The microstructure and mechanical properties of the heat-affected zone welded using the GTAW process on SDSS material used in offshore structures were analyzed. The chemical composition of the specimen material was analyzed using XRF. The microstructure of the heat-affected zone where the plate was welded was examined by SEM, and the ferrite volume fraction was assessed using the point count method of ASTM E562. A lot of ferrite was formed in the overheated weldment region, and In the weld cap where the cooling rate was fast, ferrite was not converted back to austenite and the microstructure was not uniform. From the ferrite phase fraction, it was shown that it can be applied to the pitting resistance equivalent numbers through changes in mechanical properties according to welding conditions.

In this study, we investigated the precipitation behavior of the R-phase precipitated at the initial stage of aging and its effect on the pitting corrosion of 25%Cr-7%Ni-4%Mo super duplex stainless steel. The R-phase in super duplex stainless steel was mainly precipitated at the interface of ferrite/austenite phases and inside of the ferrite phase during the initial stage of aging, and it was transformed into the σ-phase with an increase in aging time. The ferrite phase was decomposed into a new austenite phase and σ-phase. The R phase was an intermetallic compound, which represented a lower Ni and higher Mo than the matrix, and also had a higher Mo and Cr concentration than the σ phase. With an increasing aging time, the pitting potential Ep was increased slowly by the precipitation of the R-phase, and it was then steeply decreased by the precipitation of the σ-phase. The R-phase was decreased the pitting potential, but its effect was smaller than effect of σ-phase.

In this study, we investigated the precipitation behavior of the R-phase precipitated at the initial stage of aging and its effects on the mechanical properties of 25%Cr-7%Ni-2%Mo-4%W super duplex stainless steel. The R-phase was mainly precipitated at the interface of ferrite/austenite phases and inside of the ferrite phase during the initial stage of aging. It was transformed into the σ-phase with an increase of the aging time. The ferrite phase was decomposed into a new austenite(γ2)phase and the σ-phase by an aging treatment. The R phase was an intermetallic compound showing higher molybdenum and tungsten concentrations than the matrix and also showed higher molybdenum and tungsten concentrations than the σ phase. In the initial stage of aging, precipitation of the R-phase did not change the hardness, the strength and the elongation. The hardness and the strength increased upon a longer aging time, but the elongation rapidly decreased. These results show that the R-phase did not significantly affect the hardness and the strength, though it did influence the elongation.

Austenite precipitation behavior was studied with solidification rates and alloying contents, N and Cr, in duplex stainless steels by directional solidification. Directional solidification experiments were carried out with solidification rates, 1~100mm/s, and N and Cr contents, 0~0.27wt.%, 25~28wt.% respectively, in a duplex stainless steel, CD4MCU. As the solidification rate increases, the dendrite spacing reduced and the austenite phase in the ferrite matrix became finer. The volume fraction of austenite phase increased and its shape went to be round with increasing nitrogen contents in duplex stainless alloys. The Cr alloying element, even though it is a ferrite former, showed to enhance the nitrogen solubility in the alloy and caused the austenite round and finer. Also, Cr was supposed to decrease the austenite volume fraction, but it increased the austenite slightly due to increasing nitrogen solubility during solidification.

본 연구에서는 주조 2상 스테인리스강의열시효에 대한 시효온도, 시효시간 및 Nb함유량의 영향을 관찰하기 위해 기계적 성질 및 조직을 조사하였으며 Nb을 함유한 주조 2상 스테인리스강의 파괴기구를 규명하기 위해 SEM에 의한 파단면 관찰과 WDS성분분석을 통해 파괴기구의 특성을 고찰하였다. 시효온도와 시효시간이 증가함에 따라 페라니트으 미소경도가 증가하였으며 항복강도의 경우 시효온도와 시효시간에는 영향을 받지 않았으나 Nb을 함유한 재료들이 Nb을 함유치 않은 재료들에 비해 다소 낮은 항복강도 값을 보였다. 충격흡수에너지 값은 시효시간 및 시효온도의 증가에 따라 시험된 모든 재료에서 저하되었는데 0.4% Nb을 함유하는 경우 Nb을 약간 함유하거나 함유치 않은 재료들에 비해 시효시간에 따라 급격한 감소 경향을 보였다. 파단면 관찰결과 페라이트 기지 또는 페라이트/오스테나이트 상경계에서 석출된 VbC를 비롯한 탄화물들이 취성저항성을 낮추는데 크게 기여했음을 알 수 있었다.

