Study On The Mechanism Of Si In SIMP Steel For Nuclear Reactor

The accelerator driven sub-critical system(ADS)with high system safety has a strong ability to transform long-lived radioactive nuclear waste.Currently,it is thought to be the most promising method to treat the nuclear waste in the world.Reduced activation ferritic/martentic(RAFM)steels,with high thermal conductivity,small expansion coefficient,high temperature resistance and irradiation properties,are the candidate material for ADS cladding and spalling targets.Moreover,these components in ADS are in service for a long time under high temperature,strong corrosive medium and neutron radiation conditions.Therefore,degradation of microstructure and mechanical properties,high temperature corrosion and oxidation of RAFM steels for ADS components are important scientific issues to be investigated systemically.In the present study,the self-developed SIMP steel with excellent comprehensive service performance act as the research object,and the influence of silicon on the microstructure,mechanical properties,high temperature oxidation behavior and liquid Lead-Bismuth Eutectic corrosion resistance of SIMP steels under simulated service conditions were studied.The main results obtained are as follows:Three kinds of experimental steels,with different Si content of<0.03?0.58 and 1.22 wt.%(named:0Si,06Si and 12Si steel,respectively)were used in this thesis.After the normalizing and tempering treatment,these steels present the typical martensitic lath microstructure.The martensitic lath microstructure in these three kinds of experimental steels was gradually recovered and broken into subgrains after the long-term aging at 550? and 600?.Width of martensite laths can be effectively inhibited by increasing of silicon content for three kinds of steels.In addition,M23C6 and MX carbides formed in experimental steels show better structural and morphological stablility during the long-term aging.For Fe2W Laves phase,it was precipitated and grown on martensitic lath boundaries and the prior austenite grain boundaries after the incubation period.Comparing with the normalizing samples,decreasing of the room-temperature tension strength was less than 3%for 12Si steel aged for 10000 h.It is closely related to maintaining of the martensitic lath structure and morphological stability of M23C6.However,the typical martensitic lath microstructure was completely transformed into the coarse equiaxed microstructure during the aging at 650?.Meanwhile,M23C6 and MX carbides coarsened obviously.The addition of Si to SIMP steel can reduce the formation energy for Fe2W Laves phase and promote the precipitation and growth of Fe2W Laves phase.Morever,Si involved in the formation of Fe2 W Laves phase by substituting Fe element in Laves phase to form the(Fe,Si)2W ternary Laves phase.Moreover,the impact properties of SIMP steel would decrease during the(Fe,Si)2W Laves phase precipitating and coarsening.But it still meets the application requirements of engineering materials.Moreover,subsequent heat-treatment is helpful for the dissolution of(Fe,Si)2W Laves phase in SIMP steel and increase of the impact toughness.The oxidation behavior of three kinds of experimental steels with different Si content was studied in air at 700?.The results showed that the oxidation resistance of SIMP steel can be improved with the increase of silicon content.Oxidation layer growth of 12Si steel was mainly dominated by the inward diffusion of Cr ion,forming the Cr2O3 and Mn1.5Cr1.5O4 layered oxide films.While growth of the oxidation layer in 0Si and 06Si steels were dominated by the outward diffusion of Cr ion at the initial stage,and then dominated by the inward diffusion of Fe ion.This resulted in the formation of Fe2O3 outer layer and Fe-Cr spinel inner layer in 0Si and 06Si steels.Alloying element silicon solute in original experimental steel was gradually transformed into amorphous SiO2 during the long-term oxidation.For 06Si steel,SiO2 particles were dispersed at the Fe matrix/the oxide layer interface,and locally continuous bamboo-shaped SiO2 films were also observed in the Fe-Cr spinel layer.Continuous SiO2 layer of,with a thickness of?50 nm,was distributed on the interface between Fe matrix and the oxide layer in 12Si steel.This continuous SiO2 layer can effectively hinder the outward diffusion of Fe ion,leading to the excellent oxidation resistance of 12Si steel.The oxidation resistance of experimental steels with different silicon contents in the static oxygen liquid lead bismuth eutectic(LBE)at 600? has been studied.Oxidation layers on experimental steels from the surface to the inside are shown as follows:PbFe12O19(Outmost oxide layer),Fe3O4(outer oxide layer),Fe-Cr spinel inner layer and IOZ,and IOZ can be observed only in silicon-containing experimental steels.Alloying element silicon was mainly segregated at the prior austenite grain boundaries and martensite blocks boundaries in the IOZ and the internal oxide layer,forming the SiO2 band structure surrounded by continuous Cr2O3.In addition,this band structure can effectively hinder the outward diffusion of Fe ion and inward diffusion of O ion,leading to the improvement of oxidation resistance of SIMP steel in liquid LBE.The addition of 1.22 wt.%Si to experimental steel can further reduce the oxidation rate in LBE,showing the excellent resistance to liquid LBE corrosion.