Study On Microstructure And Properties Of High Nitrogen Corrosion Resistant Alloy By Centrifugal Casting

High nitrogen austenitic stainless steel has many excellent properties,such as high strength,good plasticity,good fracture toughness,good work hardening ability and excellent corrosion resistance,these attractive properties are closely related to solid solution nitrogen,because N replaces a part of Ni,high nitrogen austenitic stainless steel can also replace the relatively high price of nickel-based alloy to a certain extent.The addition of N is the main reason for improving the tensile strength and corrosion resistance of high nitrogen steel.Because of its excellent corrosion resistance,it is considered to have great application space in the field of Marine domain and chemical petroleum,which is also one of the main driving forces for the in-depth study of high nitrogen austenitic stainless steel in recent decades.In this paper,0Cr20Mn18Mo2Ni1.8N0.8 high nitrogen alloy used in non-magnetic oil drill collars was taken as the research object.After annealing and solution treatment,hardness test,microstructure observation and XRD phase composition detection were carried out to study the changes in microstructure of high nitrogen alloy and the amount of second phase precipitation under different solution temperatures after annealing.Summarize the mechanism of organizational change.The hardness of high nitrogen alloy with different cold rolling deformation was tested,and the microstructure of the section in the direction of rolling was observed.The effect of rolling deformation on grain size and grain orientation was analyzed.Electrochemical polarization performance test,impedance performance test,draw potentiodynamic polarization curve and AC impedance curve,analyze the total different deformation of the test steel corrosion resistance rule,so as to provide some reference for the actual application of the test steel environment.Tensile properties of high-nitrogen alloys with different cold-rolled deformation were experimented at room temperature and high temperature,stress-strain curves were drawn,the microstructure around the fracture was observed and analyzed,and the fracture mechanism was analyzed by observing the fracture morphology.The results are as follows:(1)After precision forging,annealing and solution processing,the grains of the high nitrogen alloy are equiaxed on the whole,only a small part of the grains have deformation twins,and there is a small amount of granular second phase in the microstructure of austenite.(2)Electrochemical tests show that,the high nitrogen alloy after solution treatment has a wider passivation platform than the annealed high nitrogen alloy.The self-corrosion current density of the sample after solid solution treatment at 1180℃reaches 4.322×10-6A/cm~2.The self-corrosion potential is-0.86 V,so the corrosion resistance of solid solution treated samples at 1180℃is better than that of other solid solution treated samples.(3)The tensile results of high nitrogen alloy at room temperature show that the grain orientation changes along the rolling direction with the increase of cold rolling deformation.Among them,30%of the alloy samples with cold rolling deformation have a large number of dislocation and deformation twins,and the deformation twin zone step phenomenon is obvious.Under the action of the stress,the twins and dislocation in the grain were bent,resulting in the tensile strength increased from 1197.1 MPa to 1548.3 MPa,and the plasticity decreased from 65.3%to 15.2%.The high temperature tensile test results show that the work hardening rate of the alloy sample is less than the softening rate,the interatomic force is weakened,The tensile strength of the alloy samples increased from 294.1 MPa to 354.8MPa and then decreased to 262.8 MPa,while the plasticity decreased from 18.5%to 12.0%and then increased to 26%.(4)With the increase of cold rolling deformation,the roughness of high nitrogen alloy surface decreases,the number of pits on surface increases,the integrity of passivated film decreases,and the grain size changes unevenly.High nitrogen alloy corrosion resistance with the increase of cold rolling deformation after the first drop rising trend The reason is that at the initial corrosion stage,with the increase of cold rolling deformation,the original passivation film on the surface of high nitrogen alloy is destroyed,and the grain size changes are inconsistent,leading to the corrosion resistance decreases.The grain size of the high nitrogen alloy matrix tends to be consistent,and more banded fiber structures are formed in the matrix,which can promote the formation of passivation film on the surface of the high nitrogen alloy,so the corrosion resistance is improved.