• Corrosion Science and Technology
• /
• 제22권2호
• /
• pp.131-136
• /
• 2023

The aim of this study was to investigate effects of microstructure control on hydrogen diffusivity, trap activation energy, and cracking behaviors of high-strength steel using a range of experimental techniques. Results of this study showed that susceptibility to hydrogen induced cracking (HIC) was significantly associated with hydrogen diffusivity and trap activation energy, which were primarily influenced by the microstructure. On the other hand, microstructural modifications had no significant impact on electrochemical polarization behavior on the surface at an early corrosion stage. To ensure high resistance to HIC of the steel, it is recommended to increase the cooling rate during normalizing to avoid formation of banded pearlite in the microstructure. However, it is also essential to establish optimal heat treatment conditions to ensure that proportions of bainite, retained austenite (RA), and martensite-austenite (MA) constituents are not too high. Additionally, post-heat treatment at below A₁ temperature is desired to decompose locally distributed RA and MA constituents.

• 대한용접접합학회:학술대회논문집
• /
• 대한용접접합학회 2009년 추계학술발표대회
• /
• pp.37-37
• /
• 2009

Due to their high corrosion resistance and improved mechanical properties super-duplex stainless steel (SDSS) are extensively used in petrochemical plants such as facilities in modern oil platform and off-shore process equipment. It is well known that the best mechanical and corrosion resistance properties of super-duplex stainless steel are obtained with a microstructure having approximately equal amounts of austenite and ferrite. And it is also known that sigma($\sigma$), chi($\chi$), secondary austenite(${\gamma}2$), chromium carbides and nitride affected adversely their properties. Therefore these phases must be avoided. However, effects of succeeding weld thermal cycle on the change of microstructure of weldment at multi-pass weld were not seldom experimentally researched. Therefore in the present work, the change of weldmetal microstructure and the effect of microstructure on pitting corrosion property at $40^{\circ}C$ by succeeding each weld thermal cycle were researched. The thermal history of root side was measured experimentally and the change of microstructure of root weld according to thermal cycle of each weld layer was evaluated. And the relationship between microstructure of root weld and pitting corrosion property at $40^{\circ}C$ was also investigated. Results of the present work are show as below. 1. The ferrite contents of root weld are gradually reduced by succeeding weld thermal cycle. 2. The 2nd phases such as sigma($\sigma$), chi($\chi$), secondary austenite(${\gamma}2$), chromium carbides and nitride are increased gradually by succeeding weld thermal cycle. 3. The pitting corrosion was detected in root weld part and weight loss by pitting corrosion is increased in proportional to the time exposed over $600^{\circ}C$ of the root weld. 4. The succeeding weld thermal cycles affect the microstructure of the former weldments and promote the formation of 2nd phases. That is, the more succeeding welds are added, the more 2nd phases are gradually increased. Consequently, it is thougth that this adversely affects pitting corrosion property.

• 한국세라믹학회지
• /
• 제36권1호
• /
• pp.97-105
• /
• 1999

탄화규소와 카본의 이성분계에서 pH, 계면활성제 및 출발원료의 입도 분포를 이용하여 성형체의 기공크기 및 기공을 제어에 관한 연구를 수행하였다. 성형체는 각각 다른 분산조건을 가지는 슬러리를 이장성형법을 이용하여 제조하였으며, 입자간의 응\ulcorner제어에 따른 상이한 성형미세구조를 가지도록 하였다. 제조된 성형체에 대하여 반응소결 공정은 1$600^{\circ}C$, 진공분위기에서 20분간 응용 실리콘 침윤 공정을 행하였다. 초기 성형 미세구조가 반응소결체의 미세구조에 미치는 영향을 관찰하기 위하여 광학 현미경 및 SEM 분석을 통하여 제조된 소결체의 미세구조를 분석하였으며, 그리고 이에 따른 반응 소결체의 기계적 특성을 평가하였다. 각각의 분산조건에 따라 성형미세구조는 다르게 나타났으며, 미세한 탄화규소를 입자를 사용하였을 경우에 기공크기가 현저하게 작아짐을 확인하였다. 제조된 반응소결체의 미세구조 분석결과 일반적인 반응소결 탄화수소에서 발견되는 bimodal 미세구조는 관찰되지 않았으며, 이는 초기 탄화규소의 카본의 비가 중요항 변수인 것으로 분석하였다. 반응소결 탄화규소의 3-점 곡강도 값은 입자간의 분산이 잘 이루어진 성형체를 사용하였을 경우 310$\pm$40 MPa으로서 응집이 일어난 성형체를 사용한 반응소결체의 260$\pm$50MPa 보다 높았다.

