Microstructural Development in a TRIP-780 Steel Joined by Friction Stir Welding (FSW): Quantitative Evaluations and Comparisons with EBSD Predictions

• Published in: Soldagem & Inspeção 2016-06, Vol.21 (2)
• Main Authors: Medina, Gladys Perez, Ferreira, Hugo Lopez, Robledo, Patricia Zambrano, Perez, Argelia Miranda, Valdes, Felipe A Reyes
• Format: Article
• Language: Portuguese
• Subjects: Electron back scatter diffraction; Friction stir welding; Heat affected zone; Microstructure; Recrystallization; Shear; Texture; TRIP steels
• ISSN: 0104-9224; 1980-6973
• DOI: 10.1590/0104-9224/SI2102.04

The present work describes the effect of FSW on the result microstructure in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM) of a TRIP-780 steel. X-ray diffraction (XRD), optical microscopy (OM) and EBSD were used for determinations retained austenite (RA) in the SZ, It was found that the amount of RA developed in SZ was relatively large, (approximately 11% to 15%). In addition, recrystallization and the formation of a grain texture were resolved using EBSD. During FSW, the SZ experienced severe plastic deformation which lead to an increase in the temperature and consequently grain recrystallization. Moreover, it was found that the recrystallized grain structure and relatively high martensite levels developed in the SZ lead to a significant drop in the mechanical properties of the steel. In addition, microhardness profiles of the welded regions indicated that the hardness in both the SZ and TMAZ were relatively elevated confirming the development of martensite in these regions. In particular, to evaluate the mechanical strength of the weld, lap shear tensile test was conducted; exhibited the fracture zone in the SZ with shear fracture with uniformly distributed elongation shear dimples.Original Abstract: O presente trabalho descreve o efeito do processamento FSW na microestrutura resultante na zona soldada, zona termomecanicamente afetada (TMAZ), zona afetada pelo calor (ZAC) e no metal base (MB) de um aco TRIP-780. Difracao de raios-X (DRX), microscopia optica (MO) e EBSD foram utilizados ??para a determinacao de austenita retida (RA) na SZ. Verificou-se que a quantidade de RA desenvolvida na SZ foi relativamente grande, (cerca de 11% a 15%). Alem disso, a recristalizacao e a formacao de textura de grao foram resolvidas usando EBSD. Durante friction stir welding, FSW (Soldagem por Atrito com Pino Nao-consumivel), a SZ experimentou severa deformacao plastica que conduziu a um aumento na temperatura e, consequentemente, recristalizacao dos graos. Verificou-se que a estrutura do grao recristalizado e niveis relativamente elevados de martensita desenvolvidos na SZ, conduziram a uma reducao significativa nas propriedades mecanicas do aco. Alem disso, os perfis de microdureza das regioes soldadas indicam que a dureza, tanto na SZ como na TMAZ foi elevada, confirmando o desenvolvimento de martensita nestas regioes. Em particular, foi visualidade na zona fundida trincas alongadas em perfil y, distribuida