Research On The Microstructure And Properties Of The First Batch Of Domestic P91 Steel Before And After 50,000 Hours Service

P91 steel is widely used in supercritical units for its excellent overall performance at high temperatures,such as the main steam and reheat steam piping and other high-temperature pressure-bearing components.Since its introduction in 1987,the first batch of domestic P91steel has been in service for over 50,000 hours in supercritical units,and P91 steel has been used in China for more than 30 years.However,compared with imported P91 steel,domestic P91 steel due to smelting,pipe making and heat treatment process,there are still problems such as uneven tissue properties and lack of high temperature long-term service data.Therefore,the study of domestic P91 steel in long-term service in the process of the evolution of the organizational properties of the law,both for China’s comprehensive assessment of the quality of domestic P91 steel pipe to provide technical support,but also for China’s high parameters,high-capacity super（super）critical unit to provide a strong guarantee of safe operation.In this paper,The microstructure,mechanical properties and fracture mechanism of the first batch of domestic P91 steel for the main steam pipeline in the power plant before and after 50,000 hours service and accelerated thermal aging at 650℃for 500 hours were investigated by means of optical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,Brinell hardness tester,Charpy impact testing machine and universal testing machine.The main obtained research results are as follows:The microstructures of as-received domestic and imported P91 steels are composed of tempered lath martensite,small M₂₃C₆particles in the lath boundary and the original austenite grain boundary,and smaller MX particles dispersed within the lath.The size of M₂₃C₆particles is slightly greater than that in imported P91 steels.Except for lower impact energy,Brinell hardness,tensile mechanical properties and small punch（SP）properties are almost equival to those of imported P91 steels.Moreover,their tensile,impact and small punch（SP）fracture mechanisms are both typical ductile fracture with microvoid coalescence.After 50,000 hours service,the size of the lath martensite in the domestic P91 steel structure became wider,the number of subgrain blocks increased.The coarsening of M₂₃C₆particles and a large size of Laves phase precipitation occurred between the lath boundary,while the size and distribution of MX phase did not changed much.Brinell hardness,small punch（SP）properties,impact toughness,room temperature and high temperature tensile mechanical properties all showed a certain decline,but its Charpy impact,room temperature and high temperature tensile as well as small punch（SP）fracture mechanism still maintained ductile fracture with more shallow and smaller dimples.After 50,000 hours of service,domestic P91 steels are accelerately thermally aged at650℃for 500h,which is totally equivalent to the 20.2 years service period.M₂₃C₆particles continuously precipitated and aggregated in the original austenite grain boundary and lath martensitic,and the existing M₂₃C₆particles continues to increase,and finally Ostwald ripening occurs.Laves phase continiously grow by cannibalizing M₂₃C₆particles,while the size and distribution of the MX phase have not much changes.With increasing of thermal aging time,the small punch（SP）mechanical properties properties decreased,and Brinell hardness further reduced near to the lower limit requirements of the national standard,but the small punch（SP）fracture mechanism is still manifested as ductile fracture characterized by a large number of dimples and tearing ridges.