Low-cycle Fatigue And Fracture Properties Of FB2 Steel Used For 620℃ Advanced Ultra-supercritical Turbine Rotor

This paper,funded by the science and technology project of Huaneng Power International,INC.named “Research on operation and optimization of materials used for 620℃ advanced ultra-supercritical（USC）unit”,mainly focused on basic mechanical properties,low-cycle fatigue property and fracture property of FB2 steel applied to 620℃ turbine rotor.This paper intends to provide technologic basis for life assessment and safety evaluation of 620℃ USC turbine totor.In this paper,experimental research on basic mechanical properties of FB2 rotor steel,including tensile properties and impact properties at room temperature（RT）and 620℃,ductile-brittle transition temperature,hardness,microstructure,effect of aging on basic properties,low-cycle fatigue property and fracture property is carried out.The main conclusions are as follows: Rp0.2 at RT exceed 700 MPa,while tensile properties in different positions of FB2 are nearly the same.Strength decreases with increasing temperature,however Rp0.2 remains at relatively high level at 620℃.The impact absorbed energy（KV2）of FB2 at RT and 620℃ are approximately 20 J and 100 J,respectively,while the fracture appearance trasition temperature(FATT₅₀)is approximately 70℃,which indicates FB2 is brittle at room temperature.The tensile property,impact property and hardness of FB2 are stable during aging,which exhibit insignificant declining tendency.FB2 shows cyclic softening under low-cycle fatigue condition at RT and elevated temperature.Cyclic strain-life relationship of FB2 can be described by Manson-Coffin formula.At a certain temperature,the stable plastic strain energy（ΔW_p）presents a power function relationship with the cycle（Nf）.ΔW_p×Nf0.5 can be used as a measurement of fatigue toughness.Fracture toughness（KIC）of FB2 at RT and 620℃ are 80.4MPa·m1/2 and 123.6MPa·m1/2,respectively.Microstructure of FB2 is fully tempered martensitic structure with a 2-grade grain size.Dispersive precipition and high density dislocations contribute to high strengh of FB2.Large BN inclusions are discovered because of the addition of boron.In this paper,cyclic strain-life curve of FB2 at RT,550℃ and 620℃ are obtained,which can be used to estimate the fatigue life.Fracture toughness of FB2 at RT,550℃ and 620℃ are obtained,which can be used to assess the safety of defects.Quantitative relationship between the plastic strain energy and cycles at RT and elevated temperature are obtained,which can be used to check the fatigue life.