Multi-scale Microstructure Control And High Temperature Mechanical Properties Of CLAM Steel Strengthened By Thermomechanical Treatmen

China Low Activation Martensitic（CLAM）steel is a kind of Reduced ActivationFerritic/Martensitic（RAFM）steel developed in China.It is one of the preferred first wall structural materials in nuclear fusion experimental reactors at this stage.Thermal aging test is usually used to evaluate the microstructure stability of martensitic heat-resistant steel under service conditions.There have been some works investigating the thermal aging behavior of CLAM steel,but only up to 20000 h.Moreover,the research on quantitative statistics and evolution law of precipitates is very lacking.In this paper,CLAM steel was thermally aged for up to 30000 h at 550and 600°C,and the stability of three precipitates during thermal aging was quantitatively studied.MX phase is counted according to TEM images;M₂₃C₆ phase and Laves phase are counted according to SEM images.In order to identify and count a large number of M₂₃C₆ and Laves phase,this paper has innovated a method:firstly,the Laves phase was counted according to SEM-BSD images;secondly,the M₂₃C₆ and Laves phases are counted as a whole according to SEM-Inlens images;finally,combining the results of the previous two steps,the statistical results of the M₂₃C₆ phase are obtained.The results show that the particle size of the Laves phase increases linearly with time before reaching the precipitation equilibrium;and for the M₂₃C₆ phase or the MX phase,it increases linearly with the one-half power of time;the coarsening rates of the Laves phase are much greater than those of the M₂₃C₆ phase or the MX phase,and is 3.4 times that of the M₂₃C₆ phase and 19 times that of the MX phase（550°C,20000h）,and 4.7 times and 21 times（600°C,20000 h）,respectively.The precipitation kinetics of the Laves phase conforms to the Johnson-Mehl-Avrami equation when thermally aged at 600°C,and the time exponent is 2.8.The Laves phase has two nucleation modes:independent nucleation and dependent nucleation adhering to the M₂₃C₆ phase.There is no unified understanding of how the growths of the two phases in the assemblage affect each other after dependent nucleation.In this paper,an enwrapping growth mechanism of the Laves phase was observed,which provides a new clue to this question.The dependent nucleation and enwrapping growth mechanism of the Laves phase is the reason for the linear growth of its particle size and the large time exponent.An important way to improve the creep strength of martensitic heat resistant steels is to use thermomechanical treatment（TMT）technology.The research on thermomechanical treatment of CLAM steel is very scarce,and the effect of thermomechanical treatment on the creep property of CLAM steel has not been reported in the literatures.In order to improve the creep property of CLAM steel,the thermal simulation testing machine Gleeble 3500 was used for the research of thermomechanical treatment process.Firstly,four different intermediate temperature TMT processes were designed for comparison.It is found that the number density of MX phase after TMT increases by two orders of magnitude,and the average particle size decreases by about 79%when the austenitization temperature increases from 1000to 1150°C,while the effect of deformation temperature and strain is relatively small;after TMT,the specimen with the largest number and the smallest size of MX phase has the best creep performance,and vice versa.Subsequently,thermomechanical treatment at a relatively low deformation temperature（600°C）was carried out.The results show that the tempering temperature has a great influence on the precipitation of MX phase,and when the tempering temperature increases from 740 to 760°C,the number density of MX phase increases by 2 to 6 times;when the austenitizing temperature increases from 1150 to 1200°C,the precipitation of MX phase does not change significantly;the reduction of deformation temperature has no obvious effect on the precipitation of MX phase.After the superior process obtained through Gleeble experiment,the hot deformation is completed by forging,and the mechanical properties are tested after tempering.The tensile test results show that,compared with the normalized and tempered CLAM steel（N+T CLAM steel）,the yield strengths of thermomechanical treatment strengthened CLAM steel（TMT CLAM steel）at room temperature and high temperatures are increased by 31-55%,the tensile strengths are increased by 30-46%,and the total elongations are decreased by 12-21%.The impact test results show that the DBTT value of N+T CLAM steel is-65°C,and that of TMT CLAM steel is-27°C.The creep tests of N+T CLAM steel shows that the stress indexes at 500,550,600and 650°C are 16.5,15.0,10.2 and 7.8,respectively;the creep activation energy is 556k J/mol;the creep deformation mechanism of N+T CLAM steel under the experimental conditions is controlled by dislocation movement,and the creep is strengthened by the dispersed second-phase particles.The double logarithmic isotherm extrapolation method and the Larson-Miller parameter method were used to predict the 100,000 h creep rupture strength at four temperatures.Under the typical design life condition of the cladding module（550°C/100000 h）,the result predicted by double logarithm method is 133 MPa,and 120 MPa by the Larson-Miller parameter method,which are comparable to the results of Eurofer 97 and F82H.The creep tests of TMT CLAM steel show that,the fracture time of TMT CLAM steel is 54 times that of N+T CLAM steel under the same temperature of 550°C and stress of 230 MPa;the stress index at 550°C is 16.7;the creep activation energy is 578k J/mol;the creep deformation mechanism is also a dislocation movement control mechanism,and the strengthening effect of the second phase particles is more obvious than that of N+T CLAM steel.Under the typical design life condition of the cladding module（550°C/100000 h）,the creep rupture strength predicted by double logarithm method is 168MPa,and the result predicted by Larson Miller parameter method is176MPa,which is 26%and 47%higher than that of N+T CLAM steel,respectively.