Nanoindentation as a strength probe—a study on the hardness dependence of indent size for fine-grained and coarse-grained ferritic steel

A nanoindentation hardness testing system, including an atomic-force microscope (AFM)-based nanoindentation tester and a calibration method using electrolytically polished single-crystal metals as references, was proposed. This was applied to a study of the mechanical properties of fine-grained ferritic steel (grain size of 1.2 μm) and coarse-grained ferritic steel (30 μm). An empirical function giving the macroscopic hardness for all four reference metals from the nanoindentation force curves was established. The converted Vickers hardness (HV*) of the coarse-grained steel is almost independent of the indent size. The fine-grained steel shows only HV* 130 with an indent of only 100 nm, compared with a macroscopic hardness of HV 210. The difference, HV 80, is considered to reflect the amount of grain-boundary strengthening. The critical indent size for the hardness transition seems to be around 1 μm, comparable to the grain size of the specimen. This result supports the explanation of grainboundary strengthening. It is also consistent with Pickering’s work on low-carbon steel, as the estimated locking parameter (k of 2.6×10⁵ N/m^3/2) in the Hall-Petch relationship is in good agreement with his value of 2.4×10⁵ N/m^3/2.