Effect of heat input and heat treatment on the microstructure and toughness of pipeline girth friction welded API 5L X65 steel

High-strength low-alloy steels (HSLA) have been considered key materials for the petroleum industry for the construction of pipelines to move oil and gas for long distances. Conventional welding processes may produce important microstructural changes, which negatively impact the mechanical behavior of the pipeline material. Alternatively, pipeline girth friction welding is an excellent choice to lessen the detrimental effect on micro-structure since it is a one-shot low heat input process. This work aims to optimize impact toughness - the most affected mechanical property -to qualify pipeline girth friction welded API 5L X65 steel joints according to the requirements of the DNV OS F101: 2013 international standard. The impact toughness optimization process relied on the evaluation of the welding parameters' effect on heat input and on post-weld heat treatment (PWHT). The results indicated that even though the single thermal cycle imposed lowered the size and fraction of martensite-austenite (M/A) constituent in the as-welded condition, impact toughness was notably reduced due to previously austenite grain growth and unfavorable crystallographic texture. However, at least part of the original base material's impact toughness can be restored by decreasing the heat input, although not sufficient to qualify the weld. Otherwise, PWHT led to grain size refinement and reduced crystallographic texture, which in turn improved impact toughness beyond the minimum level specified by the technical standard.