Effect of vacuum degassing and capsule heating on HIPed super duplex stainless steel

This study investigates the effects of vacuum and thermal degassing on powder-filled capsules of super duplex stainless steel (UNS S32505) prior to hot isostatic pressing (HIP). Degassing was performed at pressures from atmospheric to 0.01 Pa and at 300°C or 600°C. After HIP and solution treatment, tensile strength, hardness, Charpy impact toughness, microstructure, and gas content were evaluated. All samples met fully the ASTM A988/A988M specificantions. These include chemical composition limits, the α–γ phase balance of 45–50% ferrite with no intermetallics. Oxygen slightly decreased with better vacuum, while nitrogen remained largely unchanged. Tensile and hardness properties were unaffected, but Charpy impact toughness improved with better vacuum, rising from 79 J (no degassing) to 139 J (highest vacuum). SEM–EDS showed oxide inclusions as the primary factor controlling toughness, with higher inclusion density promoting crack initiation. These results highlight the importance of vacuum degassing in minimizing oxide inclusions and enhancing fracture toughness in HIPed SDSS components.