Keynote: Rapid Solidification and Microstructure Formation in Laser Additively Manufactured Martensitic Steels

Rapid Solidification and Microstructure Formation in Laser Additively Manufactured Martensitic Steels Zhe Lyu, Thomas Lienert and Leijun Li a University of Alberta, Edmonton, AB, Canada b T. J. Lienert Consulting, LLC, Los Alamos, NM, USA *Speaker: leijun@ualberta.ca Abstract Many alloy steels solidify as primary delta-ferrite, which, through a peritectic-eutectic reaction, transforms into gamma-austenite, which further transforms to lower-temperature alpha-ferrite and/or martensite. In Laser-Directed Energy Deposition (L-DED) of Grade 91 and Type 420 steels, the delta-ferrite transformation into gamma-austenite was found incomplete, leaving a fraction of delta-ferrite and much less fraction of martensite than anticipated at room temperature, with non-uniform hardness values and lower average hardness levels. An extended Scheil-type calculation was conducted to understand the kinetics of delta-ferrite decomposition. The final phase fractions were found to be dependent on the delta-to-gamma transformation kinetics that are dictated by the cooling rates for transformation, which in turn are governed by the preheat temperatures. The model results were used to develop a novel revision to the continuous cooling transformation (CCT) diagram for Grade 91 that captures the kinetics of the delta-to-gamma phase transformation. The broader implications toward rationalizing the microstructures and properties for AM (with rapid cooling rates) will be discussed.