| High nitrogen austenitic stainless steel has been widely used in modern oil and gas drilling,especially non-magnetic drill collar due to its excellent mechanical properties,low production cost and good corrosion resistance.At present,the main grades of high-nitrogen austenitic stainless steel for non-magnetic drill collars in China are still W1813N and N1310B.With the continuous development of alloying theory and the continuous improvement of drilling depth,there are serious strength shortages and intergranular corrosion during use,which can not meet the modern super High temperature,high pressure,and highly corrosive environment often encountered in deep drilling.Therefore,it is necessary to carry out research on high-nitrogen austenitic stainless steel for new non-magnetic drill collars.Based on the composition of P530,the effects of key alloying elements in high-nitrogen austenitic stainless steel on the austenite phase zone,precipitated phase zone and nitrogen solid solubility of the experimental steel were calculated by using the thermodynamic software.According to the results of thermodynamic calculation,the design composition of the experimental steel is presented,and the experimental steel which conforms to the design composition of the experimental steel is obtained by smelting.The precipitation phase of Cr=16.15%experimental steel under different aging conditions was observed by metallographic microscope.The results show that the precipitation sensitive temperature of Cr=16.15%experimental steel is 750°C850°C,The precipitated phase produced at the grain boundary can be clearly observed at 30 min after aging,and the number of precipitated phases increases sharply after 2 h of aging,the size becomes larger,and the tendency to grow into the crystal appears.The deformation obviously promoted the generation of the precipitated phase,and there were more precipitated phases at the grain boundary of the experimental steel which was kept for 1 min after deformation.The precipitated phase was determined to be Cr23C6 by scanning electron microscopy and transmission electron microscopy.The tensile test machine,impact tester and Brinell hardness tester were used to test the mechanical properties of the experimental steel under different warm forging conditions.The results show that the mechanical properties of the experimental steel gradually increase with the decrease of deformation temperature,the increase of deformation and the increase of nitrogen content,and the plastic toughness decreases gradually.After the temperature deformation,the mechanical properties and plastic toughness of the experimental steel obviously reach the standard of the material for non-magnetic drill collar.The performance index of Cr=16.15%experimental steel with deformation temperature of 600°C and deformation of 20%is Rp0.2=1102.0MPa,Rm=1231.7MPa,A=24%,Akv=50J,Brinell hardness=380.3HB,obviously high Performance on the P530.The magnetic permeability of the experimental steel was tested by a magnetometer.The results show that the magnetic permeability of the experimental steel is basically around 1.005,which is lower than the magnetic permeability standard of the non-magnetic drill collar material.With the increase of Ni equivalent/Cr equivalent,the magnetic permeability of the experimental steel gradually decreases.The deformation temperature and deformation have little effect on the magnetic permeability of the experimental steel.However,the temperature control during the deformation process is unreasonable,which leads to the precipitation of the material.The magnetic permeability is increased.Corrosion resistance of experimental steel was tested by sulfuric acid-copper sulfate method and EPR method.The results show that the experimental steel intergranular corrosion resistant performance is good,only the deformation temperature is 600?CC1 experiment of intergranular corrosion crack occurs on the surface of steel;The EPR results showed that the reactivation rate of stainless steel gradually decreased with the increase of chromium and nitrogen content,and the reactivation rate was generally lower than 5%,indicating that the experimental steel had a low tendency of intergranular corrosion and excellent corrosion resistance. |
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