Microstructure and toughness of the CGHAZ of an offshore platform steel

In this welding thermal cycle simulation, the microstructure of the coarse grain heat affected zone (CGHAZ) of an offshore platform steel exhibited a gradual transition from lath bainite/martensite (LB/LM) to granular bainite (GB) with increasing cooling time. The low temperature toughness (evaluated at −40 °C) correspondingly decreased. For the simulated CGHAZ with high value of cooling time, minimum crack initiation energy was achieved, indicating the easiness of the debonding of martensite-austenite (M-A) constituent from surrounding bainitic matrix which are frequently observed on the fractograph. Corresponding to the larger number of deflections per unit crack length, maximum ductile crack propagation energy was achieved in LB/LM which was obtained at low value of cooling time. This was linked to the high density of high misorientation packet and block boundaries, thus a small effective grain size.