Microstructural evolution mechanisms in rolled 17-4PH steel processed by ultrasonic surface rolling process

The surface of rolled 17-4 precipitation-hardening steel was subjected to the ultrasonic surface rolling process technique at room temperature. Microstructural evolution mechanisms and its intrinsic relation with processing history, namely formation process and heat treatment, were systematically analyzed via high-resolution transmission electron microscopy. The results indicated that a gradient nanocrystalline structure layer was generated at the surface, where a nanocrystalline and amorphous transitional microstructure formed on the topmost layer. The paramount role of dislocation activities in the nanocrystallization process was investigated in detail. It was found that the precipitates was also surrounded and sheared by dislocations into smaller size, which then dissolved in the matrix. Meanwhile, a phase transformation from martensite to δ-ferrite occurred in the processed surface, and superlattice and elemental redistribution or/and realignment phenomena appeared. The reason for these was that high grain boundary density provided “short circuit diffusion” channels, which raised the number of enrichment sites and enhanced the diffusion kinetics, and high dislocation density acted as fast paths, thus accelerating atomic diffusion.