Study on Corrosion Mechanism of CLAM Steel and Weld Bead in Liquid Pb-Bi at 550℃

Author:Chen Gang

Supervisor:lei yu cheng

Database:Doctor

Degree Year:2019

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Pages:162

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Lead bismuth eutectic alloy(LBE)is considered as a primary candidate coolant for lead-cooled fast reactor(LFR)as well as a coolant and a spallation target for accelerator driven sub-critical system(ADS),on account of its low melting point,high boiling point,low reactivity,low viscosity,high thermal conductivity and high neutron yield.China low activation martensitic steel(CLAM)with independent intellectual property rights and optimized composition and performance is one of the reduced-activation ferritic/martensitic steels(RAFM).CLAM steel is a candidate material for the new generation of Chinese nuclear facilities.At medium and high temperatures,liquid LBE has a strong corrosive effect on the exposed steel materials,especially on the weld bead with the weakest corrosion resistance in LBE,which will shorten the service life of the facilities.Therefore,the corrosion behavior and corrosion mechanism of CLAM steel base metal and weld bead in liquid LBE were studied,providing guidance for the practical application of CLAM steel in lead-based reactors in the future,which has important theoretical significance and practical application value.CLAM steel base metal and weld bead with tungsten inert gas welding(TIG)were immersed in oxygen-saturated liquid LBE at 550℃in this paper.The morphology of corrosion surface was observed by scanning electron microscopy(SEM)and atomic force microscope(AFM),the phase of corrosion products was detected by x-ray diffraction(XRD),and the structure of oxide layer was characterized by SEM and energy-dispersive spectrometry(EDS).The main researches are as follows:(1)Research on the effect mechanism of corrosion time on the corrosion behavior of CLAM steel base metal and weld bead in liquid LBE.The corrosion tests were carried out in 2.98 m/s LBE for 500 h,1000 h and 1500 h.The results showed that an oxide layer with a two-layer structure formed on the surface of CLAM steel base metal and weld bead corrosion samples with different corrosion time.The outer oxide layer was composed of Fe3O4 with permeable Pb and Bi,and the inner oxide layer was composed of Fe-Cr spinel(Fe,Cr)3O4.With the increase of the corrosion time from 500 h to 1500h,the oxide layer thickness of CLAM steel base metal thickened from 46μm to 66μm,and the oxide layer thickness of CLAM steel weld bead thickened from 49μm to 73μm.The oxidation corrosion kinetic curves of the oxide layer thickness of CLAM steel base metal and weld bead were obtained.(2)Research on the effect mechanism of liquid LBE velocity on the corrosion behavior of CLAM steel base metal and weld bead in liquid LBE.The first corrosion test was performed in flowing LBE with different velocities for 500 h.The flow velocities were 0 m/s,1.70 m/s,2.98 m/s,3.69 m/s and 4.77 m/s,respectively.The results showed that the thickness of the oxide layer thickened firstly and then reduced with the increase of the LBE flow velocity.The mass transfer rate was gradually accelerated with the increased flow velocity(02.98 m/s).In the process,the dissolution rate of Fe element and the diffusion and migration rate of O element were both improved,which eventually led to the aggravation of the oxidation-corrosion degree on the surface of the corrosion samples.Besides,the degree of erosion-corrosion aggravated with the increase of the LBE flow velocity(2.984.77 m/s),and the oxide layer suffered a certain degree of spalling.Therefore,the thickness of the oxide layer gradually decreased with the increase of the LBE flow velocity.The second corrosion test was performed in flowing LBE with different velocities for 1500h.The flow velocities were 1.70 m/s,2.31 m/s and 2.98 m/s,respectively.The results showed that the degree of oxidation-corrosion and erosion-corrosion on the sample surface aggravated gradually with the increase of the LBE flow velocity(1.70 m/s2.98 m/s),and the oxide layer suffered a certain degree of spalling.The oxide layer thickness of CLAM steel base metal thickened from 57μm to 66μm,and the surface roughness decreased firstly and then increased.The oxide layer thickness of CLAM steel weld bead thickened from 65μm to 73μm,and the surface roughness decreased gradually.(3)Research on the effect mechanism of post-weld heat treatment process on TIG weld bead corrosion resistance in liquid LBE.The CLAM steel weld bead was heat treated with different processes(tempering 1 h at 760°C,tempering 2 h at 760°C,quenching and tempering).Quenching and tempering(QAT)process was heated at 980℃for 0.5 h quenching and at 760℃for 2 h tempering.The results showed that the size of lath martensite of the original weld bead was large,and the distribution was not uniform.After heat treatment,the lath martensite of the weld bead became fine and distributed evenly.The microstructure of weld bead after QAT treatment was the finest.The hardness of CLAM steel original weld bead was about 420 HV.After heat treatment,the hardness of CLAM steel weld bead decreased to 255 HV,255 HV and 230 HV,respectively.And then,the CLAM steel weld bead with post-weld heat treatment was immersed in LBE with 0 m/s and 2.98 m/s for 500 h,and with 1.70 m/s and 2.98 m/s for 1500 h,respectively.The results showed that the three kinds of heat treatment process could improve the corrosion resistance of CLAM steel weld bead in LBE.QAT treatment was the best method to enhance the corrosion resistance significantly.The uniform and fine microstructure was the main factor to enhance the corrosion resistance of CLAM steel weld bead in LBE.(4)Research on the effect mechanism of welding wire with different V contents on TIG weld bead corrosion resistance in liquid LBE.CLAM steel was welded with 9.8Cr2W,9.8Cr2W0.5V,9.8Cr2W0.8V,9.8Cr2W1.0V welding wires,respectively.The results showed that the hardness of the weld bead gradually increased from 309 HV to 414 HV with the increase of V content in the weld bead.When the content of vanadium(V)was 0.43 wt%,the hardness value of the weld bead fluctuated least,and the average hardness was 409 HV.The microstructure of weld bead with 0.43 wt%V content was the finest.The four kinds of CLAM steel weld bead with different V contents were exposed in LBE with 1.70 m/s and 2.98 m/s for 500 h.The results showed that the thickness of oxide layer on the surface of weld bead samples decreased firstly and then increased with the increase of V content in corrosion tests with different flow velocities of LBE.When the content of V was 0.43 wt%,the oxide layer thickness of weld bead samples was the thinnest,and the corrosion resistance in LBE was the best.