| Ferritic/martensitic steel(F/M)is considered to be the main candidate structural material for lead-cooled fast reactor fuel cladding due to its radiation resistance,corrosion resistance and excellent mechanical properties.However,existing F/M steel still suffers from low surface hardness and poor wear resistance,such as significant corrosion and wear after prolonged exposure in flowing lead-bismuth eutectic(LBE)coolant.In order to improve the surface properties of F/M steel,laser remelting and laser cladding FeCrAl composite coating were carried out on the surface of typical F/M steel.Microstructure,hardness and wear resistance of the specimens after laser treatment were characterized by conbined ues of electron backscatter diffraction(EBSD),electron channeling contrast(ECC),energy dispersive spectrometry(EDS),secondary electronic imaging technique,Vickers Indenter and friction and wear tester.The conclusions are as follows:(1)There exist two modification zones with distinct microstructural characteristics in the F/M steel after 200W and 100W laser remelt:the remelting zone(RZ)consisting of both martensitic plates and bulk?ferrite grains.The heated affected zone(HAZ)mainly composed of martensitic plates.Both K-S and N-W orientation relationships are followed during martensitic transformation in both the zones,resulting in specific misorientation characteristics and scattered orientations.After 200W and 100W laser treatment,the hardness of RZ and HAZ surface of F/M steel is obviously higher than that of substrate.hardnesses of the RZ and HAZ of100 W specimen was 385±17 HV and 408±61 HV,respectively.The hardness of RZ and HAZ surface of 200 W specimen was 364±7 HV and 442±44 HV,respectively,significantly higher than that of matrix(267±3 HV).The increased hardness in the RZ is related to the martensitic transformation-induced grain refinement and the back stress associated with microstructural heterogeneity,while the grain refinement and high-density LABs jointly lead to the high hardness of the HAZ.The wear mechanism of both specimens is mainly abrasive wear accompanied by oxidation wear.The wear rates of 100W and 200W samples are 1.7×10-4mm3N-1m-1and 2.6×10-4mm3N-1m-1,respectively,which are much lower than that of F/M steel matrix(3.3×10-4mm3N-1m-1).It shows that the wear resistance of F/M steel is improved obviously by laser remelting.(2)Three kinds of FeCrAl oxide coatings with good metallurgical bonding on F/M steel surface have been successfully prepared by laser cladding technology,namely,FeCrAl/Al2O3,FeCrAl/Y2O3and FeCrAl/Al2O3-Y2O3coatings with thickness of about 130?m,110?m and 130?m,respectively.All produced two modification zones with distinct microstructural characteristics:cladding zone(CZ)consisting of columnar crystals(ferrite),in which a large number of second-phase particles are dispersed.The heated affected zone(HAZ)mainly composed of martensitic plates,both K-S and N-W orientation relationships are followed during martensitic transformation in both the zones.The hardness of FeCrAl/Al2O3coating is estimated about 367±9 HV,the wear rate is 2.5×10-4mm3·N-1·m-1.The main wear mechanism is adhesive wear,accompanied by oxidation wear and abrasive wear.The hardness of FeCrAl/Y2O3coating is estimated about 381±8 HV,the wear rate is 1.1×10-4mm3·N-1·m-1.The main wear mechanism is abrasive wear,accompanied by oxidation wear.The hardness of FeCrAl/Al2O3-Y2O3coating is estimated about 326±15 HV,the wear rate is 1.4×10-4mm3·N-1·m-1.The main wear mechanism is abrasive wear and oxidation wear.(3)Two kinds of FeCrAl carbide coatings(FeCrAl/Si C and FeCrAl/Ti C)have been successfully prepared on F/M steel by pulsed laser cladding,the coating thickness is about 120?m and 140?m,respectively.The CZ in the FeCrAl/Ti C specimen composed of fine equiaxed grains.The dendritic second phase is distributed along the grain boundary,and the granular second phase is also distributed inside the grain.The CZ in the FeCrAl/Si C specimen consisting of columnar ferrite,and a large number of reticular and granular second phases are uniformly distributed in the grains.The hardness of FeCrAl/Ti C coating is estimated about 349±21 HV,the wear rate is 1.1×10-4mm3·N-1·m-1.The main wear mechanism is abrasive wear and oxidation wear.The hardness of FeCrAl/Si C coating is estimated about 473±38 HV,the wear rate is 1.2×10-5mm3·N-1·m-1.The main wear mechanism is abrasive wear.The final results show that the microstructure of F/M steel after 100W and 200W laser remelt are very similar,and the hardness and wear resistance of 100W specimen are significantly higher than those of 200W specimen.Five coatings are dominated by ferrite phase,and various intermetallic compounds are formed.The hardness of FeCrAl/Si C coating is the highest,about 470 HV,and its wear rate is the lowest,nearly one order of magnitude lower than that of F/M steel substrate.It can be seen that high-performance metal-ceramic composite coatings were successfully prepared on the surface of F/M steel by selecting appropriate coating powder and supplemented by reasonable processing parameters. |
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