Effect Of Oxide Layer On Creep Fatigue Life Of 10%Cr Heat Resistant Steel

With the continuous development of social economy,new energy sources have been continuously incorporated into the power grid.The intermittent supply of new energy leads to the instability of the power output of the grid,which requires the thermal power generating unit to continuously perform peaking operations to ensure the stability of the power output.The common structural discontinuities in the unit components can cause multiaxial effects.Using the finite element method to analyze the multiaxiality of circular notched specimens with different stress concentration factors,it can be found that the maximum value of triaxiality is not located on the surface of the material.On the surface,but below the surface is not in contact with the air environment.In the past,the research and development of high temperature components such as turbine rotors did not consider the effect of oxidation on the creep fatigue life of materials.This makes it easy to over-conserve the design and life assessment of components,so that the high-temperature components can not exert their maximum potential and cause waste,and even the design margin is insufficient to bring about serious safety hazards for the safe and reliable operation of the unit.In this paper,10%Cr heat-resisting steel for ultra-supercritical turbine rotor is selected as test material,and the specific model is X12CrMoWVNbN10-1-1.High temperature test is respectively conducted under vacuum and air conditions to explore the oxidation of 10% Cr heat-resistant steel Fatigue life effect.Creep fatigue tests were conducted using four operating conditions of an ultra-supercritical turbine rotor,namely,stable operation(600° C),hot start(550° C),warm start(500° C),and cold start(300° C).The test results of the two conditions were compared and analyzed to determine the effect of oxidation on the creep fatigue life of the heat-resistant steel.The microstructure fracture morphology and cracks of the test specimens were observed and analyzed by scanning electron microscopy to explore the microstructure evolution and damage mechanism of the material under the oxidation conditions.The experimental results show that in both environments,the increase of temperature and strain amplitude will reduce the creep fatigue life of 10%Cr heat-resistant steel.Under the same experimental condition,the creep fatigue life under vacuum is always lower than the creep fatigue life under atmospheric conditions.The material has stronger resistance to deformation and fatigue resistance in an oxidizing environment.Moreover,dense oxides are formed on the surface of the material and cracks to prevent further oxidation of the material,which plays a role in protecting the material.In addition,under the vacuum environment,the crack tip always maintains a sharp structure and is prone to stress concentration.In the air,due to the existence of an oxide layer,the concentrated stress at the crack tip can be released,thereby reducing the crack growth rate.Therefore,the sharpening passivation mechanism can be used to explain the effect of oxide layer on the creep fatigue life of 10%Cr heat-resistant steel.