Experimental and micromechanical investigation of precipitate size effects on the creep behaviour of a high chromium martensitic steel

Abstract

This work involves a mechanistic investigation of the high temperature behaviour of a commercial high-Cr martensitic steel (P91), focussing particularly on the size effects of ${\text{M}}_{23}{\text{C}}_6$ carbides. To that purpose, a combination of microstructural observations, experimental measurements and crystal plasticity-based investigations of representative polycrystal aggregates of the steel microstructure are carried out. The tempered martensitic steel was found to exhibit a complex microstructure with hierarchical arrangements, including packets (10–$50\mu m$), blocks (2–$10\mu m$) and laths (0.2–$1\mu m$), and dispersed nanoscale MX-like precipitates and M₂₃C${}_6$ carbides. The effects of the tempering treatment duration on both the creep behaviour of the steel at 600 °C and the ${\text{M}}_{23}{\text{C}}_6$ carbide size were experimentally characterised. The results revealed that the size of the ${\text{M}}_{23}{\text{C}}_6$ carbides increases with tempering time, resulting in a degradation of the material’s creep resistance.

A novel multi-scale micromechanics-based modelling framework is proposed to describe the measured phenomena. It relies on representative microstructural models of the martensitic steel digitally reconstructed from EBSD measurements, and on a rate-dependent crystal plasticity formulation to describe the inelastic behaviour of the individual martensitic blocks. The latter also incorporates the effects of precipitate size into the internal slip system variable, viz. the slip resistance, through a strengthening term that is inversely proportional to the mean carbide diameter. The crystal plasticity formulation has been implemented numerically into the finite element method and calibrated from data obtained in this work. It is shown that predictions of the polycrystalline aggregate creep response are consistent with the experimental data for a relatively wide range of stress levels and temperatures. Furthermore, the predicted strong effect of carbide size on the steady state creep rate is analysed further to interpret its role on the typically observed scatter on the martensitic steel’s creep data in the 600 to 650 °C temperature range. Finally, the predicted equivalent inelastic strain distributions within the polycrystal aggregate were found to be highly heterogeneous due to the morphological heterogeneity of the martensitic blocks, and to increase with decreasing carbide size.