Avaliação na zona termicamente afetada de um aço inoxidável supermartensítico

• Ano de defesa: 2010
• Autor(a) principal: Ricardo Rodrigues Silva
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; UFMG
• Palavras-chave em Português: Engenharia Metalúrgica e de Minas; Engenharia metalúrgica; Aço inoxidavel; Metalurgia de transformação
• Link de acesso: http://hdl.handle.net/1843/BUBD-9D7H28

Nowadays, there is a strong trend for exploiting oil and gas in places that demand a high level of operation complexity. There are, for example, the presence of high levels of contaminants (mainly carbon dioxide, CO2, and sulphide, H2S), high temperature and pressures, high depth including salt layers and high distance from the shore. All these difficulties have forced companies to search for new exploration technologies. Supermartensitic stainless steels are already used as casing in production wells. Now, they appear as an alternative material for pipe line, due to their good corrosion resistance and weldability. Moreover, when compared to Super Duplex steels for this application, they are more economically due to their lower production cost. This study aimed to contribute for a better understanding of the weldability of a supermartensitic stainless steel that is being produced by V & M do Brazil. This study emphasizes the microstructure of HAZ, the most critical zone of the weld. The Thermo-Calc software was used as a tool to analyze the formation of the HAZ of this steel. The obtained data were qualitatively correlated to the microstructure of the material submitted to three different heat inputs. The three regions with different characteristics were identified in the HAZ. At the highest temperatures, Region A is characterized by the complete transformation of the material to ferrite with a strong grain growth. In Region B, a partial transformation of austenite into ferrite occurs forming a two-phase region ( + ). In the region C, complete austenitizing occurs close to the peak temperature. On cooling, this austenite transforms to martensite (not tempered). The hardness profile of HAZ for the three energy levels was similar. Higher hardness in this zone was attributed to the formation of non-tempered martensite. A circumferential joint was welded and its mechanical properties and corrosion resistance was investigated. The mechanical tests showed satisfactory results and the corrosion behavior of weld metal was similar to the base metal. Both passed in the "four point" test with a pH of 4.5 and 1 psi of H2S. The test was conducted with a stress equal to 90% of the minimum specified yield strength (655 MPa) and a 20.91 g/L solution of CH3COONa at room temperature. The results of the study showed a real possibility of using supermartensitic stainless steel in line pipe applications.