Creep Behavior Of Heat-resistant Steel Weldments By Vacuum Electron Beam For Ultra-supercritical Units

The improvement of parameters for ultra super-critical power generation units can not only improve power generation efficiency but also effectively solve the current hot issue of carbon emissions.At present,the commonly used steel types for high-temperature parts of domestic ultra super-critical power generation units are mostly imported from abroad,and further research is needed on the high-temperature performance of self-developed heat-resistant steel.In addition,in the actual service process,the high-temperature parts of the power generation units are often accompanied by creep,which leads to failure.Therefore,the research on the creep properties of welded joints of martensite heat resistant steel for the power generation units has become a hot spot.The research on the creep performance of heat-resistant steel welded joints is mostly focused on foreign developed steel,while the research on the creep performance of heat-resistant steel welded joints independently developed in China is quite insufficient.Furthermore,high-temperature uniaxial tensile testing of joint creep performance often only measures the overall creep fracture performance of the joint,and cannot obtain the creep performance of different micro-zones of the joint.Compared to the time-consuming and material-consuming of uniaxial tensile,nanoindentation creep testing only requires a small amount of material and time.Moreover,due to the high resolution of load displacement of the nanoindentation instrument,studying the creep properties of different micro-zones of the joint has strong pertinence.Therefore,in response to the above issues,this article conducts a study on the creep behavior of Co3W2 heat-resistant steel welded joints independently developed in China through a combination of high-temperature creep endurance tensile test and nanoindentation.The main work and conclusions are as follows:（1）The creep behavior of Co3W2 steel weldments under high temperature stress was studied by using high temperature creep rupture test.The fracture mode and fracture mechanism of the joint were studied by analyzing the results of the creep test and observing the microstructure of the joint after creep.The results show that:the high temperature creep fracture mode of welded joint is mainly plastic base metal fracture under high stress and brittle mode IV fracture at low stress level.The main reason for type IV cracking is that a large number of creep voids in the fine-grained heat-affected zone nucleate,grow and aggregate to form microcracks during the creep process,which eventually leads to failure.M₂₃C₆ and Laves phases provide favorable conditions for the nucleation of creep voids.（2）Feedback from the high-temperature creep fracture test using nanoindentation to measure the room temperature creep properties of different micro zones of the joint revealed that the creep strain rate sensitivity index m at the weld metal of the welded joint is the smallest,and the creep resistance performance is the best.The m of the heat-affected zone and the base metal is higher than that of the weld metal,which indicates that the heat-affected zone and the base metal are most prone to creep failure in the creep process,and the results are consistent with the high-temperature creep fracture test results.（3）The analysis and life prediction of the data obtained from the high temperature creep rupture test show that the creep fracture life strongly depends on the creep rate,and the modified Monkman-Grant equation is more suitable for the creep data in this study.Creep damage tolerance factorλ_DP value indicates that creep damage is caused by voids growth.Based on the Larson-Miller Parameter method,extrapolate the critical stress of the joint is between 195 and250 MPa.At the same time,based on the creep fracture data,the allowable stress of the 100000hour life of the welded joint at 650 and 625℃meets the industrial use requirements.（4）Based on the creep property of micro-zone,the different micro-zones of the joint are optimized by heat treatment.The optimization results indicate that compared with PWHT（Tempering）,PWNT（Normalizing）homogenizes the microstructure and hardness of different micro-zones of the joint,and the creep resistance of the micro-zones of the joint is also better optimized,and the strain rate sensitivity index tends to the same level.