Behaviors of Microstructure and Properties in API X70 SAW Weld

Uhm, Sang-Ho; Kim, Sung-Wook; Kim, Jong-Hyun; Lee, Chang-Hee; Kim, Gwang-Soo

Journal of KWJS 2005;23(2):66-74.
Published online May 12, 2005.

[연구논문]API X70 후육강관 SAW용접부 미세조직 및 인성 거동에 대한 연구

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Behaviors of Microstructure and Properties in API X70 SAW Weld

Sang-Ho Uhm, Sung-Wook Kim, Jong-Hyun Kim, Chang-Hee Lee, Gwang-Soo Kim

Abstract

API(American Petroleum Institute) steel, as a line pipe material, requires the enhanced mechanical and chemical properties with the environmental severity. Especially, the weld part(weld metal and heat affected zone)is an important region for the safety. However, the study for the behavior of microstructure and toughness in multi-pass welding is seldom. In this study, the relationship between the microstructure and toughness of welds with several welding, bending and heat-treatment conditions was examined. In particular, HIC property in the weld metal was evaluated. The microstructure and toughness in multi-passed HAZ seemed to be determined by the final welding thermal cycle and the low toughness was attributed to the MA constituents formed in the intercritically reheated region. The weld metal showed very low toughness and it was not improved by the change in bending and heat treatment conditions. Additionally, the cracks are observed in the weld metal. From these results, it was found that the choice of welding wire/flux is very important.