Isolated effects of reverted austenite and precipitates on the mechanical properties of an additive-manufactured 18Ni maraging steel

Abstract

The main characteristic of maraging steels is the combination of high strength with high toughness, which can be achieved through heat treatment cycles. Heat treatment of maraging steels is critical due to their high fraction of alloying elements such as nickel, molybdenum, cobalt, and titanium. When heated, this super-saturated solid solution system undergoes intermetallic precipitation by spinodal reaction or nucleation, growth, and martensite-to-austenite transformation. However, the literature is unclear about the predominant effect of intermetallic precipitates or reversed austenite transformation on the properties: a drop in toughness due to intermetallic precipitation or an increase in toughness and ductility due to austenite reversion. This work proposes a study to distinguish the effects of quick posterior solubilization after heat treatments on the tensile and toughness properties, obtaining a martensitic matrix with reverted austenite but without intermetallic precipitates of an additive-manufactured 18Ni maraging steel. Therefore, this study offers a novel approach to isolating the effects of intermetallic precipitates and reverted austenite by applying a rapid solubilization treatment. Furthermore, X-ray diffraction was performed before and after the tensile tests to study the austenite-to-martensite transformation after plastic deformation. The present study's findings reveal that reverted austenite is relevant for toughening and increasing ductility of the steel without intermetallic precipitates, leading to ultimate strength near 1100 MPa and 13.5% strain. Furthermore, 480 °C/3 h peak aged condition presented a CTOD value of 3.3 µm, as-built of 25 µm, contrasting to ~ 90 µm from quick solubilized samples. However, when the microstructure is subjected to heat treatments that develop fine intermetallics, the precipitates play the major and leading role in the mechanical strength of maraging steels, dominating all the aspects of the mechanical behavior of the part, even in the presence of reverted austenite.