Influência do hidrogênio na tenacidade à fratura de junta soldada de aço 9% níquel

Abstract

Rolled seamless pipes of 9% nickel steel in quenched and tempered condition has been used in oil industry for reinjection CO2 gas in oil pools, it is increasing the productive and does not turn the gas into the atmosphere. These lines of reinjection woks in a high pressure (550 bar) and with the presence of CO2, moisture and low quantities of H2S (200ppm). This work aims to study the fracture toughness of 9%Ni steel joints welding by GTAW in the root and GMAW in the filling using 625 nickel alloy as weld metal. It was performed CTOD and J- integral tests in the base metal, in the welded joint and in the coarse grains of the Heat Affected Zone (CGHAZ), in the adverse conditions that occurred in the field, being the most critical in environments with CO2 and H2S moisture and in low temperatures (-90ºC) in the decompression occasion. In addition, X-ray diffraction, microstructural analyses in optical and Scanning Electron Microscope (SEM) and a study of the fracture surface with the objective of understanding the main fracture mechanisms that occurred in this steel. The CTOD and J tests shows that the weld joint and the base metal had a high toughness and it did not decrease with low temperatures, the CTOD value continued high. However, when the material was tested in modified NACE A solution (99,8 CO2 + 0,2 H2S), the material shows to be susceptible to embrittlement mainly in the base metal, although the HAZ is a critical region because of its greater hardness, it did not suffer great embrittlement due to its proximity to the austenitic weld metal.