| At present,the ultimate temperature of martensitic heat-resistant steel P92 for the most advanced commercial ultra supercritical power plant is 628 ?.In order to further promote the construction of advanced ultra supercritical power plant,improve the power generation efficiency and reduce coal consumption,a new martensitic heat resistant steel G115 has been independently developed by CISRI,making its limit service temperature can break through to 650 ?,which is of great engineering significance.Compared with the existing 9-12% Cr martensitic steels,G115 steel increases the precipitation enhancement by adding 1% of Cu,and obtains a greater increase in the creep and rupture strength.However,due to the problem of complicated precipitates in G115 steel and preferential corrosion of Cu in sample preparation,the existing form,distribution characteristics and mechanism of Cu in steel is not clear,hindering the performance improvement and revealing strengthening mechanism.Therefore,it is urgent to develop a new analytical method for the characterization of Cu in G115 steel and to study the correlation of composition-structure-properties of Cu in G115 steel.The main contents of this paper are as follows:1.The multi-scale screening characterization of Cu in G115 steelTaking aging G115 steel as the research object,this paper proposes a multi-scale analysis method to select feature regions and feature units from macro to micro level.This method is applied to the screening and characterization of Cu.Based on the electron diffraction and energy spectrum analysis of Cu bearing feature units,it is firstly found that the Cu-rich phase with the FCC structure has been formed in the G115 steel,and its Cu content is more than 90%.Based on the statistical analysis of multi field of view of Cu bearing areas,the distribution characteristics of Cu-rich phase are revealed: Cu-rich phase is often symbiosis with M23C6 and Laves phase,also can be independently distributed in the interface,and often around thedistribution of a large number of dislocations;Cu-rich phase is spherical or oval,with an average diameter of 114 nm.2.Study on the precipitation behavior of Cu in G115 steel and the correlation of its composition-structure-properties.A series of G115 steels with Cu content of 0.5%,1%,2% and 3% have been designed.Combined with Thermo-Calc thermodynamic theory calculation,the correlation of composition-structure-properties of Cu in G115 steels has been studied through the characterization of structure and properties of tested steels with different Cu contents under different processes.The main research content of this part can be divided into the following three aspects:(1)Using Thermo-Calc thermodynamic simulation software,we calculate the equilibrium phase diagram of G115 steels with different Cu contents,and study the effect of Cu content on phase composition,content and precipitation temperature interval under thermodynamic equilibrium.The results indicate that the main precipitates in G115 steels with different Cu contents are M23C6,Laves phase,MX and Cu-rich phase.With the increase of Cu content,the transition temperature of? ? ? in G115 steels gradually decreases,and the transition temperature range gradually expands.The precipitation of Laves phase,M23C6 and MX have little change,and the solution temperature of each precipitate increases.In addition,the precipitation of Cu-rich phase increases linearly.(2)Based on the correlation between material hardness and metallographic structure under different temperatures,a method of Ac1 test has been developed under the condition of the actual heat treatment;the Ac1 of G115 steels with different Cu contents under actual process is determined;the effects of heat treatment conditions on the structure and properties are studied;according to the Ac1 test results,the heat treatment system is optimized: 1100?×1h A.C.+770?×3h A.C.In order to further explore the precipitation behavior of Cu in G115 steel,the precipitation evolution trend of Cu at different aging time and Cu contents of G115 steels has been systematically studied by multi-scale distribution analysis method.The results show that as the aging time prolongs,the content of Cu in the matrix of G115 steel decreases gradually,and Cu precipitates in the form of Cu-rich phase gradually grow up in the aging process.When the content of Cu in tempered G115 steel is 0.50%,few Cu-rich phase is precipitated.With the increase of Cu content to1.04% and above,the Cu-rich phase begins to precipitate and the amount of precipitation increases.After aging at 650 ? for 1000 h,the Cu-rich phase precipitated in different G115 steels with 0.5%~2.83% Cu contents and its amount increases with the content of Cu.Therefore,the increase of Cu content and aging process in G115 steels all promote the precipitation of the Cu-rich phase.In addition,we focus on the correlation of composition-struture-properities of G115 steels with different Cu contents under different processing conditions: with the increase of Cu content,the austenite grain size of tested steels decreases,the undissolved particles after solution treatment increased gradually,the precipitation of Laves phase are promoted;material hardness and high temperature strength during aging process increase slowly,while high temperature plastic drop.Therefore,the content of Cu in G115 steels should be reasonably controlled to ensure that the material has good high temperature and strong plasticity.In summary,this paper presents a method of multi-scale distribution analysis that can select the feature regions and units step by step.After the characterization of Cu in G115 steel based on this method,it is firstly found that G115 steel is able to form Cu-rich phase with he FCC structure,which reveals the distribution characteristics and the precipitation behavior of Cu.In addition,this paper studies the correlation of composition-structure-properties of Cu in G115 steels.This not only accumulates key multi-scale experimental data for the improvement and optimization of G115 steel,but also provides an effective way for the study of microstructure evolution and functional mechanism of other heat-resistant steels. |
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