• Journal of Welding and Joining
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• v.34 no.1
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• pp.41-46
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• 2016

With the recent increase in size of ships and offshore structures, there are more demand for thicker plates. As the thickness increases, it is known that fatigue life of the structures decrease. To improve the fatigue life, post weld treatments techniques, such as toe grinding, TIG dressing and hammer peening, are typically employed. However, these techniques require additional construction time and production cost. Therefore, it is of crucial interest steels with longer fatigue crack growth life compared to conventional steels. This study investigates fatigue crack growth rate (FCGR) behaviours of conventional EH36 steel and Ferrite-Bainite dual phase EH36 steel (F-B steel). F-B steel is known to have improved fatigue performance associated with the existence of two different phases. Ferrite-Bainite dual phase microstructures are obtained by special thermo mechanical control process (TMCP). FCGR behaviours are investigated by a series of constant stress-controlled FCGR tests. Considering all test conditions (ambient, low temperature, high stress ratio), it is shown that FCGR of F-B steel is slower than that of conventional EH36 steel. From the tensile tests and impact tests, F-B steel exhibits higher values of strength and impact energy leading to slower FCGR.

• Journal of Korea Foundry Society
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• v.14 no.3
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• pp.267-273
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• 1994

The structure of austempered ductile cast iron, called ADI, consists of graphite, retained austenite, and bainite. The bainite component of them is considered a useful structure for exriting materials for roll of rolling mill. Therefore, the ADI can be considered applicable to material for rolling contact element. The diverse tests, such as rolling contact friction test, impact test, and X-ray reflection test, were carried out to investigate the possibility of it`s application. The result of this study showed that the expected fact was confirmed. The specimen showed that the best performance had the structure of the low bainite containing the stable retained austenite of about 20%.

• Journal of Korea Foundry Society
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• v.11 no.3
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• pp.228-235
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• 1991

The Effects of the matrix structures and the amount of the retained austenite in austempered ductile iron(ADI) which contains Ni, Cu and Mo on fracture characteristics were inverstigated. The structures were changed from lower bainite to mixture of Lower and upper bainite, and then to upper bainite as austempering temperature increased. A higher austempering temperature yielded lower tensile strength and hardness, but higher impact valve and $K_{IC}$. In addition, the amount of the retained austenite increased with the austempering temperatures. Fracture surface showed a fibrous pattern and a dimple pattern with increasing the retained austenite amount.

• Journal of the Korean Society for Heat Treatment
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• v.7 no.4
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• pp.298-306
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• 1994

This study was performed to investigation the effect of austempering temperature on the mecanical properties and fracture Characteristic of the ductile cast iron with contains Cu and Mo. The obtained results of this study were as follows; Microstructure of austemped ductile cast iron obtained by austempering were low bainite with some martensite at $250^{\circ}C$, mixture of low and upper bainite at $300^{\circ}C$ and upper bainite at $350^{\circ}C$. With increasing austempering temperature, yield strength, tensile strength and hardness decreased, while the elongation and impact absorption energy increased. With increasing austempering temperature, fracture toughness value increased and mainly controlled by bolume fraction of retained austenite. The volume fraction of retained austenite increased and the fracture surface obtained fibrous and dimple with increasing austempering temperature.

• Journal of the Korean Society for Heat Treatment
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• v.30 no.2
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• pp.67-73
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• 2017

Transformation behavior and hardness change were studied in five kinds of self-control steels; standard, high V, modified Ni, W, and high C-Ni steels. In the cooling rates of $10-100^{\circ}C/min$, the primary ferrite and bainite were formed, and the amount of the former increased with decreasing cooling rate. The bainite transformation temperature, Bs, was measured as 570, 560, 590, 575, and $565^{\circ}C$ in experimental steels, respectively, which was similar to the calculated temperature. The self-control, that is, the consistency in hardness, was observed, in which the hardness increased with the decrease in Bs. In the case of hot compression testing, the lower temperature deformation led to the enhancement in hardness.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.10a
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• pp.49-52
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• 2000

The hole flanging experiments are performed on a flat circular plates with a hole in the center and the flangeability and fracture behavior of TRIP steel and ferrite-bainite duplex steel were examined. Over the ranges of conditions investigated, the flangeability of ferrite-bainite duplex steel is better than TRIP steel and the term 'hole flanging capacity' introduced in this study. Fracture behavior of TRIP steel and ferrite-bainite duplex steel was characterized by petal formation at the edge of flange. In case of crack propagation, crack tip diversion that is supposed to be responsible for flangeability occurs more severely on HSLA Steel.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.10a
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• pp.456-459
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• 2005

