Abstract
The impact fracture behavior of FH550 offshore platform steel was investigated by use of SEM,TEM and EDS.The experimental results show that the microstructure of as-rolled test steel is consist of low bainite and granular bainite,and tempered bainite is the main structure of this steel after tempering.The dimple fracture was observed on most of frature surfaces of samples,and and some inclusions,such as CaO and Al_2O_3,appear at the bottom of isometric dimples.Carbides inclusions more than 10μm were found on the cleavage fracture surface of a few of samples which could aggravate the impact toughness,and result in the fluctuation of the impact energy as brittle particles. Micropore accumulating and growing and crack necking are the main way for propagation of cracks, however,shear crack propagation may be obstructed by the particle clusters generated during plastic deformation,increasing the crack propagation work.It is also found that the fraction of high angle grain boundaries is 79.3%,and the average grain size is 7.61μm.High percentage of large angle grain boundaries and fine grain size are the key factors to obtain excellent impact toughness.
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