• 한국가스학회지
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• 제5권3호
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• pp.15-22
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• 2001

세계적으로 천연가스의 사용은 높은 효율을 가진 청정에너지로서의 장점으로 인하여 꾸준히 증가하는 추세이며, 국내에서도 급격한 경제규모의 증대와 함께 사용량이 증가하고 있다. 천연가스 배관 용접부는 금속학적 및 역학적으로 파괴에 영향을 미치는 인자들을 다양하게 가지고 있는데, 이들 인자들에 의하여 파괴역학적 인성의 열화가 발생함과 동시에 파괴시험 결과의 해석에 많은 어려움이 유발된다. 본 연구에서는 우선 국내 천연가스의 주 배관으로 사용되고 있는 API 5L X65 등급의 고장력강 용접부의 파괴역학적 안전성을 평가하기 위한 목적으로, 실제 원주 용접부와 심 용접부의 파괴인성 시험을 수행하여 미세조직의 변화를 중심으로 미시적 혹은 거시적인 관점에서의 금속학적 인자가 파괴인성에 미치는 영향에 대해 평가하였다.

• 한국해양공학회지
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• 제1권1호
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• pp.127-137
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• 1987

The evaluation of the elastic-plastic fracture toughness $J_{1C}$ was performed on the center of weld metal(CWM), the heat affected zone (HAZ) and the base metal (BM) of API 5LB steel pipes welded by the high frequency electric resistance welding. The $J_{1C}$ was evaluated by the JSME R-Curve and JSME SZW methods using the smooth and side-grooved specimens. The results are as follows; (1) The $J_{1C}$ values by the SZW method are overestimated as compared with those by the R-curve method, because the micro-crack is formed as SZW increase with the deformation at SZ after initiation of the ductile crack. (2) The everage of $J_{1C}$ values by the the R-curve and the SZW methods in side-grooved specimens tended to decrease in comparison with smooth specimens 9.42% at CWM, 4.2% at HAZ, 23.2% at BM, respectively. (3) The boundary of the fatigue pre-crack, stretched zone, and dimple regions appeared more clearly in side-grooved specimens, for the slight change of SZW in the direction of the plate thickness, as compared with smooth specimens.

• Journal of Welding and Joining
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• 제12권4호
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• pp.41-49
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• 1994

This study is to compare the welding quality of API 5L-X65 Steel- pipe's for natural gas transmission piping applied by SAW at shop and by GTAW-SMAW at site. The results can be summarized as follows; 1) Tensile strength of the welded zone by SAW(611.5 MPa) and by GTAW-SMAW(608.6 MPa) was maintained greater than that of the base metal(583.5 MPa). 2) Hardness of the welded zone and HAZ by SAW(Hv 194.8) & GTAW-SMAW (Hv 196.1) was slightly increased above that of the base metal (Hv 168.8), but less than the maximum allowable hardness(Hv 248). 3) Impact value of the welded zone by SAW(126.8 J) & GTAW-SMAW(88 J) became lower than that of the base metal(282.5 J), but was above the requirement of API 5L(68J). 4) Microscopic structure of the welded zone and HAZ by both SAW & GTAW-SMAW became fine-grained.

• 한국해양공학회지
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• 제22권2호
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• pp.50-57
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• 2008

The effect of weld improvement on the corroded fatigue life of welded structures was investigated. Toe grinding, TIG dressing and weld profiling were used as the geometric improvement methods. Fatigue tests under the corroded condition in artificial seawater were carried out to investigate the corrosion fatigue behavior of API 2W Gr.50T steel plate produced by POSCO. The test results in weld improved conditions were compared with those in as-welded condition. The test results were also compared with the design curves in UK DEn Class F. Corroded fatigue life of weld improved specimens was longer than that of as-welded specimen. Especially, the corroded fatigue life exceeded the mean SN curve in air of UK DEn Class F.

• Journal of Welding and Joining
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• 제30권6호
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• pp.6-9
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• 2012

• Corrosion Science and Technology
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• 제20권6호
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• pp.373-383
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• 2021

The effect of Cr addition to high Mn steel on flow-accelerated corrosion (FAC) behavior in a neutral aqueous environment was evaluated. For comparison, two types of conventional ferritic steels (API X70 steel and 9% Ni steel) were used. A range of experiments (electrochemical polarization and impedance tests, weight loss measurement, and metallographic observation of corrosion scale) were conducted. This study showed that high Mn steel with 3% Cr exhibited the highest resistance to FAC presumably due to the formation of a bi-layer scale structure composed of an inner Cr enriched Fe oxide and an outer Mn substituted partially with Fe oxide on the surface. Although the high Mn steels had the lowest corrosion resistance at the initial corrosion stage due to rapid dissolution kinetics of Mn elements on their surface, the kinetics of inner scale (i.e. Cr enriched Fe oxide) formation on Cr-bearing high Mn steel was faster in dynamic flowing condition compared to stagnant condition. On the other hand, the corrosion scales formed on API X70 and 9% Ni steels did not provide sufficient anti-corrosion function during the prolonged exposure to dynamic flowing conditions.

