Effect of Al content on nanoprecipitates, austenite grain growth, and toughness in coarse-grained heat-affected zones of Al–Ti–Ca deoxidized shipbuilding steels

This work focuses on the influence of Al content on the precipitation of nanoprecipitates, growth of prior austenite grains (PAGs), and impact toughness in simulated coarse-grained heat-affected zones (CGHAZs) of two experimental shipbuilding steels after being subjected to high-heat input welding at 400 kJ·cm⁻¹. The base metals (BMs) of both steels contained three types of precipitates: Type I: cubic (Ti, Nb)(C, N), Type II: precipitate with cubic (Ti, Nb)(C, N) core and Nb-rich cap, and Type III: ellipsoidal Nb-rich precipitate. In the BM of 60Al and 160Al steels, the number densities of the precipitates were 11.37 × 10⁵ and 13.88 × 10⁵ mm⁻², respectively. The 60Al and 160Al steel contained 38.12% and 6.39% Type III precipitates, respectively. The difference in the content of Type III precipitates in the 60Al steel reduced the pinning effect at the elevated temperature of the CGHAZ, which facilitated the growth of PAGs. The average PAG sizes in the CGHAZ of the 60Al and 160Al steels were 189.73 and 174.7 µm, respectively. In the 60Al steel, the low lattice mismatch among Cu₂S, TiN, and γ-Al₂O₃ facilitated the precipitation of Cu₂S and TiN onto γ-Al₂O₃ during welding, which decreased the number density of independently precipitated (Ti, Nb)(C, N) particles but increased that of γ-Al₂O₃–TiN–Cu₂S particles. Thus, abnormally large PAGs formed in the CGHAZ of the 60Al steel, and they reached a maximum size of 1 mm. These PAGs greatly reduced the microstructural homogeneity and consequently decreased the impact toughness from 134 (0.016wt% Al) to 54 J (0.006wt% Al) at −40°C.