Research On Creep-fatigue Damage Characteristic Of X12CrMoWVNbN10-1-1 Rotor Steel

With the development of global economy and technology,the energy demand unlikely shrinks rapidly.As the fossil fuels gradually exhaust and environmental problems aggravate,renewable energy technology blooms.Under such circumstence,the conventional power plant keeps improving,such as the primary energy efficiency and pollutant emissions reducing.Recently,ultra-supercritical units has been largely constructed in China.High steam temperature makes the equipments a big challenge.X12CrMoWVNbN10-1-1 steel has excellent characteristics under high temperature,and it becomes a good choice of the ultra-supercritical units.The fatigue and creep damage which occurs during the start and stop of the unit will challenge the long-term safe operation and the efficiency of the equipment.Therefore,to whole power generation industry,there are great demands to study the damage feature of X12CrMoWVNbN1-1-1 steel under creep-fatigue interaction.In this thesis,the X12CrMoWVNbN10-1-1 steel which is one of 9-12% Cr martensitic steel was studied by the creep-fatigue interaction tests at 620℃ and high temperature fatigue crack propagation tests with load-controlled loading model.The main contents and conclusions of this thesis are as follows:(1)The creep-fatigue life of X12 CrMoWVNb Nn1-1-1 steel at 620 ℃ is inversely proportional to the exponential form of the load holding time.Namely,As the load holding time increases,the fatigue life of X12 CrMoWVNb N10-1-1 steel decreases,meanwhile stabilized.If the load holding time is long enough,fatigue life is hardly ever related to stress ratio.(2)The cyclic strain of X12CrMoWVNbN10-1-1 steel was studied by strain partition method.The results show that There are the descending stage,the stable stage and the rapid increasing stage obviously for the whole strain range,the creep strain range and the anelastic strain range.The elastic strain range decreases at first and then enters the stable stage.The anelastic strain range and the plastic strain range show no obvious stage,however the fluctuation is found out around the average values.The ratchet strain,the whole strain range,the creep strain range and the inelastic strain range increase significantly with the increase of the load holding time,and decrease slightly with the increase of the stress ratio.The elastic strain range and the anelastic strain range decrease with the increase of the stress ratio,and show no relationship with the load holding time.(3)The creep-fatigue interaction coefficient μ is introduced,and the creep-fatigue interaction mechanism is studied by comparing with the macroscopic life.With the increase of the load holding time,the effect of fatigue on the creep damage is from slight inhibiting to strong promoting.With the increase of stress ratio,the promoting effect of fatigue on creep damage weakens slightly.(4)The fracture microtopography of the specimens of X12 CrMoWVNbN 10-1-1 steel under creep-fatigue test is analyzed.The results show that the failure of X12 CrMoWVNbN 10-1-1 Steel under elevated temperature is ductile rupture.The dimple surface of the pure creep specimen and specimens with long load holding time is smooth.The dimple of specimens with short load holding time are more likely damaged,and the surface around more likely grow new dimple.(5)The fatigue crack propagation rate of X12CrMoWVNbN10-1-1 steel at 620 ℃ was well fitted by the Paris law and Forman law.Meanwhile,the creep-fatigue crack data of X12 CrMoWVNb N1-1-1 steel at 600 ℃ from reference was employed to investigate the creep-fatigue crack propagation rate by the stress intensity factor range(35)K and the C* parameter.