Investigation On Microstructure Evolution And Creep Rupture Properties Of Metal Materials Used For A-USC Unit Under High Temperature Conditions

630°C-700°C Advanced Ultra-Supercritical Coal-Fired Power Generation Technology（A-USC Unit）is the frontier technology that countries around the world will focus on developing in the field of clean and efficient coal-fired power generation at present and for a long time in the future.It has important practical significance for achieving the global goal of"carbon peaking and carbon neutrality".The metal materials used for the key components of the boiler and steam turbine units in the A-USC unit are very insurmountable bottlenecks,and they are also practical engineering problems that need to be solved urgently,which mainly involve scientific issues such as the evolution of high-temperature microstructure and the degradation of mechanical properties at high temperatures.In this article,two representative types of materials are selected,including a novel W3Co3 class 11%Cr type Martensitic heat-resistant steel with the grade of 11Cr-3W-3Co heat-resistant steel,and another alternative nickel-based heat-resistant alloy material,the grade of 725 Type Ni Cr Mo Nb Al Ti nickel-based alloy.Based on the high-temperature service conditions of heat-resistant steels for 630-650°C and nickel-based superalloys for650-750°C,the nucleation and growth mechanism of the different types precipitated phases,microstructure inside the matrix and the transformation characteristics of dislocation with aging time in the long-term thermal aging or creep process were analyzed and discussed by SEM-BSE,EBSD,XRD and TEM;Furthermore,high temperature creep properties were conducted under different stress conditions of 625°C-675°C for heat-resistant steel,and the steady-state creep equation was obtained.Moreover,the evolution process of microstructure（including the type,morphology and substructure of the precipitated phase）of creep specimens was analyzed.The creep mechanism of11Cr-3W-3Co heat-resistant steel and IN725 alloy was systematically studied.The long-term aging of heat-resistant steel at 650°C showed that the Laves phase was mainly precipitated near the martensitic slat boundary,the prior austenite grain boundaries and M₂₃C₆ carbides,and as the aging time increased,the Laves phase precipitated near M₂₃C₆ would grow up by engulfing M₂₃C₆ until the carbide particles were completely swallowed.At the same time,the contents and sizes of the precipitated phases inside the matrix have changed significantly with the extension of the aging time,the high-density dislocation inside the material has been significantly reduced,and the size of the sub-grain has increased significantly,which significantly reduced the long-term creep performance of the material.The results of the high-temperature creep test of IN 725 nickel-based alloy show that the creep performance is significantly reduced after long-term aging,which is mainly due to the evolution of the internal microstructure of the material after long-term aging,The results of long-term creep deformation and fracture analysis show that the creep mechanism of 725 alloy is significantly different at different temperatures and stresses,and the creep mechanism of the material is strongly affected by the precipitated phases and applied stress at this temperature.The comparative research results of this paper further reveal the microstructure evolution of heat-resistant steel and nickel-based alloys for A-USC units in the long-term aging process,as well as the material science problem mechanism of the degradation of high-temperature mechanical properties.Test data support is provided for the design of the safety threshold of heat-resistant steel and nickel-based alloy components.It provides important theoretical significance for evaluating the creep deformation law and creep fracture life of heat-resistant steel and nickel-based alloys under high temperature stress service environment.