Effect Of Nitrogen And Copper On Structure And Properties Of Low Nickel Austenitic Stainless Steels

In order to reduce the cost of austenitic stainless steel and improve the mechanical and corrosion properties of austenitic stainless steel,the effects of nitrogen and copper on the microstructure,mechanical properties and corrosion resistance of low nickel stainless steel were studied by the method of combining theoretical calculation with experiments such as scanning electron microscope,transmission electron microscope,photoelectron spectrum analyzer.The mechanism of action of nitrogen and copper in austenitic stainless steel was revealed.A new type of nitrogen-copper composite low nickel austenitic stainless steel was developed.A new type of stainless steel was calculated to decrease to 5wt%nickel content,increase nitrogen content(0.1?0.4wt%)and copper content(0?6wt%)to the influence of austenite and ferrite phase range on the base of 316L stainless steel by using Thermo-Calc thermodynamics software and TCFe8.0 iron-based alloy database.The elastic constants,bulk modulus,shear modulus,elastic modulus of the ?-Fe,Fe-N,Fe-Ni,Fe-Ni-Cu systems and the mechanical constants reflecting the ductility of the material were calculated by First-principles.The calculation results show that N has the highest shear modulus value for ?-Fe,and the ratio of bulk modulus to shear modulus and Poisson's ratio show that Nitrogen increasing is beneficial to the strength and plasticity of ?-Fe.The doping of Cu reduced the shear modulus and elastic modulus of Fe-Ni system by 4.22%and 3.82%,respectively,the ratio of bulk modulus to shear modulus increased by 2.45%and Poisson's ratio increased by 2.16%,indicating that the doping of Cu decreases the strength of Fe-Ni system and decreases the plasticity.The calculation results determined that the optimum composition of nitrogen and copper for the austenite zone and ferrite zone of the test stainless steel was 5wt%Ni+0.3wt%N+4wt%Cu.The experimental results show that the nitrogen steel with 5 wt%nickel content has increased by 32.97%at room temperature tensile strength.The room temperature plasticity change is not obvious;the room temperature V-shaped impact energy value has increased by 43.21%;the microhardness has doubled.The structure of the test steel is single-phase austenite.Nitrogen increasing causes nitrogen to dissolve in the steel substrate and expand the crystal lattice.During the solution treatment,annealing twins are formed,resulting in lattice strengthening and twin strengthening.Nitrogen addition also promotes the precipitation and growth of carbonitrides along the grain boundaries,hindering the movement of the grain boundaries,causing internal stresses in the test steel matrix,and interacting with high-density dislocations,twins,grain boundaries,and stacking faults,which significantly increased the strength of nitrogen steel.The precipitation of Cr2N,CrN and so on in the high-nitrogen copper-containing steel structure proves this phenomenon.Electrochemical tests show that the corrosion rate of nitrogen steel is 59.2%lower than that of 316L steel.The nitrogen added to the steel is enriched and decomposed into ammonium ions and nitrate in the outer layer of the surface passivation film to improve its corrosion resistance.The average weight loss rate of intergranular corrosion of nitrogen steel is lower than the national standard of 1.1 g/m2·h,which the performance meets the national requirements.Electrochemical tests show that the nitrogen steel with 3wt%Cu has the best corrosion resistance,and the formation of Cu2O in the copper in the surface passivation film is the essential reason for improving its corrosion resistance.Arrhenius constitutive equations were used to calculate the thermal deformation activation energy of nitrogen steel,copper-containing nitrogen steel and high nitrogen copper-containing steel.The activation energy value of high nitrogen copper steel with 0.41wt%N is 17.1%higher than that of nitrogen steel with 0.24wt%N,and about 27.6%higher than that of 3wt%Cu nitrogen steel and 4wt%Cu nitrogen steel,Which is 40%higher than the activation energy value of copper steel without nitrogen,Calculations show that increasing nitrogen increases the strength of the steel,and the strength and nitrogen content increase linearly.Adding copper reduces the strength of the nitrogen steel,but it can expand the effective temperature range of hot working.The activation energy value is not Obvious different between nitrogen steels with 3wt%and 4wt%copper,the effect on the temperature range of hot working is obviously different.The combination of calculation and experiment determines that the most suitable composition of the new nitrogen-copper composite low-nickel austenitic stainless steel is 5wt%Ni+0.3wt%N+3-3.5wt%Cu.