• Structural Engineering and Mechanics
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• 제81권5호
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• pp.565-574
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• 2022

The infrastructures such as offshore, bridges, power plant, oil and gas piping and aircraft operate in a harsh environment during their service life. Structural integrity of engineering components used in these industries is paramount for the reliability and economics of operation. Two regression models based on the concept of Gaussian process regression (GPR) and Minimax probability machine regression (MPMR) were developed to predict stress intensity factor range (𝚫K). Both GPR and MPMR are in the frame work of probability distribution. Models were developed by using the fatigue crack growth data in MATLAB by appropriately modifying the tools. Fatigue crack growth experiments were carried out on Eccentrically-loaded Single Edge notch Tension (ESE(T)) specimens made of API 5L X65 Grade steel in inert and corrosive environments (2.0% and 3.5% NaCl). The experiments were carried out under constant amplitude cyclic loading with a stress ratio of 0.1 and 5.0 Hz frequency (inert environment), 0.5 Hz frequency (corrosive environment). Crack growth rate (da/dN) and stress intensity factor range (𝚫K) values were evaluated at incremental values of loading cycle and crack length. About 70 to 75% of the data has been used for training and the remaining for validation of the models. It is observed that the predicted SIF range is in good agreement with the corresponding experimental observations. Further, the performance of the models was assessed with several statistical parameters, namely, Root Mean Square Error (RMSE), Mean Absolute Error (MAE), Coefficient of Efficiency (E), Root Mean Square Error to Observation's Standard Deviation Ratio (RSR), Normalized Mean Bias Error (NMBE), Performance Index (ρ) and Variance Account Factor (VAF).

• Journal of Welding and Joining
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• 제32권1호
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• pp.93-101
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• 2014

This study aims to investigate the effect of heat input of outside SAW weld on low temperature toughness($-20^{\circ}C$) of inside SAW weld for API 5L X70 with sour gas resistance. As increasing heat input of the outside weld, low temperature toughness of the inside weld was decreased. Especially, in spite of the same heat input, the value of low temperature toughness was fluctuated. On the basis of fracture and microstructure analysis, the low temperature toughness is correlated with the fracture area ratio of shear lips and four kinds of fracture sections. These sections were divided with size and shape of dimple correlated with grain boundary ferrite and cleavage correlated acicular and polygonal ferrite in grain. Therefore, it was seen that these sections were two of final solidification area in the inside weld and the outside weld, no reheated zone and reheated zone in the inside weld. In conclusion, it is thought that the difference of low temperature toughness at the same heat input is due to the fact that each of impact test specimens could have the different microstructure, even though the notch was machined under the error tolerance of 1mm. It is because the final solidification area of the inside weld is very narrow.

• 대한금속재료학회지
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• 제47권2호
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• pp.59-70
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• 2009

This study is concerned with the correlation between microstructure and mechanical properties in heat affected zones (HAZs) of API X80 pipeline steels containing complex oxides. Three kinds of specimens were fabricated by varying alloying elements of Ti, Al, and Mg to form complex oxides, and their microstructures, Vickers hardness, Charpy impact properties were investigated. The number of complex oxides increased as the excess amount of Ti, Al, and Mg was included in the steels. The simulated HAZs containing a number of oxides showed a high volume fraction of acicular ferrite region because oxides acted as nucleation sites for acicular ferrite. According to the correlation study between thermal input, volume fraction of acicular ferrite region, and Charpy impact properties, the ductile fracture occurred predominantly when the volume fraction of acicular ferrite region was 65% or higher, and the Charpy absorbed energy was excellent over 200 J. When the volume fraction of acicular ferrite region was 35% or lower, the Charpy absorbed energy was poor below 50 J as the brittle cleavage fracture occurred. These findings suggested that the active nucleation of acicular ferrite in the oxide-containing steel HAZs was associated with the great improvement of Charpy impact properties of the HAZs.

• 대한금속재료학회지
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• 제46권12호
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• pp.788-799
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• 2008

This study is concerned with the effects of Mo, Cr, and V addition on tensile and Charpy impact properties of API X80 linepipe steels. Four kinds of steels were processed by varying Mo, Cr, and V additions, and their microstructures and tensile and Charpy impact properties were investigated. Since the addition of Mo and V promoted to form fine acicular ferrite and granular bainite, while prohibiting the coarsening of granular bainite, it increased the strength and upper shelf energy, and decreased the energy transition temperature. The Cr addition promoted the formation of coarse granular bainite and secondary phases such as martensite-austenite constituents, thereby leading to the increased effective grain size, energy transition temperature, and strength and to the decreased upper shelf energy. The steel containing 0.3wt.% Mo and 0.06wt.% V without Cr had the highest upper shelf energy and the lowest energy transition temperature because its microstructure was composed of fine acicular ferrite and granular bainite, together with a small amount of hard secondary phases, while its tensile properties maintained excellent.