Microstructure and Properties Research of Overlay Welding Triple-Phase Zone at End of N08825 Lined Steel Pipe
The lined steel pipe is a kind of tightly bonded bimetallic pipe formed by the composite forming of corrosion-resistant alloy liner and carbon steel backing pipe.The overlay welding at pipe end is an important guarantee for the weldability of engineering applications.The N08825 straight welded pipe was produced by liner unit forming,and it was inserted into the base pipe to produce the composite pipe by hydrostatic composite method.Then the overlay welding at the lined pipe end was carried out by GTAW with double-tungsten and hot wire.The residual stress distribution,microstructure,microhardness and corrosion resistance of the overlay weld and the adjacent liner and parent pipe were studied and analyzed.The results show that the yield strength of the liner straight weld is(325±20)MPa before composite forming.The inner diameter of pipe end is reduced about 0.2 mm because of residual stress after overlay welding.The hardness of the overlay weld and the adjacent liner and parent pipe meets the requirement of API Spec 5LD standard.The average pitting corrosion loss of the overlay weld of 20 batches is about 0.1201 g/cm2.The intergranular corrosion resistance of liner weld adjacent to the overlay weld is similar to that before composite forming.There is no significant difference in the corrosion rate under sensitization and non-sensitization treatment conditions,and the average value is about 0.16 mm/a.
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