Alteration in nanohardness of matrix phase associated with precipitation during long-term aging of type 316 stainless steel

Mechanical characterization for a long-term aged type 316 austenitic stainless steel was carried out using the nanoindentation technique. Samples were aged isothermally at 973 K for various times up to 39,332 h (4.5 years). The precipitation behavior was characterized with transmission electron microscope (TEM), and the hardness of the matrix phase was evaluated by the nanoindentation technique. The nanohardness of the matrix starts to decrease simultaneously with the initiation of M23C6 precipitation and is reduced significantly after 100 h aging, while the macroscopic Vickers hardness shows a considerable increase. Since the reduction in the matrix hardness is synchronized with the M23C6 precipitation, the softening results from a depression of the solid-solution hardening by solute elements such as carbon, chromium and molybdenum, which transform into M23C6. An accurate amount of precipitation hardening is evaluated by considering the reduction in the matrix hardness. The strengthening by the second phase can be understood quantitatively based on the Orowan mechanism for 183 h aging and the composite strengthening for 39,332 h, respectively.