• Journal of Welding and Joining
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• v.11 no.4
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• pp.57-69
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• 1993

In the present study, the microstructure and Charpy V notch toughness of multipass $CO_2$ FCA weldment in three different heat inputs(1-3KJ/mm)were investigated. The weldments using two different domestic FCAW wires(AWS E71T-1 and E71T-5 equivalent) in C-Mn steel were chemically analysed. The following conclusions can be inferred. 1. T-1 wire Showed a stable arc transfer, less spatter and harsh, a better bead spreading and easy slag removal, whereas T-5 wire suffered from the arc stability, which tended to increase spatter and produce a more convex bead. 2.The microsturctures of the top beads of the weldments in three different heat inputs consisted of coarse-grained boundary ferrite and Widmanstatten ferrite side plate with increasing heat inputs. The modest fraction of acicular ferrite in the two wire weldments was observed in the 2KJ/mm heat input. 3.The fine-grained reheated zones of both welds consisted of a duplex microstructure of polygonal ferrite and second phases. 4. The basic flux weldment of T-5wires showed a higher Charpy impact property than that of T-1 wires because of a higher fraction of acicular ferrite in the weld microstructure.

• Journal of Welding and Joining
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• v.29 no.1
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• pp.65-73
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• 2011

Microstructural characteristics of two high strength (600 MPa & 800 MPa) weld metals produced by flux-cored arc welding process (FCAW) were evaluated. The 600 MPa grade weld metal was consisted of 75% acicular ferrite and 25% ferrite which was formed at relatively high temperature (grain boundary ferrite, widmanstatten ferrite, polygonal ferrite). However, the 800 MPa grade weld metal was composed of about 85% acicular ferrite and 15% low temperature forming phases (bainite, martensite). The prior austenite grain size of 800 MPa grade weld metal was decreased by solute drag force. The compositions and sizes of inclusions which are the dominant factors for the formation of acicular ferrite were analyzed by a transmission electron microscopy (TEM). In both 600 MPa and 800MPa grade weld metals, the inclusions were mainly consisted of Ti-oxide and Mn-oxide, and the average size of inclusions was $0.7{\mu}m$. The 800 MPa grade weld metal exhibited higher tensile strength and similar toughness compared with the 600 MPa grade weld metal. This result is mainly due to a higher fraction of low temperature products and a lower fraction of grain boundary ferrite in the 800 MPa grade weld metal.

• Journal of Welding and Joining
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• v.14 no.6
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• pp.93-100
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• 1996

Effect of a flux composition on weld metal toughness in submerged arc welding with 60kgf/$\textrm{mm}^2$ grade C-Mo type wires was investigated and interpreted in terms of weld metal microstructure and hardenability. Flux workability was also studied by characterizing a weld bead profile. Compared to other weld metals, .weld metal used alumina basic flux with nickel showed lowest oxygen content, highest hardenability and the most acicular ferrite. The highest impact toughness of that weld metal, however, was attributed to the tough matrix due to the nickel rather than to the larger amount of acicular ferrite. Manganese silicate flux had better workability than alumina basic flux, showing broader welding conditions resulting in a depth-to-width ratio of 0.5. The composition of oxides in the weld metal was dependent on the flux composition, showing MnO-SiO$_2$-TiO in manganese silicate flux and MnO-SiO$_2$-Al$_2$O$_3$-TiO in alumina basic flux. MnO-SiO$_2$composition in both oxides was similar to a tephroite.

• Korean Journal of Materials Research
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• v.29 no.8
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• pp.511-518
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• 2019

In this study, the effect of carbon equivalent and cooling rate on microstructure and hardness of A516 steels for pressure vessel is investigated. Six kinds of specimens are fabricated by varying carbon equivalent and cooling rate, and their microstructures and hardness levels are analyzed. Specimens with low carbon equivalent consist of ferrite and pearlite. As the cooling rate increases, the size of pearlite decreases slightly. The specimens with high carbon equivalent and rapid cooling rates of 10 and $20^{\circ}C/s$ consist of not only ferrite and pearlite but also bainite structure, such as granular bainite, acicular ferrite, and bainite ferrite. As the cooling rate increases, the volume fractions of bainite structure increase and the effective grain size decreases. The effective grain sizes of granular bainite, acicular ferrite, and bainitic ferrite are ~20, ~5, and ${\sim}10{{\mu}m$, respectively. In the specimens with bainite structure, the volume fractions of acicular ferrite and bainitic ferrite, with small effective grains, increase as cooling rate increases, and so the hardness increases significantly.

