Effect Of Precipitation Behavior During Tempering Of M₂₃C₆ Phase On Properties Of CB2 Steel

Based on the requirements of energy saving and consumption reduction in economic development,the development of ultra-supercritical thermal power generators with high steam parameters is an important direction to improve the efficiency of thermal power plants.The improvement of the steam parameters of the generator set is essentially the improvement of the material properties of the high-temperature components of the unit.For CB2 steel with excellent high-temperature creep performance,high oxidation resistance and low expansion coefficient,it is undoubtedly a super-supercritical at 620?.The preferred steel for thermal power plants.For CB2 steel,precipitation strengthening provided by the M23C6 phase precipitated during tempering is the most effective strengthening route.In this paper,the effect of tempering process on the morphology,size,quantity,type and distribution of M23C6 phase in CB2 steel and the precipitation behavior of M23C6 phase on the mechanical properties of CB2 steel were studied in laboratory smelting as-cast CB2 steel.For the experimental steel after normalizing,different tempering temperatures of 690?,710?,730?,750?,770? and 790? were compared with different tempering times of 2min,10min,30min,1h,2h and 3h.he precipitation behavior of M23C6 phase in different tempering processes was observed by means of optical microscopy,scanning electron microscopy and transmission electron microscopy.The mechanism of the influence of tempering system on the precipitation behavior of M23C6 phase was clarified by Thermo-Calc,combined with hardness and room temperature stretching.The performance test was conducted to study the influence of the precipitation behavior of M23C6 phase on the performance of CB2 steel,and determine the optimum tempering temperature.In the end,I got the following conclusion:(1)After tempering,the matrix structure of CB2 steel is lath-like tempered martensite;with the increase of tempering temperature,the hardness and strength of CB2 steel gradually decrease,and the elongation and section shrinkage rate gradually increase;With the increase of tempering time,the hardness of CB2 steel decreases overall.After tempering at 730?×30min,the comprehensive performance of CB2 steel is better:Brinell hardness is 230,yield strength reaches 645MPa,tensile strength reaches 796MPa,The length ratio was 16.4%,and the reduction in area was 57.8%.(2)When the tempering temperature is lower than 730?,the M23C6 phase uniformly precipitates in the grain boundary and the inside of the grain into nearly spherical particles with an average size of about 130?150 nm;when the tempering temperature is higher than 730?,the grain boundary The growth rate of some M23C6 phase is larger than that of M23C6 phase inside the grain,and the difference of size is about 150nm.When the tempering temperature reaches 790?,the growth rate of M23C6 phase at grain boundary is further accelerated,and the carbon content in the matrix is mostly large.The form of the M23C6 phase precipitates at the grain boundary,weakens the grain boundary,and becomes a fracture source of the CB2 steel during stretching,which adversely affects its performance.(3)When the tempering time is less than 30 min,the M23C6 phase precipitates nearly spherically at the grain boundary,while the M3C phase in the matrix is not completely dissolved.When tempered for 30 min,the M3C phase is completely dissolved,and the carbon element is redistributed in the form of M23C6 phase.Precipitated,the dispersion of particles with a size of about 150?200nm is distributed in the grain boundary and the matrix;when the tempering time is more than 30min,the M23C6 phase at the grain boundary continues to grow to a certain extent and tends to be stable(300?400nm).),the average size of the M23C6 phase(150?200nm)in the grain appears to be about 160nm,which weakens the grain boundary to a certain extent and reduces the strength of the CB2 steel.