Toughness and fracture mechanism at low temperature of offshore engineering steel at different welding heat inputs

Abstract

The microstructure and low temperature toughness of offshore engineering steel which has a yield strength of 420 MPa were investigated at different welding heat inputs. As the heat input is increased from 50 kJ/cm to 200 kJ/cm, the impact energy in coarse-grained heat-affected zone (CGHAZ) is decreased from 196 to 147 J at –40 ℃. The fracture is changed from ductile to quasi-cleavage and cleavage. The prior austenite grain is increased. The size and area fraction of martensite-austenite (M-A) constituents is increased, while the number density is decreased. The capability on the crack resistance is in decreasing order for fined lath bainite (LB), interlocked LB, coarsen LB and granular bainite (GB), and it is increased by the larger misorientation angle and HAGBs. As the heat input is larger than 150 kJ/cm, the crack is induced by the M-A constituent and propagates along the weak link caused by the M-A constituents. The secondary crack is mostly deflected by the larger misorientation angle larger than 45°, and the cleavage plane prefers {001} plane. Larger welding heat input decreases the CGHAZ strength, thus leads to the increase in crack tip plastic zone (CTPZ). The existence of M-A constituent and coarsening microstructure give rise to the cleavage with the larger CTPZ.