Study On BN Precipitation And Dissolution Behavior In FB2 Rotor Steel

With the proposal of the national double-carbon target and the continuous advancement of the sustainable development strategy,energy saving and consumption reduction and coal consumption upgrading are the broad consensus of the country and all walks of life at present.It is an inevitable trend,and the material of the key high-temperature components of ultra-supercritical units is the key to restricting its development.FB2 rotor steel has excellent high temperature performance,oxidation resistance and creep fatigue resistance,etc.Among them,B and N elements play an important role in improving its creep strength,but B and N elements are easy to form clusters of BN,is unevenly distributed and difficult to control.For this reason,this paper uses thermal deformation and heat treatment to study the precipitation and remelting behavior of BN,which provides data support for the composition design and preparation process of FB2 rotor steel,and also provides technical support for the realization of the double carbon target.In this paper,by carrying out thermal compression and heat treatment tests,with the help of optical microscope,scanning electron microscope,thermal simulation testing machine and tensile testing machine,etc.,the influence of different thermal deformation and heat treatment processes on the precipitation and back-dissolving behavior of BN in FB2 rotor steel is studied.The morphology,size and distribution of BN under different thermal deformation and heat treatment conditions were analyzed,as well as the re-precipitation behavior of MX phase and M₃B₂ phase after BN re-dissolution.The main research results of this paper are as follows:（1）The changes in deformation parameters have no significant effect on the BN recovery;The deformation temperature increases,the average number of BNs increases,and the area percentage and the average size becomes smaller;The strain rate increases,the average number,area percentage,and average roundness of the BN increase,and the average roundness increased,The average size of BN is unchanged;The deformation volume increases,the average number,average size and area percentage of BN decreased first,and the average circularity increases;Thermal deformation will not promote BN precipitation to precipitate BN precipitation.（2）When solid solution at 1060°C for 4 hours,BN did not dissolve back;When solid solution at 1100°C for 4 hours,part of BN dissolved back;When solid solution at 1140°C and above for 4 hours,BN was completely dissolved;When solid solution at 1160°C for0.5h,BN did not return;When solid solution is at 1160°C for 1 hour,BN begins to dissolve back;When solid solution is at 1160°C for 2 hours or more,BN completely dissolves back;The boundary conditions for BN back-dissolution in FB2 steel are:solid solution temperature≥1160°C,holding time≥2h.Under the three cooling methods,after furnace cooling and air cooling,BN did not re-dissolve;After oil cooling,part of BN has re-dissolved;With the increase of solid solution temperature,prolongation of holding time and increase of cooling rate,the average amount of BN,average size,area percentage,and average roundness gradually decrease.（3）The increase of the isothermal temperature and the extension of the isothermal time will not lead to secondary precipitation of BN,but the size of the MX phase gradually increases,and the shape changes from irregular to nearly round.The MX phase and M₃B₂phase are distributed in the BN cycle near the holes left after melting;After FB2 was tempered at 630°C for 5 hours and tempered at 720°C for 5 hours,BN did not precipitate again.