• Journal of Welding and Joining
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• v.32 no.1
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• pp.22-27
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• 2014

In the past, cold crack was commonly observed in the HAZ(heat affected zone) of high-strength steels. Applying to TMCP(thermo-mechanical controlled process) and HSLA(high strength low alloy) steels, cold crack tends to increase the occurrence in the weld metal. It is generally understood that cold crack occurs when the following factors are present simultaneously : diffusible hydrogen in the weld metal, a susceptible microstructure and residual stress. In particular, many studies investigated the microstructural effect on the cold crack in HAZ and the cold crack in weld metals starts to receive the special attendance in modern times. The purpose of the study is to review the effect of weld microstructures (grain boundary ferrite, Widm$\ddot{a}$nstatten ferrite, acicular ferrite, bainite and martensite) on cold crack in the weld metals. Among various microstructures of weld metals, acicular ferrite produced the greatest resistance to the cold crack due to the fine interlocking nature and high-angle grain boundary of the microstructure.

• Korean Journal of Metals and Materials
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• v.46 no.4
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• pp.199-208
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• 2008

This study is concerned with effects of complex oxides on Charpy impact toughness of heat affected zone (HAZ) of API X80 linepipe steels. Three kinds of steels were fabricated by varying alloying elements such as Ti, Al, and Mg and hot-rolling conditions to form complex oxides, and their microstructures and Charpy impact properties were investigated. The number of complex oxides present in the steel containing excess Ti, Al, and Mg was twice larger than that in the conventional steels, while their size ranged from 1 to $3{\mu}m$ in the three steels. After the HAZ simulation test, the steel containing a number of oxides contained about 20 vol.% of acicular ferrite in the simulated HAZ, together with bainitic ferrite and martensite, whereas the HAZ microstructure of the conventional steels consisted of bainitic ferrite and martensite with a small amount of acicular ferrite. This formation of acicular ferrite in the oxide-containing steel was associated with the nucleation of acicular ferrite at complex oxides, thereby leading to the great (five times or more) improvement of Charpy impact toughness over the conventional steels.

• Journal of the Korean Magnetics Society
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• v.8 no.1
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• pp.1-5
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• 1998

Acicular Fe-Co alloy particles are one of the candidates for high-density magnetic recording media. We examined the effects of Co additions on the magnetic properties of Fe-Co alloy particles by using M$\'{o}$ssbauer spectroscopy, TEM, and X-ray diffraction. Acicular $Fe_n$Co (n=5, 4, 3, 2) alloy particles coated with silica, were prepared by a chemical coprecipitation method and subsequent H $_2$ reduction. The crystal structure was found to be cubic in all n ranges. The lattice constant $a_0$ decreases with increasing Co contents. Analysis of $^{57}Fe$ M\'{o}$ssbauer effect data in terms of the local configurations of Co atoms has permitted the influence of magnetic hyperfine interactions to be monitored.

• Journal of the Mineralogical Society of Korea
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• v.25 no.2
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• pp.95-104
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• 2012

Electron microscopy of the tremolite-actinolite series amphiboles from the naturally occurring asbestos locality showed the morphological diversity including fibrous, acicular, and prismatic. Very thin, long, and flexible fibers of constant width form ropy bundles or mats. Acicular particles are slightly thick, long, elastic, and easily separated from the bundle of parallel rods. Acicular fragments of lower aspect ratio are formed during the crushing of the amphibole prism. Morphological features of the amphiboles are different depending on their localities and vary in a specimen. Morphological continuum between amphibole fiber and prism requires the establishment of reliable identification criterions and sample preparation protocol based on the relation between carcinogenicity and morphological features.

• Korean Journal of Metals and Materials
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• v.50 no.3
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• pp.212-217
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• 2012

This study was carried out to investigate the effect of the hot forging ratio on the microstructure and mechanical properties of ultra high carbon steel. The microstructure of ultra high carbon steel with 1.5%wt.C consisted of a proeutectoid cementite network and acicular microstructure in pearlite matrix. With increasing hot forging ratio, the volume and thickness of the network and acicular proeutectoid cementite decreased. Lamella spacing and the thickness of eutectoid cementite decreased with increasing hot forging raito, and were broken up into particle shapes, which then became spheroidized. When the forging ratio was over 65%, the network and acicula shape of the as-cast state disappeared. With increasing hot forging ratio, hardness, tensile strength, elongation and impact value were not changed up to 50%, and then rapidly increased with the increase of the forging ratio.

• Journal of Welding and Joining
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• v.26 no.3
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• pp.51-60
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• 2008

This paper is to evaluate fracture characteristics of API 2W Gr.50 TMCP steel weldment typically applied for offshore structures, with the focus on the influence of heat input arising from flux cored arc welding. Based on the results and insights developed from this study, it is found that the toughness for both CTOD and impact exhibits a tendency to decrease as the weld heat input increases. The reheated zone of weldmetal exhibit lower hardness than solidified zone and microstructure that are liable to affect the toughness are acicular ferrite and martensite-austenite constituents (M-A). In particular, M-A is a more effective micro-phase for CTOD toughness than impact toughness.