Preparation Of Nanostructured Austenitic Steel By ECAP Method And Its Thermal Stability

The fourth generation of nuclear reactors puts higher demands on the performance of fuel assembly materials,the most critical of which is high temperature strength and radiation resistance.The successful operation of the fast reactor depends to a large extent on the cladding tube in the fast reactor fuel assembly.The cladding tube must withstand the harsh working conditions within the stack and maintain the integrity of the fuel element.At present,many reactor cladding materials are austenitic stainless steels(such as 316 L,15-15Ti).Austenitic stainless steels have good high temperature creep,fatigue resistance and low temperature toughness,but their anti-irradiation swelling performance is poor.After investigation,the radiation resistance of austenitic stainless steel can be significantly improved by optimizing the grain size and grain boundary characteristics.Good anti-irradiation swelling properties depend on optimized grain size and grain boundary characteristics,but the general view is that nanostructures have poor thermal stability.In this paper,the microstructure evolution and mechanical properties of Fe-14Cr-16 Ni and 316 L nanostructured austenitic steels were studied by Equal Channel Angular Pressing(ECAP),and then the thermal stability of the microstructure was studied by subsequent heat treatment.Austenitic steel with nanostructures was prepared by six-time ECAP extrusion at room temperature.The average grain size of Fe-14Cr-16 Ni reached ~91 nm when extruded six times.The average grain size of 316 L reached ~61 nm after six passes.In the process of extrusion at room temperature,the solid solution austenitic stainless steel is deformed by pure shearing force,and dislocation refinement,twinning refinement and phase transformation refinement process are formed to form randomly oriented nanocrystals,and A large number of dislocations are introduced into the tissue,which absorbs point defects caused by irradiation and increases the absorption of fission product traps.The results of subsequent heat treatment experiments show that:(1)Fe-14Cr-16 Nican maintain the thermal stability of microstructure and good mechanical properties at500 ℃ after short-time aging(1h),and 316 L can maintain good thermal stability at700℃;(2)Long-term aging(24h)Fe-14Cr-16 Ni can maintain good thermal stability at500℃,316 L maintains good thermal stability at 600 ℃;(3)Nanostructure Austen in subsequent heat treatment The steel has undergone high-density dislocation movement and reorganization,sub-grain formation and growth and recrystallization.The paper determined the temperature limit of the nanostructured austenitic stainless steel to maintain thermal stability,and studied its microstructure evolution and mechanical properties.It provides a basis for the further application of austenitic steel as a cladding or outer casing material.This paper has reference significance for the research of materials for the fourth generation nuclear reactor fuel assemblies.