Low Temperature Fracture Toughness of X80 Girth Welds

Akselsen, Odd M. (SINTEF, and NTNU (Norwegian University of Science and Technology)) | Østby, Erling (SINTEF) | Nyhus, Bård (SINTEF)

OnePetro 

ABSTRACT

The present work was initiated to study the low temperature brittle behaviour of welded 42'''' API X80 pipe for applications in arctic areas. The pipe was girth welded using GMAW of root and hot pass, and PGMAW of the fillers. The testing comprised of both mechanical testing and fracture toughness testing. The fracture mechanics SENT test results (-60°C) revealed quite good toughness with CTOD values from 0.3 to 5.5 mm for the fusion line, and from 2 to 5.5 mm for the weld metal. For the fusion line, the large scatter observed seems to be related to the fatigue pre-cracking, meaning that the fatigue pre-crack did not always grow to hit the fusion line. In fact, in some samples it grew towards the base metal. Therefore, the CTOD values do not necessarily be representative the coarse grained HAZ fracture toughness. Metallographic inspection confirmed very narrow HAZ due to the low heat input employed, i.e., visible HAZ width of ~ 300 μm.



INTRODUCTION

The huge foreseen oil and gas resources in the Arctic areas (Gauthier, 2009) may throw new impetus on steel development due to the harsh environments like very low temperature, strong polar winds, ice/iceberg, thaw settlement and frost heave. In addition, requirements may also be set to low weight due to long transport distances. With respect to climate conditions, steels with improved base metal toughness are sought to give sufficient robustness after welding, while the low weight focus points to higher strength. This combination of strength and toughness has always been a challenge, but considering the severe service conditions in the high north, the situation approaches the borderline for applications of "affordable" steels, containing reasonable alloying levels; i.e., not special alloys such a 9% Ni steel or stainless steel. Onshore pipelines may suffer from similar phenomena due to welding degradation of the initial low temperature base metal toughness.