• 한국세라믹학회지
• /
• 제31권2호
• /
• pp.129-136
• /
• 1994

Si3N4 with 6w/o Y2O3 and 1.5w/o Al2O3 has been gas pressure sintered and its densification behavior and the effect of the sintering variables on the microstructure and mechanical properties were investigated. Densification rate was higher at temperature below 1775$^{\circ}C$ and between 187$0^{\circ}C$ and 195$0^{\circ}C$ than between 1775$^{\circ}C$ and 187$0^{\circ}C$. The faster densification at temperature between 187$0^{\circ}C$ and 195$0^{\circ}C$ was thought to be due to the increased amount of liquid phase resulting from the increased amount of Si3N4 dissolving in the liquid. $\beta$-Si3N4 and Y-disilicate at temperatures below 1775$^{\circ}C$, and only $\beta$-Si3N4 at 187$0^{\circ}C$ and above were detected by XRD analysis. Three different two-step schedules were employed to obtain sintered body with above 99% theoretical density and to investigate the effect of the sintering variables on the density, the microstructure and the mechanical properties of the sintered body. The sintered density did not change with the heating rate, and the microstructure became coarser as the temperature increased. The strength decreased with the width of $\beta$-Si3N4 grain, while the fracture toughness increased with the square root of it. A ceramic cutting tool made of the sintered body showed an uniform flank wear after the cutting test.

• 대한토목학회논문집
• /
• 제40권2호
• /
• pp.177-182
• /
• 2020

본 연구에서는 시멘트 페이스트에 혼합된 나노 섬유가 경화된 시멘트페이스트의 압축강도와 인장강도에 미치는 영향을 연구하였다. 2종류의 나노 섬유를 사용하였다. 나일론 66 나노 섬유와 카본 나노 튜브로 보강된 나일론 66 나노 섬유를 전기방사로 제작하여 시멘트 파우더에 각각 혼합하였다. 물-시멘트비 0.5의 시멘트 페이스트 시편을 제작하고 28일간 양생하였다. 실험 결과, 나노섬유의 혼합이 시멘트 페이스트 시편의 압축강도와 인장강도를 증가시킴을 확인하였다. 나노 섬유의 보강 매카니즘을 확인하기 위해 주사전자현미경(SEM) 분석, 전계방사 투과전자 현미경(FE-TEM) 분석 및 열 중량 분석(TGA)을 수행하여 나노섬유를 포함한 시멘트 페이스트의 미세 구조를 분석하였다.

• 한국재료학회지
• /
• 제27권4호
• /
• pp.184-191
• /
• 2017

This present study deals with the effect of micro-alloying elements and transformation temperature on the correlation of microstructure and tensile properties of low-carbon steels with ferrite-pearlite microstructure. Six kinds of low-carbon steel specimens were fabricated by adding micro-alloying elements of Nb, Ti and V, and by varying isothermal transformation temperature. Ferrite grain size of the specimens containing mirco-alloying elements was smaller than that of the Base specimens because of pinning effect by the precipitates of carbonitrides at austenite grain boundaries. The pearlite interlamellar spacing and cementite thickness decreased with decreasing transformation temperature, while the pearlite volume fraction was hardly affected by micro-alloying elements and transformation temperature. The room-temperature tensile test results showed that the yield strength increased mostly with decreasing ferrite grain size and elongation was slightly improved as the ferrite grain size and pearlite interlamellar spacing decreased. All the specimens exhibited a discontinuous yielding behavior and the yield point elongation of the Nb4 and TiNbV specimens containing micro-alloying elements was larger than that of the Base specimens, presumably due to repetitive pinning and release of dislocation by the fine precipitates of carbonitrides.

