Investigation of the microstructure and toughness of 550 MPa grade pipeline after the hot-bending process

Abstract

In this work, the effects of the hot-bending process on the microstructure and low temperature (−40°C) toughness of weld metal of pipeline K65 (the highest grade of the Russian natural gas pipeline) were studied. The weld metal from the longitudinally submerged arc welding (LSAW) pipe making process was compared with that subsequently treated by the hot-bending process. It was found that the latter process led to serious deterioration in toughness, which obviously degraded the sound properties achieved in the original weld metal of LSAW pipes. Microstructural characterisation revealed that the weld metal, which originally consisted mainly of acicular ferrite in the as-deposited condition, became predominantly composed of bainitic ferrite after hot bending. It is clear that reaustenisation caused a smaller austenite grain-sized matrix, which brought about a very high volume fraction of bainite. Consequently, the low temperature toughness was deteriorated, in contrast to the excellent toughness achieved in the as-deposited weld metal.