Alloy Design Of High Co-Ni Secondary Hardening Steel By Multi-scale Simulation

In this dissertation,the mechnasm of strength,toughness and hydrogen embrittlement resistance ability in high Co-Ni secondary hardening steel was studied.Several different multi-scale simulation models were established.Based on the design and analysis of the model,the design process and standard of high Co-Ni secondary hardening steel was established.New high Co-Ni secondary hardening steel was developed based on the design process and standard.First,a multi-scale model,which was used to simulate yield strength,was established by combining thermodynamic and strength theory.Both thermodynamic simulation and transmission electron microscope?TEM?observation were used to analyze the microstructure information,including element distribution,size and volume fraction of precipitation,et al.Then,the microstructure information was imported into the strength theory,which included several different dislocation theories.The main factors,which affect the yield strength of high Co-Ni secondary hardening steel,were discussed.Second,a multi-scale model,which was used to simulate fracture toughness,was also established by combining Olson-Cohen model,thermodynamic theory and finite element method?FEM?.Combining fracture toughness test,scanning transmission electron microscope?STEM?observation and multi-scale simulation,the main factors,which affect the fracture toughness(K_IC)of high Co-Ni secondary hardening steel,were discussed.Third,a multi-scale model,which was used to simulate hydrogen embrittlement sensibility,was established by combining first principle theory,multi physics field FEM and fracture theory.First principle method was used to simulate the effect of alloy element or hydrogen on the bonding strength of bonding strength.Multiphysics field FEM was used to simulate the stress-induced hydrogen segregation at the crack tip.Both FEM and fracture theory was used to simulate the value of hydrogen embrittlement sensibility(K_ISCC).Based on both simulation and experiment,the main factors,which affect the hydrogen embrittlement sensibility of high Co-Ni secondary hardening steel,were discussed.Finally,based on experiment and simulation,the design process and standard of high Co-Ni secondary hardening steel was established.New high Co-Ni secondary hardening steel was designed based on the design process and standard.Also,based on analysis of bonding strength and therm odynamic characteristics,the formation of core-shell MC carbides at the grain boundary was designed and controlled,which could lead the further development of high Co-Ni secondary hardening steel.