Abstract
Impact toughness of the weld metal is one of the important factors affecting the quality of hot bends, which is strongly dependent on the microstructure transformation during hot bending and tempering. In this study, three kinds of weld metals with different Ni contents were selected, and then the effects of tempering temperature on the microstructure impact toughness of weld metals for hot bends were investigated by simulation conducted on a Gleeble-3500 thermal simulator. The results show that the nonmetallic inclusion particles in weld metals can become the nuclear core of acicular ferrite like in as-welded metal. So, the overlapping acicular ferrite microstructure is obtained in the weld metal after direct cooling from the reheating temperature. During tempering, the overlapping acicular ferrite microstructure is degenerated, and martensite/austenite (M/A) constituents in the acicular ferrite microstructure decompose into ferrites and carbides. The resulting carbide particles mainly distribute along the acicular ferrite grain boundaries. With the increase of the tempering temperature, the carbide particles coarsen, which decreases the impact toughness of the weld metal of hot bends. Addition of Ni to weld metals can refine the acicular ferrite and improve the impact toughness.
Highlights
Bends are some of key parts of the pipeline projects [1, 2]
Heavy-wall hot bends have been developed. e heavy-wall hot bends are usually made from longitudinal-seam submerged arc welding (LSAW) steel pipes by hot induction heated bending, on-line water cooling, and off-line tempering [5,6,7]. e microstructure and mechanical properties of the weld seam of LSAW steel pipes are different to the pipe body, and the acicular ferrite is obtained as an optimal microstructure to improve the impact toughness [8, 9]
It is well known that heat treatment is an effective method to improve the comprehensive mechanical properties of base materials and the weld joints [10,11,12]; especially, the impact toughness and strength of pipeline steels will be improved with the increase of the tempering temperature [10, 11]
Summary
Bends are some of key parts of the pipeline projects [1, 2]. Recently, in order to maximize the transport efficiency and decrease the construction and transposition costs, the gas pipeline transmission develops toward a larger diameter and/ or higher operation pressure; thereby, the high strength X80 steel grade and heavy-wall pipe over 22 mm have been applied to many long-distance transmission pipeline projects [3, 4], such as the ird West-to-East Gas Transmission Pipeline Project and the Sino-Russian Gas Transmission Pipeline Project. It is well known that heat treatment is an effective method to improve the comprehensive mechanical properties of base materials and the weld joints [10,11,12]; especially, the impact toughness and strength of pipeline steels will be improved with the increase of the tempering temperature [10, 11]. Our previous works showed that the impact toughness of weld seam metals seems to decrease trends with the increase of tempering temperature for the X80 heavy-wall hot bends [13]. For improving the impact toughness of the weld metal of heavy-wall X80 hot bends, it is necessary to further reveal the microstructure of acicular ferrite and its transformation during tempering. The evolution of the microstructure and its effects on the toughness are discussed. e results will be beneficial to improve the impact toughness of weld metals of the grade X80 hot bends
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