Microstructure and toughness of thick-gauge pipeline steel joint via double-sided friction stir welding combined with preheating

Fusion welding easily causes microstructural coarsening in the heat-affected zone (HAZ) of a thick-gauge pipeline steel joint. This is most significant in the inter-critically coarse-grained HAZ (ICCGHAZ), which considerably deteriorates the toughness of the joint. In the present work, 11-mm thick pipeline steel was joined by preheating and double-sided friction stir welding (FSW). A comparative study on the microstructure and toughness in the ICCGHAZs for FSW and gas metal arc welding (GMAW) was performed. The toughness in the ICCGHAZ for FSW was improved significantly than that in the ICCGHAZ for GMAW. Generally, the nugget zone (NZ) has a coarse microstructure in the FSW steel joint formed at the highest peak temperature. However, in the current study, the microstructure in the one-pass NZ was remarkably refined owing to the static recrystallization of ferrite. An excellent toughness was achieved in the NZ of the pipeline steel joint that employed FSW.