MICROSTRUCTURE AND MECHANICAL PROPERTIES OF WELDS IN PIPELINE STEEL

Abstract

Transport of oil and gas in various service environments, presents an enormous challenge. Pipelines are used for transportation of those natural resources. The failure of the pipeline is always associated with serious environmental damage and heavy losses. Very often the failure starts at welds that are used to join different elements of pipeline steels. Microstructural parameters like composition, structure, morphology of non-metallic inclusions and residual stresses due to welding play an important role in initiating the cracking of pipeline welds. The main objectives of this researchthesis are to analyze the microstructure and mechanical properties of submerge arc weld and establish the role of microstructure in initiation and propagation of cracks in sour (HIC) environment. In this work, the optical microscope, Electron backscattered diffraction (EBSD), X-ray diffraction and Transmission electron microscope (TEM) were used to investigate the microstructure of the X70 pipeline steel base metal, the weld bead and heat affected zone. The result obtained showed that the base metal have equiax ferrite grains while the weld bead have structure of acicular ferrite and fine bainite grains with many Al-Si-Mn-Ti oxide particles/inclusions. The heat affected zone microstructure has coarse grains with martensite and NbTi(CN) precipitate/particles. Analysis of the cross section of the weld shows that higher kernel values were observed in the weld cross section compared to the weld top surface. The kernel average misorientation (KAM) analysis at the cross section of the weld show that the amount of residual strain is higher than on the surface of the weld. The residual stress on the weld top surface was purely compressive while at the cross section mixture of tensile and compressive stress were observed. The hardness was the highest in the weld bead, a bit lower in the base metal and lowest in the heat affected zone. Higher density of voids was observed at the top of the weld bead than in the middle. It was demonstrated that the difference in hardness can be directly correlated with the differences in the grain size. HIC cracks were induced in the pipeline weld area using standard HIC test and electrochemical hydrogen-charging. The SEM observations clearly indicate that the cracking in the weld nucleated at inclusions. The cracks were seen to propagate by coalescence of small pits related to hydrogen charging. Energy dispersive (EDS) analyses showed that the cracks nucleation sites are most often located at Al-Si-Mn-Ti oxide. The EBSD measurements show that HIC can take place at a wide range of grain orientations. The map of the location of HIC crack shows that they can be both transgranular and intergranular. Experiments were carried out to understand the effect of hydrogen and role of microstructure on HIC susceptibility in the steel weld under tensile load. The results show that the ductility of the steel weld became lower as charging time increased. EBSD investigation were carried out in the heat affected zone area since all the fractured specimens were fractured in this area. The results shows that large grains and dislocation density in heat affected zone is related to the high susceptibility of the steel weld to fail under tensile load.