Stress−strain curves and derived mechanical parameters of P91 steel from spherical nanoindentation at a range of temperatures

Abstract

This work compares different measurement and analysis protocols of spherical nanoindentation tests performed at different temperatures on a ferritic/martensitic P91 grade steel, in order to derive meaningful indentation stress−strain curves (ISSC) and estimate material parameters such as indentation modulus, yield strength, work hardening exponent and ultimate tensile strength. Quasi-static multi-cycle and dynamic continuous stiffness measurement indentation using a spherical indenter with a tip radius of 20 μm has been carried out from room temperature to 600 °C in vacuum in a set-up where thermal drift has been minimised by an active surface referencing system and accurate temperature stabilization in the contact area. The methodology used to determine the contact radius is critical to achieve consistent results. The application of different combinations of definitions of contact radius and indentation-induced strain to nanoindentation data obtained by the quasi-static and dynamic measurements reveals that Tabor's approach combined with a geometrically determined contact radius best represents the ISSC relationship for the P91 characterized. This method is then extended to predict the high temperature tensile properties of the steel from nanoindentation results.