Effect of Ca Addition on Inclusions, Microstructures, and Impact Toughness of Coarse-Grained Heat-Affected Zone After High-Heat Input Welding in Mg-Deoxidized Shipbuilding Steel Plates

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.