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

Abstract

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.

1. Introduction

Welding efficiency has been improved by high-heat input welding (HHIW) technologies, which can attain a heat input over 400 kJ·cm−1. The applications of HHIW technologies include shipbuilding, construction, machinery, and other fields [1–2]. However, with increased heat input, the peak temperature of coarse-grained heat-affected zones (CGHAZs) rises, and the exposure to high temperatures is prolonged, resulting in brittle microstructures, coarse grains, and deteriorated CGHAZ toughness [3–5].

Oxide metallurgy with Ca addition can be potentially used to improve the impact toughness of CGHAZ through promoting the formation of intragranular acicular ferrite (IAF) and refinement of prior austenite grains (PAGs) [6–10], which has been confirmed by JFE Steel [11] and Kobe Steel [12]. We previously showed that the number density (ND) of complex oxysulfides increases with the increase in the Ca content, which promotes IAF nucleation and the precipitation of nanosized TiN particles and considerably improves toughness in the CGHAZ [6,8,13].

The size and distribution of austenite contribute to the toughness of CGHAZ, with large PAGs promoting the formation of brittle microstructures and inhomogeneous microstructure distribution, both of which negatively affect toughness [14–15]. During HHIW, PAG growth suffers from the inhibition caused by the dragging force of the alloying elements segregated at grain boundaries (solute drag) and the pinning force of fine particles, with the latter exhibiting a notably stronger effect [16–18]. These processes, such as particle dissolution and reprecipitation related to PAG growth, cause substantial alterations in the chemistry, size distribution, and volume fraction of precipitates during the welding process [19–20]. Especially under HHIW, the CGHAZ experiences prolonged exposure to elevated temperatures, which exacerbates the dissolution or coarsening of pinning particles.