Study on the Defects in Welded Joint of Hot Reheat Steam Pipe of Ultra Supercritical Units after Long-term Service

doi:10.3901/JME.2017.18.113

Abstract

Abstract: In order to make better use of fossil fuels and reduce emissions of CO₂, there is a demand for increasing the thermal efficiency of fossil-fired power plants by raising operating temperatures and pressure. Therefore, the ultra super-critical (USC) power plants with improved thermal efficiency have been built in the past decades. P92 steel has been recognized as excellent ferritic heat-resistant steel for USC boilers because it exhibits high creep rupture strength, excellent thermal conductivity, low coefficient of thermal expansion, and good processing ability accompanied with satisfactory resistance to stress corrosion cracking and oxidation. However, defects (cracks, for example) are easy to form in P92 steel welded joint due to relatively large viscosity and poor liquidity of molten pool caused by high content of alloy elements, which would seriously affect the safe operation of the USC unit. In this paper, the defects in welded joint of a hot reheat steam pipe of a USC unit after long-term service are studied to ensure the safe operation of the USC unit. The results show that there exist a large number of circular or elliptic white spots in welding joint and hairline cracks initiate in some of the white spots. The molybdenum levels are greatly higher in the white spots than in other zones and the hydrogen content of the zone with the white spots is also higher than that of white-spot free zone, implying that the defects in welded joint are hydrogen-induced cracks, namely white flakes. The microstructure observation on the zones with and without the white flakes reveals that the martensite lath width is smaller, the morphology of lath martensite remains better and the size of the precipitates is smaller in the former than in the latter, suggesting that the recovery extent of the zone with white flakes is lower than that of white-flake free zone.

Key words: defect, microstructure, P92 steel, steam pipe, welded joint, white flake

CLC Number:

TG132

HONG Dinghua, WANG Huanli, LI Wuping, CAI Wenhe, DONG Shuqing, LIU Maosheng, ZENG Yanping. Study on the Defects in Welded Joint of Hot Reheat Steam Pipe of Ultra Supercritical Units after Long-term Service[J]. Journal of Mechanical Engineering, 2017, 53(18): 113-120.

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