Effects Of Mn Content And Solution Temperature On The Microstructure And Properties Of 21-23%Cr Nickel-saving Duplex Stainless Stee

The use of Mn in duplex stainless steel to replace expensive Ni can significantly reduce costs.Solid solution treatment as an important heat treatment process for duplex stainless steel directly affects the ratio of the two phases,and the solid solution temperature affects the rate of diffusion of alloying elements,which in turn affects its mechanical properties and corrosion resistance.In this paper,the effect of solid solution treatment on the mechanical and corrosion resistance of 8.3%Mn-21%Cr,15%Mn-22%Cr and 16.7%Mn-23%Cr experimental steels was studied and analyzed in comparison with 2205 duplex stainless steel.Metallographic microscopy,scanning electron microscopy,transmission electron microscopy,electrochemical workstation and other experimental methods were used to characterize the metallographic organization,mechanical properties and corrosion resistance of the experimental steels,to study the mechanism of the evolution of the organization and mechanical and corrosion resistance of the experimental steels with different solid solution temperatures and Mn contents,and to provide a theoretical basis for the design and heat treatment of Ni-segmented duplex stainless steels.The main research results are as follows:1.With the increase of solid solution temperature,the austenite volume fraction decreases,while the ferrite volume fraction increases with 8.3%Mn and 16.7%Mn experimental steels and increases and then decreases with 15%Mn experimental steels,the KCr distribution coefficient decreases and the KNi distribution coefficient increases;with the increase of Mn content,the austenite phase volume fraction increases and the ferrite phase volume fraction With the increase of Mn content,the volume fraction of austenite phase increases,the volume fraction of ferrite phase decreases,and the morphology of ferrite phase changes from long and scattered distribution to sheet-like.The experimental steel with 8.3%Mn can maintain a good ratio of the two phases at different solid solution temperatures,indicating a strong ability to stabilize the two phases.2.With the increase of Mn content,the impact toughness showed a tendency to decrease;with the increase of solid solution temperature,the impact toughness showed a tendency to increase first and then decrease.The impact work of 8.3%Mn experimental steel reaches the highest 242 J at the solid solution temperature of 1050°C.Compared with 2205 experimental steel,the fracture cross-sectional area is larger,the equiaxial toughness nests are densely distributed,the hardness difference between the two phases is minimal,the dislocation density increases,the crack expansion is hindered,more energy is absorbed during fracture,and the impact toughness is higher;the increase of Mn content to 15%and 16.7%solid solution temperature The impact toughness of the experimental steel was reduced by the precipitation of moreσ-phase at 950°C,and the fracture showed brittle fracture,but as the solid solution temperature increased to 1000°C,theσ-phase disappeared and the impact toughness reached 230 J and 176 J,respectively.3.With the decrease of impact temperature,8.3%Mn,15%Mn,16.7%Mn,2205 experimental steel solid solution temperature at 1050℃,1000℃,1000℃,1050℃occurred a significant tough-brittle transition phenomenon（DBTT）.The impact temperature was reduced from 20°C to-160°C,and the impact toughness of 8.3%Mn experimental steel at 1050°C was reduced from 232.6 J to 6.4J,and its DBTT was-86.6°C;the impact temperature of 15%Mn experimental steel at 1000°C was reduced from 20°C to-60°C,and its impact toughness was reduced from 258 J to 7.1 J,and its DBTT was-1.04°C;16.7%Mn experimental steel at 1000℃impact temperature from 20℃down to-60℃,its impact toughness from 176 J down to 5.9 J,DBTT for-32℃;2205 experimental steel at1050℃impact temperature from 20℃down to-160℃,its impact toughness from 168 J down to2.4 J,DBTT for-84.2℃,indicating that at low temperature 8.3%Mn experimental steel and 2205experimental steel has strong toughness and brittleness performance compared to 2205 experimental steel.4.The tensile strength of 2205 experimental steel are around 760MPa,the elongation are around30%,the tensile strength is less affected by the solid solution temperature.the Mn content increases to 15%and 16.7%,the experimental steel with the increase of solid solution temperature post-extension decreases,the tensile strength and yield strength show a rising trend;at the same time 8.3%Mn experimental steel in different solid solution temperature have high post-extension（48%）and tensile strength（720 MPa）at different solid solution temperatures,indicating that the 8.3%Mn addition has a strong ability to stabilize the two phases,and with the increase of solid solution temperature,the tensile and yield strengths increase and the post-break elongation decreases.5.With the increase of solid solution temperature,the pitting potential（E_b）and polarization resistance（R_p）of 15%Mn experimental steel increased,while the pitting potential（E_b）and polarization resistance（R_p）of 8.3%Mn and 16.7%Mn experimental steel first increased and then decreased;with the increase of manganese content,the pitting potential of experimental steel first increased and then decreased,the impedance spectrum radius is the trend of increasing and decreasing,and the phase angle peak platform range Narrower,the corrosion rate is accelerated,which is not conducive to the formation of passivation film and repair.Among them,15%Mn experimental steel solid solution temperature at 1050℃,has a high E_b（0.9V）and 2205 experimental steel is close to,and Cr₂O₃,Fe OOH content is relatively high,K_Cr distribution coefficient is low,the alloy diffusion rate is fast,and loose porous Mn O content is relatively low,easy to form a stable passivation film.6.With the increase of solid solution temperature,the intergranular sensitivity value（R_a）decreases;with the increase of manganese content,its intergranular sensitivity value（R_a）increases,in which the solid solution temperature of 8.3%Mn experimental steel is 1100℃,the smallest intergranular sensitivity value of 0.015 is obtained,indicating that among different high manganese experimental steels,8.3%Mn experimental steel has the highest intergranular corrosion resistance at1100℃,the experimental steel has higher R_a value compared to 2205 experimental steel,which is due to the fact that this experimental steel has higher Mn content compared to 2205 experimental steel,which will aggravate the degree of corrosion and increase the intergranular corrosion tendency.