• Transactions of the Korean Society of Automotive Engineers
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• v.15 no.3
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• pp.106-113
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• 2007

The effect of chemical composition on the microstructural change and tensile property in TRIP-assisted steels with different chemical composition was investigated by using SEM, TEM, XRD and UTM. As a result of microscopic observation, the morphology of retained austenite could be identified as two types : a granular type in a steel containing higher sillicon and a film type in a steel having higher carbon. For the case of higher carbon-containing steel with a tensile strength of 860 MPa and a total elongation of 38%, film-typed retained austenite could be observed between lath bainitic ferrite. Actually, metastable retained austenite was a requisite for the good formability, which means that chemical composition plays a significant role in the microstructure and tensile property of TRIP-assisted steels. With respect to tensile property, the steels containing suitable silicon and manganese, respectively, showed a typical TRIP effect in stress-strain curve, while a steel containing higher manganese content exhibited the assimilar behavior shown in dual phase steel.

• Journal of Welding and Joining
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• v.17 no.3
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• pp.55-65
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• 1999

Metallurgical factors influencing toughness of the Intercritically Reheated Coarse-Grained Heat Affected Zone (ICCG HAZ) of multiple welded SA508-cl.3 Reactor Pressure Vessel Steel were evaluated. The recrystallized austenite formed along the prior austenite grain boundaries and late interfaced on heating to the intercritical range was transformed to bainite and/or martensite during cooling. The newly formed martensite always included some retained austenite(M-A constituents). The characteristics(amount, hardness, density, and size) of M-A constituents were found to be strongly associated with both peak temperature and cooling time(△t8/5(2)) of last pass. Toughness in the ICCG HAZ was deteriorated with increasing amount of M-A constituents which was increased with increasing the last peak temperature within the intercritical temperature range. Meanwhile, for the same intercritical peak temperature, toughness was decreased with increasing cooling time. When cooling time was short, the dominant factor influencing toughness of the ICCG HAZ was amount of M-A constituents. However, when cooling time was lengthened, the hardness difference between M-A constituents and softened matrix(tempered martensite) was found to be the dominant factor.

• Korean Journal of Materials Research
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• v.25 no.1
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• pp.9-15
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• 2015

The effect of retained and reversed austenite on the damping capacity in high manganese stainless steel with two phases of martensite and austenite was studied. The two phase structure of martensite and retained austenite was obtained by deformation for various degrees of deformation, and a two phase structure of martensite and reverse austenite was obtained by reverse annealing treatment for various temperatures after 70 % cold rolling. With the increase in the degree of deformation, the retained austenite and damping capacity rapidly decreased, with an increase in the reverse annealing temperature, the reversed austenite and damping capacity rapidly increased. With the volume fraction of the retained and reverse austenite, the damping capacity increased rapidly. At same volume of retained and reversed austenite, the damping capacity of the reversed austenite was higher than the retained austenite. Thus, the damping capacity was affected greatly by the reversed austenite.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.2
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• pp.137-148
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• 2012

In pressurized water nuclear reactors, dissimilar metal welds are susceptible to primary water stress corrosion cracking. To access this problem, accurate estimation of welding residual stresses is important. This paper provides general welding residual stress profiles in dissimilar metal nozzle butt welds using finite element analysis. By introducing a simplified shape for dissimilar metal nozzle butt welds, changes in the welding residual stress distribution can be seen using a geometry variable. Based on the results, a welding residual stress profile for dissimilar metal nozzle butt welds is proposed that modifies the existing welding residual stress profile for austenitic pipe butt welds.

• Journal of Power System Engineering
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• v.12 no.3
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• pp.72-77
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• 2008

This study was investigated to know the effect of Cu addition on austempered ductile cast iron at various temperatures and times. Test results showed that the volume fraction of retained austenite and the carbon contents of retained austenite showed the greatest value at $400^{\circ}C$. Also, in case of specimens having more Cu contents, the volume fraction of retained austenite and the carbon contents of retained austenite showed the lower value. After austempering treatment, tensile strength and Impact value increased, but elongation decreased. With increasing austempering treatment temperature, tensile strength, elongation, and impact value decreased. In case of specimen having more Cu contents, tensile strength showed the higher value, but elongation showed the lower value. Damping capacity was decreased by austempering treatment and was not affected on austempering temperature and time. In case of specimen having more Cu contents, damping capacity showed the higher value.

