Microestrutura e propriedades mecânicas do aço inoxidável austenítico Aisi 310s envelhecido em operação

Abstract

The flare is a crucial piece of equipment for the operational safety of oil platforms, as it ensures the proper disposal of toxic and flammable gases. Due to its operation at high temperatures and in oxidizing and reducing environments, it is manufactured from austenitic stainless steels. Nevertheless, the literature reports several cases of failures in equipment such as flares, generally associated with long-term exposure to high temperatures. Therefore, the present work aims to investigate the microstructural changes and mechanical properties of an oil platform flare manufactured from AISI 310S austenitic stainless steel after a long period of operation at high temperature. The analyzed component consisted of a lance composed of a rolled tube, a cast conical part, and a circumferential weld joining these elements.To this end, mechanical tests including tensile, Charpy V-notch impact, hardness, and microhardness tests were performed, in addition to microstructural analyses by optical microscopy and scanning electron microscopy, including Energy Dispersive X-ray Spectroscopy (EDS). The tensile test results showed a significant reduction in post-service ductility. The as-received AISI 310S steel exhibited an average elongation of approximately 66.9%, whereas samples from the flare triple zone and the rolled region showed average values of 35% and 8.2%, respectively. In the Charpy impact test, the as-received material presented an average absorbed energy of about 40.1 J, which is significantly higher than that observed in the flare triple zone (24 J) and, especially, in the rolled part of the flare (12 J), indicating a reduction of approximately 70% in absorbed energy compared to the initial condition. Vickers hardness tests indicated an increase of about 20 HV in the material hardness after service. Microhardness testing revealed the presence of a hardness gradient, with significantly higher values near the material surface, associated with carburization and the precipitation of hard phases. Microstructural analyses revealed intense precipitation of secondary phases, such as chromium carbides of the Cr₂₃C₆ type and sigma phase. These transformations were attributed to prolonged exposure to high temperatures and to a hydrocarbon-rich and reactive gas atmosphere, resulting in the degradation of the mechanical properties of the AISI 310S steel. It is concluded that, although AISI 310S exhibits satisfactory performance in its initial condition, metallurgical changes induced during service may adversely affect its future performance.