Hydrogen Trapping States and Apparent Hydrogen Diffusion in Laser Additively Manufactured Ultra-High Strength AerMet100 Steel as a Function of Secondary Hardening Available to Purchase

Microstructures, reversible hydrogen trapping states, and effective hydrogen diffusion coefficients (D_H,eff) of laser additively manufactured (LAM) ultra-high-strength AerMet100 steel in tempered conditions were studied by several material characterization methods, to determine diffusible, trapped, and total hydrogen content. With secondary hardening temperatures in the range of 454°C to 566°C, increasing temperature mainly promotes M₂C carbide coarsening and film-like reverted austenite thickening in the steel. Reversible hydrogen traps of tempered LAM AerMet100 steel are closely related to the precipitation behavior of highly coherent M₂C carbides. The desorption activation energy of the reversible hydrogen traps in the steel is seen to increase from 17.9±0.3 kJ/mol to 21.8±1.3 kJ/mol with temperature increasing from 454°C to 566°C. This correlates with the composition and size change of M₂C carbides at a higher tempering temperature. Hydrogen trapping capability of the steel has a peak value at a tempering temperature of 482°C corresponding to the combination of both high amount and medium trapping intensity of these reversible hydrogen traps. This results in the lowest diffusible and highest total hydrogen concentration for precharged H specimens of the steel. In addition, the D_H,eff of LAM AerMet100 steel in the overaged condition is not only influenced by the increased thickness of film-like reverted austenite but also simultaneously affected by the altered density of M₂C carbides. In comparison with the lowest D_H,eff (approximately 2.4 × 10⁻⁹ cm²/s) of LAM AerMet100 steel tempered at 482°C, a slightly higher D_H,eff of the steel tempered at a higher temperature is achieved by the apparent decrease of reversible hydrogen traps due to a decrease in density of the highly coherent M₂C carbides. These findings are important when considering achieving improved hydrogen embrittlement resistance for LAM high Co-Ni secondary hardening ultra-high-strength steel in an over-aged condition at the strength level of interest.