| The nuclear steam generator heat pipe is the core of the key components of the nuclear power unit,which determines the high parameter and high reliability of the equipment.In order to design nuclear power materials with reduced oxidation at high temperature and excellent mechanical properties and to realize the localization of nuclear power equipment materials at an early date,our group focused on Fe-Cr-Ni alloy and optimized its composition.Finally,a CHDG-B superior austenitic alloy was obtained.In this paper,the optimal solution treatment parameters of CHDG-B alloy are determined,considering the service conditions of the superior alloy for nuclear steam generator and the heat transfer tube the creep behavior at high temperature and the strain-induced precipitation behavior of the alloy are studied.The failure behavior of the alloy under the complex creep-fatigue interaction was investigated for the first time.The creep curve of CHDG-B alloy has an obvious initial creep stage and steady creep stage but an accelerated creep stage is not obvious.With the decrease of creep stress,the steady state creep duration of the alloy is prolonged.The endurance limit of CHDG-B alloy at 650?and 104 h is ?1×104650=232 MPa.After creep,nanometer NbC particles are dispersed in the crystal.However,after long creep tests,Laves phase particles were found in the dislocation pile-up.These precipitates appear later than nano-scale NbC,but larger in size and the longer the time is the more they appear.There are M23C6 particles in the crystal of CHDG-B alloy after stress relaxation.The size is about 20?m,and the shape is elliptical along the grain boundary.There are also two kinds of precipitates near the dislocation.After strain-induced precipitation at four temperatures between 600?and 750?,there are NbC particles about 10 nm in size near the dislocation.When the test temperature is 700?and 750?,the Laves phase exists,and the particle size of laves phase is obviously larger than that of NbC particle.Based on the above experimental results,it can be seen that the CHDG-B alloy has good creep properties at high temperature,and the alloying composition can effectively form the strengthened precipitated phase during the service.But the failure form of creep-fatigue is different from that of single creep.In the creep-fatigue test,the process of work hardening and recovery softening occurred in the alloy.There are a lot of dimples and creep voids on the surface of creep-fatigue fracture.However,there is no obvious morphology of intergranular fracture in the fracture surface.The steps and fatigue stripes of crack propagation were observed,and the creep-fatigue fracture was assumed to be transgranular fracture.In this experiment,the tensile holding time exists,and the fracture is dominated by creep damage.The precipitates were found to be coarse Laves phase near the dimples of fracture.The difference between creep-fatigue and single creep test is mainly due to the structure of deformation twins.Under the interactive test,more deformation twins appear in the crystals and the size of the grown deformation twins is larger.In addition to the large size twins,there are also a large number of nanoscale twins in the alloy crystals.Compared with single creep,there are also microvoids and microcracks in the process of creep-fatigue interaction failure. |
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