Mechanical and microstructural stability of P92 steel under uniaxial tension at high temperature

9 –12%Cr creep-resistant ferritic-martensitic steels are candidates for structural components of Generation IV nuclear power plants. However, they are sensitive to softening during low cycle fatigue and creep-fatigue tests, which leads to the destabilisation of the tempered martensite microstructure, inducing a decrease in further creep resistance. To better understand the softening mechanisms in an ASTM Grade 92 steel during uniaxial deformation, tensile tests were carried out at 823 K, showing an extended and stable softening stage after some work-hardening. This behaviour may be due to mechanical instability (necking), damage or microstructural (grain size) evolution. Examination of fractured and non-fractured tensile specimens (metallography, macrohardness tests, SEM, TEM) suggests that the physical mechanisms responsible for softening are mainly grain size evolution and diffuse necking.