Influence of isothermal tempering on microstructures and hydrogen-environmentally embrittlement susceptibility of laser additively manufactured ultra-high strength AerMet100 steel

Abstract

Good hydrogen-environmentally embrittlement (HEE) resistance is necessary for safe service of critical main load-bearing ultra-high strength steel (UHSS) components of advanced aircrafts in marine atmosphere environment. Controlling isothermal tempering time is one important heat treatment method to optimize HEE resistance of UHSS by designing microstructure. Influence of isothermal tempering time at 482 °C on microstructures and HEE susceptibilities of laser additively manufactured (LAM) AerMet100 steel was investigated by several material characterization methods and slow strain rate tensile (SSRT) tests in both of air and 3.5 wt% NaCl aqueous solution. Results mainly indicate that microstructure evolution behaviors of LAM AerMet100 steel successively include martensite and retained austenite decomposition, M2C carbide formation and coarsening, and film-like reverted austenite formation and thickening. Short-time tempering specimens of LAM AerMet100 steel has the highest strength-loss-index in all of the specimens due to the lowest austenite amount. The lower HEE susceptibility of LAM AerMet100 steel in long-time tempering condition is mainly corresponding to M2C carbide coarsening and film-like reverted austenite thickening. Furthermore, difference of hydrogen charging method strongly influences macro-fracture behaviors of the SSRT specimens, but micro-cracking modes of the steel in different conditions are predominantly martensite packet/plate boundary cracking. Under open-circuit potential condition, HEE cracking zone is prone to be origin from pitting corrosion sites, and the SSRT specimen in long-time tempering condition have a good ductility; in contrast, under cathodic polarization condition, HEE cracking zone is around the circumferential zones, and the tensile specimens are brittle fracture. These findings are important when considering achieving improved HEE resistance for LAM AerMet100 steel tempered at 482 °C of a longer isothermal tempering time at the strength level of interest.