ABSTRACT Duplex stainless steels are used to replace austenitic steels in industrial applications, especially in the chemical industry where corrosion resistance in chloride-containing media is an important requirement. According to literature, niobium has a great influence on the transformation of the phases of these steels (ferrite, austenite and sigma). Given the interest in evaluating this property in terms of wear resistance and corrosion of the material, the aim of this study was to evaluate the effect of niobium on the formation in austenitic-ferritic stainless steel SEW 410 Nr 1.4517 after heat treatment at 850ºC for 15, 30 and 60 minutes. The niobium contents studied were 0.2; 0.5 and 1.5 wt%. Measurements of microhardness, electrochemical tests in saline solution containing 3.5% sodium chloride and wear resistance were carried out. The results show that an increase in niobium content and sigma phase leads to an increase in hardness and wear resistance, associated with a decrease in corrosion potential. Therefore, the use of niobium in austeno-ferritic stainless steel is recommended when wear resistance is an important factor to be considered.

Austenitic-ferritic stainless steels containing 0.2% and 0.5% niobium were evaluated with respect to the microstructures, microhardness, pitting potential resistance, electrochemical impedance and wearing tests. Niobium is an alphagenic element and has a great influence on phase transformation of duplex stainless steels. In the present investigation, the samples were annealed at 1050 ºC and aged at 850 ºC to promote the formation of sigma phase. The pitting potential resistance and electrochemical impedance were evaluated in 3.5 % sodium chloride solution. The wear coefficients were calculated by micro-wear tests conducted using a fixed ball machine and aluminum oxide as abrasive. The results show that the niobium content lead to increase on Laves phase formation. This phase, in association with the sigma phase, causes a significant increase in the hardness and wear resistance with a decrease on corrosion resistance of the austenitic-ferritic stainless steels.

Os aços inoxidáveis austeno-ferríticos apresentam melhor combinação de propriedades mecânicas e resistência à corrosão que os inoxidáveis austeníticos e ferríticos. A microestrutura desses aços depende da composição química e tratamentos térmicos. Nos aços inoxidáveis austeno-ferríticos, a solidificação inicia a 1450ºC com a formação de ferrita, austenita a 1300ºC e fase sigma na faixa de 600 a 950ºC. Esta última compromete a resistência à corrosão e a tenacidade desses aços. Conforme a literatura, o nióbio tem grande influência na transformação de fases dos aços inoxidáveis austeno-ferríticos. Essa pesquisa avaliou o efeito do nióbio na microestrutura, dureza e resistência de transferência de carga de um aço inoxidável austeno-ferrítico. As amostras foram solubilizadas a 1050ºC e envelhecidas a 850ºC, para promover a formação da fase sigma. Os ensaios de corrosão foram realizados em meio de saliva artificial. Os resultados mostram que a adição de 0.5% Nb no aço provoca a formação da fase de Laves. Essa fase, associada à fase sigma, aumenta a dureza do aço, reduzindo, porém, os valores da resistência de transferência de carga.

The austenitic-ferritic stainless steels present a better combination of mechanical properties and stress corrosion resistance than the ferritic or austenitic ones. The microstructures of these steels depend on the chemical compositions and heat treatments. In these steels, solidification starts at about 1450ºC with the formation of ferrite, austenite at about 1300ºC and sigma phase in the range of 600 to 950ºC.The latter undertakes the corrosion resistance and the toughness of these steels. According to literature, niobium has a great influence in the transformation phase of austenitic-ferritic stainless steels. This study evaluated the effect of niobium in the microstructure, microhardness and charge transfer resistance of one austenitic-ferritic stainless steel. The samples were annealed at 1050ºC and aged at 850ºC to promote formation of the sigma phase. The corrosion testes were carried out in artificial saliva solution. The addition of 0.5% Nb in the steel led to the formation of the Laves phase.This phase, associated with the sigma phase, increases the hardness of the steel, although with a reduction in the values of the charge transfer resistance.