Study On Microstructure And Mechanical Properties Of C-HRA-5 Steel After Aging

Austenitic heat-resistant steel is widely used in ultra-supercritical thermal power boiler reheaters,superheaters and other parts due to its excellent high temperature resistance.With the continuous improvement of the parameters of thermal power boiler units,the requirements of steel for key parts of boiler units are becoming increasingly strict.An iron and steel company in Shanxi has developed an austenitic heat-resistant steel candidate C-HRA-5 steel that can be used as a superheater 650?-700?boiler reheater and superheater.The steel is currently in the trial production stage and has not yet been practically applied.In general,the precipitation behavior research and performance change law of the service heat-resistant steel are studied by artificial accelerated aging method.In this paper,the best solution treatment methods are found by performing different solution heat treatments on C-HRA-5 heat resistant steel at 1100?,1150?,1200?,1250?,and 1280?,and then the solution samples are aged at 650?,700?,and750?for different times.The precipitation rule of C-HRA-5 steel under different aging heat treatment methods was analyzed through the analysis of met allographic microscope,scanning electron microscope,transmission electron microscope,X-ray diffractometer,micro Vickers hardness tester,etc.And the following conclusions were mainly obtained:In this study,the best solution heat treatment method for austenitic heat-resistant steel C-HRA-5 steel is 1200?for 1 hour,and the solution structure is a single austenite structure.Transmission electron microscopy and EDS analysis showed that there was a small Z phase that was not completely dissolved in the matrix after solid solution,possibly due to the slower water cooling rate.The precipitation phases of C-HRA-5 steel after short-term aging mainly include M₂₃C₆ phase,MX phase and Z phase.After the aging time is extended to 500 h and 1000 h,there are?phase and Laves phase.The M₂₃C₆phase is mainly distributed in the grain boundary,which is a rectangular block or an irregular triangle;the Z phase is distributed in the grain boundary and the crystal,mainly in the shape of irregular long squares or small square blocks;the MX phase is distributed in the grain boundary and in the crystal,and it is spherical in the grain boundary and a small cube in the crystal;the?phase takes M₂₃C₆or MX phase as the nucleation site and it is distributed in a long rod shape within the crystal;the Laves phase appears as long bamboo leaves in the crystal.As the aging process progresses,the metastable precipitated phases Cr₃C₂and Cr₇C₃gradually transition to stable Cr₂₃C₆ precipitated phases.At the same time,the number of precipitated phases increased significantly,and nodularity occurred.The overall hardness of C-HRA-5 steel after aging at 650?,700?,and 750?tends to increase first and then decrease.During aging at different temperatures,the hardness reaches its peak after 300 h.In the early stage of aging,the precipitate phase is small in size and easily cut by dislocations,so its strengthening mechanism is mainly the dislocation cutting mechanism(Kelly mechanism);as the aging progresses,the size of the precipitate phase gradually increases,dislocation is difficult to cut through the precipitation phase,the hardness value gradually increases;after the aging time continues to increase,the precipitation phase on the austenite grain boundary is continuously distributed,and the intragranular precipitation phase particles also increase significantly.The combination of the strengthening effect of the boundary and the dispersion strengthening effect of intragranular precipitation results in the hardness of the steel reaching a peak.As the aging time continues to prolong,the precipitation phases continue to aggregate and grow,which weakens the strengthening effect.Therefore,the hardness of the steel gradually decreases.Based on the high temperature aging hardness value of C-HRA-5heat-resistant steel,a linear relationship model between hardness value and P function is established.That is,HV=869.48476-0.02757P,which is of great significance for the evaluation of the operating status of heat-resistant steel for ultra-supercritical units.