| Ultra-supercritical power generation technology is the preferred choice for clean coal power generation technology,and it is the most economical and most effective way for China to reduce CO2 emissions at this stage and in the future.In order to further increase energy conservation and emission reduction,various countries are developing advanced ultra-supercritical technology with steam parameters up to 700?.But the increase of steam parameters has raised the safety and reliability of key components of ultra-supercritical units.In the meantime,higher requirements are also put forward for the lifetime design and evaluation technology for the main components of the equipment,which has become a research hotspot in the field of clean coal power generation technology in recent years.One of the most important failure forms of key components in ultra-supercritical units at high temperatures is creep.Due to the irregular in their geometry and complex loads during operation,the key components in ultra-supercritical units are basically in a multiaxial stress state.While most of the creep data used in the design of the units were obtained by creep experiments in uniaxial stress state,the life prediction results obtained by the existing methods are different from the actual conditions of the components.Obviously,it is adverse for the life assessment and safety management of the ultra-supercritical units.Therefore,studying the creep behavior of high temperature components under multiaxial stress states,establishing the multiaxial creep constitutive model of high temperature components,and verifying with full-size components experiments,are the key scientific issues to reveal the lifetime evaluation of high temperature components in multiaxial creep conditions in ultra-supercritical units.It is of great importance to the selection,design,manufacture,operation and maintenance of key components in ultra-supercritical units.In this dissertation,a series of theoretical and experimental studies on the influence of multiaxial creep on the lifetime of key components in ultra-supercritical units have been carried out.The main research work and conclusions are as follows:(1)A series of uniaxial creep experiments of round bar specimens under different conditions were carried out for P92 steel,a commonly used material for the steam pipes in ultra-supercritical units with steam temperature of 600? and above,to provide data support for the creep model of P92 steel.Based on continuous damage mechanics Kachanov-Rabotnov(K-R)model,Norton-Bailey(N-B)and Graham-Walles(G-W)creep constitutive model were modified.The modified model can overcome the problem that traditional creep models cannot describe the whole stages of creep.The problem that the non-linear relationship between the double logarithm of the minimum creep rate and the stress occurring in the creep experiments of' P92 steel at temperatures above 600? has also been solved.Comparing with the uniaxial creep experiment results of P92 steel,it is found that both modified models can accurately predict the three stages of the creep process of P92 steel in uniaxial stress state.(2)A series of multiaxial creep experiments were conducted by using P92 steel internal tensile specimens under different internal pressure and tensile stress loading conditions with different multiaxiality to study the influence of multiaxiality on the development creep voids and provide data support for the multiaxial creep model of P92 steel.The multiaxial creep constitutive models of P92 steel were established by introducing multiaxiality and creep damage.In the modified N-B model,multiaxiality was mainly used to describe the contribution of the first principal stress and von Mises stress to damage.In the modified G-W model,multiaxiality is mainly used to describe the contribution of hydrostatic stress and von Mises stress to damage.Comparing with the multiaxial creep experiment results of P92 steel,it is found that both modified models can simulate the three phases of the creep process of P92 steel in multiaxial stress state.(3)The full-size pipe elbow creep experiment system was desined and established.Creep experiments of full-size pipe elbow with thin and thick wall-thickness subjected to in-plane bending were conducted.The multiaxial creep models obtained from the small specimens creep experiments were modified and verified.Comparing to experimental results,it showed that both the modified N-B model and the modified G-W model can simulate the creep process of P92 steel pipe elbow subjected to in-plane bending.(4)The isochronous creep rupture surface can be used to describe the trajectory of the stress state of multiaxial creep with the same uniaxial creep rupture time in the stress space.The formulae for the isochronous creep rupture trajectory of the isochronous creep rupture surface of multiaxial creep in stress space on the deviatoric plane were deduced.Using P92 steel experimental data of uniaxial and internal and tensile experiments,an isochronous creep rupture criterion for multiaxial creep of P92 steel was proposed.And it was verified by the full-size pipe elbows subjected to in-plane bending.Using the finite element analysis results of multiaxial creep model and the isochronous creep rupture criterion in the deviatoric plane,a new creep lifetime evaluation method for key components of ultra-supercritical units was proposed.(5)The existing different national design and evaluation standards or methods for the lifetime evaluation of high temperature components in power plants were compared by calculating the creep and fatigue life of the critical positions of the T-component in ultra-supercritical units.The results showed that differences existed in the lifetime evaluation results since the theory adopted by different standards were different.Under the design conditions,the damage cumulative form of T-component was mainly creep damage.The fatigue damage was basically negligible.The modified G-W multiaxial creep model was used to analyze the creep behavior of the T-component in the first three cycles under design conditions.The new creep lifetime evaluation method for key components of ultra-supercritical units was used to evaluate the creep lifetime of critical positions of the T-components. |
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