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

The effects of the Al content on the precipitation of nanoprecipitates, the growth of prior austenite grain (PAG), and the impact toughness in simulated coarse-grained heat-affected zones (CGHAZs) of two experimental shipbuilding steels after subject to high-heat input welding at 400 kJ·cm⁻¹ are studied. The base metals (BMs) of both steels contain three types of precipitates: Type-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 are 11.37 × 105 mm⁻² and 13.88 × 105 mm⁻², respectively. In 60Al steel, Type Ⅲ precipitates make up 38.12% of the total, whereas in 160Al steel, they account for only 6.39%. This variance in the amount of Type Ⅲ precipitates in 60Al steel reduces the pinning effect at the elevated temperature of CGHAZ, thereby facilitating PAG growth. The average PAG sizes in the CGHAZ of 60Al and 160Al steels are 189.73 μm and 174.7 μm, respectively. In 60Al steel, the low lattice mismatch between Cu2S, TiN and γ-Al2O3 facilitates the precipitation of Cu2S and TiN onto γ-Al2O3 during the welding, which decreases the number density of independently precipitated (Ti, Nb)(C, N) particles but increases the number density of γ-Al2O3-TiN-Cu2S particles. Thus, abnormally large PAGs are found in the CGHAZ of 60Al steel, reaching a maximum size of 1 mm. This presence of abnormally large PAGs in the CGHAZ of 60Al steel greatly reduces the microstructure homogeneity, consequently decreasing the impact toughness from 134 (0.016wt% Al) to 54 J (0.006wt% Al) at −40°C.