Evolution Of Reversed Austenite Organization In Low Carbon Martensitic Stainless Steels With Coupled Tempering History

After proper quenching and two-phase tempering heat treatment of low-carbon martensitic stainless steels,the reversed austenite organization can be obtained that is diffusely distributed in the tempered martensitic matrix,and the presence of reversed austenite can effectively improve the strength and toughness of the material.However,the evolution of reversed austenite in the tempering process of martensitic steel in the two-phase zone is more complex,which brings greater difficulties in regulating reversed austenite.In this paper,using 0013Cr4Ni Mo steel as the research material,the combination of heat treatment experiments and crystal plasticity finite element simulation is used to study the mechanism of tempering organization on the impact toughness of the material,pointing out the importance of reversed austenite,and then studying the influence of two-phase zone tempering history on the thermal stability of reversed austenite,and then constructing a numerical model for the volume fraction of reversed austenite during tempering,and finally the model is validated with experimental results.The main work of the paper is as follows:（1）The microscopic mechanisms of tissue influencing the impact toughness of the material are compared and analyzed by impact experiments and crystal plasticity finite element simulations.The results show that the impact toughness of the material has been improved by the refinement of martensitic slats,the presence of reversed austenite and the increase of the volume fraction of reversed austenite,and the carbide decrease the impact toughness of the material.（2）The effects of tempering history such as tempering temperature,holding time and cooling rate on the volume fraction of reversed austenite of0013Cr4Ni Mo steel were investigated using in situ XRD.The experimental results showed that the reversed austenite content increased with the increase of tempering temperature and holding time at the high temperature stage when the equilibrium state was not reached;the reversed austenite content at room temperature showed a parabolic change law of increasing and then decreasing with the increase of tempering temperature and holding time,and decreased with the increase of cooling rate.（3）The effects of tempering history such as tempering temperature,holding time and cooling rate on the thermal stability of 0013Cr4Ni Mo steel reversed austenite were investigated.The experimental results show that with the increase of tempering temperature,the extension of holding time and the acceleration of cooling rate,the size and quantity of M₂₃C₆increase,and a certain amount of carbon in the reversed austenite is consumed by the formation and growth of M₂₃C₆,which leads to the decrease of its thermal stability.The morphology of reversed austenite with low insulation time shows a thin strip,which gradually transforms into small pieces with the extension of holding time,reducing the interfacial resistance to the transformation of reversed austenite to martensite,which also leads to the decrease of thermal stability.（4）Based on the isothermal transformation experimental curve,the isothermal transformation calculation model of 0013Cr4Ni Mo steel tempered in the two-phase zone is established.The model is framed by the JMAK equation,and the effects of tempering temperature and holding time on the volume fraction of reversed austenite are considered;based on the continuous cooling transformation experimental curve,the continuous cooling transformation calculation model of 0013Cr4Ni Mo steel tempered in the two-phase zone is established,and the model is framed by the KM equation,and the effects of tempering temperature and tempering holding time on the volume fraction of reversed austenite are considered.（5）Combined with the experimental results to verify the numerical model of reversed austenite tissue with coupled tempering history,the results show that the numerical model has high reliability within a certain range of tempering temperature and holding time;combined with the energy spectrum,kinetic equation parameters and apparent activation energy,it indicates that the nucleation growth mechanism of reversed austenite is interfacial nucleation,the nucleation rate decays rapidly to zero,and the diffusion growth mechanism is mainly controlled by Ni element diffusion.