Avaliação do efeito de diferentes aportes de calor na microestrutura e tenacidade da região de grãos grosseiros da zona afetada pelo calor de aço TMCP

Abstract

High heat input welding is one of the alternatives adopted by shipyards and wind tower manufacturers for increasing productivity in operations for joining materials. However, the thermal cycles generated during welding may cause microstructural transformations that are detrimental to the toughness in the heat-affected zone (HAZ), especially in the coarse-grained region (CGHAZ). In this work, the grain growth and the microstructural changes in the CGHAZ of a 355 MPa yield strength steel of 35 mm thickness, produced by controlled rolling followed by accelerated cooling - TMCP (Thermo-Mechanical-Control-Process), were compared to a steel of the same grade produced by conventional rolling, both submitted to high thermal cycles in the dilatometer and Gleeble® equipments, in order to simulate high heat inputs welding. It was observed that previous austenite grain size (SGγ) in the CGHAZ follows a log-normal distribution and the TMCP steel, regardless of the heat input welding simulated used, showed (presents) SGγ values significantly lower than those obtained for the conventional steel. The lower SGγ of the TMCP steels were associated with the possible presence of TiN precipitates homogeneously distributed and coherent with the matrix, which pinning the grain growth due in regions where the maximum temperature does not exceed its dissolution temperatute. The impact toughness of the simulated CGHAZ of TMCP steel was significantly higher than that of the conventional steel. The lower SGγ, the lower volume fraction of the MA constituent and the presence of a lower hardness constituents with high fraction of high angle boundaries were the main factors to the excellent toughness of this steel. The results achieved showed that the use of thermomechanical simulators dilatometer and Gleeble® is an alternative for performance comparative weldability studies of structural steels. In addition, the use of TMCP steel may be an option to increase the productivity of wind tower manufacturers and large ships, as it allows the application of welding processes with high heat input producing welded joints with excellent properties, especially the toughness at low temperatures.