Avaliação da adição de Ti, Nb e (Ti+Nb) na microestrutura e susceptibilidade à sensitização de aços inoxidáveis ferríticos submetidos à soldagem TIG Belo Horizonte Universidade Federal de Minas Gerais Escola de Engenharia 2019

Abstract

AISI 430 ferritic stainless steels are widely used in civil construction, household appliances, exhaust gas systems in vehicles and stamped in general. However, the weldability of these steels tends to be limited. To improve its weldability, minimizing mechanical losses and corrosion resistance, it is usual to add stabilizing elements, mainly niobium and titanium. The present work evaluated the effect of addition by Ti, Nb and (Ti+Nb) and the heat input in the microstructure and susceptibility to sensitization of the ferritic stainless steels produced on an industrial scale submitted to GTAW welding. The structural characterization of the weld beads was performed by optical and scanning electron microscopy and the mechanical characterization by the Vickers microhardness profiles. The susceptibility to intergranular corrosion was evaluated through the Practical W immersion process (ASTM 763) and the double loop electrochemical potentiodynamic reactivation (DL-EPR). As expected, there was formation of martensite around the grain boundaries of AISI 430 without addition of stabilizers and Cr23C6 precipitates, resulting in generalized corrosion in the steel and in a high degree of sensitization. The addition of Ti was not sufficient to inhibit the sensitization in the AISI 430 steel samples. The addition of Nb as well as the double addition (Ti+Nb) was enough to stabilize the steel, resulting in a degree of null sensitization after samples were submitted to the welding conditions used in the study. With respect to the microstructure, the addition of Ti resulted in equiaxed grains in the WZ of AISI 430Ti and AISI 439. The condition of welding with lower heat input generated finer grains in the region of welding of steels with addition of stabilizers. The sample of steel with addition of Ti presented a higher hardness than the samples of the other steels with addition of stabilizers possibly due to the presence of very fine interand/ or intragranular precipitates of M23C6. The AISI 439 steel sample had lower hardness values for the welding conditions applied.