Heat treatment-driven stability and morphology control of reverted austenite in additively manufactured martensitic stainless steel

Additively manufactured 420 stainless steel (AM420SS) develops a refined martensitic matrix but retains highly unstable austenite. Optimizing properties require strategic heat treatments that control austenite morphology and mechanical stability while preserving solidification features that promote austenite reversion. In this work, the microstructural evolution and phase transformation in 420 stainless steels fabricated using Laser Powder Bed Fusion (LPBF) were systematically investigated under various direct tempering (DT) and quench-and-partitioning (QP) treatment conditions. Results reveal AM-specific reversion pathways during DT: high energy interfaces near melt pool boundaries and elongated lath boundaries aligned with the heat flow direction allowed growth and coalescence of austenite within the matrix, generating ∼37 % austenite after 30 min exposure at 400 °C. For the QP treatment, short exposure at 1150 °C for 30 s readily produced recrystallized and twinned austenite. The initial quench provided block, packet, lath, sub-grain, and low energy Σ3 twin boundaries as reversion pathways during partitioning. The resulting austenite morphologies from different conditions are comprehensively linked to the solidification history, carbon enrichment, strain distribution and texture evolution.