| Austenitic heat-resistant steels have been widely used in superheater and reheater tubes of thermal power plants,among which Super304H steel has been widely used in ultra-supercritical（USC）units with 600～620°C steam parameters because of its combination of excellent high-temperature strength,oxidation resistance and corrosion resistance.To avoid the explosion and leakage accidents during the long-term service and guarantee the safe operation of USC units,it is necessary to explore the microstructure evolution of the Super304H steel.C-HRA-5 is a new type of domestic-made austenitic heat-resistant steel developed for 700°C steam parameters advanced ultra-supercritical（AUSC）units.However,the service experience of C-HRA-5 steel in power plants is deficient,which still needs appraisal and evaluation.There are high content and a variety of alloy elements in C-HRA-5 steel.The investigation of its microstructure evolution,mechanical properties,strengthening,and failure mechanism can provide a reference for its safe operation.About 3%Cu element is added to both Super304H and C-HRA-5 steel,which is cost effective and exhibits excellent strengthening effect.However,there is a significant difference in their creep strength at 700°C,which might result from their different chemical composition.Therefore,the comparative study on the microstructure evolution and strengthening mechanism of the two kinds of steels is instructive for the development of domestic austenitic heat-resistant steel.The addition of Cr in Super304H and C-HRA-5 steel is to ensure a high temperature oxidation resistance performance,but Cr element may precipitate as M23C6 in the service process,which will worsen the toughness and affect the safety of the startup and shutdown of the units.Currently,there are few researches on the toughness of Super304H and C-HRA-5,thus,it is necessary to investigate the evolution of M23C6 and its impact on toughness.In this thesis,the aging tests of Super304H steel at 650°C and 700°C for 5000 h were carried out respectively,and samples were taken from the Super304H steel pipes served in a USC unit for 3～10 years.The aging tests of C-HRA-5 steel at 700°C for 5000 h and the creep tests of 140 MPa～220 MPa with more than 15000 h were carried out.Optical microscope（OM）,scanning electron microscope（SEM）and transmission electron microscope（TEM）were used to observe the microstructure,precipitation evolution,fracture morphology,and crack distribution of the samples;electron backscatter diffraction（EBSD）was used to investigate the grain characteristics and the relationship between second phases and matrix.By measuring Brinell hardness and impact toughness,and establishing creep life curves,the changes of the mechanical properties were analyzed.Based on the experimental data,firstly,the effect of microstructure on the mechanical properties,the influence of temperature on the evolution of precipitation,and the changing of the structure and precipitation of Super304H steel during the long-term service are discussed.Secondly,the type,distribution,and coarsening behavior of second phases in C-HRA-5 steel are studied;The design principle of alloy elements is analyzed;The strengthening mechanism is systematically calculated;And the failure fracture is discussed.Finally,the differences of alloy composition and precipitation characteristics between these two steels are studied;The reasons for the obvious difference in strength and toughness properties are analyzed;And the design principles of these two steels are su mmarized.The results are as follows:（1）During aging,the Cu-rich phase precipitated inside grains and M23C6 precipitated on grain boundaries of both Super304H steel and C-HRA-5 steel.The Cu-rich phase is small in size and coherent with the matrix,showing the shape of the coffee bean.The M23C6 distributed in grain boundaries of Super304H steel in a discontinuous chain-shape,but formed a continuous network in C-HRA-5 steel.Nb is precipitated as Nb（C,N）phase in Super304H steel but as Z phase in C-HRA-5 steel,which is related to the higher Cr content in C-HRA-5.Due to the addition of about 4%W element in C-HRA-5 steel,a large number of needle-shape Laves phase precipitated inside grains in the later stage of aging.（2）At 700°C,the coarsening rate of Cu-rich phase in Super304H steel is obviously faster than that at 650°C,while in C-HRA-5 steel the size of Cu-rich phase remains fine and stable,which is mainly related to the suppression by a high content of Ni element of precipitation and growth of Cu-rich phase in C-HRA-5 steel.（3）The creep strength of C-HRA-5 steel at 700°C is significantly higher than that of Super304H steel.The precipitation strengthening effect of fine Cu-rich phase,Z phase and needle-shape Laves phase is the main factor to improve the strength of C-HRA-5 steel.The strengthening mechanism of the Cu-rich phase in Super304H steel changes from shear mechanism to Orowan mechanism,and the obvious coarsening of its size leads to a significant reduction of the strengthening effect,thus the creep strength of Super304H steel is relatively low.（4）The impact toughness of both Super304H steel and C-HRA-5 steel decreased after aging,which was mainly related to the large amount of M23C6 precipitated on grain boundaries.However,the grain size of Super304H steel is fine and the M23C6 keeps discontinuous,contributing to ductile fracture of Super304H steel.While the grain size of C-HRA-5 steel is relatively large,and the amount of M23C6 is higher,forming a continuous network on grain boundaries,leading to brittle fracture.（5）In both Super304H steel and C-HRA-5 steel,the texture<111>//RD formed under stresses,suppressing the proceeding plastic deform and improving the creep resistance.While with the extension of time,a new texture<001>//RD occurred.The changing of texture is driven by the decrease of strain energy,which contributes to sliding and can improve the creep rupture ductility.（6）There are a lot of twin structures in the two steels,which can prevent the crack propagation by dispersing stress and improve the impact toughness.In the high stress region,the stress promotes the precipitation of second phases and promotes the degradation of twin structures,while the low stress has a small effect on the microstructure and second phases.In this thesis,the internal relations among the alloy design principle,microstructure evolution,and changing of strength and toughness have been established,which can provide theoretical support for the safe service of austenitic heat-resistant steels in thermal power plants,and promote the improvement of domestic austenitic heat-resistant steels,and contribute to the development of new heat-resistant steels. |
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