Role of bainitic microstructures with M-A constituent on the toughness of an HSLA steel for seismic resistant structural applications

Fracture toughness of the coarse-grained heat-affected zone (CGHAZ) of fusion welded HSLA steel is severely reduced at low temperatures with the combination of deleterious microstructures such as bainitic ferrite (BF), granular bainite (GB), polygonal ferrite (PF), and the martensite-austenite (M-A) constituents. Toughness is one of the basic requirements for seismic resistant steels. The three heat input conditions (different cooling rate) were thermo-mechanically simulated, and the combined effect of M-A constituents with bainitic microstructures on toughness was examined. As the heat input changes from low to high, a combination of acicular ferrite (AF) and BF microstructures changes to a combination of PF, GB and M-A constituent. The former microstructure combination provides better toughness (ductile), whereas the latter makes the samples more brittle. The coarse BF, GB, and PF microstructures with low angle grain boundaries (LAGBs) make them brittle by easy crack propagation. Moreover, harder microstructures such as M-A can potentially act as cleavage initiation sites. However, a high frequency of high angle grain boundaries (HAGBs) in the AF microstructure can suppress the crack propagation, which turns the fracture to completely ductile. Also, it is observed that the M-A constituent in CGHAZ was abundant in martensite rather than the austenite.