Effect Of Quenching-Partitioning On Strength And Toughness Of N-V Alloyed Super Martensitic Stainless Steel

Super martensitic stainless steel has become a key material for oil and gas exploration and transportation due to its comprehensive advantages such as good corrosion resistance,formability and weldability,but its strength and toughness level can not meet the requirements of ultra-high strength and toughness required by materials for oil and gas pipelines in deep wells and ultra-deep wells.At present,there is a new type of heat treatment quenching-partitioning process for high carbon martensitic stainless steel,which uses the concentration difference of carbon atoms for partitioning,and finally obtains more stable austenite,which can improve the toughness of the material without reducing the strength.However,the super martensitic stainless steel has a low carbon content and cannot improve the strength and toughness of the material through the carbon distribution process.N as a low-cost strong austenite formation and stabilizing element,can affect toughness by promoting the precipitation of austenite in the steel,and can increase the strength of the steel.Therefore,in this topic,the partitioning of nitrogen element is expected to simultaneously improve the strength and toughness of super martensitic stainless steel.At the same time,in order to avoid harmful Cr2 N precipitated by the addition of nitrogen,V is added to stainless steel to promote the dispersion and precipitation of VN,which can effectively improve the strength of steel.We research here that the effect of quenching and partitioning treatment on the microstructure and properties of super martensitic stainless steel to enhancing the ductility of super martensitic stainless steel while preserving their strength.The microstructure is observed and analyzed by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),Electron Backscatter Diffraction(EBSD),X-ray diffraction(XRD)and ferrite magnetometer;the mechanical property is measured through tensile test,microhardness test and fracture morphology observation.The main conclusion are as follow:(1)The matrix structure of the two test steels with different V contents are lath martensite.The lath martensite beam width of 0.12 V steel is less than 0.06 V steel,and the austenite content is also less than 0.06 V steel,indicating that the V element can be fine crystallize grains and inhibit the formation of austenite;When the V content is increased from 0.06 to 0.12,the tensile strength,yield strength,and microhardness increase by 283 MPa,121 MPa,and 47.3 HV,respectively,and the elongation decreases by 4.3%.It can be seen that the V element cannot simultaneously improve the strength and toughness of the test steel.(2)When the N content was increased from 0.1 to 0.35,the lath martensite bundle was significantly refined,the austenite volume fraction was increased,the tensile strength,yield strength,and microhardness were significantly improved,and the elongation did not change much.Among them,tensile strength increased by 558 MPa,yield strength increased by 466 MPa,micro hardness increased by 224 HV,and elongation increased by 6.5%.It shows that N element can increase its strength without reducing the toughness of the test steel.(3)The microstructure of the test steel under different quenching-partitioning systems consists of lath martensite and austenite.Compared with the as-quenched state,the austenite content in the test steel increased significantly after the partition treatment,indicating that austenite was formed after the partition treatment.The austenite content decreases with the increase of the partition temperature,the grain size also becomes smaller and smaller,and there have nano reversed austenite.(4)Compared with the quenched state,the mechanical properties of the test steel are improved after the partition treatment.Amonghem,the comprehensive mechanical properties of the test steel reached thebest when it was dispensed at 400 ? for 2minutes.At this time,the tensile strength was 1681 MPa,the yield strength was 953 MPa,the microhardness was 538.5 HV,and the elongation was 15.23%.