Composition & Processing Design And Conventional Mechanical Properties Optimization Of Reduced Activation Ferritic/Martensitic Steels

Reduced activation ferritic/martensitic(RAFM)steels is the preferred structural material for Tokamak reactor in the future.Improving the mechanical properties of RAFM steels has been a long-standing challenge that inhibits the construction of fusion engineering test reactors.In this paper,the composition,thermo-mechanical treatment and heat treatment process of RAFM steels prototype were designed to control dislocation density,martensite block,precipitation M23C6 and MX.The purpose is to improve conventional mechanical properties of RAFM steels,and then guides the further development of RAFM steels through the analysis of theirs structure and properties.In order to improve the toughness of RAFM steels on the premise of guaranteeing strength,a new type of M-RAFM steel was proposed by optimizing matrix and precipitation through the coupling design of composition and process.To improve the composition,thermodynamic calculations were used to tailor the precipitation fraction.To optimize the thermomechanical treatment and heat treatment,a modified thermomechanical control process and intermediate heat treatment were employed to refine both the precipitation and martensite blocks.The experimental results for the microstructure and mechanical properties showed that,compared with that of CNA1 and EUROFER97,the newly designed RAFM steel had a more rational microstructure and greatly improved toughness with sufficient strength.Quantitative analysis of strengthening process using Olson strengthening model shows that M-RAFM still has much room for improvement in dislocation and precipitation strengthening.On the basis of M-RAFM,four kinds of RAFM steels are proposed with different compositions based on dislocation and precipitation,including RAFM#1 in low Ti system and RAFM#2,#3 and#4 in high Ti system.In composition design,Ti element was introduced to control precipitation by adjusting C,Ta and Ti contents.Process optimization was carried out by direct tempering after hot rolling.The experimental results showed that,the strength and toughness of RAFM#1(0.078Ta-0.022Ti)have been further improved based on M-RAFM,reaching the advanced level in the world.However,the impact toughness of RAFM steels with high Ti and Ta system has been significantly deteriorated due to the slender strip structure,which can be controlled by adjusting its composition or optimizing normalizing and tempering temperature parameters.Finally,the effect of thermomechanical treatment on conventional mechanical properties of RAFM steels has been studied.Three schemes of hot rolling tempering,austenite nonrecrystallization region rolling and two-phase region(?+?)rolling were designed,which were expressed by T750,R750 and R900 respectively.Their mechanical properties showed their respective advantages:T750 had better comprehensive mechanical properties,R750 steel plasticity was obviously improved,and R900 had excellent strength and hardness.Through comparative analysis of structure and properties of three thermomechanical treatment schemes,it is found that the isthermal time,rolling temperature and reduction amount are very important for the optimum design of RAFM steels.