Effect of heat treatment on microstructure and properties of welded joint of X70 steel used for bent pipe under extremely cold service condition

doi:10.13251/j.issn.0254-6051.2022.10.029

Abstract

Abstract: X70 pipeline steel was welded through SMAW and then by simulating the induction pipe bending process, the welded joint was quenched at 950 ℃ and tempered at 500, 550 and 600 ℃. Effect of different tempering temperatures on the microstructure and properties of the welded joints were studied. The results show that the microstructure of the weld and the heat affected zone are tempered sorbite, and the low temperature impact absorbed energy at -45 ℃ of the weld and the heat affected zone reduces significantly after quenching and tempering. With the increase of the tempering temperature, the low temperature impact absorbed energy increases gradually, which is attribute to the increase of the content of carbide precipitates and acicular ferrite(AF) in the weld structure, and the carbide distribution gradually becomes uniform. The heat affected zone has the highest impact absorbed energy when tempered at 550 ℃, but it decreases obviously when tempered at 600 ℃, which is attribute to the coarsening of both the microstructure and the carbide precipitates. After quenching and tempering, the hardness of the weld is the highest, and the pipe during tensile test fractures in the base metal, the strength of the welded joint meets the requirements. Thus, the optimal matching of the impact toughness and tensile properties of the welded joint can be obtained when tempered at 550 ℃.

Key words: X70 pipeline steel, welded joint, heat treatment, low temperature impact toughness

CLC Number:

TG441.8

Zhang Xiazhou, Chen Yanqing, Zhao Yingjian, Dong Xianchun. Effect of heat treatment on microstructure and properties of welded joint of X70 steel used for bent pipe under extremely cold service condition[J]. Heat Treatment of Metals, 2022, 47(10): 173-178.

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