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

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