Surface grain boundary engineering in 304 stainless steel by means of mechanical grinding treatment-induced gradient plastic strain and annealing

Grain boundary engineering (GBE) has shown a promising application in improving resistance to intergranular corrosion (IGC) of polycrystalline metallic materials with low-stacking fault energy, such as austenitic stainless steels. However, the traditionally uniform plastic pre-straining methods, such as cold rolling, in thermomechanical processing are hard to implement on complicated surfaces. Here, we demonstrate a novel approach using gradient plastic strain induced by surface mechanical grinding treatment with a rotary tool and subsequent annealing to optimize grain boundary character distribution (GBCD) in the near-surface layer. The gradient plastic strain followed by long-time annealing (24-96 h) in 304 stainless steels achieved an optimized GBCD with over 75% frequency of coincidence site lattice (CSL) boundaries and disconnected random boundary network in the near-surface layer. The intergranular corrosion tests showed that the resulting 304 stainless steels with the optimized GBCD in the near-surface layer present an excellent resistance to IGC behavior due to a high fraction of ET' boundaries. During the annealing process, severe plastic strain near the surface results in small size grain clusters via strain recrystallization, while low-level plastic strain in the subsurface promotes the formation of high fraction of CSL boundaries and large size grain clusters via strain-induced boundary migration. After the complete depletion of gradient plastic strain, the directional growth of grain clusters promotes the further extension of surface GBE into interior region. Thus, the thickness of surface GBE region can be regulated by the annealing time.

Automated summary

Grain boundary engineering (GBE) can improve the resistance to intergranular corrosion (IGC) of polycrystalline metallic materials. The authors propose a novel method using surface mechanical grinding treatment and subsequent annealing to optimize the grain boundary character distribution (GBCD) in the near-surface layer. Experimentally, an optimized GBCD with over 75% frequency of coincidence site lattice (CSL) boundaries and disconnected random boundary network was achieved through gradient plastic strain and long-time annealing. Intergranular corrosion tests showed excellent resistance to IGC behavior in the resulting 304 stainless steels with the optimized GBCD in the near-surface layer.