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

Microstructures, reversible hydrogen trapping states, and effective hydrogen diffusion coefficients (DH,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 DH,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 DH,eff (approximately 2.4 × 10−9 cm2/s) of LAM AerMet100 steel tempered at 482°C, a slightly higher DH,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.