Comparison of two HIP processes to manufacture the DEMO divertor shielding liner

• Comparative study identifies optimal HIP route for DEMO divertor liner. • Advanced NDT confirms leak-free cooling channels. • Two-step HIP ensures cleaner interfaces and more homogeneous microstructure. • Tube process provides initial safety via primary barrier and improved channel geometry and roughness control. • Findings support reliable HIP-based fabrication for future fusion reactors. The DEMO divertor shielding liner, a critical component for neutron shielding and thermal management, requires reliable manufacturing techniques for complex cooling channel geometries. This study evaluates two Hot Isostatic Pressing (HIP) fabrication processes using Grade 91 steel mock-ups as a surrogate for EUROFER 97. Non-destructive testing, including Scanning Acoustic Microscopy and infrared thermography, confirmed continuous interfaces without detectable macroscopic defects or leakage in the cooling channels for both processes. Microstructural characterization revealed homogeneous joints with no visible interface in the two-step process, while the tube process showed ferritic phase migration and oxide-rich inclusions at the HIP interface. Local nanohardness measurements revealed elevated hardness values compared to typical tempered martensite, primarily influenced by surface condition, local work hardening, and the presence of non-metallic inclusions. Thermal-hydraulic tests highlighted higher pressure losses in the two-step mock-up due to increased surface roughness, despite similar thermal response times for both processes. Synchrotron X-ray microtomography revealed a higher defect volume fraction at the HIP interface in the tube process. Overall, the two-step process provides superior interface cleanliness and microstructural homogeneity, while the tube process offers improved control of channel geometry but requires careful surface preparation to limit interface-related defects. These findings support the selection and optimization of HIP fabrication routes for DEMO divertor components, while highlighting the need for further mechanical and irradiation-relevant qualification.