Continuous cooling transformation behaviors were studied fur low carbon HSLA steels containing three different level $(1\~3\;wt\%)$ of Ni addition. Thermo-mechanical processing (TMP) simulations to construct continuous cooling (CCT) diagram were conducted by using Gleeble system. As cooling rate increased, pearlite, granular bainite, acicular ferrite, bainitic ferrite and lath martensite were transformed from deformed austenite. Fully bainitic microstructure were developed at all cooling rate condition in high Ni containing steel due to hardenability increasing effects of Ni. Ni also influenced the transformation kinetics. At the slowest cooling rate of $0.3^{\circ}C/s$, transformation delayed with decreasing Ni contents because of the diffusion of substitutional alloy elements. However, cooling rate slightly increased to $1^{\circ}C/s$, transformation kinetics accelerated with decreasing Ni contents because nucleation of bainite was sluggish due to hardening of residual austenite.

• Transactions of Materials Processing
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• v.27 no.5
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• pp.281-288
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• 2018

This study deals with the microstructure and tensile properties of 600 and 700 MPa-grade high-strength seismic reinforced steel bars. High-strength seismic resistant reinforced steel bars (SD 600S and SD 700S) were fabricated by TempCore process, especially the SD 700S specimen was more rapid cooled than the SD 600S specimen during the TempCore process. Although two specimens had microstructure of tempered martensite in the surface region, the SD 600S specimen had ferrite-degenerated pearlite in the center region, whereas the SD 700S specimen had bainite-ferrite-degenerated pearlite in the center region. Therefore, their hardness was highest in the surface region and revealed a tendency to decrease from the surface region to the center region because tempered martensite has higher hardness than ferrite-degenerated pearlite or bainite. The SD 700S specimen revealed higher hardness in the center region than SD 600S specimen because it contained a larger amount of bainite as well as ferrite-degenerated pearlite. On the other hand, tensile test results indicated the SD 600S and SD 700S specimens revealed continuous yielding behavior because of formation of degenerated pearlite or bainite in the center region. The SD 600S specimen had a little higher tensile-to-yield ratio because the presence of ferrite and degenerated pearlite in the center region and the lower fraction of tempered martensite enhance work hardening.

• Journal of the Korean Society for Heat Treatment
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• v.4 no.1
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• pp.1-12
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• 1991

This study has been performed to find out the effect of austenitizing temperature, austempering temperature and its holding time, and tempering cycle on the mechanical properties such as impact resistance, hardness etc. of AISI $M_2$ Mo series high speed tool steel austempered or tempered after austempering treatment. The results obtained from the experiment are as follows ; (1) Optical micrograph has revealed that the transformation rate of bainite is delayed as the austenitizing temperature increases and that bainite is most apparently transformed at an austempering temperature of $290^{\circ}C$. (2) The amount of retained austenite during austempering has been analysed to be increased by the X-ray diffraction technique as the transformation product of bainite is increased. It has also been shown that the longer the holding time of austempering, the more the transformation quantity of bainite is formed, exhibiting, however, that the rate of bainitic transformation is considerably retarded after a certain period of holding time elapses. (3) Hardness measurement has shown that hardness values obtained after austempering increase with decreasing the amount of retained austenite. (4) The austempering and then tempering cycle has been formed to give hardness values which are more greatly improved as austenitizing temperature is increased. (5) The mechanical property of the specimen primary-tempered for 1 hour at $550^{\circ}C$ after austempering for 2 hours at $290^{\circ}C$ from the austenitizing temperature range of $1180^{\circ}C$ to $1210^{\circ}C$ have been estimated to be good values.

• Korean Journal of Metals and Materials
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• v.47 no.3
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• pp.155-168
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• 2009

In this study, microstructural investigation was conducted on the separation phenomenon occurring during Charpy impact tests of API X80 linepipe steels. Particular emphasis was placed on the role of microstructural phases present in the API X80 steels such as acicular ferrite, bainite, and hard secondary phases. Detailed microstructural analysis of fractured impact specimens showed that highly elongated bainite worked as prior initiation sites for separations, and that the number and length of separations increased with increasing volume fraction of bainite. In the steels having high work hardenability, tearing-shaped separations were found because the hammer-impacted region was seriously hardened during the impact test, which led to the reduction in the impact toughness. As the test temperature decreased, the tendency of separations increased, but separations were not observed when the cleavage fracture prevailed at very low temperatures. Thus, the minimization of the formation of bainite and secondary phases in the steels would be beneficial for preventing or minimizing separations because separations deteriorated low-temperature impact toughness.