The Microstructure Effect on the Fracture Toughness of Ferritic Ni-alloyed Steels

摘要

Production of oil and gas in the Brazilian pre-salt faces several technical challenges and one of the major concerns is the presence of CO_2 in high concentrations. Enhanced oil recovery (EOR) using CO_2 might be a great opportunity to increase productivity and address environmental issues avoiding the release of CO_2 to the atmosphere. The EOR working pressure can reach up to 500 bars and in case of sudden depressurization local temperature can decrease down to -60°C or lower increasing the possibility of catastrophic failure. The aim of this work is to evaluate fracture toughness of two nickel containing steels as an alternative material to manufacture low temperature toughness improved CO_2 transporting pipelines. Microstructure, effective grain size (EGS), density of high angle grain boundaries (HAGB) and fracture toughness were evaluated. The results indicated that for the as-rolled condition the large islands of the microconstituent Martensite-Austenite (M/A) in the 5~(1/2) Ni steel had a detrimental effect on fracture toughness at -100°C, while finer M/A particles and lower EGS with higher density of HAGB in the 9 Ni steel turned its fracture toughness practically temperature independent. Additionally, heat treatment has the potential to dissolve the M/A hard particles and consequently improve fracture toughness at -100°C.