The martensitic precipitation hardened stainless steels, also known as maraging stainless steels, utilise a low-carbon martensitic microstructure alongside intermetallic precipitation created by ageing treatments to produce high strength, corrosion resistant materials. However, certain environments are known to promote hydrogen embrittlement, severely reducing maximum material performance. The degree of embrittlement is known to be strongly dependent on ageing conditions; though the exact microstructural origins behind susceptibility are not so clear in existing literature, aspects relating to hydrogen transport and trapping are often highlighted in mechanistic research. In this study, a range of techniques are used to explore behavioural differences between a selection of ageing conditions of two grades of maraging steel. For each material, the peak-yield ageing treatment was selected as a baseline, with two other ageing temperatures selected to produce one ‘overaged’ and one ‘underaged’ condition with similar yield strengths. These treatments produce a myriad of microstructural differences, such as variations in austenite and grain boundary segregation. This section of work focuses on the effect of precipitation, particularly regarding precipitate coherency and its impact on hydrogen transport. Slow strain rate tensile (SSRT) testing was used to broadly characterise ductility loss when specimens are electrochemically charged in-situ. Various cathodic potentials were applied in order to produce trends of behaviour across ranges of charging current. Aspects of hydrogen-enhanced subcritical crack propagation and sub-yield crack initiation were investigated using Compact Tension (CT) specimens in Rising Step Loading (RSL), and unnotched tensile specimens in Constant Loading (CL) respectively. In all tests, the susceptibility increased significantly with decreasing ageing temperature. Transmission Electron Microscopy (TEM) was used to quantify a drop in coherency with higher ageing temperature; combined with hydrogen permeation measurements, a contribution of reduced susceptibility is linked to the lower elastic strain field trapping associated with incoherent precipitates.
