Microstructure Evolution Of Stainless Steel During Thermal Process And Molecular Dynamics Simulation Of Chromium Nitride Precipitation

As the cladding material of nuclear power plant,austenitic stainless steel is irradiated,which leads to the damage of the structure;At the same time,the second phase of cellular chromium nitride precipitates during hot working of high nitrogen austenitic stainless steel,which affects the properties of steel.Two kinds of problems are structural evolution problems during steel heat process.In this thesis,LAMMPS molecular dynamics method is used to study the microstructure evolution of austenitic stainless steel under irradiation and chromium nitride precipitation during hot working.The main work is as follows:Aiming at the phenomenon of grain boundary segregation induced by different irradiation methods,taking the Fe Cr Ni system as the research object,by changing the simulation environment temperature and the system point defect concentration,four simulation models of laser irradiation,electron irradiation,double beam simultaneous irradiation and first laser followed by electron irradiation in turn were constructed respectively.Under electron irradiation,the degree of element segregation is the most obvious,followed by sequential irradiation,followed by simultaneous irradiation,while under laser irradiation,the degree of element segregation is the weakest.It is found that the existence of vacancy defects is more conducive to the diffusion of Cr atoms,and the existence of interstitial defects is more conducive to the diffusion of Ni atoms.In view of the phenomenon of austenite grain boundary migration caused by irradiation,lattice grain boundary model with overlapping position and random grain boundary migration model were constructed respectively.The influence of alloy element concentration distribution on grain boundary migration in Fe Cr Ni system with uniform distribution of Cr and Ni atoms,Fe Ni system with unilateral distribution of Ni atoms and Fe Cr system with unilateral distribution of Cr atoms was studied by using the lattice grain boundary model of coincidence position.The results show that there is no grain boundary migration in the Fe Cr Ni system with uniform Cr and Ni distribution and the Fe Ni system with unilateral Ni atom distribution;The Fe Cr system with one-sided distribution of Cr atoms shows obvious grain boundary migration.The migration direction moves towards the direction where Cr atoms exist.The migration rate is 6.06（?）/ns,and the vacancy will slow down the grain boundary migration rate.The random grain boundary model uses 001 and 1-1-2 crystal belt axes to form the grain boundary.The results show that the migration rate of the grain boundary also increases with the increase of temperature.The migration rate is 1.98（?）/ns at 1273 K.The activation energy of grain boundary migration calculated by Arrhenius formula is 1.92 e V.In view of the precipitation behavior of chromium nitride in the hot working process of high nitrogen stainless steel of P550 steel,the best precipitation temperature of chromium nitride 1023K was set as the simulated ambient temperature.By calculating the change of bond number and coordination number of Fe,Cr and N atoms,it was found that the bond number of Fe Cr and Fe-N atoms decreased,Cr atoms and N atoms clustered into atomic clusters,and the decrease of total energy change of the system further showed that the aggregation of Cr and N atoms was the result of the chaotic system tending to be stable.The growth of Cr-N clusters can be divided into two stages:within 0-2ns,the clusters grow rapidly and are controlled by the diffusion of Cr atoms;In 2-10 ns,the structure of Cr-N cluster shows an adjustment behavior,and the diffusion ability of Cr atom and N atom is the same.Through the calculation and analysis of the radius of gyration(（?）_g,p)of the atomic cluster,it is found that the number of atoms in the node cluster of structural transformation during the growth of the Cr-N atomic cluster is n=39 and n=72,respectively.The size,coordination number and radial distribution function of node clusters（Cr-N）₃₉and（Cr-N）₇₂were further calculated.It was found that the formation of Cr-N atomic clusters was an intermediate step in the process of Cr and N atoms precipitating into Cr₂N crystals.The Cr₂N crystals precipitated from high nitrogen stainless steel were realized through the multi-step structure transformation process of Cr-N cluster formation and crystallization.