Effect Of Co Content On Microstructure And Mechanical Properties Of New Ferrite Heat-Resistant Steel(CB2)

Social and economic development makes people’s environmental awareness gradually increased.To reduce the pollution generated by thermal power plants,all countries are committed to the development of ultra-supercritical thermal power units with high steam parameters.Increasing the unit steam parameters inevitably places higher demands on the use of high temperature components.The new generation of ferrite heat resistant steel(CB2),which has high creep properties,high oxidation resistance,low coefficient of linear expansion and other advantages,has been a preferred materials used in ultra-supercritical of 620℃.In order to study the effect of Co content on microstructure and mechanical properties of CB2 steel,the contents of Co of CB2 steels were adjusted to 0.5%,1.0%,1.5%,2.0%and 3.0%.Respectively,the effects of different Co contents on microstructure and mechanical properties of CB2 steel were investigated by means of optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and secondary ion mass spectrometer(SIMS).The influence mechanism was also explained by combining the differential thermal analysis results of DSC,the simulated phase diagram of thermodynamic software Thermo-Calc and JMatPro-7.0 and the calculation results of strengthening mechanism.The following conclusions are as follows:(1)The microstructure of CB2 steel is mainly composed of martensite slab,second phase precipitated particles and high density dislocation.The type of precipitated particles in the microstructure consists mainly of grains distributed in grain boundaries and slab boundaries with a size of about 100-200nm M23C6 carbides and distributed in the grain with smaller size MX carbonitrides.(2)With the increase of Co content,the δ-ferrite content in the tested steels decreased continuously.When the Co content was 0.5%and 1.0%,the microstructure of the tested steels contained a small amount of δ-ferrite.When the Co content reached 1.5%After that,the 8ferrite was completely suppressed.With the increase of Co content,the size distribution of second phase precipitated particles in the tested steel was more concentrated.When the Co content was 1.5%,the average precipitated phase size was the smallest.(3)The tensile strength of the material increases with the increase of Co content,the reduction of area shrinkage first increases and then decreases,and the hardness value increases continuously.When the content of Co is 1.5%,the tensile strength reaches 805.13MPa,the reduction of area reaches 61.2%,the hardness is 260HB,and the original austenite grain size is small,and has excellent room temperature mechanical properties.(4)Mechanism of the effect of Co content on the microstructure and mechanical properties of CB2 steel.It is concluded that the addition of Co increases the phase area of austenite and balances the value of chromium equivalent in steel,resulting in a decrease ofδ-ferrite content in steel and an increase of the mechanical properties of CB2 steel.In addition,the calculation results of the strengthening mechanism show that the strengthening increment is the largest at 1.5%Co,which is in good agreement with the experimental results.