Study On Microstructure,properties And Nonlinear Ultrasonic Technique Of HR3C Steel During High Temperature Aging

HR3C is a new type of high chromium nickel austenitic steel widely used in the main steam pipeline and reheat steam pipeline of ultra-supercritical units.Compared with traditional austenitic steel,it has higher high temperature endurance and oxidation resistance.The excellent comprehensive properties of HR3C steel are attributed to its abundant alloying elements,but it also complicates the precipitation behavior of the second phase of HR3C at high temperature,mainly including M₂₃C₆and Nb Cr N phase.During the operation of HR3C steel,the decrease of plasticity and toughness is easy to occur,which is due to the coarsening of precipitates at grain boundaries.The appearance of these precipitates will have a significant impact on the properties of the steel.Therefore,it is necessary to analyze the changes in the organization and properties of HR3C during heat aging.Non-destructive testing techniques are also needed in the industry to detect and evaluate such changes to ensure safe operation of ultra-supercritical units.In this paper,HR3C steel was tested at 1000°C,2000h and 3000h at 700°C and750°C to obtain samples with different aging conditions,and then the microstructure and mechanical properties of the samples were analyzed.The results show that the grains of HR3C steel become finer and the twins decrease during aging,but the precipitates increase continuously.M₂₃C₆ phase is irregular square and aggregates at grain boundaries to form chains.Z-phase is tentacles and granules,mainly distributed in the crystal.The precipitation phase is distributed in the grain boundary and the crystal,which can improve the tensile strength and the high temperature endurance,but the coarsening of the precipitated phase at the grain boundary leads to a decrease in the plastic toughness.The HR3C steel samples after high temperature aging were measured by Nonlinear ultrasonic technique,and the relationship between microstructure,mechanical properties and nonlinear ultrasonic parameters was established.The results show that the nonlinear ultrasonic parameters increase with the increase of aging time and temperature,and the rising trend and the total area of the precipitated phase show a certain correlation.Compared with the original specimens,the nonlinear ultrasonic parameters of the most aged specimens increased by five times.It is worth noting that the nonlinear ultrasonic parameters have a certain correspondence with the mechanical properties of HR3C steel.When the nonlinear parameter is at a higher value,HR3C has better strength,but the impact toughness is seriously reduced,which is due to the precipitation of the phases at the grain boundary.This indicates that the aging state of HR3C steel,especially the brittleness problem,can be evaluated by nonlinear ultrasonic technique.