Effect Of Impurity Tin,Antimony,and Rare Earth Cerium On The Creep Properties Of T/P91 Heat-resistant Steel

Power generation technology with high efficiency and parameters can achieve energy conservation in order to overcome the problems of global energy shortage and environmental pollution.Thus power generation technology with high parameters is an objective for our country in developing power plant.The raising of working parameters in thermal power units depends on the improvement of heat-resistant materials.As a kind of important material that is widely used in ultra supercritical power units,T/P91 ferrite heat-resistant steel has good combination mechanical properties at high temperature.However,the researches about T/P91 steel mainly focus on the microstructure characteristics,strengthening phases evolutions during creep and the relationship of creep properties with them.The effect of impurity elements and rare earth elements on the creep properties of T/P91 steel is still lack of enough attention.Tin and antimony are two kinds of common impurities in the steel which are difficult to be removed during steelmaking and easy to enrich during scrap recycling.They tend to segregate at grain boundaries,which will deteriorate the mechanical properties of steel.Duing creep process of T/P91 steel,the segregation of impurity elements and its influence on creep properties are still unkown.In addition,the effect of rare earth elements and the combined effect of rare earth element and impurity element on the creep properties of T/P91 steel is less studied.Therefore,we study and discuss the the effect of impurity elements tin,antimony and rare earth elements cerium on the creep properties of T/P91 steel in depth.In this paper,a series of high temperature creep experiments are conducted on T/P91 steels undoped and doped with impurity element tin,antimony and rare earth element cerium as well as the T/P91 steel combination doped with antimony and cerium.According to the analysis of experimental results,the establishment of creep constitutive equation,combined with the modern analytical technique such as micro chemical components analysis of field emission gun scanning transmission electron microscopy(FEGSTEM-EDS),fractographic analysis of field emission scanning electron microscopy(FEGSEM)as well as optical microscopy analysis,the effect of impurity tin,antimony and rare earth cerium on the creep properties of T/P91 steel and their mechanism are systematically studied.Tin and antimony reduce the creep threshold of T/P91 steel stress and the apparent creep activation energy and have little influence on the creep stress exponent.The creep damage tolerance of Sn-doped steel and Sb-doped steel is lower than that of the undoped steel,indicating that they are more sensitive to stress concentration and easy to induce creep cavities.The component analysis of grain boundary shows that tin and antimony segregate to grain boundaries in T/P91 steel,which reduces the adhesion of grain boundaries.More positions for creep cavity nucleation are provided and the growth and coalescence of creep cavities are accelerated.The degradation of microstructure of T/P91 steel during creep is thus accelerated and the creep properties are deteriorated.Compared with element tin,element antimony is more inclined to segregate to grain boundary,and the apparent creep activation energy of Sb-doped steel is lower.The creep properties of T/P91 steel doped with cerium are improved obviously,and the creep apparent activation energy,the threshold stress and the stress exponent of Ce-doped steel are all much higher than that of the undoped steel.According to the analysis about the characterization of microstructure before and after creep,it is found that element cerium can inhibit the martensite coarsening and the growth of fine precipitates in T/P91 steel during creep.Cerium atoms segregate greatly to the grain boundaries,which increases the grain boundary adhesion and reduces the formation of creep cavities.Due to the two influences above,element cerium can slow down the deterioration process of the microstructure in T/P91 steel during creep,so as to improve the creep properties.Creep properties of the Ce+Sb-doped T/P91 steel are improved significantly compared with that of the Sb-doped one,and the creep cavities in the specimens after creep rupture are dramatically reduced.Grain boundary composition analysis show that cerium and antimony segregate greatly to the grain boundaries,but cerium can restrain the further segregation of antimony during creep process.In addition,when cerium and atmony both segregating to grain boundaries,the harmful effect of element antimony can be suppressed by the beneficial effect of element cerium.Togther with the stabilization effect of element cerium on the microstructure during creep,the creep properties of T/P91 steel are obviously improved.The creep behavior of the steel at every temperature is power law creep,and the steady state creep stress exponent varies with the temperature.With the variation of stress exponent with temperature being inctroduced into power law,an equation that can predict the steady state creep rate at a certain temperature and stress is established.It is found that based on the equation the predicted results are more accurate at relatively high temperatures.This equation is of great significance in engineering application.