Avaliação da soldabilidade através de simulação física e da tenacidade à fratura de juntas soldadas de aço API X80 aplicado em estruturas offshore

Abstract

Offshore structures and equipment, used for both oil extraction and wind power generation, are constantly exposed to extreme conditions, such as low temperatures, corrosion and cyclic loading. Therefore, to ensure the integrity of such components, the use of materials with excellent mechanical properties and good weldability is required. In this sense, the main committees, institutes and normative societies have created procedures to evaluate and certify the weldability of steels applied in the offshore industry. The APIRP2Z (2005), EN10225 (2009) and DNV OS F101 (2000) prequalifications are documents that were designed to ensure that steels developed for these applications present excellent toughness in the heat-affected zone (HAZ). In this context, this study aimed to evaluate the weldability of API X80 steel, 37.5 mm thick, produced by Usiminas by controlled rolling followed by accelerated cooling (TMCP - Thermomechanical Controlled Process) especially for this sector. In the first part of the thesis, the effect of the FCAW (Flux Cored Arc Welding) and SAW (Submerged Arc Welding) welding processes and the post weld heat treatment (PWHT) on the toughness of the HAZ were studied through the Charpy-V and CTOD (Crack Tip Opening Displacement) impact toughness tests. Then, the influence of different welding thermal cycles on the microstructural transformations, tensile properties and impact toughness of the coarse grain region (CGHAZ) was investigated by means of physical simulation using the dilatometer and Gleegle equipments. It was found that the steel in question, welded with heat inputs of 0.8 (FCAW) and 4.5 kJ/mm (SAW), with and without the application of PWHT, presented satisfactory weldability characteristics for the construction of offshore structures and components. Furthermore, the methodology developed in this work through physical simulation helped in determining the limiting thermal cycles for which API X80 steel presents suitable properties for its application, demonstrating that it is an adequate tool for the development of its application engineering.