Twist Extrusion Upsetting Of Low Activation Steel Precipitated Phase And Its Effect On Strong Plasticity

Reduced activation ferritic-martensitic（RAFM）steel is one of the candidate structural materials for fusion reactor cladding due to its high thermal conductivity,low coefficient of thermal expansion,good radiation resistance,excellent mechanical properties,and relatively mature industrial preparation foundation.In this paper,finite element numerical simulation as well as blocking varietal cross section twist-extrusion-upsetting under different temperature conditions are used to study the process.The influence of different process parameters,such as extrusion ratio,torsion angle,torsion channel height,different back pressures,on the deformation of low activation steel is studied with finite element simulation.By means of metallographic microscope（OM）,X-ray diffraction（XRD）and transmission electron microscope（TEM）,the microstructure and the evolution of the second phase of low activation steel under different deformation conditions are studied,and the evolution mechanism is analyzed and discussed in combination with theoretical knowledge.Finally,based on the changes of mechanical properties before and after deformation,the contribution of different strengthening mechanism to the yield strength of low activation steel is calculated.The results of finite element simulation show that the effects of different extrusion ratios and torsion angles on the cumulative strain of metal are obvious.The extrusion ratio,torsion channel height and torsion angle have significant effects on the forming load.The variation law of loading is consistent and is invariably divided into four stages:increasing in the initial stage,flat in the middle stage,sharp surge in the middle stage and flat in the tail end.The effects of extrusion ratio,torsion channel height,torsion angle and back pressure on the gradient strain distribution of the longitudinal section of the metal are great.On the whole,with the increase of extrusion ratio,torsion channel height,torsion angle and back pressure,the overall strain accumulates and the gradient strain distribution becomes more obvious.The microstructure results show that Cr₂₃C₆and Ta C are the main precipitated phases in the low activation martensite steel after twist-extrusion-upsetting deformation,and their contents tend to decrease with the increase of temperature.After heat treatment,the average diameters of M₂₃C₆and MX phases are 109nm and 22nm respectively,and the volume fractions are 2.51%and 0.48%respectively.The average diameters of M₂₃C₆phase at 600℃,670℃and 740℃are 55nm,60nm and 70nm respectively,and the corresponding volume fractions are 2.7%,2.69%and 2.0%.At 600℃,670℃and740℃,the average diameter of MX phase after deformation is 13.64nm,13.96nm and19.15nm respectively,and the corresponding volume fraction is 0.5%,0.3%and 0.32%.The results of hardness and tensile tests show that the mechanical properties of low activation steel can be significantly improved by twist-extrusion-upsetting deformation.The microhardness and yield strength of the specimens decreased with the increase of deformation temperature.After deformation at 600℃,the microhardness of the specimens reached 322HV and the yield strength was 966.4MPa,which were 46.36%and 82.34%higher than those of the specimens treated with heat treatment.In addition,through the discussion and analysis of the strengthening mechanism before and after deformation,the results show that the fine grain strengthening,precipitation strengthening and dislocation strengthening are the main strengthening mechanisms in the process of twist-extrusion-upsetting deformation of low activation steel.