Inclusions and microstructures in coarse-grained heat-affected zone of Al-Ti-Ca deoxidized shipbuilding steels with different Al contents after high-heat input welding

The inclusions, microstructure evolution and impact toughness of the coarse-grained heat-affected zone (CGHAZ) of Al-Ti-Ca deoxidized shipbuilding steel plates with different Al contents after high-heat input welding of 400 kJ cm-1 are investigated. The characteristics of inclusions and their influence on the refinement of microstructure and impact toughness in CGHAZ was elucidated. Increasing the Al content from 0.016 wt% to 0.022 wt % and 0.043 wt% results in an increase in the average size of prior austenite grain and the brittle phase fractions, while decreasing the content of intragranular acicular ferrites (IAFs). The results of the thermodynamic calculations, high-temperature laser scanning confocal microscopy and differential scanning calorimetry all show that increasing the Al content can diminish the austenitic stable region, elevate the transition temperature from austenite to ferrite and suppress the nucleation of IAF. In the present steels, the IAF formation is successfully induced by the specific inclusions, namely Al2O3-CaO-Ti2O3-MnS(-CaS), in which the calcium aluminate inclusions are embedded with Ti-oxides and CaS-MnS precipitates. The effective inclusions have the number density of 93 mm-2 in 16Al steel and 27 mm-2 in 22Al steel, but they are rarely found in 43Al steel. On average, an effective inclusion can induce 3.4 IAFs in 16Al steel and 2.8 IAFs in 22Al steel. Thus, as the Al content increases from 0.016 wt% to 0.22 wt% and 0.043 wt%, the impact toughness of CGHAZ at -40 degrees C decreases from 134 J to 78 J and 24 J with the welding heat input of 400 kJ cm-1.