Microstructure And Properties Of HR3C/T92 Long-term Service Dissimilar Joint

Ultra supercritical units are still the main source of electricity around the world.Due to the long-term service under high temperature and high pressure,the evolution of microstructure and properties of units' material should become the cause of concern.Normally,the weld is the weakest part of the pipeline.What's more,to adapt the actual working condition,there are lots of HR3C/T92 dissimilar joints in the reheater of the USC units,so it's important to study the HR3C/T92 dissimilar joint after service.The HR3C/T92 dissimilar joint had serviced for 51020 h under 605? has been studied.The microstructure and mechanical properties of this dissimilar joint has been studied by optical microscope,scanning electron microscope,electron probe,transmission electron microscope,impact tester and creep and rupture strength test machine.The effect of long-term service has been analyzed.The results show that the base metal and the weld both precipitate lots of precipitates.The M23C6 phase,Nb Cr N phase and Nb N phase appear in the HR3 C base metal;MX phase appear on the base metal of the weld;a large number of MX phase appear in the T92 base metal The chain-like precipitates appeared on the grain boundary of HR3 C base metal had coarsened.Some precipitates appeared on the twin boundaries and grew into the grain.There are lots of particle precipitates precipitated in T92 base metal.The precipitates of the weld grew along the branch grain boundary.The result shows that grain orientations of different parts of the weld aren't converge.The high-temperature constant strain creep tests are performed on HR3C/T92 joint.The result shows that compare to the as received material,HR3 C base metal still has good creep-resistant properties while creep-resistant properties of the T92 base metaldrop more.The creep properties of the weld are similar to T92 base metal.The microhardness of the heat affect zone of HR3 C steel and weld rises sharply while that of T92 steel and weld rises a bit.The impact strength of service HR3 C steel has a drop of 90% compared to the supply state HR3 C steel and the impact fracture is clearly a brittle fracture.The impact strength of service T92 steel has a drop of 30% compared to the supply state T92 steel and the impact fracture of the T92 steel is a ductile fracture.The impact fracture of weld is a mixed fracture.The HR3 C base metal still has good creep porperties but it has already showed embrittlement.The particle precipitates on the grain boundary of service HR3 C are determined as M23C6,and the precipitates grow into the grain are determined as M7C3 by TEM.