Combined effect of M/A constituent and grain boundary on the impact toughness of CGHAZ and ICCGHAZ of E550 grade offshore engineering steel.

Abstract

The present paper investigated the relationship between low temperature impact toughness and microstructure of bainite in coarse-grained heat affected zone (CGHAZ) and intercritically rehazed CGHAZ (ICCGHAZ) of an offshore engineering steel from both the microstructure morphological and crystallographic aspects. In this work, six groups of samples simulated CGHAZ and ICCGHAZ were designated at three different cooling rates. The Charpy test results showed that the toughness in CGHAZ decreases dramatically with decrease of cooling rate, which was attributed to the microstructural evolution from lath bainite to granular bainite, accompanying with the size increase of Bain zone and the change of M/A morphology from film to block. The increase in hardenability by cooling rate promotes more crystallographic variants from different Bain groups. Meanwhile, the combination with controlled inter-spacing of block boundaries by self-accommodation below the critical Griffith crack length, micro-crack can be arrested by these high angle grain boundaries thereby suppressed brittle fracture initiation and increased fracture properties. However, the variation in toughness of ICCGHAZ is not a concern, since obtaining excellent toughness is scarcely accessible even if the matrix microstructure is analogous to CGHAZ. It was due to the formation of coarse M/A constituents (~2 μm) necklacing at the prior austenite grain boundary. The visualized crystallography suggested that the impact toughness was partially correlated to the configuration manner and the size of Bain zones as well via promoting highly misoriented angle (>45°) boundaries, which in turn effectively deflected or arrested the brittle crack propagation.