Optimization Of Dimension And Heat Treatment Technology And Research On Reliability Of The Insulator Steel Pin

As an important connecting and bearing component of high voltage transmission line,disk suspension insulator steel pin should have high strength,toughness,plasticity and low-cycle fatigue resistant property.However,in severe in villainous weather such as raining,snowing and gale,fatigue fracture of the steel pin has occurred for many times,resulting in the accidents of wire dropping and string dropping.Undoubtedly,severe economic loss was brought about.Main reason is that the heat treatment process is not up to standard,the microstructure is not reasonable,or the external dimension of the steel pin is not optimal.Therefore,based on physical and chemical tests of the in-service steel pin,size optimization of the steel pin,research on the microstructure and mechanical properties of the steel pin materials under different heat treatment conditions and reliability evaluation of the steel pin are carried out for formulating the technical specifications for the quality evaluation and operation evaluation of the steel pin of high-voltage transmission line insulators used for quality evaluation of the steel pins of the different-grade transmission lines.It is of great significance to guide monitoring and management and further to ensure running safety of the transmission lines.In this dissertation,four kinds of typical domestic steel pin products,named A1,A2,B1 and B2,of different strength grades were employed to study their running states in bad weather via investigation of microstructure and mechanical properties of the steel pins,combining with finite element numerical simulation.Taking four kinds of commonly-used steel pin materials as the objects,their microstructural characteristics and mechanical properties under different heat treatments were discussed.Based on the low cycle fatigue resistant properties,the heat treatment processes of the steel pins were optimally chosen.Finally,the size of the steel pin ball joint fillet was optimized in terms of finite element numerical simulation.Main conclusions are drawn as following:The inclusions of sulfides and spherical oxides are axially distributed in these steel pins.The sulfide grade is high,and also the microstructures are unreasonable with coarse ferrite areas in all four kinds of steel pins.Moreover,the banded structure can be apparently observed in A2.The mechanical properties of A1,A2 and B1 are poor,especially the plasticity and impact toughness.The finite element simulation results show that large stress concentrations exist in the ball head neck of these four kinds of steel pins,and those of A1,A2 and B1 are close to or even exceeds the R_p0.2values of the corresponding metal materials,indicating an obvious potential safety hazard.The normalized Q235 steel has low strength and high toughness and plasticity.It has a good cycle stability under the 85%R_m stress.It maintains high strength and plasticity after 40000 cycles.The quenched and low-temperature tempered 45 steel has high strength and hardness,but low toughness and plasticity.Fracture occurs only after 600cycles under the 90%R_m stress.The quenched and medium temperature tempered 45steel has good comprehensive mechanical properties.After 5000 cycles,it can still keep high strength and plasticity.After quenching and high-temperature tempering,the 45steel has the best comprehensive mechanical properties and the low-cycle fatigue resistant property.The strengths of the quenched and low temperature tempered 20Mn2and 20Mn V steels are high,but the toughness and plasticity of them are low.Under 90%R_m stress,fracture of them occurs after about 3000 cycles.The results show that the microstructures of quenched and high temperature tempered 20Mn2 and 20Mn V steel are of good conditions,and the comprehensive mechanical properties of them are excellent.After 5000 cycles under the same stress level,the strength and plasticity of them still maintain a very high level.Therefore,the optimal heat treatment processes of normalizing(Q235 steel)and high-temperature tempering after quenching were fixed in order to improve the low-cycle fatigue resistant properties of the steel pins.According to finite element numerical simulation of the ball head fillet of the insulator steel pin,the optimal fillet radius is 7.1 mm,rather than 3.5 mm recommended in the current standard.At this time,the maximum stress of the steel pin ball head neck is reduced by 22.4%,and therefore the service reliability of the steel pin is significantly improved.