| Super martensitic stainless steel has gradually become a key material for oil and gas exploitation and transportation due to its comprehensive advantages such as excellent weldability and good corrosion resistance.However,with the significant increase of energy demand,its strength and toughness can no longer meet the mechanical properties required for oil and gas pipes in deep and ultra-deep Wells.The quenching and partitioning（Q&P）process uses the concentration difference of carbon atoms to partition,and eventually obtains more stable austenite,which can improve the toughness of the material without reducing the strength.However,due to the low carbon content of super martensitic stainless steel,the strength and toughness of the material cannot be improved by carbon partition process.Nitrogen,as a cheap element for the formation and stabilization of strong austenite,can improve the comprehensive mechanical properties of the material through solid solution strengthening,precipitation strengthening and fine grain strengthening,without causing adverse effects on the corrosion resistance and weldability.Therefore,nitrogen partition is used to replace the traditional carbon partition in this project,and the strength and toughness of super martensitic stainless steel is expected to be improved simultaneously.In order to avoid the decrease of corrosion resistance caused by the precipitation of harmful phase Cr2N,when vanadium is added to steel,vanadium and nitrogen will preferentially form VN and disperse in the tissue.At the same time,this study also increased the content of manganese to increase austenite content,so as to further improve the plasticity and toughness of super martensitic stainless steel.Therefore,this topic takes the super martensitic stainless steel alloyed with N,Mn and V as the research object,and designs a reasonable quenching and partitioning process.Use metallographic microscope,scanning electron microscope,transmission electron microscope,electron backscatter diffraction and backscatter diffraction to explore the influence of Q&P process and different alloying elements on phase composition and microstructure evolution;Through tensile test and microhardness test,the influence of Q&P process and different alloying elements on mechanical properties is clarified,and the main rules obtained are as follows:（1）The microstructure of the test steel was composed of lath martensite+retained austenite+reversed austenite after Q&P heat treatment with different patitioning processes.After 400℃×2min partitioning,the composite mechanical properties of the material are the best,and the distribution of reversed austenite in the structure is more uniform and the content is higher.The higher or lower partitioning temperature and the longer partitioning time are not conducive to the formation of reversed austenite.（2）In the process of partitioning,nitrogen diffuses from supersaturated martensite and concentrates at martensit-martensite and martensit-austenite boundaries.The enrichment of nitrogen provides a high local driving force for the formation of reversed austenite,in which the martensitic phase and austenite phase conform to the Kurdjumov-Sachs orientation relationship.The reversed austenite distributed between the lath martensite can absorb the deformation work during deformation and prevent the crack propagation between the lath martensite.（3）The austenite volume fraction in the test steel increases with the increase of nitrogen content from 0%to 0.23%and 0.35%when nitrogen element is used as a strong austenite stabilizing and forming element.In addition,the addition of nitrogen can refine the size of martensite lath obviously.EBSD analysis of tensile fracture of test steel shows that the austenite structure with different stability has more continuous TRIP effect during tensile process,which is beneficial to the improvement of plasticity of test steel.（4）The increase of Mn content in the test steel will lead to the increase of the volume fraction of retained austenite and the increase of grain size,which will lead to the great difference of mechanical properties.When vanadium is added to the test steel,it reacts with nitrogen to form VN,which is dispersed in the tissue to avoid the precipitation of the harmful phase Cr2N.However,the change of vanadium content has no obvious effect on the microstructure and mechanical properties of the steel.In addition,the precipitation of VN is also due to the improvement of the mechanical properties of the material. |
PDF Full Text Request