Creep Damage Mechanism And Evaluation Method Of The Ultra Supercritical Austenitic Heat Resistant Steel

To further improve the thermal efficiency and reduce the harmful exhaust emissions,the thermal power units need to increase its steam temperature and steam pressure so as to improve the power generation efficiency,protect the environment as well as to achieve the energy-saving.In recent years,the new type austenitic heat-resistant steels represented by Super304H and HR3C steel,have been widely employed in the boiler tubes of ultra-supercritical units,due to their excellent high temperature performance.However,under high temperature,high pressure and long-term service,the creep damage will inevitably occur in austenitic steel for those boiler tubes,such as the excessive coarsening of precipitates at grain boundaries and the initiation as well as the propagation of intergranular cracks.To prevent and control this catastrophic failure,it is necessary to systematically investigate the evolution process of creep damage of the new austenitic steel,so as to explore the failure mechanism of the austenitic steel and the evolution law of creep damage of its microstructure.In addition,here we also explore the creep damage evaluation of the austenitic heat-resistant steel based on the new characterization method,which is of great significance to ensure the safe operation of high-temperature components of the ultra-supercritical unit.In this paper,the creep fracture tests of Super304H and HR3C steel specimens were firstly carried out at 650℃/210MPa to obtain the creep fracture life of these two materials.Then,a series of interrupted creep tests were carried out to obtain the specimens with different degrees of creep damage.The microscopic analysis of those Super304H specimens shows that the precipitations of second phase of Super304H gradually increase at the grain boundaries with the extension of creep time,and the creep voids preferentially appear at the triangular grain boundaries and are distributed around the M₂₃C₆ phase rather than around the larger Nb（C,N）phase.The analysis of those HR3C steel specimens shows that Cr atoms at grain boundaries are redistributed with the precipitation of M₂₃C₆ phase,and the dislocation density in the grains also increased significantly at the end of the creep stage,especially in the area around the precipitated phases.The increase of dislocation density represents the increase of the degree of lattice de-bonding and the absence of atomic arrangement rules,which creates conditions for the formation of creep holes,microcracks and other defects.Meanwhile,the creep damage characterization and evaluation of Super304H steel and HR3C steel were studied based on EBSD technique.It was found that GROD and GOS increased by 220%in the middle and late creep period,while KAM and GND only increased by 108.97%and 82.53%,respectively.The results show that GROD and GOS have stronger evaluation ability and are more suitable for the creep degradation evaluation of Super304H steel.The further study of HR3C steel shows that the GROD value is larger near the grain boundary than that in the grain,which means that GROD can be further used to characterize the intergranular damage of HR3C steel.Thus,the characterization method based on the misorientation parameter can effectively evaluate the intergranular damage of austenitic steel during creep.In this paper,the mechanical properties of Super304H steel and HR3C steel were also tested based on the automatic ball indentation method.The results show that the mechanical properties of these two austenitic steels do not increase or decrease monotonously during creep,but increase first and then decrease.Combined with the analysis of microstructure observation,it is inferred that the large size Z phase and the precipitated second phase M₂₃C₆particles in the early creep stage can hinder the slip of dislocation lines,which leads to the increase of mechanical properties of austenitic steel.With the increase of creep time,the precipitation strengthening effect decreases,and the recovery softening phenomenon takes the dominant role,which leads to the decline of the mechanical properties of the austenitic steel at the later stage of creep.It should also be pointed out that the whole life stage of austenitic steel cannot be evaluated by automatic ball indentation method alone because the test data change little in the second stage of creep.Finally,the creep damage evaluation of austenitic steels were carried out by using the nonlinear ultrasonic technique.The nonlinear ultrasonic test was carried out for the creep damage of Super304H and HR3C austenitic steels.The results show that the amplitude of the nonlinear parameters of these two austenitic steels increase monotonically within the creep life range,and the increase of the nonlinear parameters is very significant at the end of the creep life.The results also show that nonlinear ultrasonic technique can be used to characterize and predict the creep damage of austenitic steel,because the formation of microdefects and the accumulation of dislocation in the creep process of austenitic steel will lead to the distortion of ultrasonic propagation,thus enhancing the nonlinear effect of the material.Here,the polynomial function and exponential function are used to fit the data obtained,so as to study the damage evaluation and prediction methods of two kinds of austenitic heat resistant steels.The results show that the fitting effect of exponential function is better than polynomial function.For Super304H steel,compared with the two-term exponential function,the single-term exponential function has better fitting effect.For HR3C steel,the fitting effect of single-term exponential function is good only in the late creep stage,but the two-term exponential function fits well in the early and middle creep stages.Based on the above results,we constructed a piecewise function for predicting the creep damage degree of HR3C steel.The function uses a two-term exponential function to predict the life of the HR3C steel in the range of early and middle creep life,and a single-term exponential function to predict at the end of creep life.