Hydrogen trap states and binding energies were determined for AERMET 100 (Fe-13.4Co-11Ni-3Cr-1.2Mo-0.2C), an ultrahigh-strength steel using thermal desorption methods. Three major H desorption peaks were identified in the precipitation-hardened microstructure, associated with three distinct metallurgical trap states, and apparent activation energies for desorption were determined for each. The lattice diffusivity (DL) associated with interstitial H was measured experimentally and verified through trapping theory to yield H-trap binding energies (Eb). Solid-solution elements in AERMET 100 reduce DL by decreasing the pre-exponential diffusion coefficient, while the activation energy for migration was similar to that of pure Fe. M2C precipitates were the major reversible trap states, with Eb of 11.4 to 11.6kJ/mol and confirmed by heat treatment that eliminated these precipitates and the associated H-desorption peak. A strong trap state with Eb of 61.3 to 62.2kJ/mol was likely associated with martensite interfaces, austenite grain boundaries, and mixed dislocation cores. Undissolved metal carbides and highly misoriented grain boundaries trap H with a binding energy of 89.1 to 89.9kJ/mol. Severe transgranular H embrittlement in peak-aged AERMET 100 at a low threshold-stress intensity was due to H repartitioning from a high density of homogeneously distributed and reversible M2C traps to the crack tip under the influence of high hydrostatic tensile stress.

Hydrogen Trap States in Ultra-High Strength AERMET 100 Steel. D.Li, R.P.Gangloff, J.R.Scully: Metallurgical and Materials Transactions A, 2004, 35[3], 849-64