| With the increasing demand for electric energy in China,the development of green energy by modern science and technology cannot meet the growing demand for electric energy,thermal power generation is still the most important power generation method in China in the coming decades.As the country's requirements for thermal power generation efficiency gradually increase,it is very important to develop more advanced ultra-supercritical thermal power generating units.However,with the improvement of steam parameters of thermal power generating units,hot end components such as superheaters and reheaters of ultra-supercritical thermal power generators are facing more severe working environments,in addition to the critical requirements for high temperature mechanical properties of metal materials,the material's resistance to high temperature oxidation and corrosion has received more extensive attention.Therefore,it is particularly important to study the oxidation behavior of the high temperature materials of thermal power generating units in the steam environment.This paper intends to modify the composition of traditional commercial HR3C austenitic heat-resistant steel,and select three different compositions of HR3C austenitic heat-resistant steel.A self-built steam oxidation device is used to perform steam oxidation experiments on several austenitic heat-resistant steels for 1000 hours under steam conditions at 650? and 700?.The oxidative weight gain of the four groups of samples at different temperatures was recorded and the kinetic curve was plotted.The morphology of surface and cross-section,oxide composition and microstructure of each sample were observed and analyzed by SEM,EDS,TEM and optical microscopy,the oxides formed on the surface of the alloy after oxidation were characterized by XRD,and the steam oxidation mechanism of HR3C austenitic heat-resistant steel at 650? and 700? was revealed.The results show that:The oxidation weight gain of the four groups of HR3C austenitic heat-resistant steel under steam condition at 650? is lower than that of the same group at 700?,temperature has a significant effect on the oxidation behavior of HR3C austenitic heat-resistant steel,accelerating the growth and peeling of oxide film.Under the steam condition of 650?,the modified HR3C-3#heat-resistant steel has the smallest oxidation weight gain,and has good resistance to steam oxidation and high microhardness.The modified HR3C-2#heat-resistant steel showed unstable oxidation at both temperatures,the outer oxide layer was loose porous island-shaped Fe2O3,and the inner oxide layer were mixed oxides rich in Fe,Cr and Ni.Four groups of austenitic heat-resistant steels showed different degrees of grain refinement,and the grain refinement of the modified HR3C-2#heat-resistant steel was the most obvious.Different from the dense Cr2O3 protective film formed on the surface under steam condition of 650?,the modified HR3C-1#heat-resistant steel and the modified HR3C-3#heat-resistant steel form a large amount of Nb2O5 on the surface of oxidation at 700?.Similar to the modified HR3C-2#heat-resistant steel in the later stage of oxidation,the outer oxide layer is island-shaped Fe2O3,and the inner oxide layer are mixed oxides rich in Fe,Cr and Ni.In addition,the effect of Nb on the steam oxidation behavior and mechanical properties of HR3C austenitic heat-resistant steel was further explored.Under the condition of 650? steam,when the content of Nb is within a certain range(less than 0.59wt.%),the steam oxidation resistance of the alloy increases with the increase of Nb content;when the content of Nb is over high(2.93wt.%),which will greatly reduce the steam oxidation resistance of the alloy.The addition of Nb and Co promotes grain refinement.Proper amount of Nb will increase the microhardness of the alloy,and an excessive amount of Nb will reduce the mechanical properties of the alloy. |
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