Effect of Cr content on softening mechanism of the heat-affected zone of X80 GMAW automatic girth weld

Abstract

Softening of the heat-affected zone (HAZ) could lead to strain concentration in the girth weld joint, which endangers the service safety of high strength pipelines, while the chemical composition and original microstructure of pipeline steel play a determining role in its softening behavior. In this study, X80 pipeline steels with different Cr contents but identical manufacturing processes were girth welded using automatic gas metal arc welding (GMAW). Microhardness, microstructure, grain size, dislocation density and precipitation of both base metal and the HAZ were analyzed to reveal the influence of Cr content on the softening mechanism. The results showed that with the increase of Cr content from 0.115 % to 0.216 %, the HAZ softening ratio decreased from 11.00 % to 3.48 %. The steel with higher Cr content exhibited fine acicular ferrite (AF) dominated microstructure, which introduced higher dislocation density and refined polygonal ferrite grains in the softening zone, whereas lower Cr content led to granular bainite (GB) dominated structure with inferior softening resistance. Precipitates showed no significant difference between the two steels. Relatively higher Cr content and AF fraction in base metal is beneficial to its anti-softening behavior in the HAZ.