Stress corrosion cracking of E690 steel as a welded joint in a simulated marine atmosphere containing sulphur dioxide

Effects of heat input on microstructure and fracture toughness of simulated coarse-grained heat affected zone for HSLA steels

Deep penetration fiber laser-arc hybrid welding of thick HSLA steel

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.

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

Simulated inter-critical heat affected zones (ICHAZ) of a large thickness bainitic steel were prepared with three different peak temperatures (Tp) of 750 °C, 780 °C and 800 °C. The microstructure and crystallographic feature were investigated. Toughness of simulated ICHAZ specimens was assessed using instrumental Charpy impact test at − 40 °C. It was found that the volume fraction of newly formed bainite increased and the effective grain size decreased with the increase in inter-critical Tp. The toughness decreased slightly with the Tp of 750 °C compared with the base metal (BM), while the toughness improved sharply when the Tp exceeded 780 °C. The network-like martensite was primarily responsible for the low crack initiation energy with the Tp of 750 °C. In contrast, the fine bainite formed with the Tp above 780 °C was effective in increasing the crack nucleation resistance, resulting in the high initiation energy. The matrix softening as well as the grain refinement played important roles in improving the crack propagation energy. Furthermore, it was confirmed that the presence of a multi-phase microstructure can lead to the scattering of toughness.

Yanlei Zhou

Papers

Microstructure and toughness of the CGHAZ of an offshore platform steel