Mechanical Testing of Austenitic Stainless Steels at 4 K: A Novel Approach for Characterizing Unstable Crack Propagation in Cryogenic Applications

The mechanical characterization of austenitic stainless steels at cryogenic temperatures is crucial for the structural components of superconducting magnets in fusion devices and particle accelerators. This study presents and compares mechanical testing results for FXM-19 (Nitronic 50) and 316LNH (316LN with nitrogen in the upper limit) while comparing their behaviour against crack propagation, with emphasis placed on a novel analytical procedure for characterizing unstable crack propagation (UCP) during fracture toughness (FT) testing at 4 K. FXM-19 demonstrated excellent yield strength, making it one of the most desirable alloys for use as structural materials in high magnetic fields at cryogenic temperatures (superconductor base projects). It also exhibited UCP behaviour, which is very complex to characterize at very low temperatures due to the existence of discontinuous plastic flow or serrated yielding in this kind of materials. A new methodology was developed to distinguish between unstable crack tearing and serrations at 4 K. The study reveals that FXM-19 consistently exhibits unstable crack propagation after stable tearing, whereas 316LNH maintains stable crack propagation with exceptional toughness. These findings have critical implications for material selection in safety-critical cryogenic applications, where stable and predictable fracture behaviour is often preferred.