Ultrasonic nanocrystal surface modification for strength improvement and suppression of hydrogen permeation in multi-layered steel

Hydrogen embrittlement of multi-layered steel (MLS) is significantly affected by a degree of hydrogen permeation from surface layers to interfaces. Here, the surface microstructure of multi-layered steel was modified for the first time using ultrasonic nanocrystal surface modification (UNSM), and the roles of UNSM-treated surface layers in strength and hydrogen permeation behavior were investigated by examining microstructural evolutions of the surface layer. Since the UNSM induced the compressive residual stress, grain refinement, and deformation twin formation at the specimen surface, the yield strength greatly improved via synergetic contributions of the grain refinement effect and dislocation strengthening. In addition, the UNSM-affected zone of 150-210 mu m along depth direction effectively suppressed the hydrogen permeation by supplying compressive residual stresses and hydrogen trapping sites including grain boundaries, dislocations, and twin boundaries. (c) 2021 Elsevier B.V. All rights reserved.