Study On Low Cycle Fatigue Performance Of A New Nuclear Cladding Material

Compared with traditional zirconium alloys,high chromium ferrite/martensitic steel has better high temperature oxidation resistance,corrosion resistance,stress corrosion cracking resistance and mature manufacturing properties,so it has become one of the fourth generation of nuclear energy system cladding candidate materials.The temperature field and alternating stress field of nuclear power plant make cladding materials bear certain alternating loads,and its fatigue performance is one of the important performance indexes to measure the comprehensive performance of cladding.At present,researches on ferritic/martensitic steel mainly focus on the law and mechanism of the influence of material composition and preparation process on material properties.However,there are few reports on the intrinsic mechanical properties of thin-walled clad pipes.This is mainly due to the special geometry of thin-walled clad pipes,especially when the thickness is less than 0.5mm,it is very difficult to directly measure the mechanical properties.As a result,the intrinsic mechanical property data of thin-walled pipes are scarc e.In this paper,a new type of ferrite/martensite steel thin-walled pipe provided by a research institute was selected as the research object.The mechanical properties of the pipe were directly tested by self-designed clamps.Uniaxial tensile tests and strain controlled low cycle fatigue tests were conducted at 25℃ and 500℃ respectively.The fatigue fracture was analyzed by microscopic characterization method.The uniaxial tensile test results show that the mechanical strength of the pipe at 500℃ is degraded seriously.The yield limit decreases from 552.5MPa to 494.8MPa,the tensile limit decreases from 1106.3MPa to 659.2MPa,a decrease of 40.4%,and the elongation after fracture decreases from 42.6%to 14.9%.T he low-cycle fatigue test results show that the fatigue life of the pipe is closely related to the strain amplitude.The fatigue life of the material can reach more than 5×104 at 0.3%strain,and rapidly decreases to about 1×104 when the strain increases to 0.4%.Meanwhile,the fatigue life of the material is greatly affected by temperature at low strain amplitude.The fatigue life of the pipe is 30,596 times at 25℃ and 0.35%strain,but drops to more than 5000 times at 500℃.With the increase of strain amplitude,the influence of temperature on the fatigue life gradually decreases.Cyclic softening occurs at both temperatures,but the softening behavior is not obvious at 25℃,0.3%and 0.35%strain amplitudes.Slight cyclic hardening occurs at the beginning of the cycle,and the softening rate increases slightly with the increase of strain amplitudes.When the strain increases from 0.3%to 0.6%at 25℃,the softening rate increases from 2%to 5%.However,the cyclic softening behavior is obvious at 500℃,and the softening rate increases by an order of magnitude,with the softening rate between 15%-20%corresponding to the strain amplitude of 0.3%-0.6%.The Manson-Coffin formula,the tensile lag energy formula and the finite element method are used to predict the fatigue life of pipes.The three methods have obtained good prediction results,which are basically consistent with the test data.The prediction effect of Manson-Coffin formula is better than the other two methods at two temperatures.Through SEM observation of fatigue fracture,it is found that the pipe at 25℃ and 500℃presents typical fatigue fracture morphology,which is composed of crack nucleation region,steady-state expansion region and final fracture region.The macro fatigue fracture shows the characteristics of transgranular cleavage fracture.At high temperature,a large number of coarse oxide particles are distributed in the steady-state expansion area,and the fall off and fracture of the oxides will aggravate the crack propagation,which may be one of the important reasons for the decrease of fatigue life of the pipes at high temperature.TEM observation and EDS analysis show that these oxides are mainly composed of Cr304.In addition,a large number of fatigue bands were found on the fatigue fracture surface,and the spacing of fatigue bands on the fracture surface was larger under high strain amplitude,indicating that the crack growth rate was larger and the fatigue life of the pipe was lower under high strain amplitude.In this paper,the mechanical properties of new ferritic/martensitic steel thin-walled pipes are provided,and the fatigue fracture mechanism is analyzed,which has important reference value for the promotion and application of new thin-walled clad pipes and the safety assessment of nuclear engineering.