δ-Ferrite Formation and Its Effect on the Mechanical Properties of Heavy-Section AISI 316 Stainless Steel Casting

δ-Ferrite formation across the thickness of a heavy-section AISI 316 casting slab and the effect of its decomposition on the bending properties have been studied. Cellular-shaped δ-ferrite with the faction of less than 3 pct is observed at the surface position of the cast while skeletal-shaped δ-ferrite with the faction of up to 8 pct is present at the center position, which is attributed to the combined effects of macro-segregation and cooling rate. Extensive decompositions of δ-ferrite through direct transformation (δ → σ) and eutectoid transformation (δ → σ + γ₂) are demonstrated. Micro-cracks could be observed within the decomposed δ-ferrite on the convex surface of bending samples from both the surface and center position of the cast, but occurrence of cracking is only present in the sample from center position of the cast. Removal of the brittle σ phase within δ-ferrite by a preheat treatment at 1050 °C could effectively improve the bending performance.