기존 탄소강과 스테인리스강의 재료적 특성(인장강도, 탄성계수, 파단형태 등)이 상이하기 때문에 탄소강과 다른 응력도 -변형도 곡선을 필요로 한다. 본 논문에서는 Ramberg-Osgood을 바탕으로 Rasmussen이 제안한 스테인리스강에 대한 곡선을 Arrayago 등에 의해 제시된 변수와 응력도-변형도 수정식을 유한요소해석 결과의 예측내력과 실험내력을 비교한다.

Stainless steel has many advantages such as excellent durability, ductility and corrosion resistance. Recently, the use of stainless steel has been increased in a variety of structural members and industrial fields. But, stainless steels are not included is the structural materials of Korean building standards and corresponding design standards are not specified. In this paper, finite element analysis has been performed to investigate the ultimate strength of single shear duplex stainless steel (STS329FLD) four-bolted connection with extended variables. As a result, strength reduction ratio by curling was estimated quantitatively.

최근에 오스테나이트와 페라이트의 조직을 갖는 듀플렉스계 스테인리스강이 개발되었고 오스테나이트계 스테인리스강에 비해 고강도, 응력부식균열에 대한 고내식성과 재료비절감으로 다양한 산업분야에서 사용량이 증가되고 있다. 그러나, 현재 스테인리스강은 한국 건축구조기준에 구조용재료로 지정되어 있지 않고 구조설계기준도 마련되어 있지 않는 실정이다. 이 연구에서는 2행 1열 듀플렉스계 스테인리스강(STS329FLD) 볼트접합부에 대한 구조적 성능을 조사하기 위한 실험적 연구를 수행하였다. 주요변수는 전단접합형태(일면전단과 이면 전단)과 하중방향 연단거리이다. 종국파단형태는 전형적인 블록전단파단, 인장파단과 면외변형이 관찰되었다. 일면전단 접합부에서 면외변 형발생으로 최대 20%까지 내력저하가 발생되었다. 실험최대내력은 현행설계기준 AISC/AISI/KBC, EC3와 AIJ 및 기존연구자에 의한 제안된 식에 의한 예측내력과 비교하였다. 면외변형이 발생하지 않은 접합부에 대해서 실제 전단파단단면을 고려한 Clement & The의 식에 의한 예측 내력이 실험내력에 가장 근접했고 면외변형을 동반한 접합부에 대해서는 면외변형을 고려해 제안한 Kim & Lim식에 의해서 과대평가하는 것으로 나타났다.

In this study, structural behaviors such as ultimate strength and fracture shape of duplex stainless steel (STS329FLD) fillet-welded connections according to welding direction were investigated through monotonic tensile test. Main variables are weld length and welding direction against the loading direction. Specimens of SDTFW series (transverse to the loading direction) and SDLFW series (longitudinal to the loading direction) failed by tensile fracture and shear fracture, respectively. Ultimate strengths got higher with the increase of weld length and SDTFW series showed higher ultimate strengths than SDLFW series.

In this paper, block shear fracture behavior in base metal of fillet-welded connection fabricated with duplex stainless steel (STS329FLD) were investigated through experimental procedures. Main variables are weld lengths in the longitudinal and the transverse directions of applied force. As a result, test specimens failed by typical block shear facture (the combination of tensile fracture and shear-out fracture) in base metal not weld metal. Ultimate strength of the specimens tended to get higher with the increase of the weld lengths.

Recently, the use of duplex stainless steel which has a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel has been increased in a variety of industrial fields. Especially, STS329FLD with lower nickel content and higher yield strength compared with austenitic stainless steel has been developed. In this paper, experiments have been conducted to investigate the structural behaviors of single shear duplex stainless steel(STS329FLD) bolted connection with four bolts. Specimens with relatively long end distance failed by block shear fracture accompanied with curling (out-of-plane deformation) and the curling reduced the ultimate strength.