Effect of Ferrite Content, Cr/Ni Equivalence Ratio on Residual Stress and Microhardness of SS316L WAAM

Abstract

This study characterizes the microstructural evolution and coupled mechanical response along the build height of the SS316L Wire Arc Additive Manufacturing (WAAM). Correlation between elemental segregation, solidification mode, ferrite content, microhardness and residual stress at top, middle and bottom regions. A progressively Creq/Nieq ratio (1.31 to 1.75) towards the bottom region altered the solidification pathway from primarily austenitic in the top to an austenitic-ferrite mode at the middle and ferrite-stabilized solidification in the bottom region. This shift corresponds with ferrite number (FN) increases from ∼1 to ∼9 and a residual stress transition from tensile stress (80 ± 5 MPa) and lower hardness (216 ± 1.07 HV0.5) at the top to high compressive stress (220 ± 7 MPa) and high hardness (270 ± 2.12 HV0.5) near the substrate. Microstructural observations confirmed this gradient with finer cellular austenite evolving into a mixed dendritic columnar structure containing lathy, skeletal and vermicular δ-ferrite. The results demonstrate that thermal history and solute redistribution inherently produce a functionally graded microstructure and property profile in SS316L WAAM.