| Abstract Scope |
Additive manufacturing (AM) is considered an attractive pathway for processing precipitation hardened (PH) martensitic grade stainless steels across a wide range of maritime and turbomachinery applications. Since these alloys display excellent weldability in the wrought condition, fusion-based AM techniques are commonly used to produce relatively defect-free components. In order to achieve the microstructures needed to meet the desired mechanical and corrosion properties in these alloy systems, solutionizing and aging heat treatments are required and typically coupled with hot isostatic pressing (HIP) to achieve defect free structures. However, there is little to no uniformity or predictability in how different material heats respond to these different heat treatments, resulting in significant uncertainty in the microstructure and corresponding mechanical properties results. These differences in heat treatment response can be generally correlated with small changes in composition, but the highly segregated as-deposited microstructures in these complex multi-component alloy systems are driving complex phase transformations that are affected by changes in composition. Many of these phase transformations are driven by variations in nitrogen and oxygen, leading to the formation of carbo-nitride and oxide phases during HIP post-processing or solutionizing heat treatments. By impacting the microstructure of these AM processed alloys in unpredictable ways under the same heat treatment conditions, significant uncertainty in the mechanical properties results. The evolution of these phase transformations and the role that different alloying elements play in the formation of these minor phases will be critical to developing heat treatment standards for these important alloy systems. Since many of the differences result from small changes in minor alloying elements, such as Mn, Nb, N, and O, current standards for both composition and heat treatment cycles are insufficient and more flexible heat treatment practices designed for each heat should be pursued. |