ABSTRACT The risk of brittle fracture is an important aspect when oil and gas exploration move to Arctic regions. Therefore, the present work addresses examination of fracture surfaces in scanning electron microscope to identify brittle fracture mechanisms. The study is based on Charpy and CTOD bend testing at -60°C of weld thermal simulated specimens corresponding to both coarse grained HAZ (CGHAZ) and intercritically reheated coarse grained HAZ (ICCGHAZ). Brittle fracture was obtained by both test methods, and SEM investigation of the fracture surfaces revealed that slag particles like sulphides and oxides had initiated cleavage in the CGHAZ specimens, and that martensite-austenite (MA) particles probably were responsible for cleavage in the ICCGHAZ specimens. In both types of weld simulated structures, the crack initiation points were located close to the pre-crack tip.
INTRODUCTION The increased focus on oil and gas exploration in the high north will inevitably give renewed attention to the low temperature embrittlement of structural steels. The ductile to brittle fracture transition temperature that occurs in such steels will consequently be very critical due to low service temperature. There are low alloy steels available with sufficiently low transition temperature, but they tend to loose their excellent properties after welding, primarily the toughness. If steels are to be made with the substantial improvements in weldability, further developments in steel processing technology, combined with optimization in alloying practice, must be made. In parallel, a better understanding of brittle fracture mechanisms of different weld microstructures must be sought. Such information has been collected from scanning electron microscopy (SEM) examination of fracture surfaces of Charpy V or CTOD (crack tip opening displacement) tests taken from either real weldments or weld thermal simulation. However, this may not be straight forward because of difficulties in finding the first point of fracture initiation (first microcrack to form).