Post-printing heat treatment routes for additively manufactured 420 stainless steel

Abstract

The introduction of new steel alloys into additive manufacturing requires the development of appropriate post-processing strategies to tailor microstructure and mechanical performance. Additively manufactured 420 stainless steel (AM420SS) has recently emerged as a martensitic steel alloy suitable for processing by laser powder bed fusion (LPBF). Despite exhibiting mechanical properties comparable to those of conventionally heat-treated counterparts in the as-printed condition, post-printing treatments remain necessary to maximize in-service performance and broaden industrial applicability, either by exploiting the solidification-induced microstructure or by homogenizing the microstructure to eliminate solidification features while enabling controlled phase constitution. In this thesis, four post-printing strategies are established. The first strategy is a quench-and-partitioning (QP) treatment designed to homogenize the microstructure and modify the austenite grain morphology. The results reveal that a short austenitization duration followed by initial quenching to 150°C during QP promotes the formation of Σ3 twin boundaries in the high-temperature austenite state and leads to the development of inter-lath austenite within individual parent grains under ambient conditions. The second strategy involves a systematic investigation of direct tempering (DT) for austenite reversion. Microstructural characterization shows that increasing the DT temperature from 300°C to 500°C drives a transition in reverted austenite from Type I (located between neighboring martensitic laths) to Type II (located along parent austenite grain boundaries). The third strategy is a quench-and-tempering (QT) treatment that transforms the microstructure into fine and fully-martensitic grain state. Mechanical property evaluation of QT and DT samples tempered at 400°C for 30 minutes demonstrates that transformation-induced plasticity is activated only in DT-treated samples containing both Type I and Type II reverted austenite microstructure. In addition to these thermal treatments, the fourth strategy is through thermo-mechanical processing which examines softening mechanisms active during the high-temperature deformation. Dynamic recrystallization is the dominant mechanism at intermediate temperature and strain rate conditions, while dynamic recovery through grain growth takes over at high temperatures. Overall, the findings of this thesis demonstrate that the microstructure and mechanical performance of LPBF-fabricated AM420SS can be engineered through different post-printing routes to meet the requirements of high-strength applications.