The concept of “EBSD strain analysis” and its application to creep and creep-fatigue damage assessment of ferritic and austenitic heat resistant steels

Abstract

The concept of EBSD (Electron Back Scattering Diffraction) strain analysis was proposed and applied to investigate the parameter for accumulated permanent strain during creep or creep-fatigue damage process. The EBSD strain analysis was based on the dislocation theory with a simplified model of estimating dislocation density within the EBSD unit pixel and dislocation mobile distance. EBSD analyses were conducted on experimentally creep and creep-fatigue damaged austenitic SUS304HTB boiler tube steel and ferritic Mod.9Cr piping steel. Creep damage was imposed under 650°C, 130 and 100 MPa stress, and creep-fatigue damage was imposed by an electric hydraulic servo testing machine with induction heater under the condition of 650°C, 1% strain range and 10 min tensile strain hold. The longitudinal mid-section of specimen was observed by EBSD equipment with IPF (Inverse Pole Figure) maps, KAM maps, GAM (Grain Average Misorientation) maps and GOS (Grain Orientation Spread) maps. Apparent reduction of misorientation and growth of block size was observed for damaged Mod.9Cr steel, while increase of misorientation was observed for SUS304HTB steel. Among EBSD strain parameters M_ε,ΔKAMave, M_ε,ΔGAMave and M_ε,GOSave, those two contradictory trends were turned to the almost similar linear correlations with accumulated permanent strain both for creep and creep-fatigue by M_ε,GOSave.