Short-term creep strain localization in 12%Cr turbine steel under biaxial stress states at 600 °C via digital image correlation

Abstract

FV566 martensite steel is an important material for turbine components whose extreme operating conditions potentially give rise to the creep deformation. This paper investigates the short-term creep strain localization of this steel under biaxial stress states at 600 °C. Short-term creep tests utilizing digital image correlation (DIC) were performed on different geometries of specimens with various applied stresses to investigate the strain and damage localization. Using the macroscopic creep data obtained from uniaxial specimens, finite element (FE) models were developed to evaluate stress redistribution and damage evolution. Contrast to the uniaxial state, strain heterogeneities at biaxial states were very similar under different stress levels and the notch tends to extend the creep life. The ‘crack’ growth rate was low during the steady-state creep and increases significantly at the tertiary stage creep, especially close to failure time. Further microstructure characterization involving SEM and EBSD had shown that the formation, growth and coalescence of martensite cracking and microcavities in near fracture frontier area during inelastic deformation are the main creep damage mechanism. By linking DIC, FE and microstructure results, the short-term creep fracture mechanism under biaxial stress states was thoroughly discussed.