Microstructural Evolution of T91 in High Temperature Conditions and under Combined Effect of Stress and Temperature

Abstract

ASTM A213 T91 steel is widely used in power plants and petrochemical industry for long-term service components. Due to its high resistance to creep, thermomechanical fatigue and corrosion, the use of grade 91 steel allows usual plant service parameters to be raised up to ultra-supercritical conditions (600 °C, 300 bar) so that performances are remarkably increased. The strongest factors that affect performances are the time of exposure, the temperature and the applied stress: such parameters can dramatically decrease the service life of a plant component. The improved mechanical properties of grade 91 are strictly related to its specific microstructure: a tempered martensite matrix with fine precipitates embedded in. Two typologies of secondary phases are present: M23C6 carbides (where M = Cr/Fe/Mo/Mn) and finely dispersed MX-type carbonitrides (where M = V/Nb and X = C/N). This study is focused on the microstructure evolution of grade 91 steel under creep conditions. First, three sets of laboratory-aged specimens heated in oven at 550 °C, 600 °C and 650 °C were examined; the exposure time was up to 50,000 h. Furthermore, the influence of stress on the microstructure in two sets of samples was evaluated: a first batch of specimens cut from an ex-service tube of a petrochemical plant (over 100,000 h of service at 580 °C and 19–25 bar) and a second set of samples coming from another ex-service tube under ultra-supercritical conditions (605 °C, 252 bar) in a power plant. The microstructures were characterized by optical, scanning electron and transmission electron microscopy and the results were compared to the literature. Some interesting trends were evidenced, in terms of secondary phases precipitation and coarsening, as well as martensite recovery. Furthermore, the applied stress seems to influence size and number of Laves phase particles.