Microstructural and Crystallographic Analysis of the Charpy-V Impact Specimens Fracture of Simulated Coarse Grain HAZ Region of TMCP Steel

Abstract:

This work investigated the fracture of Charpy-V Impact test specimens of weld coarse grained heat affected zone (CGZAC) of two 355 MPa class steels. One of the steels was produced by controlled rolling followed by accelerated cooling (TMCP – Thermo-mechanical-Controlled-Process) and another by conventional rolling followed by normalizing heat treatment. The CGZACs were produced by termo-mechanical simulation in the Gleeble equipment under high thermal input welding conditions. The analyzes were carried out in cross section of the fracture plane, using optical and scanning electron microscopy applying Electron Backscatter Diffraction (EBSD) technique. It was observed that the grain size of the previous austenite, close to fracture of the TMCP steel, was considerably smaller than that of the conventional steel and that the cleavage fracture unit was related to smaller structures present inside the austenitic grain. By means of EBSD, it was verified that TMCP steel presented a higher density of high angle grain boundary and smaller crystallographic packages when compared to conventional steel, factors that improve the Charpy-V impact resistance of the material. These characteristics may be associated with the pinning effect of the austenitic grain by coherent TiN precipitates

Key-words:
CGHAZ; TMCP Steels; Gleeble; Charpy-V Impact Test; EBSD