An investigation of the effect of pre-straining on the creep behaviour of a P91 steel at 600 °C using impression creep testing

Abstract

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in a grade 91 steel using impression creep testing. The grade 91 material investigated has been tested in two different microstructural conditions, in the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant. The two microstructural conditions were confirmed by hardness mapping and Electron Back Scattered Diffraction (EBSD). Previous investigations have used pre-strained uniaxial creep specimens to investigate this effect, but the present work has utilised the specialised small-scale impression creep testing technique to test material obtained at a number of positions along the axes of failed hot tensile specimens. This allowed impression creep samples to be extracted at various pre-strains for investigation and for a wide range of hot tensile pre-strain to be investigated. The two microstructural conditions have shown a divergence in behaviour, with the normal martensitic material showing little change in creep strength with increasing pre-strain and the aberrant material increasing markedly in creep strength with increasing pre-strain.