Carburization of cladding materials has long been a concern for the nuclear industry and has led to the restricted use of high-thermal conductivity fuels such as uranium carbides. With the rise of small modular reactors (SMRs) that frequently implement a graphite core-block, carburization of reactor components is once more in the foreground as a potential failure mechanism. To ensure commercial viability for SMRs, neutron-friendly cladding materials such as Zr-based alloys are required. In this work, the carburization kinetics of Zircaloy-4 (Zry-4), for the temperature range 1073–1673 K (covering typical operating temperatures and off-normal scenarios) are established. The following Arrhenius relationship for the parabolic constant describing ZrC growth is derived: K_p (in μm²/s) = 609.35 exp(−1.505 × 10⁵/RT)). Overall, the ZrC growth is sluggish below 1473 K which is within the operational temperature range of SMRs. In all cases the ZrC that forms from solid state reaction is hypo-stoichiometric, as confirmed through XRD. The hardness and elastic modulus of carburized Zry-4 are also examined and it is shown that despite the formation of a ZrC layer, C ingress in the Zry-4 bulk does not impact the mechanical response after carburization at 1073 K and 1473 K for 96 h.

The traditional manufacturing approach to produce engineering components can have a high energy cost, high material waste, longer delivery times, and specific geometries that may be unattainable. The recent developments in additive manufacturing might provide the opportunity to produce complex engineering components, reduced manufacturing costs, and reduced delivery times. Direct Energy Deposition (DED) offers excellent possibilities such as fabrication of metal components with complex geometries, repair of high-value equipment, development of functionally graded materials, and large-scale additive fabrication or repair. This work focuses on the fabrication of AISI 420 martensitic steel using DED for the application of the aerospace, automotive, and medical industries. AISI 420 martensitic steel (12.7%Cr, 0.4%C in wt.%) was successfully deposited onto 316L substrate by a Laser Engineered Net Shaping (LENS^®) process carried out in an open atmosphere. The cross-sectional examination by electron microscopy and XRD confirms the dual-phase microstructure of martensitic needles in random orientation and approximately 22 wt. % of austenite lamellar phase by Rietveld refinement and quantitative phase analysis. There are no cracks observed throughout the materials. However, the area fraction of porosity was found to be 0.4%, with the max size of 2μm. Preliminary mechanical characterization by micro-Vickers hardness tests shows uniform hardness about 725 (HV) trend across the build. The microstructure, the chemical composition of the phases, and the mechanical properties of the steel could be affected by the post-heat treatment, which is very sensitive. The team investigates to optimize the heat-treating method to improve the microstructure and mechanical properties.