• Journal of Ocean Engineering and Technology
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• v.17 no.2
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• pp.40-46
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• 2003

The corrosion resistance of super duplex stainless steel on both its fibrous and dispersed phase was investigated. These structures consist of various volume fraction and distribution of austenite structure, which were obtained by changing the heat treatment temperature and cycle. The fibrous phase had higher austenite volume fraction than that of the dispersed phase at the same temperature. Corrosion resistance of super duplex stainless steel was evaluated through an immersion test and an impingement test, using 35% HCI and sea water, respectively. Super duplex stainless steel was compared with STS316L and STS304. The corrosion resistance of super duplex stainless steel was superior to ST316L and STS304. The dispersed phase of super duplex stainless steel was more stabilized than the fibrous phase in corrosion. The magnitude of corrosion rate was in order STS304, STS316L, fibrous phase of super duplex stainless steel and dispersed phase of super duplex stainless steel.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2012.06a
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• pp.173-173
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• 2012

Duplex STS는 응력부식 저항이 큰 페라이트상과 우수한 내식성을 제공하는 오스테나이트상이 미세하게 1:1로 결합하여 강도가 오스테나이트 STS 보다 최소 1.7배 이상 높을 뿐 아니라 공식(pitting)과 응력부식 저항성이 우수해 최근에 주목받고 있는 고내식 고강도 재료이다. STS의 내식성을 평가하는 여러 지수 중 Pitting에 대한 내식성을 평가하는 지수로서 PREN (Pitting Resistance Equivalent Number)이 있다. PREN =%Cr + 3.3%(Mo + 0.5%W) + 16%N PREN이 30 이상이면 해안지역에서 사용가능하나, PREN이 40 이상인 경우에는 원자력발전소, 탈황 설비, 해수설비 및 화학Plant 등 고내식 환경에서 주로 사용가능하다. PREN이 40 이상인 Super Duplex STS은 다량의 Mo와 N을 첨가하여 만든 제품으로, 최근 10여 년 동안 해수 냉각 설비, 해수 담수화 설비, 탈황 설비, 석유화학 설비 및 원전용 CASK 등의 다양한 분야에 그 사용량이 꾸준히 증가하고 있는 상황이다. 본 연구에서는 Super Duplex STS의 TIG용접에서 실드가스 중의 $N_2$의 첨가가 PREN에 미치는 영향을 검토하였다. 실드가스 중 $N_2$가 용접금속으로 침입하는 메커니즘을 규명하고, 용접조건 변화에 따른 용접금속 내 N의 함량을 측정하여 PREN을 계산하고, 용접금속의 기계적 특성과 미세조직을 검토하였다.

• Corrosion Science and Technology
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• v.22 no.6
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• pp.435-446
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• 2023

Corrosion behaviors of laser-welded super duplex stainless steel (SDSS) tubes after exposure to an actual power plant environment for one year and those of fin-tube welded SDSS were evaluated. Results showed that corrosion damage on the back side of the SDSS tube in the direction of hot air was higher than that on the front side regardless of weldment location. However, corrosion damage showed no difference between weldment and base metal due to recovery of phase fraction in the weldment through post weld heat treatment (PWHT). Nevertheless, the SDSS tube showed severe corrosion damage along grain boundary due to surface phase transformation (δ → γ) and Cr₂N precipitation caused by PWHT with a high N₂ atmosphere. Corrosion resistance of the SDSS tube was recovered when degraded surface was removed. Corrosion sensitivity of a fin-tube increased significantly due to pre-existing crevice, unbalanced phase fraction, and σ phase precipitation adjacent to the fusion line. Although corrosion resistance was improved by recovered phase fraction and sufficient dissolution of σ phase during PWHT, corrosion reaction was concentrated at the pre-existing crevice. These results suggest that welding conditions for fin-tube steel should be optimized to improve corrosion resistance by removing pre-existing crevice in the weldment.