• 한국재료학회지
• /
• 제25권8호
• /
• pp.370-375
• /
• 2015

In this study, to confirm the effect of alloying elements on the phase transformation and conditions of the friction stir process, we processed two materials, SS400 and SM45C steels, by a friction stir process (FSP) under various conditions. We analyzed the mechanical properties and microstructure of the friction stir processed zone of SS400 and SM45C steels processed under 400RPM - 100mm/min conditions. We detected no macro (tunnel defect) or micro (void, micro crack) defects in the specimens. The grain refinement in the specimens occurred by dynamic recrystallization and stirring. The microstructure at the friction stir processed zone of the SS400 specimen consisted of an ${\alpha}$-phase. On the other hand, the microstructure at the friction stir processed zone of the SM45 specimen consisted of an ${\alpha}$-phase, $Fe_3C$ and martensite due to a high cooling rate and high carbon content. Furthermore, the hardness and impact absorption energy of the friction stir processed zone were higher than those of base metals. The hardness and impact absorption energy of FSPed SM45C were higher than that of FSPed SS400. Our results confirmed the effect of alloying elements on the phase transformation and mechanical properties of the friction stir processed zone.

• 대한금속재료학회지
• /
• 제49권9호
• /
• pp.678-685
• /
• 2011

The aim of this study was to determine the effect of applied pressure and sintering temperature on the microstructure and mechanical properties for spark plasma sintering (SPS) from commercial pure titanium (CP-Ti) powders. Spark plasma sintering is a relatively new sintering technique in powder metallurgy which is capable of sintering metal and ceramic powers quickly to full density at a fairly low temperature due to its unique features. SPS of -200 mesh or -400 mesh CP-Ti powders was carried out in an $Ar+H_2$ mixed gas flowing atmosphere between $650^{\circ}C$ and $750^{\circ}C$ under 10 to 80 MPa pressure. When SPS was carried out at relatively low temperatures ($650^{\circ}C$ to $750^{\circ}C$), the high (>60 MPa) pressure had a marked effect on densification and grain growth suppression. The full density of titanium was achieved at temperatures and pressures above $700^{\circ}C$ and 60 MPa by spark plasma sintering. The crystalline phase and microstructure of titanium sintered up to $700^{\circ}C$ consisted of ${\alpha}$-Ti and equiaxed grains. Vickers hardness ranging from 293 to 362 Hv and strength ranging from 304 to 410 MPa were achieved for spark plasma sintered titanium.

• 한국초전도ㆍ저온공학회논문지
• /
• 제5권3호
• /
• pp.23-25
• /
• 2003

Effect of chemically modified precursor solution on YBCO coated conductor prepared by MOD-TFA method was investigated. YBCO thin films were deposited on (l00)-oriented single crystalline LaAlO$_3$ substrates by conventional MOD-TFA process. The microstructures of YBCO thin films contain maze-like patterns. The origin of this microstructure was delineated by compositional inhomogeneity during the pyrolysis process and it was shown that addition of diethanolamine (DEA) improve the microstructure of grown YBCO films. In addition, it was demonstrated that the chemical modification of precursor solution makes no harmful effect on biaxial texture of YBCO thin films.

• 소성∙가공
• /
• 제14권1호
• /
• pp.49-56
• /
• 2005

In the semi-solid die casting process, an important thing is the flow behavior of semi-solid materials. The flow patterns of the semi-solid material can make the defects during die filling. To control the flow patterns is very important and difficult. In this paper, the flow behavior of the semi-solid A356 alloy material during die filing at various die gate shapes has been observed with the grain size controlled material. The effect of the gate shape on the die filling characteristics was investigated. The filling tests in each plunger stroke were experimented, and also simulated on the semi-solid material die casting process by MAGMAsoft. According to the filling tests and computer simulation, the effect of the gate shape on liquid segregation has been investigated.