• Korean Journal of Materials Research
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• v.27 no.9
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• pp.477-483
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• 2017

This present study deals with the microstructure and tensile properties of 600 MPa-grade high strength and seismic resistant reinforcing steels. The high strength reinforcing steel (SD 600) was fabricated by Tempcore processing, while the seismic resistant reinforcing steel (SD 600S) was air-cooled after hot-rolling treatment. The microstructure analysis results showed that the SD 600 steel specimen consisted of a tempered martensite and ferrite-pearlite structure after Tempcore processing, while the SD 600S steel specimen had a fully ferrite-pearlite structure. The room-temperature tensile test results indicate that, because of the enhanced solid solution and precipitation strengthening caused by relatively higher contents of C, Mn, Si and V in the SD 600S steel specimen, this specimen, with fully ferrite-pearlite structure, had yield and tensile strengths higher than those of the SD 600 specimen. On the other hand, the hardness of the SD 600 and SD 600S steel specimens changed in different ways according to location, dependent on the microstructure, ferrite grain size, and volume fraction.

• Journal of the Korean Society for Heat Treatment
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• v.9 no.1
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• pp.53-61
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• 1996

Effects of carbon and Ti on damping capacity are investigated in an Fe-17%Mn alloy. The suppressive force of carbon against ${\gamma}{\rightarrow}{\varepsilon}$ transformation increases linearly with an increase in its content, lowering Ms temperature and volume fraction of ${\varepsilon}$ martensite. Carbon deteriorates damping capacity by reducing the interfacial area of damping sources and mobility of the boundaries contributing to anelastic deformation. The reduction in damping capacity is accelerated when carbon-containing alloy is aged at higher temperatures above room temperature. The effect of Ti on damping capacity is found to be benificial in carbon-containing alloy, which is attributed to the depletion of carbon solute due to the formation of TiC.

• Journal of the Korean Society for Heat Treatment
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• v.24 no.5
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• pp.251-261
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• 2011

The changes in microstructure and texture during annealing were examined in a series of 0.015% C-1.5% Mn cold-rolled sheet steels with 0~0.5% Mo. Orientation distribution function data were calculated from the (110), (200), (211) pole figures determined on the rolled plane of cold-rolled and annealed steel sheets. Regardless of Mo content and annealing conditions, martensite volume fraction was less than 1.0%, not affecting the texture evolution. Textural change at the cooling stage after heating at $820^{\circ}C$ for 67 sec was not observed. Increasing the Mo content and annealing temperature markedly strengthened the intensities of ${\gamma}$-fiber texture, resulting in the increase in $r_m$ value. The desirable texture evolution for deep drawability in the 0.5% Mo steel may be mainly caused by the grain refining effect of Mo carbide in the hot-rolled steel sheet.

• Transactions of Materials Processing
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• v.20 no.4
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• pp.309-315
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• 2011

The behavior of metallic materials at high strain rates shows different characteristics from those in quasi-static deformation. Therefore, the strain rate should be considered when simulating crash events. The objective of this paper is to evaluate the dynamic tensile characteristics of SPRC440 as a function of the volume fraction of phases. As-received SPRC440 is composed of ferrite and pearlite phases. However, ferrite and martensite phases were observed after heat treatment at $730^{\circ}C$ and $780^{\circ}C$ for 5 minutes, as expected by calculations based on the curves from dilatometry tests. High cross-head speed tensile tests were performed to acquire strain-stress curves at various strain rates ranging from 0.001 to $300\;s^{-1}$, which are typical in real vehicle crashes. It was observed that the flow stress increases with the strain rate and this trend was more pronounced in the as-received specimens consisting of ferrite and pearlite phases. It is speculated that the dislocation density in each phase has an influence on the strain rate sensitivity.

• Nuclear Engineering and Technology
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• v.40 no.2
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• pp.99-106
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• 2008

Through mechanical alloying, hot isostatic pressing and hot rolling, a 9%Cr Fe-based oxide dispersion-strengthened alloy sample was fabricated. The tensile strength of the alloy is significantly improved when the microstructure is modified during the post-consolidation process. The alloy samples were strengthened as the cooling rates increased, though the elongation was somewhat reduced. With a cooling rate of $800^{\circ}C/s$ after normalization at $1150^{\circ}C$, the alloy sample showed a tensile strength of 1450 MPa, which is about twice that of the hot rolled sample; however, at $600^{\circ}C$ the tensile strength dramatically decreased to 620 MPa. Optical microscope and transmission electron microscope were used to investigate the microstructural changes of the specimens. The resultant strengthening of the alloy sample could be mainly attributed to the interstitially dissolved nitrogen, the fraction of the tempered martensite, the fine grain and the presence of a smaller precipitate. The decrease in the tensile strength was mainly caused by the precipitation of vanadium-rich nitride.

