• Journal of Welding and Joining
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• v.7 no.3
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• pp.44-54
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• 1989

This study was undertaken to evaluate the allowable welding heat input range for high strength steels manufactured by various processes and to compare the weldability of TMCP steel for high heat input welding with that of conventional Ti-added normalized steel. The allowable welding heat input ranges for conventional 50kg/$mm^2$ steel to guarantee D or E grade of ship structural steel were below 150 and 80kJ/cm respectively. Such a limit in welding heat input was closely related with the formation of undesirable microstructures, such as grain boundary ferrite and ferrite side plate in the coarse grain HAZ. In case of 60 and 80kg/$mm^2$ quenched and tempered steels, for securing toughness in weldments over toughness requirements for base metal, each welding heat input had to be restricted below 60 and 40kJ/cm, that was mainly due to coarsened polygonal ferrite in weld metal and lower temperature transformation products in coarse grain HAZ. The TMCP steel could be appropriate as a grade E ship hull steel up to 200kJ/cm, but the Ti-added normalized steel could be applied only below 130kJ/cm under the same rule. This difference was partly owing to whether uniform and fine intragranular ferrite microstructure was well developed in HAZ or not.

• Proceedings of the KWS Conference
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• 2006.05a
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• pp.140-142
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• 2006

The bend ductility of Type 410S ferritic stainless steel overlay weld on carbon steel was investigated. Overlay weld that was stabilized with Nb had large columnar ferrite grain and Nb precipitate on grain boundary. And that caused fracture when bend test without concern of PWHT condition. Proper bend ductility at as-welded condition was achieved by refining ferrite grain with addition of $0.04{\sim}0.09%$ Al and $0.2{\sim}0.5%$ Ti that make oxide, carbide and nitride at high temperature.

• International Journal of Korean Welding Society
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• v.3 no.2
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• pp.46-52
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• 2003

The refinement of microstructure and phase transformation near the interface of pure copper/carbon steel dissimilar metals joints with various friction welding parameters have been studied in this paper. The microstructure of copper and carbon steel joints were changed to be a finer grain compared to those of the base metals due to the frictional heat and plastic deformation. The microstructure of copper side experienced wide range of deformed region from the weld interface and divided into very fine equaxied grains and elongated grains. Especially, the microstructures near the interface on carbon steel were transformed from ferrite and pearlite dual structure to fine ferrite, grain boundary pearlite and martensite due to the welding thermal cycle and rapid cooling rate after welding. These microstructures were varied with each friction welding parameters. The recrystallization on copper side is reason for softening in copper side and martensite transformation could explain the remarkable hardening region in carbon steel side.

• Korean Journal of Materials Research
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• v.28 no.12
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• pp.702-708
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• 2018

This study deals with the effect of microstructure factors on the strain aging properties of API X70 pipeline steels with different microstructure fractions and grain sizes. The grain size and microstructure fraction of the API pipeline steels are analyzed by optical and scanning electron microscopy and electron backscatter diffraction analysis. Tensile tests before and after 1 % pre-strain and thermal aging treatment are conducted to simulate pipe forming and coating processes. All the steels are composed mostly of polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite. After 1 % pre-strain and thermal aging treatment, the tensile test results reveal that yield strength, tensile strength and yield ratio increase, while uniform elongation decreases with an increasing thermal aging temperature. The increment of yield and tensile strengths are affected by the fraction of bainitic ferrite with high dislocation density because the mobility of dislocations is inhibited by interaction between interstitial atoms and dislocations in bainitic ferrite. On the other hand, the variation of yield ratio and uniform elongation is the smallest in the steel with the largest grain size because of the decrease in the grain boundary area for dislocation pile-ups and the presence of many dislocations inside large grains after 1 % pre-strain.

• Korean Journal of Materials Research
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• v.27 no.11
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• pp.636-642
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• 2017

In the present study the microstructure of low-carbon steels fabricated by controlled rolling and accelerated cooling processes was characterized and identified based on various microstructure analysis methods including optical and scanning electron microscopy, and electron backscatter diffraction(EBSD). Although low-carbon steels are usually composed of ${\alpha}-ferrite$ and cementite($Fe_3C$) phases, they can have complex microstructures consisting of ferrites with different size, morphology, and dislocation density, and secondary phases dependent on rolling and accelerated cooling conditions. The microstructure of low-carbon steels investigated in this study was basically classified into polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite based on the inverse pole figure, image quality, grain boundary, kernel average misorientation(KAM), and grain orientation spread(GOS) maps, obtained from EBSD analysis. From these results, it can be said that the EBSD analysis provides a valuable tool to identify and quantify the complex microstructure of low-carbon steels fabricated by controlled rolling and accelerated cooling processes.

