Microstructure and creep behaviour of P92 steel after HPT

The influence of the application of severe plastic deformation on microstructure changes and creep deformation behaviour was investigated in P92 creep-resistant pipe steel. P92 steel in its as-received state, with a microstructure typical of tempered martensite, was subjected to high-pressure torsion at room temperature. An investigation of the microstructure using an electron backscatter diffraction technique revealed that severe plastic deformation led to the subsequent formation of an ultrafine-grained microstructure, containing {110} fibre texture and randomly misoriented boundaries. The microstructure characteristics and hardness measurements exhibited saturated values at an equivalent strain of higher than about 20. The creep behaviour of the ultrafine-grained specimen produced in the saturated region exhibited a minimum creep rate that was about two orders of magnitude faster, and a larger creep ductility, in comparison with the as-received state. Creep exposure at 873 K with a time to fracture of about 3 h led to a slight grain coarsening and to the formation of Laves phase.