Austenite decomposition behavior adjacent to δ-ferrite in a Si-modified Fe-Cr-Ni austenitic stainless steel during thermal aging at 550 °C

The δ-ferrite decomposition has been widely reported in conventional Fe-Cr-Ni austenitic stainless steel during thermal aging. However, a novel phenomenon is found in a Si-modified austenitic stainless steel, that δ-ferrite decomposition is suppressed, replaced by the decomposition of adjacent austenite. Herein, the decomposition behavior of austenite adjacent to δ-ferrite and its effect on impact toughness in a Si-modified austenitic stainless steel during aging at 550 °C up to 3000 h have been investigated. During thermal aging, austenite adjacent to δ-ferrite is decomposed in the following sequence: (1) γ → M ₂₃C ₆ + α transformation takes place. The preferential formation of secondary M ₂₃C ₆ carbides not only rejects Si atoms into the surrounding austenite but also produces a C-depleted zone. The transformation of austenite to ferrite is induced in the Si-riched and C-depleted micro-zone. (2) The growth of secondary M ₂₃C ₆ carbides induces a continuous rejection of Ni and Si atoms into M ₂₃C ₆/γ and M ₂₃C ₆/α interfaces, and the strong attractive interaction between Ni and Si provides the chemical driving force for nucleation of M ₆C carbides and G-phase. (3) With prolonged aging time, the lower C, high Ni, and high Si concentrations in the frontier of decomposed austenite will promote the preferential precipitation of M ₆C carbides rather than M ₂₃C ₆ carbides. During impact deformation, microcracks caused by strain incompatibility between secondary M ₂₃C ₆ and α-ferrite, resulting in a slight decrease in impact toughness. As austenite decomposition proceeds, the higher strain incompatibility across M ₆ C/α interfaces leads to brittle cleavage fracture, resulting in a significant decrease in impact toughness.