CLAM Steel Of High Temperature Creep Characteristics Research

Energy is an important material base for human survival and development.China’s sustained and rapid economic and social development is inseparable from the energy security.Fusion energy can be the most ideal energy for the future development of human beings.At present,the proven nuclear resources reserve can sustain human needs for more than 10,000 years.China low activation martensitic steel is one of the main candidates for the first wall material of future commercial fusion reactors,it has the advantages of lower radiation swelling rate,thermal expansion coefficient and high thermal conductivity,and has relatively mature industrial production technology at the same time.CLAM steel has developed very rapidly in the past ten years,but there are few data on long-term aging and creepThe microstructure of CLAM steel is controlled by quenching and tempering.The creep test of CLAM steel is carried out at the stress range of 90MPa~280MPa and the temperature range of 500℃~650℃.The creep behavior of CLAM steel is discussed by using creep data.The microstructural evolution of CLAM steel during creep is studied.The creep curve of CLAM steel is drawn,the creep deformation mechanism and fracture mechanism are discussed,and the creep life evaluation of CLAM steel is carried out by two methods.Detection by optical microscopy,scanning electron microscopy and transmission electron microscopy analysis of the microstructure variation of diffusion and distribution,CLAM steel alloy during creep martensitic laths and sub structure evolution and the evolution of the precipitates.In this paper,the creep behavior of CLAM steel is investigated.It is found that when the experimental temperature is constant,the decrease of stress will prolong the first stage of creep,and the accelerated creep stage will be about 1/10 of the total test time.Under the same stress condition,the steady-state creep rate increases by nearly three orders of magnitude with the increase of temperature at 550℃ to 600℃,indicating that the steel has a strong temperature sensitivity.The variation of stress exponent of CLAM steel at 550℃~650℃ is decreasing with the increase of temperature.The mechanism of creep deformation of CLAM steel at 650℃ is mainly the strengthening mechanism of second phase particles by combining theory.The creep life of CLAM steel is predicted by double logarithmic isothermal extrapolation method and Larson-Miller parameter method,and two prediction models are established.After comparing the two models,the calculated values of Larson-Miller model are in good agreement with the experimental values,and the prediction accuracy is higher.It is predicted that the creep fracture strength of CLAM steel is about 129 MPa at 550℃ for 100 thousand hours,and the creep rupture strength is about 65 MPa at 600℃ for 100 thousand hours.In the initial state and the creep fracture of CLAM steel samples with SEM and TEM to observe the microstructure of CLAM steel showed that after heat treatment the initial state of the grains are equiaxed,the size is about 7~12 m,the microstructure is martensite.A large number of precipitates in grain boundary,sub grain boundary,lath boundary and matrix,which are distributed in the grain boundary,sub grain boundary and lath boundary precipitates in large size,mainly for the M23C6 carbide precipitates;small size dispersed in the matrix,MX carbon nitride.At the same time,a small amount of insoluble particles in the solid solution process can be observed,and relatively large TaC particles are distributed in the matrix.After high temperature creep,the microstructure of deformation zone of CLAM steel is sub crystalline,and the response of subgrain structure will accelerate creep fracture.The response of the subgrain is mainly due to the strain induced recovery due to the stress,while the response of the subgrain is governed by the static recovery and the strain induced recovery at lower stress.The microstructure of high stress condition is relatively stable,without the phenomenon of abnormal growth;low coarsening should occur obvious stress conditions that precipitated phase and sub crystal,the creep stress will affect the recovery and growth of subgrains.MX carbon nitride is relatively stable during the high temperature creep test,but the coarsening is not obvious.M23C6 carbide coarsening and dislocation density decrease.The Laves phase is precipitated at 550℃ and 600℃,and the size of the Laves phase is small.The effect of CLAM is similar to M23C6,which plays a role in precipitation strengthening.By comparing the CLAM creep deformation microstructure and aging of the regional comparison found that the evolution rate of microstructure evolution rate and microstructure of CLAM steel during creep is significantly faster than the aging,showed that the stress will accelerate the microstructure evolution and promote the formation of Laves phase.