Simultaneous enhancement of mechanical properties and resistance to hydrogen-assisted degradation by multiple precipitation and nano-twinning in medium manganese steel

The microstructure, mechanical properties and resistance to hydrogen-assisted degradation of a medium manganese stainless steel (Fe-0.17C–10Mn–18Cr–5Ni-0.9V-0.26 N, in wt. %) were investigated. The investigated steel was 65% cold rolled and subsequently annealed at 1000 °C for 3 min to promote a very fine-grained austenitic microstructure of ∼1.3 μm with various V/Cr-rich precipitates in the size range of 20–400 nm. This steel took up 2.48 ppm hydrogen during in-situ immersion slow strain rate tensile (SSRT) testing at a strain rate of 10⁻⁶ s⁻¹ in 5% NaCl. Insignificant changes in its strength and ductility values were observed, which demonstrates a high resistance to hydrogen-assisted degradation. The analysis of thermal desorption spectroscopy (TDS) curves of the SSRT specimens revealed that hydrogen uptake occurs on irreversible hydrogen traps (precipitates), which inferred from the corresponding high temperature desorption peaks. Transmission electron microscopy conducted on interrupted tensile samples revealed the formation of nano-deformation twins of ∼20 nm, which explains the outstanding strain hardening behavior. The findings demonstrate an excellent hydrogen trapping ability due to multiple types of precipitates in the fine-grained austenitic microstructure, and an outstanding strength-ductility balance (yield strength: 600 MPa, ultimate tensile strength: 975 MPa, total elongation: >45%) that is explained by deformation-induced nano-twins. The synergetic effects of various types of precipitates, fine grain size and deformation mechanism on the resistance to hydrogen-assisted degradation and mechanical behavior are discussed.