn- 1.8 Mo iron.
    Surfaces of the test pieces were finished with mechanical grinding or electro-spark machining up to roughness of Ra:0.30 ~ 0.35. Seizure properties were evaluated with using a high peripheral speed wear tester. Higher load was applied for evaluating seizure property, while lower load was applied for friction coefficient. Weight loss was also measured after testing. Surfaces of the specimen after seizure test were observed with SEM.
    The specimens with higher amounts of graphite showed lower friction at the early stage of seizure tests. When the testing load was increased, seizure was significantly occurred on the specimen with graphite.
    The effect of graphite for lubrication was also diminished in the case of wear tests under lower load except the early stage of testing.
    The electro-spark machining for surface finishing lead to exfoliation of graphite from the surface resulting in increase in friction coefficient. The specimen with higher amount of carbides showed superior wear properties.
  • A Proposition of a Kinetics Model for Stress-induced Martensitic Transformation in Fe-28Mn-6Si-5Cr Shape Memory Alloy

    pp. 378-383

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    DOI:10.2355/tetsutohagane.101.378

    Shape memory alloys (SMAs) have been widely-used for many applications because of its unique characteristic. Among various kinds of SMAs, an Fe-based alloy indicates the excellent formability, machinability and weldability. In addition, its production cost is lower than other alloys. Therefore, it is attempted that the alloy is applied to structural members such as the joint and dampers. When the Fe-based alloy is used as the members, a model of the transformation kinetics with higher precision is indispensable to simulate the deformation behavior of the members for its design. In the past, the model of forward and reverse transformation kinetics has been proposed; however, the small initial value for the volume fraction of martensite must be introduced. In this study, the precise model for transformation kinetics will be derived from an idea of a nucleation site and the transformation probability based on thermodynamics. Then, a validity of the proposed model is confirmed by a comparison with the past-proposed model. As a result, the proposed model can express the result calculated by the model proposed in the past. Additionally, it is successful that weak points of the previous model can be improved.
  • Development of Numerical Model to Predict Cleavage Fracture Toughness of Ferrite-Pearlite Steels

    pp. 384-393

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    DOI:10.2355/tetsutohagane.101.384

    A numerical model to quantitatively predict cleavage fracture initiation in ferrite-pearlite steel is proposed. The model is based on microscopic fracture process of three stages; Stage-I: formation of fracture origin in pearlite colony, Stage-II: propagation of the pearlite crack into ferrite matrix, and Stage-III: propagation of the crack across ferrite grain boundary. In the proposed model, fracture conditions are formulated by the probability of pearlite cracking based on the experimental results on Stage-I and the concept of fracture stress of ferrite matrix on Stage-II and Stage-III. Ferrite grains and cementite particles are assigned based on their distributions into each volume element. Applied plastic strain and stress of each volume element are calculated by finite element analysis. Cleavage fracture is assumed to initiate at the time when the fracture conditions of the all stages are satisfied in any one of the volume elements. Cleavage fracture toughness of three point bend test is simulated by the proposed model. The numerical predicted results of fracture toughness show good agreement with experimental ones. The bottleneck process of cleavage fracture is then evaluated by the number of arrested micro-cracks until all of the fracture process is satisfied. Influence of ferrite and pearlite size on cleavage fracture toughness is evaluated. It is shown that steel with finer pearlite colony is tougher, and then the developed model can reproduce the size effect of cleavage fracture toughness. Based on the aforementioned results, the validation and the effectiveness of the proposed model are found out.
  • Effect of NiAl Precipitates on Strength and Damping Capacity of Fe-Cr-Al Alloys

    pp. 394-399

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    DOI:10.2355/tetsutohagane.101.394

    Strength and damping capacity of Fe-Cr-Al magneto-mechanical damping alloys doped with different amounts of Ni and Al were investigated. After slow cooling followed by homogenization, the coarse NiAl phase with the B2 structure were precipitated in the Fe-Cr-Al matrix. The precipitation of the NiAl phase was effective in increasing strength, while the damping capacity showed a low value. The coarse NiAl precipitates suppressed the motion of the magnetic domain wall necessary for magneto-mechanical damping, resulting in a decrease in internal friction. In contrast, the NiAl precipitates became fine after an appropriate heat treatment. The alloys with fine NiAl precipitates exhibited high yield stress around 1 GPa and high internal friction more than 0.04.
  • An Experimental Evaluation for Rate Sensitivity of Fe-28Mn-6Si-5Cr Shape Memory Alloy under Bending Deformation

    pp. 400-405

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    DOI:10.2355/tetsutohagane.101.400

    Among various kinds of shape memory alloys, an Fe-based alloy indicates excellent formability, machinability and weldability. Additionally, its production cost is lower than other alloys. Therefore, it is attempted that the alloy be applied to structural members such as joints for pipelines, splice plates for railways, bolts and nuts, etc. When the alloy is used for the members, it is conceivable to deform flexurally at higher deformation rate because of earthquake, typhoon and related natural phenomena. Thus, it is important to investigate rate sensitivity of the alloy. In this study, a three-point bending test by using a thin plate made of Fe-28Mn-6Si-5Cr alloy is conducted at various normalized deformation rate such as 3.5×10–5, 3.5×10–4, 3.5×10–3, 3.5×10–2, 12.6 and 27.4 s–1. The rate sensitivity of its bending deformation under loading and shape memory effect by heating after unloading are investigated. As a result, it is shown that the positive rate sensitivity under loading, which means the load level increases with increasing in the deflection rate, can be observed in the alloy. However, it is hard to conclude that the shape memory effect depends on deflection rate clearly in the quasi-static region.

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