• Journal of Welding and Joining
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• v.31 no.6
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• pp.44-49
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• 2013

The influence of thermo-mechanical cycling on the microstructure and strength in the weld coarse-grained heat affected zone (CGHAZ) of Ti-Nb added low carbon HSLA steel was explored through Vickers hardness tests, nanoindentation experiments, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Undeformed and deformed CGHAZs were simulated using Gleeble simulator with different heat inputs of 30kJ/cm and 300kJ/cm. At high heat input of 300kJ/cm, the CGHAZ consisted of ferrite and pearlite and then their grain sizes were not affected by deformation. At low heat input of 30kJ/cm, the CGHAZ consisted of lath martensite and then the sizes of prior austenite grain, packet and lath width decreased with deformation. In addition, the fraction of particle increased with deformation and this is because the precipitation kinetics was accelerated by deformation. Meanwhile, the Vickers and nanoindentation hardness of deformed CGHAZ with 30kJ/cm heat input were higher than those of undeformed CGHAZ, which are due to the effect of grain refinement and precipitation strengthening.

• Journal of Welding and Joining
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• v.31 no.6
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• pp.8-16
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• 2013

As the energy consumption increases rapidly, power generation needs the high energy efficiency continuously. To achieve the high efficiency of power generation, the materials used have to endure the higher temperature and pressure. The 9-12%Cr steels possess good mechanical properties, corrosion resistance, and creep strength in high temperature due to high Cr contents. Therefore, the 9-12%Cr steels are widely used for the high-temperature components in power plants. Even though the steels usually have a fully martensitic microstructure, they are susceptible to the formation of ${\delta}$-ferrite specifically during the welding process. The formation of ${\delta}$-ferrite has several detrimental effects on creep, ductility and toughness. Therefore, it is necessary to avoid its formation. As the volume fraction of ${\delta}$-ferrite is less than 2% in microstructure, it has the isolated island morphology and causes no significant degradation on mechanical properties. For ${\delta}$-ferrite above 2%, it has a polygonal shape affecting the detrimental influence on the mechanical properties. The formation of ${\delta}$-ferrite is affected by two factors: a chemical composition and a welding heat input. The most effective ways to get a fully martensite microstructure are to reduce the chromium equivalent less than 13.5, to keep the difference between the chromium and nickel equivalent less than 8, and to reduce the welding heat input.

• Coupled systems mechanics
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• v.6 no.1
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• pp.41-54
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• 2017

Shape-memory alloys (SMA) have interesting behaviors and important mechanical properties due to the solid-solid phase transformation. These phenomena are dominated by the evolution of microstructures. In recent years, the microstructures in SMAs have been studied extensively and modeled using molecular dynamics (MD) simulations. However, it remains difficult to identify the crystal variants in the simulation results, which consist of large numbers of atoms. In the present work, a method is developed to identify the austenite phase and the monoclinic martensite crystal variants in MD results. The transformation matrix of each lattice is calculated to determine the corresponding crystal variant. Evolution of the volume fraction of the crystal variants and the microstructure in Ni-Ti SMAs under thermal and mechanical boundary conditions are examined. The method is validated by comparing MD-simulated interface normals with theoretical solutions. In addition, the results show that, in certain cases, the interatomic potential used in the current study leads to inconsistent monoclinic lattices compared with crystallographic theory. Thus, a specific modification is applied and the applicability of the potential is discussed.

• Journal of Welding and Joining
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• v.28 no.3
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• pp.99-103
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• 2010

It is well known that martensite-austenite (M-A) constituents are formed in the intercritically reheated coarse grained heat affected zone (ICCGHAZ) of a multipass weld and they act on the local brittle zone (LBZ) in the welded structures. To investigate the effect of M-A constituents on the tensile properties of ICCGHAZ, specimens with M-A constituents of different volume fraction and size were prepared through the multipass welding cycles simulated by a Gleeble simulator and then tensile test was carried out. The results indicated that finely distributed M-A constituents contributed to decrease the yield ratio, which is mainly due to the increased tensile strength.

• Korean Journal of Metals and Materials
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• v.48 no.1
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• pp.19-27
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• 2010

This paper investigates the influence of aging time on creep properties via a small punch creep test and evaluates the microstructural change of P122 steel at $600^{\circ}C$. The area fraction of precipitates was quantitatively analyzed to identify the relationship between the creep rupture life and precipitates was coarsening behavior of precipitates along the grain boundaries was also investigated for various aging times. It is found that this coarsening behavior led to a loss of solution hardening and rewulte in a hardness drop and a reduction of creep life.