Mechanism of δ-ferrite decomposition in high Si-bearing austenitic stainless steel weld metal during aging at 550°C

Abstract

The 15Cr-9Ni-Nb austenitic stainless steel weld metal with 3.5 wt% Si content was designed and prepared via tungsten inert-gas arc welding and then stabilized heat treatment (SHT) at 900 °C for 3 h. The mechanism of δ-ferrite decomposition during the aging process at 550 °C was investigated for both the as-welded (AW) and the SHT weld metals via scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and high-resolution transmission electron microscopy (HRTEM), and its impact toughness effect was also discussed. The original δ-ferrite for both the AW and SHT weld metals decomposed to the σ-phase, A16Mn6Si7-type G-phase, and more stable δ-ferrite during aging. The coarse σ-phases were distributed at the δ/γ interface and the relatively fine G-phases (A16Mn6Si7) were at the δ/γ interface or inside the δ-ferrite. Si promoted the formation of the G-phases and σ-phases during aging, consumed a large amount of Cr, Ni, Si, and Mn from the δ-ferrite, and stabilized the remaining δ-ferrite. The impact toughness of the AW weld metal sharply dropped during aging due to the coarse σ-phases promoting the formation and propagation of intergranular cracks. The SHT reduced the δ-ferrite in the weld metal, thus avoiding the formation of a large number of σ-phases during aging and therefore improving the impact toughness.