• Korean Journal of Materials Research
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• v.26 no.11
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• pp.583-589
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• 2016

In this study, recrystallization behaviors in the two-phase (${\alpha}+{\gamma}$) region of micro-alloyed steels such as Base, Nb, TiNbV and CAlN were investigated in terms of flow stress, microstructure and associated grain boundary characteristics. The flow stress of all specimens reached peak stress and gradually decreased, which means that recrystallization or recovery of proeutectoid deformed ferrite and recovery or transformation to ferrite of deformed austenite occurred by thermal activation. The precipitation of carbide or nitride via the addition of micro-alloying elements, because it reduced prior austenite grain size upon austenitization, promoted transformation of austenite to ferrite and increased flow stress. The strain-induced precipitation under deformation in the two-phase region, on the other hand, increased the flow stress when the micro-alloying elements were dissolved during austenitization. The recrystallization of the Nb specimen was more effectively retarded than that of the TiNbV specimen during deformation in the two-phase region.

• Korean Journal of Metals and Materials
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• v.49 no.2
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• pp.93-103
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• 2011

Precipitation behavior of high-nitrogen duplex Fe-24Cr-7Mn-4Ni-4Mo-0.43N stainless steel aged at $850^{\circ}C$ was investigated using scanning transmission electron microscopy. Based on the analyses of selected area diffraction patterns, four kinds of precipitates (intermetallic sigma (${\sigma}$) and chi (${\chi}$), $Cr_2N$ and secondary austenite) were identified. At the ferrite/austenite phase boundary, the ${\sigma}$ phase and secondary austenite were formed via ${\alpha}{\rightarrow}{\gamma}+{\sigma}$ eutectoid reaction. The precipitation of $Cr_2N$ occurred at the austenite grain boundary as well as the interior of the ferrite. The intermetallic ${\chi}$ phase also formed within the ferrite and showed a cube-cube orientation relationship with the ferrite. Further aging produced a lamellar structure composed of $Cr_2N$ and austenite along the ferrite/austenite boundary and enhanced the precipitation of the ${\chi}$ phase. The crystallographic features of the precipitates were also examined in terms of the orientation relationship with the austenite or ferrite matrix.

• Korean Journal of Metals and Materials
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• v.50 no.1
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• pp.52-58
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• 2012

Hydrogen-delayed fracture (HDF) was analyzed from the deposited weld metals of 600-MPa and 800-MPa flux-cored arc (FCA) welding wires, and then from the diffusible hydrogen behavior of the weld zone. Two types of deposited weld metal, that is, rutile weld metal and alkali weld metal, were used for each strength level. Constant loading test (CLT) and thermal desorption spectrometry (TDS) analysis were conducted on the hydrogen pre-charged specimens electrochemically for 72 h. The effects of microstructures such as acicular ferrite, grain-boundary ferrite, and low-temperature-transformation phase on the time-to-failure and amount of diffusible hydrogen were analyzed. The fracture time for hydrogen-purged specimens in the constant loading tests decreased as the grain size of acicular ferrite decreased. The major trapping site for diffusible hydrogen was the grain boundary, as determined by calculating the activation energies for hydrogen detrapping. As the strength was increased and alkali weld metal was used, the resistance to HDF decreased.

• Korean Journal of Materials Research
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• v.13 no.4
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• pp.228-232
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• 2003

The effects of$ Nd_2$$O_3$addition on the properties of Mn-Zn ferrite were investigated in the doping concentration range from 0.05 to 0.25 wt%. All samples were prepared by standard fabrication of ferrite ceramics. With increasing the Neodymium oxides, specific density and initial permeability increased on the whole. But, the tendencies such as upper result had the measured value on limitation and characteristics saturated or decreased properties after that. With increasing the content of Neodymium oxides. both the real and imaginary component of complex permeability and the magnetic loss(tan$\delta$) increased. Because reason that magnetic loss increases is high ratio that a real department increases than imaginary department. Magnetic loss increased none the less for increasing the real department related with magnetic permeability. But, the magnetic loss of ferrite doped with the Neodymium oxides were lower than that of none doped Mn-Zn ferrite. The small amount of percent Neodymium oxides in Mn-Zn ferrite composition led to enhancement of resistivity in bulk, and more so in the grain boundary.

• Journal of the Korean Society of Industry Convergence
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• v.6 no.4
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• pp.299-305
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• 2003

This research is for the relationship with heat treatment cooling temprature and the characteristic of Mechanical properties of Boron-Addition-Steel, the main material and SM25C steel, the sub material, structure viewing fractography, hardness test, tensite test and are carried out after the manufacturing small-specimen treated with heat of $750^{\circ}C$, $850^{\circ}C$, $1050^{\circ}C$. The influence to the Mechanical properties accompanied by AISI51B20, Boron-Addition-steel shows the following result. 1. The influenc of heat treatment by the content of cabon-steel is dominant. Addition of boron result is Strengthening structure effectively by segregation and improving over all mechanical characters such as good. it results from the increase of temacity by the stability of inter granular with improvement of harden-ability. 2. Boron-Addition-Steel exist in the from of martensite structure accompanied by the ferrite precipitition centering around grain boundary, and is improved to Hv 200. 3. The height of harden-ability and fatigue stress the influence of heat results from crystal structure of martensite by difference of strength level in the structure of ferrite and doesn't have am effect on sensibility of temperature, and turns out to defend on production and growth of Matrix-structure-factor.