Abstract
Effect of Ca addition on inclusions, microstructures, and impact toughness of coarse‐grained heat‐affected zone (CGHAZ) in Mg‐deoxidized shipbuilding steel plates after high‐heat input welding (HHIW) of 400 kJ cm−1 are investigated. Characteristics of inclusions and their effect on microstructures and impact toughness are elucidated. With the addition of Ca, number density of inclusions decreases from 384 to 273 mm−2, but average size increases from 1.23 to 1.32 μm. Number densities of inclusions such as MgAl2O4, MgAl2O4 + MnS, Mg–Al–Ti–O + MnS, MgO + MnS, and TiN + MnS, which are observed to induce intragranular acicular ferrites (IAFs) formation, decrease from 3.0, 15.3, 18.8, 16.5, and 18.1 to 0.2, 1.2, 4.4, 3.0, and 6.3 mm−2, respectively. Observed Ca‐containing inclusions of diameter over 2.5 μm in Mg–Ca steel cannot serve as the effective nucleation sites for IAFs formation. Contents of IAFs and polygonal ferrites decrease from 73.20% and 19.96% to 54.34% and 5.63%, respectively. Frequency of high‐angle grain boundaries decreases from 70.5% to 51.3%. The CGHAZ fracture morphology changes from ductile fracture to brittle fracture with the area proportion of the fibrous zone decreasing from 56.5% to 0%. Hence, the impact toughness of CGHAZ decreases significantly from 175 to 23 J at −20 °C after HHIW.
Published Version
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