| With the continuous development of nuclear power technology,liquid lead or lead-bismuth alloy is used as refrigerant in the lead-cooled fast reactor,which has unique advantages in inherent safety,recycling ability of renewable energy,discharging lower-radioactivity waste,reactor system and refueling time,etc is attracting widespread attention of the society.Current materials can not meet the requirements of structural materials for lead-cooled fast reactor.In order to improve the corrosion and radiation resistance of steel,1.5% Si is added to T91 steel.The addition of Si makes T91 steel have the ability to form a compact passivation film in lead-bismuth and to improve its lead-bismuth corrosion resistance.However,9-12%Cr martensitic heat-resistant steel have a tendency of producing δ-ferrite.The addition of Si increase the tendency of producing ferrite.When we produce large-size T91 steel ingots appear a problem of the content of δ-ferrite is exceeding Standard.The presence of δ-ferrite usually reduces the impact toughness,corrosion resistance,and persistent strength of steel,so we should control the presence of ferrite in steel.δ-Ferrite content.The main contents and results are as follows:(1)Study on Reasons why the content of δ-ferrite in large-size T91 steel ingots excessive is exceeding Standard.Prediction the content of δ-Ferrite content in T91 Steel by Schafler Diagram.Then the equilibrium and non-equilibrium phase diagrams of high silicon martensitic heat-resistant steel are calculated with Thermo-calc software.According to the calculation results,the slag remelting process of steel is simulated by MELTFLOW-ESR software.The study provides theoretical basis for solving the problem of the content of δ-ferrite in large-size T91 steel ingot is exceeding Standard.(2)Verify the accuracy of the simulation results by experiment.The process of δ-ferrite formation is directly observed.The electroslag ingot in T91 steel produced in industry is discussed and explained by DSC test,high temperature in-situ test and theoretical calculation results.The test results show that the component segregation occurred during solidification and the uneven cooling rate of the ingot cause the content of δ-ferrite in large-size T91 steel ingot is exceeding Standard.(3)Design test to eliminate T91 steel δ-Ferrite according to the reason why the content of δ-ferrite in large-size T91 steel ingot is exceeding Standard.The results show that heating the sample to 1220℃ for 48 hours,then cooling to 1100℃ for 6hours and then hot forging can obtain the large blocks of δ-ferrite in material breaks into small pieces,further promoting δ-Ferrite is converted to austenite by this process,which can make content of δ-Ferrite in the T91 steel ingot could be controlled below1%.(4)Inspection to eliminate the influence of process on mechanical properties of material.The mechanical properties and structure of the material before and after ABSTRACT eliminating the process were designed and compared.The results show that after eliminating the process,the impact absorption energy of the sample is increased from27.5J to 131 J,the tensile strength is increased from 731.5MPa to 790.5MPa,the yield strength is increased from 538.5MPa to 603 MPa,the impact fracture of the sample is changed from dissociation type to dimple type,and the brittle fracture to ductile fracture.In conclusion,mechanical properties,especially impact properties of T91 steel after eliminating process are significantly improved.Because all of 9-12% Cr martensitic heat-resistant steels are produced δ-The tendency to ferrite,while δ-Ferrite can affect the properties of 9-12% Cr martensitic heat-resistant steel,Relevant research results can be the key factor to solve the bottleneck of large-scale development of T91 steel electroslag ingot,so the results of this study can be used for reference in the industrial production of all 9-12% Cr martensitic heat-resistant steel. |