• Journal of Korea Foundry Society
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• v.17 no.2
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• pp.151-157
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• 1997

다섯 조성의 서로 다른 Mo, Ni 및 Mn함량을 지닌 3 mass% C-20 mass% Cr 공정조성의 백주철을 질소분위기하에서, 1173, 1273 및 1373 $^{\circ}K$의 온도로 3.6, 7.2, 14.4 및 28.8 ks동안 불안정화열처리를 행한 후 공냉시켜 잔류오스테나이트를 측정한 결과 합금원소함량, 불안정화열처리조건 및 $V{\Upsilon}$사이에 다음과 같은 관계를 얻었다. 즉 Mo, Ni 및 Mn을 첨가한 결과, 주방상태에서 79.89${\sim}$91.65 vol.%범위의 $V{\Upsilon}$가 얻어 졌으며 퍼얼라이트는 전혀 생성되지 않았다. 1173 $^{\circ}K$에서 불안정화열처리를 한 경우에는 무수히 작은 이차탄화물이 석출하였으나 온도가 높아 질수록 그 수는 점차 감소하여 1373 $^{\circ}K$에서는 거의 관찰되지 않았다. 1173 및 1373 $^{\circ}K$에서 불안정화열처리를 한 경우, 불안정화열처리시간의 증가에 따른 $V{\Upsilon}$의 변화는 미미하였으나 1273 $^{\circ}K$의 경우에는 감소하였다. 또한 Mo, Ni 및 Mn의 첨가량이 많아짐에 따라 $V{\Upsilon}$는 증가하였으며 이 두 함수사이의 관계식을 정립시키기 위하여 P인자를 도입하였다.

• Journal of the Korean Society for Heat Treatment
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• v.13 no.4
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• pp.253-258
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• 2000

The present study aimed to develop the TRIP(transformation induced plasticity) aided high strength low carbon steel sheets using reverse transformation process. The cold-rolled 0.14C-6.5Mn steel was reverse-transformed by slow heating to intercritical temperature region and air cooling to room temperature. An excellant combination of tensile strength and elongation of $98.3kgf/mm^2$ and 44.4% appears. This combination comes from TRIP phenomena of retained austenite during deformation. The stability of retained austenite Is very Important for the good ductility and it depends on diffusion of carbon and manganese during reverse transformation. The air cooling after holding at intercritical temperature retards the formation of pearlite and provides the carbon enrichment in retained austenite, resulting the increase of elongation in cold-roiled TRIP steel.

• Proceedings of the KAIS Fall Conference
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• 2003.06a
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• pp.83-86
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• 2003

SKD11은 자이로 펌프의 로타와 붓싱 재질로 사용되는데, 자이로펌프는 임펠러의 회전이 없고 원판의 기울기 운동으로 흡입과 토출을 반복하는 고압용적식 펌프이다. 이 펌프의 내구성을 향상시키기 위하여 우수한 내압성과 내마모성을 갖는 재질선정이 요구한다. 따라서 현재 가장 널리 사용되고있는 SKD11에 심냉처리를 통하여 미세조직의 개선을 수행하였고, 가스질화처리에 의한 표면경화로 내압성과 내마모성을 증가시키고자 하였다. SKD11의 미세조직 변화를 조사하기 위하여 광학현미경과 X-선 회절기를 사용하여 미세조직 관찰과 상변화를 조사하였으며, Rockwell 경도기틀 사용하여 각 변수에 따른 경도변화를 조사하였다. 연구 결과 기지조직은 모두 마르텐사이트로 나타났으며, 이 이외에 각각의 조건에 따라 크롬탄화물, PFZ, 잔류오스테나이트가 확인되었다. SKD11을 액화질소로 냉각한 후 500℃에서의 템퍼링으로 가장 큰 경도값을 얻을 수 있었다. 또한 가스질화처리에 의하여 표면경도를 크게 증가시킬 수 있었다.

• Journal of Korea Foundry Society
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• v.32 no.3
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• pp.138-143
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• 2012

This steel has been synthesized integrating concepts from Austempering Ductile Cast Iron (ADI) technology. While ADI has excellent mechanical and physical properties, the Young's modules of ADI is approximately 20% lower than steel. In addition, the presence of graphite nodules in ADI can be sites of crack initiation, where fracture takes place at graphite matrix interface. Because of this limitations of ADI, there has been a growing interest in austempered steels as structural materials in resent years. In this investigation, a new steel with microstructure composed of ferrite and austenite and with simultaneous high tensile strength (1,150 MPa) and high ductility (33%) was developed. The goal of this investigation is to obtain a better understanding of deformation and transformation behaviour in high carbon retained austenite(${\gamma}_{HC}$) and over-saturated ferrite(${\alpha}$) during the plastic deformation. A detailed study of the microstructure of this steel was carried out by means of X-ray diffraction (XRD) and electron back scattering diffraction (EBSD) technic. In this way it was shown that BCC phase (BCC) took up the larger part of the nominal strain whereas the a part of retained austenite responded to the mechanincal load by partial martensite transformation, and misorientation change in the retained austenite after plastic strain could be attributed to the large elongation.

• Journal of the Korean Institute of Gas
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• v.2 no.3
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• pp.78-87
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• 1998

The mechanical properties, heat treatment and the history for the development of Ni-bearing ferritic steels for the application at low temperatures are reviewed. Ni-bearing ferritic steels are classified into $2-3\%$ Ni steels, $5.5\%$ Ni steel, $9\%$ Ni steel and $13\%$ Ni steel., of which $9\%$ Ni steels are most widely used for the large LNG storage tanks owing to their encellent fracture toughness up to $-196^{\circ}C$. The effect of retained austenite on the tensile properties and toughness was precisely discussed. As the size of LNG storage tank increases, thicker plates are needed. Thus, the recent efforts for the improvement of low temperature toughness and homogeneity are also introduced.