Effect of heat treatment on microstructure and properties of welded joints of X80 steel used for bent pipe under extremely cold service conditions

doi:10.13251/j.issn.0254-6051.2020.10.016

Abstract

Abstract: Effects of different heat treatment processes on the microstructure and low-temperature impact property of the welded joints of X80 steel used for bent pipe were studied by means of microstructure analysis, low-temperature impact test, and fracture analysis. The results show that, when normalized in the range of 890-980 ℃, the microstructure of welded joints gradually coarsens with the increase of normalizing temperature, and the number of M-A components and carbides rises, the aggregation extent increases gradually, and the low-temperature impact property decreases. When tempered in the range of 620-680 ℃, with the increase of tempering temperature, the mixed microstructure of bainite and acicular ferrite in the welded joints undergoes recovery and recrystallization, the content of carbide precipitates and proeutectoid ferrite increases, and the hard and brittle M-A components decompose gradually, the impact absorbed energy at low temperature increases first and then decreases, and the microhardness decreases gradually. The microstructure of the welded joints after heat treatment is mainly composed of acicular ferrite (AF) and granular bainite (GB), while a small amount of proeutectoid ferrite (PF) and dispersed M-A components are present. When normalized at 920 ℃ and then tempered at 660 ℃, the impact absorbed energy at low temperature of the welded joints is the best (148 J), while the microhardness is fluctuated slightly, and the radiation zone of the impact fracture is quasi-cleavage fracture, without large-scale cracks and with partial dimples, showing that the bent pipe treated by this heat treatment process can meet the service requirements under the extreme cold condition of -45 ℃.

Key words: X80 pipeline steel, microstructure of welded joints, heat treatment, low temperature toughness

CLC Number:

Bi Zongyue, Wei Feng, Zhang Wanpeng, Li Jie, Zhang Min. Effect of heat treatment on microstructure and properties of welded joints of X80 steel used for bent pipe under extremely cold service conditions[J]. Heat Treatment of Metals, 2020, 45(10): 79-84.

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