Grain size strengthening effect vs. indentation size effect in advanced high Cr heat-resistant steel

초록

It is well known that excellent strength of advanced 12% Cr ferritic/martensitic steel (which has received attentions as strong candidates as materials for ultra-super-critical boilers) is attributed to the combined strengthening mechanisms of the matrix and grain boundaries. While many studies have been made to reveal the source of its high matrix strength (e.g. strain hardening due to high dislocation density, solid-solution strengthening by W and Mo, precipitation strengthening by M23C6, MX and Laves phase), the contributions of grain boundary (i.e. grain size) strengthening effects to the macroscopic strength performance have not been clearly understood due to the complex nature of its grain boundary system. Here we performed a series of nanoindentation experiments on the advanced 12% Cr heat-resistant steel having extremely fine and complex lath-martensitic microstructures, in order to explore the role of the boundaries in strengthening mechanism of the virgin sample and in strength degradation mechanism during thermal aging. The results from nanoindentation experiments and microstructure observations are discussed in terms of the grain boundary contributions to the variations in nanoindentation hardness with the maximum indentation load (Pmax) and Lason-Miller parameter (LMP), exhibiting interesting size-dependences.