Improving the yield strength of an antibacterial 304Cu austenitic stainless steel by the reversion treatment

As an implant material, Cu-bearing austenitic stainless steels can possess the antibacterial property, but their mechanical strength is low. In order to improve the yield strength of a 304Cu (17%Cr–7%Ni–3%Cu) alloy through substantial grain refinement, a research investigation has been taken up to conduct the reversion annealing treatment comprising a heavy (71%) cold rolling reduction followed by annealing at various temperatures (650–950 °C) and durations (1–5400 s). The microstructure evolution was examined by electron backscatter diffraction and further characterized by magnetic measurements, and mechanical properties were determined by tensile and hardness testing. The precipitation of Cu was confirmed by transmission electron microscopy. It was found that the reversion of deformation-induced martensite to austenite took place by the shear mechanism, followed by subgrain formation and continuous recrystallization resulting in quite non-uniform grain size distribution. The finest reversed grains were around 0.6 μm in size, but also much larger austenite grains and a small fraction of unreversed martensite existed in the final structure despite annealing at least up to 800 °C. Coherent Cu particles were observed after aging for 1.5 h at 700 and 650 °C, while the yield strength could be improved to 507 and 791 MPa, respectively, i.e. by ~2–3 times that of the annealed steel. The ductility of the steel remains still high, the fracture elongation being 36%.